Relationship between surface fields over Indian ocean and monsoon rainfall over homogeneous zones of India

The monthly mean atmospheric fields and surface parameters of NCEP/NCAR reanalysis for the period 1948-1998 have been studied to examine the characteristics of monsoon circulation features, sea surface temperature (SST), sea level pressure, surface wind stress and latent heat flux over the Indian Ocean and nearby seas during deficient, normal and excess rain years. The entire period of study has been classified into deficient, normal and excess rain years for all India as well as for each of the five homogeneous zones separately based on the observed seasonal mean rainfall. On the basis of the mean characteristics of the surface fields, the oceanic region covering the Indian Ocean and adjacent seas has been divided into four regional sectors. Using various statistical means the relation between the surface fields over the four regional sectors and the monsoon rainfall over five homogeneous zones of Indian landmass has been examined. Attempt have been made to identify some surface parameters which can be used as predictors for seasonal mean monsoon rainfall over the entire India and also over some homogeneous zones.

Some aspects of the West African monsoon circulation As deduced from a geostationary satellite

This paper focusses on some aspects or the West African monsoonal circulation observed during the period 15 July-l0 August 1979 of the PGGE, as derived from the satellite cloud windvectors. Temporal averages of the computed winsfields reveal that the flow at the low level is southerly (monsoonal), and Its line of discontinuity with the continental northeasterly was found at approximately 16°-18°N, lying about 300 km south of the accepted mean position. At both the middle and upper tropospheres the flow is easterly with axis about 12o-14,N and, latitude 8 No respectively, such that it is a circulation south of the axis and northwards, it is anticyclonic. The satellite-observed tropospheric circulation IS then discussed in relation to the, weather manifestations over the sub-region typical of the July / August period. The mass fields obtained from our gridded satellite-winds indicate that inflow into the land area occur mainly at the lowest layer (1000:850 hPa), whereas at the upper, levels (that is, above 850 hPa) it is predominantly an outflow, The tropospheric average gives a net mass for divergence from within the area, The significance of this result in relation to the observed weather phenomenology of a temporary cessation of the monsoon precipitations occurring about the peak of the season IS also discussed.

Usability of the low frequency mode in southwest monsoon circulation for long range prediction of rainfall activity over central India

pro pagation have been ctudicd on the hasi, of upper air data of a few sta tions,The frequency of occurrence Ill' significant periodicity in th is mode i.. rchuively high for Visakhapatnam andMadras. 1 here appears a large inter-annual variability of the maximum amplitudes of rhe filtered series with nospecial preference to any latitudinal bell. Northward propagation of this mode also slums large inn.....-annunlvarisbility. In some ~ears the propaga tion " as totally absent. The phusc changes in the filtered ser ies o fVisakbapnma rn match ed with the cha nges in weekly ra infall activity over central India and thi.. may, pcrhups.be used to foreshadow the activ ity of the monsoon over central India ,

Interaction of mid-latitude systems in the southern Hemisphere with southwest monsoon

The movement of cold fronts with associated westerly waves in the lower troposphere across southern Africa and adjoining southwest Indian Ocean during the months of May to August for the years 1977 to 1981 has been examined in relation to the chief features of southwest monsoon. The deep frontal systems which penetrate north or latitude 25° S cause considerable fluctuations in the intensity of south to north pressure ridge along the east coast of southern Africa and Mascarene high. During the period of movement deep frontal systems from the west coast of South Africa to the Mozambique channel, the pressure index falls leading to decrease in cross equatorial flow. With further eastward movement of the system across Mozambique channel the pressure .index rises and causes increase in cross equatorial flow in Arabian Sea, strengthening equatorial westerlies and .increase in horizontal shear. The study has revealed a definite association between variation of pressure index with onset and various phases of the monsoon circulation. This association could be of help in understanding and forecasting of these monsoon features.

Observed and model simulated inter-annual variability of the Indian monsoon

The Center for Ocean-Land-Atmosphere (COLA) general circulation model has been integrated seven times with observed global sea surface temperature (SST) for the years 1979-98. The model-simulated annual cycle, the seasonal mean and the interannual variability of the summer monsoon rainfall and circulation over the Indian region are compared with the corresponding observations. It if found that, although this model has shown remarkable success in simulating the local and global response of tropical SST anomalies, the model shows poor skill in simulating the interannual variability of monsoon rainfall over India. While it is true that the correlation between the observed tropical Pacific SST and the Indian summer monsoon rainfall for the most recent 20 years itself is considerably over India is largely related to the systematic errors of the model in simulating the climatological mean monsoon circulation and rainfall, especially over the oceanic regions.

Satellite estimated water vapour in relation to Asian monsoon Circulation

Water vapour plays a crucial role in various exchange and transport processes in the atmosphere and its knowledge in the tropics is extremely important for input to various global circulation models. The vast oceans of earth's surface provide a large source of moisture and continuously modify the thermodynamics of the atmosphere through latent heat flux and condensational heating. In the tropics, especially in the Indian ocean the water vapour is highly heterogeneousin nature, and is one of the parameters which is responsible for cloud formation, associated with tropical systems like monsoon flows, depression, cyclones etc. The present paper reviews the various information’s available from deferent geostationary and polar orbiting satellites about water vapour affecting the southwest monsoon region around the Indian Ocean and Indian subcontinent. The temperature and moisture data from TIROS operational vertical sounder (TOVS) and INSAT-2E water vapour channel are examined to study water vapour distribution. Their usefulness in characterizing the Asian south-west (SW) monsoon circulation is focused. The Western Indian ocean showed an increase in mid-tropospheric moisture (700-500hPa) over about 8 to 10 days prior to the onset over Kerala coast. NOAA/TOVS layer tmperature and humidity is used to extrapolate the humidity profile at standard pressure levels. It is also used to compute latent and sensible heat flux. Total integrated water vapour from SSM/I is also used for estimating latent heat fluxes and for the diagnostics of NWP models. Recently, INSAT-2E water vapour channel was used to monitor the monsoon circiulation features. The new WV channel brought out clearly the feeding of various air masses, especially water vapour associated with monsoon onset and monsoon lows.

Seasonal predictability of the Indian Summer Monsoon : What role do land surface conditions play?

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 this paper, results from a programme of seasonal forecast ensembles are used to address, specifically, the influence of western Eurasian land surface conditions on the variability and hence predictability of the Indian summer monsoon. The factors that are important for establishing spring time land surface conditions over western Eurasia, particularly snow amounts are also investigated. The results have shown that high snow amounts over western Eurasia are linked to La Nina, suggesting that the El-Nino/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 temperature. An ensemble of summer integrations with climatological sea surface temperature (SST) has been used to investigate the impact of these anomalous land surface conditions on monsoon variability. The results have shown that the monsoon circulation is substantially weakened in association with above normal snow amounts over western Eurasia, whilst All India Rainfall is slightly increased. Results from a parallel ensemble with observed SSTs show an opposite response in All India Rainfall, suggesting that the forcing by SST anomalies is potentially dominating the monsoon's inter-annual variability. The 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, although the mechanisms involved have yet to be identified. 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 mid-latitude circulation.

Searching for a fingerprint of global warming in the Asian summer monsoon

This study investigates possible trends in several large scale indices that describe the Asian summer monsoon. Results from recent atmospheric general circulation experiments are used to provide clues as to how the monsoon might be changing due to the effects of global warming. Interestingly, this study has found that the large-scale wind shear monsoon indices have been decreasing at a rate of 0.1-0.3% per year (based on NCEP/CAR reanalysis 1958-98) in quantitative agreement with recent results from doubled CO2 simulations made using several state-of-the-art climate models. Nevertheless, despite the weakening of the monsoon circulation, all-India rainfall shows no clear trend in either the model results or in the observation reanalysis from 1958-98. Multiple regression is used to separate out the "dynamical" contribution from the observed all-India rainfall index, and a clear increasing trend then emerges in the "non-dynamical" residual. A simple dimensionless Multivariate Monsoon Index (MMI) is proposed that could be of use in monitoring global warming changes in the monsoon.

A view of global monsoons in observed and model climatology

A 17 - year (1979-95) January and July climatology obtained from a T 62/ 28 -level version of the National Centers for Environmental Prediction (NCEP) global spectral operational model is compared with a mean observed climatology for the same period obtained from its reanalysis project, with a view to finding out how well it captures some of the well-thrown characteristics of the global monsoon circulation generated seasonally by differential heating of the earth's surface by the sun in the course of its annual oscillation about the equator. Good correspondence between the two is found in the fields of mean monthly anomaly (deviation of monthly mean from the annual mean) of surface temperature, surface pressure, atmospheric circulation and total rainfall over most parts of the globe, barring a few exceptions mostly in circulation and rainfall. Large diversity in the distribution and intensity of monsoon found over different regions due to land-sea configurations, cold and warm ocean surfaces and high mountain ranges appears to be well reflected in model and observed climatology. However, the concept of a single equatorial trough moving from one hemisphere to the other to cause advance and onset of monsoon appears to fail especially over warm oceans, where there appears to be evidence in favour of two troughs, one in each hemisphere. It is the equatorial trough in the summer hemisphere that moves to bring up the monsoon in that hemisphere. There appears to be some evidence to suggest an east-west movement of monsoons between major continents and oceans.

Simulation of monsoon circulation and cyclones with different types of orography

In this study a number of sensitivity experiments have been conducted with different types of orography in the ECMWF spectral General Circulation Model (GCM). The basic aim is to simulate the large scale features of the Indian summer monsoon and the movement of cyclones. Different types of digital filters have been used to represent the orography as close to the observed values as possible. A comparative study shows that the Lanczos filter gives the best results. Thus, the Lanczos filter has been used in subsequent sensitivity experiments. One cyclone over the Bay of Bengal during pre-monsoon month of May, and another during the must active month of August have been selected for numerical experiments. The large scale features of the monsoonal rainfall were found to be close to those observed, when standard deviation envelope orography was used. After 3 days of model integration, the rainfall distribution improved compared to the initial stages of integration. Although the predicted cyclone followed the observed track, the rate of movement was very sluggish. There was slight deepening of the systems with an increase of orography The movement of the systems was also found to be slightly faster with the enhancement of orography.