Impact of Global Warming on the Western North Pacific Circulation Anomaly during Developing El Nino

El Nino stimulates an anomalous cyclone over the North Pacific during its developing phase. Using 30 CGCMs and 11 AGCMs from CMIP5, we find a weakly strengthened anomalous North Pacific cyclone (NPC) in a warmer climate in CGCMs, and intermodel uncertainty exists. A similar change of the anomalous NPC is found in AGCMs with increased mean state SST but with a stronger amplitude of enhancement. Based on a simple Gill model, the diabatic heating anomaly, mean state static stability, and meridional gradient of relative vorticity are identified to be responsible for the change of the anomalous NPC. Analyses of the CMIP5 models suggest that the change of the anomalous NPC is largely determined by the competition between the enhanced diabatic heating anomaly and the enhanced mean state static stability. The amplitude of enhancement of the anomalous NPC is strongly modulated by the change of precipitation anomaly over the equatorial central-eastern Pacific, which depends on the changes of mean state SST and the El Nino-related SST anomaly. Compared with a uniform warming, an El Nino-like mean state SST warming favors a much stronger enhancement of the anomalous NPC, by enhancing the mean state precipitation and latent heating anomaly associated with the precipitation anomaly over the equatorial Pacific. However, the air-sea coupling acts to weaken the SST anomaly associated with El Nino in the CGCMs, which further reduces the enhancement of the anomalous NPC.