INTRODUCTION:
The reasons for the variability of the North American summertime circulation and rainfall are not well understood at the present time. The occurrence of extreme summertime drought in 1988, and spring/summer floods in 1993 over United States have however highlighted the range of variability of the North American summer climate, and spurred interest in investigating the causes of such extreme variability. Low-frequency variations in warm-season circulation, drought, and streamflow profoundly impact US agriculture and water resources management. An improved understanding of the involved forcing and mechanisms will advance the development of the warm-season hydroclimate forecasts. The primary focus of our research is to obtain an improved description of the climatological evolution and interannual variability of the northern summer season circulation and rainfall over the Americas.

PROJECT GOALS
Our goal is to provide a dynamics based understanding of the mechanism(s) controlling the circulation variability on the continental scale, and hydrologic based understanding of the mesoscale rainfall variability on the diurnal and seasonal time scales. Of particular interest, is the interaction between the large-scale circulation variability and the hydrologic cycle over the Mississippi basin, in part, through the modulation of the low-level jet that transports significant amounts of moisture from the Gulf of Mexico into the Great Plains.

METHODOLOGY
We propose to attain these research objectives using a strategy that includes: a) dynamical diagnosis of the recurrent interannual variability of the summertime circulation in the expanded PACS sector (that includes North America) using both the operational analyses and the reanalyzed data sets, b) diagnostic modeling to identify the mechanisms generating recurrent interannual variability in this domain that includes the eastern Pacific ITCZ, the SST cold-tongue, the Mexican and Central American monsoon regions, and the United States, and c) the diagnosis of the moisture budget from the regional Eta model's 3-hourly analyses and forecasts to provide an improved description of the diurnal and seasonal variability of moisture transports over North America, and the moisture source for the Mexican monsoon rainfall.

RESULTS AND ACCOMPLISHMENTS
Progress has been made in objectively extracting the structure of the US warm-season precipitation, Palmer drought index, and U.S. Geological Survey streamflow anomalies linked to the Pacific decadal SST variability modes during the 1950–90s. The important finding here is that the impact of decadal SST variability modes is comparable to ENSO’s over large continental sectors.

Diurnal cycles of precipitation and moisture flux from the eta model analysis reveal, perhaps, for the first time a coherent picture of the circulation over the Gulf of California and the Sierra Madre Occidental, strongly suggesting the role of local moisture sources.

FUTURE WORK
The externally forced perturbations to US hydroclimate can be generated in at least two ways: from modulation of moisture flux into the continent, and from the redistribution of precipitation due to stormtrack displacements. Future research will focus on the structure and causes of recurrent stationary and transient moisture flux variability, since fluxes provide a key link between circulation and precipitation, and highlight features of the low-level flow that are crucial for moisture transports.

PUBLICATIONS RESULTING FROM THIS RESEARCH:
Mathew Barlow (Ph.D., March ‘99): Climatological evolution and decadal variability of North

American warm season hydroclimate. UCAR postdoctoral fellow at International Research Inst. for Climate Prediction (IRI), LDEO, Columbia University, NY.

Barlow, M., S. Nigam, and E.H. Berbery, 1998: Evolution of the North American Monsoon System. J. Climate, 11, 2238-2257.

Nigam, S., M. Barlow, and E.H. Berbery, 1999: Pacific decadal SST variability: Impact on U.S. drought and streamflow. Eos, Transactions, American Geophysical Union, Vol. 80, 51 (Dec. 21), 621-625.

Barlow, M., S. Nigam, and E.H. Berbery, 2000: ENSO, Pacific decadal variability and U.S. summertime precipitation, drought, and streamflow. (Submitted to the J. Climate on May 7, 1999; tentatively accepted; submitted in final form in mid-February, 2000).

CONTACTS:

Sumant Nigam
nigam@atmos.umd.edu
phone: (301) 405 5381
fax: (301) 314 9482

Ernesto Hugo Berbery
berbery@atmos.umd.edu
phone: (301) 405 5351
fax: (301) 314 9482

INSTITUTION:

Department of Meteorology
University of Maryland
College Park, MD 20742-2425

http://metosrv2.umd.edu/~nigam

http://metosrv2.umd.edu/~berbery

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