Precipitation and circulation covariability in the extratropics

Abstract

Extratropical precipitation is primarily produced by cold and warm fronts associated with surface cyclones and upper-level troughs. The growth of these midlatitude storms is partially controlled by the dry baroclinicity of the troposphere, which in turn can be roughly quantified by the intensity of the upper-level zonal flow. Orographic rainfall, an important component of the precipitation in several midlatitude regions, is also partially determined by the intensity of the cross-mountain midlevel winds. Thus, at monthly and longer time scales, variations of precipitation and zonal flow aloft (as well as wind shear) at a given location should exhibit some degree of coherence. In this work the local covariability of these variables is documented over intermonthly and interannual time scales, using global precipitation products and atmospheric reanalysis from 1979 to 2004. The spatial correspondence between the precipitation and two indices of synoptic activity in the extratropics is also documented.

The local correlation (r(o)) between monthly anomalies of precipitation and upper-level (300 hPa) zonal flow varies in space, from moderately and even highly significant values (r(o) similar to 0.3 to 0.7) over the midlatitude oceans to near zero over the interior of continental areas. Broadly similar results are found when considering the monthly variance of the high-pass-filtered meridional wind (an index of eddy activity) or the midlevel Eady growth rate. The local correlation map between precipitation and low-level (850 hPa) zonal flow is similar to its upper-level counterpart, but the correlations over open ocean are somewhat weaker, while orographic effects show up more clearly. The correlations are positive and large upstream of the major north-south-oriented mountain ranges, as strong westerlies promote upslope rain in addition to storm-related precipitation. In contrast, the correlation tends to be negative downstream of the ranges, as strong westerlies enhance the rain shadow effects over the lee side.