Role of internal tide mixing in keeping the deep Andaman Sea warmer than the Bay of Bengal

Jithin, A. K.; Francis, P. A.

doi:10.1038/s41598-020-68708-6

Cited by 20 publications

(8 citation statements)

References 55 publications

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“…It has been proposed recently that the rate of tide-induced vertical mixing in deep waters of the Andaman Sea is about two-orders of magnitude higher than that in the Bay of Bengal and that this may contribute to downward diffusion of heat in the Andaman Sea, in part accounting for the high deepwater temperature (Jithin and Francis, 2020). However, it must be pointed out that the greatly enhanced vertical mixing is by itself a consequence of the lack of vertical stratification below the sill depth in the Andaman Sea, thereby making the proposed mechanism secondary.…”

Section: Hydrography and Oxygen Distributionmentioning

confidence: 99%

“…The Mediterranean-type marginal seas of the NWIO have been fairly well investigated (Morcos, 1970;Grasshoff, 1975Grasshoff, , 1976Brewer and Dyrssen, 1985;Yao et al, 2014a,b;Al-Yamani and Naqvi, 2019). In comparison, oceanographic processes of the Andaman Sea are not so well known (Sen Gupta et al, 1981;Sarma and Narvekar, 2001;Jithin and Francis, 2020). All three marginal seas are connected to open ocean basins having some of the most intense mesopelagic oxygen minimum zones (OMZs) of the world oceans, although the redox status of these OMZs are quite different (Rao et al, 1994;Naqvi et al, 2006;Bristow et al, 2017).…”

Section: Introductionmentioning

confidence: 99%

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Deoxygenation in Marginal Seas of the Indian Ocean

Naqvi

2021

Front. Mar. Sci.

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This article describes oxygen distributions and recent deoxygenation trends in three marginal seas – Persian Gulf and Red Sea in the Northwestern Indian Ocean (NWIO) and Andaman Sea in the Northeastern Indian Ocean (NEIO). Vertically mixed water column in the shallow Persian Gulf is generally well-oxygenated, especially in winter. Biogeochemistry and ecosystems of Persian Gulf are being subjected to enormous anthropogenic stresses including large loading of nutrients and organic matter, enhancing oxygen demand and causing hypoxia (oxygen < 1.4 ml l–1) in central and southern Gulf in summer. The larger and deeper Red Sea is relatively less affected by human activities. Despite its deep water having remarkably uniform thermohaline characteristics, the central and southern Red Sea has a well-developed perennial oxygen minimum at mid-depths. The available data point to ongoing deoxygenation in the northern Red Sea. Model simulations show that an amplified warming in the marginal seas of the NWIO may cause an intensification of the Arabian Sea oxygen minimum zone (OMZ). Increases in particulate organic carbon and decreases in oxygen contents of the outflows may also have a similar effect. In the Andaman Sea, waters above the sill depth (∼1.4 km) have characteristics similar to those in the Bay of Bengal, including an intense OMZ. As in the case of the Bay of Bengal, oxygen concentrations within the Andaman Sea OMZ appear to have declined slightly but significantly between early 1960s and 1995. The exceedingly isothermal and isohaline water that fills the deep Andaman Basin is also remarkably homogenous in terms of its oxygen content. A very slight but statistically significant decrease in oxygen content of this water also seems to have occurred over three decades preceding 1995. New information is badly needed to assess the extent of further change that may have occurred over the past 25 years. There have been some reports of coastal “dead zones” having developed in the Indian Ocean marginal seas, but they are probably under-reported and the effects of hypoxia on the rich and diverse tropical ecosystems – coral reefs, seagrasses, and mangroves – in these seas remain to be investigated.

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Section: Hydrography and Oxygen Distributionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Deoxygenation in Marginal Seas of the Indian Ocean

Naqvi

2021

Front. Mar. Sci.

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“…The Andaman Sea, located in the northeastern Indian Ocean, is a semi-closed marginal sea, and is bounded by Myanmar to the north, in addition to Thailand and Malaysia in the east, while being partly isolated by the Andaman and Nicobar Islands from the BoB ( Dutta et al, 2007 ). The Andaman Sea is connected to the eastern BoB through shallow passages including the Preparis Channel in the north, the Ten Degree Channel, and the Great Channel in the south ( Jithin and Francis, 2020 ). A large freshwater influx and seasonal monsoon winds lead to the region containing the Andaman Sea and BoB being distinct from other water bodies in tropical regions ( Han and Mccreary, 2001 ; Mahadevan, 2016 ).…”

Section: Introductionmentioning

confidence: 99%

Microbial community structures and important taxa across oxygen gradients in the Andaman Sea and eastern Bay of Bengal epipelagic waters

Guo

Zhang

et al. 2022

Front. Microbiol.

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In oceanic oxygen minimum zones (OMZs), the abundances of aerobic organisms significantly decrease and energy shifts from higher trophic levels to microorganisms, while the microbial communities become critical drivers of marine biogeochemical cycling activities. However, little is known of the microbial ecology of the Andaman Sea and eastern Bay of Bengal (BoB) OMZs. In the present study, a total of 131 samples which from the Andaman Sea and eastern BoB epipelagic waters were analyzed. The microbial community distribution patterns across oxygen gradients, including oxygenic zones (OZs, dissolved oxygen [DO] ≥ 2 mg/L), oxygen limited zones (OLZs, 0.7 mg/L < DO < 2 mg/L), and OMZs (DO ≤ 0.7 mg/L), were investigated. Mantel tests and Spearman’s correlation analysis revealed that DO was the most important driver of microbial community structures among several environmental factors. Microbial diversity, richness, and evenness were highest in the OLZs and lowest in the OZs. The microbial community compositions of OZ and OMZ waters were significantly different. Random forest analysis revealed 24 bioindicator taxa that differentiated OZ, OLZ, and OMZ water communities. These bioindicator taxa included Burkholderiaceae, HOC36, SAR11 Clade IV, Thioglobaceae, Nitrospinaceae, SAR86, and UBA10353. Further, co-occurrence network analysis revealed that SAR202, AEGEAN-169, UBA10353, SAR406, and Rhodobacteraceae were keystone taxa among the entire interaction network of the microbial communities. Functional prediction further indicated that the relative abundances of microbial populations involved in nitrogen and sulfur cycling were higher in OMZs. Several microbial taxa, including the Thioglobaceae, Nitrospinaceae, SAR202, SAR406, WPS-2, UBA10353, and Woeseiaceae, may be involved in nitrogen and/or sulfur cycling, while also contributing to oxygen consumption in these waters. This study consequently provides new insights into the microbial community structures and potentially important taxa that contribute to oxygen consumption in the Andaman Sea and eastern BoB OMZ.

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“…Koch-Larrouy et al (2007) and Nagai and Hibiya (2015) have shown, for the Indonesian region, that IT induces a surface cooling of -0.5 °C on average and that this decreases cloud convection in the atmosphere on a local scale, which in turn reduces precipitation by 20% and thus plays an important role on the climate on a regional scale. Furthermore, Jithin and Francis (2020) showed that IT can also affect the temperature in deep waters (> 1600 m), leading to a warming of the order of 1-2 °C.…”

Section: Introductionmentioning

confidence: 99%

Internal tides off the Amazon shelf Part I : importance for the structuring of ocean temperature during two contrasted seasons

Assene

Koch-Larrouy

Dadou

et al. 2023

Preprint

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Abstract. Tides and internal tides (IT) in the ocean can significantly affect local to regional ocean temperature, including sea surface temperature (SST), via physical processes such as diffusion (vertical mixing) and advection (vertical and horizontal) of water masses. Offshore of the Amazon River, strong IT have been detected by satellite observations and well modelled ; however, their impact on temperature, SST and the identification of the associated processes have not been studied so far. In this work, we use high resolution (1/36°) numerical simulations with and without the tides from an ocean circulation model (NEMO). This model explicitly resolves the internal tides (IT) and is therefore suitable to assess how they can affect ocean temperature in the studied area. We distinguish the analysis for two contrasted seasons, from April to June (AMJ) and from August to October (ASO), since the seasonal stratification off the Amazon River modulates the IT’s response and their influence in temperature. The generation and the propagation of the IT in the model are in good agreement with observations. The SST reproduced by the simulation including tides is in better agreement with satellite SST data compared to the simulation without tides. During ASO season, stronger meso-scale currents, deeper and weaker pycnocline are observed in contrast to the AMJ season. The observed coastal upwelling during ASO season is better reproduced by the model including tides, whereas the no-tide simulation is too warm by +0.3 °C for the SST. In the subsurface above the thermocline, the tide simulation is cooler by −1.2 °C, and warmer below the thermocline by +1.2 °C compared to the simulation without the tides. The IT induce vertical mixing on their generation site along the shelf break and on their propagation pathways towards the open ocean. This process mainly explains the cooler temperature at the ocean surface and is combined with vertical and horizontal advection to explain the cooling in the subsurface water above the thermocline and a warming in the deeper layers below the thermocline. The surface cooling induced in turn an increase of the net heat flux from the atmosphere to the ocean surface, which could induce significant changes in the local and even for the regional tropical Atlantic atmospheric circulation and precipitation. We therefore demonstrate that IT, via vertical mixing and advection along their propagation pathways, and tides over the continental shelf, can play a role on the temperature structure off the Amazon River mouth, particularly in the coastal cooling enhanced by the IT. Keywords: internal tides, Amazon continental shelf and slope, temperature, modeling, satellite data, mixing, heat flux.

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Role of internal tide mixing in keeping the deep Andaman Sea warmer than the Bay of Bengal

Cited by 20 publications

References 55 publications

Deoxygenation in Marginal Seas of the Indian Ocean

Deoxygenation in Marginal Seas of the Indian Ocean

Microbial community structures and important taxa across oxygen gradients in the Andaman Sea and eastern Bay of Bengal epipelagic waters

Internal tides off the Amazon shelf Part I : importance for the structuring of ocean temperature during two contrasted seasons

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