Zonal gradient of the winter sea level atmospheric pressure at 50°N: An indicator of atmospheric forcing of North Pacific surface conditions

Abstract

The Aleutian Low dominates the winter atmospheric circulation of the midlatitude North Pacific. The long‐term average of the lowest value of the winter (December, January, and February) mean sea level pressure, the AL, is located near 50°N, 177°E. Anomalies in the winter atmospheric circulation are primarily reflected in the magnitude and longitude of the lowest sea level pressure and also in the zonal gradient of sea level pressure at 50°N. West of the AL the zonal gradient is negative, indicating a northerly component of the geostrophic wind. East of the AL the zonal gradient is positive, indicating a southerly component of the geostrophic wind. Because of their pertinence to forcing of near‐surface ocean conditions, the zonal gradients at 50°N for the winters of 1947–1990 have been analyzed, and anomalous distributions have been identified. In this series, two distinct gradient patterns can be recognized. In the first pattern, proceeding eastward from the minimum near 140°E, the gradient rises monotonically to its maximum near 140°W. The magnitudes of the northerly and southerly components of the geostrophic winds are at their maximum at 140°E and 140°W longitudes, respectively. In the second group, again proceeding eastward from the minimum near 140°E, the gradient first rises to a secondary maximum, usually east of the AL and then declines to a secondary minimum before rising again to its maximum value near 130°W. In terms of the geostrophic wind, proceeding eastward from the longitude of the AL, the southerly component first increases then decreases and, in extreme cases, may reverse direction to a northerly component before increasing again to its maximum near the eastern boundary of the ocean. Results of model studies point to a possible linkage between patterns of the zonal gradient and tropical disturbances. To gain an appreciation of how an anomalous atmospheric circulation may affect near‐surface ocean conditions, the characteristic features of the zonal pressure gradient are examined and compared to previous studies. These include the effects on surface currents, on the turbulent flux of heat across the sea surface, and on the precipitation over the northwestern United States, British Columbia, and Alaska. The comparisons show that in the absence of direct observations, the zonal gradient of the mean winter (December, January, and February) sea level pressure at 50°N can be used to infer the consequences of an anomalous atmospheric circulation on ocean conditions in the high midlatitudes of the North Pacific.