Barrier Layer in the Indian Ocean during Central Pacific and Eastern Pacific El Niños: Observations and Mechanism

Lili Zeng, SCSIO South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China, Xiaolin Zhang, University of Colorado, Boulder, United States and Xuemeng Tian, Ocean University of China, China
El Niño and Southern Oscillation (ENSO) is a coupled ocean-atmospheric interannual variability in the equatorial Pacific Ocean with a period of 2 to 7 years. During El Niño condition, the easterly trade wind weakens, the positive sea surface temperature anomaly appears in the eastern Pacific Ocean, and the warm pool in the western Pacific moves eastward. As a result of warm water, the deep convection occurs in the western Pacific Ocean, which further leads to heavy rainfall. The huge rainfall in the western Pacific Ocean will generate a shallower mixing layer and a “barrier layer”. As proposed in Cronin and McPhaden (2002) the barrier layer could mainly result from (1) heavy rainfall, especially (but not only) under light wind conditions; (2) horizontal advection of a remote Barrier Layer; (3) the tilting of near-vertical salinity contours, requiring vertical shear in horizontal currents; and (4) the vertical stretching of the upper water column, assuming the preexistence of a barrier layer. There are two types of El Nino events, namely central Pacific El Nino and Eastern Pacific El Nino, which have different impact on the Indian Ocean windstress, precipitation forcing. By using insitu data (RAMA data), assimilation data, in this work, we are trying to see how the formation of barrier layer changes during Eastern Pacific El Niños compared to Central Pacific El Niños. The quantified estimation of the contribution from each mechanism is currently under investigation, and an up-to date analysis will be presented.