Causes of climate extremes-generating ocean states

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The winters of 2013/14 and early 2015 across the Northern hemisphere included chronic drought in California, extreme cold and snowy winters in northeast North America and severe floods in the U.K. Observational analyses and modeling have linked these disparate climate extremes to sea surface temperature (SST) anomalies. The SST anomalies in the tropical Indian and Pacific Oceans that have been invoked are distinct from those associated with El Nino or La Nina. The SST anomalies were warm in the Southern Indian Ocean and Western Tropical Pacific and neutral to cool around the maritime continent and central Equatorial Pacific Ocean. A recent analog to that state was the winter of 1990/91. A second example of an extremes-generating ocean state is that of persistent, multiyear cold conditions in the Central Equatorial Pacific Ocean. For the last century and a half for which SST records exists, extreme drought in western North America has been associated with such Tropical Pacific Ocean conditions. 1999 to 2002 was one such recent state. In contrast to El Nino and La Nina events, the ocean dynamics and thermodynamics of these two extremes-generating states have not been extensively studied. This project will conduct a detailed analysis of the ocean dynamic and thermodynamic processes that cause the oceans to adopt these SST patterns. The ocean states to be studied generate droughts, floods, extreme cold and snow across the globe, particularly in North America and Europe. The societal impacts of these events have been large. Improving their prediction and the understanding of their frequency of occurrence and intensity are key to enabling adaptation and disaster preparedness. Prediction requires understanding and the proposed work will advance understanding of the oceanic causes of the SST anomalies responsible for them, thus enabling advances in prediction and characterization of ocean-forced weather and climate extremes. The results of the study will improve our capability to model and forecast such hazards and events. Further, results generated in this work will readily be communicated to the communities that can advance efforts for prediction, societal planning and adaptation via lead investigator's close involvement in NOAA's Drought task force and the National Integrated Drought Information System's Task Force. The data created will be served via the International Research Institute of Columbia University's Data Library and be available as a research resource to the entire community.The project is centered on the analysis of the momentum and heat budgets of the upper oceans contained within three ocean reanalyses: the European Center for Medium Range Weather Forecasts' Ocean Reanalysis (1958 to present), the consortium for Estimating the Circulation and Climate of the Ocean (Version 4, 1993 to present) and the Geophysical Fluid Dynamics Laboratory's Ensemble Climate Data Assimilation system (1960 to present). The contributions of surface fluxes, vertical mixing and dynamical ocean heat convergence by the mean flow and transient eddies will be evaluated for the extremes generating states using standard numerical procedures. Dynamical and mixing contributions will be related to changes in upper ocean heat content and to changes in currents and vertical motion which in turn will be related to wind stress forcing. Guided by a series of hypotheses, the causes of the extremes-generating ocean states will be determined to the extent allowed by data accuracy and sample size limitation. Long term trends in the ocean reanalyses and surface fluxes, as well as ensembles of climate model simulations, will be examined for any evidence that human-driven climate change is altering the probability of extremes-generating ocean states and the physical reasons why. The project will provide a comprehensive assessment of the oceanographic and ocean-atmosphere causes of the ocean states associated with two important extremes-generating SST anomalies. This work, largely observationally-based, will determine the physics underlying oceanic causes of recent climate and weather extremes.