Rates and Mechanisms of Water Mass Transformation in the Labrador Sea as Inferred from Tracer Observations
Rates and Mechanisms of Water Mass Transformation in the Labrador Sea as Inferred from Tracer Observations
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根据示踪剂观测推断拉布拉多海水体转化的速率和机制
DOI：
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发表时间：
2002
期刊：
影响因子：
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通讯作者：
M. Visbeck
中科院分区：
文献类型：
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作者：
S. Khatiwala;P. Schlosser;M. Visbeck
水文和瞬变示踪剂（3 H和3 He）观测从拉布拉多海中部1991年和1996年之间收集的时间序列记录的复杂变化的示踪剂领域的结果，在20世纪90年代的对流活动的变化。在1991年和1993年之间，随着大气强迫的加强，对流渗透到逐渐增加的深度，在1993年冬季达到2300米。在此期间，拉布拉多海水的潜在温度（θ）/盐度（S）特性几乎保持不变，因为地表冷却和淡水的向下混合通过挖掘和更温暖和更咸的东北大西洋深层水的向上混合而平衡。结果表明，1991年至1993年期间水柱（150-2500 m）热含量的净变化与估计的平均热损失（110 W m−2）相比可以忽略不计，这意味着热量横向汇聚到拉布拉多海中部几乎在令人惊讶的短时间内平衡了大气冷却。有趣的是，拉布拉多海水的~ 3 H-~ 3 He年龄在这段对流增强期间增加。从1995年开始，冬季变得温和，对流仅限于800米以上。1994 ~ 1996年，3 H-3 He年龄的演化类似于一个停滞水体的演化。相反，θ和S在此期间的增加意味着示踪剂与边界的交换通过涡致翻转环流和沿等密度线的涡搅动进行[交换系数为O（500 m2 s−1）]。
作者构建了拉布拉多海的淡水收支，并定量地证明了海冰融水是拉布拉多海大陆架和内陆盐度年循环的主要原因。结果表明，运输的淡水涡旋到中央拉布拉多海（140厘米之间的3月至9月）可以很容易地解释所观察到的季节性淡化。最后，作者讨论了涡旋引起的翻转环流在新通风的拉布拉多海水向边界流系统的输送和扩散方面的作用，并将其强度（2-3 Sv）与诊断的拉布拉多海水浮力强迫形成率进行了比较。
Time series of hydrographic and transient tracer (3H and 3He) observations from the central Labrador Sea collected between 1991 and 1996 are presented to document the complex changes in the tracer fields as a result of variations in convective activity during the 1990s. Between 1991 and 1993, as atmospheric forcing intensified, convection penetrated to progressively increasing depths, reaching 2300 m in the winter of 1993. Over that period the potential temperature (θ)/salinity (S) properties of Labrador Sea Water stayed nearly constant as surface cooling and downward mixing of freshwater was balanced by excavating and upward mixing of the warmer and saltier Northeast Atlantic Deep Water. It is shown that the net change in heat content of the water column (150–2500 m) between 1991 and 1993 was negligible compared to the estimated mean heat loss over that period (110 W m−2), implying that the lateral convergence of heat into the central Labrador Sea nearly balances the atmospheric cooling on a surprisingly short timescale. Interestingly, the 3H–3He age of Labrador Sea Water increased during this period of intensifying convection. Starting in 1995, winters were milder and convection was restricted to the upper 800 m. Between 1994 and 1996, the evolution of 3H–3He age is similar to that of a stagnant water body. In contrast, the increase in θ and S over that period implies exchange of tracers with the boundaries via both an eddy-induced overturning circulation and along-isopycnal stirring by eddies [with an exchange coefficient of O(500 m2 s−1)].
The authors construct a freshwater budget for the Labrador Sea and quantitatively demonstrate that sea ice meltwater is the dominant cause of the large annual cycle of salinity in the Labrador Sea, both on the shelf and the interior. It is shown that the transport of freshwater by eddies into the central Labrador Sea (140 cm between March and September) can readily account for the observed seasonal freshening. Finally, the authors discuss the role of the eddy-induced overturning circulation with regard to transport and dispersal of the newly ventilated Labrador Sea Water to the boundary current system and compare its strength (2–3 Sv) to the diagnosed buoyancy-forced formation rate of Labrador Sea Water.