Influence of upper-ocean stratification on tropical cyclone-induced surface cooling in the Bay of Bengal

The barrier layer (BL) is a layer of water separating the thermocline from the density mixed layer in the upper ocean, which has the capability of reducing the negative feedback effect caused by tropical cyclone (TC) acting on the upper ocean and back on the TC itself. This study analyzed in-situ Argo floats measurements, data-assimilated HYCOM/NCODA reanalysis, and the longer-term (1961–2010) variations of Ocean Reanalysis System 4 (ORAS4) based BL in the TC main development region (MDR) to characterize the BL in the western North Pacific (WNP) for different temporal scales and to understand its role in resisting TC induced sea surface cooling. First, the result indicates that the effect of BL on TC enhancement in the MDR of WNP might be overestimated. Further analysis based on partial correlation shows that the BL plays a key role in resisting the cooling response only while BL is strong (BL thickness ≥ 5 m) and TC wind forcing is weak. Meanwhile, the distribution of BL demonstrates markedly the mesoscale characteristic. BL with thickness 0–5 m occupies the highest proportion (~67.55%), while thicker BL (BL thickness (BLT) larger than 5 m) takes up about 25–30%. Besides, there are ~3% with BL thicker than 30 m. For life length, BLT with 0–5 m is limited to 5 days, while BL with thickness more than 30 m can persist for more than 30 days. The scenario is attributed to diverse processes that result in different characteristic temporal scales of BL. Additionally, the analysis of coverage region and average BLT in the recent decade shows a serious situation: both the spatial coverage and BLT increase sharply from 2001 to 2010, which implies that TC–BL interactions might occur more frequently and more vigorously in future if the changing trend of BL remains unchanged.

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Upper-Ocean Response to the Super Tropical Cyclone Phailin (2013) over the Freshwater Region of the Bay of Bengal

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0228.1 ◽

2019 ◽

Vol 49 (5) ◽

pp. 1201-1228 ◽

Cited By ~ 6

Author(s):

Yun Qiu ◽

Weiqing Han ◽

Xinyu Lin ◽

B. Jason West ◽

Yuanlong Li ◽

...

Keyword(s):

Tropical Cyclone ◽

Bay Of Bengal ◽

Heat Fluxes ◽

Sea Surface ◽

Sea Surface Salinity ◽

Upper Ocean ◽

Surface Salinity ◽

Ocean Response ◽

Upper Ocean Response ◽

The Impact

AbstractThis 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.

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Coupled effects of ocean current on wind stress in the Bay of Bengal: Eddy energetics and upper ocean stratification

Deep Sea Research Part II Topical Studies in Oceanography ◽

10.1016/j.dsr2.2019.07.005 ◽

2019 ◽

Vol 168 ◽

pp. 104617 ◽

Cited By ~ 3

Author(s):

Hyodae Seo ◽

Aneesh C. Subramanian ◽

Hajoon Song ◽

Jasti S. Chowdary

Keyword(s):

Wind Stress ◽

Bay Of Bengal ◽

Ocean Current ◽

Upper Ocean ◽

Ocean Stratification

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Eddy-induced Track Reversal and Upper Ocean Physical-Biogeochemical Response of Tropical Cyclone Madi in the Bay of Bengal

10.5194/os-2018-133 ◽

2019 ◽

Author(s):

Riyanka Roy Chowdhury ◽

S. Prasanna Kumar ◽

Arun Chakraborty

Keyword(s):

Life Cycle ◽

Tropical Cyclone ◽

Bay Of Bengal ◽

Net Primary Productivity ◽

Heat Content ◽

Remote Sensing Data ◽

Fold Increase ◽

Upper Ocean ◽

Western Boundary ◽

Cyclonic Storm

Abstract. The life cycle of the tropical cyclone Madi in the southwestern Bay of Bengal (BoB) during 6th to 12th December 2013 was studied using a suite of ocean and atmospheric data. Madi formed as a depression on 6th December and intensified into a very severe cyclonic storm by 8th December. What was distinct about Madi was its (1) swift weakening from very severe cyclone to a severe cyclone while moving towards north on 9th, (2) abrupt track reversal close to 180-degree in a southwestward direction on 10th, and (3) rapid decay in the open ocean by 12th December while still moving southwestward. Using both in situ and remote sensing data, we show that oceanic cyclonic eddies played a leading role in the ensuing series of events that followed its genesis. The sudden weakening of the cyclone before its track reversal was facilitated by the oceanic cyclonic (cold-core) eddy, which reduced the ocean heat content and cooled the upper ocean through upward eddy-pumping of subsurface waters. When Madi moved over cyclonic eddy-core, its further northward movement was arrested. Subsequently, the prevailing northeasterly winds assisted the slow moving system to change its track to a southwesterly path. While travelling towards southwestward direction, the system rapidly decayed when it passed over the regions of cyclonic eddies located near the western boundary of the BoB. Though Madi was a category-2 cyclone, it had a profound impact on the physical and biogeochemical state of the upper ocean. Cyclone-induced enhancement in the chlorophyll a ranged from 5 to 7-fold, while increase in the net primary productivity ranged from 2.5 to 8-fold. Similarly, the CO2 out-gassing into the atmosphere showed a 3.7-fold increase compared to the pre-cyclone values. Our study points to the crucial role oceanic eddies play in the life cycle of cyclone in the BoB. Eddies being ubiquitous and tropical cyclones occur twice a year in the BoB, there is an urgent need to incorporate them in the models for the better prediction of the cyclone track and intensity.

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