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Role of Low Clouds in Summertime Atmosphere–Ocean Interactions over the North Pacific

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  • 1 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
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Abstract

The summer-to-summer variability of the areal extent of marine stratiform cloudiness (MSC; stratus, stratocumulus, and fog) over the North Pacific is examined for the period of record 1952–92 using a dataset based on surface observations. Variability is largest in two regions: the central and western Pacific along 35°N coincident with a strong meridional gradient in climatological MSC amount, and the eastern Pacific near 15°N downstream of the persistent stratocumulus deck off Baja California. The MSC amount in both regions tends to be negatively correlated with local sea surface temperature (SST), suggestive of a positive cloud feedback on SST. The MSC amounts in the two regions also tend to be negatively correlated by virtue of their relationship to the basin-wide sea level pressure (SLP) field: a strengthening of the seasonal mean subtropical anticyclone is accompanied by increased cloudiness in the trade wind regime and decreased cloudiness in the southerly flow farther toward the west. These relationships are reflected in the leading modes derived from empirical orthogonal function analysis and singular value decomposition analysis of the MSC, SST, and SLP fields.

From the 1950s to the 1980s, summertime MSC amounts increased in the central and western Pacific and decreased in the trade wind region, while SST exhibited the opposite tendencies. Although these trends contributed to the relationships described above, similar patterns are obtained when the analysis is performed on 1-yr difference fields (e.g., 1953 minus 1952, 1954 minus 1953, etc.). Hence, it appears that MSC plays an important role in atmosphere–ocean coupling over the North Pacific during the summer season when latent and sensible heat fluxes are not as dominant and the coupling between atmospheric circulation and SST is not as strong as in winter.

* Current affiliation: National Center for Atmospheric Research, Boulder, Colorado.

Corresponding author address: Joel Norris, Advanced Studies Program, National Center for Atmospheric Research, P.O. 3000, Boulder, CO 80307-3000.

Email: jnorris@ucar.edu

Abstract

The summer-to-summer variability of the areal extent of marine stratiform cloudiness (MSC; stratus, stratocumulus, and fog) over the North Pacific is examined for the period of record 1952–92 using a dataset based on surface observations. Variability is largest in two regions: the central and western Pacific along 35°N coincident with a strong meridional gradient in climatological MSC amount, and the eastern Pacific near 15°N downstream of the persistent stratocumulus deck off Baja California. The MSC amount in both regions tends to be negatively correlated with local sea surface temperature (SST), suggestive of a positive cloud feedback on SST. The MSC amounts in the two regions also tend to be negatively correlated by virtue of their relationship to the basin-wide sea level pressure (SLP) field: a strengthening of the seasonal mean subtropical anticyclone is accompanied by increased cloudiness in the trade wind regime and decreased cloudiness in the southerly flow farther toward the west. These relationships are reflected in the leading modes derived from empirical orthogonal function analysis and singular value decomposition analysis of the MSC, SST, and SLP fields.

From the 1950s to the 1980s, summertime MSC amounts increased in the central and western Pacific and decreased in the trade wind region, while SST exhibited the opposite tendencies. Although these trends contributed to the relationships described above, similar patterns are obtained when the analysis is performed on 1-yr difference fields (e.g., 1953 minus 1952, 1954 minus 1953, etc.). Hence, it appears that MSC plays an important role in atmosphere–ocean coupling over the North Pacific during the summer season when latent and sensible heat fluxes are not as dominant and the coupling between atmospheric circulation and SST is not as strong as in winter.

* Current affiliation: National Center for Atmospheric Research, Boulder, Colorado.

Corresponding author address: Joel Norris, Advanced Studies Program, National Center for Atmospheric Research, P.O. 3000, Boulder, CO 80307-3000.

Email: jnorris@ucar.edu

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