Vertical structure variability in the equatorial Pacific before and after the Pacific climate shift of the 1970s.
Byung-Kwon Moon, Sang-Wook Yeh and Boris Dewitte
Based on the linear theory, oceanic baroclinic modes in the equatorial Pacific are estimated using results from the Simple Ocean Data Assimilation (SODA) system. The analysis for 1950-1997 suggests that the vertical stratification increases at the upper levels after the late 1970s due to changes in the vertical temperature structure. After the late 1970s, the variability of the higher-order baroclinic mode contributions to current anomalies and surface pressure significantly increases in the central equatorial Pacific. This is associated with an increase of the dominant period of the ENSO variability. Simple coupled model experiments indicate that when atmospheric forcing projects more onto the higher baroclinic modes according to the results of the SODA decomposition, the amplitude and dominant period of ENSO increase similarly to what is observed at the Pacific decadal shift. Our results support that the changes in the behavior of ENSO after the late 1970s are associated with oceanic vertical structural changes in temperature that tend to increase the contribution of higher-order baroclinic modes.