Hydrological Signatures Relating the Asian Summer Monsoon and ENSO

Abstract

Using the NCEP-NCAR reanalysis for the 1950-2000 period, differences in the atmospheric hydrological cycle between the extremes of ENSO (i.e., La Nina minus El Nino) are examined. Zonal vertically integrated moisture transport (VIMT) across 1008E accounts for about half of the variability in net moisture convergence in the north Indian Ocean region between ENSO extremes when all ENSO events are considered. Changes in VIMT across 1008E are associated with large changes in the strength of the Pacific Ocean trade wind regime during ENSO. The bulk of the remaining VIMT anomalies are from the Arabian Sea and appear to be associated with sea level pressure variations in the northern and western parts of the Indian Ocean Basin. This initial analysis, therefore, suggests that the interaction between the monsoon and ENSO may be more complex than the direct modulation of VIMT by the Pacific Ocean trade winds alone. The analysis is refined further by comparing the differences of the Indian and Pacific Ocean hydrological cycles between ENSO extremes when they occur concurrently with anomalous monsoons (ENSO-anomalous monsoon years (EAM)), and when the monsoon is normal (ENSO-normal monsoon years (ENM)). For both EAM and ENM years, similar differences exist in VIMT across 1008E between ENSO extremes. However, major differences are noted in VIMT anomalies from the west and south into the north Indian Ocean region. Thus, the principal difference in moisture convergence in the north Indian Ocean between EAM and ENM years is associated primarily with VIMT anomalies in the western Indian Ocean region and not those in the eastern Indian or Pacific Oceans. To test the hypothesis that Pacific Ocean SST anomalies occurring prior to the monsoon may be important in influencing the eventual nature of the monsoon, the analysis is extended backward to the spring period. While May SST differences in the Nino-3 region between ENSO extremes are found to be similar for both EAM and ENM years, VIMT differences in both the Indian Ocean and the central and western Pacific Oceans are sig- nificantly larger during EAM years than ENM years. May SST differences in the central subtropical Pacific Ocean are also significantly larger during EAM than ENM years. These results show that the anomalous SST gradient between the eastern equatorial and the central subtropical Pacific Ocean prior to the monsoon onset, together with its associated VIMTs anomalies, may be important factors in determining the degree of connection between monsoon and ENSO. In addition, the circulation in the Indian Ocean prior to and during the monsoon onset shares a strong association with the eventual intensity of the monsoon-ENSO coupling.