JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, C01006, doi:10.1029/2007JC004150, 2008 Click Here for Full Article

Hydrography of the West Spitsbergen Current, Branch: Autumn 2001 Edward D. Cokelet,1 Nicole Tervalon,2,3 and James G. Bellingham3 Received 7 February 2007; revised 15 August 2007; accepted 24 September 2007; published 10 January 2008.

[1] The Atlantic inflow is the primary source of water and heat to the . A large portion enters through where it splits into the Yermak and Svalbard Branches. In October to November 2001 the USCGC Healy occupied five oceanographic transects across the Svalbard Branch west and north of Spitsbergen. Temperature and salinity sections show the warm, salty Atlantic Water cooling and freshening as it flows along the continental slope for 553 km. The temperature in the upper 500 m of the water column decreased by 0.25°C/(100 km) implying a heat flux across the sea surface of up to 520 W/m2 if one assumes a nominal current of speed of 0.1 m/s. A temperature-salinity (T-S) analysis shows that the shape of the T-S curves can be explained in terms of the ratio of the atmospheric-to-ice-melt heat flux. The observations imply that ice melt accounts for progressively more surface cooling as the flow moves toward the Arctic Ocean. An autonomous underwater vehicle completed a vertical section in the region of maximum water mass gradient and revealed layer interleaving in fine detail. Data from this cruise extend a long time series of Atlantic layer source temperature beginning in 1910 (Saloranta and Haugan, 2001). The extended observations show that after the temperature of the Atlantic layer inflow reached a peak in 1992, it cooled to a minimum in 1998 and rose again through 2001. Citation: Cokelet, E. D., N. Tervalon, and J. G. Bellingham (2008), Hydrography of the West Spitsbergen Current, Svalbard Branch: Autumn 2001, J. Geophys. Res., 113, C01006, doi:10.1029/2007JC004150.

1. Introduction marines have been put to good use to measure below the ice [Morison et al., 1998], but besides being expensive to [2] The climate of the Arctic has become of increasing operate, they cannot measure near the ice surface nor to interest in recent years. Study of the region is complicated the deepest oceanographic depths. because the inhospitable, ice-covered Arctic Ocean occu- [4] Work begin in the 1990s to develop an autonomous pies the bulk of it. The Arctic Ocean seems to have underwater vehicle (AUV) with sufficient cruising duration, undergone a change in recent times with the reduction of depth range and oceanographic instrumentation to measure ice cover and warming of its upper layers. This has led to Arctic Ocean water properties [Bellingham et al., 2000]. heightened observational and modeling activities [Aagaard Such a robotic vehicle could avoid the ice by working et al., 1996; Carmack et al., 1995; Karcher et al., 2003; below it and would not require the logistical support of Lindsay and Zhang, 2005; Morison et al., 1998, 2000; human endeavors. The AUV’s design mission was to Steele and Boyd, 1998; Zhang et al., 1998]. sample the flow of warm Atlantic Water along the conti- [3] Measurements in the Arctic Ocean are difficult to nental slope of the from Spitsbergen to 1000 km make because of its shifting ice cover and the logistical east in a series of undulations along and across the current. challenge of transporting personnel and material over great This Atlantic Layer Tracking Experiment (ALTEX) was distances. Ice camps have been set up on the as designed to study this sparsely sampled conduit of warm slowly drifting observatories below which the ocean can be water—a primary source of water and heat to the Arctic sampled. Dedicated scientific icebreakers have been pressed Ocean. In October 2001 the prototype ALTEX AUV was into service, and it was in 1987 that Polarstern made the tested in the Arctic Ocean aboard the USCGC Healy— first oceanographic section across the Nansen Basin north a new icebreaker. The AUV systems were tested in both from the Barents Sea [Anderson et al., 1989]. Naval sub- open and ice-covered waters under full arctic conditions. When the ship was not engaged in AUV operations, a secondary cruise objective was to occupy oceanographic 1NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington, USA. sections in the region of the Atlantic layer inflow. The 2Presently at Mount Madonna School, Watsonville, California, USA. Atlantic inflow has often been measured along the west side 3Monterey Bay Aquarium Research Institute, Moss Landing, California, of Spitsbergen (see Saloranta and Haugan [2001], for a USA. multiyear statistical analysis), but observations along the north side and the continental slope farther east are far less Copyright 2008 by the American Geophysical Union. 0148-0227/08/2007JC004150$09.00

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Figure 1. Chart of the cruise track (blue) and CTD stations (red) for USCGC Healy cruise HY-01-03 for the period 11 October to 2 November 2001. CTD sections are numbered increasing in the flow direction of the West Spitsbergen Current’s Svalbard Branch. AUV missions on 24 October were located between CTD casts 22 and 23. The ice edge as determined by the National Ice Center [2005] is shown for three dates during the cruise: 19 October (green), 26 October (magenta), and 2 November (red). The path of the West Spitsbergen Current (WSC) is shown schematically after the work of Rudels et al. [2005]. Bathymetric data are from the International Bathymetric Chart of the Arctic Ocean [Jakobsson et al., 2000]. frequent. The purpose of this paper is to report on those ward extension of the WSC in the north of the Kara Sea, sections. and they flow together along the Arctic Basin’s rim and [5] It has long been known that Fram Strait, separating oceanic ridges. Our oceanographic sections sampled the Greenland and Spitsbergen, provides the deep connection Svalbard Branch of the WSC. between the Atlantic and Arctic Oceans. The major south- ward outflow from the Arctic Ocean occurs in the East 2. Methods Greenland Current, and the northward inflow is in the West Spitsbergen Current (WSC). The WSC follows the conti- [6] The measurements were made on USCGC icebreaker nental slope which splits at the junction of the Yermak Healy cruise HLY 01-03, 8 October to 5 November 2001. Plateau and the Spitsbergen continental shelf about 79°N Healy departed Tromso, Norway, sailed north across the [Figure 1; Aagaard et al., 1987; Farrelly et al., 1985; Perkin Barents Sea, and entered the Arctic Ocean near CTD cast 1 and Lewis, 1984]. The Svalbard Branch of the WSC follows on 11 October (Figure 1). Healy completed her final CTD the upper slope along the 400-m contour, across the neck of cast (52) on 2 November and returned to Tromso. Eight of the Yermak Plateau, and along the north side of Spitsbergen. the 52 CTD casts (9–12, 14, 27–29) were conducted to The Yermak Branch follows the lower continental slope at support AUV operations to shallow depths or at isolated about the 1000-m contour, along the seaward edge of the locations or times and were not appropriate for inclusion plateau, and then rejoins the Svalbard Branch somewhere to into regular transects. The remaining 44 casts defined five the northeast of Spitsbergen. More recently it has been transects roughly perpendicular to the shelf break and documented that a second major inflow into the Arctic extending into the Arctic Ocean basin (Figure 1). These Ocean occurs through the Barents Sea [Loeng, 1991; were numbered 1 to 5 increasing in the flow direction of the Schauer et al., 2002]. This colder inflow rejoins the east- West Spitsbergen Current’s Svalbard branch. Section 2 is

2of16 C01006 COKELET ET AL.: W. SPITSBERGEN CURRENT: SVALBARD BRANCH C01006 short because the ice was closing in and the cruise was [10] Air temperature was recorded by an automated nearing its end. Some of the CTD casts were in ice-covered system on the ship’s mast. Ice edge locations were obtained waters. Owing to ice navigation, weather conditions and from the National Ice Center [2005]. For the period under AUV operations, the CTD sections could not always be study they came from Radarsat images, hand-edited by ice completed in a continuous manner. The times required to analysts using meteorology, climatology, and ice drift as a complete each section were as follows: Section 1—53 hr, guide. Section 2—8 hr, Section 3A (casts 13–17)—65 hr, Section 3B [11] The ALTEX AUV was under development at (casts 17–26)—49 hr, Section 4—21 hr, and Section 5— MBARI for work in the Arctic Ocean [Bellingham et al., 25 hr. 2000]. It was 4–6 m long depending on its configuration [7] CTD casts were conducted with a Sea-Bird 911plus and 0.53 m in diameter. It had a theoretical depth range of CTD system with dual temperature and conductivity sen- 4500 m, but was not deployed deeper than 500 m on this sors, dual pumps and TC ducts, a Sea-Bird SBE-13B cruise. For this test cruise, silver-zinc batteries gave it a oxygen sensor, a Wet Labs CS-25-660 (red) C-STAR theoretical duration of 12 h and a range of 65 km at 1.5 m/s. Transmissometer, and a Chelsea Aquatracka III fluorometer. The AUV carried a number of oceanographic instruments The CTD sampled at 24 Hz, but data pairs were averaged including dual Sea-Bird SBE 3F temperature and SBE 4C internally and transmitted at 12 Hz to the shipboard data conductivity sensors that sampled at 4 Hz [Tervalon and acquisition system. Measurements were recorded as the Henthorn, 2002]. Their advertised accuracies were CTD was lowered at 20 m/min over the upper 300–400 m ±0.001°C and ±0.004 psu, respectively. It also carried an of each cast to below the depth of large vertical temperature MBARI in situ ultraviolet spectrometer (ISUS) system to gradients (usually at the bottom of the Atlantic Water layer) measure dissolved nitrate [Johnson and Coletti, 2002], and then at 60 m/min to within 20 m of the ocean bottom. a WET Labs optical backscatter meter, a Sea-Bird SBE In post-processing the data were averaged into 1-m incre- 43 dissolved oxygen sensor and an ice-profiling sonar ments. Sometimes the CTD was operated in very cold [Tervalon and Henthorn, 2002]. Underwater navigation conditions with air and water temperatures as low as 22 near the pole is difficult because the Earth’s magnetic field and 1.8°C, respectively. The water in the CTD’s tubing lines are nearly vertical. The ALTEX AUV tested a few froze occasionally, and the pump stopped, even though the different heading and navigation devices on this cruise CTD was kept in a heated area and moved quickly outside [McEwen et al., 2003]. before deployment. In those instances the CTD was lowered [12] AUV data presented in this paper are from runs 3–5 into the Atlantic Water to warm before being brought back on 24 October 2001 (year-day 297) for the periods 10:54– to the surface for cast initiation. 12:21, 13:27–14:39, and 15:00–16:11 GMT, each covering [8] The CTD cage carried a twelve-bottle rosette. Water about 6 km. The AUV executed a series of vertical undu- samples were acquired on the up-cast usually at the greatest lations between the surface and 500 m with typical descent depth and then at pressures of 3000, 2000, 1500, 1000, 750, and ascent angles of 30° to the horizontal. With temperature 500, 400, 300, 200, 150, 100, 50, 20, and 5 dbar (decibar = and salinity sampled at 4 Hz and a 1.5 m/s vehicle speed, 104 Pa 1 m of water depth). When the number of samples the vertical sampling resolution was 0.22 m. The combined exceeded 12, first the 20 dbar and then the 1500 dbar AUV track from post-processed navigation measurements sample were excluded. One salinity sample was collected over about 18 km on day 297 is available in McEwen et al. per cast alternately at the deepest and shallowest depths. [2003, Figure 12]. Samples were placed in glass bottles, their caps wrapped in plastic tape, and transported shore side where they were 3. Results and Discussion analyzed using a Guildline Autosal laboratory salinometer 3.1. Hydrographic Sections by the University of Washington’s Oceanography Technical Services Marine Chemistry Laboratory. These values were [13] Figure 2 shows a sequence of oceanographic cross- subsequently used to correct the CTD salinities by offsets of sections of potential temperature (q0) following the Svalbard 0.0079 to 0.0236 psu. Nutrient samples (492) were collected Branch as it flows into the Arctic Ocean from Section 1 in from the water bottles and frozen in plastic bottles at 80°C the south to Section 5 in the northeast. The Atlantic Water’s for transport to Seattle. The Marine Chemistry Laboratory warm core is concentrated over the continental slope at the analyzed them for nitrate, nitrite, ammonium, phosphate, right side of each section. Atlantic Water (AW) is distin- and silicate to WOCE (World Ocean Climate Experiment) guished from Arctic water masses by its warm temperature standards. Oxygen samples (201) were taken on a variety of and high salinity (q0 2°C, S 34.88 psu, [Aagaard et al., casts to calibrate the CTD oxygen sensors. They were 1985]). The temperature is cooler both below and above the analyzed onboard using an automated Winkler titration AW. Subzero temperatures are found at the deepest depths system. The oxygen sensor data were noisy and sometimes where Arctic water resides. Ice melting and atmospheric clearly erroneous, possibly owing to sensor freeze-damage; heat exchange cool the surface waters. Potential density (sq) therefore attempts to calibrate the sensors were abandoned. isolines in Figure 2 pass through the Atlantic Water and [9] Healy carried a 150-kHz acoustic Doppler current intersect the ocean surface marking conduits for isopycnal profiler (ADCP), but this being the ship’s first scientific heat transfer, especially along Section 1. field season, no provision had yet been made for accurate [14] Figure 3 contains the sequence of salinity sections and essential ship’s heading-data input. Additionally, the corresponding to the temperature sections of Figure 2. ADCP failed a few days into the cruise; therefore no useful Atlantic Water is found in the upper 500 m of all the ADCP measurements were available. sections. In general, the surface water freshens between oceanographic Sections 1 and 5 due to ice melt as will be

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Figure 2. Potential temperature (q0 °C referred to 0 dbar pressure) for Sections 1–5 looking downstream in the West Spitsbergen Current, Svalbard Branch, 11 October to 2 November 2001. Warm Atlantic Water flows along the right edge of each section that stretches from deep water on the left, over the continental slope to the shelf break at about 200 dbar (200 m). Potential density, sq, isolines are shown in black every 0.1 kg m3. Note the change in vertical scale at 500 dbar. The black horizontal line above Section 3 indicates the span of the AUV transect. CTD cast locations are labeled along the top of each section with corresponding plot marks along the sea bottom.

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Figure 3. Salinity (psu) for Sections 1–5 looking downstream in the West Spitsbergen Current, Svalbard Branch, 11 October to 2 November 2001. Salty Atlantic Water flows along the right edge of 3 each section. Potential density, sq, isolines are shown in black every 0.1 kg m . Note the change in vertical scale at 500 dbar and in the salinity scale at 34.80 psu. The black horizontal line above Section 3 indicates the span of the AUV transect. CTD cast locations are labeled along the top of each section with corresponding plot marks along the sea bottom.

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[16] To provide integrated measures of the variation along the WSC, consider the mean temperature and salinity aver- aged from 100 to 500 dbar in the vertical and from the shelf break (defined here to be the shallowest station deeper than 170 m on each section) to 65 km seaward. This range covers the core of the Atlantic Water. The 65-km distance was chosen because it is the maximum width of Section 4 when 100-m-deep station 36 is excluded. (Section 2 extends only 22 km which is too narrow to form representative averages in the AW; therefore its means are probably biased high in temperature and salinity.) Using historical hydrographic data south of 79°N, Saloranta and Haugan [2004] chose to study the WSC confined between the 500 and 1200 m depth contours. However, that range excludes the core of the Atlantic Water in our study area; therefore we chose our averaging interval as just described. Figure 4. The maximum and mean Atlantic Water core [17] The lower curves of Figure 4 show how the mean AW temperatures and salinities in the West Spitsbergen Current, core temperature and salinity vary along the flow direction. Svalbard Branch, between Section 1 (0 km) and section 5 Overall the mean temperature decreases from 3.41°Cat (553 km). Averages are restricted to 100–500-m sampling Section 1 (0 km) to 2.28°C at Section 5 (553 km). The mean depths with bottom depths exceeding 170 m, and to 65 km salinity decreases from 35.03 to 34.95 psu. Linear least seaward of the shelf break. square fits to the mean temperature and salinity give longi- tudinal gradients of 0.19°C/100 km and 0.013 psu/100 km, respectively. These are similar to 0.20°C/100 km discussed in Part 3.4. In Section 1 the waters above the and 0.010 psu/100 km as determined by Saloranta and Atlantic Water are less saline due to near-surface water Haugan [2004] using a statistical ensemble of hydrographic that has exited the Arctic Ocean via the East Greenland casts during the summer months (August–October) of Current and recirculated in the and Fram 1949 to 1999. Their stations were south of ours between Strait [Rudels et al., 1996, 2005]. The upper layers of the 74°N and 79°N in the WSC. This implies that the WSC cools Arctic Ocean are freshened by river runoff, ice melt and and freshens at approximately the same rate for over Pacific Ocean inflow through Bering Strait [Anderson and 1200 km. It is of historical interest to note that Helland- Jones, 1992; Rudels et al., 1996; Untersteiner, 1988]. Hansen and Nansen [1912, p. 26] computed a longitudinal Freezing and brine rejection on the continental shelves gradient of 0.20°C/100 km in the Atlantic Water on the of the Arctic Ocean form dense water that flows into the west side of Spitsbergen between 75°N and 79°N from basin and mixes, attaining a density level between the measurements in 1910. Atlantic Water and the surface. The resulting cold halo- [18] Saloranta and Haugan [2004] considered two more cline in the central Arctic Ocean keeps the warm Atlantic depth ranges (0–100 and 0–500 dbar) which represent the Water away from direct contact with sea ice, thus reducing surface layer and the combined surface and Atlantic layers. ice melt and strong heat loss to the atmosphere [Aagaard Table 1 shows our late-summer 2001, longitudinal mean et al., 1981; Steele et al., 1995]. temperature and salinity gradients, fit via the least squares 3.2. Atlantic Water Cooling Along the WSC method for the three depth ranges. The largest discrepancies between our single-cruise values and their multiyear ones [15] The Atlantic Water cools and freshens as it enters the occur when the upper layer is included. We measured a Arctic Basin owing to atmospheric cooling, ice melt, and cooling of 0.51°C/100 km and a freshening of 0.077 psu/ mixing. To measure the amount of change, we define 100 km versus their 0.32 and 0.028, respectively. Atlantic Water cores for each section. The temperature However, our values fall within their statistical range. The and salinity core locations are those positions and depths greater cooling and freshening are consistent with our at which the potential temperature and salinity reach their measurements being farther north and into the Arctic Ocean maximum values. These often occur at the same oceano- where the effects of sea ice are more important. graphic station, but the salinity maximum is always deeper. [19] The rate of cooling along the West Spitsbergen The distance along each core is the cumulative great circle Current can be used to estimate the heat flux to the ocean distance between sections at the core locations. Figure 4 is a plot of the core values versus distance along the West Spitsbergen Current. The maximum core temperature and salinity decrease monotonically along the WSC’s course. The core temperature drops fairly steadily from 5.71 to Table 1. Longitudinal Temperature and Salinity Gradients and the 5.03°C over the first 360 km between Sections 1 (at 0 km) Vertical Heat Flux in the Atlantic Water Core Along the Svalbard and 4 and then falls to 3.32°C over the last 193 km to Branch of the West Spitsbergen Current 2 Section 5. The core salinity drops from 35.14 to 35.06 psu Layer, z1-z2, dbar dq/dy, °C/100 km dS/dy, psu/100 km Q, W/m between Sections 1 and 3, holds constant to Section 4, and 0–100 0.51 0.077 210 drops to 35.01 psu at Section 5. 100–500 0.19 0.013 310 0–500 0.25 0.024 520

6of16 C01006 COKELET ET AL.: W. SPITSBERGEN CURRENT: SVALBARD BRANCH C01006 surface. The heat content, q, of a material element of fluid [21] If one requires that the vertical heat flux from the is related to the mean, v, and turbulent, v0, velocity fields Atlantic Water core be taken up only by the atmosphere and by sea ice directly above, then the total heat flux (0–500 dbar) would be 520 W/m2 for a 0.1 m/s current (Table 1). This is @q probably an upper bound on the actual local heat flux to the þ v Árq þrÁv0q0 ¼ 0 ð1Þ @t ocean surface because neglected horizontal processes in the upper ocean come into play. The surface layer may absorb where the overbar indicates an ensemble mean over turbu- the heat flux across a wider area than that directly above the lent timescales. The molecular diffusion of heat is neglected AW core owing to horizontal processes such as isopycnal compared to the turbulent eddy (or Reynolds) heat flux v0q0 mixing. Boyd and D’Asaro [1994] suggest that mesoscale in (1). After Boyd and D’Asaro [1994], integration of (1) eddies diffuse heat along isopycnals to the sea surface. The over a fixed volume, V, with Cartesian coordinates x isopycnals on Section 1 (Figure 2) pass through the Atlantic increasing offshore, y increasing along the current and z Water and outcrop 140 km seaward of the shelf break, increasing downward yields indicating that AW can communicate with the sea surface Z Z Z some distance away. In addition, freezing and brine exclu- @ sion on the continental shelf can lead to horizontal intru- q dxdydz ¼ Q dxdy v Árq dxdydz ð2Þ @t sions that maintain the cold halocline above the Atlantic V Surface V Water, absorbing and spreading its upward-diffused heat seaward [Aagaard et al., 1981]. where it is assumed that only the heat flux across upper surface of integration, Q, is important. Assuming that the 3.3. Mesoscale Disturbances fluid is incompressible, the local heat content is steady and [22] The local mean temperature minimum (2.77°Cat the current velocity does not vary downstream and applying 280 km) at Section 3 in Figure 4 is due to a mesoscale the divergence theorem to the second term on the right hand disturbance. Whether it is an eddy, a meander, or a distur- side of equation (2) yields bance along the advancing ice edge cannot be determined. Z Z Figure 1 shows the ice edge advancing southward across @q this section between 19 and 26 October, just when it was Q dxdy ¼ r C v dxdz ð3Þ w w @y being sampled. Figure 5 shows the geostrophic velocity Surface A referred to the deepest common depth between station pairs on each section. On Section 3 the velocity is westward  where q = rw Cw q, rw is the fluid density, Cw is the specific between stations 20 and 21 and eastward between stations heat of seawater, q is the ensemble mean potential 22 and 24. If it is an eddy, then it is a counterclockwise, temperature, v is the ensemble mean downstream velocity, cyclonic one centered between stations 21 and 22. The and A is the current’s cross-sectional area. Equation (3) disturbance seems to have pushed the warm Atlantic Layer shows symbolically how heat loss across the ocean surface core shoreward and recirculated seaward a warm core at to the atmosphere and to sea-ice melting leads to a 200 dbar between stations 20 and 21. The net effect has longitudinal heat flux divergence of the , i.e., been to lower the mean temperature on the inner 65 km of the water cools downstream if the current is uniform. the section used to construct Figure 4. Eddies of 50–60 km @q [20] To estimate the right-hand side of (3) from @y we diameter and 10–25 cm/s speeds have been observed in the assume a nominal barotropic current of 0.1 m/s that is WSC on the west side of Spitsbergen [Bourke et al., 1987; representative of current meter observations [Fahrbach et Gascard et al., 1995; Johannessen et al., 1987]. Two CTD al., 2001] and has been used previously [Saloranta and casts (27 and 28, not shown in Figure 1) in support of AUV Haugan, 2004]. Other authors [Aagaard et al., 1987; Rudels operations were conducted after, but near, Section 3. They et al., 2005] have used current speeds of one-half to twice allow an estimate of temperature variability. Cast 27 lay this value which gives an idea of the uncertainty in such between casts 20 and 21, and cast 28 lay between casts calculations. Tabulated values of Cw are available [Gill, 21 and 22. The mean temperature in the Atlantic Water fell 1982]. The resulting vertical heat fluxes are given in the last about 0.3°C in 3 days when the five sites are compared. column of Table 1. They show that a vertical heat flux [23] The near-bottom-referenced geostrophic velocity of 310 W/m2 from the Atlantic Water core (100–500 dbar, field (Figure 5) is adequate to show the presence of 0–65 km from the shelf break) to the layer above is required mesoscale disturbances, but current-meter measurements to account for the measured cooling. This is very close to indicate that barotropic currents extend to the bottom in the 50-year mean of 330 W/m2 found farther south in the the WSC [Fahrbach et al., 2001]. Therefore bottom- WSC [Saloranta and Haugan, 2004]. Aagaard et al. [1987] referenced geostrophic currents do not represent the abso- studied approximately the same geographic area at about the lute geostrophic velocity field. No absolute reference same time of year as we. Their sections taken 20–27 No- velocities were available for this cruise because the ADCP vember 1977 cover the WSC from our Sections 1 to 4. failed. Section 1 (Figure 5) shows another possible cyclonic Aagaard et al. [1987] estimated a heat flux of 230 W/m2 eddy centered between stations 47 and 46 with an inter- from the 100–200-m layer using a current speed of 0.2 m/s sected diameter of 50 km. If one assumes that both as measured by a current meter west of Spitsbergen at 79°N. disturbances are eddies superimposed upon locally uniform Using their current speed and layer thickness for compar- background velocity fields, then their maximum rotational ison, we obtain a heat flux of 240 W/m2. speeds are 15–20 cm/s—values within the range of previ- ous estimates [Bourke et al., 1987; Gascard et al., 1995;

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Figure 5. Geostrophic velocity (vg, cm/s) for Sections 1–5 looking downstream in the West Spitsbergen Current, Svalbard Branch, 11 October to 2 November 2001. The geostrophic velocity is referred to the deepest common depth between station pairs on each section. Note the change in vertical scale at 500 dbar. The black horizontal line above Section 3 indicates the span of the AUV transect.

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Spitsbergen at Section 2. Section 3 shows a trace of Atlantic Water at its seaward end at 250 dbar. This is possibly AW from the Yermak Branch of the West Spitsbergen Current that has either wound around the inside of the Yermak Plateau or crossed over the top. The only UAIW encoun- tered is on Section 3 at 100 dbar, and it could be defined away with a slight shift of its lower salinity boundary in Figure 6. [26] The cruise’s temperature-salinity diagram reminds one of a lady’s high-heeled shoe (Figure 6). Its outer envelope can be described in terms of water mass end- members. One bounding extreme at the upper right (the top of the heel) represents the maximum temperature and salinity found in the Atlantic Water. Another at the lower right (the tip of the heel) is due to the deepest LAIW encountered. A third extreme along the bottom (the sole) is due to the freezing point of seawater. At its right end (the rear of the sole) are a temperature minimum and a Figure 6. Potential temperature-salinity (q0-S) diagram salinity maximum on the boundary between Polar Inter- 3 for the 52 CTD casts with potential density (s0, kg/m ) mediate Water and Polar Water. This is the extreme of the isolines overlaid. Each circle represents a sample every cold halocline water that forms by mixing between Atlan- 1 dbar and is color-coded for pressure—0–100 dbar (black), tic Water and shelf water. The cold halocline water serves 100–200 dbar (red), 200–500 dbar (green), 500–1000 dbar as a low-salinity cap that insulates the sea ice from the (purple), and 1000–3000 dbar (cyan). The core of the warm Atlantic Water over the central Arctic Ocean Atlantic Water is at the upper right with warm temperature [Aagaard et al., 1981]. The lower left (the toe) of the (q0 2°C) and high salinity (S 34.8 psu). Other water figure is elongated toward the freezing point of pure fresh masses are Lower Arctic Intermediate Water (LAIW), Upper water. Arctic Intermediate Water (UAIW), Deep Water (DW), Polar [27] The shape of the individual T-S profiles can be Intermediate Water (PIW), Polar Water (PW), and Arctic related to the flux of heat lost from the ocean due to Surface Water (ASW) [Aagaard et al., 1985]. The freezing atmospheric cooling and ice melting. The salinity, S,ofa point (FP) curve for seawater is drawn along the bottom. mass of water of thickness Dz influenced by ice melting will change in time by Johannessen et al., 1983, 1987]. A third, less-energetic, dS Q ðÞS S anticyclonic disturbance may lie along Section 5, centered ¼ i i ð4Þ near station 5. dt LirwDz

3.4. T-S Relationships and Water Masses [Boyd and D’Asaro, 1994; Moore and Wallace, 1988] [24] Temperature-salinity diagrams and the water mass where Qi is the heat flux required to melt the ice, Si 5 psu classifications that they provide are useful tools in the Arctic is the sea ice salinity, and Li = 4281 J/kg/°C[Steele and Ocean. Figure 6 shows the potential temperature-salinity Morison, 1993] is the latent heat of fusion of pure ice. The diagram for the entire cruise with observations color-coded temperature, T, of a mass of water cooled by a heat flux, Q = by depth range. Arctic water masses as defined by Aagaard Qa+ Qi, through the sea surface due both to atmospheric et al. [1985; modified from Swift and Aagaard, 1981] are sensible heat transfer, Qa, and ice melt, Qi, will change in outlined in boxes, and potential density, s0, isolines are time by shown for reference. At depth the properties fall into a narrow T-S range within the Lower Arctic Intermediate dT Qa þ Qi Water (LAIW). They approach but do not reach Deep Water ¼ ð5Þ dt r CwDz (DW) values because the water is too shallow. Warm, salty w Atlantic Water (AW) dominates between 200 and 500 dbar. [Boyd and D’Asaro, 1994; Moore and Wallace, 1988] Temperature and salinity maxima lie in the AW between the where the small amount of heat required to warm the ice to surface and 200 dbar. Between 100 and 200 dbar, the its freezing point and the contribution of salinity to latent properties fall into the AW and Arctic Surface Water heat have been neglected. The ratio of (5) to (4) is a curve in (ASW) categories and just enter the Upper Arctic Interme- T-S space whose slope is given by diate Water (UAIW). Near-surface (0–100 dbar) values come from the ASW, Polar Intermediate Water (PIW) and dT Q þ Q L Q 1 Polar Water (PW) masses. ¼ a i i 79:21þ a : ð6Þ [25] Figure 7 shows the hydrographic sections color- dS Qi CwðÞS Si Qi S Si coded by water mass. On every section Atlantic Water dominates above 500 dbar. Arctic Surface Water is found This curve is appropriate for water near the sea surface on every section, but Polar and Polar Intermediate Water do where ice melt and atmospheric cooling occur. Integrating not appear until the Arctic Ocean is entered north of from different initial points (S0, T0), each representing the

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Figure 7. Water mass type for Sections 1–5 looking downstream in the West Spitsbergen Current, Svalbard Branch, 11 October to 2 November 2001. The water masses are Polar Water (PW), Polar Intermediate Water (PIW), Arctic Surface Water (ASW), Atlantic Water (AW), Upper Arctic Intermediate Water (UAIW), Lower Arctic Intermediate Water (LAIW), and Deep Water (DW) [Aagaard et al., 1985]. Note the change in vertical scale at 500 dbar. The black horizontal line above Section 3 indicates the span of the AUV transect. CTD cast locations are labeled along the top of each section with corresponding plot marks along the sea bottom.

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salinity and temperature where the water meets the surface, yields a family of curves given by Qa ðÞS Si T ¼ T0 þ 79:21þ ln : ð7Þ Qi ðÞS0 Si

[28] Some of the patterns on the T-S diagram in Figure 6 can be explained in terms of equation (7). The shapes of the T-S curves change markedly along the flow between Sections 1 and 5. Consider, therefore, separate T-S diagrams for Sections 1, 3, and 5 (Figure 8). Individual casts are shown as continuous curves and labeled by cast number. Dashed lines represent one family of curves, beginning at a virtual AW maximum (S0 = 35.15, T0 = 6.0), along which seawater can be cooled and freshened owing to various ratios of atmospheric cooling, Qa, to ice melt, Qi. For fixed Qa/Qi, the slopes of the curves given by equation (6) are independent of (S0, T0) and only weakly dependent on S because of the small range of oceanic salinities encountered [Boyd and D’Asaro, 1994]. Therefore the slopes drawn are similar for any starting point near the ocean surface where cooling commences. [29] Each T-S diagram (Figure 8) shows that the most gradual T-S slope achievable via surface cooling occurs when atmospheric sensible heat transfer vanishes in relation to ice melt (Qa/Qi = 0). Conversely, the steepest slope occurs when ice melt vanishes (Qa/Qi = 1). Atlantic Water is a primary end-member source for Arctic Intermediate Water (AIW) which is much colder but only slightly fresher. AIW is formed by cooling of AW at the surface in the Iceland and gyres leading to deep convection [Aagaard et al., 1985]. Figure 8 shows that the T-S path (the shoe’s heel shaft) from AW to LAIW and UAIW lies along a steep curve corresponding to Qa/Qi 10, implying that atmospheric cooling plays a dominant role. [30] For the T-S diagram of Section 1 (Figure 8a) the casts nearest Spitsbergen (41–45) have slopes corres- ponding to Qa/Qi 1, consistent with Arctic Surface Water forming as a result of equal heat loss to the atmosphere and to ice melting. Melting seems to play a smaller role (Qa/ Qi 2) farther into Fram Strait (casts 46–52) where surface cooling begins at a lower temperature (Figure 2). Figure 9 shows time series of the air and sea-surface temperature (SST) during the cruise. Section 1 was occupied last on 31 October to 2 November. The air temperature was well below the freezing point, and the SST was well above— conditions commensurate with atmospheric cooling and ice melting. However, Healy was not in sea ice, and the ice edge (Figure 1) was north of Section 1 until she departed the

Figure 8. Potential temperature-salinity (q0-S) diagrams area. Therefore no ice was available to melt locally. The for CTD casts on Sections (a) 1, (b) 3, and (c) 5 with ASW on this section probably formed in the Arctic Ocean 3 potential density (s0, kg/m ) isolines overlaid. Individual and was transported into the region by the southward- casts are color-coded and numbered. Red, dashed lines flowing (EGC) in Fram Strait or represent one family of curves along which seawater is from mixing in the Nordic Seas. The EGC pushed the ice cooled and freshened near the surface by various ratios of edge south to 76°Nat5°W (Figure 1). 31 atmospheric cooling, Qa, to ice melt, Qi. Similar curves with [ ] Section 3 is a transition region. The temperature time the same slopes can begin anywhere in the near-surface series (Figure 9) shows that the first set of casts (13–18) waters. Also shown in black are the water mass boundaries was taken in ice-forming conditions with water at the and the freezing point of seawater. freezing point and air temperatures at or below it. The second set (19–26) was taken in melting conditions with the water above the freezing point, although in contact with

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curves become more gradual. Together these observations imply that ice melt becomes progressively more important in cooling and freshening the inflowing surface water at this time of year. In those situations where the ship was outside the ice edge as defined from satellite measurements, the melting may have come from ice bands and isolated floes, or meltwater may have been transported into the area. 3.5. Thermohaline Intrusions [34] In March–April 1981, Lewis and Perkin [1983] orchestrated a series of light aircraft landings on the sea ice north of Spitsbergen. At each site of their Eurasian Basin Experiment (EUBEX), they conducted a CTD cast to 1000 m. Several casts showed strong interleaving of ther- mohaline intrusions, and temperature profiles had a ‘‘three peaked’’ structure, i.e., three mid-depth maxima [Perkin and Lewis, 1984]. May and Kelley [2001] analyzed the inter- leaving layers at depths of 200 to 600 m and were able to Figure 9. Time series of air (black), sea surface (SST, red) trace individual intrusions over distances of up to 100 km. and freezing point (blue) temperatures during USCGC Our Sections 3 and 4 were in the same general area, but Healy cruise HY-01-03. Green vertical lines denote the south of 83°N where the previous authors observed these sampling intervals for CTD Sections 1–5 and the AUV features. We found many examples of interleaving with section. several mid-depth temperature and salinity maxima but no characteristic ‘‘three peaked’’ profiles. Intrusions could not be traced between stations separated by as little as 10 km in cold air. Ice covered the northern part of the section and our data set. Apparently no large-scale thermohaline intru- advanced through it as shown by the 19 and 26 October ice sions occur in the Svalbard Branch of the WSC between the edge locations in Figure 1. The early T-S curves (13–20, shelf break at 200 m and the edge of the Yermak Plateau at Figure 8b) have cold, fresher Polar Water at the surface 1000 m. Likewise Rudels et al. [2000] found continuous (Figure 7) tending toward the 2°C Atlantic Water at 200 m intrusions in the Eurasian Basin but no continuity between and gradual slopes outside the range explainable by Qa/Qi. individual CTD stations in Fram Strait. They concluded that The gradual T-S slopes in the Polar Water are not due to intrusions can form at fronts where two water masses meet heat loss via atmospheric cooling or ice melt directly, but and can evolve and spread if the water masses move in the rather to a mixture with cold, salty water at the PIW same direction into low-energy areas removed from bound- boundary. The latter may be formed by freezing and brine ary currents, such as the Eurasian Basin. exclusion on the shelf or by cooling AW along a curve of slope corresponding to Qa/Qi 2 [Figure 8b; Aagaard et 3.6. AUV Section al., 1981; Steele and Morison, 1993; Steele et al., 1995]. [35] The ALTEX AUV was tested on several occasions Later curves nearer Spitsbergen (casts 23–26, Figure 8b) during the cruise. Upon the completion of Section 3, Healy are similar to those of Section 1 that have Arctic Surface returned to the vicinity of CTD cast 22 (Figure 1), and Water at the surface (Figure 7) tending toward 4°Cinthe the AUV was launched on a back-to-back, three-mission AW and Qa/Qi 1. The T-S curves for casts 21 and 22 sequence in ice-free conditions. Each mission was limited to (Figure 8b) lie between these two groups in a transition zone two dives to 500 m and two ascents covering about 6 km from Arctic Ocean conditions in the north to WSC con- horizontally. Between missions it was interrogated and ditions in the south. re-oriented from a small boat. The three-mission sequence, [32] Section 5 was sampled in ice-free conditions at the as indicated by the black horizontal line above Section 3 in beginning of the cruise (Figure 1). Surface water temper- Figures 2, 3, 5, and 7, carried the AUV to the vicinity of atures were above freezing, and no ice was forming CTD cast 23. Fortunately, this provided a combined, fine- (Figure 9). The T-S curves have two main temperature scale vertical section across the largest gradient in T-S maxima (Figure 8c). For all but cast 8, the slopes are such properties encountered on the entire cruise (compare casts that Qa/Qi lies between 0 and 1 near the surface. This implies 22 and 23 in Figure 8b). that the heat loss due to ice melt exceeded atmospheric [36] Figure 10 shows sections of potential temperature, cooling at some previous time and location because there salinity, and water mass type. The AUV began in Polar was no ice present during sampling. Negative T-S slopes Water and passed through Polar Intermediate Water, Arctic from the first temperature maximum to the first minimum Surface Water, Atlantic Water and Lower Arctic Intermedi- imply mixing between ASW and PIW. Below the tempera- ate Water in just 18 km (Figure 10c). Layers are interleaved ture minimum, the curves are similar to the northern stations as shown most clearly by multiple temperature extrema in on Section 3. The northernmost, cast-8 T-S curve belongs to the T-S diagram (Figure 11). A warm, salty layer across the the same family as the northern Section-3 curves (Figure 8b). transect centered about 120 dbar (Figures 10a and 10b) is [33] In general the surface density, temperature and identifiable as a sequence of temperature maxima in the salinity decrease along the flow direction between Sections 1 Atlantic Water for 27.7 s0 27.9 (Figure 11). The and 5 (Figure 8). Also the slopes of the near-surface T-S shallowest temperature maximum can be traced on the T-S

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than portrayed. Horizontal aliasing away from the pycno- cline appears to be less of a problem. No density overturns could be distinguished above instrumental noise on vertical density profiles. [37] Isopycnals tilt down from north to south, implying a mean baroclinic pressure gradient (Figure 10); therefore one might expect intrusions originating from the north. However, the mesoscale disturbance centered at this loca- tion (see section 3.3) might be geostrophic such that the sea surface tilts oppositely, and the pressure gradient is balanced by the .

4. Conclusions

[38] Although there is a long history of Arctic oceano- graphic exploration going back over a century, and the West Spitsbergen Current is a primary source of water and heat to the Arctic Ocean, only six oceanographic cruises have sampled along it from the west to the north side of Spitsbergen [Aagaard et al., 1987; Anderson and Jones, 1992; Gascard et al., 1995; Pfirman and Thiede, 1992; Rudels et al., 2000; Schauer et al., 2004; Schytt, 1982]. This Healy cruise with its five transects adds one more to that number. It has allowed a computation of Atlantic Water cooling and vertical heat transfer. The cooling (0.19°C/ 100 km) measured on this cruise is nearly identical to that (0.20°C/100 km) found farther south from a 50-year statistical ensemble [Saloranta and Haugan, 2004]. It implies a vertical heat flux from the Atlantic Water to the upper ocean of 310 W/m2 and to the ocean surface of 520 W/m2 (Table 1) based upon an assumed 0.1 m/s current. [39] Temperature-salinity diagrams have proven useful tools to delineate water properties and map water masses. The slope of T-S profiles in the West Spitsbergen Current have been interpreted in terms of the external processes of atmo- spheric cooling and ice melt and the internal process of mixing between end-members. Three important end-members in this region are Atlantic Water, Lower Arctic Intermediate Water, and Polar Intermediate Water at the temperature minimum of the cold halocline. The examination of temperature time series during the cruise reveals that although the air temperature was

Figure 10. Sections of (a) potential temperature, (b) salinity, 3 and (c) water mass type with potential density (s0,kg/m) isolines overlaid for the combined AUV mission on 24 October 2001. Each ascent and descent leg is numbered. Vertical white lines mark the breaks between the three-mission sequence. Note the change in the salinity scale at 34.80 psu.

diagram from legs 1 to 12 in the density range 27.3 s0 27.7. Using this information one can trace the same feature in the temperature section along a series of oscillations centered around 50 dbar (Figure 10a). However, the tem- perature and salinity extrema there are quite localized. Alternating highs and lows in the pycnocline displacement are associated with alternating AUV legs. Perhaps even at the fine-scale resolution of about 3 km provided by the AUV at this depth, oscillations on the pycnocline may be aliased, and horizontal length scales may be even smaller Figure 11. Potential temperature-salinity (q0-S) diagrams for legs 1, 2, 5, and 12 of the AUV section.

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underwater navigation should be accurate enough to mea- sure ocean currents with an AUV-mounted acoustic Doppler current profiler to an accuracy of 0.01–0.02 m/s. The power source should be adequate for mission durations of days to weeks. AUVs have great potential as oceanographic tools to ply the Arctic waters, unencumbered by the sea ice above and the logistical requirements of manned expeditions. [41] In the 1990s extensive warming was noticed in the Arctic Ocean [Carmack et al., 1995; Quadfaseletal., 1991]. Saloranta and Haugan [2001] thought it prudent to study the interannual variability of the West Spitsbergen Current by constructing time series of temperature and salinity for representative hydrographic stations from 1910 to 1997. They assembled ensembles of the available hydro- graphic data west and north of Spitsbergen in four regions above the continental slope in the core of the current. Data were chosen for August–October of each year when the upper layer waters of the WSC are warmest. Sample depths of 100–300 m were selected because they are in the Atlantic Water but below surface influences. Schauer et al. [2004] subsequently extended the time series in ensem- ble E1 to 2000. Ensemble E1 is to the west of Spitsbergen between 78.90 and 79.55°N. Our hydrographic stations 42– 44 from Section 1 are at 78.86°N, just 4.4 km south of E1 which spans 72 km north–south. Owing to sparse water column data in this region, it is worthwhile to include these stations in E1. Station 39 from our Section 2 falls into ensemble E3, north of Spitsbergen. Our observations are from the end of the seasonal time window, 30–31 October. [42] Figure 12 shows the temperature and salinity history for ensembles E1 (red) and E3 (green) from 1969. Table 2 gives the recent numerical values for future reference. Noteworthy in Figure 12 is the continuous arctic oceano- Figure 12. Time series of autumn (a) temperature and graphic record since 1969, now extended to 2001 for E . (b) salinity in the Svalbard Branch of the West Spitsbergen 1 The observations at E3 are intermittent but mostly in phase Current. Ensembles E1 and E3 are from the west and north with those at E . Through 1997, Saloranta and Haugan side of Spitsbergen, respectively, over the continental slope 1 [2001] noted temperature maxima at E1 in 1970, 1984, and averaged between 100 and 300 m. The time series of 1992 with minima in 1977, 1987, and 1995. After 1997 the Saloranta and Haugan’s [2001] (small circles) has been core temperature of the Atlantic Water decreased to a low in extended by Schauer et al.’s [2004] observations for 1997– 1998 and has since increased to a value in 2001 roughly 2000 (squares) and our 2001 values (stars). E1 values for comparable to highs in 1970 and 1984, but lower than in the 1997 (large circle) are a mean of Saloranta and Haugan’s 1990s. Figure 12 shows that the warming observed in the and Schauer et al.’s values. incoming Atlantic layer in the 1990s [Quadfasel et al., 1991] was at the peak of a recurring cycle of warming and cooling. The salinity in E1 (Figure 12b) behaves somewhat often below freezing, sea ice was melting owing to heat like the temperature (correlation coefficient r = 0.58, sig- transfer from the warm Atlantic Water below. The near-surface nificantly different from 0 with 95% confidence) so that T-S slopes imply that ice melt becomes progressively more important in cooling and freshening the inflowing surface wateratthistimeofyear. [40] An autonomous underwater vehicle was used to Table 2. Mean Temperature and Salinity for Hydrographic Station study the oceanography of the Atlantic layer inflow—albeit Ensembles E1 and E3 as Defined by Saloranta and Haugan [2001] on a test run of only 18 km. It gives a small-scale snapshot E1 E1 Number E3 E3 of the interleaving of Atlantic, Arctic Surface, Polar, and Number of Temperature Salinity of Temperature Salinity Polar Intermediate Waters. Several test deployments, some Year Observations (°C) (psu) Observations (°C) (psu) with out-and-back missions of up to 4-km length under sea 1997a 2 3.50 35.036 - - - b ice, demonstrate that the ALTEX AUV can perform in the 1998 1 3.00 35.030 - - - 1999b 1 4.00 35.060 - - - Arctic [McEwen et al., 2003]. Our results show that it can 2000b 1 3.80 35.060 - - - be used to measure hydrography, much as a series of CTD 2001c 3 4.28 35.049 1 3.99 35.054 casts from a ship would. However, more AUV develop- aAverage of Saloranta and Haugan [2001] and Schauer et al. [2004]. mental work is needed. Launch and recovery should be bSchauer et al. [2004]. simple and fast, requiring no small boat operations. Ideally, cThis paper.

14 of 16 C01006 COKELET ET AL.: W. SPITSBERGEN CURRENT: SVALBARD BRANCH C01006 warm and salty years in the Svalbard Branch tend to go Bourke, R. H., M. D. Tunnicliffe, J. L. Newton, R. G. Paquette, and T. O. together. Manley (1987), Eddy near the Molloy Deep revisited, J. Geophys. Res., 92, 6773–6776. [43] The 3-year-running-mean temperature time series in Boyd, T. J., and E. A. D’Asaro (1994), Cooling of the West Spitsbergen ensemble E1 is correlated at zero lag with the winter North Current: Wintertime observations west of Svalbard, J. Geophys. Res., 99, Atlantic Oscillation (NAO) index as noted by Saloranta and 22,597–22,618. Carmack, E. C., R. W. MacDonald, R. G. Perkin, F. A. McLaughlin, and Haugan [Hurrell, 1995, up-to-date winter extended NAO R. J. Pearson (1995), Evidence for warming of Atlantic water in the index data provided by the Climate Analysis Section, southern Canadian Basin of the Arctic Ocean: Results from the Larsen- NCAR, Boulder, Colorado, USA; Saloranta and Haugan, 93 Expedition, Geophys. Res. Lett., 22, 1061–1064. Fahrbach, E., J. Meincke, S. Osterhus, G. Rohardt, U. Schauer, V. Tverberg, 2001]. The correlation is not large, but it is significantly and J. Verduin (2001), Direct measurements of volume transports through different from zero. With 4 years of data added (1998–2001), Fram Strait, Polar Res., 20, 217–224. the correlation increased slightly from 0.40 (96% confidence) Farrelly, B., T. Gammelsrod, L. G. Golmen, and B. Sjoberg (1985), to 0.41 (98% confidence). As Saloranta and Haugan found, Hydrographic conditions in the Fram Strait, summer 1982, Polar Res., 3, 227–238. the 3-year-running-mean salinity in E1 is not significantly Gascard, J.-C., C. Richez, and C. Rouault (1995), New insights on large-scale related to the winter NAO index (r = 0.04). oceanography in Fram Strait: The West Spitsbergen Current, in Arctic [44] Two other long time series are available for the Oceanography: Marginal Ice Zones and Continental Shelves, edited by W. O. Smith and J. M. Grebmeier, pp. 131–182, AGU, Washington, DC. Atlantic inflow in the West Spitsbergen Current. Grotefendt Gill, A. E. (1982), Atmosphere-Ocean Dynamics, 662 pp., Academic Press, et al. [1998] compiled time series of winter temperatures at New York. 76°N in Fram Strait between 1973 and 1995, but only at the Grotefendt, K., K. Logemann, D. Quadfasel, and S. Ronski (1998), Is the Arctic Ocean warming?, J. Geophys. Res., 103, 27,679–27,687. sea surface. Blindheim et al. [2000] presented temperature Helland-Hansen, B., and F. Nansen (1912), The sea west of Spitsbergen, the and salinity hydrographic data for the Sorkapp section across oceanographic observations of the Isachsen Spitsbergen Expedition in Fram Strait at 76.3°N for 1967–1996, but in 2100–2300 m 1910, Videnskapsselskapets. Skrifter I. Matematisk-Naturvidenskabelig of water, seaward of the warm core. The E time series is Klasse, 12, 1–89. 1 Hurrell, J. W. (1995), Decadal trends in the North Atlantic Oscillation: important because it is based upon direct in situ measure- Regional temperatures and precipitation, Science, 269, 676–679. ments within the warm core of the Atlantic layer inflow over a Jakobsson, M., N. Cherkis, J. Woodward, R. Macnab, and B. Coakley period of 90 years with annual data since 1969. (2000), New grid of Arctic aids scientists and mapmakers, Eos Trans. AGU, 81, 89–93. Johannessen, O. M., J. A. Johannessen, J. H. Morison, B. A. Farrelly, and [45] Acknowledgments. This project was supported by the National E. A. S. Svendsen (1983), Oceanographic conditions in the marginal ice Science Foundation under grant number OPP-9910290, the Packard zone north of Svalbard in early fall 1979 with emphasis on mesoscale Foundation, NSF Polar Programs through the North Pole Environmental processes, J. Geophys. Res., 88, 2755–2769. Observatories Project at the University of Washington, and the NOAA Johannessen, J. A., et al. (1987), Mesoscale eddies in the Fram Strait Arctic Research Office. We acknowledge James E. Overland for formulat- marginal ice zone during the 1983 and 1984 Marginal Ice Zone ing this project and securing the funding, Tuomo Saloranta for providing Experiment, J. Geophys. Res., 92, 6754–6772. the West Spitsbergen Current ensemble data, Ursula Schauer for recent Johnson, K. S., and L. J. Coletti (2002), In situ ultraviolet spectrophoto- WSC ensemble E1 data, Peter Proctor for CTD processing, Karen Birchfield metry for high resolution and long-term monitoring of nitrate, bromide for graphics support, Kathy Krogslund and the University of Washington and bisulfide in the ocean, Deep-Sea Res. I, 49, 1291–1305. Marine Chemistry Laboratory for salinity and nutrient analyses, Eugenia Karcher, M. J., R. Gerdes, F. Kauker, and C. Koberle (2003), Arctic warm- Dowling of the National Ice Center for information on ice edge processing, ing: Evolution and spreading of the 1990s warm event in the Nordic seas and the MBARI AUV crew of William J. Kirkwood, Drew Gashler, Hans and the Arctic Ocean, J. Geophys. Res., 108. Thomas, Rob McEwen, Mark Sibenac, Rich Henthorn, Farley Shane, D. J. Lewis, E. L., and R. G. Perkin (1983), Supercooling and energy exchange Osborne, Mark Talkovic and Todd Walsh. We thank Captain David near the Arctic Ocean surface, J. Geophys. Res., 88, 7681–7685. Visneski and crew of the USCGC Healy. We give special thanks to Lindsay, R. W., and J. Zhang (2005), The thinning of Arctic sea ice, 1988– MBARI’s Amy West and Mike Pinto and to Healy’s Marine Science 2003: Have we passed a tipping point?, J. Clim., 18, 4879–4894. Technicians who conducted the CTD casts in cold, windy, dark conditions. Loeng, H. (1991), Features of the physical oceanographic conditions of the Peter Haugan read an early version of this manuscript and offered Barents Sea, Polar Res., 10, 5–18. suggestions for improvement. We dedicate this paper to the memory of May, B. D., and D. E. Kelley (2001), Growth and steady state stages of Joseph Andrew ‘‘Drew’’ Gashler who died tragically in an avalanche in thermohaline intrusions in the Arctic Ocean, J. Geophys. Res., 106, 2004. His good sense and skillful management of AUVoperations helped to 16,783–16,794. make this work possible. This is contribution No. 2928 from the Pacific McEwen, R., H. Thomas, D. Weber, and F. 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