Monsoons

Coordinator: Julia Slingo

Aims

To investigate the variability and predictability of the Asian summer monsoon on intraseasonal to interannual timescales and to identify those factors that influence that predictability.

Achievements

1. A common circulation pattern which links rainfall variability over India on subseasonal and interannual timescales has been identified and used to demonstrate that predictability of the seasonal mean monsoon behaviour requires only that the effects of the slowly varying components of the climate system be correctly simulated.

The relationship between subseasonal and interannual variability of the Indian Summer Monsoon has been investigated using analysis of the dominant modes of variability in the 40-year NCEP/NCAR Reanalysis, with complementary satellite and surface based precipitation data. The hypothesis that the characteristics of monsoon subseasonal variability (i.e. weather regimes) are modulated on interannual timescales in a systematic and therefore predictable manner has been tested. The null hypothesis is that predictability of the seasonal mean monsoon behaviour requires only 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 closely related to the observed seasonal mean All India Rainfall (AIR). A counterpart of this mode has also been identified at subseasonal timescales which projects strongly on to the daily AIR, confirming that a common mode of monsoon variability exists on seasonal and subseasonal timescales (Figure 1). It has been shown that the temporal behaviour of this subseasonal mode does not show any evidence of bimodality (i.e. transitions between weather regimes) and that anomalous monsoons are not associated with changes in weather regimes (Figure 2). Further analysis has confirmed that low frequency modulation of the basic state is primarily responsible, supporting the null hypothesis that predictability of the seasonal mean monsoon requires only that the effects of the slowly varying components of the climate system be correctly simulated. Thus model improvements to reduce systematic errors in the mean simulation and the response to low frequency boundary forcing may improve the prospects for dynamical seasonal prediction.

2. Using an ensemble of seasonal forecasts, the influence of Eurasian land surface conditions (specifically springtime snow anomalies) on Indian monsoon variability has been demonstrated which has implications for the future development of seasonal prediction for India.

The oceans, with their much larger heat capacity, are generally considered to provide the long term memory of the climate system required for seasonal prediction. However, although a wide range of studies have confirmed the importance of tropical SSTs, particularly El Niņo, there is also evidence that the land surface conditions may influence the monsoon. In particular, anomalous springtime snow amounts over Eurasia may provide long term memory to the climate system by affecting the land surface energy and moisture budgets. In turn the anomalous land surface conditions introduced by snow anomalies may influence monsoon variability.

In collaboration with ECMWF and ICTP, a programme of seasonal ensemble experiments has been developed to address, specifically, the role of the Eurasian land surface conditions, as distinct from SSTs, in influencing the predictability of the Indian summer monsoon. The results have shown that high snow amounts over western Eurasia are linked to La Niņa, suggesting that the El Niņo/Southern Oscillation (ENSO) has an influence on the wintertime climate of Eurasia. The signature of these snow depth anomalies is carried through to the summer in terms of changes in soil wetness and surface temperatures. In turn, the monsoon circulation is substantially weakened in association with above normal snow amounts over western Eurasia, whilst All India Rainfall is slightly increased.

These results have demonstrated that land surface conditions can have a significant impact on the large scale monsoon circulation and to a lesser extent on Indian Summer Monsoon rainfall. It is suggested that interactions between the mid-latitude circulation and the monsoon may hold the key to understanding the link between Eurasian land surface conditions and monsoon variability. If that is the case then predictability of this relationship is likely to be limited, due to the high level of internal variability of the midlatitude circulation.

Contributors: H. Annamalai, Bernd Becker, Julia Slingo, Laura Ferranti (ECMWF), Franco Molteni (ICTP), Ken Sperber (PCMDI)

Publications

Slingo, J. M. and partners, 2000: SHIVA: Studies of the Hydrology, Influence and Variability of the Asian Summer Monsoon. Final Report of FP4 Project, published by the European Commission.
Becker, B. D., J. M. Slingo, L. Ferranti and F. Molteni, 2000: Seasonal predictability of the Indian Summer Monsoon: What role do land surface conditions play? Mausam, (in press).
Sperber, K. R., J. M. Slingo and H. Annamalai, 2000: Predictability and the relationship between subseasonal and interannual variability during the Asian Summer Monsoon. Q. J. R. Meteorol. Soc., (in press).
Slingo, J. M. and H. Annamalai, 2000: 1997: The El Niņo of the century and the response of the Indian Summer Monsoon. Mon. Weath. Rev. (in press)
Ferranti, L., J. M. Slingo, T. N. Palmer and B. J. Hoskins, 1999: The effect of land surface feedbacks on the monsoon circulation. Q. J. R. Meteorol. Soc., 125, 1527-1550.
Annamalai, H., J. M. Slingo, K.R. Sperber and K. Hodges, 1999: The mean evolution and variability of the Asian Summer Monsoon: Comparison of ECMWF and NCEP/NCAR Reanalyses. Mon. Weath. Rev., 127, 1157-1186.