Volume 107, Issue 1
March 1998, pages 1-105
pp 1-2 March 1998
pp 3-3 March 1998
pp 5-18 March 1998
Moisture profiles have been estimated over the region bounded by the latitudes 40°N and 40°S and longitudes 30°E to 130°E using INSAT digital infra red cloud imagery data. The representativeness of these profiles in representing moisture field associated with the development and movement of synoptic scale systems during the period September 15th, 1996 to March 31st, 1997 has been examined. It has been shown that the changes in the moisture field associated with the withdrawal of the southwest and northeast monsoons from the Indian sub-continent, development and movement of synoptic scale sytems (depressions, tropical cyclones and waves in easterlies) and equatorial troughs in the Indian Ocean could be clearly seen in humidity profiles. The initial development of tropical systems is first seen in the humidity field in the upper troposphere. These profiles could be used in monitoring the initial development and subsequent movement of tropical systems. Further the data on moisture distribution from the data gap regions of the Indian Ocean could be used as an additional source of moisture in numerical analysis and prognosis.
pp 19-32 March 1998
In this paper satellite-derived radiative energy budget such as shortwave radiative heating, longwave radiative heating and net radiation balance have been studied for the post-onset phase of summer monsoon 1979. Since clouds play an important role in determining diabatic heating field as well as being a reflection of status of the monsoon itself, the day to day evolution of clouds from TIROS-N satellite has been made. Satellite-derived radiative heating rates from surface to 100 hPa were computed for each 100 hPa thickness layer. These heating rates were then compared with the observed latitudinal distribution of total radiative heating rates over the domain of the study.
From the results of our study it was found that the characteristic features such as net radiative heating rates of the order of 0.2°C/day at upper tropospheric layer (100–200 hPa) and cooling throughout the lower tropospheric layers with relatively less cooling between 500–700 hPa layer observed in a case of satellite-derived radiative energy budget agree well with the characteristic features of observational radiative energy budget over the domain of the study. Therefore, it is suggested that radiative energy budget derived from satellite observations can be used with great potential and confidence for the evolution of the complete life cycle of the monsoon over the Indian region for different years.
pp 33-43 March 1998
Detailed analysis of the surface winds over the Indian Ocean derived from ERS-1 scatterometer data during the years 1993 and 1994 has been used to understand and unambiguously identify the onset phase of south-west monsoon. Five day (pentad) averaged wind vectors for the period April to June during both years have been examined to study the exact reversal of wind direction as well as the increase in wind speed over the Arabian Sea in relation to the onset of monsoon over the Indian west coast (Kerala). The related upper level humidity available from other satellites has also been analysed.
The results of our analysis clearly show a consistent dramatic reversal in wind direction over the western Arabian Sea three weeks in advance of the onset of monsoon. The wind speed shows a large increase coinciding with the onset of monsoon. These findings together show the dominant role of sea surface winds in establishing the monsoon circulation. The study confirms that the cross equatorial current phenomenon becomes more important after the onset of monsoon.
pp 45-64 March 1998
The role of intraseasonal oscillations (ISOs) in modulating synoptic and interannual variations of surface winds over the Indian monsoon region is studied using daily averaged National Centers for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) reanalyses for the period 1987–1996. Two dominant ISOs are found in all years, with a period between 30–60 days and 10–20 days respectively. Although the ISOs themselves explain only about 10–25% of the daily variance, the spatial structure of variance of the ISOs is found to be nearly identical to that of high frequency activity (synoptic disturbances), indicating a significant control by the ISOs in determining the synoptic variations. Zonal and meridional propagation characteristics of the two modes and their interannual variability are studied in detail.
The synoptic structure of the 30–60 day mode is similar in all years and is shown to be intimately related to the strong (‘active’) or weak (‘break’) phases of the Indian summer monsoon circulation. The peak (trough) phase of the mode in the north Bay of Bengal corresponds to the ‘active’ (‘break’) phase of monsoon strengthening (weakening) the entire large scale monsoon circulation. The ISOs modulate synoptic activity through the intensification or weakening of the large scale monsoon flow (monsoon trough). The peak wind anomalies associated with these ISOs could be as large as 30% of the seasonal mean winds in many regions. The vorticity pattern associated with the 30–60 day mode has a bi-modal meridional structure similar to the one associated with the seasonal mean winds but with a smaller meridional scale. The spatial structure of the 30–60 day mode is consistent with fluctuations of the tropical convergence zone (TCZ) between one continental and an equatorial Indian Ocean position. The 10–20 day mode has maximum amplitude in the north Bay of Bengal, where it is comparable to that of the 30–60 day mode. Elsewhere in the Indian Ocean, this mode is almost always weaker than the 30–60 day mode. In the Bay of Bengal region, the wind curl anomalies associated with the peak phases of the ISOs could be as large as 50% of the seasonal mean wind curl. Hence, ISOs in this region could drive significant ISOs in the ocean and might influence the seasonal mean currents in the Bay.
On the interannual time scale, the NCEP/NCAR reanalysed wind stress is compared with the Florida State University monthly mean stress. The seasonal mean stress as well as interannual standard deviation of monthly stress from the two analyses agree well, indicating absence of any serious systematic bias in the NCEP/NCAR reanalysed winds. It is also found that the composite structure of the 30–60 day mode is strikingly similar to the dominant mode of interannual variability of the seasonal mean winds indicating a strong link between the ISOs and the seasonal mean. The ISO influences the seasonal mean and its interannual variability either through increased/decreased residence time of the TCZ in the continental position or through occurrence of stronger/weaker active/break spells. Thus, the ISOs seem to modulate all variability in this region from synoptic to interannual scales.
pp 65-90 March 1998
Ensemble seasonal integrations are carried out with the COLA GCM, with a view to understand the dynamical connection between warm SST anomalies in the equatorial central-eastern Pacific Ocean and the upper level stationary wave anomalies seen during drought years over the Indian summer monsoon region. In addition, experiments with and without orography are performed in order to examine the role of the Himalayas in modulating the El Niño induced stationary wave anomalies over the summer monsoon region.
The GCM simulations show a statistically significant weakening of the summer monsoon activity over India in response to the SST forcing in the equatorial Pacific Ocean. This weakening of the summer monsoon appears to be largely related to modifications of the local Hadley and Walker cells over the summer monsoon region. In addition, it is seen that the anomalous ENSO divergent forcing over the tropical Pacific Ocean can act as a potential source for Rossby wave dispersion. Here one finds the possibility of meridionally propagating Rossby waves, which emanate from the ENSO forcing region, to interact with the subtropical westerlies and generate anomalous highs and lows in the subtropics and extratropics. The quasi-stationary perturbations seen over west Asia, Pakistan and northwest India during drought years, seem to be generated by the above mechanism. An alternate mechanism that could be important for the persistence of the quasi-stationary perturbations seems to be based on the dynamic excitation of middle latitude normal modes which can extract energy from the zonally varying unstable basic flow.
It is seen from the GCM simulations, that the Himalayan orography plays a crucial role in anchoring the El Niño induced extratropical westerly troughs far to the west in the high latitude belt. In the absence of orography it is seen that the ENSO induced extra-tropical cyclonic anomalies tend to intrude southward into the monsoon region thereby destroying the regional scale circulations completely. Another effect due to the Himalayas is to generate lee waves on the eastern side of the topographic barrier which encircle the globe in the subtropics and midlatitudes.
pp 91-95 March 1998
An Ocean-Atmosphere Index (OAI) for ENSO is developed using data on Southern Oscillation Index (SOI) and sea surface temperature (SST) over eastern equatorial Pacific. Seasonal values of OAI, SOI and SST have been computed for the seasons September-October-November (SON), December-January-February (DJF), March-April-May (MAM) and June-July-August (JJA). Similarly SON to DJF, DJF to MAM, MAM to JJA and JJA to SON tendencies have been worked out for SOI, SST and OAI. The relationships between Indian Monsoon Rainfall (IMR) and SOI/SST/OAI, (i) for the seasons SON, DJF and MAM before and after the monsoon and JJA concurrent with the monsoon and (ii) for SON to DJF and DJF to MAM tendencies before and after the monsoon, and MAM to JJA tendency concurrent with the monsoon have been explored. It is found that IMR is more influenced by SOI before the monsoon than it is influenced by SST before the monsoon and IMR affects SST after monsoon more strongly than it affects SOI after the monsoon. It is also observed that DJF to MAM tendencies for SOI, SST and OAI before monsoon are significantly related to IMR, among which the relationship between IMR and DJF to MAM tendency for OAI is the best.
pp 97-105 March 1998
Surface features such as soil moisture and vegetation have a profound impact on the surface energy balance and the atmospheric boundary layer. To quantify this effect for a tropical location, a detailed field experiment, VEBEX, was designed and successfully executed in a tropical site at Bangalore, India. VEBEX was a joint experiment between the North Carolina State University, Indian Institute of Science (IISc), and the University of Agricultural Science (UAS) at Bangalore, India. Continuous surface meteorological measurements were taken over an entire crop period (pre-sowing to post-harvest). During different stages of the plant growth, intensive observations of surface turbulence, and measurements of physiological and soil moisture measurements were also conducted. The results obtained provide an insight into the unusually strong variability for the tropics. Interpretation of the observations and an overview of the analysis procedure and future research initiatives are also presented.