The cause of an extremely low salinity anomaly in the Bay of Bengal during 2012 spring

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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Wind forcing of salinity anomalies in the Denmark Strait overflow

Ocean Science Discussions ◽

10.5194/osd-8-1403-2011 ◽

2011 ◽

Vol 8 (3) ◽

pp. 1403-1440 ◽

Cited By ~ 1

Author(s):

S. Hall ◽

S. R. Dye ◽

K. J. Heywood ◽

M. R. Wadley

Keyword(s):

General Circulation ◽

Thermohaline Circulation ◽

Circulation Model ◽

Ocean General Circulation Model ◽

Low Salinity ◽

The North ◽

East Greenland Current ◽

Denmark Strait ◽

Salinity Anomaly ◽

Salinity Water

Abstract. The overflow of dense water from the Nordic Seas to the North Atlantic through Denmark Strait is an important part of the global thermohaline circulation. The salinity of the overflow plume has been measured by an array of current meters across the continental slope off the coast of Angmagssalik, southeast Greenland since September 1998. During 2004 the salinity of the overflow plume changed dramatically, with the entire width of the array (70 km) freshening between January 2004 and July 2004, with a significant negative salinity anomaly of about 0.06 in May. The event in May represents a fresh anomaly of over 3 standard deviations from the mean since recording began in 1998. We show that the OCCAM 1/12° Ocean General Circulation Model not only reproduces the 2004 freshening event (r=0.96, p<0.01), but also correlates well with salinity observations over a previous 6 year period (r=0.54, p<0.01). Consequently the physical processes causing the 2004 anomaly and prior variability in salinity are investigated using the model output. Our results reject the hypotheses that the anomaly is caused by processes occurring between the overflow sill and the moorings, or by an increase in upstream net freshwater input. Instead, we show that the 2004 salinity anomaly is caused by an increase in volume flux of low salinity water, with a potential density greater than 27.60 kg m−3, flowing towards the Denmark Strait sill in the East Greenland Current. This is caused by an increase of southward wind stress upstream of the sill at around 75° N 20° W four and a half months earlier, and an associated spin-up of the Greenland Sea Gyre.

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

Journal of Physical Oceanography ◽

10.1175/2008jpo3879.1 ◽

2009 ◽

Vol 39 (5) ◽

pp. 1184-1199 ◽

Cited By ~ 8

Author(s):

K. Nisha ◽

Suryachandra A. Rao ◽

V. V. Gopalakrishna ◽

R. R. Rao ◽

M. S. Girishkumar ◽

...

Keyword(s):

Bay Of Bengal ◽

Arabian Sea ◽

Surface Wind ◽

Surface Heat ◽

The Other ◽

Near Surface ◽

Low Salinity ◽

Southeastern Arabian Sea ◽

Surface Wind Field ◽

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 relatively cooler near-surface thermal regime persisted owing to prolonged upwelling until November 2005. In addition, the observed local surface wind field was relatively stronger, and the net surface heat gain to the ocean was weaker over the Lakshadweep Sea during the postmonsoon season of 2005. Thus, in winter 2005/06, the combination of prolonged upwelling and stronger surface wind field resulting in anomalous net surface heat loss caused weaker secondary warming of the near-surface waters in the Lakshadweep Sea. This led to a weaker horizontal sea surface temperature (SST) gradient between the Lakshadweep Sea and the intruding Bay of Bengal waters and, hence, a reduced number of thermal inversions compared to other winters despite the presence of stronger vertical haline stratification.

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Characteristics of coastal waters of the western Bay of Bengal during different monsoon seasons

Marine and Freshwater Research ◽

10.1071/mf9921517 ◽

1992 ◽

Vol 43 (6) ◽

pp. 1517 ◽

Cited By ~ 3

Author(s):

A Suryanarayana ◽

CS Murty ◽

DP Rao

Keyword(s):

Fresh Water ◽

Bay Of Bengal ◽

Northern Region ◽

Eastern Coast ◽

Low Salinity ◽

The North ◽

Frontal Zones ◽

Salinity Water ◽

Western Bay Of Bengal ◽

Upwelling Index

Physical characteristics of waters along the eastern coast of India up to offshore distances of 400 km have been investigated during the seasons of the north-easterly and south-westerly monsoons in light of relevant observed meteorological forcings. Analysis of wind data for five years (1980-84) showed high magnitudes of onshore-directed wind-stress impulse in the northern region throughout the year. Under the influence of this impulse, surface waters from offshore move towards the coast and sink along the shore under the direct wind setup. This feature is also indicated by negative values of the upwelling index. Fresh water, discharged through multiple outlets along the coast, mixes within the upper 50 m of the water column and spreads south-westward as a lens of low-salinity water. Further, fresh water advects offshore to distances of about 150 km. The transient, cross-shelf, haline frontal zones observed along the western margin of the Bay of Bengal separate the northern dilute waters from the southern saline waters.

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Revisiting the Causal Connection between the Great Salinity Anomaly of the 1970s and the Shutdown of Labrador Sea Deep Convection

Journal of Climate ◽

10.1175/jcli-d-20-0327.1 ◽

2021 ◽

Vol 34 (2) ◽

pp. 675-696

Author(s):

Who M. Kim ◽

Stephen Yeager ◽

Gokhan Danabasoglu

Keyword(s):

Heat Flux ◽

North Atlantic ◽

Surface Heat Flux ◽

Deep Convection ◽

Surface Heat ◽

Labrador Sea ◽

Alternative View ◽

Low Salinity ◽

Salinity Anomaly ◽

Great Salinity Anomaly

AbstractThe Great Salinity Anomaly (GSA) of the 1970s is the most pronounced decadal-scale low-salinity event observed in the subpolar North Atlantic (SPNA). Using various simulations with the Community Earth System Model, here we offer an alternative view on some aspects of the GSA. Specifically, we examine the relative roles of reduced surface heat flux associated with the negative phase of the North Atlantic Oscillation (NAO) and extreme Fram Strait sea ice export (FSSIE) in the late 1960s as possible drivers of the shutdown of Labrador Sea (LS) deep convection. Through composite analysis of a long control simulation, the individual oceanic impacts of extreme FSSIE and surface heat flux events in the LS are isolated. A dominant role for the surface heat flux events for the suppression of convection and freshening in the interior LS is found, while the FSSIE events play a surprisingly minor role. The interior freshening results from reduced mixing of fresher upper ocean with saltier deep ocean. In addition, we find that the downstream propagation of the freshwater anomaly across the SPNA is potentially induced by the persistent negative NAO forcing in the 1960s through an adjustment of thermohaline circulation, with the extreme FSSIE-induced low-salinity anomaly mostly remaining in the boundary currents in the western SPNA. Our results suggest a prominent driving role of the NAO-related heat flux forcing for key aspects of the observed GSA, including the shutdown of LS convection and transbasin propagation of low-salinity waters.

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Ocean Preconditioning of Cyclone Nargis in the Bay of Bengal: Interaction between Rossby Waves, Surface Fresh Waters, and Sea Surface Temperatures*

Journal of Physical Oceanography ◽

10.1175/2011jpo4437.1 ◽

2011 ◽

Vol 41 (9) ◽

pp. 1741-1755 ◽

Cited By ~ 41

Author(s):

Lisan Yu ◽

Michael J. McPhaden

Keyword(s):

Mixed Layer ◽

Bay Of Bengal ◽

Rossby Waves ◽

Sea Surface ◽

Near Surface ◽

Sst Front ◽

Fresh Waters ◽

Low Salinity ◽

Heat Potential ◽

Cyclone Nargis

Abstract An in-depth data analysis was conducted to understand the occurrence of a strong sea surface temperature (SST) front in the central Bay of Bengal before the formation of Cyclone Nargis in April 2008. Nargis changed its course after encountering the front and tracked along the front until making landfall. One unique feature of this SST front was its coupling with high sea surface height anomalies (SSHAs), which is unusual for a basin where SST is normally uncorrelated with SSHA. The high SSHAs were associated with downwelling Rossby waves, and the interaction between downwelling and surface fresh waters was a key mechanism to account for the observed SST–SSHA coupling. The near-surface salinity field in the bay is characterized by strong stratification and a pronounced horizontal gradient, with low salinity in the northeast. During the passage of downwelling Rossby waves, freshening of the surface layer was observed when surface velocities were southwestward. Horizontal convergence of freshwater associated with downwelling Rossby waves increased the buoyancy of the upper layer and caused the mixed layer to shoal to within a few meters of the surface. Surface heating trapped in the thin mixed layer caused the fresh layer to warm, whereas the increase in buoyancy from low-salinity waters enhanced the high SSHA associated with Rossby waves. Thus, high SST coincided with high SSHA. The dominant role of salinity in controlling high SSHA suggests that caution should be exercised when computing hurricane heat potential in the bay from SSHA. This situation is different from most tropical oceans, where temperature has the dominant effect on SSHA.

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