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

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 differences. The major cloud frequency shifts are between stratiform clouds and large cumulus east of 130°W and between small cumulus and deep convective clouds west of 130°W. East of 160°E, frequency of deep convective cloud increases rapidly above a knee in the curve of frequency versus SST located near 25.5°C in July?November and near 27°C January?May. Since this temperature difference is similar to the difference in midtropospheric mean temperature between the same seasons in the same region, this relationship suggests strong control of deep convection by mean static stability in this region. As expected, a strong and linear relationship exists between anomalies in OLR and in the frequency of deep convective clouds observed at the surface.