Dominant Anomaly Patterns in the Near-Surface Baroclinicity and Accompanying Anomalies in the Atmosphere and Oceans. Part I: North Atlantic Basin

Headnote

ABSTRACT

Variability in the monthly mean flow and storm track in the North Atlantic basin is examined with a focus on the near-surface baroclinicity, B = B^sup x^i + B^sup y^j. Dominant patterns of anomalous B found from empirical orthogonal function (EOF) analyses generally show patterns of shift and changes in the strength of B. Composited anomalies in the monthly mean wind at various pressure levels based on the signals in the EOFs display robust accompanying anomalies in the mean flow up to 50 hPa in the winter and up to 100 hPa in other seasons. Anomalous eddy fields accompanying the anomalous B^sup x^ patterns exhibit, broadly speaking, structures anticipated from linear theories of baroclinic instabilities and suggest a tendency for anomalous wave fluxes to accelerate/decelerate the surface westerly accordingly. Atmospheric anomalies accompanying B^sup y^ anomalies have patterns different from those that accompany B^sup x^ anomalies but are as large as those found for B^sup x^. Anomalies in the sea surface temperature (SST) found for the anomalous patterns of B^sup x^ often show large values of small spatial scales along the Gulf Stream (GS), indicating that a meridional shift in the position of the GS and/or changes in the heat transport by the GS may be responsible for the anomalous B^sup x^ and concomitant tropospheric and lower-stratospheric anomalies. Anomalies in the net surface heat flux, SST in preceding months, and meridional eddy heat flux in the lower troposphere support this interpretation.

(ProQuest: ... denotes formulae omitted.)

1. Introduction

Baroclinicity in the lower atmosphere in classic theories of atmospheric stability is defined by the effect of the horizontal temperature gradient, in combination with the Coriolis force, on the vertical shear of the horizontal velocity and the vertical stability of the atmosphere (Charney 1947; Eady 1949). In its original form, the Eady maximum growth rate for baroclinic instability ßoRMax is defined by

...

in a zonally homogeneous steady mean state, where U is the mean zonal flow, /is the Coriolis parameter, and N is the Brunt-Väisälä frequency. Charney's formula is slightly different from the Eady's but still incorporates the same effects. Lindzen and Farrell (1980) first applied the Eady parameter to atmospheric data to successfully estimate the maximum growth rate of baroclinic instability in the troposphere....