Hydrographic and transport variability on the Halifax section

John W. Loder, Charles G. Hannah, Brian D. Petrie, and Elizabeth A. GonzalezBedford Institute of Oceanography, Fisheries and Oceans Canada, Dartmouth, Nova Scotia, Canada

Received 13 December 2001; revised 7 November 2002; accepted 13 December 2002; published 17 September 2003.

[1] Archived data and geostrophic computations are used to examine variability inhydrographic properties and along-shelf transport on the Scotian Shelf, with focus on theHalifax section and a decadal-scale hydrographic anomaly during the late 1950s and early/mid 1960s. The long-term annual cycle shows strong seasonal variations in baroclinictransport on the inner shelf and at the shelf edge, and an associated steric change inadjusted sea level (ASL) at Halifax. Regional wind-forcing and barotropic currents makesmaller contributions to the annual cycle in ASL. In contrast, regional wind-forcingcontributes about 40% of the ASL variability for periods of 6–30 days. Hydrographicsections indicate that the 1950s/1960s anomaly arose from episodic extensions ofLabrador Slope Water along the shelf edge followed by on-shelf intrusions, primarilyduring the fall-spring periods of 1958–1959 and 1963–1964. It ended with excursions ofWarm Slope Water into the region in 1967–1968. An analysis of monthly temperatureanomalies indicates that slope temperatures can account for 50% of the variance of deepshelf temperatures 3–6 months later. Correlations between the hydrographic andbaroclinic transport variations indicate increased southwestward flow associated withlower temperatures and salinities, but no evidence was found for large local current eventsat the onset and termination times of the 1950s/1960s anomaly. Positive correlationsbetween wind-adjusted ASL and steric anomalies point to the potential for sea level beinghelpful in monitoring baroclinic transport variability. INDEX TERMS: 4223 Oceanography:

General: Descriptive and regional oceanography; 4219 Oceanography: General: Continental shelf processes;

4215 Oceanography: General: Climate and interannual variability (3309); KEYWORDS: Scotian Shelf, Scotian

Slope, Halifax section, hydrographic variability, transport variability

Citation: Loder, J. W., C. G. Hannah, B. D. Petrie, and E. A. Gonzalez, Hydrographic and transport variability on the Halifax section,

J. Geophys. Res., 108(C11), 8003, doi:10.1029/2001JC001267, 2003.

1. Introduction

[2] Recent findings that the seasonal circulation in theScotian Shelf region is primarily baroclinic [Han et al.,1997; Hannah et al., 2001; Sheng et al., 2001], togetherwith evidence of substantial decadal-scale and interannualchanges in hydrographic properties of advective origin[Petrie and Drinkwater, 1993; Drinkwater et al., 1999;Smith et al., 2001], point to the potential for significantdecadal-scale and interannual circulation changes. However,a major difficulty in the identification of such changes isthe absence of long time series of current measurements.In particular, there are few current observations from thelate 1950s and 1960s when there was a pronounced hydro-graphic anomaly extending from the Gulf of St. Lawrenceto the Middle Atlantic Bight [Petrie and Drinkwater, 1993](hereinafter referred to as PD93). On the other hand,diagnostic model computations from multiyear compositehydrographic fields indicate a substantial increase in theequatorward transport of subpolar water on the ScotianShelf and Slope during this period [Greatbatch et al.,1991; Loder et al., 2001].

[3] While the hydrographic database for the Scotian Shelfhas provided a realistic representation of the long-termannual variation in 3-D temperature and salinity [e.g., Petrieet al., 1996; Loder et al., 1997], as well as indices ofinterannual variability at some sites (e.g., PD93), it isgenerally not adequate for providing 3-D seasonal distribu-tions in individual years. The most frequent sampling ofScotian Shelf hydrographic structure comes from repeatedoccupations of the ‘‘Halifax section,’’ a set of seven stationsextending across the shelf and slope from Halifax to the2700-m isobath (Figure 1). The section was occupiedapproximately seasonally from 1950 to the mid 1970swhich includes the period of the anomalous conditions inthe late 1950s and early 1960s. The sampling was thendiscontinued because of concerns about aliasing of high-frequency variability [Mann and Needler, 1967]. Thoughthe recent identification of decadal-scale hydrographicchanges indicates that the concerns were overestimated forhydrographic properties, questions about aliasing effectsremain for estimates of high-frequency hydrographic vari-ability and baroclinic circulation (through geostrophic com-putations) from sparse data.[4] Our goal is to describe and understand the temporal

and cross-shelf variability of the 1950s/1960s hydrographicand circulation changes on the Scotian Shelf, drawing on

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C11, 8003, doi:10.1029/2001JC001267, 2003

Copyright 2003 by the American Geophysical Union.0148-0227/03/2001JC001267

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the full temporal resolution of Halifax section, coastal sealevel and wind stress observations. Attention is focused onthe onset and termination of the anomalous conditions, andthe transport variability in the region’s two major flows: theNova Scotian Current on the inner shelf and the shelf-edgecurrent. We also examine the relationships among sea level,wind stress, steric height, and transport estimates, andevaluate the extent of the coupling between shelf and slopehydrographic anomalies.[5] We start with a description of the data sets and

analysis methods in section 2. We then discuss the long-term (climatological) annual cycle in the hydrography andother variables (section 3), as background for the interan-nual variability evaluations. The interannual variabilityresults are presented in section 4, starting with the variabil-ity indices, then focusing on particular events and the shelf-slope coupling, and finally examining the inter-relationsamong the variability indices. The results are summarizedand discussed in section 5.

2. Methods

2.1. Observational Data and Fields

[6] The primary information source is the historical oceantemperature and salinity database [e.g., Petrie et al., 1996]which includes data from dedicated occupations of theHalifax section [Taylor, 1961, 1966; de la Ronde, 1972;Drinkwater and Taylor, 1982; Dobson, 1975, 1977, 1978].Measurements were taken with reversing thermometers andbottle samples prior to 1967, after which a conductivity-temperature-depth profiler was generally used. During mostoccupations of the Halifax section (typically completed in aday), profiles were obtained at seven stations (Figure 1).Sampling was usually over the whole water column for theshelf stations (stations 1–5), but often only over the upper300–1000 m for the slope stations.[7] Using 4-D optimal linear interpolation [Bretherton et

al., 1976], cross-shelf fields of temperature, salinity, anddensity were estimated for the long-term annual cycle

(over 1950–1996) on three cross-shelf sections (Figure 1):(1) the Halifax section extending offshore to the 2700-misobath, and to a depth of 500 or 1000 m; (2) the Lahavesection (6 stations) on the western Scotian Shelf extendingoffshore to the 400-m isobath; and (3) the SW NovaScotia section (six stations) extending from Cape Sableacross Browns Bank to the middle of Northeast Channel(maximum depth of 245 m). The spatial grid had 5-kmhorizontal spacing and standard vertical positions of in-creasing separation with depth (and extending below theseafloor to allow near-bottom interpolation). The time gridpoint was taken as the mid-day of each section occupation,or the first day of each month for the annual cycle. Thehorizontal correlation scales in the assumed covariancefunction [Loder et al., 1997] were specified as 60 and30 km in the along- and cross-shelf directions. Specifiedvertical correlation scales increased from 10 m at the surfaceto 75 m at 500 m, and the temporal correlation scale was45 days.[8] On the basis of data availability, two sets of fields

were estimated for individual occupations of the Halifaxsection (with nearby data included) during 1950–1996:(1) a first set extending offshore to station 6 near the1000-m isobath and to a depth of 300 m, which gave morerealizations; and (2) a second set extending offshore tostation 7 and to a depth of 500 m, which provided greaterspatial coverage over the slope. The spatial correlation scaleswere the same as in the long-term estimates and the temporalcorrelation scale was 20 days. Sensitivity studies indicatedthat the details of the hydrographic and velocity structurewere often dependent on the correlation scales, but the nettransport estimates had limited sensitivity to these scales.Two types of data screening were used. First, a visualinspection of the data distribution was used to eliminateoccupations that had substantial areas without data. Second,hydrographic indices were included in subsequent analysesonly when their average relative error (ratio of standarddeviation of estimated value to the standard deviation of thefield) was under 0.45 (typically, the relative error was 0.2).[9] Time series of wind stress from Sable Island and

adjusted sea level (ASL) from Halifax (Figure 1) wereused to interpret the transport variability inferred from thehydrographic sections. The wind stresses were computedfrom hourly wind observations using a quadratic stress lawand the drag coefficients from Smith [1988], and thenaveraged to daily values. Hourly sea level data from Halifaxwere adjusted for the inverse barometer effect using localatmospheric pressure data, and then averaged to dailyvalues. The mean and trend during 1955–1970 (the studyfocal period) were removed from each time series, providingtime series of daily de-meaned, de-trended values. Theremoval of the trend over this period was especially impor-tant for sea level because of a secular change of 7.1 cm,about half of which can be accounted for by regionalsubsidence [Douglas, 1991]. The annual cycle was computedby averaging these values over all years, and smoothing witha 30-day box-car filter. Daily anomalies of both wind stressand ASL were then obtained by subtracting the long-termcycle from the daily de-meaned, de-trended values. Box-caraverages of the anomalies were used to obtain bi-daily,weekly, and monthly anomalies for use in the interannualvariability investigations.

Figure 1. Map of the Scotian Shelf showing majortopographic features and the location of the cross-shelfsections used in the study. Station locations and cross-shelfzones on the Halifax section are indicated. The innerportions of the Lahave and SWNS sections are shoreward ofthe perpendicular bar on the section lines.

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[10] Finally, in order to examine shelf-slope hydrographiccoupling with maximum temporal resolution, monthly sub-surface temperature anomalies were constructed for theinner shelf (75–175 m; area 14 of Petrie et al. [1996]),Emerald Basin (100–200 m; area 12), and the Slope Water(100–200 m; area 33). There was at least 1 observation in466 (472, 472) months for the inner shelf (Emerald Basin,Slope Water) during the 615 months from January 1950 toMarch 2001. The means and annual cycles were estimatedand removed, and gaps in the time series were filled bylinear interpolation.

2.2. Baroclinic Velocities and Transports

[11] Estimates of the along-shelf ‘baroclinic’ flow (asso-ciated with horizontal density gradients) were obtained fromgeostrophic computations on the cross-shelf density fields.For each density field, a baroclinic velocity field wascomputed to balance the sum of the local depth-varyingpressure gradient (due to density gradients) and the depth-invariant pressure gradient (due to a sea surface slope) thatgives zero total pressure gradient (and hence no motion) atthe seafloor or a specified reference level. The associatedsea surface elevation obtained by integrating the surfaceslope along the section is referred to as the ‘‘steric eleva-tion’’ (relative to the offshore starting point of the integra-tion and the reference level).[12] Baroclinic transports were computed through verti-

cal, followed by horizontal, summations of the baroclinicvelocities over specified grid intervals (and so may notrepresent the peak transport in a particular direction).Cumulative transport refers to the net value of depth-integrated transport between specified horizontal positions.Barotropic transport is taken to be the component estimatedwhen the near-bottom velocity is used for the entire watercolumn.[13] In the examination of the (long-term) annual cycles,

the reference level was 500 or 1000 m for the Halifaxsection, and the seafloor for the other sections. In theindividual-occupation evaluations for the Halifax section,it was 300 m (first set) or 500 m (second set). Estimates oflong-term hydrography, velocity, transport and steric eleva-tion were also obtained for the date of each occupation ofthe Halifax section using the same reference level, and theresulting fields were subtracted from the individual-occu-pation fields to obtain anomaly fields. In addition, long-termcoastal steric heights relative to 250 m in Emerald Basinwere computed using composite monthly-mean density datafrom station 1 for 0–75 m, station 2 for 75–150 m, andstation 3 for 150–250 m.

2.3. Statistical Analyses and Circulation Model

[14] Variability indices for the Halifax section were com-puted from the hydrographic and velocity fields (or theiranomalies) by spatial averaging over specified areas. Thesection was divided into four cross-shelf and two verticalzones. The cross-shelf zones were designated Inner (coast tostation 3), Mid (station 3–5), Outer (station 5–6), andOffshore (station 6–7) (Figure 1), with the Inner zoneexpected to include most of the Nova Scotian Current andthe Outer zone most of the shelf-edge current. Spatially-averaged hydrographic indices were computed for theUpper (100 m and above) and Lower (125 m and below)

portions of the estimated fields in these areas. Indices ofdepth-integrated transport and cross-shelf steric elevationdifferences were computed for each of the four cross-shelfzones (and combinations thereof). An index was retained inthe analyses when the zone-averaged relative errors for allcontributing zones (e.g., local zone for local hydrography,or both vertical zones for transport) were less than 0.45.High-resolution temperature and salinity time series frommoored meters on the inner shelf [Anderson and Smith,1989] and at the shelf break [Smith and Petrie, 1982] wereused to quantify the overall temperature and density vari-ability. Temperature standard deviations ranged from 0.5�Cin the Inner zone to 1.0�C in the Outer zone, which implyuncertainties of 0.25–0.5�C in the temperature indices.Representative uncertainties in the transport indices wereestimated to be 0.1 Sv in the Inner zone, 0.35 Sv in theOuter zone, and 0.5 Sv in the Offshore zone.[15] The hydrographic, steric elevation and transport

indices, together with the observed Halifax ASL anomaliesand estimates of wind-forced anomalies from either aregional circulation model or a wind stress regressionrelation, were used in standard statistical evaluations(variances, correlations, regressions, coherence, etc.) ofthe relationships among variables. Model predictions ofregional wind-forced contributions to transport and sea levelwere obtained with the Lynch et al. [1992] linear barotropicfinite element model (as used by Greenberg et al. [1997]),on a grid extending from the eastern Scotian Shelf to CapeCod. Spatially-variable friction coefficients were taken fromnonlinear model solutions for tidal and seasonal-meandensity and wind-forcing [Hannah et al., 2001], with theeffect of an additional background current magnitude of10 cm/s included. The wind stress anomalies from SableIsland were used to force the model solutions, with zeroelevation specified on the upstream boundary. Thus themodel-predicted wind influences are only associated withstress over the Scotian Shelf, and do not include ‘‘upstream’’influences which can account for 30–40% of the coastal sealevel variance at Halifax for periods of 2–20 days [e.g.,Schwing, 1989]. As an alternative, a linear regressionrelation between Halifax ASL and Sable Island wind stresswas also used to estimate the wind-forced contribution toHalifax ASL anomalies. For the interpretation of the annualcycle of Halifax ASL, the circulation model was used withthe long-term monthly-mean stresses from Sable Island toestimate the regional wind-forced contribution.

3. Annual Cycle

3.1. Hydrography and Baroclinic Velocity

[16] The long-term annual cycle of hydrography andbaroclinic velocity on the Halifax section is illustrated bythe cross-shelf fields in Figure 2. Aspects of this climatol-ogy have been described by several authors [e.g., Smith etal., 1978; Drinkwater et al., 1979; Umoh and Thompson,1994; Petrie et al., 1996; Loder et al., 1997], so only a briefsummary will be given here.[17] The basic vertical structure of the shelf hydrogra-

phy is an upper layer of relatively-fresh water originatingfrom the Gulf of St. Lawrence and the NewfoundlandShelf/Slope, overlying more saline water of offshore(slope) origin in Emerald Basin (Figure 2). In winter

LODER ET AL.: HALIFAX SECTION VARIABILITY GLO 4 - 3

and spring, temperature and salinity show similar struc-ture, with values generally increasing with depth anddistance offshore, and the largest horizontal gradientsoccurring on the inner shelf and at the shelf edge. Fromspring through fall, a warm shallow near-surface layerevolves associated with solar heating, resulting in aremnant cold intermediate layer that extends offshore tothe shelf edge with minimum temperatures on the innershelf. The seasonal variations of the upper and interme-diate layers are determined largely by air-sea interactionsand along-shelf advection [e.g., Umoh and Thompson,1994], while the properties of the deep basin waters aredetermined primarily by exchanges through the ScotianGulf [e.g., Smith et al., 1978]. Over the upper continentalslope, there is a wedge of the relatively fresh upper-layershelf water, overlying a layer of Warm Slope Water(temperature of 9�–12�C, salinities above 35) centeredat 200–300 m, and Labrador Slope Water (4�–9�C,34.5–35) at depth [Gatien, 1976].[18] The dominant features of the baroclinic velocity

fields on the Halifax section are surface-intensified south-

westward flows on the inner shelf and near the shelfedge over the upper slope (Figure 2). The inner-shelfflow, centered near the 150-m isobath and extending fromstation 1–3, is strongest in winter (peak near 30 cm/s)and weakest in summer (peak of 10–15 cm/s). This is theNova Scotian Current that was described by Drinkwater etal. [1979] who found substantially lower geostrophictransports due to a shallower (100-m) reference level.The upper-slope flow is strongest (20–30 cm/s) in winterand spring, and weakest (10–20 cm/s) in summer andfall, with its central position fluctuating between the 200-and 500-m isobaths. This flow was not detected in theDrinkwater et al. [1979] analysis, but is consistent withthe downstream remnant of the shelf-edge LabradorCurrent [Han et al., 1997; Loder et al., 1998] and thecyclonic Slope Water gyre over the Scotian Slope [Csanady,1979]. The velocity fields also show other weaker andgenerally smaller-scale features whose reliability is uncer-tain. An exception is the predominantly northeastward(but weak) flow over the northern half of Emerald Bank(station 3–5) which is consistent with the shelf-edge flow

Figure 2. Annual cycle of temperature, salinity and along-shelf baroclinic velocity (positive south-westward, green/yellow) on the Halifax section, extending offshore to station 7 with a reference level of500 m. Contour intervals are 2�C, 0.5 and 10 cm/s, respectively. Distributions are shown for 1 February(day 32, winter), 1 May (day 121, spring), 1 August (day 213, summer), and 1 November (day 305, fall).Standard station locations are indicated in the upper left panel.

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making an onshore meander that moves counterclockwisearound Emerald Basin [e.g., Thompson and Griffin, 1998;Hannah et al., 2001].[19] The annual cycles of baroclinic transport on the

Halifax section and the two downstream sections on thewestern Scotian Shelf are summarized in Figure 3. In

each case, the variation in cumulative transport is shownfor the inner shelf, and for the whole shelf extendingbeyond the shelf edge. For the inner shelf, the (south-westward) transports are much larger on the Halifaxsection where they vary from about 1.1 Sv in winter to0.5 Sv in summer, than on the western Scotian Shelfwhere they are in the �0.1 to 0.3 Sv range. This along-shelf decrease is consistent with that described by Hannahet al. [1996, 2001] from numerical model diagnoses of3-D climatological density fields, and with their suggestionthat the western Scotian Shelf is a dynamical barrier to theequatorward penetration of subpolar water. The wintertransport estimates for the inner Halifax section are withinthe uncertainty estimates of measured transports (Figure 3)computed from the CASP current meter array (lateNovember 1985 to early April 1986 [Anderson and Smith,1989]). Similarly, the present estimates for the inner SWNova Scotia section compare favorably with measuredvalues from the Cape Sable array (April 1979 to March1980 [Smith, 1983]). The larger magnitudes of the veloc-ities and transports on the inner Halifax section comparedto those estimated by Drinkwater et al. [1979] point to theimportance of the baroclinic pressure gradient field below100 m (note the sloping near-bottom isohalines in Figure 2[see also Sheng et al., 2001].[20] The cumulative transports for the whole shelf are

larger than those for the inner shelf, reflecting the south-westward shelf-edge flow. On the Halifax section, there is anet increase of 0.2–0.4 Sv out to the 340-m isobath withlargest magnitude in winter, and a further increase of 0.5–1.4 Sv (referenced to the seafloor) out to the 1000-m isobathin winter and spring (Figure 3). The result is a largeseasonal variation in total southwestward (baroclinic) trans-port over the shelf (Inner+Mid+Outer), with winter andspring peaks of 2 and 2.2 Sv, and an early summerminimum of 0.4 Sv. The analyses also indicate that, in theOffshore zone (stations 6–7; Figure 2), the flows aregenerally weaker and tend to be more northeastward.However, the reliability of the slope transport estimates islimited by the strong variability and undersampling of thehydrographic structure, as well as uncertainty in the refer-ence level velocity.[21] On the Lahave section, the transport out to the 400-m

isobath also increases (compared to the inner shelf) by0.2–0.4 Sv with a peak increase in winter, resulting in aseasonally varying shelf-wide transport of 0.2–0.7 Svsouthwestward. The shelf-wide transport (to the middleof Northeast Channel) on the SW Nova Scotia section(across the entrance to the eastern Gulf of Maine) alsovaries seasonally, with winter and summer maxima of 0.8and 0.6 Sv, respectively, primarily associated with theshelf-edge/channel flow. These maxima are qualitativelyconsistent with moored measurements in Northeast Chan-nel [Ramp et al., 1986] but larger because the measure-ments did not include the upper 75 m. The presenttransport estimates also differ from Smith’s [1983] esti-mates, because the latter did not capture most of the shelf-edge flow [Hannah et al., 2001, Figure 11]. The magni-tudes and spatial patterns of the present estimates aresimilar (as expected) to those in recent numerical modeldiagnoses of the seasonal-mean circulation [Han et al.,1997; Hannah et al., 2001], indicating the importance of

Figure 3. Annual cycle in cumulative baroclinic transportshoreward of selected isobaths on (a) the Halifax section,(b) the Lahave section, and (c) the SW Nova Scotia section.See Figure 1 for locations. Solid lines indicate transports onthe Inner shelf, and dashed and dash-dotted lines totaltransports between the coast and positions on the upperslope. The transports (positive southwestward) are refer-enced to a level of no motion at the seafloor in all cases. Themeasured transports (solid dots) for the Inner Halifaxsection from Anderson and Smith [1989] are included inFigure 3a, and for part of the SWNS section from Smith[1983] in Figure 3c, with their statistical uncertaintyindicated by the vertical lines. 1 Sv = 106 m3s�1.

LODER ET AL.: HALIFAX SECTION VARIABILITY GLO 4 - 5

the shelf-edge flow and variability off Halifax to down-stream regions [Loder et al., 1998].

3.2. Steric Elevation and Observed Sea Level

[22] The southwestward flows described above (Figures 2and 3) arise from generally lighter (less saline) water andhence higher (computed) surface elevations as one proceedsonshore across the shelf. The cross-shelf steric elevationdifferences (Figure 4a) associated with the baroclinic trans-port variations on the Halifax section (Figure 3) showseasonal variations with peak coastal set-ups in winter andan annual range of 11–14 cm depending on the offshorereference position. By zone, the largest seasonal change isthe 11-cm range across the Inner shelf (Nova ScotianCurrent), but there is also a 5-cm range across the Outershelf (shelf-edge current).[23] The annual variation in steric elevation difference

across the Inner shelf is similar to the long-term annualcycle of observed ASL at Halifax (Figure 4b), providingfurther support for the predominant role of baroclinic flowin the seasonal circulation (the steric elevation differenceswith other zones included show less similarity to the ASL).The annual ASL cycle has a broad maximum from Octoberto January, a broad minimum from April to August, andan annual range of about 10 cm. The broad maximum isconsistent with the October peak in ocean heat content onthe inner shelf and in Emerald Basin (areas 12 and 14 ofPetrie et al. [1996]; also see Umoh and Thompson [1994]),and with the December–January salinity minimum associ-ated with the passage of the annual freshwater pulse fromthe Gulf of St. Lawrence [Sutcliffe et al., 1976].[24] While there is approximate agreement (rms misfit of

2.5 cm) between the annual cycles of Halifax ASL andsteric elevation difference across the Inner shelf, there iseven better correspondence (rms misfit of 1.9 cm) betweenthe annual cycles of ASL and steric height calculated fromthe monthly-mean density profiles at station 1–3 (Figure 4b).The model estimate of the regional wind-forced contributionto the annual Halifax sea level cycle is only 1–2 cm, and itsinclusion (shown as the wind-adjusted steric height inFigure 4b) does not improve the correspondence (rms misfitof 2.3 cm) with the ASL (probably because the windinfluence is at the noise level in the ASL and stericestimates). This small effect of wind is consistent withThompson’s [1986] finding of a secondary influence ofregional (compared to ocean-scale) wind-forcing on seasonalsea level variability.[25] The difference between the annual cycles of

observed Halifax ASL and wind-adjusted steric height hasa notable seasonal pattern (Figure 4c). During winter(November–March), the ASL exceeds the adjusted stericheight by 3 cm on average, while during April–October, theASL is generally lower than the adjusted steric height by anaverage of 1 cm. This seasonal change may partly reflect aseasonal variation in the barotropic (associated with a non-zero velocity at the seafloor) component of shelf or slopeflow, involving more southwestward flow in winter.Assuming that the associated elevation is confined to theInner shelf and linearly distributed between the coast andstation 3, each centimeter of coastal sea level changecorresponds to a current change of 0.015 m s�1 and atransport change of 0.11 Sv. Current measurements within

10 m of the seafloor on the shelf part of the Halifax sectionare only available for the 100-m isobath in April–October,and indicate a northeastward mean flow of 0.017 m s�1

[Gregory and Bussard, 1996]. The associated coastal set-down of 1 cm is consistent with the ASL-steric difference inFigure 4c, suggesting a small northeastward barotropictransport in spring-fall. During winter, the deepest availablemeasurements from the Inner shelf (at heights of 18–110 mabove bottom at four CASP sites [Anderson and Smith,1989]) indicate southwestward mean currents of 0.01–0.1 m s�1, and if taken to represent barotropic flow, implya coastal set-up of about 3 cm that also fits reasonably wellwith the ASL-steric difference (Figure 4c). However, thepresent geostrophic estimates (Figure 2) as well as diagnos-tic model studies [e.g., Han et al., 1997] indicate significantbaroclinic shear at the CASP measurement depths, sothat the above values probably overestimate the barotropicflow. Thus the seasonal change in ASL-steric difference(Figure 4c) probably reflects a combination of a seasonalbarotropic transport change of up to (but probably less than)0.4 Sv in the Nova Scotian Current, the annual cyclesof baroclinic (Figure 4a) and barotropic [Greenberg andPetrie, 1988] current variations on the outer shelf, andlarger-scale influences.

4. Variability About the Halifax SectionAnnual Cycle

4.1. Hydrographic and Baroclinic TransportVariability Indices

[26] The hydrographic variability about the annual cycleon the Halifax section is illustrated by the zone-averagedanomalies of temperature in Figure 5. The period 1950–1970 had the best temporal and spatial resolution, withoccupations typically seasonally with the slope included.The dominant anomalous feature identified by PD93 of ashelf-wide reduction in temperature by about 5�C during thelate 1950s and early-to-mid 1960s is apparent. The salinity(not shown here) had a similar variation with a range of 1.The variations have similar patterns in the Inner, Mid, andOuter zones, with increased amplitudes in the latter (con-sistent with the findings of PD93). In view of the reducedsampling since 1970 and the large anomaly in the late 1950sand 1960s, we focus on the 1955–1970 period.[27] The detailed timing of the 1950s/1960s hydrographic

anomaly, and the geostrophic transport variations (500-manalysis) are shown in Figure 6. The temperature anomalyfor the Inner-Lower zone was about 1�C below normal inthe mid 1950s, decreased an additional 2�C during thewinter of 1958–1959, slowly recovered in the early1960s, decreased 3–4�C during the winter of 1963–1964followed by a slow recovery, and returned to near-normalvalues in 1968. Both the 1958–1959 and 1963–1964decreases were preceded (3–6 months) by periods ofbelow-normal temperatures at the shelf edge (Figure 6c,Outer-Upper), and the 1968 warming was associated withabove-normal shelf-edge temperatures.[28] In spite of the large changes in hydrographic prop-

erties, the baroclinic transport anomalies on the Innershelf were typically under 0.5 Sv (compared to the long-term annual variation of 0.5–1.1 Sv), indicating consider-able persistence of the Nova Scotian Current. The absolute

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Figure 4. Annual cycles in (a) cross-shelf steric elevation difference across various zones of the Halifaxsection; (b) smoothed Halifax ASL (thick line) for 1955–1970 with standard errors in the monthly meansindicated by vertical lines, steric heights (solid dots with standard errors indicated by vertical lines)estimated from monthly mean density profiles at stations 1–3, steric heights adjusted for regional windcontributions (triangles), and steric elevation difference across the Inner shelf (squares; repeated fromFigure 4a); and (c) the difference between monthly mean ASL and the wind-adjusted steric height, withcombined standard errors (solid dots and vertical lines). The broken lines in Figure 4c are the coastalelevations for barotropic currents estimated from measured currents: for winter, from the CASP data[Anderson and Smith, 1989], and for April–October, from Gregory and Bussard [1996].

LODER ET AL.: HALIFAX SECTION VARIABILITY GLO 4 - 7

transport estimates for the Outer shelf were generally (butnot always) southwestward, with magnitudes again typicallyunder 0.5 Sv, while the absolute estimates for the Offshorezone were approximately evenly distributed between north-eastward and southwestward flow, with magnitudestypically of order 1 Sv and ranging up to 2 Sv. Consideringthe coarse spatial sampling and estimated uncertainties (ear-lier), many of the Outer and Offshore transport estimates arenot statistically significant, so they must be interpreted withcaution. For example, it was found that the transport anomalyestimates for the Outer and Offshore zones were generallynot reliable, apparently because of the combined uncertaintiesin the individual-section and long-term estimates.[29] On the basis of the PD93 finding that cold temper-

atures on the Scotian Shelf were associated with increasedtransport on the Newfoundland Shelf, and the Loder et al.[2001] finding of increased southwestward transport on theScotian Slope during the cold period, large positive (nega-tive) transports might be expected to be associated with thenegative (positive) temperature anomalies. These are notreadily apparent in Figure 6. Correlation analyses (Table 1)between the Inner/Mid temperature anomalies and thetransport estimates do show significant negative correla-tions, but they are not large. They indicate that the present

Figure 6. Indices of temperature (from Figure 5) andalong-shelf baroclinic transport variability for various zoneson the Halifax section from 1955–1970. The temperatureand Inner-shelf transport indices are anomalies relative tothe annual cycle, while absolute transports (positive south-westward) from the 500-m analysis are used for the Outerand Offshore zones.

Table 1. Correlation Coefficients Between Temperature Anoma-

lies and Baroclinic Transport Indices (Figure 6) for the Halifax

Section During 1950–1996a

Temperature Anomalies

Inner-Lower Mid-Lower

Inner transport anomaly �0.29 –Outer transport �0.44 �0.44Offshore transport �0.47 �0.40

aAll the correlations are significant with p = 0.99.

Figure 5. Indices of temperature variability (relative to theannual cycle based on 1950–1996) for the various Upper(�100 m; solid dots) and Lower (�125 m; open dots) cross-shelf zones from occupations of the Halifax section. Theindices for the Inner, Mid, and Outer zones are from the300-m analysis, and those for the Offshore zone fromthe 500-m analysis.

GLO 4 - 8 LODER ET AL.: HALIFAX SECTION VARIABILITY

estimates of Outer and Offshore transport variability canaccount for about 20% of the temperature variance on theshelf below 125 m. It is noteworthy that the correlationbetween shelf temperatures and Inner transport anomalies iseven lower, indicating that variability in the Nova ScotianCurrent is not a predominant factor in the low-frequencyshelf hydrographic variability.

4.2. Onset of the 1950s/1960s AnomalousHydrographic Conditions

[30] We next describe the hydrographic transition periodsin more detail by examining individual hydrographic sec-tions. In October 1958 (Figure 7), shelf and upper-slopetemperatures were near or just below normal, while thoseoffshore were 4�C below normal indicating an increasedpresence of Labrador Slope Water. By February 1959, alarge change in upper-slope properties had occurred, withtemperatures at 100–200 m reduced to about 2�C, morethan 6�C below normal. Transports on the Outer shelf werenear-normal (southwestward) during these occupations(Figure 6d), while those Offshore were weak (<0.5 Sv;Figure 6e). There was a strong seasonal cooling of Uppershelf waters during this period, and a slight cooling of thedeep waters in Emerald Basin but the latter remained onlyslightly cooler than normal.[31] By May 1959 (Figure 7), upper-slope temperatures

had warmed to only about 2�C below normal, while deepEmerald Basin water had cooled to 5.4�C [Taylor, 1961],about 3�C below normal. Similar shelf and upper-slopedistributions (below the seasonal surface layer) persistedthrough the July and October 1959 occupations. The abovesequence indicates that a major part of the onset of thehydrographic anomaly occurred between October 1958 andMay 1959, and involved an upper-slope change followed bya shelf-basin change. The spring-fall 1959 occupations(Figure 7) show a cooler-than-normal intermediate layerextending from the coast to the slope, indicating the broadextent of the anomaly.[32] A somewhat similar sequence of events occurred

during the winter of 1963–1964. In September 1963(Figure 8), shelf temperatures at depth had warmed to nearnormal while shelf-edge temperatures had cooled sincespring 1963 to about 4�C below normal (Figure 6c). Asimilar subsurface pattern existed in December 1963 (notshown). By March 1964, the slope cooling had intensified(anomalies exceeding �6�C) and broadened, and shelftemperatures at depth had fallen to about 4�C below normal.The shelf anomaly persisted through July 1964, September1964 (not shown) and subsequent years with some amelio-ration. The slope anomaly was considerably reduced by July1964, but re-intensified in fall 1964 and remained until1966–1967 (Figure 5).[33] The 1958–1959 and 1963–1964 sequences indicate

that the onset and re-intensification of the Scotian Shelfhydrographic anomaly both occurred through a combinationof a more widespread fall-winter distribution of LabradorWater over the slope, followed by intrusions onto the shelf.However, examination of other winters indicates that anom-alously cold shelf-edge water can also occur without majoreffects on inner-shelf conditions (Figure 5c). For example,cool shelf-edge anomalies exceeding �2�C were observedin the winters of 1954 and 1956, with limited influences on

Emerald Basin bottom waters. Thus it appears that the onsetof anomalous shelf hydrographic properties requires bothanomalous water at the shelf edge and an on-shelf intrusionmechanism.[34] The present analyses have not found direct evidence

of increased southwestward transport over the Scotian Slopeduring 1958–1959 or 1963–1964. Nevertheless there ismounting collective evidence that increased Labrador Cur-rent transport around the Grand Bank (PD93) was a majorfactor in the origin of the anomalous conditions. Evalua-tions of decadal-scale changes have indicated relatively-cold and -fresh upper-ocean anomalies centered on theScotian Slope but extending to the Newfoundland Slopeduring the late 1950s [Levitus, 1989; Kushnir, 1994], andincreased equatorward transports in the Slope Water[Greatbatch et al., 1991]. Studies of year-to-year variabilityhave indicated that, in 1959, there was a major coldanomaly in the Slope Water [Volkmann, 1962; Worthington,1964], as well as a large westward flow of Labrador CurrentWater south of the Grand Bank [Pickart et al., 1999]. Whilemany questions remain regarding the temporal variabilityand mechanisms involved in the large-scale circulationfluctuations, it seems likely that they were an importantfactor to the anomalous hydrographic conditions on theScotian Shelf/Slope in the 1950s/1960s.

4.3. End of the 1950s/1960s AnomalousHydrographic Conditions

[35] The below-normal subsurface temperatures persistedon the inner shelf and upper slope of the Halifax section intoApril 1967 (Figure 9); the temperature anomaly decreasedover the slope and at depth in Emerald Basin from Novem-ber 1966 to January 1967, and subsequently intensifiedfrom January to April 1967. There was then a 14-monthgap until the section’s next occupation in June 1968, atwhich time the deep-shelf and upper-slope temperatures hadwarmed (by more than 3�C) to values about 1�C belownormal, and Warm Slope Water had returned to the shelfedge. The warming continued through October 1968 whenshelf temperatures returned to near normal and upper-slopetemperatures were more than 4�C above normal associatedwith a further northward excursion of Warm Slope Water(Figure 9). The Outer transport estimates from the April1967 and June and October 1968 sections were small(Figure 6d), while the corresponding Offshore estimates(Figure 6e) were northeastward with magnitude near 1.5 Svin June and October 1968.[36] Other hydrographic data provide indications of the

evolution between April 1967 and June 1968. For the upperslope, they indicate the presence of Warm Slope Water(10.5�C) from May to August 1967, followed by a return ofLabrador Slope Water (3.2�–6.2�C) in November 1967, andthen a return of warmer waters (8.7�C) as early as lateJanuary 1968. For Emerald Basin they indicate somewarming at depth by August 1967 (5.7�–7.8�C), a slightrelapse in January 1968 (5.7�–6.9�C), and then warmerwaters (7�–8.2�C) dominating by May 1968.[37] Collectively, these observations indicate that the end

of the anomalous shelf hydrographic conditions was asso-ciated with increasing prevalence of Warm Slope Water nearthe shelf edge starting in May 1967 and intensifying duringearly 1968. The sparse geostrophic estimates suggest north-

LODER ET AL.: HALIFAX SECTION VARIABILITY GLO 4 - 9

Figure 7. Temperature distributions from individual occupations of the Halifax section during 1958–1959 (left panels), and anomalies relative to the annual cycle on the same calendar day (right panels). Thecontour interval is 2�C.

GLO 4 - 10 LODER ET AL.: HALIFAX SECTION VARIABILITY

eastward flow over the middle slope (Offshore zone) in1968, consistent with the change in hydrographic properties.The hydrographic data indicate that the warming at depth inEmerald Basin progressed gradually between summer 1967and fall 1968. By December 1968 the deep salinity inEmerald Basin had increased to above-normal values over35, which Petrie and Smith [1977] suggested may havebeen associated with a measured onshore deep current of0.02 m/s and eastward wind stress of 0.054 Pa duringOctober–December. Above- or near-normal shelf temper-atures then persisted through the 1970s, with variable butgenerally above-normal slope temperatures.[38] It is unclear whether the penetration of hydrographic

anomalies from the shelf edge into Emerald Basin ispredominantly related to the long-term seasonal-meancirculation or other variability. Using moored measurementsat the shelf edge, Smith and Petrie [1982] found that theon-shelf fluxes were dominated by the seasonal-meancontribution. Recent diagnostic modeling studies [Hannahet al., 2001; Shore et al., 2000] have identified a persistenton-shelf meander of the shelf-edge flow around/over

Emerald Bank. On the other hand, wind-driven shelf-edge upwelling [Petrie, 1983] followed by topographicchanneling [Greenberg et al., 1997] is a candidate mech-anism for episodic on-shelf penetration. This is supportedby the occurrence of upwelling-favorable wind stressanomalies during the winters of 1958–1989, 1963–1964and 1967–1968, although these were not the only winterswith such anomalies. Another possible factor in the end ofanomalous conditions in Emerald Basin is the circulationassociated with the 10-cm negative anomaly in Halifax-Yarmouth sea level difference during late spring 1967(relatively high sea level at Yarmouth [Sandstrom, 1980,Figure 8], when there was a large negative baroclinictransport anomaly in the Inner zone and northeastwardtransport in the Offshore zone (April 1967; Figure 6).

4.4. Shelf-Slope Coupling

[39] In this subsection, we use the monthly subsurfacetemperature anomalies from the inner shelf, Emerald Basinand the Slope Water during 1950–2001 to quantitativelyevaluate the inter-relation between shelf and slope hydro-

Figure 8. Temperature distributions from individual occupations of the Halifax section during 1963–1964 (left panels), and anomalies relative to the annual cycle on the same calendar day (right panels). Thecontour interval is 2�C.

LODER ET AL.: HALIFAX SECTION VARIABILITY GLO 4 - 11

Figure 9. Temperature distributions from individual occupations of the Halifax section during 1966–1968 (left panels), and anomalies relative to the annual cycle on the same calendar day (right panels). Thecontour interval is 2�C.

GLO 4 - 12 LODER ET AL.: HALIFAX SECTION VARIABILITY

graphic variability. The mean temperatures (removed) were3.9�, 7.4�, and 9.1�C for the inner shelf, Emerald Basin andSlope Water, respectively, while the annual cycles hadranges of 1.8�, 0.6�, and 1.2�C.[40] The Slope Water has the largest overall, interannual

and seasonal variance (Table 2). The inner shelf andEmerald Basin have similar interannual variance but theinner shelf has greater seasonal variance, likely the result ofits shallower depth and location in a coastal upwelling zone[Petrie et al., 1987].[41] Figures 10a–10c show that the temperature anoma-

lies are closely related at the lower frequencies and this issupported by correlation analyses (Table 3). For the low-pass filtered time series (interannual band), the Slope Watertemperature fluctuations can account for 83% of the vari-ance in the Basin and 64% of the variance in the inner shelf.The time lags of 3–5 months between the Slope and Basinanomalies, and 5 months between Slope and inner shelf(unfiltered) anomalies confirm that changes over the slopeprecede changes in the Basin, and imply a reasonableadvective speed of 1 cm/s. On the other hand, the analysesindicate that the peak low-frequency correlation between theSlope and inner shelf anomalies occurs for lag near 0;however, the cross-correlation peak between these seriesis quite broad and at 5 months lag, the Slope series canaccount for 59% of the low-frequency variance on the innershelf (which is not significantly less than for zero lag).[42] To further evaluate the predictability of basin and

inner-shelf temperature anomalies by the Slope Wateranomalies, we constructed a filter that predicts the temper-ature at time t = t* from the Slope temperature at timest � t* (a causal filter). Two filter families were tried:[43] 1. Filter with a delay of m months and with filter

weights that decay exponentially with timescale t months.The filter weight is zero for m � 1 months previous to thecurrent time t*, a maximum m months before t* and decaysexponentially beyond that.[44] 2. A simple average of the previous n months. Both

filters had unit amplitude and were not tuned for gain.[45] For the Slope-Basin connection, filters with delays

m = 0� 3 months and decay timescales of t = 3� 18 monthsyielded predictions that captured 50–55% of the variance ofthe Basin temperature anomalies, compared with 37% for thebest linear regression. An example is shown in Figure 10d.The simple average gave results similar to those from thedecay filter with t = n and m = 0 months, but captured a fewper cent less variance. If either filter was used to predictthe Slope anomalies from the Basin anomalies the resultswere much worse, which supports our conclusion that the

Slope Water is influencing the Basin temperatures and notvice versa. For the connection between the Slope andinner-shelf anomalies, filters with lags of 0–3 monthsand decay timescales of 1–24 months yielded predictedtime series that capture 25–32% of the variance of theinner-shelf time series (Figure 10e), compared with 21%for the best linear regression. This is about half of thefraction of variance predicted for the Basin time seriesfrom the Slope anomalies.

4.5. Comparison With Observed and Wind-ForcedSea Level Indices

[46] The continuous time series of Halifax ASL and SableIsland wind stress allow further interpretation of the tem-perature and transport indices:[47] 1. The annual cycle of baroclinic transport is clearly

apparent in coastal sea level (Figure 4) so other variations inbaroclinic transport may be detectable as well.[48] 2. Wind-forced transport variability can be expected

to modify the density field [e.g., Petrie et al., 1987]. Storm-band wind variations can cause high-frequency variabilityin baroclinic flow that could be aliased into apparent lower-frequency variability, while low-frequency wind variabilitycould be a forcing of the low-frequency transport variationsof interest. On the other hand, Thompson and Sheng’s[1997] finding of a baroclinic current mode on the innershelf (near station 2 on the Halifax section) with nocorrelation with wind or sea level suggests that wind-forcing is a secondary influence on baroclinic transportvariability.[49] 3. Coastal sea level may provide indications of

important barotropic flows associated with shelf waves orlarge-scale ocean circulation fluctuations that need to beincluded in transport variability indices.[50] We start with a comparison for the period 1955–

1970 of the anomalies of observed ASL and model-predicted wind-forced sea level at Halifax. A significantcontribution from regional wind-forcing to observed ASL isclearly apparent in the monthly anomalies (Figure 11a),consistent with previous findings [Sandstrom, 1980;Schwing, 1989]. Coherence analysis (not shown) usingbi-daily anomalies indicates broad coherence (0.5–0.8,mean = 0.64 ± 0.12) at periods of 6–30 days, with gainsbetween 1 and 2 (mean = 1.4 ± 0.3) and phase lags of 24–48 hours (mean = 30 ± 8 hours) for ASL relative to windstress, with both gain and phase generally increasing withperiod. Thus over these periods, about 40% of the variancein ASL can be explained by regional winds. This isconsistent with the results of Schwing [1989], who foundthat about 90% of the subtidal sea level variance at periodsof 1–40 days during the winter of 1985–1986 could berelated to wind and upstream forcing, with about half ofthis due to regional wind-forcing.[51] The relation of the cross-shelf (coastal relative to

500 m at station 6) steric anomalies to the ASL anomaliesduring 1955–1970 is illustrated in Figure 11b where theASL anomalies are shown as both monthly means (contin-uous curves) and weekly means for the weeks centered onthe section occupations. The steric anomalies have compa-rable magnitudes to the ASL anomalies, with similarfluctuations apparent at times and a significant positivecorrelation (r2 = 0.13; p = 0.98, linear slope = 0.32) between

Table 2. Root-Mean Square Temperatures (�C) From the 100–

200 m Monthly Anomalies for the Inner Shelf, Emerald Basin, and

Slope Water During 1950–2001a

Time Series Inner Basin Slope

Unfiltered 1.7 1.5 2.2Low-pass 1.3 1.2 1.5High-pass 1.1 0.8 1.5aThe low-pass filtered time series (interannual variability) were created

using a 13-month running-average filter. The high-pass filtered time series(seasonal variability) are the difference between the unfiltered and low-passfiltered time series.

LODER ET AL.: HALIFAX SECTION VARIABILITY GLO 4 - 13

Figure 10. Monthly temperature anomalies (dots) and 13-month running means (solid line) for (a) SlopeWater (100–200 m), (b) Emerald Basin (100–200 m), and (c) the Inner Halifax section (75–175 m).Predicted temperature anomalies (solid lines) are also shown for (d) Emerald Basin and (e) the InnerHalifax section from the decay filter (lag m = 0 and decay scale t = 9 months), together with thetemperature anomalies (dots). Note the different temperature scale in Figure 10a.

GLO 4 - 14 LODER ET AL.: HALIFAX SECTION VARIABILITY

the (weekly) ASL and the steric anomalies. However, it isclear that the Halifax section is severely undersampledrelative to the sea level variability. During the hydrographictransition periods discussed above, there was no consistencybetween the variations in ASL and steric anomalies, nor anylarge and persistent ASL anomalies. It is noteworthy that thelargest ASL anomaly during the 1955–1970 period was apositive one with a peak (set-up) of 16 cm during Febru-ary–April 1958 which is only partly explained by regionalwind stress forcing. This points to a larger-scale influence inthe year preceding the onset of the anomalous shelf hydro-graphic conditions, possibly resulting in increased south-westward flow over the shelf and/or slope.[52] A similar analysis for the inner shelf only (coastal

relative to station 3) resulted in more realizations than whenthe entire shelf was considered (53 versus 34). The linear

slope relating steric elevation difference to ASL was 0.63with a correlation r2 = 0.32, both values higher than with theentire shelf included. When the inner-shelf estimates werereduced to the subset coincident with entire-shelf realiza-tions, the slope was 0.6 and the correlation was reduced tor2 = 0.19. The fact that the gain for the inner shelf is twicethat for the entire shelf is consistent with an offsettingcontribution to cross-shelf elevation anomalies from thenortheastward flow on the inshore side of Emerald Bank.[53] The wind and the inner-shelf steric height anomalies

are only weakly correlated (r2 =0.12), and thus are largelyindependent of each other. Adding their contributionsindicates that they can explain 60–70% of the sub-tidalASL variance (40% from wind and 20–30% from steric).Together with Schwing’s [1989] result that upstream remoteforcing can explain 30–40% of the ASL variance forperiods of 2–20 days, this suggests that nearly all thesubtidal ASL variance can be explained by the combinationof regional wind, upstream remote and steric influences.However, a multiple regression analysis involving a higher-resolution time series of hydrographic (steric) variability isneeded to resolve this partitioning.[54] These results indicate that there is a relationship

between Halifax sea level anomalies and steric changesacross the entire Halifax Section, but it is not strong.Several factors may contribute to this: (1) There are four flowarteries across the Halifax section (southwestward flows onthe inner shelf and upper slope, northeastward flows onthe mid shelf and slope) whose signatures may be partlyoffset in coastal sea level; (2) aliased high-frequencyhydrographic variability may be contaminating the stericanomaly estimates; (3) the ASL anomalies are dominated

Table 3. Results of Lagged Correlation Analyses Between Slope

Water Subsurface Temperature Anomalies and Those for Emerald

Basin and the Inner Shelf a

Zero Lag Maximum Correlation

r2 r2 Lag, months

Emerald BasinUnfiltered 0.24 0.37 3Low-pass 0.79 0.83 3–5

Inner ShelfUnfiltered 0.19 0.21 5Low-pass 0.64 0.64 �2 to 1

aA positive lag means that the basin or shelf lags the Slope Water. Herer2 is the fraction of the variance accounted for. The high-pass filteredanomalies were not significantly correlated.

Figure 11. Monthly ASL anomalies at Halifax for 1955–1970 (thick shaded line) compared with(a) predicted wind-forced monthly anomalies from the circulation model (solid black line), and with(b) cross-shelf steric elevation differences for the Halifax section (solid dots) and observed weekly ASLanomalies (solid diamonds) for the time of each section occupation. The steric anomalies are relative to500 m at station 6.

LODER ET AL.: HALIFAX SECTION VARIABILITY GLO 4 - 15

by wind and upstream barotropic influences [Schwing,1989]; and (4) larger-scale changes in northwest Atlanticsea level may be contributing to the lower-frequency ASLanomalies.

4.6. Potential Barotropic Transport MagnitudesFrom ASL Anomalies

[55] To provide an indication of the potential magnitudeof the low-frequency barotropic circulation variability,barotropic transport anomalies were estimated as follows.Two ‘‘residual ASL anomalies’’ were computed for eachoccupation of the Halifax section by subtracting from theASL anomalies the wind-forced anomalies (from a regres-sion relation with Sable Island wind stress) and the stericelevation anomalies computed for either the inner shelf orthe entire shelf (relative to 500 m at station 6). Then,estimates of the inner-shelf transport anomalies were calcu-lated assuming that the ‘‘residual ASL anomalies’’ weredistributed linearly across either the inner or entire shelf.This resulted in four different estimates of the barotropictransport on the inner shelf for each occupation of thesection. The RMS magnitudes of these residual transportanomalies were 0.6–0.8 Sv for the inner-shelf gradientcases, and about 0.3 Sv for the entire-shelf gradient cases.The substantial magnitudes of these residual transports arequalitatively consistent with previous studies of importantcontributions from remotely forced shelf waves to transportvariability on the inner Scotian Shelf [Schwing, 1992;Thompson and Sheng, 1997]. However, they should beconsidered more as upper bounds than likely values, dueto the possibility of larger-scale contributions to the ASLanomalies together with uncertainties in the removal ofwind and steric effects.[56] From the same analysis, barotropic transport esti-

mates were made for the Outer shelf using the stericelevation anomalies relative to 500 m at station 6 and the‘‘residual ASL anomaly’’ distributed linearly across theentire shelf. The magnitudes ranged from �0.7 to 1.5 Svwith an RMS amplitude of 0.5 Sv. While the comparison ofthese estimates with the events seen in the temperature andsalinity time series did not indicate major barotropic currentchanges at the times of the shelf hydrographic changes, theestimated magnitudes point to the potential for significantinfluences.

5. Summary and Discussion

[57] The annual cycle of baroclinic circulation on theHalifax section shows two major southwestward flowarteries, both with strong seasonal variations consistent withrecent diagnoses from circulation models [Han et al., 1997;Hannah et al., 2001]. The clearest picture is available forthe Nova Scotian Current on the inner shelf which hasa seasonal variation in baroclinic transport ranging from0.5 Sv in summer to 1.1 Sv in winter, magnitudes which aresubstantially larger than earlier geostrophic estimates[Drinkwater et al., 1979] which only considered the upper100 m. Comparison of the associated coastal steric elevationchange with the annual cycle of observed sea level (ASL) atHalifax indicates that the steric change is the predominantcontributor to the 10-cm annual sea level range, withsmaller contributions from wind, barotropic flow, and

offshore factors. Limited moored current measurementsfrom the inner shelf together with the ASL indicate thatthe barotropic transport varies seasonally with a range of upto 0.4 Sv and a southwestward peak in winter.[58] The second major southwestward flow artery is at the

shelf edge, representing a downstream extension of thesubpolar western boundary current in the North Atlantic.Its baroclinic transport is more uncertain, because of datasparsity and hydrographic variability over the upper slope.Estimates for its (long-term) baroclinic transport out to the1000-m isobath range from about 0.2 Sv in summer to about1.4 Sv in spring. Although these transports are of compa-rable magnitude to those for the Nova Scotian Current, theshelf-edge current width and associated cross-shelf eleva-tion change (5 cm) are only about half of those for the latter.As a result of this and the current’s offshore location, it hasnot been possible to estimate its annual barotropic variationfrom Halifax ASL. It should be noted that the geostrophicestimates here were referenced to 1000 m, whereas previousdynamic height estimates [Csanady, 1979] relative to thebottom and extending further offshore in the Slope Waterindicate an elevation change of about 10 cm across theslope/shelf-edge, suggesting that the slope baroclinic flow isnot fully resolved in the present estimates.[59] The annual cycle of baroclinic velocity on the two

sections across the western Scotian Shelf indicates smallertransports, particularly on the inner shelf. This is consistentwith recent circulation model diagnoses [e.g., Hannah et al.,2001] that the Nova Scotian Current off Halifax includes acontribution from an onshore meander of the shelf-edgecurrent (that then returns to the outer shelf) and that there isa tendency for part of the shelf-edge current to turn offshoreover the western Scotian Slope.[60] The individual occupations of the Halifax section

indicate that the onset and termination of the 1950/1960sanomaly occurred through a series of events involvingchanges in the type of Slope Water at the shelf edge,followed by intrusions onto the shelf. The onset wasassociated with an increased presence of Labrador SlopeWater over the upper slope in October 1958 and February1959, followed by cooling at depth in Emerald Basin duringwinter-spring of 1959. The anomaly re-intensified duringthe winter of 1963–1964 after increased presence of Labra-dor Slope Water over the slope between September 1963 andMarch 1964. In both cases, the shelf bottom waters anoma-lies were about �4�C after shelf-edge anomalies of about�6�C. The anomalous period ended during 1966–1968,associated with increased presence of Warm Slope Waterover the upper slope in summer 1967 and winter 1968. Thebaroclinic transport, Halifax ASL, and wind stress indices donot indicate any major events during these transition periods,with the exceptions that there were upwelling-favorablewind stress anomalies during the winters of 1958–1959,1963–1964, and 1967–1968, and there was a large north-eastward transport in the Offshore region associated withprevalence of Warm Slope Water in 1968.[61] The analysis of shelf-slope coupling using monthly

subsurface temperature anomalies provides firm support forthe shelf anomalies lagging the slope anomalies by 3–5months, consistent with the slope origin proposed by PD93.The analysis also suggests the Slope Water temperatures canbe used to provide some short-term (3–5 months) predict-

GLO 4 - 16 LODER ET AL.: HALIFAX SECTION VARIABILITY

ability of the deep shelf-water temperatures, possibly up to50% of the variance for deep water in Emerald Basin. Thesubstantial residual variance is consistent with the anoma-lous hydrographic conditions occurring over the upper slopein some years, without anomalies subsequently occurring onthe shelf. This suggests that the mechanisms for on-shelfpenetration of the anomalies include an intermittent com-ponent such as shelf-edge upwelling [Petrie, 1983] associ-ated with the above-noted wind stress anomalies, in additionto the on-shelf meander of the shelf-edge current aroundEmerald Bank [Hannah et al., 2001]. The advective lag canconfound attempts to relate the slope and shelf anomaliessince only 25% of the variance can be accounted for withzero lag.[62] The baroclinic transport estimates for individual

occupations of the Halifax section are negatively correlatedwith temperature indices for the Lower-Inner and Lower-Mid zones. This is consistent with Loder et al.’s [2001]results of increased southwestward shelf-edge transportfrom diagnosis of decadal-average density fields, but thepresent results indicate that the increased transport was notpersistent. Rather, the annual cycle suggests a large seasonalvariation and the individual-section estimates indicate con-siderable interannual or higher-frequency variability.[63] The wind stress, Halifax ASL, and steric anomalies

are inter-related. Our analysis confirmed that 40–45% ofthe subtidal variability in Halifax ASL is due to regionallocal wind stress, and showed that the steric anomalies cancontribute 20–30% to the variance. The remaining varianceis consistent with Schwing’s [1989] finding of significantcontributions from upstream. If the upstream contributionsto Halifax ASL involve elevation gradients across the shelf,then the barotropic transport anomalies could be significantto both of the major flow arteries. Overall, the results pointto the potential for monitoring the baroclinic and barotropiccomponents of the Nova Scotia Current using Halifax ASL,local winds, and temperature and salinity measurementsfrom the coast to the middle of Emerald Basin, but thatadditional information would be needed for monitoring theshelf-edge current.[64] While the present analyses have not resolved the

extent and structure of transport variations along the ScotianSlope contributing to the 1950s/1960s anomalous condi-tions, they nevertheless support the important role ofincreased occurrence of Labrador Slope Water. A somewhatsimilar event of shorter duration in the late 1990s (Figure 10)appears to have had a similar origin. Cold water from thecontinental slope was observed entering the deep basins ofthe Scotian Shelf and the Gulf of Maine beginning in thefall of 1997 and persisting for through 1998 [Drinkwater etal., 1999] [see also Pershing et al., 2001], associated withincreased occurrence of Labrador Slope Water in the region.The major difference was that there was a series of on-shelfintrusion events in the late 1950s and early 1960s, and onlythe one in 1998.[65] A major factor in the study’s limited conclusions

regarding baroclinic transport anomalies and their role in the1950/1960s hydrographic anomaly is the sparsity of thehydrographic sampling on the Halifax section. The long-standing concerns regarding the aliasing of high-frequencytemporal variations [Mann and Needler, 1967] are a greaterproblem for geostrophic transport computations than for

hydrographic anomalies. A second limitation on the geo-strophic flow estimates is the coarse station spacing in themajor flow arteries, particularly over the steep topographyat the shelf edge. This results in a low signal-to-noise ratiofor the baroclinic transport estimates. In the future, thecombination of elevation estimates from altimetry, higherspatial resolution of the shelf-edge hydrography andbaroclinic models provide the potential for improvedunderstanding the dynamical relationships that governtemperature and transport on the Halifax Section.

[66] Acknowledgments. We are grateful to our many colleagues whohave contributed to the historical hydrographic database, Halifax sea leveland Sable Island wind stress time series, and the optimal interpolationprocedure. We particularly thank Shawn Oakey for assistance with theanalysis and displays, Ken Drinkwater for data contributions and discus-sions, Hal Sandstrom for internal review comments, and two anonymousreviewers for constructive comments that prompted us to explore severalissues further and solidify our conclusions. This is a contribution to theGLOBEC Canada program funded jointly by DFO and NSERC, the U. S.GLOBEC program funded jointly by NOAA and NSF, and the CanadianPERD Climate Change Impacts on the Energy Sector program.

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�����������������������E. A. Gonzalez, C. G. Hannah, J. W. Loder, and B. D. Petrie, Bedford

Institute of Oceanography, Fisheries and Oceans Canada, P.O. Box 1006,Dartmouth, N.S., Canada B2Y 4A2. (HannahC@mar.dfo-mpo.gc.ca;loderj@mar.dfo-mpo.gc.ca; PetrieB@mar.dfo-mpo.gc.ca)

GLO 4 - 18 LODER ET AL.: HALIFAX SECTION VARIABILITY