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Luc Rainville
,
Craig M. Lee
,
K. Arulananthan
,
S. U. P. Jinadasa
,
Harindra J. S. Fernando
,
W. N. C. Priyadarshani
, and
Hemantha Wijesekera

Abstract

We present high-resolution sustained, persistent observations of the ocean around Sri Lanka from autonomous gliders collected over several years, a region with complex, variable circulation patterns connecting the Bay of Bengal and the Arabian Sea to each other and the rest of the Indian Ocean. The Seaglider surveys resolve seasonal to interannual variability in vertical and horizontal structure, allowing quantification of volume, heat, and freshwater fluxes, as well as the transformations and transports of key water mass classes across sections normal to the east (2014–15) and south (2016–19) coasts of Sri Lanka. The resulting transports point to the importance of both surface and subsurface flows and show that the direct pathway along the Sri Lankan coast plays a significant role in the exchanges of waters between the Arabian Sea and the Bay of Bengal. Significant section-to-section variability highlights the need for sustained, long-term observations to quantify the circulation pathways and dynamics associated with exchange between the Bay of Bengal and Arabian Sea and provides context for interpreting observations collected as “snapshots” of more limited duration.

Significance Statement

The strong seasonal variations of the wind in the Indian Ocean create large and rapid changes in the ocean’s properties near Sri Lanka. This variable and poorly observed circulation is very important for how temperature and salinity are distributed across the northern Indian Ocean, both at the surface and at depths. Long-term and repeated surveys from autonomous Seagliders allow us to understand how freshwater inflow, atmospheric forcing, and underlying ocean variability act to produce observed contrasts (spatial and seasonal) in upper-ocean structure of the Bay of Bengal and Arabian Sea.

Open access
Yun Qiu
,
Weiqing Han
,
Xinyu Lin
,
B. Jason West
,
Yuanlong Li
,
Wen Xing
,
Xiaolin Zhang
,
K. Arulananthan
, and
Xiaogang Guo

Abstract

This study investigates the impact of salinity stratification on the upper-ocean response to a category 5 tropical cyclone, Phailin, that crossed the northern Bay of Bengal (BOB) from 8 to 13 October 2013. A drastic increase of up to 5.0 psu in sea surface salinity (SSS) was observed after Phailin’s passage, whereas a weak drop of below 0.5°C was observed in sea surface temperature (SST). Rightward biases were apparent in surface current and SSS but not evident in SST. Phailin-induced SST variations can be divided into the warming and cooling stages, corresponding to the existence of the thick barrier layer (BL) and temperature inversion before and erosion after Phailin’s passage, respectively. During the warming stage, SST increased due to strong entrainment of warmer water from the BL, which overcame the cooling induced by surface heat fluxes and horizontal advection. During the cooling stage, the entrainment and upwelling dominated the SST decrease. The preexistence of the BL, which reduced entrainment cooling by ~1.09°C day−1, significantly weakened the overall Phailin-induced SST cooling. The Hybrid Coordinate Ocean Model (HYCOM) experiments confirm the crucial roles of entrainment and upwelling in the Phailin-induced dramatic SSS increase and weak SST decrease. Analyses of upper-ocean stratification associated with 16 super TCs that occurred in the BOB during 1980–2015 show that intensifications of 13 TCs were associated with a thick isothermal layer, and 5 out of the 13 were associated with a thick BL. The calculation of TC intensity with and without considering subsurface temperature demonstrates the importance of large upper-ocean heat storage in TC growth.

Open access
Hemantha W. Wijesekera
,
Emily Shroyer
,
Amit Tandon
,
M. Ravichandran
,
Debasis Sengupta
,
S. U. P. Jinadasa
,
Harindra J. S. Fernando
,
Neeraj Agrawal
,
K. Arulananthan
,
G. S. Bhat
,
Mark Baumgartner
,
Jared Buckley
,
Luca Centurioni
,
Patrick Conry
,
J. Thomas Farrar
,
Arnold L. Gordon
,
Verena Hormann
,
Ewa Jarosz
,
Tommy G. Jensen
,
Shaun Johnston
,
Matthias Lankhorst
,
Craig M. Lee
,
Laura S. Leo
,
Iossif Lozovatsky
,
Andrew J. Lucas
,
Jennifer Mackinnon
,
Amala Mahadevan
,
Jonathan Nash
,
Melissa M. Omand
,
Hieu Pham
,
Robert Pinkel
,
Luc Rainville
,
Sanjiv Ramachandran
,
Daniel L. Rudnick
,
Sutanu Sarkar
,
Uwe Send
,
Rashmi Sharma
,
Harper Simmons
,
Kathleen M. Stafford
,
Louis St. Laurent
,
Karan Venayagamoorthy
,
Ramasamy Venkatesan
,
William J. Teague
,
David W. Wang
,
Amy F. Waterhouse
,
Robert Weller
, and
Caitlin B. Whalen

Abstract

Air–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange along the periphery of Sri Lanka, which includes the 100-km-wide East India Coastal Current (EICC) carrying low-salinity water out of the BoB and an adjacent, broad northward flow (∼300 km wide) that carries high-salinity water into BoB during the northeast monsoon. Atmospheric boundary layer (ABL) observations during the decaying phase of the Madden–Julian oscillation (MJO) permit the study of multiscale atmospheric processes associated with non-MJO phenomena and their impacts on the marine boundary layer. Underway analyses that integrate observations and numerical simulations shed light on how air–sea interactions control the ABL and upper-ocean processes.

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