North Pacific cloud feedbacks inferred from synoptic-scale dynamic and thermodynamic relationships

Abstract

Daily satellite cloud observations and reanalysis dynamical parameters are analyzed to determine how midtropospheric vertical velocity and advection over the sea surface temperature gradient control midlatitude North Pacific cloud properties. Optically thick clouds with high tops are generated by synoptic ascent, but two different cloud regimes occur under synoptic descent. When vertical motion is downward during summer, extensive stratocumulus cloudiness is associated with near-surface northerly wind, while frequent cloudless pixels occur with southerly wind. Examination of ship-reported cloud types indicates that midlatitude stratocumulus breaks up as the boundary layer decouples when it is advected equatorward over warmer water. Cumulus is prevalent under conditions of synoptic descent and cold advection during winter. Poleward advection of subtropical air over colder water causes stratification of the near-surface layer that inhibits upward mixing of moisture and suppresses cloudiness until a fog eventually forms. Averaging of cloud and radiation data into intervals of 500-hPa vertical velocity and advection over the SST gradient enables the cloud response to changes in temperature and the stratification of the lower troposphere to be investigated independent of the dynamics. Vertically uniform warming results in decreased cloud amount and optical thickness over a large range of dynamical conditions. Further calculations indicate that a decrease in the variance of vertical velocity would lead to a small decrease in mean cloud optical thickness and cloud-top height. These results suggest that reflection of solar radiation back to space by midlatitude oceanic clouds will decrease as a direct response to global warming, thus producing an overall positive feedback on the climate system. An additional decrease in solar reflection would occur were the storm track also to weaken, whereas an intensification of the storm track would partially cancel the cloud response to warming. © 2005 American Meteorological Society.