Different Influences on Tropical Pacific SST Gradient from Natural Forcing and Anthropogenic Forcing

Several simulations under natural forcing, greenhouse gas forcing, and full forcing (both natural forcing and anthropogenic forcing) over the last 2000 years using the Community Earth System Model (CESM) were used to investigate the different changes of tropical Pacific sea surface temperature (SST) gradient and corresponding mechanisms. Under the full forcing, during the present warming period (PWP; 1901-1999), Pacific SST shows larger warming over the whole tropical Pacific region than the subtropical Pacific region. This pattern is composed by an El Niņo-like SST gradient due to the greenhouse gas forcing and a La Niņa-like SST gradient due to the natural forcing. Two sensitivity experiments, the PWP under greenhouse gas forcing and Medieval Warming Period (MWP; 751-1250) under natural forcing, were used to examine the mechanisms. The results showed that, under the greenhouse forcing, the larger warming over eastern tropical Pacific is induced by increased surface net heat flux mainly contributed by short-wave radiation and latent heat flux. Under the natural forcing, the larger warming over western tropical Pacific is induced by changes of vertical heat transportation caused by the surface wind anomalies.