Low-Frequency Variability of Temperature in the Vicinity of the Equatorial Pacific Thermocline in SODA: Role of Equatorial Wave Dynamics and ENSO Asymmetry

The Simple Ocean Data Assimilation (SODA) reanalysis (1958-2001) is used to investigate the decadal variability in the equatorial thermocline in the Pacific. Whereas the thermocline depth exhibits weak variation at decadal time scales, the temperature change in the vicinity of the thermocline in the western Pacific is significant and has a vertical scale of; 150 m. Based on a modal decomposition of the model variability, it is shown that such temperature change can be interpreted to a large extent as vertical displacements of the isotherms associated with the Kelvin and first meridional Rossby waves of the first three baroclinic modes. This indicates that decadal change at the subsurface in the warm pool region may be forced by the winds, consistent with the results of a multimode linear model simulation. The decadal mode of vertical temperature can be described by the first two dominant statistical modes (EOFs): the first mode is associated with changes in the slope of the thermocline (swallowing in the western-central Pacific and deepening in the eastern Pacific), representative of the 1976/77 climate shift and ahead of the ENSO modulation; and the second mode corresponds to a basinwide uplift of the thermocline and behind the ENSO modulation. It is further shown that the subsurface temperature in the warm pool region is negatively skewed, which results from the ENSO asymmetry. The results are consistent with the hypothesis of change in mean state resulting from the residual effect of the asymmetric ENSO variability.The authors thank Axel Timmermann for stimulating discussions during the ENSO workshop "Exploring El Nino: Beyond the current understanding and predictability barrier'' in Seoul on 12-13 September 2006. Soon-II An, Sang-Wook Yeh, Byung-Kwon Moon, and Boris Dewitte benefited from support of the Centre National de la Recherche Scientifique (CNRS) through a Science and Technology Amicable Research (STAR) program. S.- I. An was supported from the "National Comprehensive Measures against Climate Change'' program by the Ministry of Environment, Korea (Grant 1600-1637-301-210-13). Sulian Thual benefited from support of Institut de Recherche pour le Developpement (IRD) and the Agence National de la Recherche (ANR) within the Per