The Impact of Madden-Julian Oscillation on the Asian Summer Monsoon Precipitation in Pakistan

Since Pakistan locates in the west edge of Asian Summer Monsoon Region, the amount of precipitation there is severely influenced by the variability of monsoon activity. In some dry years such as in 1991 or 2002, Pakistan suffered from strong drought, causing great economic and life losses, while, in wet years such as 1994 or 2003, flood came to Pakistan, washing away whole cities. In order to reduce the damage of these water hazards, seasonal prediction is greatly effective, but due to the complicated mechanism of monsoon, it is still very challenging problem. In this research, we suppose ocean as one of the sources for seasonal (a few months) precipitation trend and try to show the relationship between oceans and monsoonal precipitation, which is essential for the improvement of seasonal prediction. This research utilizes JRA25 reanalysis dataset and National Oceanic & Atmospheric Administration (NOAA)'s OLR datasets. Correlation analysis between sea surface temperature and OLR over Pakistan on July showed that there is significant relationship between Arabian Sea and monsoonal precipitation in Pakistan. From the empirical Orthogonal Functions analysis conducted on zonally averaged mass stream function over Arabian Sea, the anomaly of atmospheric circulations over Arabian Sea is mainly composed of the anomaly of subtropical jets in Northern and Southern Hemisphere, and of the jet in lower troposphere around equator. Especially, the strength of Northern subtropical jet with Hadley-like circulation, which is derived from principle component, is strongly correlated with precipitation activity in Pakistan. On July of extremely dry years, downdraft in mid-latitude region associated with subtropical jet and Hadley-like circulation are strengthened, so that monsoonal precipitation is suppressed, In wet years, opposite condition is seen. The strength of those circulations greatly changes intra-seasonally, and, as Sajani et al. (2007) indicates, Madden-Julian Oscillation is a good source of these intra-seasonal variability. In order to reveal the MJO's influence on these local meridional circulations, composite analysis based on the Real-time Multivariate MJO Index (Wheeler & Hendon, 2004) and correlation analysis was conducted. These analyses showed that MJO indirectly influences on the meridional circulations over Arabian Sea, through Walker Circulation over Indian Ocean. This influence is not only limited to the phases where convection is located in Indian Ocean, but it continues after convection reached at Pacific Ocean. As the figure shows, the impact of the convection around equator on the atmospheric circulations is widened when the (active / inactive) convection locates at the updraft region of Walker circulation and the perturbation of the Walker Circulation might have an influence on the meridional circulation over Arabian Sea. This impact changes from strengthening effect, little effect, and weakening effect as the convection area propagates eastward. Furthermore, the difference in the extent of eastward propagation and the monsoonal precipitation were also examined.