Summary: Changes in water vapor transports of the ascending branch
of the tropical circulation
and Richard P. Allan2
Received 4 May 2011; revised 1 July 2011; accepted 13 July 2011; published 24 September 2011.
 Recent studies have found contradicting results on whether tropical atmospheric
circulation (TAC) has intensified or weakened in recent decades. Here we reinvestigate
recent changes in TAC derived from moisture transports into the tropics using high
temporal and spatial resolution reanalyses from ERAInterim. We found a significant
strengthening of both the lowerlevel inward transports and the midlevel outward
transports over the recent two decades. However, the signal in the total budget is weak
because the strengthening of the inflow and the strengthening of the outflow neutralize
each other, at least to some extent. We found atmospheric humidity to be relatively stable,
so we suggest that the intensification is mainly caused by an intensification of the
windrelated circulation strength. The exact quantitative values were found to depend
heavily on whether the calculations are based on mean or instantaneous values. We
highlight the importance of using the instantaneous values for transport calculations, as
they represent the coincidence of high wind speeds and high atmospheric humidity.
Citation: Zahn, M., and R. P. Allan (2011), Changes in water vapor transports of the ascending branch of the tropical
circulation, J. Geophys. Res., 116, D18111, doi:10.1029/2011JD016206.