A Time-Varying Greenhouse Warming Pattern and the Tropical–Extratropical Circulation Linkage in the Pacific Ocean

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Abstract
Recently, Cai and Whetton provided modeling evidence that the greenhouse warming pattern has undergone a systematic change from a pattern with maximum warming in subtropical and mid- to high latitudes to one that is El Niño–like from the 1960s onward. They suggest that the mechanism for the change is the transmission of the large extratropical warming to the equatorial east Pacific via modeled tropical–extratropical Pacific circulation pathways. The present study addresses several associated issues. How is the systematic change manifested in empirical orthogonal functions? How do the meridional heat balances respond to the systematic change? Does the proposed mechanism operate in the absence of greenhouse forcing? It is shown that the warming signals are represented by two empirical orthogonal functions, the first of these reflecting a long-term trend in the period considered, and the second showing the change in trend from the 1960s onward. Consistent with the time-varying warming pattern, the relative importance of various heat exchange processes in the tropical Pacific Ocean also undergoes systematic changes. Prior to the 1960s, advective heat flux from the extratropics is the heat source for warming the tropical subthermocline (80–270 m). This subthermocline warming weakens the thermocline and reduces the diffusive heat transfer down through the subthermocline. From the 1960s onward, as substantial subthermocline warming upwells, the El Niño–like pattern develops, strengthening the thermocline; consequently, the downward diffusive heat transfer to the subthermocline enhances reversing the trend prior to the 1960s, and eventually becomes the dominant source for subthermocline heating. The dynamical process, whereby extratropical anomalies are transmitted to the Tropics, operates in a run without external forcing, in association with a mode of ENSO-like interdecadal oscillation. In the equatorial central-eastern Pacific, the associated anomalies upwell and initiate an ocean–atmosphere feedback that changes the equatorial west–east sea surface temperature gradient and easterly winds, reinforcing the upwelled anomalies. The commonality of the modeled interannual ENSO cycles and the interdecadal ENSO-like variability is also discussed.