Volume 101, Issue 1
March 1992, pages 1-107
pp 1-11 March 1992
Parasnis has observed in a presidential address that geophysics is not a Popperian science in a major way. That is, hypotheses are not consciously put forth in a falsifiable format and much of the effort goes in seeking supporting evidence for favoured hypotheses. Parker evolved a parameter extremization strategy, initially to tackle the problem of non-uniqueness in geophysical inference. Later he based a hypothesis testing proposal on it, which is refreshingly Popperian. It has not been adopted widely, partly because it requires global extrema, and not local and this has been regarded as a problem with no solution. Attention is drawn towards tunnelling algorithm, which solves the problem of global optimization successfully, makes Parker’s Popperian proposal practical and extends the range of Popperian geophysics.
pp 13-25 March 1992
A hierarchy of climate models have been developed and applied to the problem of doubling the CO2 concentration in the atmosphere. Currently available general circulation models include the most complete treatment of the global wanning and are capable of providing changes in several of the meteorological parameters in time scales of half a century or even more. Much skill is gradually being achieved now for future climate simulations. In this paper, we have attempted to describe the response of the National Center for Atmospheric Research Climate Community Model (NCAR CCM), whose performance for northern hemispheric climate simulations was reported to be very satisfactory to Indian region. The seasonal (winter and summer) changes in surface temperature, rainfall and soil moisture expected over the Indian sub-continent due to doubling of CO2 in the atmosphere as inferred from model output statistics are discussed. A probable scenario for sea level rise along the Indian coastline by the year 2030 AD as a result of ocean water’s expansion due to global warming is outlined. These projections should not be treated as predictions of what is going to happen over the Indian sub-continent. Rather, they merely illustrate to what extent we might be affected by the future climate change.
pp 27-34 March 1992
The time series of Indian summer monsoon rainfall for the period 1871–1989 has been analysed using the method of deterministic chaos. It is found that a strange attractor underlies the time series implying the existence of a prediction function. This function has been approximated by a second-degree polynomial, involving the rainfalls of the past seven years and the coefficients have been estimated by least squares fit. The interannual variations of actual and computed rainfalls have been presented for a comparative study.
pp 35-46 March 1992
The main advantages of constant potential enthalpy as a vertical coordinate are weaker horizontal velocity gradients in frontal regions and a higher vertical resolution. A disadvantage is the intersection of isentropes with the ground and folding of these surfaces. A numerical model is proposed to overcome the difficulties imposed by the intersection of isentropes with the ground. The model contains a physical and computational domain. The top and bottom surfaces of the computational domain are isentropes whereas the physical domain of the flow confined above by a free surface of constant pressure, and the bottom of this domain is the surface of the earth. In the present study the top surfaces of these two domains coincide (θT, PTare constants in space and time). The model was tested for the study of frontogenesis and cyclogenesis and integrated for 7 days. The results correspond to enstrophy-conserving finite difference scheme.
pp 47-66 March 1992
A two-dimensional, nonlinear, vertically integrated model was used to simulate depth-mean wind-driven circulation in the upper Ekman layers of the Bay of Bengal and Andaman Sea. The model resolution was one third of a degree in the latitude and longitude directions. Monthly mean wind stress components used to drive the model were obtained from the climatic monthly mean wind data compiled by Hastenrath and Lamb. A steady-state solution was obtained after numerical integration of the model for 15 days. The sensitivity of the model to two types of open boundary conditions, namely, a radiation type and clamped type, was tested. A comparison of simulated results for January with available ship drift data showed that the application of the latter along the open boundary could reproduce all the observed features near the boundary and the interior of the model domain. The model was integrated for 365 days to study the circulation during the southwest and northeast monsoon seasons. The model was successful in simulating the broad features of circulation including gyres and eddies observed during both the seasons, the development of north equatorial current during the northeast monsoon period and eastward moving monsoon drift current up to 90°E during the southwest monsoon season. During the latter season, two anticyclonic gyres were observed in the central and the southern parts of the Bay. A cyclonic type of circulation was prevalent in the central and western parts of the Bay of Bengal during the northeast monsoon months of November and December. The simulated western boundary current along the east coast of India, flows northward and southward during the southwest and northeast monsoon seasons respectively. It is presumed that this western boundary current, simulated during both the seasons, is locally wind-driven.
pp 67-75 March 1992
The distribution of temperature and salinity in the upper 500 m of the northwestern Bay of Bengal, adjoining the east coast of India, during the retreat of southwest monsoon (September) of 1983 is presented. This study reveals coastal upwelling (limited to the upper 40 m) induced by the local winds. Waters of higher surface salinity near the coast characterize the upwelling. The freshwater influx near the head of the Bay diluted the surface salinity to as low as 26.0 × 10−3. The surface circulation was weak and led to a net transport of 2.0 × 106m3.s−1 directed towards northeast.
pp 77-88 March 1992
An algorithm for the solution of a nonlinear problem of phase boundary movement and evolution of temperature distribution due to the perturbation in the basal heat flux has been discussed. The reduction of the problem to a system of nonlinear ordinary differential equations with the help of a Fourier series method leads to a stiff system. This stiffness is taken care of by the use of a modified Euler’s method. Various cases of basal heat flow variation have been considered to show the performance and stability of the technique for such a nonlinear system. The first case of step-wise function is taken to analyse the performance of the technique, and the study has been extended to other general cases of linear increase, periodic variation, and box and triangular function type variations in the heat flux. In the step-wise case the phase boundary attains a constant position rapidly if the supplied heat flux is sufficiently large. The effect of periodicity in the heat flow is clearly depicted in the phase boundary movement, where the phase boundary oscillates about the mean position at large times. The absence of any constant level in the case of linear increase in heat flux is due to a very large value of heat flux. In the cases of box car and triangular heat flux the boundary starts moving downward after the cessation of excess heat flux but does not immediately return to its original preperturbation state, instead approaches it at large times. This technique may be applied to more general cases of heat flow variation.
pp 89-98 March 1992
A systematic study of the major ion chemistry of the Ganga source waters—the Bhagirathi, Alaknanda and their tributaries—has been carried out to assess the chemical weathering processes in the high altitude Himalaya. Among major ions, Ca, Mg, HCO3 and SO4 are the most abundant in these river waters. These results suggest that weathering of carbonate rocks by carbonic and sulphuric acids dominates in these drainage basins. On an average, silicate weathering can contribute up to ∼ 30% of the total cations.
The concentration of total dissolved salts in the Bhagirathi and the Alaknanda is 104 and 115mg/l, respectively. The chemical denudation rate in the drainage basins of the Bhagirathi and the Alaknanda is, respectively, 110 and 137 tons/km2/yr, significantly higher than that derived for the entire Ganga basin, indicating intense chemical erosion of the Himalaya.
pp 99-107 March 1992
Tholeiitic pillow basalt from the South Andaman island, an integral part of the outer sedimentary arc of the Sunda-Burmese double chain arc system in the Bay of Bengal, is characterized by the occurrence of several morphologies of quenched crystals of plagioclase and pyroxene. Plagioclase shows a swallow tail, belt-buckle, rosette and closely spaced fan-spherulites pattern while pyroxene has elongate parallel chain, dendritic, spherulitic and finely ornamented feathery spherulitic habit. Most of these textures are identical to those reported from submarine basalts, lunar basalts, spinifex textured rocks and experimentally produced textures. The occurrence of these quench textures in the Andaman basalt suggests that they were formed by rapid cooling at 30–70°C/h in a submarine environment.