Changes in water vapor transports of the ascending branch of the tropical circulation

Abstract

[2] Recent and future changes in the statistical characteristics of the tropical atmospheric circulation (TAC) pattern and associated changes in the tropical hydrological cycle do not only locally affect the weather properties in the tropical areas but they have an influence on the extratropical regions. The general TAC pattern consists of convective regions of upward, ascending air movement (ASC) and of regions of downward, descending air motion (DESC), with low‐level flow into ASC and midlevel outflow into DESC commonly referred to as the Hadley Cell circulation. Along with the upward air motion, ASC contains most of the tropical precipitation and in its annual cycle by and large follows the Sun’s zenith. Part of the water vapor originates from evaporation in areas close by, but additionally large amounts of water are transported by atmospheric movements from DESC, causing a largely positive tropical water balance (precipitation‐evaporation) at the cost of the dry subtropics. A number of recent studies have addressed past and possible future changes in the moisture budget or in other components of the hydrological cycle over the tropics or over parts of the tropics. An increase in low‐level atmospheric water vapor of 7% per degree of warming is derived from theoretical considerations (Clausius‐Clapeyron relation, e.g., Wentz and Schabel[2000]; Trenberth et al. [2003]; Held and Soden