The variation of warm season precipitation over the Northern Plains: the spring-fall mode and the contribution of the Great Plains low-level jet
Both the seasonal and interannual variations of the warm season precipitation over the Great Plains are crucial to agriculture. The largest amount of season precipitation occurs during late spring over the Northern Plains and can be well represented by the spring-fall mode through the empirical orthogonal function (EOF) analysis. A distinct precipitation center with May maximum that covers Iowa, Missouri, Kansas, and South Dakota is delineated in the spring-fall mode. The spring-fall mode of divergence circulation and water vapor flux forms a vertical reversal of structure with convergence at lower-levels and divergence at upper-levels accompanied with the water vapor convergence centered over the Northern Plains. The precipitation center is maintained by the divergence circulation and the water vapor flux according to their consistent locations revealed in the spring-fall mode. An upper-level trough established over the central-western United States in early spring deepens and leads to a baroclinic (westward-tilting) structure of the atmosphere in late spring. Embedded in this unstable environment, the Great Plains low-level jet (GPLLJ) associated with the convective activity is enhanced and contributes more than 75% to the May-June precipitation over the Northern Plains. Due to different precipitation behaviors, the rainy season over the central United States is separated into May-June and July-August to determine the cause of the interannual variation of the Northern Plains rainfall. In both seasons, a teleconnectional wave train pattern over the Pacific-North America region appears in the EOF analysis and the correlation map associated with the year-to-year variation of precipitation and the occurrence frequency of coupled type (C-type) LLJ. The Fourier scale separation is introduced to isolate the short-wave regime (wavenumbers 4-25) of the anomalous pattern. The short-wave regime streamfunction shows a well-organized North Pacific short-wave train connecting to North America in a vertically uniform structure. The correlation coefficient of the short-wave train and the C-type LLJ activity are highly significant at 0.81 during both seasons. Based upon the close relationship between C-type LLJ and the Great Plains precipitation, it is indicated that C-type LLJ plays a role as the downscale process in connecting the teleconnectional short-wave train to the precipitation anomalies. In addition, the North Atlantic Oscillation (NAO) during July-August is found to be highly correlated with the occurrence frequency of C-type LLJ (with a correlation coefficient of 0.7) through a possible seesaw pattern between central North America and Greenland. The 1993 summer flood is an example of the combined effect from the coexistence of extreme short-wave train and NAO phases.