The sensitivity of MBL clouds to ENSO and global warming - A regional model study

The atmosphere over the eastern subtropical Pacific Ocean is characterized by extensive stratocumulus decks, which undergo fairly large interannual variations connected with the ENSO cycle in the Pacific. Current comprehensive climate models (including the coupled ocean-atmosphere GCMs evaluated in the IPCC AR4 report) generally do a quite poor job in simulating either the mean cloudiness or the interannual cloudiness variations in this region. The fact that current models display such inadequacies is significant since the climate feedbacks associated with subtropical clouds play an important role in determining global climate sensitivity. This paper will report on analysis of cloud feedback processes in the eastern Pacific as simulated by the University of Hawaii’s IPRC Regional Atmospheric model (iRAM). In contrast to most GCMs the iRAM, when forced by realistic ocean temperatures and lateral boundary conditions, has been shown to simulate quite accurately the large-scale cloud climatology in the eastern Pacific region. In this project iRAM was run for an extensive hindcast run (1982-2009), a period for which we have detailed satellite data for comparison. Then the model was run for several global warming scenarios with boundary conditions appropriate for late 21st century conditions. The cloud feedbacks diagnosed in the global warming run will be compared with those evident in the interannual variations in the hindcast run. In particular, we will compare the cloud response to strong ENSO extremes such as the 1997/98 El Niño to the cloud response seen in the global warming scenarios.