A Global Climatology of Tropical Moisture Exports

Abstract

In a recent paper, a climatology of tropical moisture exports (TMEs) to the Northern Hemisphere (NH) was constructed on the basis of 7-day forward trajectories, started daily from the tropical lower troposphere, which were required to reach a water vapor flux of at least 100 g kg 21 m s 21 somewhere north of 358N. It was shown that TMEs contribute significantly to regional precipitation. Here, the authors complement and extend this work by (i) using 6-hourly European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) data from 1979 to 2010 instead of the earlier 40-yr ECMWF Re-Analysis (ERA-40), (ii) extending the climatology to the Southern Hemisphere (SH), and (iii) relating TME events to atmospheric rivers (ARs) previously discussed in the literature. The main conclusions are as follows: First, the TME climatology is not sensitive to the reanalysis dataset. Second, SH TME shows four activity centers: the central and eastern Pacific Ocean (1708-908W, near the South Pacific convergence zone), eastern South America and the adjacent Atlantic Ocean (608W-08, near the South Atlantic convergence zone), the western Indian Ocean (308-808E), and western Australia (1108-1408E). Third, TME activity in the SH peaks in austral summer because of higher moisture contents and possibly because of increased Rossby wave activity from the NH. Fourth, El Niñ o warm events are associated with reduced activity over the South Pacific and increased activity over the South Atlantic and around Australia, while correlations with the southern annular mode are generally weak. Finally, around 90% of all ARs affecting the U.S. West Coast during December-May are connected to TME events, revealing their importance for heavy rain and flooding.

Comparison of Eulerian and Lagrangian moisture source diagnostics – the flood event in eastern Europe in May 2010

“…7a). The subsequent advection of this tropical moisture towards Europe indicated by the trajectories is also diagnosed as a specific tropical moisture export event by the objective Lagrangian analysis performed by Knippertz et al (2013).…”

The role of the Amazon Basin moisture in the atmospheric branch of the hydrological cycle: a Lagrangian analysis

“…In that way, the most recently developed Lagrangian techniques are being extensively applied for evaluating the origin of the water that precipitates over a continental area (e.g. Stohl and James, 2005;Dirmeyer and Brubaker, 2007;Gimeno et al, 2013;Knippertz et al, 2013).…”

Different atmospheric moisture divergence responses to extreme and moderate El Niños

“…The variability of the oceanic water supply greatly influences water availability for all regions. Excessive transports are usually major causes for extreme weather and flood events (Knippertz and Wernli 2010;Galarneau et al 2010;Chang et al 2012;Knippertz et al 2013), while interrupted transports can lead to droughts and subsequent socioeconomic stresses (Cai et al 2012(Cai et al , 2014. Hence, a clear understanding of the mechanisms that force observed changes to the hydrological cycle is of major importance.…”