Friday, 13 November 2009: 12:00 AM
The Intra-Americas Sea low-level jet (IA-LLJ) has been studied mainly for the summer and winter season. In contrast, the spring conditions have been less studied. Here we analyze the boreal spring variability of the IA-LLJ and its relation with precipitation and tornadic activity in the region of the lower Mississippi, Tennessee and Ohio river basins (MORB). The main mode of variability of the spring IA-LLJ is obtained from the combined empirical orthogonal function (EOF) of the zonal and meridional wind at 925-hPa. The first EOF of the IA-LLJ is a strengthening of the climatological flow with stronger easterlies in the Caribbean and stronger southeasterlies in the Gulf of Mexico. The first mode of variability of the IA-LLJ is related mainly to the Pacific North American (PNA) pattern and to an increase in precipitation over the eastern half of North America as the moisture fluxes associated with the IA-LLJ increase. The precipitation in the MORB region is mostly associated with changes in the IA-LLJ intensity as the PNA modulates the pressure in the southeast of the US. Tornadic activity in nine states spanning the MORB region is also significantly correlated with the PNA index in March. Nonetheless, the PDO and the Niño indices are also highly correlated with tornadic activity. Among the environmental factors that influence tornadic activity are a southwesterly wind shear, dry transients at the mid-troposphere, moist transients at low levels, and an increase in convective available potential energy (CAPE).
Center Affiliation: RSMAS
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