Predicting the extremes of Indian summer monsoon rainfall with coupled ocean-atmosphere models

An analysis of the retrospective predictions by seven coupled ocean-atmosphere models from major forecasting centres of Europe and USA, aimed at assessing their ability in predicting the interannual variation of the Indian summer monsoon rainfall (ISMR), particularly the extremes (i.e. droughts and excess rainfall seasons) is presented in this article. On the whole, the skill in prediction of extremes is not bad since most of the models are able to predict the sign of the ISMR anomaly for a majority of the extremes. There is a remarkable coherence between the models in successes and failures of the predictions, with all the models generating loud false alarms for the normal monsoon season of 1997 and the excess monsoon season of 1983. It is well known that the El Niño and Southern Oscillation (ENSO) and the Equatorial Indian Ocean Oscillation (EQUINOO) play an important role in the interannual variation of ISMR and particularly the extremes. The prediction of the phases of these modes and their link with the monsoon has also been assessed. It is found that models are able to simulate ENSO-monsoon link realistically, whereas the EQUINOO-ISMR link is simulated realistically by only one model-the ECMWF model. Furthermore, it is found that in most models this link is opposite to the observed, with the predicted ISMR being negatively (instead of positively) correlated with the rainfall over the western equatorial Indian Ocean and positively (instead of negatively) correlated with the rainfall over the eastern equatorial Indian Ocean. Analysis of the seasons for which the predictions of almost all the models have large errors has suggested the facets of ENSO and EQUINOO and the links with the monsoon that need to be improved for improving monsoon predictions by these models

On forecasting the Indian summer monsoon: The intriguing season of 2002

This year, the rainfall over India during the first half of the summer monsoon season was 30 below normal. This has naturally led to a lot of concern and speculation about the causes. We have shown that the deficit in rainfall is a part of the natural variability. Analysis of the past data suggests that there is a 78 chance that seasonal mean rainfall this year will be 10 or more below the long-term average value. We discuss briefly how forecasts for seasonal rainfall are generated, whether this event could have been foreseen, and share our perspective on the problems and prospects of forecasting the summer monsoon rainfall over the Indian region

Impact of the moisture transport formulation on the simulated tropical rainfall in a general circulation model

The impact of numerical modeling of moisture transport on the simulation of the seasonal mean pattern of precipitation in the tropics is studied. The NCAR CCM2 with spectral and semi- Lagrangian moisture transport has been used for this purpose. The differences in the numerical modeling of moisture transport are found to have a significant impact on the simulation of the seasonal mean patterns. The major differences while using the spectral method (vis-a-vis the semi-Lagrangian method) are (1) a decrease in rainfall over the Indian monsoon region, (2) a decrease in rainfall over the west Pacific region and (3) an increase in rainfall over the central and east Pacific regions. There are substantial differences in the amount of precipitable water vapor simulated by the two moisture transport techniques. It is shown that the difference in precipitable water vapor between the two simulations is associated with changes in the vertical moist static stability (VMS) of the atmosphere, and differences in the simulated precipitation patterns

Impact of convective downdrafts on model simulations: results from aqua-planet integrations

The role of convective scale downdrafts has been examined, using the NCAR-CAM3.0 aqua-planet configuration. We find that, convective downdrafts make the atmosphere more unstable thus increasing the convective available potential energy (CAPE) of the atmosphere. It is noticed that, although the rate of CAPE consumption increases with the incorporation of downdrafts, the generation of CAPE increases with a higher rate. Also, it is noted that there is a reduction in the deep convective rainfall, with the inclusion of downdrafts, which is primarily due to the re-evaporation of precipitation within the downdrafts. There is a large increase in the low cloud fraction and the shortwave cloud forcing with the inclusion of convective scale downdrafts in the cumulus scheme, which along with the evaporation within the downdraft causes cooling in the troposphere

Morlet wavelet analysis of tropical convection over space and time: Study of poleward propagations of Intertropical Convergence Zone (ITCZ)

A methodology to use continuous Morlet wavelet transform over space and time has been developed. By means of this method, the power spectra in 4-dimensional space (viz. space, the spatial-scale, time and the temporal-scale) can been estimated. This is applied to study the spatial and temporal scales associated with the poleward propagations of the ITCZ (InterTropical Convergence Zone) over the Indian and West Pacific longitudes. The method has an added advantage that we can obtain the predominant spatial scales over a particular region, the temporal scales associated with them and their time of occurrence. We find that poleward propagations of ITCZ have a predominant spatial scale of about 30degrees. However the predominant temporal scale varies from year to year. Other regions which do not have significant poleward propagations do not show these ( spatial and temporal) characteristics