Varieties of Shallow Temperature Maximum Waters in the Western ...

GEOPHYSICAL RESEARCH LETTERS, VOL. 28, NO. 18, PAGES 3441-3444, SEPTEMBER 15, 2001

Varieties of Shallow Temperature Maximum Waters in the Western Canadian Basin of the Arctic Ocean Koji Shimada Japan Marine Science and Technology Center, Yokosuka, Japan

Eddy C. Carmack Institute of Ocean Sciences, Sidney, Canada

Kiyoshi Hatakeyama and Takatoshi Takizawa Japan Marine Science and Technology Center, Yokosuka, Japan

Abstract. The properties and spreading pathways of shallow temperature maximum waters (STMs) in the western Canadian Basin are investigated using CTD and mooring data obtained in 1997-98 as part of the SHEBA (Surface Heat Budget of the Arctic Ocean) drift experiment and available historical data. Three distinct varieties of STM are recognized on the basis of salinity range: (1) Surface Mixed Layer Water (SMLW) with S < 30 psu; (2) Eastern Chukchi Summer Water (ECSW) with 31 < S < 32 psu; and (3) Western Chukchi Summer Water (WCSW) with S > 32 psu. These STMs carry sufficient heat within the upper layers of the ocean to significantly affect the rates of ice cover and decay. For example, during the winter of 1997-98 anomalously warm STM ( > 0 ◦ C) originating from ECSW was observed to spread northwards along the Northwind Ridge and Chukchi Plateau, where the maximum reduction of the ice covers was subsequently observed in late summer, 1998 [Maslanik et al., 1999]. Regional climate variability and ice cover in the western Canadian Basin are thus affected not only by anomalous atmospheric circulation patterns, but also by the circulation of upper ocean water masses.

pattern was much larger than those of the Northwind Ridge and Chukchi Plateau? Here we examine the upper ocean circulation in the western Canadian Basin and argue that the spatial distribution of shallow temperature maximum waters (STMs) in upper ocean may contribute to an additional precondition for the local retreat of sea ice cover.

Study Area and Data Figure 1 shows the bathymetry of the study area and the CTD and mooring stations that were conducted during the SHEBA drift experiment in 1997-1998. A SeaBird (SBE19) conductivity, temperature and depth probes were used to obtain temperature and salinity measurements along the SHEBA drift (CCGS Des Groseilliers). The instrument was

Introduction Maslanik et al. [1999] reported a record reduction in sea ice cover in the western Canadian Basin during summer, 1998, and suggested that the combined effects of two mechanisms would be responsible for such anomalous conditions. One was the precondition of the water column by light ice conditions prevailing at the end of summer, 1997, which enhanced the in-situ warming of the surface mixed layer by solar radiation [McPhee et al., 1998]. The other was a persistent anomalous atmospheric condition from autumn 1997 through summer 1998, characterized by a decrease in northerly winds and an associated weakening or displacement of anticyclonic Beaufort Gyre. However, a key question remains: why did the maximum retreat in ice cover occur along the flanks of the Northwind Ridge and Chukchi Plateau (example, SSMI ice concentration map provided from NECP), even though the atmospheric circulation Copyright 2001 by the American Geophysical Union. Paper number 2001GL013168. 0094-8276/01/2001GL013168$05.00

Figure 1. CTD casts with SHEBA drift track and location of mooring NWR97. Year days of 1997 are also marked on the drift track. Three selected CTD casts (star symbols) for θ -S plots in bottom figure. International Bathymetric Chart of the Arctic Ocean (IBCAO) is used for plots of bottom topography (Jakobsson et al., 2000).

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SHIMADA ET AL.: SHALLOW TEMPERATURE MAXIMUM WATERS IN THE CANADIAN BASIN

Figure 2. Vertical section of temperature (top), salinity (middle), and seafloor topography along SHEBA drift track. Major salinity fronts (F1, F2, and F3) are found in this area. F1 corresponded to the water mass front of Mackenzie Water by Guay and Falkner (1997). CTD cast B is used for the estimation of stored heat by ECSW.

lost at sea in August 1998, so no post-cruise calibration was conducted. From a comparison between the SBE-19 and a well quality controlled CTD data from the CCGS Louis S. St-Laurent in 1998, the maximum errors for temperature and salinity are conservatively estimated about 0.01 ◦ C and 0.01 psu, respectively. This is sufficiently accurate to discuss upper ocean temperatures and salinities above the Atlantic Layer. A year long record of temperature and salinity was acquired by SBE-16 CTD sensors on the eastern slope of the Northwind during the SHEBA period. The maximum error for temperature and salinity were less than 0.01 ◦ C and 0.1psu, respectively. In addition, XCTD observation was made on a section extending from Barrow Canyon to the Northwind Ridge from the CCGS Louis S. St-Laurent in late September 1998. The accuracy of temperature and salinity is estimated to be 0.03 ◦ C and 0.05 psu, respectively. We also adopt the temperature and salinity data sets from Arctic Ocean Section 1994 [Aagaard et al., 1996; Carmack et al., 1997; Swift et al., 1997] and from the EWG Arctic Ocean Atlases [Environmental Working Group, 1997].

according to the classification of Coachman et al. [1975]. The third and most saline type of STM (32 < S < 33 psu) is called Western Chukchi Summer Water (WCSW), which corresponds to the Summer Shelf Water that enters the Arctic Basin via Herald Canyon according to the classification of Coachman et al. [1975]. WCSW was the coldest STM observed among the three. Even the historical highest temperature of WCSW, found on the slope north of the Herald Canyon (Station 9) in the 1994 Arctic Ocean Section [Aagaard et al., 1996; Carmack et al., 1997; Swift et al., 1997] was only -0.6 ◦ C. Next we examine the spatial distribution of the three STMs from the vertical section of temperature and salinity along the SHEBA drift track (Figure 2). The SMLW occurs in the region immediately east of the Northwind Ridge (days 340-400). In this area, in fact, two shallow temperature maxima were observed. The upper STM was classified as SMLW. Note that the winter surface mixed layer just above SMLW was slightly warm and fresh relative to that of the surrounding regions. This proves that SMLW affect the vertical flux of heat and salt in the winter surface mixed layer, and is suggested to be an important precondition for the retreat in ice cover in next summer [McPhee et al., 1998; Kadko, 2000]. A salinity front (F1 in Figure 2) comprised of the western boundary of the SMLW was observed, especially on the Northwind Ridge. This steep front might also delimit water enriched in Mackenzie Water as characterized by high concentrations of barium [Guay and Falkner, 1997] and light values of 18 O [Macdonald et al., 1999]. Baroclinic flow associated with this thermohaline structure would be expected to advect SMLW from the source region northwards along the flanks of the Northwind Ridge and the Chukchi Plateau. We thus suggest that the spatial distribution of the temperature of SMLW would not be strictly established locally by short wave radiation during the preceding summer. The ECSW type was observed at all stations except on the northern and southwestern Chukchi Plateau. The spatial distribution of temperature, however, was not uniform. Anomalously warm ECSW, with temperatures over 0 ◦ C, was only observed on the Northwind Ridge (days 390-440). Within the salinity range of ECSW, a salinity front (F2 in Figure 2) was found on the Northwind Ridge. This spatial distribution and structure may also be associated with the spreading pathway of ECSW. As with SMLW, a baroclinic

Results θ-S diagrams at three selected stations (Figure 1: bottom) show that the STMs can be grouped into three categories according to salinity range (i.e., the origin of water masses). The first and freshest type of STM (S < 30 psu) is Summer Mixed Layer Water (SMLW), which derives from warming by short wave radiation in open water areas during summer [Kadko, 2000] and from the Mackenzie River [Guay and Falkner, 1997]. The second and warmest type of STM (31 < S < 32 psu) is Eastern Chukchi Summer Water (ECSW), which corresponds to Alaskan Coastal Water

Figure 3. Spatial distributions of potential temperature on S=31.5 and dynamic height (dynamic m) of the 50dbar surface relative to 500dbar surface from the winter climatology of EWG Arctic Ocean Atlas.


Figure 4. Timeseries of temperature and salinity at the mooring NWR97 site (Figure 1) from October 16, 1997 to October 6, 1998. Minimum values of pressure at mean sensor levels are 49, 104, 150, and 200dbar (marked as star symbols). The variation of pressure at the top sensor was less than 5 dbar. The ice condition is depicted using the NSIDC ice charts.

flow likely transports ECSW northward along the flanks of the Northwind Ridge and Chukchi Plateau. As such, a salinity front in the salinity range of ECSW (F3 in Figure 2) was also found on the Chukchi Plateau. Here, the isohalines of 31 < S < 32 psu are seen to outcrop into the surface mixed layer. Consequently, the westward spreading of ECSW was terminated at the Chukchi Plateau. The WCSW type was found only in the west of the Northwind Ridge. It was much colder than either SMLW or ECSW. Hence, at least in the region of the Northwind Ridge and Chukchi Plateau during the SHEBA drift experiment, the influence of WCSW on the upper ocean heat content was insignificant. The major pathway of WCSW is suggested to be via Herald Canyon in the west of Chukchi Plateau. Vertical sections from the 1994 Arctic Ocean Section (not shown) give additional information on the spatial distribution of WCSW. WCSW is observed to extend from the slope north of Herald Canyon to the southern portion of the eastern side of the Mendeleyev Ridge. The termination of WCSW above the Mendeleyev Ridge corresponds to the location of the water mass front that separates the Pacific and Atlantic assemblies discussed by McLaughlin et al. [1996] and Morison et al. [1998].

Figure 5. Vertical section of temperature and salinity from the shelf slope north of the Barrow Canyon to the Northwind Ridge in September 26-28, 1998. The ice condition is depicted using the XCTD observation log sheet. XCTD cast 31 is used for the estimation of stored heat by ECSW.

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Figure 6. Schematic illustration for the circulation of the three shallow temperature maximum waters (SMLW, ECSW, WCSW) in the western Arctic Ocean.

The spatial distribution of all three STMs is associated with the major topographic features of the region. Vertical sections of temperature and salinity show that the Northwind Ridge and Chukchi Plateau are key areas where both ECSW and SMLW are advected from south to north in the western Canadian Basin. Figure 3 shows the spatial distribution of potential temperature on S=31.5 psu within the salinity range of ECSW and dynamic height of 50dbar level relative to 500dbar from the EWG Arctic Ocean Atlas for winter period data (December-May) [Environmental Working Group, 1997]. Here, we use the digital grid datasets interpolated adopting the method of Spectral Objective Analysis (SA). A core of the warm water is located on the Northwind Ridge. This spatial pattern agrees with the spreading pattern of anomalously warm ECSW (> 0 ◦ C) in Figure 2. This implies that the advective pathway of ECSW along the Northwind Ridge and the Chukchi Plateau typifies the annual circulation and forms a local, subsurface hot spot in the western Canadian Basin. The corresponding dynamic height reveals that the expected baroclinic flow field is nearly parallel to the seafloor topography above the Northwind Ridge and the Chukchi Plateau. This pattern agrees well with the speculated pathway of ECSW and resultant spatial distribution of temperature. The seafloor topography in this region is thus crucially important for the upper ocean heat and sea ice distributions in the western Canadian Basin. CTD observations discussed above (Figure 2) were made across the Northwind Ridge during winter (January to March, 1998). In the upstream region where ECSW reaches the slope near Point Barrow, the duration of the warm event is pulse like, limited to a few months from late summer through autumn. Therefore, it might be speculated that the warm water event associated with ECSW above the Northwind Ridge could also be pulse like; this, however, is not true. Figure 4 shows a time series of temperature and salinity from four CTD sensors (49dbar, 104dbar, 150dbar, and 200dbar) on mooring NWR97. Within the salinity range of ECSW (31 < S < 32 psu) the warm event (∼ 0 ◦ C) lasted until the following summer. Then, in late July 1998, when sea ice over the mooring site began to decrease, temperature in the salinity range of ECSW dropped, on the average of about 1 ◦ C. This suggests that the huge amount of heat stored in ECSW above the Northwind Ridge was windmixed into the surface layer, and subsequently accelerated the retreat of ice cover. In fact, in the ice covered area on the northern half of the Northwind Ridge (> 76.8◦ N)

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the ECSW maintained warm temperatures nearly 0 ◦ C into late September (Figure 5). Kadko [2000], using the radioactive half-life of beryllium, argued that upper water column temperature returned to near freezing upon mixing. In our study, on the other hand, the ECSW still warm beyond winter period. From a one-dimensional viewpoint, the stored heat within the ECSW (40-100m) near the Northwind Ridge can be estimated about 140MJm2 according to a comparison of the late summer temperature profile from the XCTD survey (74.96◦ N, 155.68◦ W, Sep. 26, 1998, Figure 5) with the winter temperature profile near the same location from the SHEBA site (75.00◦ N, 156.53◦ W, Feb. 1, 1998, Figure 2). If the stored heat is used only for melt of ice, the upper limit of the melt can be estimated about 50cm. This value coincided closely with the insolation (150MJm2 ) to the ocean through leads and thin ice during AIDJEX melt season in 1975 in the case of 9/10s ice coverage [Maykut and McPhee, 1995]. The lifetime of the warm ECSW remains unknown, and further studies using geochemical tracers should be required.

Summary and discussion We have described the distribution of three STMs in the western Canadian Basin. The circulation pattern of these three major summer water masses is summarized in Figure 6, wherein both eastward and westward advections of ECSW in the southern Beaufort Sea are illustrated. We speculate that the westward advection would occur mainly above the outer region of the shelf and slope, where the upper ocean circulation is governed by wind (ice) driven circulation. If this hypothesis is correct, the dynamics of offshore transport of ECSW from the slope, in particular between Barrow and Prudhoe Bay, will be a key issue to understand the circulation of ECSW. One possible process was introduced by Kubokawa [1991], who found that both potential vorticity difference across the front and transport of low potential vorticity through straits or submarine canyons control the nonlinear evolution of the flow downstream. If the former is large while the later is low, the direction of frontal waves tends to be in upstream propagation and the outflow forms large eddies in the vicinity of the strait. Furthermore, the seasonal variability in the flow direction on the slope near Point Barrow seems to be significant to support the heat transport by ECSW [Aagaard and Roach, 1990]. This would be associated with the relative strength of wind driven currents (offshore) compared to buoyancy boundary currents (onshore). This will be discussed in a subsequent paper using direct current measurement data near the Point Barrow. Acknowledgments. We are greatly indebted to D. Sieberg, D. Tuele, and C. Darnell for the Northwind Ridge mooring program, and to C. Ashjian, B. Campbell, I. Melnikov, S. Moore, B. Sherr, P. Wheeler, B. Welch for CTD casts at SHEBA site. Their efforts were indispensable to perform our study. We also express our special thanks to the leaders of the cruises J. Swift (SHEBA 1997) and R. Macdonald (SHEBA 1998), the captains and crew members of ships associated with SHEBA (CCGS Des Grosilliers; CCGS Louis S. St-Laurent). We also thank the leaders of the SHEBA project, D. Moritz, D. Perovich, and A. Heiberg. The authors deeply thank K. Aagaard, T. Weingartner, J. Morison, M. Steele, B. van Hardenberg, H. Melling, G. Holloway, F. McLaughlin, T. Kikuchi and S. Nishino for their valuable comments. Bon van Hardenberg and Matt Grinder conducted CTD data processing from the SHEBA site.

References Aagaard, K., The Beaufort Undercurrent. in The Alaskan Beaufort Sea: Ecosystems and Environment, edited by P. Barnes and E. Reimnitz, pp. 47-71, Academic Press, New York, 1984. Aagaard, K., and A.T. Roach, Arctic Ocean-Shelf exchange: Measurements in Barrow Canyon, J. Geophys. Res., 95, 18,163-18,175, 1990. Aagaard, K., L. A. Barrie, E. C. Carmack, C. Garrity, E. P. Jones, D. Lubin, R. W. Macdonald, J. H. Swift, W. B. Tucker, P. A. Wheeler, and R. H. Whritner, U.S., Canadian researchers explore Arctic Ocean, Eos Trans. AGU, 77, 209,213, 1996. Carmack, E., K. Aagaard, J. Swift, R. Perkin, F. McLaughlin, R. Macdonald, P. Jones, J. Smith, K. Ellis, and L. Kilius, Changes in temperature and tracer distributions within the Arctic Ocean: Results from the 1994 Arctic Ocean Section. Deep Sea Res., 44, 1487-1502, 1997. Coachman, L. K., K. Aagaard, and R. B. Tripp, Bering Strait: the regional physical oceanography, 172pp, University of Washington Press, Seattle, 1975. Environmental Working Group, Joint US-Russian Atlas of the Arctic Ocean, Oceanography Atlas for the Winter Period, National Ocean Data Center, Washington, DC, 1997. Guay, C. K., and K. K. Falkner, Barium as a tracer of Arctic halocline and river waters, Deep Sea Res., 44, 1543-1569, 1997. Jakobsson,M.,N.Z.Cherkis,J.Woodward,R.Macnab,and B.Coakley, New grid of Arctic bathymetry aids scientists and mapmakers, Eos Trans. AGU, 81, 89,93,96, 2000. Kadko, D., Modeling the evolution of the Arctic mixed layer during the fall 1997 Surface Heat Budget of the Arctic Ocean (SHEBA) Project using measurements of 7Be, J. Geophys. Res., 105, 3369-3378, 2000. Kubokawa, A., On the behavior of outflows with low potential vorticity from a sea strait, Tellus, 43A, 168-176, 1991. Macdonald, R. W., E. C. Carmack, F. A. McLaughlin, K. K. Falkner, and J. H. Swift, Connections among ice, runoff and atmospheric forcing in the Beaufort Gyre, Geophys. Res. Lett., 26, 2223-2226, 1999. Maslanik, J. A., M. C. Serreze, and T. Agnew, On the record reduction in 1998 Western Arctic sea-ice cover, Geophys. Res. Lett., 26, 1905-1908, 1999. McLaughlin, F. A., E. C. Carmack, and R. W. Macdonald, Physical and geochemical properties across the Atlantic/Pacific water mass front in the southern Canadian Basin, J. Geophys. Res., 101, 1183-1197, 1996. McPhee, M. G., Stanton, T. P., Morison, J. H., and Martinson, D. G., Freshening of the upper ocean in the Arctic: Is perennial sea ice disappearing? Geophys. Res. Lett., 25, 1729-1731, 1998. Morison, J.H., M. Steele and R. Andersen, Hydrography of the upper Arctic Ocean measured from the nuclear submarine USS Pargo. Deep Sea Res., 45, 15-38, 1998. Maykut, G.A., and M.G. McPhee, Solar heating of the Arctic mixed layer. J. Geophys. Res., 100, 24,691-24,703, 1995. Schlitzer, R., Ocean Data View, http://www.awi-bremerhaven. de/GEO/ODV, 2000. Steele, M. and T. Boyd, Retreat of the cold halocline layer in the Arctic Ocean. J. Geophys. Res., 103, 10419-10435, 1998. Swift, J., E. Jones, K. Aagaard, E. Carmack, M. Hingston, R. Macdonald, F. McLaughlin, and R. Perkin, Waters of the Makarov and Canada Basins, Deep Sea Res., 44, 1503-1530, 1997. K. Shimada, K. Hatakeyama, and T. Takizawa, Japan Marine Science and Technology Center, Yokosuka, 237-0061, Japan. (e-mail: [email protected], [email protected], [email protected]) E. C. Carmack, Institute of Ocean Sciences, 9860 West Saanich Road, Sidney, B.C. Canada V8L 4B2. (e-mail: [email protected]) (Received March 12, 2001; revised May 25, 2001; accepted June 28, 2001.)