Intraseasonal Variability of the South Asian Monsoon

David M. Lawrence


The entire thesis can be downloaded in PDF format here:

Chapter 1: Preface and Introduction
Chapter 2: Data and Methods
Chapter 3: Structure and Evolution of the Intraseasonal Oscillation
Chapter 4: Interannual Variations of the Intraseasonal Oscillation
Chapter 5: Modulation of Synoptic-scale Convection
Chapter 6: Conclusions

Animation of JJAS ISO
Animation of DJFM ISO
Cover Page


The South Asian monsoon exhibits pronounced intraseasonal variability on timescales ranging from a few days to more than a month. A principal purpose of this study is to provide a comprehensive analysis of the low-frequency (25 to 80 days) monsoon intraseasonal variability and to determine its structure in both space and time. Large-scale active and break periods of rainfall are associated with the slowly evolving Intraseasonal Oscillation (ISO) that is characterized during northern summer by an apparent northward propagation of convection emanating from the central equatorial Indian Ocean. The northern winter ISO is dominated by steady eastward propagation of equatorial convection and lacks significant poleward propagation and therefore is examined in the context of comparison with the summer ISO. A cross-correlation and lagged regression analysis is utilized to investigate the temporal and spatial evolution of convection and large-scale circulation patterns associated with the ISO. Outgoing longwave radiation (OLR) filtered to eastward wavenumbers 1 to 3 and periods 25 to 80 days is used as the predictor for two independent regression analyses which focus on the northern summer (JJAS) and northern winter (DJFM) periods, respectively. The evolution of ISO convection and the large-scale circulation in northern summer appears similar in many respects to that of northern winter and can be thought of in terms of propagating equatorial modes. Surface frictional convergence into a Rossby cell that is excited by equatorial ISO convection generates a band of convection that is oriented southeast to northwest and stretches from the equator to about 20o N. Viewed along any meridian the mode appears to propagate northward while equatorial convection propagates to the east.

The summer ISO is examined in further detail and is found to contain two predominant modes of evolution that occur with approximately the same frequency. The two modes differ primarily in how the equatorial Indian Ocean convection develops. The first mode exhibits an initially stationary development of low-frequency equatorial Indian Ocean convection (23 events) while the second mode evolves as a steady eastward propagation of convection (27 events). Both modes are characterized by northward propagation onto the Indian subcontinent subsequent to the development of strong convection at around 90o E. In all cases, northward propagation does not occur in the absence of eventual eastward propagation of convection along the equator.

Interannual variations of summertime ISO activity are investigated. Indices of the level of ISO activity, particularly the off-equatorial component of ISO convection, are developed using OLR for 1975-1997. Interannual variations in ISO activity are found to be related to year-to-year changes in the number of discrete events rather than changes in the characteristic period. Seasons characterized by strong and numerous ISOs exhibit significantly more low precipitation days and consequently deficient seasonal rainfall totals than seasons characterized by little or no ISO activity. Seasonal ISO activity is found to be essentially uncorrelated to the El Niño Southern Oscillation (ENSO) or any other contemporaneous sea surface temperature (SST) anomalies. The summertime ISO activity does exhibit a reasonably strong inverse relationship with South Asian monsoon strength. Over the 22-year period examined here, the relationship between South Asian monsoon strength and ISO activity is stronger than its well documented relationship with ENSO. The year-to-year variations of ISO activity also exhibit a clear biennial timescale suggesting that the tropospheric biennial oscillation (TBO) may play a role in determining ISO activity although the physical mechanism is not readily apparent.

A second dominant mode of intraseasonal variability is made up of synoptic-scale westward propagating convective disturbances with timescales between 5 and 10 days. During the active ISO phase over India, westward propagating synoptic-scale wave activity is above normal. The development of high-frequency convective disturbances over the Bay of Bengal and peninsular India is attributed to instability that is favored in regions of strong easterly vertical wind shear in conjunction with equatorial heating. As ISO convection moves into the western Pacific Ocean region westward propagating synoptic-scale waves are excited from which point they propagate to the west across southeast Asia into the Bay of Bengal bringing episodes of significant rainfall during the suppressed, and normally dry, ISO phase over India.