Climatology of the moisture budget in the Great Lakes Region as viewed from the North American Regional Reanalysis

Moisture budget in the Great Lakes region and its seasonal and interannual variability were analyzed using the North America Regional Reanalysis (NARR). The NARR data set, which has much higher spatial and temporal resolution and was derived based on more sophisticated land surface model and advanced data assimilation algorithm than its global counterparts, enables more accurate depictions of moisture budget components and their variability. In all but three years during the 28-year (1979-2007) NARR data period, the regional mean annual evaporation is found to be insufficient to account for the mean annual precipitation and its variability. The additional moisture comes from transport mostly form south/southwest into the region. Majority of this moisture influx occur in the lowest 3000 m of the atmosphere and the heights of the maximum influx, which appear below 1000 m, coincide with the typical heights of maximum southerly low-level jets. The interannual variability of the moisture deficit, which is dominated by variability of precipitation, can be largely explained by similar variability of the moisture flux convergence. This points to the critical importance of accurate simulation of large-scale moisture transport, in addition to local hydrological cycle, for accurate weather and climate predictions for the Great Lakes region. In all seasons, the evaporation exhibits an upward trend that is consistent with a warming trend. In winter and spring, little or no trend is seen in the moisture flux divergence, but the precipitation displays an increasing trend, consistent with the increase in evaporation. In summer and autumn, moisture flux convergence exhibits a sharp downward trend, which compensate with the increase in evaporation, leaving only a weak downward trend in precipitation.