RESEARCH ARTICLE10.1002/2013JC009480

The oceanic response of the Turkish Straits System to anextreme drop in atmospheric pressureJeffrey W. Book1, Ewa Jarosz1, Jacopo Chiggiato2,3, and S€ukr€u Besiktepe2,4

1Naval Research Laboratory, Stennis Space Center, Mississippi, USA, 2NATO Centre for Maritime Research andExperimentation, La Spezia, Italy, 3Consiglio Nazionale delle Ricerche—Istituto di Scienze Marine, Venezia, Italy, 4Instituteof Marine Sciences and Technology, Dokuz Eyl€ul University, Izmir, Turkey

Abstract Moorings across all four entry/exit sections of the Dardanelles Strait and the Bosphorus Straitsimultaneously measured the response of the Turkish Straits System to the passage of a severe cyclonic stormthat included an atmospheric pressure drop of more than 30 mbar in less than 48 h. The bottom pressureresponse at the Aegean Sea side of the Dardanelles Strait was consistent with an inverted barometer response,but the response at the other sections did not follow an inverted barometer, leading to a large bottom pres-sure gradient through the Turkish Straits System. Upper-layer flow toward the Aegean Sea was reversed by thestorm and flow toward the Black Sea was greatly enhanced. Bottom pressure across the Sea of Marmarapeaked 6 h after the passage of the storm’s minimum pressure. The response on the Dardanelles side was acombination of sea elevation and pycnocline depth rise, and the response on the Bosphorus side was an evengreater sea elevation rise and a drop in pycnocline depth. The peak in bottom pressure in the Sea of Marmarawas followed by another reverse in the flow through the Dardanelles Strait as flow was then directed awayfrom the Sea of Marmara in both straits. A simple conceptual model without wind is able to explain fluctua-tions in bottom pressure in the Sea of Marmara to a 0.89–0.96 level of correlation. This stresses the importanceof atmospheric pressure dynamics in driving the mass flux of the Turkish Strait System for extreme storms.

1. Introduction

On 21 November 2008, the atmospheric pressure over the Dardanelles Strait, the Sea of Marmara, and theBosphorus Strait suddenly dropped by more than 30 mbar in less than 48 h and then just as abruptly returnedto normal atmospheric pressure conditions in the next 48 h (Figure 1). The explosively developing cycloneresponsible for this atmospheric pressure drop would meet the criteria of a ‘‘meteorological bomb’’ set forthby Sanders and Gyakum [1980] if the pressure drop is evaluated at a fixed position, but it does not satisfy theirtechnical definition of a meteorological bomb since we are not following changes in either central or relativepressure of the moving cyclone system. Also, the cyclogenesis is mainly occurring over land and thereforelacking the air-sea heat exchange component that helps to make conventional meteorological bombs almostentirely maritime in their global distributions [Lim and Simmonds, 2002]. Nevertheless, this event was a verystrong and destructive cyclonic storm (Figure 2), sinking Kadikoy Pier in the Bosphorus Strait in Istanbul andimposing damaging high winds in many places throughout Turkey before proceeding to the north andimpacting the Baltic region with extreme conditions. The normal outflow of surface Black Sea waters to theSea of Marmara through the Bosphorus was completely reversed by this event [Jarosz et al., 2011].

This storm occurred during the time period when the Naval Research Laboratory, the NATO UnderseaResearch Centre (now Centre for Maritime Research and Experimentation), and the Turkish Navy Office ofNavigation, Hydrography, and Oceanography were collaboratively collecting data from four mooring sec-tions [Jarosz et al., 2011, 2012] covering each of the main entry and exit points of both the Dardanelles andBosphorus Straits (Figure 3). This observational array provides a unique opportunity to examine the oceanicresponse to the storm over the whole Turkish Straits System and examine the basin exchange dynamicsthat are important for determining this response. Fundamentally, a storm of this nature can directly influ-ence oceanic flow through straits by forcing from wind stress and by forcing from atmospheric pressurechanges. Of these, the effect of wind stress is more typically considered [e.g., see Valle-Levinson and Blanco,2004; Woodgate et al., 2005; Pullen et al., 2011], but for basins connected by flow restrictions the invertedbarometer effect can be imperfect and atmospheric pressure should not be neglected [Lyu et al., 2002;Le Traon and Gauzelin, 1997; Candela et al., 1989]. In this paper, we focus on the atmospheric pressure effect

Key Points:� Atmospheric pressure can be a

primary factor in response of straitsto storms� Differing inverted barometer

responses between basins can drivestrong currents� Atmospheric pressure of cyclones

cause a complex response in theTurkish Straits

Correspondence to:J. W. Book,book@nrlssc.navy.mil

Citation:Book, J. W., E. Jarosz, J. Chiggiato, andS. Besiktepe (2014), The oceanicresponse of the Turkish Straits Systemto an extreme drop in atmosphericpressure, J. Geophys. Res. Oceans, 119,3629–3644, doi:10.1002/2013JC009480.

Received 2 OCT 2013

Accepted 12 MAY 2014

Accepted article online 14 MAY 2014

Published online 11 JUN 2014

BOOK ET AL. VC 2014. American Geophysical Union. All Rights Reserved. 3629

Journal of Geophysical Research: Oceans

PUBLICATIONS

in a study of the strong November2008 storm over the Turkish Straitsand examine the general dynamics ofextreme atmospheric pressure sys-tems over restricted straits. All timesused in this paper are given in theUTC standard.

2. Synoptic AtmosphericConditions

The severe storm affecting the TurkishStraits System region on 22 November2008 was triggered by a southwardexcursion of the jet stream associatedwith North-Atlantic blocking to thewest and a large-scale depression overnortheastern Europe, with consequentmeridional advection of arctic airmasses to southern Europe. The onsetof this synoptic situation becamemature on 21 November (Figure 2,top), when the outbreak of arctic airmasses approached the northern Medi-terranean sector where the interactionwith the local orography and with thewarm, moist Mediterranean air favoredbaroclinic instabilities. An extensivelow-pressure system quickly developedover southeastern Europe. This cycloneeventually shifted to the north and 2days later deepened to 954 mbar atsea level over Estonia.

The intrusion of this cold frontal sys-tem over the Balkans triggered galeforce southwesterlies over the TurkishStraits System region (Figure 2) with aslight increase in surface temperatureand a drop of atmospheric pressure atsea level of more than 24 mbar in 24 h(Figure 2 (bottom) and Figure 1). Thecold front passed during the after-noon of 22 November, along withmoderate postfrontal northwesterliesand a significant temperature drop ofaround 10�C.

3. Ocean Bottom PressureReaction

Hydrostatic ocean bottom pressurevariations (p

0

b) are made up of a sumof atmospheric pressure variations(p0atm), pressure variations due to the

Figure 2. Atmospheric model analyses for (top) 21 November 12:00 UTC and (bot-tom) 22 November 12:00 UTC showing air temperature at 850 mbar (color), windat 850 mbar (arrows), and geopotential height anomaly at 850 mbar (white con-tour lines). Data come from National Centers for Environmental Prediction FinalOperational Model Global Tropospheric Analyses [National Centers for Environmen-tal Prediction, 2000].

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Figure 1. Measured atmospheric pressure variations for 18–28 November UTC2008, from 13 meteorological stations surrounding the Turkish Straits System. Datakindly provided courtesy of Turkish State Meteorological Service.

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rising and falling sea surface height (g0), and pressure variations created by changes in the water column’svertical density structure (qðzÞ0),

p0b5p0atm1q0gg01gð0

2Hq0dz; (1)

26˚

26˚

27˚

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Dardanelles Strait

Bosphorus Strait

Black Sea

AegeanSea

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Dardanelles

M1

M2

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Dardanelles

M3

M4

29˚00' 29˚10'

41˚10'Northern

Bosphorus

M7 M8

29˚00'

41˚00'

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Bosphorus

M5M6

Figure 3. The Turkish Straits System. Dark triangles give the locations of bottom mounted ADCP and pressure sensors located in all theentrances/exits of the straits. M1, M3, M5, and M7 were located in the deepest part of the channel and are the moorings primarily used inthis paper. M2, M4, M6, and M8 were similarly instrumented but located off the main channels. Four line moorings measuring temperatureand salinity were deployed near the four deep channel bottom moorings. Further details can be found in Jarosz et al. [2011, 2012]. Circlesindicate the positions of 12 of the meteorological stations used in Figure 1. The 13th meteorological station is located (40.77�N, 30.38�E)near the farthest east reaches of the Sea of Marmara, not shown on these maps.

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with q0 as a reference density, g as the gravitational acceleration, H as the bottom depth, z as the verticalcoordinate, and primes indicating anomalies with respect to each variable’s time mean. The invertedbarometer response occurs when the second term on the right-hand side cancels the first term on theright-hand side, both locally and globally [Ponte et al., 1991; Wunsch and Stammer, 1997] leading to no otherocean response to the atmospheric pressure changes. If this approximation is true then atmospheric pres-sure changes can be completely ignored when studying the dynamical response of the ocean to passingstorms. However, for nearly enclosed seas or any location with flow restrictions, the inverted barometer bal-ance cannot be completely achieved at all frequencies of atmospheric pressure change due to restrictionsin mass exchange with neighboring bodies of water [Wunsch and Stammer, 1997].

The Strait of Gibraltar provides such a restriction for the Mediterranean and has been shown to cause adeparture from a perfect inverted barometer response at high frequencies when considering the entireMediterranean mean sea level [Le Traon and Gauzelin, 1997], but for particular regions such as the Aegean[Tsimplis and Vlahakis, 1994], the local high-frequency response can be very close to a theoretical invertedbarometer [e.g., see Wunsch and Stammer, 1997, section 13]. Using TOPEX/POSEIDON altimetry data, Ducetet al. [1999] show that the sea level of the Black Sea does not respond as an inverted barometer at periodsfaster than 40 days (periods faster than 5 days were not observable in their data set) due to the strong flowrestriction imposed by the Bosphorus Strait. However, in an examination of sea level data from tide gaugesdistributed between the Aegean and Black Seas, Alpar and Y€uce [1998] found significant coherencebetween these records and barometric pressure at periods of 3.5–6.7 days suggesting that an invertedbarometer response can influence subtidal variations of sea level in the Turkish Straits System.

During the severe storm in November 2008, bottom-mounted pressure sensors recorded pb in all theentrances and exits of the individual straits, simultaneous to the routine recording of patm by local meteoro-logical stations. Therefore, we can evaluate the details of the inverted barometer response for this particularstorm across the entire Turkish Straits System. p

0

b records were obtained from the bottom pressuresrecorded by Sea-Bird Electronics Seagauge Wave and Tide Recorders contained in trawl-resistant bottommounts (Figure 3) deployed in the main channels of the straits (off-channel pressure records were alsoobtained but were nearly identical to the main channel records and not used). p0atm records were obtainedby averaging the atmospheric pressure anomalies from all 13 stations shown in Figure 1.

If pressure variations due to density structure are not too large and if an inverted barometer response holds,then p

0

b will be nearly zero. In this case, p0

b2p0atm will approximate q0gg0 and be nearly equal to 2p0atm. Fig-ure 4 (top) shows p

0

b2p0atm for each strait entrance/exit during the storm compared to 2p0atm estimatedfrom the average pressure variations of the 13 meteorological stations. During the time period of theatmospheric pressure event (21–24 November), the general envelope of the response at the Aegean side ofthe Dardanelles matched an inverted barometer response fairly well, peaking at just over 0.3 dbar at thesame time as atmospheric pressure reached its minimum value of 0.3 dbar midday on the 22nd. There aresmall departures in p

0

b2p0atm tracking 2p0atm during the main storm event, but differences increase after 24November with respect to both higher-frequency and lower-frequency oscillations. The higher-frequencyvariations are mainly due to a small portion of the semidiurnal tidal barotropic energy that is present inpotential energy form in the southern Dardanelles. This potential energy portion of the barotropic tidalenergy is almost entirely transformed into kinetic forms and also partially dissipated through the Straitbefore reaching the Sea of Marmara [Book et al., 2010] due to its relatively low amplitude and high fre-quency. The lower-frequency departure in p

0

b2p0atm tracking 2p0atm that occurs around 26 November is likelyassociated with a second wind event that will be briefly discussed later in this paper.

The response at the Sea of Marmara sides of both the Dardanelles and Bosphorus also showed a peak simi-lar to an inverted barometer response, but these peaks were delayed by about 6 h with respect to theatmospheric pressure minimum. The similar response at both ends of the Sea of Marmara suggests that thisbody is responding uniformly to the atmospheric pressure forcing of the storm, at least with regards tohydrostatic bottom pressure variations. The response at the Black Sea side of the Bosphorus shows little orno relationship to an inverted barometer.

The differences in the inverted barometer response on both sides of the Dardanelles and on both sides ofthe Bosphorus cause significant bottom pressure differences to develop along the two straits. In the Darda-nelles, the Aegean side responds immediately to the drop in atmospheric pressure but the Sea of Marmara

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response is delayed, creating a negative bottom pressure difference from the Marmara to the Aegean dur-ing the initial hours of the storm that reaches a value of 20.2 dbar near midday on the 22nd (Figure 4, bot-tom). As the response on the Sea of Marmara side of the Dardanelles reaches its peak on the 23rd, theAegean response has already declined in synch with the atmospheric pressure. Thus, the bottom pressuredifference reverses along the Dardanelles Strait, reaching a positive 0.2 dbar difference from the Marmarato the Aegean (Figure 4, bottom). The Black Sea side of the Bosphorus has a relatively stable 0.3 dbar dropin pressure (Figure 4, top) sustained over several days, but the rise in pressure at the Sea of Marmara side ofthe Bosphorus in response to the atmospheric pressure (although delayed) adds up to another 0.3 dbar inpressure difference, creating a peak of 20.6 dbar bottom pressure difference from the Black Sea to the Seaof Marmara on the 23rd (Figure 4, bottom).

Pressure variations due to density structure that have so far been neglected, can be estimated for the stormperiod from Sea-Bird Electronics MicroCAT temperature/salinity/pressure sensors mounted on line mooringsnear each of the main channel bottom moorings [Jarosz et al., 2011, 2012] (Figure 3). In the Turkish StraitsSystem, two different water masses are separated by a strong pycnocline at a depth of �25 m and fluctua-tions in the pycnocline interface can produce fluctuations in the last term of equation (1). Low salinity (�18psu) waters of Black Sea origin occupy the upper layer of the System and salty (�38.5 psu) waters of Aegeanorigin occupy the lower layer of the System. The two-layer stratification and two-layer current system in theTurkish Straits System is driven by the density differences between the adjoining seas and the stability ofthis two-layer structure is due to their salinities [Besiktepe et al., 1994]. Measurements of salinity (which con-trols density structure) made at discrete depths on the four line moorings are shown in Figure 5.

Unfortunately, due to safety requirements and instrument failure and loss at some locations, no line moor-ing sensors were located in the low-salinity, upper layer for the Aegean side of the Dardanelles or for theSea of Marmara side of the Bosphorus. To partially compensate for this, measurements from conductivitycells enclosed in each bottom ADCP mooring (shown in light cyan and blue) were also added to Figure 5

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Figure 4. (top) Measured p0

b2p0atm values for the Aegean side of the Dardanelles (blue), Sea of Marmara side of the Dardanelles (black),Sea of Marmara side of the Bosphorus (green), and Black Sea side of the Bosphorus (magenta). The red line is the inverse of the observedatmospheric pressure variation. (bottom) Measured bottom pressure differences along the Dardanelles Strait (blue) and along the Bospho-rus Strait (magenta). Positive differences indicate higher pressures toward the north.

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since the variation in salinity structure with depth is much greater than cross-channel variations in salinity.These enclosed sensor’s response times are reduced compared to the response times of line mooring sen-sors, but are still able to track relatively high frequency trends as demonstrated, for example, by M7 and M8salinities shown in Figure 5 (bottom right). Notably, since mooring M6 was deployed at a much shallowerdepth than the other moorings, its bottom sensor can be used to track a drop in salinity at the Sea of Mar-mara side of the Bosphorus that the line mooring sensors were unable to capture.

Before the storm, the northern Dardanelles (Figure 5, top right) had a thicker Black Sea water layer thanusual with the sensor at 39 m depth reaching a salinity of 32.34 on 19 November (not shown), which is typi-cal of salinities at 21 m depth during other periods. However, during the storm the situation completelyreverses to a Black Sea water layer thinner than usual with an observed 10 psu increase at the 16 m depthupper sensor. From 22 November through midday on the 24th, there is a weak decrease (�2–4 psu) in salin-ity for the sensors down to 67 m in the southern Dardanelles (Figure 5, top left) and down to 26 m in thenorthern Dardanelles, but salinities at 16 m in the northern Dardanelles stay well above their average value(28.42 psu) until 27 November. The northern Bosphorus salinity observations (Figure 5, bottom right) showonly a moderate increase in salinity at the two upper level sensors (20 and 30 m depth) accompanied by amoderate decrease in salinity in the lower layer sensor (57 m) and the sensors enclosed in the ADCP moor-ings (66 m and 78 m). This is suggestive of storm enhanced mixing between the layers. The two availablesensors in the southern Bosphorus (Figure 5, bottom left) show high salinities with a minor decrease duringthe storm. This decrease (not visible at the scale used in Figure 5, bottom left) was much less than the

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Figure 5. Salinities as measured by MicroCAT instruments on line moorings TC1–4 deployed near M1, M3, M5, and M7 moorings and by conductivity cells enclosed in each bottomADCP mooring (see Figure 3) for the four sections as labeled. The depth of each sensor is labeled in color within the plots, with the salinities from the enclosed bottom sensors alwayscolored blue for the off-channel site and light cyan for the main-channel site. The salinity scales in each plot are varied according to the ranges of available measurements for each loca-tion. No measurements were shallow enough to observe the thin, low-salinity, upper layer of the southern Dardanelles (top left). Further details of the mooring configurations can befound in Jarosz et al. [2011, 2012].

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decrease observed in the northern Bosphorus bottom layer at similar depth levels. However, as mentionedbefore, the sensor enclosed in mooring M6 at a depth of 27 m recorded a dramatic 12 psu decrease in salin-ity at a fixed depth level in the southern Bosphorus during the peak of the storm.

Using the assumption that density varies linearly between measured depths and is constant above theupper-most sensor and below the lowest sensor, values for g

Ð 02H q0dz can be estimated (Figure 6, top) for all

sections except the southern Dardanelles where no measurements were shallow enough to observe fluctu-ations in the low-salinity upper layer waters. The decrease in upper layer thickness in the northern Darda-nelles leads to a relative steady positive contribution to p

0

b from the density integral, averaging 0.17 dbarfrom 22 to 24 November. This means that roughly half of the delayed peak in p

0

b2p0atm at the Dardanellesside of the Marmara (Figure 4) can be attributed to a rise in the pycnocline depth and only half can beattributed to a rise in sea level (Figure 6, bottom) at this location. Additionally, this implies that the actualsea-level anomaly was below zero before and after the peak, and that p

0

b2p0atm in the southwest Sea of Mar-mara remains positive over much of the storm only due to horizontal redistribution of mass and the g

Ð 02H q0

dz term.

For the southern Bosphorus, only four sensors are available and three of these are located at nearly thesame depth level. Therefore, estimated values for g

Ð 02H q0dz are essentially derived from a weighted sum of

two records. This sum suggests that the density integral dropped by more than 0.3 dbar during the storm(Figure 6, top) in the southern Bosphorus due to the increase in upper layer thickness. This implies that thedelayed peak in p

0

b2p0atm at the Bosphorus side of the Marmara (Figure 4) is composed of a sea level rise(Figure 6, bottom) twice the value of the inverted atmospheric pressure peak and a drop in pycnoclinedepth that cancels approximately half of the pressure contribution of the sea level rise, i.e., mass is beingredistributed during the storm such that the pycnocline rises in the southwest Sea of Marmara and falls inthe northeast Sea of Marmara.

Results from a Regional Ocean Modeling System (ROMS) run for the Sea of Marmara as described in Chig-giato et al. [2012] can be used to examine further details of the pycnocline depth changes that occurred

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Figure 6. (top) Estimated gÐ 02H q0dz values for the Sea of Marmara side of the Dardanelles (black), the Sea of Marmara side of the Bospho-

rus (green), and the Black Sea side of the Bosphorus (magenta). (bottom) Estimated values for q0gg0 calculated from p0

b2p0atm2gÐ 02H q0dz

for the Dardanelles (black) and Bosphorus (green) sides of the Sea of Marmara. The red line is the inverse of the observed atmosphericpressure variation.

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during this strong November storm. During the peak of the oceanic response to the storm, the depth of ther25 isopycnal (Figure 7) in the model rose about 15 m all along the northwestern side of the Sea while fall-ing about 15 m throughout much of the eastern half of the Sea. The model simulation matched the rise inpycnocline depth in the southwestern Sea of Marmara as estimated by the observations (black line, top, Fig-ure 6) reasonably closely, but underpredicted the magnitude of the deepening of the pycnocline in thenortheastern Sea of Marmara as estimated by the limited observations (green line, top, Figure 6). The modelresults suggest a complex pycnocline depth response and mass redistribution in the Sea of Marmara follow-ing its mesoscale circulation patterns, but also supports the concept of the development of a pycnocline tiltfrom west to east during the storm in general agreement with wind-driven flow dynamics [Monismith, 1986;Cushman-Roisin et al., 1994]. The model also suggests that the locations of the northern Dardanelles andsouthern Bosphorus mooring sections are reasonably representative of the main changes in pycnoclinedepth for the respective two sides of the Sea of Marmara.

For the northern Bosphorus, the decrease in salinity in the lower layer is approximately compensated by anincrease in salinity in the upper layer leading to only small density integral values until 23 November. Usingthe continuity equation, the time rate of change of the density structure integral (divided by g) can bedirectly equated to the vertically integrated mass flux divergence, 2

Ð 02H r! � ðq u!Þdz. This lends quantitative

support to the suggestion that the salinity changes in the northern Bosphorus are due to enhanced verticalmixing rather than flow convergence/divergence, since the latter would require a change in the verticallyintegrated density structure and that was not observed in the Bosphorus until the 23rd.

4. Ocean Velocity Reaction

Velocities were measured in each section (Figure 3) by bottom-mounted, upward-looking Acoustic DopplerCurrent Profilers (ADCPs). Details of the instrumentation and observations are provided in Jarosz et al. [2011,2012]. For this portion of the work, we focus on measurements made at moorings M1, M3, M5, and M7located in the deepest part of each channel. Figure 8 shows the velocities as observed during the stormperiod at each of these moorings. Velocities measured east of the main channels (moorings M2, M4, M6, andM8, not shown) show very similar structure to the velocity patterns observed in the deep channels. Along-strait directions were determined by calculating the depth median of the direction of the time-averaged flowfor each mooring, with time averaging done over the August/September 2008 to February 2009 portions ofthe observations (i.e., the entire observational period for the Bosphorus deployments and the matching firsthalf of the Dardanelles deployments). This technique selects the general flow direction of the lower layertoward the Black Sea, because, in all cases these layers are (on average) thicker than the respective upperlayers (although this is barely true for the northern Bosphorus section where the thicknesses of the layers aresimilar). The directions determined by this technique are 90.5�T, 41.0�T, 1.6�T, and 46.6�T, for the southernDardanelles, northern Dardanelles, southern Bosphorus, and northern Bosphorus respectively.

The prestorm strait flows on 20 November (Figure 9) followed the general mean flow patterns as describedin Jarosz et al. [2011, 2012], i.e., the flows had a clearly defined two layer structure, with relatively thin

Figure 7. Average r25 anomaly depth for the period 22 November 12:00 UTC to 23 November 12:00 UTC at the peak of the oceanic stormresponse, calculated from a ROMS simulation of the Sea of Marmara as described in Chiggiato et al. [2012]. The anomaly is calculated fromthe average r25 depth over the period 18–28 November. The depth of the r25 isopycnal is a reasonable proxy for the pycnocline depthbetween the waters of Black Sea and Aegean Sea origin, and thus the model predicts a deepening of this interface by 15 m on the easternside of the Sea of Marmara and a shallowing of this interface by 15 m on the western side, in response to the storm.

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upper-layer flows toward the Aegean in the southern Dardanelles and southern Bosphorus, and thickerupper-layer flows toward the Aegean in the northern Dardanelles and northern Bosphorus. The lower layerflows toward the Black Sea are typically much thicker in the Dardanelles sections than their counterpartupper-layer flows, and also are relatively thick in the Bosphorus sections. The northern Bosphorus sectionuniquely has flow toward the Black Sea concentrated as a midwater column jet and the southern Darda-nelles has weak reversals near the bottom forming an occasional three-layer flow structure. On 20 Novem-ber, flow of Black Sea water toward the Aegean in the upper layer was somewhat weaker than average andflow of Aegean Sea water toward the Black Sea was somewhat stronger than average. The median differ-ence from the means presented in Jarosz et al. [2011, 2012] was 17 cm/s of additional flow toward the BlackSea across all layers. The largest difference occurred in the upper-layer flow in the southern Bosphorustoward the Sea of Marmara which was about 45 cm/s weaker than average.

The immediate response to the storm on 21 November was blockage of the upper-layer flow toward theAegean by 18:00 UTC in all the ADCP observed depth levels in all four sections (Figures 8 and 9). The upperdepth cells for the ADCPs are 4–8 m below the surface, so it is possible that some flow toward the Aegeanpersisted if it was confined to a very thin surface layer. The reversal of the upper-layer flow was strongestfor the southern Bosphorus (representing a �100 cm/s change over a day), relatively strong for the northernDardanelles, but only a weak reversal for the southern Dardanelles and northern Bosphorus. Reversals ofthe upper layer flow, identified locally as Orkoz, have been observed before in the Bosphorus [Besiktepeet al., 1994], but never before with current meters at both ends of the strait. Reversal of the upper layer flowat both ends of the Bosphorus was observed in the daily average on the 21st (Figure 9, top right) of themain channel moorings, documenting complete blockage of the upper layer flow over the entire Bosphorusfor a sustained period during the storm. The reversal in the northern Dardanelles was associated with a

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Figure 8. Depth-time plots of along-strait (positive flows are directed toward the Black Sea) currents observed by bottom-mounted ADCPon the moorings deployed in the deepest parts of the channel (M1, M3, M5, and M7) for each mooring section as labeled. A 2150 to150 cm/s color scale was used for all plots. Very few observed velocities exceed this range in the two Dardanelles sections or in the south-ern Bosphorus section, but the flow in the northern Bosphorus section exceeded 150 cm/s for 5% of the observations shown andexceeded 200 cm/s for 1% of the observations shown. Thus, the midlayer jet-like core for the northern Bosphorus where these strongvelocities occur is color saturated in order to use a consistent color scale with the other sections that show less range in velocity.

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rapid increase in salinity in the upper-layer sensors (Figure 5, top right). The observed change in salinity forthe northern Bosphorus was delayed and more modest, and the strong drop in salinity observed at 27 mdepth in the southern Bosphorus did not occur at the same time as the first strong reversals in upper-layerflow that were observed there (although it is possible that the filtering effect from the mooring enclosurecould have contributed to the delay in the salinity signal). The lower-layer flow response on the 21st wasnot as strong as the upper-layer flow response, although flow in the northern Bosphorus outflow jet didbegin to strengthen (Figure 8).

As the storm reached its minimum atmospheric pressure on the 22nd, the ocean velocity response greatlystrengthened. Flow in the upper layer in the southern Bosphorus and northern Dardanelles reached100 cm/s back toward the Black Sea (Figure 9). The flow toward the Black Sea was surface intensified forboth ends of the Dardanelles Strait, and occupied the entire water column for the southern Bosphoruspeaking there with a subsurface core exceeding 150 cm/s. This was not the case for the northern Bosphoruswhere weak upper-layer flow toward the Sea of Marmara was reestablished. However, the midlevel, jet-likeflow in the northern Bosphorus toward the Black Sea was greatly intensified and vertically thickened, nearlyreaching a daily average value of 200 cm/s (Figures 8 and 9). The intensification of the flow in the Bosphorustoward the Black Sea is in general agreement with the strong bottom pressure difference observed by thepressure sensors (Figure 4). Weak flow toward the Aegean occurred in the southern Dardanelles in thelower layer creating a complete reversal of direction from the typical southern Dardanelles flow pattern.

Early on 23 November, the flow reversed again in the Dardanelles, now flowing toward the Aegeanthroughout the water column in the northern Dardanelles and in the upper half of the water column in thesouthern Dardanelles (Figure 8). During this period, flow in the Bosphorus continued to be strongly

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Figure 9. Daily-average, along-strait velocity structures with depth for the Aegean side of the Dardanelles (blue), Sea of Marmara side ofthe Dardanelles (black), Sea of Marmara side of the Bosphorus (green), and Black Sea side of the Bosphorus (magenta). Note cross sectionalareas of the four sections are not equal and therefore velocity ratios between sections are not equivalent to transport ratios.

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intensified toward the Black Sea with similar structure as on the 22nd. This period of flow away from theSea of Marmara through both straits is in agreement with continuation of strong bottom pressure differen-ces along the Bosphorus and reversal of the bottom pressure difference along the Dardanelles (Figure 4).Around midday on the 23rd, the rereversal in the Dardanelles stopped and flow toward the Sea of Marmarawas restored (Figure 8). Flow in the southern Bosphorus also returned to a more typical flow pattern butwith a weak and diffuse upper-layer flow. The change in the northern Bosphorus was very abrupt, with thesuper strong midlevel jet drastically decreasing in size and strength over a period of 2 h and then continu-ing on with a structure more similar to its long-term mean state.

Throughout the 24th and 25th the Turkish Straits System returned to more normal flow conditions(Figure 9). However, there was another blockage event on the 26th and it was not until the 27th that flowin the upper layer toward the Aegean was reestablished to its normal strength (Figure 8). Observed salinitychanges in the northern Dardanelles and northern Bosphorus (Figure 5) support this with a reestablishmentof stratification on the 24th, a slight increase in salinity on the 26th in the upper layers, and a return to nor-mal stratification conditions on the 27th. Meteorological station measurements (not shown) indicate a sec-ond peak in southwesterly winds occurred on the 25th and 26th which might be responsible for the secondblockage event. The second peak in wind seemed to be stronger in the wind stations located near the Dar-danelles than those located near the Bosphorus and the atmospheric pressure anomaly from this secondaryevent was weak (Figure 1).

5. Conceptual Model

How much of the observed bottom pressure variations and mass flux through the straits can be solelyexplained by the rapidly changing atmospheric pressure of the November 2008 storm? To examine thisquestion, we used a conceptual model of atmospheric pressure forcing of the barotropic mode of the Turk-ish Straits System. The model is an implementation of the two basin, two strait model of Candela et al.[1989] with nonlinear friction similar to the correction suggested by Malacic [1993]. The Candela et al. [1989]model has been implemented for the Turkish Strait System before by €Ozsoy et al. [1998], but our implemen-tation differs from this previous study in an important way. Based on previous findings of an invertedbarometer response in the Aegean Sea for fast variations in atmospheric pressure [Tsimplis and Vlahakis,1994] and the specific finding of an inverted barometer response at the Aegean Sea side of the DardanellesStrait during the November 2008 storm (Figure 4), the Aegean Sea boundary condition in the conceptualmodel is assumed to follow a perfect inverted barometer. Thus, the ocean pressure anomaly of the Aegeanis set to be zero in the model. The driving force of the model across the Dardanelles Strait then becomesthe entire atmospheric pressure signal of the storm, differing starkly from the boundary condition used by€Ozsoy et al. [1998] of g50 in the Aegean which allows only the spatial gradient of the atmospheric pressure(very small for the November 2008 storm, see Figure 1) to drive this previous model implementation.

In contrast, the Black Sea in our implementation is given a boundary condition of g50. With this we areassuming that flow through the Bosphorus and river flow into the Black Sea cannot significantly alter thesea surface elevation of the entire Black Sea over the short time period of the November 2008 storm. This isa reasonable assumption considering that the surface area of the Black Sea is more than 36 times that ofthe Sea of Marmara. The sea surface elevation of the Sea of Marmara can be relatively quickly altered byflow through the Dardanelles and Bosphorus Straits. For example, a steady 50 cm/s excess flow through theDardanelles can raise the sea surface in the Sea of Marmara by 10 cm in about 4 h. Therefore, the sea sur-face elevation of the Sea of Marmara is allowed to adjust according to the fluxes through the straits in theconceptual model. As with previous versions of this model, we treat the basins (e.g., the Sea of Marmara) asa whole and do not consider variations across them, i.e., this model is not meant to represent the complex3-D physics of the Sea of Marmara, but only to examine the specific question of the role of temporal atmos-pheric pressure changes in driving mass flux and bulk pressure variation of the system. Figure 10 provides aschematic diagram for this conceptual model of the Turkish Straits System.

Following Candela et al. [1989], the momentum balances for the flow through both straits are assumed tobe dictated by the pressure differences on both sides and a frictional drag. We model the frictional momen-tum sink as quadratic bottom friction drag. We purposefully neglect the role of wind stress in an attempt to

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study the role of atmospheric pres-sure alone in producing the observedvariations. Discrete forms of themomentum and mass conservationequations are then,

gMt 5gM

t211ADuD

t212ABuBt21

AMDt; (2)

uDt 5uD

t21102patm

t 2q0ggMt

q0LDDt

2Cd

HDuD

t21juDt21jDt;

(3)

uBt 5uB

t211patm

t 1q0ggMt 2patm

t

q0LBDt

2Cd

HBuB

t21juBt21jDt;

(4)

with gM as the sea surface elevationof the Sea of Marmara, uD and uB asthe respective average flows throughthe Dardanelles and BosphorusStraits, AD and AB as the cross-sectional areas of the respectivestraits, AM as the surface area of theSea of Marmara, t as the time, patm as

the atmospheric pressure, q0 as a reference density, g as the gravitational acceleration, LD and LB as thelengths of the respective straits, Cd as the drag coefficient, and HD and HB as the bottom depths for therespective straits. For numerical reasons, the bottom drag is approximated using the velocities from the pre-vious time step based on the assumption that the velocity change over a single time step should be small.Cross sectional area values from the Sea of Marmara side of the Dardanelles and the Sea of Marmara side ofthe Bosphorus are used in order to compare the model results with observed flows from moorings M3 andM5. The drag coefficient was set to be 0.006. Other choices for Cd were also used and the results were notsensitive to this choice as will be discussed.

The conceptual model is run for-ward in time starting with gM50on 20 November at 14:00 and noprior flow through the straits. Themodel was forced by theobserved atmospheric pressureanomalies at hourly time stepsuntil 25 November. Results areshown in Figures 11 and 12. ForgM , which represents a basin wideaverage of sea surface elevations,we compare to both sets ofðp0b2p0atmÞ=ðq0gÞ observations madeat the Dardanelles and the Bos-phorus sides of the Sea of Mar-mara, as appropriate for abarotropic model that does notattempt to simulate baroclinicpressure changes or associatedsteric elevation changes. The close

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Figure 11. Estimates of sea-level elevation changes in the Sea of Marmara from theconceptual model solution (gM in blue), from the northern Dardanelles pressuremeasurements (ðp0b2p0atmÞ=ðq0gÞ in black), and from the southern Bosphorus pres-sure measurements (ðp0b2p0atmÞ=ðq0gÞ in green).

p’ = 0 p’ = patm + ρ0 g ηM

p’ = patm

ηMη = −patm/(ρ

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Sea of Marmar lack SeaAegean Sea

Figure 10. Schematic diagram for the conceptual model of the Turkish Straits Sys-tem. A time varying atmospheric pressure anomaly (patm) is applied uniformly overthe system. The sea surface elevation anomaly (g) for the Aegean Sea acts as aninverted barometer producing an oceanic pressure anomaly (p0) of zero. The seasurface elevation anomaly for the Black Sea is clamped at zero, creating an oceanicpressure anomaly equal to the atmospheric pressure anomaly. The sea surfaceanomaly for the Sea of Marmara (gM) is allowed to adjust according to the flowthrough the Dardanelles (uD) and Bosphorus (uB) Straits as determined by the pres-sure gradients along the straits.

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agreement between these twoobserved time series taken fromopposite sides of the Sea sug-gests that variation of this com-ponent across the Sea ofMarmara may be small duringthe storm, and that the meanbarotropic Sea of Marmararesponse is likely close to whatwas observed at these twolocations.

Given the simplicity of the concep-tual model, the level of agreementwith the data is remarkable. The cor-relation coefficient between themodel gM and the northern Darda-nelles measurements is 0.92 and thecorrelation coefficient between themodel gM and the southern Bospho-rus measurements is 0.94, both ofwhich are within the 95% confidence

limits of the correlation coefficient between the two measurements themselves (0.92). The model very accu-rately captures the timings of the rise and fall of the sea level anomaly and also simulates the occurrence ofa secondary peak in sea level after the storm on the 24th (Figure 11). The model does not completely cap-ture the magnitude of the observed peak, the timing of the secondary peak for the northern Dardanelles, orthe minimum observed on the 24th.

The conceptual model velocities do not agree as closely with the observations (Figure 12) compared to theagreement between the model and the observed elevations. However, the model does simulate the qualita-tive behavior of the flows even though it changes less abruptly than the observed flows. The correlationcoefficients are 0.81 for the northern Dardanelles flows and 0.78 for the southern Bosphorus flows. Themodel results were not very sensitive to the choice of value for Cd although some level of bottom drag wasrequired in order to accurately simulate the timing of the sea elevation fall and to prevent the currents fromreaching unrealistically large values. For example, reducing Cd to 0.003 changed all the correlation coeffi-cients by less than 0.004, much less than the expected error in any of the calculations. The flow for the Bos-phorus was the most sensitive to Cd, becoming more peaked in its variation as the frictional drag wasreduced. Friction plays an important role in this complex strait [Gregg and €Ozsoy, 2002] and certainly usinga single flow value and a quadratic drag formulation does not adequately capture the dissipation ofmomentum that takes place inside the strait.

The conceptual model illustrates the concept of atmospheric pressure driven flow due to constricted flowpassages rather than due to gradients in atmospheric pressure. As the atmospheric pressure drops on the21st, flow is driven through the Dardanelles Strait to the Sea of Marmara (Figure 12) because the atmos-pheric pressure drop is compensated by an inverted barometer response in the Aegean but not in the Seaof Marmara. This causes the sea level elevation to rise in the Sea of Marmara which in turn drives flowthrough the Bosphorus to the Black Sea. Eventually, sea level rise in the Sea of Marmara compensates forthe low atmospheric pressure and the flow through the Dardanelles Strait to the Sea of Marmara begins toabate. As the atmospheric pressure climbs back toward normal values on the 23rd, the pressure gradientfor the Dardanelles is reversed and the sea level high in the Sea of Marmara drains out through both theDardanelles and Bosphorus (Figures 11 and 12). The system drains faster than the atmospheric pressurerises, causing an overshoot and a secondary peak on the 24th. Thus, the conceptual model is able to quali-tatively explain many elements of the observations from Figures 4 and 8.

The conceptual model simulates the sea level response of the storm for the Marmara very well without anywinds, as well as a major portion of the excitation and reversals of flow through the straits. Sea level changein the Black Sea is conceptually excluded in the model and the observed sea level drop in the Black Sea

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Figure 12. Depth-averaged along-strait flow in the Dardanelles from the conceptualmodel solution (uD in blue), in the northern Dardanelles from ADCP measurements(black), in the Bosphorus from the conceptual model solution (uB in magenta), andin the southern Bosphorus from ADCP measurements (green).

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side of the Bosphorus (Figure 4) could be caused by the storm winds. This in turn could be contributing tostronger flows in the Bosphorus than the model was able to simulate. However, the simplicity of the frictionparameterization could also be an important factor as the model also underpredicted both the maximumand minimum flows in the Dardanelles. The second blockage event on the 25th and 26th (Figure 8) is alsolikely to be associated with different dynamics because atmospheric pressure variations were weak duringthat period, although some residual effects from the earlier storm could still be impacting the system. Theimplication of the conceptual model is that wind driven setup of the sea surface is relatively less importantthan atmospheric pressure dynamics for driving the max flux between basins during an extreme storm, ifthe basins are connected to an outside body of water that can respond as an inverted barometer as in theTurkish Strait System.

6. Conclusions

On 21–24 November 2008, the atmospheric pressure over the Turkish Straits System fell by more than 30mbar and returned to normal values all within 72 h in association with a strong and destructive cyclonicstorm (Figure 1). The oceanic response to the storm was also very energetic, reversing the upper-layer flowin the straits, greatly accelerating the lower layer flow in the straits, changing the density structure in theSea of Marmara, and enhancing mixing in the Bosphorus. The reaction process was directly related to thediffering responses of the three basins to abrupt changes in atmospheric pressure.

Bottom pressure observations at the Aegean side of the Dardanelles Strait imply a sea-level response con-sistent with an inverted barometer reaction. However, bottom pressure observations at the Sea of Marmaraside and on both sides of the Bosphorus indicate that the Black Sea and Sea of Marmara sea levels did notreact as an inverted barometer. On the Black Sea side of the Bosphorus, the sea elevation dropped (due towind or other influences) and when coupled with the atmospheric pressure drop produced a bottom pres-sure difference of more than 0.6 dbar across the entire Turkish Straits System during the peak of the storm(i.e., the sum of the two lines in Figure 4, bottom). In this case of rapid atmospheric pressure change, theDardanelles Strait is playing a primary role of constricting the inverted barometer response, and the physicscannot be modeled by only taking into account the Bosphorus as was done by Ducet et al. [1999] for longertime periods.

The pressure difference between the Aegean and the Sea of Marmara caused a reversal of the upper layerflow in the Dardanelles and created a mass flux toward the Sea of Marmara. This pressure difference was ini-tially equal to nearly the entire atmospheric pressure anomaly of the storm because the Aegean Sea hadresponded as an inverted barometer but the Sea of Marmara had not, due to the restriction of the Darda-nelles Strait. A mass flux was also created from the Sea of Marmara to the Black Sea due to the pressure dif-ference along the Bosphorus, but this flux was associated with the rising sea level elevation in the Sea ofMarmara (caused by the mass flux through the Dardanelles) and wind effects rather than directly related toatmospheric pressure forcing.

Within 6 h of the atmospheric pressure minimum, q0gg01gÐ 02H q0dz reached positive values in the Sea of

Marmara nearly equal to the absolute value of the atmospheric pressure minimum (Figure 4, top). However,this cannot be considered a delayed inverted barometer response, because in the southwestern Sea of Mar-mara 50% or more of the signal was associated with the rise (or disappearance) of the pycnocline depth,and in the northeastern Sea of Marmara a much larger rise in g0 than an inverted barometer response was�50% compensated by a corresponding drop in pycnocline depth (Figure 6). This baroclinic response inthe Sea of Marmara which redistributed mass and altered the pycnocline tilt was likely caused by the wind.

As g0 peaked, flow through the Bosphorus toward the Black Sea reached its maximum value (Figure 8). Theflow was unidirectional in the southern Bosphorus, but was concentrated in a midlevel outflow jet(>150 cm/s) below weak inflow in the northern Bosphorus (Figure 9). Inflow toward the Sea of Marmara atthe northern Bosphorus section and lack of any outflow at the southern Bosphorus section implies thatrecirculation was occurring within the strait and this may have contributed to the enhanced mixing thatwas observed there (Figure 5).

As the atmospheric pressure rose back to normal levels, the bottom pressure difference along the Darda-nelles reversed sign and the flow along this strait briefly reversed back toward the Aegean. Flow was

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directed away from the Sea of Marmara in both straits as the excess accumulated mass in the basin drainedback to the Aegean and continued to flow onward to the Black Sea. There is evidence for overshooting ofthis process and another brief period of flow reversal occurred in the Dardanelles. Throughout the entirestorm period, there was a steady positive flux of waters to the Black Sea. A very simple conceptual modeldriven by the atmospheric pressure fluctuations without wind stress is able to explain fluctuations in bot-tom pressure in the Sea of Marmara to a 0.89–0.96 level of correlation (Figure 11) and also qualitativelyagree with the behavior of observed velocity fluxes through the straits. This implies that atmospheric pres-sure was a primary factor controlling the ocean reaction to this storm.

Although ‘‘explosive’’ atmospheric cyclones and accompanying extreme pressure drops (for some otherexamples, see Moscatello et al. [2008] or Katsafados et al. [2011]) such as this one are not frequent, between1 and 10 events (average 5.5 occurrences of explosive cyclones per year) do occur every year in the Mediter-ranean [Kouroutzoglou et al., 2011], and cyclogenesis is particularly concentrated in the Aegean and BlackSea regions [Trigo et al., 1999, 2002] with cyclones from these and other regions affecting Turkey [Karacaet al., 2000]. Our case study shows the importance of considering differing atmospheric pressure responsesbetween basins for such storms, even when the low pressure systems do not have significant spatial gra-dients in atmospheric pressure over a given region. Neglecting basin to basin exchange driven by the differ-ent responses to atmospheric pressure in operational models could lead to inaccurate predictions, even ifwind driven transport is accounted for. Parameterizations of the induced pressure gradient through straitsor constrictions between basins (like the conceptual model used here for the Turkish Straits Systems or likethe simple model used by Lyu et al. [2002] for the Japan/East Sea) could potentially be coupled to morecomplex operational models of individual basins to correct for this issue. Over synoptic time scales, atmos-pheric pressure driven responses like the ones documented here for the November 2008 storm over theTurkish Straits System could be playing important roles for many coastal areas and semienclosed seas andpotentially contributing to the destruction caused by extreme storms.

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sensitivity on sea surface temperature, Nat. Hazards Earth Syst. Sci., 11, 1233–1246, doi:10.5194/nhess-11-1233-2011.Kouroutzoglou, J., H. A. Flocas, K. Keay, I. Simmonds, and M. Hatzaki (2011), Climatological aspects of explosive cyclones in the Mediterra-

nean, Int. J. Climatol., 31, 1785–1802, doi:10.1002/joc.2203.Le Traon, P.-Y., and P. Gauzelin (1997), Response of the Mediterranean mean sea level to atmospheric pressure forcing, J. Geophys. Res.,

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sphere events, Mon. Weather Rev., 130, 2188–2209.Lyu, S. J., K. Kim, and H. T. Perkins (2002), Atmospheric pressure-forced subinertial variations in the transport through the Korea Strait, Geo-

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AcknowledgmentsWe thank the Turkish Navy Office ofNavigation, Hydrography andOceanography, the crew of the NRVAlliance, and all those involved withthe observational effort during thefield experiments. Special thanks isgiven to Mark Hulbert, Andrew Quaid,Steve Sova, and Wesley Goode fortheir expert technical work that madeit possible to deploy moorings in suchenvironmentally challenging locations.We thank M. Kemal Cambazo�glu forhis help in researching the localimpact of this storm and his helpfulcomments during early stages of thiswork. We also thank the twoanonymous reviewers whosecomments helped improve themanuscript. J. W. Book and E. Jaroszwere supported by the Office of NavalResearch as part of the researchprogram ‘‘Exchange Processes inOcean Straits.’’ This is NRL contributionNRL/JA/7330–13-1921. The data usedin this paper can be shared with otherscientists if written permissions areobtained from the Turkish Navy Officeof Navigation, Hydrography andOceanography and the NATO Centrefor Maritime Research andExperimentation.

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BOOK ET AL. VC 2014. American Geophysical Union. All Rights Reserved. 3644