The 12–24-Day Mode of Global Precipitation

Abstract

Global precipitation estimates derived from satellite data at the Goddard Laboratory for Atmospheres for 1979?80 were used to explore time variations in global precipitation. Time series of the area-averaged precipitation [P] over the Asian-Australian (AA) monsoon (60°E?12°W), and the extra-AA monsoon (120°W?60°E) hemispheres were used in describing the variations. A distinct seesawlike intraseasonal variation of precipitation between these two hemispheres emerges from the two time series. Two intraseasonal (30?60 and 12?24 day) modes stand our in die spectral analysis of the two [P] time series. The 30?60 day mode is well known, while the 12?24-day mode is identified here for the first time. Using data generated by the Global Data Assimilation System of the National Meteorological Center, an effort was made to investigate the characteristics of the 12?24-day mode of global precipitation via potential functions for the 200-mb wind, water vapour transport, and precipitation. It is found that the 12?24-day mode exhibits a wavenumber 1 structure and propagates eastward. The seesaw intraseasonal variation of precipitation between the AA and extra-AA monsoon hemispheres is caused not only by the 30?60-day mode but also by the 12?24-day mode.