Relationships between Large-Scale Vertical Velocity, Static Stability, and Cloud Radiative Forcing over Northern Hemisphere Extratropical Oceans*

Abstract

This paper identifies dynamical and thermodynamical factors that govern the seasonal and interocean differences in cloud cover and cloud radiative forcing (CRF) over the storm track regions of the northern extratropical Pacific and Atlantic Oceans. An outstanding problem of interest is the fact that cloud cover is larger in the summer than winter in the North Pacific, while the converse is true in the North Atlantic. This paper considers separately January and July in the North Pacific and North Atlantic and finds that, on daily timescales, rising motion associated with synoptic-scale events such as cyclones produces greater CRF. However, CRF does not vary much with vertical velocity in regions of subsidence. In addition, increased moist static stability is associated on daily and monthly mean timescales with increased cloud cover and shortwave CRF. These results imply that, on monthly mean timescales, if we hold moist static stability constant, CRF should increase with increasing vertical velocity variance. This effect, by itself, would tend to increase CRF during winter, since the variance of vertical velocity is much larger during winter than summer. This is consistent with what is observed in the North Atlantic. In the North Pacific, however, the mean moist static stability is much larger during summer, and this effect tends to counteract the summertime decrease in vertical velocity variance, resulting in greater summertime cloud cover. Extending the argument to explain interocean differences in cloudiness or CRF during the same season, this paper finds that the North Pacific and North Atlantic have approximately the same CRF (or cloud cover) during winter because the mean vertical velocity variance and moist static stability are approximately the same. The North Pacific is more cloudy than the North Atlantic during summer because, while the mean vertical velocity variance is approximately the same, mean moist static stability is much greater in the North Pacific. Finally, spatial variations in both parameters within a given ocean basin tend to either reinforce each other or compete in their effect on CRF.