A DIAGNOSIS OF INTERPENTADAL CIRCULATION CHANGES IN THE NORTH-ATLANTIC
A DIAGNOSIS OF INTERPENTADAL CIRCULATION CHANGES IN THE NORTH-ATLANTIC
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DOI：
10.1029/91jc02423
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发表时间：
1991-12-15
影响因子：
3.6
通讯作者：
LEVITUS, S
中科院分区：
文献类型：
--
作者：
GREATBATCH, RJ;FANNING, AF;LEVITUS, S
本文叙述了北大西洋环流的诊断计算。三个基本的情况被认为是：气候平均状态和环流的五分之一，1955-1959年和1970-1974年。Levitus（1982，1989）的密度数据与Hellerman和Rosenstein（1983）的年平均风应力场（用于气候案例）和来自20世纪50年代和70年代综合海洋-大气数据集的风应力数据一起用作输入。结果表明，墨西哥湾流在1970-1974候比1955-1959候弱约30 Sv。其中约20希沃特是由于副热带环流的环流急剧减弱。这可以追溯到与大西洋中脊西侧海底地形有关的海底压力扭矩的变化。同样的一般区域也被证明是重要的，以提高运输的气候平均副热带环流以上的平底Sverdrup关系预测。其余的10西弗是由于加拿大大西洋沿岸以南和墨西哥湾流以北的大陆坡地区的气旋环流减弱。这也与底部压力扭矩的变化有关。我们发现，在1500米深度以上的密度场的变化贡献了约一半的运输变化。目前尚不清楚对其余一半的估计有多可靠。这是因为它取决于深度大于1500米的分析密度场的变化，而这些变化可能是数据不足或不可靠的结果。我们的计算没有诊断出副极地环流的总输送量的显着变化。为了解释的结果，我们已经分裂的斜压性和救济（JEBAR）项的联合效果分为两部分：一部分与底部压力扭矩和补偿的一部分与密度分层的影响，可变的底部地形。这导致体积传输流函数PSI自然地划分为三个部分; PSI = PSI-W + PSI-C + PSI-B。PSI-W仅使用风强迫计算，并假设海洋密度均匀。PSI-C是这与使用平底Sverdrup关系计算的流函数PSI-S之间的差。它由JEBAR中与密度补偿相关的部分驱动。最后，PSI-B是PSI和PSI-S之间的差异，并且是由底部压力扭矩驱动的PSI的部分。（PSI-C + PSI-B）则给出JEBAR项对PSI的总贡献。我们发现，对于气候平均副极地环流，密度补偿是特别重要的，与底部压力扭矩取代环流向南，而不是增强其运输。对于副热带环流，密度补偿起的作用较小。两个五元组之间的差几乎都发生在底部压力扭矩部分。
Diagnostic calculations of the circulation in the North Atlantic are described. Three basic cases are considered: the climatological mean state and the circulation in the pentads 1955-1959 and 1970-1974. Density data from Levitus (1982, 1989) are used as input together with the annual mean wind stress field of Hellerman and Rosenstein (1983) for the climatological case and wind stress data derived from the Comprehensive Ocean-Atmosphere Data Set for the 1950s and 1970s. The results suggest that the Gulf Stream was some 30 Sv weaker in the 1970-1974 pentad than in the pentad 1955-1959. About 20 Sv of this is due to a dramatic weakening of the circulation of the subtropical gyre. This is traced to a change in bottom pressure torque associated with the bottom topography on the western side of the Mid-Atlantic Ridge. This same general area is also shown to be important for enhancing the transport of the climatological mean subtropical gyre above that predicted by the flat-bottomed Sverdrup relation. The remaining 10 Sv is due to a weakening of the cyclonic gyre in the continental slope region south of Atlantic Canada and north of the Gulf Stream. This too is associated with a change in bottom pressure torque. We find that changes in the density field above 1500 m depth contribute about half of the transport change. It is not clear how reliable is the estimate of the remaining half. This is because it is dependent on changes in the analyzed density field at depths greater than 1500 m, and these could be a result of insufficient or unreliable data. No significant change in the total transport of the subpolar gyre is diagnosed by our calculations. In order to interpret the results we have split the joint effect of baroclinicity and relief (JEBAR) term into two parts: a part associated with bottom pressure torque and a part associated with compensation by the density stratification for the effect of variable bottom topography. This leads to a natural division of the volume transport stream function PSI into three parts; PSI = PSI-W + PSI-C + PSI-B. PSI-W is calculated using wind forcing alone and assumes a uniform density ocean. PSI-C is the difference between this and the stream function, PSI-S, calculated using the flat-bottomed Sverdrup relation. It is driven by that part of JEBAR associated with density compensation. Finally, PSI-B is the difference between PSI and PSI-S and is that part of PSI driven by bottom pressure torque. (PSI-C + PSI-B) then gives the total contribution to PSI from the JEBAR term. We find that for the climatological mean subpolar gyre, density compensation is particularly important, with bottom pressure torque displacing the gyre southward rather than enhancing its transport. For the subtropical gyre, density compensation plays less of a role. Almost all the difference between the two pentads occurs in the bottom pressure torque part.