Basinwide Integrated Volume Transports in an Eddy-Filled Ocean
Basinwide Integrated Volume Transports in an Eddy-Filled Ocean
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DOI：
10.1175/2009jpo4185.1
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发表时间：
2009-12-01
影响因子：
3.5
通讯作者：
Johns, W. E.
中科院分区：
文献类型：
--
作者：
Kanzow, T.;Johnson, H. L.;Johns, W. E.
北大西洋副热带大西洋经向翻转环流（AMOC）强度的时间演变受到远端强迫、盆地尺度经向相干、气候相关的输送异常（如高纬度深水形成率的变化）和局地强迫输送异常（如漩涡或罗斯比波）的影响，这些异常可能与小的经向相干尺度有关，可视为噪声。本文的重点是在撞击大西洋西部边界时，当地涡旋和罗斯比波对北纬26.5度AMOC的影响程度。自2004年4月以来，通过结合佛罗里达洋流、埃克曼和洋中输送，以及后者从巴哈马和摩洛哥海岸之间的连续密度测量中获得的数据，对该纬度AMOC进行了连续估计，分别代表：大西洋在这个纬度的东西边界。在距西部边界100公里范围内，从现场密度测量的动态高度和测高高度来看，向边界方向的海面高度变异性减少了三倍。因此，2004年4月至2006年10月连续观测到的小于1000米的全盆地纬向综合上大洋中输运仅变化3.0 Sv (1 Sv = 10(6) m(3) s(-1)) RMS。相反，从离岸16、40和500公里的西部边界站到东部边界的海洋中上游运输量分别变化3.6、6.0和10.7 Sv RMS。向西边界的涡旋能量的减少在一个非线性减少重力模型中得到再现，这表明边界捕获波可能解释了沿海地区观测到的变率下降，因为它们为与撞击西边界的涡旋有关的快速向赤道输出输送异常提供了一种机制。线性罗斯比波的解析模型表明，对向边界方向的温跃层厚度变率的减小有一个简单的标度。物理上，振幅的减小可以理解为沿边界压力梯度加速流体并沿边界迅速传播压力异常。结果表明，在年际至年代际尺度上，局地涡旋场不主导26.5°N海域中上层输送或AMOC变率。
The temporal evolution of the strength of the Atlantic Meridional Overturning Circulation (AMOC) in the subtropical North Atlantic is affected by both remotely forced, basin-scale meridionally coherent, climate-relevant transport anomalies, such as changes in high-latitude deep water formation rates, and locally forced transport anomalies, such as eddies or Rossby waves, possibly associated with small meridional coherence scales, which can be considered as noise. The focus of this paper is on the extent to which local eddies and Rossby waves when impinging on the western boundary of the Atlantic affect the temporal variability of the AMOC at 26.5 degrees N. Continuous estimates of the AMOC at this latitude have been made since April 2004 by combining the Florida Current, Ekman, and midocean transports with the latter obtained from continuous density measurements between the coasts of the Bahamas and Morocco, representing, respectively, the western and eastern boundaries of the Atlantic at this latitude.Within 100 km of the western boundary there is a threefold decrease in sea surface height variability toward the boundary, observed in both dynamic heights from in situ density measurements and altimetric heights. As a consequence, the basinwide zonally integrated upper midocean transport shallower than 1000 m-as observed continuously between April 2004 and October 2006-varies by only 3.0 Sv (1 Sv = 10(6) m(3) s(-1)) RMS. Instead, upper midocean transports integrated from western boundary stations 16, 40, and 500 km offshore to the eastern boundary vary by 3.6, 6.0, and 10.7 Sv RMS, respectively.The reduction in eddy energy toward the western boundary is reproduced in a nonlinear reduced-gravity model suggesting that boundary-trapped waves may account for the observed decline in variability in the coastal zone because they provide a mechanism for the fast equatorward export of transport anomalies associated with eddies impinging on the western boundary. An analytical model of linear Rossby waves suggests a simple scaling for the reduction in thermocline thickness variability toward the boundary. Physically, the reduction in amplitude is understood as along-boundary pressure gradients accelerating the fluid and rapidly propagating pressure anomalies along the boundary. The results suggest that the local eddy field does not dominate upper midocean transport or AMOC variability at 26.5 degrees N on interannual to decadal time scales.