Current Highlight

Advances in the Americas Drought Predictability Research

(1) Hydroclimate trends in Southwest U.S.
A recent study, jointly supported by NOAA and NSF, examines how the hydroclimate in the southwest United States will change in the 21st century based on multiple climate models participating in the IPCC Fourth Assessment Report (AR4). It is found that 18 out of the 19 models predict a drier climate for the southwest U.S (see Fig.1). Significantly, the projected trend in "precipitation minus evaporation", an indicator of drought potential, implies that droughts as severe as the 1930's Dust Bowl drought can become common occurrences in this region as early as in the next few decades (Seager et al., 2007). The drying trend is found to be driven by both the change in atmospheric moisture content and a change in atmospheric circulation pattern under global warming. In future climate, rain-producing weather storms that normally travel through the southwest U.S. will shift northward, making the region drier. The reason for this changing weather pattern in the warmer climate remains to be investigated.

Fig. 1 The trend in "precipitation minus evaporation" for the southwest U.S. simulated by the climate models for IPCC AR4. From Seager et al. (2007)

(2) Warm Season Predictability of Great Plains Hydroclimate
The role of low level jets in warm season precipitation variability over the Great Plains has recently been highlighted. A deficit of precipitation over central US during the warm season is linked to reduced moisture fluxes from the Great Plains Low-Level Jet (GPLLJ) and the Caribbean Low Level Jet (CLLJ; Weaver and Nigam, 2007) . In turn, GPLLJ variability is linked to coherent, large-scale, upper level height patterns over the Pacific, and Atlantic basins; while for the CLLJ the variability of the Intra America Sea is of particular relevance. Concerning precipitation-land-surface relationships, surface-water balance over the region shows that model evapotranspiration over the region is being overestimated (Ruiz-Barradas and Nigam, 2007). This supports the idea that external moisture fluxes are of primary importance for precipitation variability in contrast to the local recycling of precipitation.

(3) Precipitation trends in Carribean/Central-America region
Recent work showed that in eastern equatorial South America the leading mechanism for drought is the upped-ante mechanism, of general importance in regions at the margin of convective zones. This, together with anthropogenic change, could play an important role in the substantial negative precipitation trends found in the Carribean/Central-America region in summer in both land station (approximately 50 year records) and satellite (since 1979) observations. An analytic prototype of convective margins for tropical land regions subject to inflow from adjacent ocean regions has recently been developed (Lintner and Neelin, 2007). The approach yields an expression for the location of the convective margin from which it is straightforward to deduce how climate perturbations impact the margin. The simple model shows the interplay of inflow from a nonconvecting ocean region, continental heating and associated low-level convergence, and the location of the onset of convection as moisture reaches a sufficiently high value. Observational analysis for ENSO teleconnections to equatorial South America indicates the usefulness of the prototype for situations where the 1-D inflow is a reasonable assumption.

References:
Lintner BR and JD Neelin, 2007: A prototype for convective margin shifts. Geophys. Res. Lett., Vol. 34, No. 5, L05812

Ruiz-Barradas, A., and S. Nigam, 2007: Terrestrial Water Balance over the Great Plains in Regional and Global Reanalyses and Model Simulations. In preparation.

Seager, R., M. Ting, I. Held, Y. Kushnir, J. Lu, G. Vecchi, H.-P. Huang, N. Harnik, A. Leetmaa, N.-C. Lau, C. Li, J. Velez, N. Naik, 2007, Model projections of an imminent transition to a more arid climate in southwestern North America, Science, 316, 1181-1184

Weaver, S., and S. Nigam, 2007: Variability of the Great Plains Low-Level Jet: Large Scale Circulation Context and Hydroclimate Impacts. Journal of Climate. In press.

First VOCALS Modeling Workshop

Improved understanding of intraseasonal variability in the tropical northeast Pacific

Development of an experimental seasonal hydrologic prediction system for the continental U.S.

Detecting changes in water storage using GRACE data