Simulating Multi-Season Past Droughts
Abstract

Abstract Author: Dongxiao Zhang and Michael J. McPhaden

Abstract Title: Trends in the Pacific Shallow Overturning Circulation, Tropical Pacific SST and the Hydrological Cycle

Abstract: It is generally agreed that the persistent tropical Pacific SST cold anomalies, or La Niña-like state, has been the dominant driver of the droughts in the U.S. on decadal to multidecadal time scales. It is still uncertain what mechanisms generate persistent tropical SSTs and how predictable they are. Climate models have projected the trend of more precipitation in the tropics and drier conditions in the subtropics, a pattern can affect future U.S. drought conditions under global warming. Here we show a connection between the shallow meridional overturning circulation in the Pacific, the tropical Pacific SST and precipitation in two IPCC climate models, as well as in observations. The long term trends of the shallow circulation in the climate models, in both 20th century simulation and in A1B scenario runs, are significantly weaker than observed during the last half of 20th century. Much weaker trends are also found in the eastern tropical Pacific SST and precipitation in the models compared to observations. Our analysis suggests that the weakening trend of the shallow overturning circulation is related to the enhanced warming of the eastern tropical Pacific, relative to the rising of global mean temperature, and intensified tropical rainfall. This changes may involve positive feedbacks between ocean circulation, SST, and precipitation similar to that in the Pacific Decadal Oscillation. The current generation climate models may have underestimated the intensification of hydrological cycle associated with the global warming because they poorly represent the weakening trend in the Pacific shallow overturning circulation and associated changes in tropical SST and precipitation.