K. Krishna Kumar

Diurnal asymmetry of surface temperature trends over India

Diurnal asymmetry of the recent surface warming, reported for several continental areas, is believed to be related to changes in cloudiness, humidity, atmospheric circulation patterns, winds and soil moisture. This paper presents linear trend analyses of maximum and minimum temperature data at 121 stations in India during the period 1901–87. While the mean temperature trends over India are similar to the global and hemispheric trends, there are marked differences in their diurnal manifestation from those reported for other areas. The increase in the mean temperatures over India is almost solely contributed by the maximum temperatures, with the minimum temperatures remaining practically trendless, leading to an increase in the diurnal range of temperatures. These trends do not show any significant urban or altitude bias. There are some differences in the trends on smaller spatial and temporal scales, but the increase of maximum temperature is predominant over a major part of India, particularly in winter and post-monsoon seasons.

Interannual and Interdecadal Variability of Thailand Summer Monsoon Season

Summer monsoon rains are a critical factor in Thailand’s water resources and agricultural planning and management. In fact, they have a significant impact on the country’s economic health. Consequently, understanding the variability of the summer monsoon rains over Thailand is important for instituting effective mitigating strategies against extreme rainfall fluctuations. To this end, the authors systematically investigated the relationships between summer monsoon precipitation from the central and northern regions of Thailand and large-scale climate features. It was found that Pacific sea surface temperatures (SSTs), in particular, El Nino–Southern Oscillation (ENSO), have a negative relationship with the summer monsoon rainfall over Thailand in recent decades. However, the relationship between summer rainfall and ENSO was weak prior to 1980. It is hypothesized that the ENSO teleconnection depends on the SST configuration in the tropical Pacific Ocean, that is, an eastern Pacific–based El Nino pattern, such as is the case in most of the post-1980 El Nino events, tends to place the descending limb of the Walker circulation over the Thailand–Indonesian region, thereby significantly reducing convection and consequently, rainfall over Thailand. It is believed that this recent shift in the Walker circulation is instrumental for the nonstationarity in ENSO–monsoon relationships in Thailand. El Ninos of 1997 and 2002 corroborate this hypothesis. This has implications for monsoon rainfall forecasting and, consequently, for resources planning and management.

Space-Time Evolution of Meteorological Features Associated with the Onset of Indian Summer Monsoon

Abstract To study the climatological structure of the atmospheric fields during the onset phase of the Indian summer monsoon, a composite analysis of different meteorological parameters over Indian stations is carried out. The composites are constructed relative to a uniform set of onset dates over south Kerala. Over the peninsular Indian stations, the rainfall composites show sudden and sharp increases with onset except in the case of east coast stations, where rainfall does not substantially change with the onset of the summer monsoon. The composite wind analysis demonstrates how the upper-tropospheric subtropical westerlies weaken and shift poleward and the tropical easterlies strengthen and spread north with the onset of the monsoon. The onset vortex that takes the monsoon northward along the west coast in many years is clearly discernible between 600 and 400 hPa in the composite streamline charts. The relative humidity builds up suddenly in the vertical a few days before the onset at the respective s...

Sensitivity of the Simulated Monsoons of 1987 and 1988 to Convective Parameterization Schemes in MM5

Abstract In this study the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) version 3.5.2 was used to simulate the Indian summer monsoon during the two contrasting years of 1987 and 1988, a dry year and a wet year, respectively. Three different convection parameterization schemes of Betts–Miller–Janjic, Kain–Fritsch, and Grell were used to study the sensitivity of monsoon to cumulus effects. The model was integrated for a period of 6 months, starting from three different initial conditions of 0000 UTC on 1, 2, and 3 May of each year using the NCEP–NCAR reanalysis data as input. The 6-hourly reanalysis data were used to provide the lateral boundary conditions, and the observed weekly Reynolds sea surface temperature, linearly interpolated to 6 h, was used as the lower boundary forcing. The results show that all three cumulus schemes were able to simulate the interannual and intraseasonal variabilities in the monsoon with reasonable acc...

ENSO‐monsoon relationships in a greenhouse warming scenario

Recent studies based on observed climatic data indicate weakening of the relationship between El Nino-Southern Oscillation (ENSO) and Indian summer monsoon rainfall, possibly due to global warming. Transient climate change simulations of a coupled ocean-atmosphere GCM (ECHAM4/OPYC3) project a change towards enhanced ENSO activities in the tropical Pacific, as well as increase in mean monsoon rainfall and variability over India. However, the interannual correlations between the two are strong throughout the 240 year simulation. Analysis of monsoon viz-a-viz ENSO in the model simulations suggest a diminished impact of warm ENSO (El Nino) events on monsoon, while the impact of cold ENSO (La Nina) events remains unchanged in the scenario. Anomalous warming over the Eurasian landmass as well as enhanced moisture conditions over the Indian monsoon region in the global warming scenario have possibly contributed to the weakening of the impact of warm ENSO events on monsoon.

Epochal changes in Indian Monsoon‐ENSO precursors

Precursors for the Indian monsoon are known to be highly epoch specific in their skills in predicting the monsoon on seasonal time scales. We show that the various precursors are correlated with the monsoon, only when they are correlated with ENSO, which happened in the recent period 1951–1990, but not in the 1990s and the period 1911–50. This accounts for the skill in monsoon prediction during 1951–90. We find that ENSO and its precursors tend toward higher amplitude and 3–5 year periods in the 1951–1990 epoch, and toward decreased amplitude and 5–7 year periods in the 1911–1950 epoch. We argue that the shift to lower frequency and amplitude in the earlier epoch diminished the association between the monsoon precursors and ENSO, leading to diminished skill in predicting the monsoon in that epoch. However, the simultaneous relationship between the monsoon and ENSO has been stable over the past 140 years, suggesting that the monsoon—ENSO teleconnections are robust once ENSO is established. Changes in the frequency of ENSO have implications for statistical prediction schemes for ENSO and the monsoon.

Projected Climate Change in the Hindu Kush–Himalayan Region By Using the High-resolution Regional Climate Model PRECIS

Abstract The Hindu Kush–Himalayan (HKH) region is characterized by a variety of climatic conditions from tropical to alpine. It has been documented that the rates of warming in the HKH region are significantly higher than the global average and that the warming is occurring at much higher rates in the high-altitude regions than in the low-altitude regions. Mountainous environments are considered sensitive indicators of climate change. Hence this study examined the potential impact of global warming on the HKH region by applying Hadley Centres high-resolution regional climate model PRECIS (Providing Regional Climates for Impact Studies) to 3 subregions: the western, central, and eastern Himalaya. The physical mechanisms that drive warming are different for the 3 regions, and the western Himalaya has 2 major rainy seasons, whereas the central and eastern Himalaya have only one. This study therefore focused on the common rainy season (June–September), during which all 3 regions receive the highest proportion of their annual rainfall. The 3 PRECIS simulations that correspond to the Intergovernmental Panel on Climate Changes A1B emissions scenario were carried out for a continuous period from 1961 to 2098. They were validated with high-resolution (0.25° latitude × 0.25° longitude) data provided by the Asian Precipitation—Highly Resolved Observational Data Integration Towards Evaluation of the Water Resources (APHRODITE) project and by the US National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) reanalysis data. The model was reasonably effective in simulating the monsoon climate over the HKH region. The climate projections were examined over the short (2011–2040), medium (2041–2070), and long term (2071–2098). The model projections indicate that significant warming will occur throughout the HKH region toward the end of the 21st century. Summer monsoon precipitation is expected to be 20–40% higher in 2071–2098 than it was in the baseline period (1961–1990). The 3 Quantifying Uncertainty in Model Predictions simulations show large differences in projections in the western Himalaya.

Decadal variations in NDVI and food production in India

In this study we use long-term satellite, climate, and crop observations to document the spatial distribution of the recent stagnation in food grain production affecting the water-limited tropics (WLT), a region where 1.5 billion people live and depend on local agriculture that is constrained by chronic water shortages. Overall, our analysis shows that the recent stagnation in food production is corroborated by satellite data. The growth rate in annually integrated vegetation greenness, a measure of crop growth, has declined significantly (p < 0.10) in 23% of the WLT cropland area during the last decade, while statistically significant increases in the growth rates account for less than 2%. In

Pre-monsoon maximum and minimum temperatures over India in relation to the summer monsoon rainfall

The pre-monsoon thermal field over the Indian landmass has an important bearing on the land‐sea heating contrast in the region, consequently influencing the establishment, advance and overall performance of the Indian summer monsoon rainfall. This paper examines the relationship between the pre-monsoon thermal field over India and the following summer monsoon rainfall, in order to identify possible predictors for long-range forecasting of Indian summer monsoon rainfall. Based on the spatial patterns of correlations of monsoon rainfall with maximum and minimum temperatures at 121 stations well distributed over India, during the recent period 1951‐80, two predictors showing a significant contribution to the variance in monsoon rainfall have been identified. They are (i) March minimum temperature in east peninsular India and (ii) May minimum temperature in west central India. These two predictors have performed very well in terms of their significant contribution to the multiple regression models during 1951‐1987, vis-a `-vis several other known predictors. They have also shown a consistently significant relationship with the monsoon rainfall during the recent period, from the mid-1940s till the end of the data period. A stepwise regression model for long-range forecasting of all-India summer monsoon rainfall, involving three regional predictors, has been developed, and has shown a multiple correlation of 089. # 1997 by the Royal Meteorological Society. Int. J. Climatol., 17: 1115‐1127 (1997)

Comparison of Penman and Thornthwaite methods of estimating potential evapotranspiration for Indian conditions

SummaryThornthwaites (1948) empirical method of estimating potential evapotranspiration (PE) has been preferred by several scientists in India to Penmans (1948) theoretical combination approach, because of the formers simplicity. However, in view of the doubts expressed in various quarters regarding the applicability of Thornthwaites method for monsoon climates, a comparison is made of the performance of these two methods over different parts of India, using about 26 years of data at 15 stations spread over the country. Various aspects of the manifestations and their differences are presented. It is found that Thornthwaites method gives considerably higher estimates ofPE and shows lower inter-annual variability than Penmans method during the southwest monsoon season. A systematic annual variation of the difference between the two methods is also noticed which is found to be mainly due to the actual vapor pressure and sunshine duration included in Penmans method.ZusammenfassungThornthwaites (1948) empirische Methode zur Bestimmung der potentiellen Evapotranspiration (PE) wurde auf Grund ihrer Einfachheit von vielen indischen Wissenschaftlern der theoretisch-empirischen von Penman (1948) vorgezogen. Da die Anwendbarkeit der Methode Thornthwaites für Monsunklimate von manchen Kreisen in Zweifel gezogen wird, wird hier ein Vergleich zwischen beiden Methoden mit Daten von etwa 26 Jahren von 15 Stationen, die über ganz Indien verteilt sind, gezogen. Es zeigt sich, daß Thornthwaites Methode während des Südwestmonsuns höhere Werte mit geringerer Schwankung zwischen den Jahren liefert als die Penmans. Ebenso wurde ein systematischer Unterschied in der jährlichen Variation zwischen beiden Methoden festgestellt, der sich vor allem auf den tatsächlichen Dampfdruck und die Sonnenscheindauer, die in Penmans Methode berücksichtigt werden, zurückführen läßt.