GFDL - Geophysical Fluid Dynamics Laboratory

A link between the hiatus in global warming and North American drought

March 13th, 2015

Thomas L. Delworth, Fanrong Zeng, Anthony Rosati, Gabriel Vecchi, and Andrew Wittenberg . Journal of Climate. DOI: 10.1175/JCLI-D-14-00616.1

Summary

The authors use three GFDL climate models (CM2.1, CM2.5_FLOR, CM2.5_FLOR_FA) to study the mechanisms behind the hiatus in global warming over the last decade and their possible relationship to southwestern U.S. drought. This study suggests that a majority of the drought in the southwestern U.S. over the last decade is the result of persistent anomalous wind conditions in the tropical Pacific, and is likely due to natural variability.

By modifying the winds in the models to reproduce the observed winds only in the tropical Pacific, the authors show that these changing winds can drive both the hiatus in global warming and drought conditions over the southwestern U.S. (see figure). If the observed wind changes are a result of natural variability, this suggests that both the hiatus and the drought over the southwestern U.S. are the product of natural variability of the climate system. If, however, the observed wind changes are a response to changing radiative forcings, this would have important implications for future climate in the southwestern US.

Additional experiments show that southwestern drought over the next decade will be strongly influenced by the behavior of the winds in the tropical Pacific. Further, idealized experiments show that this mechanism for producing a hiatus is self-limiting, and only persists for a decade or two in our models.

Potential future changes in water resources are an immense societal challenge. Improving our ability to understand observed changes is a vital contribution towards gaining confidence in our ability to reliably project future changes. The knowledge that a majority of the observed drought is likely due to natural variability could have important implications for water resource and infrastructure planning.

 The figure above shows changes in precipitation, calculated as annual mean precipitation for the period 2002-2012 minus the annual mean precipitation for the period 1979-2000. The units are mm of precipitation per day. Shown in the upper left (panel a) are the results from observations, showing relatively drier conditions (yellow to brown shading) over much of the western U.S. for the 2002-2012 period. The other three panels (b-d) show results from various GFDL models when we insert into the models the unusually strong easterly winds that were observed in the tropical Pacific over the 2002-2012 period. All three models reproduce the observed drying over the western U.S. in response to the observed stronger easterly winds in the tropical Pacific. These same simulations also reproduce the hiatus in global warming in response to these wind changes.
The figure above shows changes in precipitation, calculated as annual mean precipitation for the period 2002-2012 minus the annual mean precipitation for the period 1979-2000. The units are mm of precipitation per day. Shown in the upper left (panel a) are the results from observations, showing relatively drier conditions (yellow to brown shading) over much of the western U.S. for the 2002-2012 period. The other three panels (b-d) show results from various GFDL models when we insert into the models the unusually strong easterly winds that were observed in the tropical Pacific over the 2002-2012 period. All three models reproduce the observed drying over the western U.S. in response to the observed stronger easterly winds in the tropical Pacific. These same simulations also reproduce the hiatus in global warming in response to these wind changes.