Marine Low-Cloud Anomalies Associated with ENSO

Abstract

As a contribution to understanding the possible impact of altered climate regimes on marine clouds, and hence on cloud radiative forcing, ship-observed marine low clouds and precipitation frequency for individual seasons are regressed at zero lag on an index of El Niño?Southern Oscillation (ENSO) for the period December 1955? January 1996 for ocean areas between 40°S and 70°N. Seasonal anomalies of atmospheric circulation parameters, static stability, and SST are also examined in order to illuminate physical mechanisms responsible for observed ENSO cloud variations. The following extratropical regions exhibit significant ENSO cloud anomalies and are discussed in detail: winter?spring North Pacific, summer North Pacific, winter western North Atlantic, autumn northeastern Atlantic, and western Mediterranean Sea. In all of these regions except the summer North Pacific, cloud anomalies are related to jet stream and storm track anomalies associated with atmospheric teleconnection patterns. The summer North Pacific anomalies are also connected with jet stream and storm track anomalies, but these are associated with a persistent SST anomaly rather than an atmospheric teleconnection. ENSO anomalies in the western and eastern equatorial Pacific are analyzed in greater detail than in previous work, as well as those in the Arabian Sea during winter and summer monsoons. With the exception of the Arabian Sea region in winter, cloud anomalies are consistently related to 1) changes in storm tracks and/or 2) changes in low-level static stability and temperature advection.