Rectification of El Nino Southern Oscillation into Climate Changes in the Tropical Pacific on Decadal and Longer Time-scales

To investigate the effect of ENSO events on the tropical Pacific mean climate on decadal and longer time-scales, a pair of forced experiments are conducted with the NCAR Pacific basin model. In one of them, the climatological surface wind stress of 1950—2011 is applied while in the other, the surface wind stress used is climatological winds of 1950-2011 plus the interannual monthly anomalies over the period. Thus the long-term means of the surface wind stress in the two runs are identical. The two experiments also use the same linear relaxation boundary conditions---the SST is linearly restored to the same prescribed SST. The two runs, however, are found to yield significantly different mean climate for the tropical Pacific. The mean state of the run with interannual fluctuations in the surface winds is found to have a cooler warm-pool, warmer thermocline water, and warmer eastern surface Pacific than the run without interannual fluctuations in the surface winds. The warming of the eastern Pacific has a pattern that highly resembles the observed decadal warming pattern including the off-equator maximum. The run with interannula fluctatuions in the surface winds simulate the ENSO anomalies in the upper ocean realistically. The differences in the mean state of the upper ocean in the two runs thus indicate that ENSO events collectively have a significant effect on the mean state of the tropical Pacific on decadal and longer time-scales through the nonlinearity in the ocean dynamics.