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On the Relationships among Low-Cloud Structure, Sea Surface Temperature, and Atmospheric Circulation in the Summertime Northeast Pacific

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

The long-term record of observations from Ocean Weather Station (OWS) November (N), which operated at 30°N, 140°W from 1949 to 1974, is analyzed to document the relationships among boundary layer cloud structure, sea surface temperatures (SSTs), and atmospheric circulation. During the oceanic summer season, June through September, OWS N lay in the steady trade wind flow of the northeast Pacific. Boundary layer air parcels, which pass through the location of N, are typically in transition from the solid stratus or stratocumulus of the North Pacific to trade cumulus that is characteristic of the subtropics. Cloud observations indicate that low-cloud amount is high, averaging 70%, despite the absence of a well-mixed boundary layer. Low-cloud type code 8, cumulus and stratocumulus with bases at different levels, is the most frequently reported cloud type at all hours of the day. These observations suggest that along the stratus to trade cumulus transition, high cloud amount can exist long after the boundary layer ceases to be well mixed.

An analysis of summertime interannual variability suggests that low-cloud amount near ship N is better correlated with SST and upper air temperatures 24–30 h upwind than with the local SST and upper air temperature. This nonlocal relationship between boundary layer cloudiness and environmental parameters suggests that the Lagrangian histories of boundary layer air parcels must be considered for the accurate prediction of boundary layer cloudiness. These nonlocal relationships may explain the apparent propagation of SST and cloudiness anomalies along a Lagrangian trajectory.

On an interannual timescale, low cloud amount at N is also correlated with many large-scale variables associated with atmospheric circulation, such as temperature advection, the strength of the subtropical high, surface wind speeds, and surface wind steadiness. These multiple relationships imply a more complex picture than a simple relationship between boundary layer cloudiness and SST. In particular, variations in atmospheric circulation associated with the surface wind patterns may play an important role in modulating both boundary layer cloudiness and SST.

Abstract

The long-term record of observations from Ocean Weather Station (OWS) November (N), which operated at 30°N, 140°W from 1949 to 1974, is analyzed to document the relationships among boundary layer cloud structure, sea surface temperatures (SSTs), and atmospheric circulation. During the oceanic summer season, June through September, OWS N lay in the steady trade wind flow of the northeast Pacific. Boundary layer air parcels, which pass through the location of N, are typically in transition from the solid stratus or stratocumulus of the North Pacific to trade cumulus that is characteristic of the subtropics. Cloud observations indicate that low-cloud amount is high, averaging 70%, despite the absence of a well-mixed boundary layer. Low-cloud type code 8, cumulus and stratocumulus with bases at different levels, is the most frequently reported cloud type at all hours of the day. These observations suggest that along the stratus to trade cumulus transition, high cloud amount can exist long after the boundary layer ceases to be well mixed.

An analysis of summertime interannual variability suggests that low-cloud amount near ship N is better correlated with SST and upper air temperatures 24–30 h upwind than with the local SST and upper air temperature. This nonlocal relationship between boundary layer cloudiness and environmental parameters suggests that the Lagrangian histories of boundary layer air parcels must be considered for the accurate prediction of boundary layer cloudiness. These nonlocal relationships may explain the apparent propagation of SST and cloudiness anomalies along a Lagrangian trajectory.

On an interannual timescale, low cloud amount at N is also correlated with many large-scale variables associated with atmospheric circulation, such as temperature advection, the strength of the subtropical high, surface wind speeds, and surface wind steadiness. These multiple relationships imply a more complex picture than a simple relationship between boundary layer cloudiness and SST. In particular, variations in atmospheric circulation associated with the surface wind patterns may play an important role in modulating both boundary layer cloudiness and SST.

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