What is the dominant wind direction for air-sea heat exchange?
In the mid-latitudes the weather changes quickly. Changing winds influence the heat exchange between the atmosphere and the ocean. A new study by Fumiaki Ogawa and Thomas Spengler shows how important it is to consider short-term phenomena like extratropical cyclones and cold air outbreaks when calculating air-sea fluxes. Read their account below.
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Written by Fumiaki Ogawa and Thomas Spengler from the Bjerknes Centre for Climate Research and the Geophysical Institute at the University of Bergen.
Understanding air-sea heat exchange is key to comprehending our global聽climate system.听聽that the often-employed view using climatological聽mean temperature and wind fields to characterize this exchange only holds for聽tropical and subtropical regions. In the mid and higher latitudes, however, the聽climatological air-sea heat exchange is mostly attributable to weather聽phenomena on sub-weekly scales.
Wind direction for maximum surface sensible heat flux (shading). Annual 6-hourly data.
The atmosphere and ocean exchange energy through surface latent and sensible heat fluxes.听The largest amounts of this exchange are observed in the subtropics and along the western聽boundary currents in the midlatitudes as well as adjacent to the sea ice edge in the high聽latitudes.
Surface sensible or latent heat fluxes are proportional to the product of the near-surface wind聽speed and the difference in temperature or humidity between the surface and the overlying聽atmosphere, respectively. As the climatological-mean heat fluxes are remarkably well聽described when using the climatological-mean wind components in the bulk flux formulas,聽previous studies employed steady-state and linear thinking to describe the atmospheric聽response to sea surface temperature anomalies and fronts.
Prevailing wind direction for surface upward sensible heat flux based on 6-hourly (left) and monthly (right) mean data. The hatched mask indicates where the climatological-mean sea ice concentration exceeds 30 percent.
We find, that the mean wind in the lower latitudes is indeed well described by the mean聽westward wind components (green shading in the figure above). In the mid and high latitudes,聽however, a significant fraction of the climatological-mean wind speed is associated with wind聽variations on synoptic time scales. The prevailing wind direction associated with the most聽intense upward heat flux in the midlatitudes thus features a significant equatorward wind聽component (yellow/orange shading in the left panel), which is in contrast to the climatological聽mean eastward wind direction in the midlatitude (red shading in the right panel).
These findings pinpoint the necessity to consider the role of sub-weekly weather phenomena,聽such as extratropical cyclones and cold air outbreaks in the mid and higher latitudes, to聽accurately describe the air鈥搒ea heat exchange in a physically consistent way.
Reference
Ogawa, F., and Spengler, T., 2019:聽.听Journal of Climate, 32, 5601-5617 .
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