| Description |
The main objectives of this study were to identify the leading propagating patterns of atmospheric variability over the Midwest, and to determine the relationships of these patterns with Midwest precipitation. Complex Hilbert empirical orthogonal function (HEOF) analysis was performed on daily mean 850-hPa horizontal moisture transport, 850-hPa temperature advection, jet relative frequency, and the difference between 850-hPa and 250-hPa vorticity advection. Atmospheric fields were derived from the 6-hourly NCEP-NCAR reanalysis on a year-round and within-season basis. Additionally, the HEOFs were phase-shifted to maximize the correlation between the real part of the score series and area-weighted power-transformed Midwest precipitation. In the year-round analysis, the leading HEOF of combined jet relative relative frequency and 850-hPa horizontal moisture transport captured the seasonal migration of the jet and attendant low-level circulation features. The second HEOF showed high jet relative frequency over the Midwest on the upstream side of a trough, and moisture transport from the Gulf of Mexico into the Midwest. The leading within-season HEOF of combined jet relative relative frequency and 850-hPa horizontal moisture transport showed a similar pattern in winter, spring, and fall. In all seasons, the monthly mean scores of the leading HEOF of combined jet relative relative frequency and 850-hPa horizontal moisture transport were better estimates of Midwest precipitation than the Pacific-North American pattern, North Atlantic Oscillation, and El Ni˜no-Southern Oscillation teleconnection indices. In addition, this study examined the relationship between the leading winter propagating patterns of variability and lake effect precipitation over the Great Lakes region. Here, the leading HEOF of combined jet relative relative frequency and 850-hPa horizontal moisture transport was phase-shifted to maximize the correlation between the real part and a lake effect precipitation fraction time series. The phase-shifted HEOF did not resolve the mesoscale features of lake effect snow, but did position the synoptic-scale circulation so that flow developed the expected northerly component over the Great Lakes. |
