B. N. Goswami

A dipole mode in the tropical Indian Ocean

For the tropical Pacific and Atlantic oceans, internal modes of variability that lead to climatic oscillations have been recognized, but in the Indian Ocean region a similar ocean–atmosphere interaction causing interannual climate variability has not yet been found. Here we report an analysis of observational data over the past 40 years, showing a dipole mode in the Indian Ocean: a pattern of internal variability with anomalously low sea surface temperatures off Sumatra and high sea surface temperatures in the western Indian Ocean, with accompanying wind and precipitation anomalies. The spatio-temporal links between sea surface temperatures and winds reveal a strong coupling through the precipitation field and ocean dynamics. This air–sea interaction process is unique and inherent in the Indian Ocean, and is shown to be independent of the El Niño/Southern Oscillation. The discovery of this dipole mode that accounts for about 12% of the sea surface temperature variability in the Indian Ocean—and, in its active years, also causes severe rainfall in eastern Africa and droughts in Indonesia—brightens the prospects for a long-term forecast of rainfall anomalies in the affected countries.

Increasing trend of extreme rain events over India in a warming environment.

Against a backdrop of rising global surface temperature, the stability of the Indian monsoon rainfall over the past century has been a puzzle. By using a daily rainfall data set, we show (i) significant rising trends in the frequency and the magnitude of extreme rain events and (ii) a significant decreasing trend in the frequency of moderate events over central India during the monsoon seasons from 1951 to 2000. The seasonal mean rainfall does not show a significant trend, because the contribution from increasing heavy events is offset by decreasing moderate events. A substantial increase in hazards related to heavy rain is expected over central India in the future.

Intraseasonal Oscillations and Interannual Variability of the Indian Summer Monsoon

Abstract How and to what extent the intraseasonal oscillations (ISOs) influence the seasonal mean and its interannual variability of the Indian summer monsoon is investigated using 42-yr (1956–97) daily circulation data from National Centers for Environmental Prediction–National Center for Atmospheric Research 40-Year Reanalysis and satellite-derived outgoing longwave radiation data for the period of 1974–97. Based on zonal winds at 850 hPa over the Bay of Bengal, a criterion is devised to define “active” and “break” monsoon conditions. The underlying spatial structure of a typical ISO cycle in circulation and convection that is invariant over the years is constructed using a composite technique. A typical ISO has large-scale horizontal structure similar to the seasonal mean and intensifies (weakens) the mean flow during its active (break) phase. A typical active (break) phase is also associated with enhanced (decreased) cyclonic low-level vorticity and convection and anomalous upward (downward) motion in...

A physical mechanism for North Atlantic SST influence on the Indian summer monsoon

A link between the Atlantic Multidecadal Oscillation (AMO) and multidecadal variability of the Indian summer monsoon rainfall is unraveled and a long sought physical mechanism linking Atlantic climate and monsoon has been identified. The AMO produces persistent weakening (strengthening) of the meridional gradient of tropospheric temperature (TT) by setting up negative (positive) TT anomaly over Eurasia during northern late summer/autumn resulting in early (late) withdrawal of the south west monsoon and persistent decrease (increase) of seasonal monsoon rainfall. On inter-annual time scales, strong North Atlantic Oscillation (NAO) or North Annular mode (NAM) influences the monsoon by producing similar TT anomaly over Eurasia. The AMO achieves the interdecadal modulation of the monsoon by modulating the frequency of occurrence of strong NAO/NAM events. This mechanism also provides a basis for explaining the observed teleconnection between North Atlantic temperature and the Asian monsoon in paleoclimatic proxies. Citation: Goswami, B. N., M. S. Madhusoodanan, C. P. Neema, and D. Sengupta (2006), A physical mechanism for North Atlantic SST influence on the Indian summer monsoon

Coherent Intraseasonal Oscillations of Ocean and Atmosphere during the Asian Summer Monsoon

The space-time evolution of the ocean and atmosphere associated with 1998-2000 monsoon intraseasonal oscillations (ISO) in the Indian Ocean and west Pacific is studied using validated sea surface temperature (SST) and surface wind speed from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager, and satellite outgoing longwave radiation. Monsoon ISO consist of alternating episodes of active and suppressed atmospheric convection moving northward in the eastern Indian Ocean and the South China Sea. Negative/positive SST anomalies generated by fluctuations of net heat flux at the ocean surface move northward following regions of active/suppressed convection. Such coherent evolution of SST, surface heat flux and convection suggests that air-sea interaction might be important in monsoon ISO.

Quasi-Periodic Oscillations in a Symmetric General Circulation Model

Abstract Long lime integrations with a symmetric version of the CLAS Climate Model with hydrology have shown that the Hadley circulation has well defined strong and weak episodes. This oscillation of the Hadley circulation seems to occur in two dominant range of periodicities, one with periods between 10 and 15 days and another with periods between 20 and 40 days. This quasi-periodic behavior of the Hadley circulation is seen only when the moist convective heating is determined by dynamics. If the latent beating in the atmosphere is prescribed and kept artificially fixed, this oscillation of the Hadley circulation disappears. Thus, this oscillation of the Hadley circulation appears as a result of interactions between moist convective and dynamical processes. A wave phenomenon is seen in the lower atmosphere that propagates toward the position of maximum radiative beating. This wave perturbation has a length scale of about 15–20 degrees latitude in the north-south direction. This phenomenon has large ampli...

The “Year” of Tropical Convection (May 2008–April 2010): Climate Variability and Weather Highlights

The representation of tropical convection remains a serious challenge to the skillfulness of our weather and climate prediction systems. To address this challenge, the World Climate Research Programme (WCRP) and The Observing System Research and Predictability Experiment (THORPEX) of the World Weather Research Programme (WWRP) are conducting a joint research activity consisting of a focus period approach along with an integrated research framework tailored to exploit the vast amounts of existing observations, expanding computational resources, and the development of new, high-resolution modeling frameworks. The objective of the Year of Tropical Convection (YOTC) is to use these constructs to advance the characterization, modeling, parameterization, and prediction of multiscale tropical convection, including relevant two-way interactions between tropical and extratropical systems. This article highlights the diverse array of scientifically interesting and socially important weather and climate events assoc...

Drying of Indian subcontinent by rapid Indian Ocean warming and a weakening land-sea thermal gradient

There are large uncertainties looming over the status and fate of the South Asian summer monsoon, with several studies debating whether the monsoon is weakening or strengthening in a changing climate. Our analysis using multiple observed datasets demonstrates a significant weakening trend in summer rainfall during 1901-2012 over the central-east and northern regions of India, along the Ganges-Brahmaputra-Meghna basins and the Himalayan foothills, where agriculture is still largely rain-fed. Earlier studies have suggested an increase in moisture availability and land-sea thermal gradient in the tropics due to anthropogenic warming, favouring an increase in tropical rainfall. Here we show that the land-sea thermal gradient over South Asia has been decreasing, due to rapid warming in the Indian Ocean and a relatively subdued warming over the subcontinent. Using long-term observations and coupled model experiments, we provide compelling evidence that the enhanced Indian Ocean warming potentially weakens the land-sea thermal contrast, dampens the summer monsoon Hadley circulation, and thereby reduces the rainfall over parts of South Asia.

South Asian monsoon

As the word “monsoon” (derived from an Arabic word meaning seasons) indicates, the South Asian (SA) summer monsoon is part of an annually reversing wind system (Figure 2.1(b, e) (Ramage, 1971); (Rao, 1976)). The winds at low levels during the summer monsoon season are characterized by the strongest westerlies anywhere at 850 hPa over the Arabian Sea, known as the low-level westerly jet (LLJ) (Figure 2.1e), and a large-scale cyclonic vorticity extending from the north Bay of Bengal (BoB) to western India known as the “monsoon trough” (Figure 2.1(e) (Rao, 1976)). The easterly jet (Figure 2.1(f)) centered around 5oN and the Tibetan anticyclone centered around 30oN are important features of upper level winds over the monsoon region during northern summer. Millions of inhabitants of the region, however, attach much greater importance to the associated seasonal changes of rainfall. Wet summers and dry winters (Figure 2.1 (a, d)) associated with the seasonal changes of low-level winds are crucial for agricultural production and the economy of the region. The monsoon, or the seasonal changes of winds and rainfall, in the region could be interpreted as a result of northward seasonal migration of the east-west oriented precipitation belt (Tropical Convergence Zone, TCZ) from southern hemisphere in winter to northern hemisphere in summer (Gadgil, 2003). The largest northward excursion of the rain belt takes place over the Indian monsoon region where it moves from a mean position of about 5oS in winter (Figure 2.1 (a)) to about 20oN in northern summer (Figure 2.1(d)) (Waliser and Gautier, 1993).

Interannual Variations of Sea Surface Temperature over the Arabian Sea and the Indian Monsoon: A New Perspective

Abstract The interannual variation of surface fields over the Arabian Sea and Bay of Bengal are studied using data between 1900 and 1979. It is emphasized that the monthly mean sea surface temperature (SST) over the north Indian Ocean and monsoon rainfall are significantly affected by synoptic systems and other intraseasonal variations. To highlight the interannual signals it is important to remove the large-amplitude high-frequency noise and very low frequency long-term trends, if any. By suitable spatial and temporal averaging of the SST and the rainfall data and by removing the long-term trend from the SST data, we have been able to show that there exists a homogeneous region in the southeastern Arabian Sea over which the March–April (MA) SST anomalies are significantly correlated with the seasonal (June–September) rainfall over India. A potential of this premonsoon signal for predicting the seasonal rainfall over India is indicated. It is shown that the correlation between the SST and the seasonal mon...