Seasonal and Interannual Variations in Stratiform and Convective Clouds over the Tropical Pacific and Indian Oceans from Ship Observations

Abstract

Abstract Anomalies in frequency of occurrence of stratiform and convective cloud types identified by volunteer observing ships are related to anomalies in SST and surface wind divergence for the tropical Pacific and Indian Oceans for the period December 1952–December 1992. Cloud type frequency anomalies have also been related to outgoing longwave radiation (OLR) anomalies for the period 1979–91. The strongest cloud frequency signals are associated with the seasonal shift in the ITCZ and with the first empirical orthogonal function of SST after removal of the annual cycle. The latter corresponds to the annually averaged SST signature of ENSO. Data are separated into two seasons to display these signatures: January–May corresponding to warm eastern Pacific equatorial SST and July–November corresponding to cool eastern Pacific equatorial SST. Relationships between cloud type frequencies, SST, and divergence are generally similar for spatial variations within each season, and for seasonal and ENSO-related dif...