Hydroclimate and variability in the Great Lakes region as derived from the North American Regional Reanalysis

We investigated the seasonal and interannual variability of the moisture budget in the Great Lakes region of the United States using the North American Regional Reanalysis (NARR) data set from 1979 through 2007. The much higher spatial and temporal resolution and improved precipitation and land surface data assimilation of the NARR data set compared with its global counterparts enable more accurate depictions of the moisture budget and hydrological cycle in the Great Lakes region. The analyses reveal that in the past three decades except for two drought years, the evaporation over the region is insufficient to account for the total precipitation. Transport mechanisms supply additional moisture, with a net gain in moisture associated with meridional transport by southerly winds overcoming a net loss in moisture due to zonal transport by westerly winds. The interannual variability of the moisture deficit (the difference between evaporation and precipitation) is associated mainly with the interannual variability in the moisture flux convergence. These results highlight the critical importance of remote moisture sources and large-scale moisture transport to the hydrological cycle in the Great Lakes region. The trend analyses show an upward trend for evaporation that is consistent with warming over the region in all seasons during the NARR data period. Precipitation exhibits an increasing trend in spring and winter, with the largest increase in winter, but no clear trends in summer and autumn.