On the Influence of Warm Pool SST Variability and Wind Stress Forcing on ENSO in SODA

In order to understand the change in oceanic variability associated with the mid-1970s's climate shift, we analyze the contribution of momentum forcing to the leading baroclinic modes over the tropical Pacific using a Simple Ocean Data Assimilation (SODA) version 2.0.2 for the period of 1958-97. Specifically, we look at the statistical relationship between wind projection coefficients and climate indices, and attempt to provide a physical explanation for the observed changes. It is found that the wind stress projection coefficients according to the oceanic baroclinic modes are different in terms of their magnitude and phase in the tropical Pacific, reflecting a specific forcing associated with each mode after and before the 1976 climate shift. Compared to before the 1970s, the first baroclinic mode is more effective in impacting the interannual sea surface temperature through equatorial wave dynamics and there is an increased delayed response of the second baroclinic mode variability to the interannual atmospheric forcing after the late 1970s. This reflects changes in ENSO feedbacks processes associated with the climate shift. Our analysis further indicates that after the late 1970s there is a decrease in the wind stress forcing projecting onto the Ekman layer which is associated with increased mixed layer depth. This result suggests that the changes in the ENSO properties before and after the late 1970s are largely associated with the changes in the way that the wind stress forcing projected onto a multi-mode linear system in the tropical Pacific Ocean on interannual timescales. The variability of warm pool sea surface temperature is related to wind stress forcing in the western Pacific. Two leading EOF modes of warm pool SST variability are highly correlated with each oceanic baroclinic mode.