Role of the Atmospheric Moisture Budget in Defining the Precipitation Seasonality of the Great Lakes Region

Precipitation in the Great Lakes region has a distinct seasonal cycle that peaks in early summer, followed by a decline in August and a secondary peak in September. This seasonality is often not captured by models, which necessitates understanding of the driving mechanisms to ascertain the model biases. This study analyzes the atmospheric moisture budget using reanalysis datasets to assess the role of regional evapotranspiration and moisture influx from remote origins in defining the precipitation seasonality, and to understand how the Great Lakes modulate spatial patterns and magnitudes of these components. Specifically, the land–water thermal contrast yields large seasonal variations in the evaporative fluxes and creates distinctive localized spatial patterns of moisture flux divergence. We find considerable month-to-month variations in both evapotranspiration and the net moisture transport through the boundaries, where they play a cooperative (contrasting) role in amplifying (dampening) the moisture content available for precipitation and total precipitable water. Our seasonal analysis suggests that the misrepresentation of the budget quantities in models, for example, in simulation of moisture transport processes and parameterization schemes, can result in an anomalous precipitation behavior and, in some cases, violation of the atmospheric moisture mass balance, resulting in large residual magnitudes. We also identify conspicuous differences in the representation of moisture budget components in the various reanalyses, which can alter their representation of the regional hydroclimates.