A Climatology of the Aleutian High

Abstract

Three global datasets are used to investigate climatological properties of the high pressure system commonly found in the boreal winter stratosphere over the Aleutian Islands. Based on a detailed examination of 10 years (1985?1994) of data from the European Centre for Medium-Range Weather Forecasts in the layer 250?10 hpa, the following definition of the ?Aleutian High? is proposed: 1) 10-hPa heights exceeding 30.8 km in the sector 40°?80°N, 120°E?100°W, 2) during 1 October?31 March, 3) with areal extent greater than 50° long ? 10° lat., 4) with relative vorticity of less than ?2.5 ? 10?5 s?1, and 5) lasting at least 5 days. More than 60% of days during December, January, and February satisfied these criteria. The 711 total days were averaged together, yielding an Aleutian High Composite (AHC). These ECMWF dates are used to create an AHC with data from the National Centers for Environmental Prediction (formerly the National Meteorological Center.) The definition is applied (with slight modification) to the data from the Limb Infrared Monitor of the Stratosphere experiment, allowing extension into the mesosphere. The AHC structures of geopotential height, temperature, winds, and potential vorticity are examined in detail, and differences among the composites are discussed. The Aleutian High and polar vortex constitute a ridge/trough pair that tilts westward with altitude from the surface into the mesosphere. The geopotential height difference between the Aleutian high and the polar vortex maximizes at ?3 km near the stratopause. Warm and cold temperature perturbations underly these features to the west, giving rise to a poleward heat flux. Axes of high heights and warm air trend continuously equatorward and downward into the subtropical western Pacific. During much of the boreal winter, stratospheric and mesospheric flows are highly zonally asymmetric. At 10 hPa the AHC structure shows zonal eastward flows of less than 10 m s?1 near 180°, but greater than 40 m s?1 near 80°E, with ?20 m s?1 northward flow near 130°E and southward flow near 100°W. Parcel descent rates exceeding 2 km day?1 are diagnosed ?60° upstream of the warm anomaly. The Aleutian High thus helps determine preferred longitude bands of winter subsidence and of material exchange between the subtropics and extratropics.