A Link Between the Hiatus in Global Warming and North American Drought

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Monday, 5 January 2015: 11:45 AM
122BC (Phoenix Convention Center - West and North Buildings)
Thomas L. Delworth, NOAA/GFDL, Princeton, NJ; and F. Zeng, A. Rosati, G. Vecchi, and A. T. Wittenberg

Portions of western North America have experienced prolonged, severe drought over the last decade, with significant impacts across a range of sectors. This drought has occurred at the same time as the so-called global warming hiatus a decade long period with little increase in global mean surface temperature. Using simulations with medium and high-resolution climate models, as well as observational analyses, we find a strong link between the hiatus and recent decadal-scale drought over North America, with the connection arising from the impact of anomalously strong easterly winds on the tropical Pacific Ocean. These anomalous easterly winds induce enhanced upwelling of cold sub-surface water in the eastern Pacific. This relatively cold water creates a broad area of negative sea surface temperature anomalies over the eastern Pacific. These SST anomalies, in turn, induce a global scale atmospheric response, including atmospheric ridging in the mid troposphere extending from the central North Pacific through the western and southern U.S. This ridging systematically reduces the likelihood of precipitation over western North America. In our simulations the increased tropical easterly winds, in association with changing radiative forcing, increase the likelihood of a decade with a 15% or more reduction in precipitation from 3% to 46%, with most of the increased likelihood coming from the enhanced winds.

If the enhanced tropical easterly winds are the result of natural variability, this implies that the recent decadal drought over portions of western North America is also a result of natural climate variability. The drought should recede, at least temporarily, if the persistent easterly winds in the tropical Pacific return to conditions observed in prior decades. It is critical to better understand the physical processes that have led to the sustained increase in tropical easterly winds over the last decade or so. Such understanding may help to quantify the likelihood of continued drought.