EL NIŅO AND ITS TROPICAL TELECONNECTIONS DURING THE HOLOCENE AND LAST GLACIAL MAXIMUM

The mean state of the tropical Pacific and its interannual variability during the Holocene and Last Glacial Maximum are investigated using multi-century simulations with the NCAR Climate System Model (CSM). The CSM is a global coupled atmosphere-ocean-sea ice model.

Better-resolved ocean sediment records for the eastern Pacific cold tongue and the western Pacific warm pool have challenged past interpretations of the east-west sea surface temperature (SST) gradient and degree of SST cooling during the Holocene and Last Glacial Maximum. The CSM reproduces the recently published data on changes of the mean state of the tropical Pacific. For the Holocene, the model results show a sharpening of the zonal SST gradient across the tropical Pacific (more cooling in the central and eastern Pacific than the western Pacific) and increased upwelling in the central Pacific. For the Last Glacial Maximum, differential cooling across the tropical Pacific results in a reduced zonal SST gradient.

Proxy data suggest that the intensity and frequency of ENSO was decreased compared to modern during the Holocene. The CSM predicts weaker El Niņos/La Niņas compared to present for 11,000-3500 years ago and stronger El Niņos/La Niņas for the Last Glacial Maximum. During the Holocene, there are more occurrences of small and less occurrences of large El Niņos and La Niņos. Changes in intensities for the Last Glacial Maximum are traced to a sharpening of the tropical thermocline as a result of cooling at Southern Hemisphere middle and high latitudes. Changes in intensities for 11,000-3500 years ago are traced to strengthening of zonal wind stresses across the tropical Pacific Ocean by the stronger summer Asian monsoon and weakening of the tropical thermocline. Thus, for the Holocene and the Last Glacial Maximum, changes in both the atmospheric and oceanic mean climate conditions outside the tropical Pacific region need to be considered for understanding changes in ENSO SST variability. In addition, to understand proxy evidence of weaker precipitation variability in the Pacific region requires a determination of both changes in El Niņo/La Niņa and changes in the atmospheric circulation and hydrologic cycle.