M. S. Girishkumar

Observed interannual variability of near-surface salinity in the Bay of Bengal

An in situ gridded data of salinity, comprising Argo and CTD profiles, has been used to study the interannual variability of near-surface salinity (within 30 m from sea surface) in the Bay of Bengal (BoB) during the years 2005-2013. In addition to the broad agreement with earlier studies on the north-to-south gradient of surface salinity and general features of seasonal variability of salinity, the data also revealed few episodes of enhanced freshening in the BoB. The observations showed distinct anomalous low salinity (< 2 psu) waters in the northern BoB during June-February of the years 2006-2007 (Y67), 2011-2012 (Y12), and 2012-2013 (Y23). The anomalous freshening during these years showed similar life cycle, such as, it starts in the northern BoB during July-September of current summer and extends up to February-March of next winter with a southward propagation. Analysis showed that the oceanic and atmospheric conditions associated with positive Indian Ocean Dipole (pIOD) lead to these freshening events, and IOD rather than El NiA±o/Southern Oscillation (ENSO) controls the interannual variability of salinity in the BoB. The mixed layer salt budget analysis indicated the dominant role of local fresh water flux (horizontal advection) on the observed salinity tendency during summer (winter) monsoon season. Enhanced precipitation associated with pIOD lead to enhanced freshening in northern BoB during June-September, which remained to this region with prevailing summer monsoon circulation. The weakening or absence of southward east India coastal current (EICC) during October-December of these freshening years trapped anomalous freshwater in the northern BoB. Key Points Anomalous freshening observed in BoB in 3 years Positive IOD initiate the freshening events in northern BoB Salt budget analysis resolve contributions from different processes © 2015. American Geophysical Union. All Rights Reserved.

Observed chlorophyll-a bloom in the southern Bay of Bengal during winter 2006–2007

The analysis of 6-year chlorophyll-a data provided by the Moderate Resolution Imaging Spectral (MODIS) radiometer revealed anomalous chlorophyll-a bloom in the southern Bay of Bengal during the winter 2006–2007. The plausible causative mechanisms for such a large chlorophyll-a during the winter 2006–2007 are analysed through surface wind field, surface net heat flux and sea surface height anomaly (SSHA) data. The chlorophyll-a bloom developed in November 2006 near the northern tip of Sumatra. It was intensified and propagated slowly westward during December 2006 to January 2007 and then weakened by February 2007. The combined effect of shallowing of thermocline and weakening of barrier layer due to anomalous westward propagating upwelling Rossby waves associated with the Indian Ocean Dipole (IOD) event and relatively strong wind field causing entrainment of subsurface nutrient-rich water to euphotic zone leads to the bloom in the southern Bay of Bengal.

Influence of the Atlantic zonal mode on monsoon depressions in the Bay of Bengal during boreal summer

The influence of the Atlantic Zonal Mode (AZM) or the Atlantic NiA±o on monsoon depressions in the Bay of Bengal during the boreal summer (June-August) is studied. Our analysis shows that there is a statistically significant difference in the number of monsoon depressions in the Bay of Bengal between the warm and cold phases of the AZM; more (fewer) monsoon depressions form during the cold (warm) phase of AZM. It also shows that there are differences in spatial pattern of trajectories of monsoon depressions; during the cold phase of AZM, the tracks are relatively long and seem to cluster along the axis of core monsoon region compared to the warm phase of AZM. The analysis indicates an increase (a reduction) in low-level cyclonic vorticity and midtropospheric humidity but a reduction (an increase) in vertical wind shear due to anomalous circulation pattern. All of these changes are favorable for the enhancement (suppression) of monsoon depressions during the cold (warm) phase of the AZM. Our analysis further shows a teleconnection pathway by which the AZM can influence the remote Indian Ocean. This could have implications for enhancing monsoon prediction skill, especially during non-El NiA±o-Southern Oscillation years. Key Points Influence of the AZM on monsoon depressions during boreal summer Cold (warm) AZM increases (decreases) number of monsoon depressions The difference in large-scale atmospheric conditions in the BoB during AZM

Influence of Pacific Decadal Oscillation on the relationship between ENSO and tropical cyclone activity in the Bay of Bengal during October–December

The relationship between ENSO and tropical cyclones (TCs) activity in the Bay of Bengal (BoB) during October–December under cold (1950–1974) and warm (1975–2006) phase of Pacific Decadal Oscillation (PDO) is investigated. A statistically significant difference in the formation of total number of TCs and intense TCs (Category-1 and above) between El Niño and La Niña years is observed when the PDO was in warm phase. Our analysis shows that, there is a tendency to form more number of TCs during La Niña years (2.62 TCs per season) than during El Niño years (1.6 TCs per season) under warm phase of PDO. Moreover, the difference is quite high for intense TCs cases, such as, relatively more number of intense TCs forms in the BoB during La Niña years (1.4 TCs per season) compared to El Niño years (0.10 TCs per season) under warm phase of PDO. However, the difference in the formation of total number of TCs and intense TCs between La Niña and El Niño years is not significant under cold phase of PDO. Significant enhancement in low level cyclonic vorticity and mid-troposphere humidity during La Niña years compared to El Niño years when the PDO was in warm phase, rather than the PDO was in cold phase leads to this difference. Our analysis further shows that how the ENSO related teleconnection to the Indian Ocean region differ under warm and cold phase of PDO.

Reduced Near-Surface Thermal Inversions in 2005–06 in the Southeastern Arabian Sea (Lakshadweep Sea)*

Abstract Repeat XBT transects made at near-fortnightly intervals in the Lakshadweep Sea (southeastern Arabian Sea) and ocean data assimilation products are examined to describe the year-to-year variability in the observed near-surface thermal inversions during the winter seasons of 2002–06. Despite the existence of a large low-salinity water intrusion into the Lakshadweep Sea, there was an unusually lower number of near-surface thermal inversions during the winter 2005/06 compared to the other winters. The possible causative mechanisms are examined. During the summer monsoon of 2005 and the following winter season, unusually heavy rainfall occurred over the southwestern Bay of Bengal and the Lakshadweep Sea compared to other years in the study. Furthermore, during the winter of 2005, both the East India Coastal Current and the Winter Monsoon Current were stronger compared to the other years, transporting larger quantities of low salinity waters from the Bay of Bengal into the Lakshadweep Sea where a relat...

Evaluation of ASCAT-Based Daily Gridded Winds in the Tropical Indian Ocean*

AbstractThe quality of daily gridded Advanced Scatterometer (ASCAT; DASCAT) blended winds is examined in the tropical Indian Ocean using 3-day running mean gridded Quick Scatterometer (QuikSCAT; QSCAT) winds and in situ daily winds from the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA). The primary objective of this study is to examine whether DASCAT is a reliable replacement for the widely used QSCAT wind products. Spatial distributions of DASCAT and QSCAT winds show good agreement in speed and direction, except over a few localized regions. The study finds a significant spatial coherence between rainfall and the regions of discrepancy between DASCAT and QSCAT. Comparison of DASCAT and QSCAT wind products with RAMA moorings indicates that DASCAT better captures the overall wind variability compared to QSCAT, especially during rainy and low wind (<5 m s−1) conditions. The root-mean-square of the RAMA–DASCAT (RAMA–QSCAT) difference during rainfall in the zonal an...

Do cold, low salinity waters pass through the Indo-Sri Lanka Channel during winter?

During winter, along the east coast of India, the near-surface flow is characterized by the southward-flowing East India Coastal Current (EICC) which bends around Sri Lanka and enters into the south-eastern Arabian Sea (AS). This current carries cooler, low-salinity waters from the head Bay of Bengal (BoB) into the south-eastern AS. But due to a lack of any direct in situ measurements, it is not clear whether any part of this current that flows through the Indo-Sri Lanka Channel (ISLC) is significant. An attempt is made in this study to look for any observational evidence for the southward flow of cooler, low salinity waters through the ISLC during winter. In the absence of direct in situ measurements on the observed currents in the non-navigable shallow ISLC, the observed high resolution, advanced very high resolution radiometer (AVHRR) sea surface temperature (SST), and sea-viewing wide field-of-view sensor (SeaWiFS) chlorophyll-a and historic sea surface salinity (SSS) data are utilized as tracers to track any southward water flow through the Pamban Pass and Adams Bridge in the ISLC. The analysis suggests that both the non-navigable shallow Pamban Pass and the Adams Bridge in the ISLC act as barriers and limit the southward flow of cooler, low salinity waters into the Gulf of Mannar in the south during winter.

Mixed Layer Temperature Budget for the Northward Propagating Summer Monsoon Intraseasonal Oscillation (MISO) in the Central Bay of Bengal

Composite analyses of mixed layer temperature (MLT) budget terms from near surface meteorological and oceanic observations in the central Bay of Bengal are utilized to evaluate the modulation of air-sea interactions and MLT processes in response to the summer monsoon intraseasonal oscillation (MISO). For this purpose, we use moored buoy data at 15°N, 12°N and 8°N along 90°E together with TropFlux meteorological parameters and the Ocean Surface Current Analyses Real-time (OSCAR) current product. Our analysis shows a strong cooling tendency in MLT with maximum amplitude in the central and northern BoB during the northward propagation of enhanced convective activity associated with the active phase of the MISO; conversely, warming occurs during the suppressed phase of the MISO. The surface mixed layer is generally heated during convectively inactive phases of the MISO primarily due to increased net surface heat flux into the ocean. During convectively active MISO phases, the surface mixed layer is cooled by the combined influence of net surface heat loss to the atmosphere and entrainment cooling at the base of mixed layer. The variability of net surface heat flux is primarily due to modulation of latent heat flux and shortwave radiation. Shortwave is controlled primarily by an enhancement or reduction of cloudiness during the active and inactive MISO phases and latent heat flux is controlled primarily by variations in air-sea humidity difference.