This is one of 16 Rapid Response (RAPID) projects funded as the result of a Dear Colleague Letter (NSF 11-006) encouraging diagnostic analyses of climate model simulations prepared for the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5). Research conducted in these projects is expected to lead to more detailed model intercomparisons, better understanding of robust model behaviors, and better understanding and quantification of uncertainty in future climate simulations.
Analysis conducted in this project examines the veracity and robustness of the climate change simulations for the AR5 as part of the Coupled Model Intercomparison Project version 5 (CMIP5), particularly with respect to projected hydroclimatic changes in the water limited region of southwestern North America. Warming trends are already particularly pronounced in this region in the spring season, which is a critical issue for surface hydrology due to the sensitivity of snowpack to spring warming, and the relative lack of precipitation during this season. Therefore climate change at this time of year has a potentially dramatic effects on snow-fed rivers and on soil moisture during the start of the growing season. The projected hydrologic changes associated with these trends are one of the most pronounced and significant signals that appeared in an earlier generation of climate model simulations prepared for the IPCC Fourth Assessment Report (AR4), so it is important to examine CMIP5 simulations carefully to examine the hydroclimatic processes simulated in current model. The research also examines the simulation of summer rainfall in the region, and in this season simulations used for AR4 were not consistent from one model to another. Thus, the extent to which the new generation of models produces a consistent response of summertime rainfall in the southwest to greenhouse gas increases is considered. Model outputs to be examined include maximum and minimum temperature, focusing on the spring and summer seasons, surface fluxes in the spring and summer seasons, soil moisture, snowfall in mountainous regions in the winter and spring seasons, and pecipitation from winter through summer.
The broader impact of the project lies in its support of the IPCC AR5, which is intended to provide information on climate change and its consequences to decision makers worldwide. The hydroclimate of southwestern North America is of great societal interest given the large population and population growth in the region, as well as the importance of the region for agriculture and the sensitive ecosystems contained within it.
We have examined precipitation (P) and evapotranspiration (E) on land areas across southwestern North America using new (CMIP5) coupled climate model simulations. The project was motivated by results using the previous generation of coupled climate models, showing that the surface water budget (P-E) is projected to decline significantly in this semiarid region as the result of projected climate change -- i.e., the Southwest is projected to become drier. We were interested in (1) seeing if this result is reproducible in newer model simulations; (2) checking for consistency with observations of current hydrologic change (a project ongoing under separate support); (3) understanding the physical mechanisms for surface drying by considering the P and E terms separately, and considering the northern and southern parts of "southwestern North America" (corresponding to southwestern U.S. and northwestern Mexico). One of the principal hydrologic reasons for considering the U.S. and Mexico separately is that major rivers in the southwestern U.S. are snowmelt-dominated, whereas snowpack plays a minimal role in Mexico. Snowpack is projected to diminish significantly in climate model projections, a trend that is already seen in observations. We found that: drying trends, defined as negative trends in the difference P-E, are indeed found in CMIP5 projections for the 21st Century across southwestern North America in both the warm and cold seasons. However the relative importance in trends of P or E varies both geographically and (in the U.S. portion of the Southwest) seasonally. In the southwestern U.S., most of the trend in P-E occurs in the cold half of the year (December through May), associated with increasing evapotranspiration. This is consistent with ongoing observed trends in snowpack (downward) and temperature (upward). The projected cold season precipitation trend is very small. A small projected downward trend in summer precipitation adds somewhat to the overall annual decrease in P-E. In northwestern Mexico, the trend in P-E occurs year-round, associated with decreasing precipitation. Climate models project very large decreases in precipitation in northwestern Mexico -- much more pronounced than the precipitation trends projected for the southwestern U.S. These results are based on a limited sample of CMIP5 simulations. If the results hold up to additional analysis, there are important implications for water management across southwestern North America. On the U.S. side of the border, trends in P-E are driven by the E term, which is highly temperature sensitive. Water resource managers need to know that the temperature trend projections are considered to be more robust than precipitation projections (consistent with the observation that these trends are already seen in data), so confidence in the projected trend in P-E is relatively high, and management efforts should be aimed at minimizing evaporation from surface water bodies. On the Mexican side of the border, changes in P-E are largely driven by P, meaning that the projections imply that the annual supply of water from precipitation will decrease across a region that is already strongly water limited. Future improvements in precipitation simulation will clarify this discouraging projection.