The inter‐decadal change in anomalous summertime water vapour transport modes over the tropical Indian Ocean–western Pacific in the mid‐1980s

Abstract

Using the monthly reanalysis data sets of National Center for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) for 1961–2015, the inter‐decadal change in inter‐annual water vapour transport over the tropical Indian Ocean–western Pacific during summer and the related mechanisms were analysed in this article. The results show that two major modes of anomalous water vapour transport over the tropical Indian Ocean–western Pacific (60°–140°E, 10°S–30°N) experienced a significant inter‐decadal change in the mid‐1980s. The first mode (EOF1) shows that the anticyclonic water vapour transport over the northwestern Pacific moves significantly southwards after the mid‐1980s. Before the mid‐1980s, the anomalous water vapour originates from the subtropical western Pacific and moves through the South China Sea to eastern China. However, the anomalous water vapour mainly originates from the tropical western Pacific and moves through the South China Sea to eastern China after the mid‐1980s. The second mode (EOF2) reflects the enhanced effect of the anomalous water vapour transport over the tropical Indian Ocean on East Asia after the mid‐1980s. Before the mid‐1980s, the distribution of the water vapour anomalies is uneven over the tropical Indian Ocean and the magnitude of water vapour is relatively small, so it has a weak effect on rainfall over East Asia. After the mid‐1980s, anomalous anticyclonic water vapour transport originates from the tropical Indian Ocean and moves along the Arabia Sea, Indian Peninsula and Indo‐China Peninsula and then forms an anomalous cyclonic water vapour transport over south China. Furthermore, the possible causes of the inter‐decadal transition in anomalous water vapour transport are investigated for the two major modes. The significant inter‐decadal transition in anomalous water vapour transport is associated with the enhanced effect of the sea surface temperature (SST). Before the mid‐1980s, the relationships between the two major modes and the SST of the tropical oceans are relatively weak. After the mid‐1980s, the EOF1 becomes significantly regulated by the El Niño‐Southern Oscillation (ENSO) in the previous winter, and the positive (negative) phase of the principal component of the first mode (PC1) corresponds to significantly positive (negative) SST anomalies over the tropical northern Indian Ocean and Maritime Continent during the same period. For EOF2, the anomalous water vapour transport over the tropical Indian Ocean is related to the enhanced inter‐annual variability of tropical Indian Ocean dipole (TIOD) after the mid‐1980s.