Observed Trends in Subtropical Stratocumulus and Associated Meteorology

Abstract:

The importance of low-level cloud feedbacks to climate sensitivity motivates an investigation of how low-level cloud amount, liquid water path, and related meteorological conditions have changed in recent decades in subtropical stratocumulus regions. Using a satellite cloud dataset corrected for inhomogeneities, we find that during 1984-2009 low-level cloud amount significantly increased over the northeast and southeast Pacific, significantly decreased over the northeast Atlantic, and weakly increased over the southeast Atlantic and southeast Indian oceans. During 1988-2012, liquid water path decreased over the northeast Pacific, significantly increased over the southeast Pacific and northeast and southeast Atlantic, and weakly increased over the southeast Indian oceans. Examination of meteorological parameters from four re-analyses indicates that positive trends in low-level cloud amount are associated with decreasing trends in sea surface temperature and increasing trends in inversion strength, subsidence and cold-air advection, and vice-versa. Relationships between liquid water path and meteorological conditions are weaker, but increasing trends in liquid water path are associated with increasing trends in sea surface temperature and decreasing trends in inversion strength, subsidence, and cold-air advection, and vice-versa. A multi-linear regression model based on these four meteorological variables well captures the sign and to certain extent magnitude of observed cloud amount trends in almost all stratocumulus regions, but a similarly constructed model largely fails to reproduce the observed liquid water path trends. Differing signs of cloud trends and differing contributions from meteorological parameters between regions suggest that observed changes in subtropical stratocumulus since the 1980s are primarily due to natural variability rather than a systematic response to climate change.