Predictability and the relationship between subseasonal and interannual variability during the Asian summer monsoon

The relationship between subseasonal and interannual variability of the Asi an summer monsoon has been investigated through analysis of the dominant mo des of variability in the 40-year NCEP/NCAR Re-analysis, with complementary satellite and surface-based precipitation data. The hypothesis that the ch aracteristics of monsoon subseasonal variability (i.e. weather regimes) are modulated on interannual time-scales in a systematic and therefore predict able manner has been tested. The null hypothesis is that predictability of the seasonal mean monsoon behaviour requires that the effects of the slowly varying components of the climate system be correctly simulated. An interannual mode of monsoon variability has been identified which is clo sely related to the observed seasonal mean all-India Rainfall (AIR). A coun terpart of this mode has also been identified at subseasonal timescales whi ch projects strongly on to the daily AIR, confirming that a common mode of monsoon variability exists on sub-seasonal and interannual time-scales. It has been shown that the temporal behaviour of this subseasonal mode, as des cribed by the probability distribution function (PDF) of the principal comp onent time series, does not show any evidence of bimodality, the shape of t he PDF being Gaussian. Further, it has been shown that the mean of the PDF is systematically and significantly perturbed towards negative (positive) v alues in weak (strong) monsoon years as categorized in terms of the seasona l mean AIR. This translation in the mean of the PDF, rather than a change i n shape of the PDF, suggests that anomalous monsoons are not associated wit h changes in weather regimes. Further analysis has confirmed that low-frequ ency modulation of the basic state is primarily responsible for these shift s in the subseasonal PDFs, supporting the null hypothesis that predictabili ty of the seasonal mean monsoon requires that the effects of the slowly var ying components of the climate system be correctly simulated. Thus, model i mprovements to reduce systematic errors in the mean simulation and the resp onse to low-frequency boundary forcing may improve the prospects for dynami cal seasonal prediction. However, the results indicate that only a subset of the subseasonal modes a re systematically perturbed either by the El Nino Southern Oscillation or i n weak vs. strong monsoon years, suggesting that predictability is likely t o be limited by the chaotic, internal variability of the monsoon system.