Inter-hemispheric and inter-zonal moisture transports and monsoon regimes

Abstract

The principal objectives here are to quantify the strengths of global monsoon circulations in terms of cross-equatorial moisture transport and to verify its behaviour during two extreme years of precipitation over the Amazon Basin. The National Centers for Environmental Prediction / National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data sets for the period 1950–2012 are used in this study and the results are compared with calculations from ERA-Interim data. During the South Asian summer monsoon (JJA), the transport is 82.1 × 107 kg s−1 between 30° and 90°E from the Southern Hemisphere (SH) to the Northern Hemisphere (NH) and reverses to 41.4 × 107 kg s−1 from NH to SH in northern winter (DJF). Over South America between 30° and 90°W, the values are 31.4 × 107 kg s−1 in DJF from NH to SH and 21.5 × 107 kg s−1 in JJA from SH to NH. Although there is no wind reversal from summer to winter seasons over the South American monsoon region, the moisture transport across the equator reverses. The transports during a weak and a strong monsoon in the Amazon Basin are distinctly different suggesting that the cross-equatorial moisture transport can be used as a Monsoon Intensity Index (MII). For the South American monsoon, the DJF index is 28.5 × 107 kg s−1 in the dry year 2004–2005 and 45.1 × 107 kg s−1 in the wet year 2011–2012 as against the climatological value of 31.4 × 107 kg s−1. There is an indication of an inverse relation between the MIIs over the Amazon Basin and the Australian sectors. The annual mean moisture transport across the equator is from SH to NH and is equivalent to three Amazon River discharges. © 2016 Royal Meteorological Society