The problem of obtaining the area average rainfall from scattered raingauge data on to a regular grid has been addressed. A new interpolation formula has been obtained using finite element method giving due consideration to missing observations in any raingauge station data time series. An important feature of this method is that there is no need to redesign the triangulation of the given region in case: (a) if number of observations increase in this region, or (b) if observations are missing in the data time series of any station chosen as a node in triangulating this region. Their contribution to the areal precipitation has been incorporated in a mathematically consistent manner. As an application of this method, daily rainfall time series of 119 raingauge stations for four successive summer monsoons (1986-1989) are utilized to obtain the daily precipitation values on a spatial grid with 1.8° × 3.6° (lat/lon) resolution. From the daily gridded rainfall, pentads (5-day averages) of rainfall are obtained. These pentads are arranged in a time series and a principal component analysis (PCA) is performed. The first four principal components explain 63% of the total variance of rainfall over India. The oscillations depicted by these modes agree well with earlier studies. It is concluded that a few dominant PCs of rainfall calculated from pentads (as compared to seasonal and annual average rainfall) explain a larger part of the total variance of rainfall. This method is essentially related to large-scale rainfall depiction. It could, therefore, easily be used to produce global gridded precipitation fields from raingauge measurements. The spatial interpolation code is available on request from the first author.

During the Indian Ocean Experiment (INDOEX) Intensive Field Phase, 17 superpressure balloons drifting at lower tropospheric levels were launched from Goa to sample the motion and meteorological characteristics of polluted air masses originating from the Indian subcontinent. The mass flow, as expected, is attracted to the Intertropical Convergence Zone (ITCZ), which is reached in about 7 days. Successive trajectories show evidence of shifts in the flow regime, consistent with Meteosat‐5 imagery and illustrative of the Madden–Julian oscillation. Balloons also show evidence of coastal anticyclonic circulations off the western coast of India, linked to both the orography of the Ghâts and the diurnal coastal wind mesoscale wind systems. These mesoscale circulations may be important in the transport of anthropogenic or mineral pollutants across the coast. The meteorological data gathered on board the balloons were systematically compared with European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses interpolated in time and space at the balloons' successive locations every half‐hour. The agreement is generally good in terms of wind velocities, temperature, and humidity. However, above the marine boundary layer, balloon data seem to indicate a cold bias of analyzed temperatures and a northeasterly bias of horizontal winds. The underestimation of variability in the analyzed winds can be partly explained by mesh size smoothing in the analysis system.

Over the tropics the atmospheric general circulation models usually fail in predicting horizontal wind divergence, which is closely related to atmospheric heating and to the vertical exchanges associated with convection. With the aim of forcing atmospheric models we present here a reconstruction of wind divergences based on the links between infrared brightness temperatures, convective activity, and large‐scale divergence. In practice, wind divergences are reconstructed from brightness temperatures using correlations obtained from numerical simulations performed with a general circulation model. When building those correlations, a distinction must be made between the brightness temperatures of opaque clouds and those of semitransparent clouds, only the former being directly associated with convection. In order to filter out semitransparent clouds we use radiative thresholds in the water vapor channel in addition to the window channel. We apply our approach to Meteosat‐5 data over the Indian Ocean. Comparison with wind divergences reconstructed independently from Meteosat water vapor winds partially validates our retrieval. Comparison with European Center for Medium‐Range Weather Forecasts analyses indicates that much can be gained by adding information on the wind divergence in the tropics to force an atmospheric model.

'Meteorology Facility, TERLS/SHAR, Vikram Sarabhai Space Centre, ISRO, Thiruvananthapuram 695 022, India Space Physics Laboratory, Vikram Sarabhai Space Centre. ISRO, Thiruvananthapuram 695 022, India Department of Physics, Goa University, Goa 403 206, India Physical Oceanography Division, National Institute of Oceanography, Goa 403 004, India Meteorological Observatory, Panjim, Goa 403 001, India National Centre for Medium Range Weather Forecasting, Mausam Bhavan Complex, New Delhi 110 003, India INDOEX-India Programme Office, ISTRAC, Bangalore 560 058, India Nalional Physical Laboratory, New Delhi 110 012, India Laboratory for Dynamic Meteorology, Ecole Normale superieure, 75005 Paris, France Laboratory for Dynamic Meteorology, Ecole Polytechnique, 91128 Palaiseau Cedex, France Balloon Division, Toulouse Space Centre, DSO/ED/BANE, 18, Avenue Edouard Belin, 31401, Toulouse Cedex, France

During the IFP-99 of the INDOEX programme, an Indo-French joint scientific team conducted 17 constant balloon flights from Goa mainly to study the various features of the continental air mass flow from the Indian subcontinent towards the ITCZ induced by the NE winter circulation and thereby to derive the transportation of the continental aerosols and trace gases to the Indian Ocean regions. Out of the 17 flights, 15 were successful. Data obtained from the Constant Altitude Balloon Experiment are analysed to study the features of the wind fields and thermodynamic parameters of the marine boundary layer at ~ 900 hPa level over the oceanic regions adjoining west-coast of Peninsular India. From the balloon co-ordinates, the instantaneous vector wind as well as its zonal and meridional components are estimated. Also estimated are the ambient air temperature and relative humidity at 30 min interval. These data are examined for the spatial features. Mean wind showed presence of three broad flow channels, from India, Africa and West Asia, that are important in influencing the Arabian Sea environment and provide potential pathways for transport of continental pollutants. Variations associated with the M.) oscillations are present in the zonal wind, close to the equator. On several occasions wind showed occurrence of eddies with anticyclonic circulation having temporal scales of one to five days. Temperature and humidity structure showed large dry areas over the Arabian Sea associated with the flow channels. Rather humid regions occur over coastal India and near the ITCZ.

We study data from the Central Equatorial Pacific Experiment, in particular the soundings providing temperature, pressure, wind and ozone profiles, and we link them to the large‐scale conditions deduced from the analyses of the European Centre for Medium‐Range Weather Forecasts. to the west of the date line, the mean wind shows a strong shear at the tropopause between the easterlies of the upper troposphere and the westerlies of the lower stratosphere. These are conditions propitious to the formation of critical levels for quasi‐stationary gravity waves generated by deep convection. the measured wind profiles show the main characteristics of the encounter of a wave with its critical level and, in the ozone and potential‐temperature profiles, the existence of a conspicuous mixing layer in the vicinity of this critical level. Our interpretation that the mixing layer is a consequence of the critical level is supported by numerical simulations with a nonlinear time‐dependent two‐dimensional model. the occurrence of a mixing layer in the tropopause structure seems to be characteristic of large‐scale equatorial convective areas during westerly phases of the quasi‐biennial oscillation.

The mechanisms responsible for the mean state and the seasonal and interannual variations of the coupled tropical Pacific-global atmosphere system are investigated by analyzing a thirty year simulation, where the LMD global atmospheric model and the LODYC tropical Pacific model are coupled using the delocalized physics method. No flux correction is needed over the tropical region. The coupled model reaches its regime state roughly after one year of integration in spite of the fact that the ocean is initialized from rest. Departures from the mean state are characterized by oscillations with dominant periodicites at annual, biennial and quadriennial time scales. In our model, equatorial sea surface temperature and wind stress fluctuations evolved in phase. In the Central Pacific during boreal autumn, the sea surface temperature is cold, the wind stress is strong, and the Inter Tropical Convergence Zone (ITCZ) is shifted northwards. The northward shift of the ITCZ enhances atmospheric and oceanic subsidence between the equator and the latitude of organized convention. In turn, the stronger oceanic subsidence reinforces equatorward convergence of water masses at the thermocline depth which, being not balanced by equatorial upwelling, deepens the equatorial thermocline. An equivalent view is that the deepening of the thermocline proceeds from the weakening of the meridional draining of near-surface equatorial waters. The inverse picture prevails during spring, when the equatorial sea surface temperatures are warm. Thus temperature anomalies tend to appear at the thermocline level, in phase opposition to the surface conditions. These subsurface temperature fluctuations propagate from the Central Pacific eastwards along the thermocline; when reaching the surface in the Eastern Pacific, they trigger the reversal of sea surface temperature anomalies. The whole oscillation is synchronized by the apparent meridional motion of the sun, through the seasonal oscillation of the ITCZ. This possible mechanism is partly supported by the observed seasonal reversal of vorticity between the equator and the ITCZ, and by observational evidence of eastward propagating subsurface temperature anomalies at the thermocline level.

The thirty year simulation of the coupled global atmosphere-tropical Pacific Ocean general circulation model of the Laboratoire de Métérologie Dynamique and the Laboratoire d’Océanographie Dynamique et de Climatologie presented in Part I is further investigated in order to understand the mechanisms of interannual variability. The model does simulate interannual events with ENSO characteristics; the dominant periodicity is quasi-biennial, though strong events are separated by four year intervals. The mechanism that is responsible for seasonal oscillations, identified in Part I, is also active in interannual variability with the difference that now the Western Pacific is dynamically involved. A warm interannual phase is associated with an equatorward shift of the ITCZ in the Western and Central Pacific. The coupling between the ITCZ and the ocean circulation is then responsible for the cooling of the equatorial subsurface by the draining mechanism. Cold subsurface temperature anomalies then propagate eastward along the mean equatorial thermocline. Upon reaching the Eastern Pacific where the mean thermocline is shallow, cold subsurface anomalies affect surface temperatures and reverse the phase of the oscillation. The preferred season for efficient eastward propagation of thermocline depth temperature anomalies is boreal autumn, when draining of equatorial waters towards higher latitudes is weaker than in spring by a factor of six. In that way, the annual cycle acts as a dam that synchronizes lower frequency oscillations.

The response of the Indian summer monsoon to the direct effect of sulphate aerosols is examined using a general circulation model in addition to a chemical transport model. Three scenarios for sulphur dioxide emissions are considered, yielding to past, present, and future concentrations of sulphate aerosols. The induced changes in monsoon precipitation and dynamics are investigated from ensemble means of a set of independent simulations. A general decrease in precipitation over India and South-East Asia is observed when future aerosol concentrations are included in the model. The changes in precipitation are associated with a weakening of the pressure gradient over the Indian region and changes in the large-scale circulation. The response of the monsoon to aerosol forcing is different from the response to the 1987-1988 ENSO sea surface temperature forcing.

Several recent works attribute the formation of polar stratospheric clouds (PSCs) to the occurrence of localized orographic waves. Using ECMWF analyses, we investigate the large scale stratospheric flow conditions in a number of cases where PSCs have been detected both in the Arctic and in the Antarctic. We show that PSCs appear within strong planetary scale uplifts of isentropic surfaces. The adiabatic cooling of air parcels travelling within such planetary scale uplifts while conserving their humidity and trace constituents, seems to be the main mechanism for PSC formation. The PSC distribution would then follow a planetary structure, even though local orographic waves could still play an additional role when planetary scale conditions are met.

The sensitivity of the interannual variations of the summer monsoons to imposed cloudiness has been studied with a general circulation model using the initial conditions prepared from the European Centre for Medium-Range Forecasts analyses of 1 May 1987 and 1988. The cloud optical properties in this global model are calculated from prognostically computed cloud liquid water. The model successfully simulates the contrasting behavior of these two successive monsoons. However, when the optical properties of the observed clouds are specified in the model runs, the simulations show some degradation over India and its vicinity. The main cause of this degradation is the reduced land–sea temperature contrast resulting from the radiative effects of the observed clouds imposed in such simulations. It is argued that the high concentration of condensed water content of clouds over the Indian land areas will serve to limit heating of the land, thereby reducing the thermal contrast that gives rise to a weak Somali jet. A countermonsoon circulation is, therefore, simulated in the vector difference field of 850-hPa winds from the model runs with externally specified clouds. This countermonsoon circulation is associated with an equatorial heat source that is the response of the model to the radiative effects of the imposed clouds. Indeed, there are at least two clear points that can be made: 1) the cloud–SST patterns, together, affect the interannual variability; and 2) with both clouds and SST imposed, the model simulation is less sensitive to initial conditions. Additionally, the study emphasizes the importance of dynamically consistent clouds developing in response to the dynamical, thermal, and moist state of the atmosphere during model integrations.

Le cycle de l'eau joue un role central dans la dynamique du climat terrestre. La vapeur d'eau dans l'atmosphere est le plus abondant et le plus efficace des gaz a effet de serre, loin devant le dioxyde de carbone. Elle joue donc un role de premier plan dans le bilan radiatif global de la planete: le flux vers le bas dans l'infrarouge du a l'effet de serre de la vapeur d'eau (qui correspond a un rechauffement de la surface) est en moyenne de l'ordre de 100 W m-2 , valeur a comparer aux 240 W m-2 du flux solaire net absorbe; a cet effet de serre de la vapeur d'eau, il faut ajouter l'effet de serre des nuages, qui correspond a un flux vers le bas dans l'infrarouge de 31 W m-2 environ. En plus de leur role dans l'effet de serre, les nuages reflechissent l'energie solaire; cet effet entraine un deficit d'ensoleillement des basses couches, ou en d'autres termes un refroidissement de la surface, qui correspond a un flux vers le haut de 47 W m-2. Les effets energetiques du cycle de l'eau ne se limitent d'ailleurs pas aux aspects radiatifs, car la vapeur d'eau transporte aussi de l'energie sous forme de chaleur latente. Cette energie latente, alimentee par l'evaporation a la surface, est transportee par les mouvements de l'air et se libere lors de la condensation sous forme de gouttelettes d'eau ou de cristaux de neige ou de glace. Le flux d'energie correspondant, donne par l'evaporation moyenne globale, se situe aux environs de 82 W m-2 . On voit que l'ensemble de ces processus mettent en jeu des flux d'energie voisins du flux d'energie solaire.

Lagged relationships between the Indian summer monsoon and several climate variables are investigated. The variables examined are gridded fields of snow cover (14 years), sea surface temperature (41 years) and 500 hPa geopotential height north of 20 °N (42 years). We also used series of global air temperature (108 years) and Southern Oscillation index (112 years). Precipitation over all India during June–September over a 112 year period are used as Indian monsoon index. Emphasis is put on early monsoon precursors. In agreement with the tendency for a low frequency oscillation in the ocean-atmosphere system, several precursor patterns are identified as early as the year preceding the monsoon. The most important key regions and seasons of largest correlations are selected and the corresponding series are used to perform a monsoon prediction. The prediction shows however a relatively moderate score mainly due to the not highly significant correlations. To improve the predictions we filtered the variables into their biennial (1.5–3.5 years) and low frequency (3.5–7.5 years) modes. Correlations between the monsoon and the filtered variables are higher than those obtained without filtering especially for the biennial mode. The two modes are out-of-phase before the monsoon and in-phase during and after. This phasing is found in all variables except for snow cover for which the two modes are in-phase before the monsoon and out-of-phase during and after. It is suggested that such phasing may be important for the formation of snow and could explain the higher correlations when variables are concomitant or are lagging the monsoon. Early predictions of the monsoon based on those two modes show improved scores with highly significant correlations with the actual monsoon.

Results from 90-day simulations with the LMD GCM are described, where sea surface temperatures of 1987 or 1988 years are respectively prescribed. The initial states correspond to 1 June 1987 and 1 June 1988. The simulated precipitation rates over India show a strong contrast between the two years, with drought occurring during summer 1987 and abundant rainfall during summer 1988. The dry regime simulated during 1957 corresponds to an eastward displacement of the outflow at 200 mb and a weaker westerly flow at the surface as compared with 1988, both features being in agreement with reality. Because it is more difficult for models to simulate rainfall differences than to simulate wind variations between the two years, the changes in simulated rainfall over India are studied in more detail. In particular, more integrations are carried out to test the sensitivity of rainfall variations to initial conditions, and the result is that the decrease of rainfall in 1987 compared to 1988 is a robust feature of the model.Very early, the importance of evapotranspiration in simulating land rainfall was emphasized. Additional integrations are performed in order to study the impact of the new vegetation scheme introduced in the LMD GCM. It is shown that the contrast in rainfall between the two years is better simulated when the evapotranspiration rate of vegetation cover is represented. When vegetation is not represented in the model, the model does not simulate accurately the interannual variation of the precipitation rates.

The relationship between interannual variability of rainfall in north-east Brazil and tropical sea-surface temperature is studied using observations and model simulations. The simulated precipitation is the average of seven independent realizations performed using the Laboratoire de Meteorologie Dynamique atmospheric general model forced by the 1970-1988 observed sea-surface temperature. The model reproduces very well the rainfall anomalies (correlation of 0 . 91 between observed and modelled anomalies). The study confirms that precipitation in north-east Brazil is highly correlated to the sea-surface temperature in the tropical Atlantic and Pacific oceans. Using the singular value decomposition method we find that Nordeste rainfall is modulated by two independent oscillations, both governed by the Atlantic dipole, but one involving only the Pacific, the other one having a period of about 10 years. Correlations between precipitation in north-east Brazil during February-May and the sea-surface temperature 6 months earlier indicate that both modes are essential to estimate the quality of the rainy season.

The atmospheric circulation of Titan is investigated with a general circulation model. The representation of the large-scale dynamics is based on a grid point model developed and used at Laboratoire de Météorologie Dynamique for climate studies. The code also includes an accurate representation of radiative heating and cooling by molecular gases and haze as well as a parametrization of the vertical turbulent mixing of momentum and potential temperature. Long-term simulations of the atmospheric circulation are presented. Starting from a state of rest, the model spontaneously produces a strong superrotation with prograde equatorial winds (i.e., in the same sense as the assumed rotation of the solid body) increasing from the surface to reach 100 m sec-1 near the 1-mbar pressure level. Those equatorial winds are in very good agreement with some indirect observations, especially those of the 1989 occultation of Star 28-Sgr by Titan. On the other hand, the model simulates latitudinal temperature contrasts in the stratosphere that are significantly weaker than those observed by Voyager 1 which, we suggest, may be partly due to the nonrepresentation of the spatial and temporal variations of the abundances of molecular species and haze. We present diagnostics of the simulated atmospheric circulation underlying the importance of the seasonal cycle and a tentative explanation for the creation and maintenance of the atmospheric superrotation based on a careful angular momentum budget.

The STRATÉOLE experiment is designed to study the wintertime Antarctic lower stratosphere polar vortex and its springtime breakdown. To this end, it is planned to fly a large number (around 200) of long-lived (3 months), small isopycnic drifting balloons instrumented with temperature and pressure sensors, GPS and transmitters. The main goal of STRATÉOLE experiment is to provide an unprecedented documentation of the wind field in the vicinity of the vortex edge in order to study vortex porosity and erosion, filamentation and mixing properties of the air masses. In addition, by the use of other sensors on some gondola, like radiometers and tuneable laser diodes, STRATÉOLE will also provide in-situ and/or column-integrated trace species measurements (like NO2, O3, CH4, H2O, aerosols...) and information on the radiation budget of the Antarctic lower stratosphere during this period. This will permit to obtain a better understanding of mechanisms responsible for ozone depletion occurring during the springtime vortex dilution.

Fluctuations of solar irradiance can have a significant impact on terrestrial climate only if they are strong enough and last long enough to induce significant warming of the upper ocean. This is hardly true for the eleven year cycle, whose amplitude in terms of mean absorbed solar flux is less than 0.1 Wm −2. The Maunder Minimum case is somewhat different, as the solar luminosity in the second half of the XVIIth Century may have been less than its present value by as much as 0.4%, i.e. about 1 Wm −2. The Maunder Minimum is therefore a possible candidate for explaining the socalled Little Ice Age episode. It is, however, very difficult to really assess the relevance of such an hypothesis, as the chronology of the Little Ice Age itself is somewhat blurred, and we lack quantitative data on various possible other perturbation mechanisms that could have induced cooling, such as volcanic aerosols or a significant change in the deep ocean circulation. Concerning the Little Ice Age itself, the present data situation is not so good : XVIIth Century paleotemperature data are scarce, and the most abundant data concern glacier extensions, which cannot be directly used for climate model validation ; and we cannot expect fast improvement of this situation.