Modeling work has shown that persistent droughts in Southwestern North America are forced by multiyear La Niñas in the tropical Pacific Ocean with a warm subtropical North Atlantic also playing a role in some cases. These persistent droughts, including the severe one that began after the 1997/98 El Niño, place colossal strain on regional water resources, impact agriculture, fires, ecosystems and the regional economy leading to billions of dollars in expenses in disaster relief. In addition the most recent generation of Intergovernmental Panel on Climate Change (IPCC) model climate projections (the Assessment Report 4, AR4) robustly predicts that Southwestern North America will become more arid as part of a general subtropical drying caused by an intensifying hydrological cycle and a poleward shift of the Hadley Cell border and mid-latitude storm tracks. This drying is projected to become comparable in amplitude to naturally occurring drought by mid-century. Prediction of hydroclimate variability and change on the interannual to decadal timescale, if skillful, would allow advance planning across water-sensitive parts of the region’s economic and social systems.
In a collaborative effort with the NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL) we propose to 1) examine the mechanisms and predictability of tropical SST-forced drought on interannual to decadal timescales and 2) examine anthropogenic-induced regional drying in models and observations to determine its mechanisms, if this is occurring and when it provides a useful predictable signal that needs to be adapted to. This work will rely on the GFDL Climate Model 2.1 (CM2.1) which realistically produces multiyear La Niñas that force drought in Southwestern North America. The predictability of these will be examined in a perfect model environment allowing assessment of potential predictability with uncertainty estimates. Similar predictability experiments will be performed for multidecadal changes in tropical Pacific climate within CM2.1 that appear analogous to the 1976/77 climate shift. To determine actual predictability we will examine initialized (from the observed atmosphere-ocean state) climate change projections that will be performed as part of IPCC AR5. These experiments will include changes in radiative forcing and include hindcasts and predictions of the next years to decades. They will be examined for actual predictive skill that comes from the initial conditions as well as the relative amplitudes and character of natural variability and forced climate change.