Indian Ocean and monsoon coupled interactions in a warming environment

Several studies have drawn attention to the steady warming of the equatorial and tropical Indian Ocean (IO) sea surface temperature (SST) observed during recent decades. An intriguing aspect of the IO SST warming trend is that it is has been accompanied by a pronounced weakening of the large-scale boreal summer monsoon circulation. Based on a detailed diagnostic analysis of observed datasets, reanalysis products and IPCC AR4 coupled model output, this study examines how the observed changes in the summer monsoon circulation could have contributed to this SST warming trend. The present results reveal that the weakening trend of the summer monsoon cross-equatorial flow has favored a reorientation of surface westerlies towards the equatorial IO during recent decades, relative to summer monsoons of earlier decades, which were dominated by stronger cross-equatorial flow. Our analysis suggests that the weakening of the summer monsoon cross-equatorial flow has in turn significantly accelerated the SST warming in the central equatorial IO. While the trend in the equatorial westerlies has promoted downwelling and thermocline deepening in the eastern equatorial IO, the central equatorial IO warming is attributed to reduced upwelling in response to a weakening trend of the wind-stress curl. The observed trends in Indian monsoon rainfall and the near-equatorial SST warming are shown to be closely related to variations in the meridional gradient of the monsoon zonal winds. An examination of the twentieth century simulations from 22 IPCC AR4 models, suggests that some models capture the recent equatorial IO warming associated with the weakened summer monsoon circulation reasonably well. The individual member models, however, exhibit significant inter-model variations in representing the observed response of the IO and monsoon coupled system.     

Drifting and Meandering of Olive Ridley Sea Turtles in the Bay of Bengal: Role of Oceanic Rossby Waves

Olive Ridley turtles in the Bay of Bengal are previously thought to migrate southward from their nesting ground, along the east coast of India (Orissa coast), towards Sri Lanka. Surprisingly, three of the four Platform Transmitter Terminal (PTT) attached turtles in April-June 2001 meandered off the east coast of India for about two months. It is found that these turtles meandered at the peripherals of cold core cyclonic eddy surrounded by warm core eddies on either side. Concentrations of prey for the turtles in those frontal regions are known to be abundant. Only one of the four PTT attached turtles migrated to the south along the frontal regions in the direction of geostrophic currents. It is found that the locations of these thermal fronts in the Bay of Bengal are primarily determined by the Oceanic Rossby waves and local Ekman pumping.     

Trends and variability of meteorological drought over the districts of India using standardized precipitation index

Meteorological drought during the southwest monsoon season and for the northeast monsoon season over five meteorological subdivisions of India for the period 1901–2015 has been examined using district and all India standardized precipitation index (SPI). Whenever all India southwest monsoon rainfall was less than ?10% or below normal, for those years all India SPI was found as ?1 or less. Composite analysis of SPI for the below normal years, viz., less than ?15% and ?20% of normal rainfall years indicate that during those years more than 30% of country’s area was under drought condition, whenever all India southwest monsoon rainfall was –15% or less than normal. Trend analysis of monthly SPI for the monsoon months identified the districts experiencing significant increase in drought occurrences. Significant positive correlation has been found with the meteorological drought over most of the districts of central, northern and peninsular India, while negative correlation was seen over the districts of eastern India with NINO 3.4 SST. For the first time, meteorological drought analysis over districts and its association with equatorial pacific SST and probability analysis has been done for the northeast monsoon over the affected regions of south peninsular India. Temporal correlation of all India southwest monsoon SPI and south peninsular India northeast monsoon SPI has been done with the global SST to identify the teleconnection of drought in India with global parameters.     

High resolution facies record on late Holocene flood plain sediments from lower reaches of Narmada Valley,Western India

A high resolution quantitative granulometric record for site Uchediya [21°43′2.22″ N, 73° 6′26.22″ E; 10 m a. s. l.] gives understanding towards accretion history of the late Holocene flood plain in the lower reaches of Narmada River. Two sediment facies (sandy and muddy) and seven subfacies (sandy subfacies: St_MS+FS+CS, Sm_FS+MS, Sl_FS+VFS, and St_{MS + CS}; muddy subfacies: Fm_SILT+VFS+FS, Fm_SILT+VFS (O) and Fm_SILT+VFS (T)) are identified based on cluster analysis supplemented with sedimentary structures observed in field and other laboratory data. Changes in hydrodynamics are further deduced based on various sedimentological parameters and their ratios leading to arrive at a depositional model.     

Late holocene storm records from lower reaches of Narmada valley,Western India

Storms from the Arabian Sea are the most significant meteorological feature in western India that brings extreme rainy days together with catastrophic flooding. The present study reports two such palaeo-storm horizons at 1.16 m and 3.2 m above the present day water level in the Narmada channel, 56 km inland based on sedimentology and foraminiferal records. Both the horizons show similar sediment facies and foraminiferal assemblage. The present findings instigate to look for such new sites and build palaeo-storm records for western India.     

Analysis of the Tsunami of December 26, 2004, on the Kerala Coast of India—Part II: Arrival Times

The Tsunami of December 26, 2004, in the Indian Ocean arrived on the coast of Kerala in southwest India some three hours after the tsunami was generated. The tsunami activity persisted throughout that day and, in some locations, even into the early morning of the next day. Based on interviews with eye witnesses, arrival times of tsunami waves are presented here followed by some preliminary analysis of the results.     

Research report—A geographical information system for oil spills sensitivity mapping in the Shetland Islands (United Kingdom)

Geographical Information Systems (GIS) have been making important contributions to environmental monitoring and management. This paper discusses their use in oil spill sensitivity mapping in the Brent System Pipeline Nearshore Section in the Shetland Islands. The study demonstrates the efficiency and effectiveness of the GIS approach in determining the critical areas that need to be protected in the event of an oil spill. It concludes with a discussion on the future development in this field.     

Dynamics of intensification of the boreal summer monsoon flow during IOD events

The present study focuses on understanding the dynamics of intensification of the boreal summer monsoon cross-equatorial flow generally observed during positive-Indian Ocean Dipole (IOD) events, by taking 1994 as a case study. In particular, the influence of the anomalous divergent motions during 1994 (i.e., east–west circulation over equatorial Indian Ocean and the monsoon Hadley-type circulation) on the intensification of summer monsoon cross-equatorial flow is investigated. This problem is examined using diagnostic analyses and simulation experiments from a multi-level global atmospheric model forced with observed diabatic heating. The results suggest that the transfer of kinetic energy (KE) from the divergent motions to the rotational flows can be very effective during IOD periods like 1994; and provides a plausible explanation for the increase of KE of the monsoon cross-equatorial flow over Bay-of-Bengal and adjoining areas. The study also investigates the enhanced activity of westward propagating disturbances observed during 1994 over the monsoon region.