Anomalous Sea Surface Temperatures and Local Air–Sea Energy Exchange on Intraannual Timescales in the Northeastern Subtropical Pacific

Abstract

Here 11 years of surface data (1961?72, excluding 1963) taken at ocean weather ship N (OWS N) are analyzed. OWS N is located in the subtropical eastern Pacific Ocean (140°W, 30°N). Bulk formulas are employed to calculate each component of the surface heat flux (sensible, latent, longwave, and shortwave) from the 3-h measurements of sea surface temperature (SST), air temperature, surface humidity, wind speed, and cloudiness. Analyses are performed on fluxes averaged over daily and monthly intervals. Results indicate a large fraction of the variance in net surface energy flux is associated with anomalies in the latent heat flux; the latter are principally due to variability in the surface wind speed. Cross correlation and regression analyses of monthly anomalies of SST and SST tendency (?SST/?t) with the surface heat flux components indicate over 50% of the variance in SST-tendency anomalies is accounted for by local anomalies in the net surface energy flux. In the summer, the summed variance in the four components of the surface heat flux is explained almost completely by two modes of variability that are nearly orthogonal. The first (second) mode is defined as the combination of surface flux components that optimally covaries with the SST-tendency anomaly (SST anomaly) and it contains 74% (26%) of the summed variance in all of the surface heat flux components. In addition, the net heat flux anomaly associated with the the SST-tendency anomaly, which results from the summing of the individual components that define the first mode, accounts for virtually all (96%) of the variance in the net surface flux; it is dominated by the latent heat flux component. The second mode is dominated by the variability in the shortwave flux (mainly due to changes in the cloudiness), but the opposing anomalies in latent and longwave flux largely cancel the anomalies in the shortwave. Hence, the net heat flux associated with the flux components that covary with the SST anomalies is too small to generate significant variability in SST. The physical scenario consistent with the analyses presented is as follows. Throughout the year, variability that is inherent to the atmosphere causes net surface flux anomalies (mainly due to anomalies in evaporation driven by wind speed anomalies) that account for over 50% of the variability in SST. During the summer months, the changes in the SST that are driven by the aforementioned atmospheric variability, in turn, force changes in the lower troposphere (e.g., in the low-level cloudiness) that are announced by a redistribution of the surface heat flux components, though these changes in the lower atmosphere do not further affect the ocean because there is an insignificant change in the net surface heat flux. The results obtained from the observations are confirmed using a one-dimensional ocean mixed layer model. Model results also indicate that heat flux anomalies due to entrainment processes act in the same sense as the net surface heat flux anomalies but are small (about 7% of the variance) compared to the surface heat flux anomalies. Anomalies in ocean advection contribute significantly to SST anomalies only during late wintertime and only for seasonally averaged and longer timescales.