California is in its fourth year of an epic drought, thought to be caused primarily by a drop in winter precipitation. In this seminar, Professor Richard Seager will review the key mechanisms underlying the drought and discuss whether the El Nino event this year could spell an end to it.
Abstract
California is in its fourth year of an epic drought. The prime cause of the drought is a drop in winter precipitation summing to more than a year of missing precipitation. Atmosphere modeling strongly suggests that the persistent west coast ridge that blocked Pacific storms and caused the precipitation drop was forced by a combination of SST anomalies in the Indian and tropical Pacific Oceans, of presumed natural origin. In addition to the precipitation drop, California was warm during much of the drought and, from the surface moisture perspective, this was responsible for about a quarter of the drought with the long term warming trend accounting for at least some of that. The drought is not representative of expected human-driven climate change in California. Models project the California wet season to become shorter and sharper. The midwinter projected wetting is partly a simple thermodynamic case of ‘wet getting wetter’ but is also importantly influenced by a shift to more southwesterly prevailing winds at the west coast. This wind shift occurs as part of a change in the intermediate scale stationary wave field that is itself a response to accelerated subtropical upper troposphere westerlies. Modeling and analysis suggests, however, that the midwinter wetting of California is likely overestimated by the CMIP5 models. Finally the implications of the current El Nino for California precipitation in the upcoming 2015/16 winter will be discussed and whether it has the potential to end the drought.
Biography
Richard Seager is currently the Palisades Geophysical Institute/Lamont Research Professor at the Lamont Doherty Earth Observatory of Columbia University in Palisades, New York. He gained his undergraduate degree at Liverpool University in England and came to the United States in 1983 as a graduate student at Columbia. Richard completed his PhD in 1990 under the supervision of Professor Mark Cane and Dr. Steve Zebiak. His project involved using tropical atmosphere and ocean models to understand key features of the tropical climate. In 1991-2 he completed a postdoctoral appointment at the University of Washington before returning to Lamont, this time to stay.
Throughout his career he has used numerical models, observations and proxy reconstructions of past climates to understand the physical mechanisms responsible for climate variability and change on seasonal to glacial-interglacial timescales. He has a particular interest in how the variation of the tropical atmosphere-ocean system organize climate on a global scale. Richard has also studied the reasons why the mean climate of the planet is the way it is and why Europe has mild winters, why there is a tropical Pacific warm pool, why there are subtropical anticyclones etc. His recent work has focused on the mechanisms of persistent North American drought and its relation to tropical Pacific and tropical Atlantic Ocean temperature variations. This work has led him into studies of Medieval megadroughts in the American West and studies of the hydrological future of the West.