Dynamics of mean and subtidal flow on the New England shelf
Dynamics of mean and subtidal flow on the New England shelf
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DOI：
10.1029/2002jc001417
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发表时间：
2003-08
影响因子：
--
通讯作者：
R. Shearman;S. Lentz
中科院分区：
文献类型：
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作者：
R. Shearman;S. Lentz
从1996年8月到1997年6月部署的海岸混合和光学实验系系阵列的洋流和水文观测资料用于描述速度变化和评估新英格兰大陆架上从几天到几个月不等的时间尺度上的环流动力学。潮下（数天至数周）的洋流变化沿等深线呈极化，并以偶发的西流爆发为主。沿等深线潮下流以地转和正压流为主，与沿45°T方向（与局地等深线方向逆时针65°）的大尺度沿岸风应力波动相关。潮下近地表地转输与风致Ekman输运的拟合估计然而，近底地转输量远大于由底应力引起的Ekman输量。低频流（月、长时间尺度）在各站点、各深度均以西向流为主，秋季向西流最强。低频沿等深线流主要是地转流，斜压和正压分量大小相近。深度平均地转输在数量上与来自风和底应力的Ekman输运一致。在潮下和低频时间尺度上测量的底部应力都很弱，几乎比风应力小一个数量级。以地转为主的沿等深线流的低频波动可归因于陆架间密度场的变化，这种变化与地表加热的季节循环有关。在秋季，热风切变最强，因为横断等深线温度梯度与持续的横断等深线盐度梯度协同作用，增强了横断等深线密度梯度（即近海海水变暖变淡）。在冬季，作为对地表冷却的响应，跨等深线温度梯度反转，减少跨等深线密度梯度（即近海较冷和较淡水）。
[1] Current and hydrographic observations from the Coastal Mixing and Optics experiment moored array, deployed from August 1996 through June 1997, are used to describe the velocity variability and evaluate the dynamics of circulation over the New England shelf on timescales ranging from a few days to several months. Subtidal (days to weeks) current variability was polarized along-isobath and dominated by episodic bursts of westward flow. The along-isobath subtidal flow was primarily geostrophic and barotropic, and was correlated with large-scale along-coast wind stress fluctuations oriented 45°T (65° counterclockwise from the local isobath orientation). Subtidal near-surface ageostrophic transport matched estimates of wind-driven Ekman transport; however, near-bottom ageostrophic transport was much larger than estimates of Ekman transport from bottom stress. Low-frequency (monthly and longer timescales) flow was generally westward and off-shelf at all sites and depths, with the strongest westward flow during the fall. Low-frequency along-isobath currents were primarily geostrophic with baroclinic and barotropic components of similar magnitude. Depth-averaged ageostrophic transport was quantitatively consistent with Ekman transport from wind and bottom stress. Measured bottom stress at both subtidal and low-frequency timescales was weak, nearly an order of magnitude smaller than the wind stress. Low-frequency fluctuations in the predominantly geostrophic along-isobath flow were attributable to variations in the cross-shelf density field associated with the seasonal cycle in surface heating. During the fall, thermal wind shear was strongest, because the cross-isobath temperature gradient was acting in concert with the persistent cross-isobath salinity gradient to enhance the cross-isobath density gradient (i.e., warmer and fresher water inshore). During the winter, in response to surface cooling, the cross-isobath temperature gradient reversed sign, reducing the cross-isobath density gradient (i.e., cooler and fresher water inshore).