Three-Dimensional Water Vapor and Cloud Variations Associated with the Madden–Julian Oscillation during Northern Hemisphere Winter

Abstract

The Madden?Julian oscillation (MJO) is the dominant form of intraseasonal variability in the Tropics. In previous studies, intraseasonal variability has usually been characterized in terms of wind or convection anomalies, while the structure of MJO-related moisture variations has been greatly unexplored. This work focuses on the behavior of moisture and related hydrological fields associated with MJO events during Northern Hemisphere winter. Five-day averaged (1979?99) Television Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) moisture soundings from the NASA Pathfinder data were used, providing global coverage at specific pressure levels. The TOVS moisture, as well as the International Satellite Cloud Climatology Project (ISCCP) cloud fraction anomaly data, were composited based on MJO events selected via an index constructed from Xie?Arkin bandpassed pentad rainfall data. Analysis of the three-dimensional structure and evolution of precipitation, water vapor, and clouds over the life cycle of the MJO shows a rich set of relationships between the variables. The composite evolution of moisture shows markedly different vertical structures as a function of longitude. There is a clear westward tilt with the height of the moisture maximum associated with MJO disturbances propagating eastward across the Indian Ocean. These disturbances evolve into nearly vertically uniform moist anomalies as they reach the western Pacific. Near-surface (≥850 mb) water vapor leads precipitation by 1 pentad over the Indian Ocean and western Pacific. Upper-level water vapor lags the peak in precipitation by 1?2 pentads, as the upper troposphere is moistened following intense convection. In the eastern Pacific, the moisture variations then become confined to the lower levels (<?700 mb), with upper-level water vapor nearly out of phase. ISCCP total cloud fraction is highly correlated with humidity, and also leads observed precipitation. There is a longitudinal displacement between the maxima in rainfall and the maxima in cloud fraction, with higher cloudiness at the (western) trailing edges of the rainfall maxima. The cloud-top heights also show consistent changes over the course of the composite MJO life cycle, with an increasing (decreasing) strength of middle (high) cloud variations as the disturbances propagate eastward to the western Pacific. Averaged over all phases of the MJO, dry anomalies dominate over moist anomalies that occur in the active phase of the disturbance. The bias toward dry anomalies is strongest between 15°N and 15°S in the Western Hemisphere and suggests a low-frequency rectification of intraseasonal variability onto the mean background moisture state of the atmosphere.