北大西洋海表面温度锋与大西洋风暴路径及大气大尺度异常的关系研究

运用NCEP、Had ISST再分析资料,北大西洋涛动(NAO)月指数序列,探讨了海表面温度(SST)锋的时空变化特征,揭示了北大西洋SST锋的主要气候变率及其与北大西洋风暴轴和大气大尺度环流异常的关系。研究表明,剔除季节循环后的SST锋显示其最主要变率为锋区的向南/北摆动,其对应的风暴轴发生相应的西南/东北移动,并同时在北大西洋上空对应一个跨海盆的位势高度负/正异常。这种环流异常可引起高纬度海平面气压(SLP)的反气旋/气旋式环流,这有利于增强海表面风对大洋副极地环流的负/正涡度异常输入,进一步减弱/加强了高纬度上层冷水向SST锋区的输送。北大西洋SST锋的另一主要模态为锋区在南北方向的分支和合并。当SST锋异常在40°N~45°N以单支形式加强时,对流层位势高度场和SLP南北梯度增大,对应NAO正位相,此时风暴轴也为单支型;同时SLP异常场促使冰岛附近具有气旋式风应力异常,亚速尔地区具有反气旋式风应力异常,导致副极地环流和副热带环流均加强,增加高纬度冷水和低纬度暖水在锋区的输入,从而进一步增强40°N~45°N附近的SST锋区。当SST锋异常在40°N~45°N纬带南北发生分支时,风暴轴也同时出现北强南弱的南北分支,此时对应了负位相NAO,来自北南的冷暖水输送减弱,SST锋也发生减弱分支。此外,位于大洋内区的SST锋东端也存在一个偶极子型的模态,尽管其解释方差相对较小,但仍与偏东北的NAO型具有显著相关。谱分析表明,北大西洋SST锋与风暴轴具有1~3年和年代际共振,与中高纬大尺度环流也存在周期1~3年的共变信号,其中准一年共变信号体现了SST锋和NAO之间的对应关系。进一步诊断分析表明,SST锋上空的近表层大气斜压性和经向温度梯度随着SST锋的增强而增强,经向热通量的向北输送导致涡动有效位能的增加;海洋的非绝热加热产生更强的垂直热量通量,这有利于涡动有效位能释放成为涡动动能,从而表现为该区域的风暴轴加强,并进一步影响风暴轴中的天气尺度扰动与下游大尺度环流异常的相互作用过程。

Relationship between North Atlantic SST Front and Storm Track as well as Large-scale Atmosphere Anomalies

Temporal and spatial variations of the storm track and SST (Sea Surface Temperature) frontal zone in the North Atlantic, as well as climate variability of the SST frontal zone and its relationship with the storm track and large-scale atmospheric circulation anomaly, are analyzed by using the NCEP and HadISST (Hadley Centre Sea Ice and Sea Surface Temperature data set) reanalysis data and NAO (North Atlantic Oscillation) monthly index data. Results show that after the seasonal cycle is filtered out, the main variability of the SST frontal zone is shown by its south/north shift, which corresponds to the southwestward/northeastward moving of the storm track and negative/positive geopotential height anomaly. This negative/positive geopotential height anomaly can lead to anticyclonic/cyclonic circulation at high latitudes, which is favorable for the development of negative/positive eddy anomaly in the ocean and eventually weakens/strengthens the SST gradient. Furthermore, when the SST frontal zone near 40°N-45°N intensifies as a single zone, the meridional gradients of geopotential height and SLP (Sea Level Pressure) become sharper, corresponding to the NAO positive phase. Meanwhile, the storm track demonstrates a single-track pattern, and the SLP anomaly results in cyclonic wind stress curl near the Iceland and anticyclonic wind stress curl near the Azores. As a result, stronger oceanic circulation develops in the subtropics and sub-polar region, which has a positive feedback to the SST gradient and storm track located near 40°N-45°N. When the SST frontal zone near 40°N-45°N splits into two bands, the storm track also bifurcates into two branches and the SST gradient becomes weaker, corresponding to the NAO negative phase. Although the variance contribution of SST front in the middle of the ocean is small, it is still significantly correlated with NAO. Spectral analysis suggests that the North Atlantic SST gradient anomaly has a covariant cycle of 1-3 years with the storm track anomaly and large scale atmospheric anomaly. The SST gradient anomaly also has a decadal covariant cycle with the storm track anomaly. The quasi-annual cycle shows that there exist certain covariations between SST gradient anomaly and storm track and NAO. Atmospheric baroclinicity, air temperature gradient, and poleward and upward transport of eddy heat flux are strong near the SST frontal zone, which are favorable for the conversion from mean available potential energy to eddy kinetic energy, and eventually lead to the establishment and strengthening of the storm track and affect the relationship between synoptic turbulence and large-scale circulation anomaly.