Freshening of Subsurface Waters in the Northwest Pacific Subtropical Gyre: Observations and Dynamics
Freshening of Subsurface Waters in the Northwest Pacific Subtropical Gyre: Observations and Dynamics
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DOI：
10.1175/jpo-d-13-03.1
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发表时间：
2013-12
影响因子：
3.5
通讯作者：
Youfang Yan;E. Chassignet;Y. Qi;W. Dewar
中科院分区：
文献类型：
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作者：
Youfang Yan;E. Chassignet;Y. Qi;W. Dewar
沿等密度表面在中低纬度地区传播的地下盐度异常已被证明在调节海洋和气候变化方面发挥着重要作用。在这项研究中，利用观测、数值输出和预测模型研究了西北太平洋副热带环流地下盐度异常的持续清新和向西南传播及其动力机制。对观测结果的分析表明，2003-11 年间，西北太平洋副热带环流 24.5 西格玛表面附近出现了明显的地下淡水化，盐度下降了约 0.25 PSU。研究发现，这种清新与露头区域（北纬 25 度 - 35 度，东经 130 度 - 160 度）的盐度异常的表面强迫有关。推导了基于露头等重线盐度守恒假设的预测模型，并用于检查这种表面强迫机制。从预测模型以及观测和数值输出得出的地下盐度空间结构之间的相似性表明，地下盐度异常在露头区域通风。地表强迫产生的振幅约为0.25 PSU的盐度异常在露头带俯冲，然后在大约一年内向西南传播，并伴随着潜在的涡度异常，到达吕宋海峡以东（类似于北纬15度）。当异常达到北纬15度时，它们转向并逐渐向东移向太平洋中部，与南副热带环流的东向逆流有关。
Subsurface salinity anomalies propagating between mid- and low latitudes along isopycnal surfaces have been shown to play an important role in modulating ocean and climate variability. In this study, a sustained freshening and southwestward propagation of subsurface salinity anomalies in the northwest Pacific subtropical gyre and its dynamical mechanism are investigated using observations, numerical outputs, and a predictive model. Analyses of the observations show a pronounced subsurface freshening with salinity decreasing about 0.25 PSU near the 24.5-sigma surface in the northwest Pacific subtropical gyre during 2003-11. This freshening is found to be related to the surface forcing of salinity anomalies in the outcrop zone (25 degrees-35 degrees N, 130 degrees-160 degrees E). A predictive model based on the assumption of salinity conservation along the outcrop isopycnals is derived and used to examine this surface-forcing mechanism. The resemblance between the spatial structures of the subsurface salinity derived from the predictive model and from observations and numerical outputs suggests that subsurface salinity anomalies are ventilated over the outcrop zone. A salinity anomaly with an amplitude of about 0.25 PSU generated by the surface forcing is subducted in the outcrop zone and then propagates southwestward, accompanied by potential vorticity anomalies, to the east of Luzon Strait (similar to 15 degrees N) in roughly one year. When the anomalies reach 15 degrees N, they turn and move gradually eastward toward the central Pacific, associated with an eastward countercurrent on the southern subtropical gyre.