Trans-Atlantic responses of Calanus finmarchicus populations to basin-scale forcing associated with...

Progress in Oceanography 58 (2003) 301–312www.elsevier.com/locate/pocean

Trans-Atlantic responses ofCalanus finmarchicuspopulations to basin-scale forcing associated with the North

Atlantic Oscillation�

C.H. Greenea,∗, A.J. Pershinga, A. Conversib, B. Planquec, C. Hannahd,D. Sameotod, E. Headd, P.C. Smithd, P.C. Reide, J. Jossif, D. Mountaing,

M.C. Benfieldh, P.H. Wiebei, E. Durbinj

a Ocean Resources and Ecosystems Program, Cornell University, 2130 Snee Hall, Ithaca, NY, 14853, USAb National Research Council, Institute for Marine Sciences, Lesina, Italy and Marine Science Research Center,

State University of New York, Stony Brook, NY, USAc IFREMER, Nantes, France

d Department of Fisheries and Oceans, Bedford Institute of Oceanography, Dartmouth, NS, Canadae Sir Alister Hardy Foundation for Ocean Science, Plymouth, UK

f Northeast Fisheries Center, National Marine Fisheries Service, Narragansett, RI, USAg NMFS, Woods Hole, MA, USA

h Louisiana State University, Baton Rouge, LA, USAi Woods Hole Oceanographic Institution, Woods Hole, MA, USA

j Graduate School of Oceanography, University of Rhode Island, Narragansett, RI, USA

Abstract

Populations of the copepod speciesCalanus finmarchicus often dominate the springtime biomass and secondaryproduction of shelf ecosystems throughout the North Atlantic Ocean. Recently, it has been hypothesised that interannualto interdecadal fluctuations observed in such populations are driven primarily by climate-associated changes in oceancirculation. Here, we compare evidence from the North Sea and Gulf of Maine/Western Scotian Shelf (GoM/WSS)linking fluctuations inC. finmarchicus abundance to changes in ocean circulation associated with the North AtlanticOscillation (NAO). A particularly striking contrast emerges from this Trans-Atlantic comparison: whereas the NorthSeaC. finmarchicus population exhibits a negative correlation with the NAO index, the GoM/WSS population exhibitsa more complex, positive association with the index. The physical processes underlying these contrasting populationresponses are discussed in the context of regional- to basin-scale circulation changes associated with the NAO. 2003 Elsevier Ltd. All rights reserved.

Keywords: Calanus finmarchicus; Climate variability; Gulf of Maine; North Atlantic Oscillation; North Sea

∗ Corresponding author. Tel.:+1-607-255-5449; fax:+1-607-254-4780.E-mail address: [email protected] (C.H. Greene).

� MERCINA Working Group (Marine Ecosystem Responses to Climate In the North Atlantic).

0079-6611/$ - see front matter 2003 Elsevier Ltd. All rights reserved.doi:10.1016/j.pocean.2003.08.009

Alessandra Conversi

RefMan # 1217

302 C.H. Greene et al. / Progress in Oceanography 58 (2003) 301–312

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302

2. Trans-Atlantic patterns in C. finmarchicus abundance and the NAO . . . . . . . . . . . . . . . . . . . . . . . 302

3. Basin- and regional-scale changes in climate, ocean circulation, and the NAO . . . . . . . . . . . . . . . . . 304

4. Population responses of C. finmarchicus to changes in ocean circulation . . . . . . . . . . . . . . . . . . . . . 3064.1. The North Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3064.2. The Gulf of Maine/Western Scotian Shelf region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307

5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310

1. Introduction

Calanus finmarchicus dominates the zooplankton biomass found throughout the cold temperate and sub-polar oceanic waters of the North Atlantic Ocean (Raymont, 1983; van der Spoel & Heyman, 1983; Mauch-line, 1998). Although considered an oceanic species, C. finmarchicus frequently achieves high levels ofabundance in many North Atlantic shelf ecosystems. For example, although not self-sustaining, expatriatepopulations of C. finmarchicus dominate the springtime biomass and secondary production in the shelfecosystems of the North Sea, the Northeast Atlantic, and Gulf of Maine/Western Scotian Shelf (GOM/WSS)region of the Northwest Atlantic. Since these expatriate shelf populations must be replenished every yearor every few years by advection from some oceanic source (Longhurst, 1998), it has been hypothesisedthat their interannual to interdecadal fluctuations in abundance are driven primarily by climate-associatedchanges in ocean circulation (Greene & Pershing, 2000). Such fluctuations in the abundance of C. finmarch-icus have been documented in the North Sea and attributed to changes in ocean circulation patterns associa-ted with the North Atlantic Oscillation (NAO) (Reid, Planque, & Edwards, 1998; Stephens, Jordan,Taylor, & Proctor, 1998; Heath, Backhaus, Richardson, McKenzie, Slagstad, Gallego et al., 1999). Morerecently, fluctuations in abundance observed in the GOM/WSS region also have been linked to NAO-associated changes in ocean circulation (Greene and Pershing, 2000; Conversi, Piontkovski and Hameed,2001; MERCINA, 2001). Here, we compare these trans-Atlantic responses of C. finmarchicus to basin-scale NAO forcing and document how regional- to basin-scale changes in ocean circulation account forthe striking differences observed in the population ecology of this species in shelf ecosystems on bothsides of the North Atlantic.

2. Trans-Atlantic patterns in C. finmarchicus abundance and the NAO

The North Sea population of C. finmarchicus has exhibited a general decline in abundance since theearly 1970s (Fig. 1a). This negative trend has been observed in the Irish Sea and shelf waters north andwest of Scotland and Ireland as well (Fromentin & Planque, 1996; Planque & Taylor, 1998). In contrast,the GoM population of C. finmarchicus increased from relatively low levels of abundance during the 1960sto very high levels of abundance during the 1980s (Fig. 1b) (Jossi & Goulet, 1993; Greene & Pershing,2000). This positive trend halted abruptly in 1990. From 1991 to 1994, the abundance levels of C. finmarch-icus in the GoM were an order of magnitude lower than those observed during the 1980s and comparableto levels observed during the 1960s. A brief increase in the population occurred during the mid-1990s, but

303C.H. Greene et al. / Progress in Oceanography 58 (2003) 301–312

Fig. 1. Time series of C. finmarchicus abundance from (a) the North Sea and (b) the Gulf of Maine/Scotian Shelf region. Timeseries are based on Continuous Plankton Recorder (CPR) data collected for C. finmarchicus from the North Sea since the late 1940sand from the Gulf of Maine/Scotian Shelf region since the late 1950s. Data from the Gulf of Maine are indicated by circles andfrom the Scotian Shelf by triangles. Methods of sample collection and processing have been described by Hardy (1939); Colebrook(1975), and Jossi and Goulet (1993). (c) Time series of the NAO index from 1955 to 1999. The NAO index is the mean difference,expressed as an anomaly from the long-term mean, in atmospheric pressure during winter between the subtropical high pressuresystem of the North Atlantic, measured in Lisbon, Portugal, and the subpolar low pressure system, measured in Stykkisholmur,Iceland (Hurrell, 1995). (d) Cross-correlation analyses between C. finmarchicus abundance and the NAO index. Results of the analysesof data from the North Sea are indicated by rectangles and from the Gulf of Maine during 1961–1989 and 1961–1999, respectivelyby triangles and circles. (e) Regression between C. finmarchicus abundance in the North Sea and the NAO index with no time lag.(f) Regression between C. finmarchicus abundance in the Gulf of Maine and the NAO index with a time lag of 4 years, 1961–1989(dashed) and 1961–1999 (continuous).

this was followed by another decline to extremely low levels in 1998. The abundance levels of C. finmarch-icus on the WSS largely mirrored those observed in the GoM for the periods of 1961–1975 and 1991–1994(Fig. 1b); however, the brief resurgence of the GoM population during the mid-1990s was not observed onthe WSS (Sameoto, 2001). Unfortunately, a gap in the time series from the WSS does not allow comparisonsduring the critical decade of the 1980s, when the C. finmarchicus population in the GoM exhibited itshighest abundance.

Relationships between C. finmarchicus abundance trends in these ecosystems and the NAO can be exam-ined by comparing the above time series with a time series of the NAO index (Fig. 1c). A cross-correlation


analysis between the abundance of C. finmarchicus in the North Sea and the NAO index reveals a strongnegative correlation, with little evidence for a time lag exceeding 1 year (Fig. 1d and e). In contrast, asimilar cross-correlation analysis between C. finmarchicus abundance in the GoM and the NAO indexreveals a weak positive correlation from 1961 to 1989, with evidence for a time lag of approximately 4years (Fig. 1d and f). However, this correlation breaks down and becomes non-significant when data fromthe 1990s are included (Fig. 1d and f). To reconcile these contrasting population responses, it is necessaryto examine how basin-scale changes in climate and ocean circulation associated with different phases ofthe NAO manifest themselves in physical and biological processes at the regional scale.

3. Basin- and regional-scale changes in climate, ocean circulation, and the NAO

The phase of the NAO has important effects on climate throughout the North Atlantic. During positivephases, the planetary westerly winds intensify, and the North Atlantic storm track shifts to the north. Thesechanges lead to milder conditions in the western Atlantic and east coast of the United States; colder,stormier conditions in the northwest Atlantic and Greenland; and milder conditions in the northeast Atlanticand coasts of northern Europe and Great Britain (Hurrell, 1995; Dickson, Lazier, Meincke, Rhines, &Swift, 1996). In contrast, during negative phases of the NAO, the westerly winds diminish in intensity andthe storm track shifts to the south. These changes lead to colder, stormier conditions in the western Atlanticand east coast of the United States; milder conditions in the northwest Atlantic and Greenland; and colder,drier conditions in the northeast Atlantic and coasts of northern Europe and Great Britain (Hurrell, 1995;Dickson, Lazier, Meincke, Rhines & Swift, 1996).

The phase of the NAO also has important effects on the formation and spreading of deep water, as wellas the near-surface circulation patterns of the North Atlantic. In the subpolar northwest Atlantic, duringpositive phases, convection is deeper and more intense in the Labrador Sea; a relatively cool, fresh, andthick layer of Labrador Sea Water is formed and volume transport in the Deep Western Boundary Currentincreases, while volume transport in the shallow baroclinic component of the Labrador Current decreases(Fig. 2a) (Dickson, Lazier, Meincke, Rhines & Swift, 1996; Dickson, 1997; Curry, McCartney, & Joyce,1998; Loder, Shore, Hannah, & Petrie, 2001). In the subpolar northeast Atlantic, convection during positivephases of the NAO is shallower and weaker in the Greenland Sea; the deep-water layers are warmer,saltier, and thinner; and the formation and volume transports of Greenland Sea Deep Water and Norwegian

Fig. 2. Ocean circulation responses to (a) positive and (b) negative phases of the NAO. Areas with increased deep convection andwater mass formation are darkly shaded; those with decreased deep convection and water-mass formation are lightly shaded. Watermasses and currents are labeled as follows: GSDW, Greenland Sea Deep Water; LC, Labrador Current; LSW, Labrador Sea Water.


Sea Deep Water decrease (Fig. 2a) (Dickson, Lazier, Meincke, Rhines & Swift, 1996; Dickson, 1997).When the phase of the NAO reverses to negative, these patterns of convection and deep-water formationin the Labrador and Greenland Seas also reverse (Fig. 2b). Hence, convection becomes shallower andweaker in the Labrador Sea, the Labrador Sea Water layer becomes warmer, saltier and thinner, and volumetransport in the Deep Western Boundary Current decreases while volume transport in the near-surfacebaroclinic component of the Labrador Current increases (Dickson, Lazier, Meincke, Rhines & Swift, 1996;Dickson, 1997; Curry, McCartney & Joyce, 1998; Loder, Shore, Hannah & Petrie, 2001). Synchronouswith the above events in the Labrador Sea, convection becomes deeper and more intense in the GreenlandSea; the deep-water layers are cooler, fresher, and thicker; and the formation and volume transports ofGreenland Sea Deep Water and Norwegian Sea Deep Water increase (Dickson, Lazier, Meincke, Rhines &Swift, 1996; Dickson, 1997). The deep, intense convection observed in the Greenland Sea during negativephases of the NAO is mirrored by shallow, intense ventilation in the western Sargasso Sea and increasedformation and spreading of 18° Mode Water (Dickson, Lazier, Meincke, Rhines & Swift, 1996).

Interactions between the major western boundary currents of the subpolar and subtropical gyres havealso been linked to the NAO (Rossby & Benway, 2000; Loder, Shore, Hannah & Petrie, 2001). Associatedwith these NAO-linked gyral interactions are changes in the Coupled Slope Water System (CSWS) of theNorthwest Atlantic (Pickart, McKee, Torres and Harrington, 1999; MERCINA, 2001). During positivephases of the NAO, the CSWS tends to operate in its maximum modal state, with relatively warm andsalty Atlantic Temperate Slope Water (ATSW) taking a position adjacent to the shelf break and forminga front near the mouth of the Gulf of St. Lawrence with Labrador Subarctic Slope Water (LSSW) (Fig.3a) (MERCINA, 2001). This advance of ATSW coincides with a decrease in shallow Labrador Currenttransport around the Tail of the Grand Banks and an enhanced hydrographic signature of Labrador SeaWater in the Deep Western Boundary Current (Pickart, McKee, Torres & Harrington, 1999). When thephase of the NAO reverses to negative, the CSWS typically shifts to its minimum modal state (Fig. 3b).This mode is characterised by relatively cool and fresh LSSW advancing along the shelf break as far tothe southwest as the Middle Atlantic Bight (MERCINA, 2001). This advance of LSSW along the shelfbreak displaces the warmer, saltier ATSW further offshore and coincides with an increase in shallowLabrador Current transport around the Tail of the Grand Banks and a reduced hydrographic signature ofLabrador Sea Water in the Deep Western Boundary Current (Pickart, McKee, Torres & Harrington, 1999).

Fig. 3. Modes of the Coupled Slope Water System: (a) maximum mode; (b) minimum mode. Circled numbers indicate 1997–1998advance of LSSW frontal boundary along the continental margin: (1) September 1997; (2) January 1998; (3) February 1998; (4)August 1998. Water masses and currents are labelled as follows: ATSW, Atlantic Temperate Slope Water; GS, Gulf Stream; LSSW,Labrador Subarctic Slope Water.


4. Population responses of C. finmarchicus to changes in ocean circulation

4.1. The North Sea

The changes in climate and ocean circulation patterns associated with the NAO provide a context forinterpreting the contrasting population responses of C. finmarchicus on the two sides of the North Atlantic.The response of this species in the North Sea has been attributed to several physical mechanisms associatedwith the NAO, each operating on a different time scale (Fig. 4). The first of these mechanisms involvesyear-to-year fluctuations of advective transport into the North Sea. Positive NAO years are characterisedby an increased inflow of relatively warm upper-layer water from the North Atlantic into the North Sea(Fig. 4) (Reid, Borges, & Svendsen, 2001a). This increased inflow of warmer North Atlantic water ishypothesised to supply the North Sea with fewer recruits of C. finmarchicus from the oceanic habitat wherethey diapause during autumn and winter (Hirche, 1996). It also provides a less favourable environmentfor growth and reproduction of this cold-water species (Fromentin & Planque, 1996). In contrast, negativeNAO years are characterised by an increased inflow of relatively cool deep water from the Norwegian Seainto the North Sea (Fig. 4) (Reid, Borges & Svendsen, 2001a). This increased inflow of cooler water ishypothesised to supply the North Sea with an increased number of recruits from the oceanic diapausehabitat and provide the population with cooler, more favourable conditions for growth and reproduction.Both of these advection-based hypotheses provide reasonable explanations for the strong negative corre-lation between the abundance of C. finmarchicus in the North Sea and the NAO index (Fig. 1d and e)(Planque & Reid, 1998), as well as the general decline in abundance of the species observed there sincethe early 1970s (Fig. 1a) (Fromentin & Planque, 1996; Planque & Taylor, 1998; Reid, Planque &Edwards, 1998).

Additional mechanisms have been proposed that link abundance patterns of C. finmarchicus in the NorthSea to longer-term changes in ocean circulation associated with the NAO. One of these mechanismsinvolves changes in regional deep-water circulation. Heath et al. (1999) hypothesised that the main sourceof late copepodites in diapause that recruits to the North Sea shelf in spring occurs at depths greater than

Fig. 4. NAO-associated changes in ocean circulation patterns thought to affect C. finmarchicus abundance in the North Sea. Thefirst mechanism involves year-to-year fluctuations in advective transport into the North Sea. Positive NAO years are characterisedby an inflow of relatively warm water from the North Atlantic into the North Sea. Negative NAO years show an inflow of relativelycool water from the Norwegian Sea. A second mechanism involves changes in deep-water circulation. During negative NAO con-ditions, the production and transport of Norwegian Sea Deep Water (NSDW) increases, leading to an increased flux of this watermass across the Iceland-Scotland Ridge through the Faroe-Shetland Channel (FSC). The third mechanism involves an intensificationof a jet at the shelf edge, which transports warm water into the vicinity of the British Isles and North Sea.


600 m in the Faroe-Shetland Channel. During the predominantly positive NAO conditions of the pastquarter century (Hurrell, Kushnir, & Visbeck, 2001), the production and transport of Norwegian Sea DeepWater has steadily declined, leading to a reduced flux of this water mass across the Iceland-Scotland Ridgethrough the Faroe-Shetland Channel (Fig. 4) (Hansen, Turrell, & Osterhus, 2001). These changes in deep-water circulation have been hypothesised to reduce the supply of late copepodites available for recruitmentinto the North Sea, further augmenting the decline in the abundance of C. finmarchicus observed there inrecent years.

Another proposed mechanism involves the intensification of the slope current, which transports warmwater into the vicinity of the British Isles and the North Sea (Fig. 4) (Reid, Holliday, & Smyth, 2001b).Reid, Borges and Svendsen (2001a) hypothesised that several years of highly positive NAO conditionsduring the late 1980s and early 1990s were necessary to intensify the flow of this shelf edge jet. Sincethe jet advects both warm water and warm-water species into the region, it has resulted in a regime shiftthat is unfavourable to C. finmarchicus (Reid et al., 2001a,b). As in the previous case, this mechanismwould further augment the recent declining trend in the abundance of C. finmarchicus observed in theNorth Sea.

Although each of the mechanisms described above would tend to reinforce the declining trend in C.finmarchicus abundance in the North Sea, the longer-term mechanisms may interact with the annual oneto complicate year-to-year predictability. For example, Planque and Reid (1998) found that their predictionsof the abundance of C. finmarchicus in the North Sea, based on a regression with the NAO index, provedaccurate through most of the CPR time series until the mid-1990s. A pronounced drop in the NAO indexduring the winter of 1996 led them to predict a year of high C. finmarchicus abundance after a steadydecline through the early part of the decade. This predicted increase did not occur, and one explanationmay be that a single-year drop in the NAO index, even as in this case the largest drop of the century, maynot be sufficient to overcome the effects of changing circulation patterns that were years in the making.

4.2. The Gulf of Maine/Western Scotian Shelf region

The more complex, positive association between C. finmarchicus in the GoM/WSS region and the NAOindex has also been attributed primarily to advective–supply mechanisms (Greene and Pershing, 2000;MERCINA, 2001). These mechanisms, suggested after recent observations emerged from the Global OceanEcosystem Dynamics (GLOBEC) Northwest Atlantic/Georges Bank Field Programme, link fluctuations inC. finmarchicus abundance to NAO-triggered modal shifts in the northwest Atlantic’s CSWS. In the sectionthat follows, we will first describe the events observed during the years of the GLOBEC Field Programme,then place these observations in the context of relevant time-series data collected over the past half century.

The drop of the century in the NAO index during the winter of 1996 led to a modal shift in the CSWS,an intensification of the shallow baroclinic component of the Labrador Current around the Tail of the GrandBanks, and the most extensive intrusion of LSSW into the GoM/WSS region since the 1960s (MERCINA,2001). This cooler, fresher LSSW steadily advanced along the Scotian Shelf break and replaced or signifi-cantly altered the deep basin waters of the WSS and GoM over the subsequent 2 years (Fig. 3b). By theearly autumn of 1998, all of the warmer, saltier ATSW, typical of the GoM deep basins since the 1970s,had been replaced by LSSW or rendered significantly cooler and fresher through mixing. Coinciding withthis major advective event and change in the regional hydrography, C. finmarchicus exhibited an order-of-magnitude decline in abundance in the GoM during 1998 (Fig. 5). The NAO index returned to positivevalues after the winter of 1996, and by December 1999 the LSSW had been replaced in most of the WSSand GoM deep basins by ATSW (MERCINA, 2001). Again, coincident with the physical changes, C.finmarchicus in the GoM/WSS region during December 1999 returned to a level comparable to thatobserved before the dramatic decline of 1998 (Fig. 5).

The oceanographic events observed in the Northwest Atlantic during the latter part of the 1990s provide


1997 1998 1999

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Georges BasinJordan BasinWilkinson Basin

C. finmarchicus C5 Integrated Abundance

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Fig. 5. Depth-integrated abundance of C. finmarchicus fifth copepodites sampled in the deep basins of the Gulf of Maine duringthe autumns of 1997 to 1999. One night-time MOCNESS sample was collected from each basin on each of the five cruises. Abundancesfrom samples taken in Georges Basin are indicated by circles, those from Jordan Basin by squares, and from Wilkinson Basin bytriangles. Mean values from all basins during a given year are indicated by a diamond with bars corresponding to ±1 SE.

an important piece of the puzzle for interpreting the historical abundance of C. finmarchicus in theGoM/WSS region over the past half century. During the decade of the 1960s, the NAO index was predomi-nantly negative and the CSWS operated in its minimum modal state (Fig. 6) (MERCINA, 2001). This wasan unusual period during the past 50 years when the deep waters of the GOM/WSS region were derivedprimarily from cool and fresh LSSW (Loder et al., 2001; MERCINA, 2001) and numbers of C. finmarchicuswere relatively low (Greene and Pershing, 2000; MERCINA, 2001). Subsequent to the 1960s, the NAOindex shifted to a predominantly positive phase and the CSWS to its maximum modal state (Fig. 6).Associated with these changes, the deep waters of the GoM/WSS region became warmer and saltier, derivedprimarily from ATSW, and C. finmarchicus numbers increased significantly. These coincident increases inthe NAO index and C. finmarchicus account for the significant positive correlation observed between themduring the period of 1961–1989 (Greene & Pershing, 2000). In the context of these long-term trends, theevents of the late 1990s fit the pattern consistently: the NAO index dropped dramatically during the winterof 1996, the coupled slope water system shifted from its maximum to minimum modal state during 1997,and C. finmarchicus abundance declined by an order of magnitude during 1998.

The puzzle is more complicated than this, however. Although the CSWS typically undergoes modalshifts in response to large phase changes in the NAO index, the time lag of the response and the intensityof the phase change necessary to elicit a response (i.e. threshold effect) appear to be variable. After the1960s, the coupled slope water system shifted from its maximum to minimum modal state on only a fewbrief occasions, including 1981–1983, 1988–1991, and 1996–1998 (Fig. 6) (MERCINA, 2001). While the1996–1998 modal shift was clearly in response to the large phase change in the NAO index during the


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Time Series from the Gulf of Maine Region

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c. Gulf of Maine C. finmarchicus Abundance

b. Regional Slope Water Temperature Index

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Fig. 6. Time series from the North Atlantic. (a) Annual values of the winter NAO index. (b) Annual values of the Regional SlopeWater Temperature index. (c) Annual values of the Calanus finmarchicus abundance index in the GoM. Positive anomalies in eachof the time series are lightly shaded and negative anomalies darkly shaded. Lines connecting the figures are offset by the time lagsdetermined in the cross-correlation analyses. The NAO index is defined in the caption to Fig. 1. The Regional Slope Water Temperatureindex was developed as an indicator of the modal state of the CSWS, with positive (negative) values corresponding to maximum(minimum) modal state conditions (MERCINA, 2001). It is the dominant mode derived from a principal component analysis of eightslope–water temperature–anomaly time series from the GoM/WSS region. The Calanus finmarchicus abundance index is the meanabundance anomaly for this species calculated each year as the mean difference between log-transformed observed abundances andlog-transformed expected abundances (MERCINA, 2001). Expected abundances were determined from the climatological mean sea-sonal cycle of C. finmarchicus in the Gulf of Maine. These abundance index values, connected by the solid line, have been calculateddifferently from the abundance values shown here and in Fig. 1b as circles. Those values were calculated by the standardisedprocedures described by Colebrook (1975) and therefore are comparable to other values presented in the literature. The abundanceindex values displayed only in this figure are valuable in that they can be used to fill in gaps in the time series when data areinsufficient to meet the SAHFOS standards.

winter of 1996, the circumstances leading up to the modal shifts in the early and late 1980s are less clear.Both of these modal shifts were weaker than the one observed in the late 1990s. In addition, although bothof them occurred within a few years of a positive to negative phase change in the NAO index, the timelag in each case was different (Fig. 6) (MERCINA, 2001). We conclude from analyses of these time-seriesdata that, although physical responses of the CSWS are significantly correlated with the NAO index, alarge proportion of the variance in the relationship remains unexplained as a result of variable time lagsand potential threshold effects (MERCINA, 2001). This conclusion is consistent with the weak positivecorrelation observed between C. finmarchicus abundance in the GoM and the NAO index from 1961 to1989, a relationship that subsequently became non-significant when data from the 1990s were included(Fig. 1d and f).

While retrospective analyses of the time series data yield correlative relationships that are informative,they provide few details of the underlying physical and biological processes linking population responsesof C. finmarchicus in the GoM with NAO-associated modal shifts in the CSWS. We hypothesise thatspringtime incursions of slope water onto the WSS play a central role in linking the two. Head, Harrisand Petrie (1999) have shown that slope-water incursions can be an important source of C. finmarchicus


recruiting to the WSS. Since the major inflows into the GoM from the WSS also occur during spring(Smith, Houghton, Fairbanks & Mountain, 2001), the timing is right for these incursions of slope waterto affect the advective supply of C. finmarchicus into the GoM.

There are several potential mechanisms that could explain the relationship between the modal state ofthe CSWS and C. finmarchicus in the GoM/WSS region. During the minimum modal state of the CSWS,LSSW advances southwestward along the WSS, displacing ATSW offshore (Fig. 3b). If the abundance ofC. finmarchicus is lower in LSSW than in ATSW, then source waters with reduced abundance of thisspecies could provide a simple explanation for the regional decline of C. finmarchicus during minimummodes of the CSWS. We are unaware of any evidence supporting this hypothesis at present. An alternativehypothesis is that cross-frontal exchange is impeded by intensification of the slope-water front duringminimum modal conditions, reducing the frequency and/or intensity of slope-water incursions. Pickart,McKee, Torres and Harrington (1999) provided evidence that the gradient of this front is intensified duringminimum modal conditions; however, we are unaware of any observations confirming that the frequencyand/or intensity of slope-water incursions are reduced under these conditions. Clearly, future field studieswill be necessary to fill in the gaps in our knowledge.

5. Discussion

We have reviewed evidence from the North Sea and GoM/WSS linking fluctuations in the abundanceof C. finmarchicus to changes in ocean circulation associated with the NAO. A noteworthy finding of thisreview is the regional dependence of the population responses of C. finmarchicus. In the North Sea, thespecies exhibits a negative correlation with the NAO index, whereas the GoM/WSS population exhibits amore complex, positive association with the index. Since ocean–circulation responses to the NAO vary indifferent regions of the North Atlantic, it should come as no surprise that the population responses of C.finmarchicus vary by region as well.

The responsiveness of C. finmarchicus to changes in ocean circulation, combined with its ecologicalimportance throughout the North Atlantic, makes this species a good indicator of climate change and asuitable target species for global climate-change research (GLOBEC, 1991; MERCINA, 2001). While thisreview has focused primarily on linking regional population responses of C. finmarchicus to circulationchanges associated with the NAO, our findings are also relevant to predicting the future responses of thespecies to global climate change.

During the past quarter century, the NAO index has exhibited a persistently increasing trend, a trendthat some have associated with greenhouse warming (Hurrell, Kushnir & Visbeck, 2001) and the recentlydocumented rise in oceanic heat content (Hoerling, Hurrell, & Xu, 2001; Barnett, Pierce, & Schnur, 2001;Levitus et al., 2001). While we have shown that positive NAO conditions appear unfavourable for C.finmarchicus in the North Sea and favourable in the GoM/WSS region, the increased climate variabilitypredicted by some models as a consequence of rising greenhouse-gas concentrations (IPCC, 2001) makeslong-term predictability difficult at present. The NAO has exhibited unusual behaviour during the latterhalf of the 1990s, including a shift in the subpolar low-pressure centre towards the Greenland Sea(Ulbrich & Christoph, 1999). Several investigators have suggested that rising greenhouse-gas concentrationsmay be responsible for this unusual behaviour (Langenberg, 2000). In addition, the extreme drop of theNAO index during 1996 may represent an unusual event or perhaps a sign of the larger swings in climatethat we might expect in a greenhouse future. The uncertainty surrounding these recent events also raisesthe issue of what we might expect if the NAO were to enter a long, persistently negative phase. Palaeocli-mate records indicate that such conditions have occurred in the past (Appenzeller, Stocker, & Anklin, 1998;Jones, Osborn, & Briffa, 2001), and some investigators have suggested that we might expect a return tosuch conditions in a greenhouse future (Wood, Keen, Mitchell & Gregory, 1999; Hillaire-Marcel, de Vernal,


Bilodeau, & Weaver, 2001). Ultimately, the fate of C. finmarchicus populations on both sides of the NorthAtlantic will depend on how regional circulation patterns respond to natural, as well as anthropogenicallyforced changes in global climate.

Acknowledgements

We thank the organisers for the opportunity to present this paper at the 70th Anniversary Symposiumof the CPR survey held in Edinburgh, Scotland. This paper is contribution #341 from the US GLOBECNW Atlantic/Georges Bank Programme, with support provided by the NOAA Coastal Ocean Programmeand the NSF Biological Oceanography Programme. The manuscript was prepared while Charles H Greenewas in residence as a sabbatical fellow at the National Center for Ecological Analysis and Synthesis, aCenter funded by NSF (Grant #DEB-0072909) and the University of California, Santa Barbara.

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