ENSO相联系的热带太平洋上层海洋异常环流

本文使用SODA(simple ocean data assimilation)海洋同化资料,系统分析了厄尔尼诺-南方涛动(El Ni?o-Southern Oscillation,ENSO)循环中冷暖位相期间热带太平洋上层海洋环流的演变规律,探讨了形成海洋环流异常的新机制。结果表明,在厄尔尼诺成熟期,热带中东太平洋赤道潜流最弱,赤道两侧出现反气旋性环流异常;西太平洋赤道外热带海域出现气旋性环流异常,该区南、北赤道流、棉兰老流、黑潮、新几内亚沿岸潜流及南赤道逆流增强;北赤道逆流区出现异常气旋性环流串,北赤道逆流接近正常。在厄尔尼诺衰退期和拉尼娜发展期,热带中西太平洋赤道潜流达到极强,赤道两侧出现气旋性环流异常;西太平洋赤道外热带海域异常环流减弱,该处主要流场的强度减弱或处于正常状态;北赤道逆流区反转为异常西向流。结果表明, ENSO循环期间的上层海洋环流异常受到热带太平洋温跃层深度异常产生的压强梯度力异常调控,在赤道外热带海洋温跃层深度异常和科里奥利力共同作用产生大尺度海洋环流异常,而在赤道海域,海洋温跃层深度异常和Gill效应造成赤道潜流异常以及关于赤道对称的气旋或反气旋性环流异常。

ENSO-RELATED UPPER-LAYER OCEAN CIRCULATION ANOMALY IN THE TROPICAL PACIFIC OCEAN

Variability of the tropical Pacific Ocean thermocline and upper-layer circulations under the ENSO cycle was analyzed using the Simple Ocean Data Assimilation (SODA) with the mechanisms underlying the ocean circulation explored. Results show that, during the mature phase of El Niño events, the Equatorial Undercurrent is the weakest in the central-eastern Pacific Ocean and anti-cyclonic circulation anomalies were found in the off-equatorial region. In the western Pacific Ocean, cyclonic circulation anomaly is clearly away from the equator, in which currents including the North Equatorial Current, South Equatorial Current, Mindanao Current, Kuroshio, New Guinea Coastal Undercurrent, and South Equatorial Countercurrent are the strongest. The North Equatorial Countercurrent shows no significant anomaly although a cluster of cyclonic circulation anomaly exists within the North Equatorial Undercurrent region. By contrast, during the decaying phase of an El Niño event or developing phase of a La Niña event, the North Equatorial Undercurrent is the strongest in the tropical central-western Pacific Ocean. Cyclonic circulation anomalies are clear in the off-equatorial region. Off-equatorial current anomalies in the tropical western Pacific Ocean are weak or neutral, and the North Equatorial Countercurrent turns to show westward anomaly. The above processes are reversed during La Niña. We suggested that these upper ocean circulation anomalies under the ENSO cycle can be driven by internal oceanic pressure gradient due to thermocline depth anomaly (TCA). A combination of off-equatorial TCA and the Coriolis forcing leads to a large-scale ocean circulation anomaly, while the equatorial TCA and Gill effect together result in the North Equatorial countercurrent anomaly and cyclonic or anti-cyclonic ocean circulation anomaly that is symmetric at the equator.