Sources of global warming in upper ocean temperature during El Nino

Global average sea surface temperature (SST) from 40 degreesS to 60 degrees N fluctuates +/-0.3 degreesC on interannual period scales, with global warm ing (cooling) during El Nino (La Nina). About 90% of the global warming dur ing El Nino occurs in the tropical global ocean from 20 degreesS to 20 degr eesN, half because of large SST anomalies in the tropical Pacific associate d with El Nino and the other half because of warm SST anomalies occurring o ver similar to 80% of the tropical global ocean. From examination of Nation al Centers for Environmental Prediction [Kalnay et al., 1996] and Comprehen sive Ocean-Atmosphere Data Set [Woodruff et al., 1993] reanalyses, tropical global warming during El Nino is associated with higher troposphere moistu re content and cloud cover, with reduced trade wind intensity occurring dur ing the onset phase of EI Nino. During this onset phase the tropical global average diabatic heat storage tendency in the layer above the main pycnocl ine is 1-3 Wm(-2) above normal. Its principal source is a reduction in the poleward Ekman heat flux out of the tropical ocean of 2-5 Wm(-2). Subsequen tly, peak tropical global warming during El Nino is dissipated by an increa se in the flux of latent heat to the troposphere of 2-5 W m(-2), with reduc ed shortwave and longwave radiative fluxes in response to increased cloud c over tending to cancel each other. In the extratropical global ocean the re duction in poleward Ekman heat flux out of the tropics during the onset of El Nino tends to be balanced by reduction in the flux of latent heat to the troposphere. Thus global warming and cooling during Earth's internal mode of interannual climate variability arise from fluctuations in the global hy drological balance, not the global radiation balance. Since it occurs in th e absence of extraterrestrial and anthropogenic forcing, global warming on decadal, interdecadal, and centennial period scales may also occur in assoc iation with Earth's internal modes of climate variability on those scales.