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Dependence of winter precipitation over Portugal on NAO and baroclinic wave activity

Ulbrich, U., Christoph, M., Pinto, J. G. and Corte‐Real, J. (1999) Dependence of winter precipitation over Portugal on NAO and baroclinic wave activity. International Journal of Climatology, 19 (4). pp. 379-390. ISSN 0899-8418

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To link to this article DOI: 10.1002/(SICI)1097-0088(19990330)19:4<379::AID-JOC357>3.0.CO;2-8

Abstract/Summary

The relationship between winter (DJF) rainfall over Portugal and the variable large scale circulation is addressed. It is shown that the poles of the sea level pressure (SLP) field variability associated with rainfall variability are shifted about 15° northward with respect to those used in standard definitions of the North Atlantic Oscillation (NAO). It is suggested that the influence of NAO on rainfall dominantly arises from the associated advection of humidity from the Atlantic Ocean. Rainfall is also related to different aspects of baroclinic wave activity, the variability of the latter quantity in turn being largely dependent on the NAO. A negative NAO index (leading to increased westerly surface geostrophic winds into Portugal) is associated with an increased number of deep (ps<980 hPa) surface lows over the central North Atlantic and of intermediate (980<ps<1000 hPa) surface lows over North-western Europe. It is suggested that these distant surface lows have no direct influence on local Portuguese precipitation, but rather contribute to advection at their southern flanks. The other aspect of baroclinic wave activity varying with the NAO is the mid-tropospheric storm track (defined by the 500 hPa bandpass-filtered geopotential height variance). A possible local influence of the storm track due to vertical motions ahead of the upper air troughs cannot be unambiguously separated from the effect of advection. A separate influence of local surface cyclones over the Iberian peninsula which may, for instance, arise from the large scale ascent of air, is revealed by the statistics: for a given advection, rainfall amounts for months with local cyclone cores over the considered region tend to exceed those without. Copyright © 1999 Royal Meteorological Society

Item Type:Article
Refereed:Yes
Divisions:No Reading authors. Back catalogue items
Faculty of Science > School of Mathematical and Physical Sciences > Department of Meteorology
ID Code:32835
Uncontrolled Keywords:precipitation; North Atlantic Oscillation; Portugal; surface low; storm track; baroclinic wave; EOF-analysis
Publisher:John Wiley & Sons

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