Interactions between the responses of north American climate to El Nino-La Nina and to the secular warming trend in the Indian-Western Pacific Oceans

The modulation of El Nino and La Nina responses by the long-term sea surface temperature ( SST) warming trend in the Indian-Western Pacific ( IWP) Oceans has been investigated using a large suite of sensitivity integrations with an atmospheric general circulation model. These model runs entail the prescription of anomalous SST conditions corresponding to composite El Nino or La Nina episodes, to SST increases associated with secular warming in IWP, and to combinations of IWP warming and El Nino/La Nina. These SST forcings are derived from the output of coupled model experiments for climate settings of the 1951-2000 and 2001-50 epochs. Emphasis is placed on the wintertime responses in 200-mb height and various indicators of surface climate in the North American sector. The model responses to El Nino and La Nina forcings are in agreement with the observed interannual anomalies associated with warm and cold episodes. The wintertime model responses in North America to IWP warming bear a distinct positive ( negative) spatial correlation with the corresponding responses to La Nina ( El Nino). Hence, the amplitude of the combined responses to IWP warming and La Nina is notably higher than that to IWP warming and El Nino. The model projections indicate that, as the SST continues to rise in the IWP sector during the twenty-first century, the strength of various meteorological anomalies accompanying La Nina ( El Nino) will increase ( decrease) with time. The response of the North American climate and the zonal mean circulation to the combined effects of IWP forcing and La Nina ( El Nino) is approximately equal to the linear sum of the separate effects of IWP warming and La Nina ( El Nino). The summertime responses to IWP warming bear some similarity to the meteorological anomalies accompanying extended droughts and heat waves over the continental United States.