OBSERVATIONS OF VERTICAL CURRENTS AND CONVECTION IN THE CENTRAL GREENLAND SEA DURING THE WINTER OF 1988-1989
OBSERVATIONS OF VERTICAL CURRENTS AND CONVECTION IN THE CENTRAL GREENLAND SEA DURING THE WINTER OF 1988-1989
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DOI：
10.1029/93jc00658
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发表时间：
1993-08
影响因子：
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通讯作者：
F. Schott;M. Visbeck;J. Fischer
中科院分区：
文献类型：
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作者：
F. Schott;M. Visbeck;J. Fischer
1988-1989年冬季，在格陵兰海中部停泊了5台声学多普勒海流剖面仪(ADCP)，以测量可能与深层混合和对流有关的垂直流。两个ADCP在300米高空与热敏电阻串结合，两个在200米处向下俯视，一个在1400米高空向上俯视，在10月和11月的两次强冷风事件中出现第一个垂直速度方差极大值，当时浅混合层的冷却和湍流在温跃层中产生内波。从11月下旬开始，边缘冰带在格陵兰海中部向东扩展，在12月下旬达到最大范围。1月中旬，格陵兰海中部开放了一个不结冰的海湾，留下了一块楔形的冰，沿着扬·马延洋流的轴线卷曲在它周围，然后向东北方向移动，一直存在到1989年4月。在冰下形成了冰点温度下的混合层，在冰盖期间，混合层的厚度从60m增加到120m，对应的平均热损失约为40W m−2。在冰盖期间，垂直洋流最小。无冰湾开放后，在2月1日和15日前后，伴随着冷却事件，混合层加深到>；350m，并伴随着强烈的小尺度垂直速度变化。在这一阶段，更多起源于大西洋的咸水向上混合，进一步降低了稳定性，在格陵兰海中部产生了一个水平范围为>；50公里的均质水池，为随后的对流到更深的地方预留了条件。在3月6-16日期间观测到了个别对流事件，与1400m的下沉速度有关，约3 cm的S−L。其中一个事件被确认为约300m水平尺度的烟羽，这与最近提出的尺度论证和模式结果以及先前在西地中海狮子湾的类似观测结果相一致。深对流发生在无冰海湾的中心，因此卤水排斥似乎不是其产生的必要条件。1400米处的羽流温度一般高于均匀的地表水池，这表明在下降的过程中，周围较温暖的海水被卷走了。对流事件期间的平均垂直速度与零很难区分，这表明羽状物是一种混合剂，而不是引起水团的平均向下输送。然而，与受混合层物理控制的表层水池不同，400-1400m之间的水体并没有被零星的羽流水平均匀分布在大片区域内。总体而言，与狮子湾的结果相比，1988-1989年冬季格陵兰海中部的对流活动较弱，仅限于中等深度。
During the winter of 1988–1989 five acoustic Doppler current profilers (ADCPs) were moored in the central Greenland Sea to measure vertical currents that might occur in conjunction with deep mixing and convection. Two ADCPs were looking up from about 300 m and combined with thermistor strings in the depth range 60–260 m, two were looking downward from 200 m, and one was looking upward from 1400 m. First maxima of vertical velocity variance occurred at two events of strong cold winds in October and November when cooling and turbulence in the shallow mixed layer generated internal waves in the thermocline. Beginning in late November the marginal ice zone expanded eastward over the central Greenland Sea, reaching its maximum extent in late December. In mid-January a bay of ice-free water opened over the central Greenland Sea, leaving a wedge of ice, the “is odden,” curled around it along the axis of the Jan Mayen Current and then northeastward and existing well into April 1989. Below the ice a mixed layer at freezing temperatures developed that increased in thickness from 60 to 120 m during the period of ice cover, corresponding to an average heat loss of about 40 W m−2. Through brine rejection, mixed-layer salinity increased steadily, reducing stability to underlying weakly stratified layers (Roach et al., 1993). During the ice cover period, vertical currents were at a minimum. After the opening of the ice-free bay, successive mixed-layer deepening to >350 m occurred in conjunction with cooling events around February 1 and 15, accompanied by strong small-scale vertical velocity variations. Upward mixing of more saline waters of Atlantic origin during this phase reduced the stability further, generating a pool of homogeneous water of >50 km horizontal extent in the central Greenland Sea, preconditioned for subsequent convection to greater depths. Individual convection events were observed during March 6–16, associated with downward velocities at the 1400-m level of about 3 cm s−l. One event was identified as a plume of about 300-m horizontal scale, in agreement with recently advanced scaling arguments and model results, and with earlier similar observations in the Gulf of Lions, western Mediterranean. The deep convection occurred in the center of the ice-free bay; hence brine rejection did not seem necessary for its generation. Plume temperatures at 1400 m were generally higher than that of the homogeneous surface pool, suggesting entrainment of surrounding warmer waters on the way down. Mean vertical velocity over a period of convection events was indistinguishable from zero, suggesting that plumes served as a mixing agent rather than causing mean downward transport of water masses. However, different from the surface pool that was governed by mixed-layer physics, the water between 400 and 1400 m was not horizontally homogenized in a large patch by the sporadic plumes. Overall, and compared to results from the Gulf of Lions, convection activity in the central Greenland Sea was weak and limited to intermediate depths in winter 1988–1989.