Progress in understanding of Indian Ocean circulation, variability, air–sea exchange, and impacts on biogeochemistry

Phillips, Helen E.; Tandon, Amit; Furue, Ryo; Hood, Raleigh R.; Ummenhofer, Caroline C.; Benthuysen, Jessica A.; Menezes, Viviane V.; Hu, Shijian; Webber, Benjamin G. M.; Sanchez-Franks, Alejandra; Cherian, Deepak; Shroyer, Emily L.; Feng, Ming; Wijesekera, H. W.; Chatterjee, Abhisek; Yu, Lisan; Hermes, Juliet; Murtugudde, Raghu; Tozuka, Tomoki; Su, Danielle; Singh, Arvind; Centurioni, Luca; Prakash, Satya; Wiggert, Jerry D.

doi:10.5194/os-17-1677-2021

Cited by 69 publications

(28 citation statements)

References 708 publications

(1,013 reference statements)

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“…Along the northwest and west coasts of Australia flow several coastal currents [see the review paper by Phillips et al (2021) and references therein]. The Leeuwin Current flows poleward along the west coast; it is a permanent current with well known annual variability.…”

Section: Introductionmentioning

confidence: 99%

A Seasonal Undercurrent Along the Northwest Coast of Australia

Furue

2022

Front. Mar. Sci.

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In the North West Shelf region of Australia is a surface current (Holloway Current), which flows southwestward along the shelf break. This paper describes a seasonal undercurrent below the Holloway Current. A 5-day climatology is constructed from the output of an eddy-resolving oceanic general circulation model (OGCM). A seasonal northeastward-flowing undercurrent is found on the upper continental slope during the climatological April–May. This undercurrent reverses during February–March. During its annual cycle, the phase of the undercurrent tends to propagate southwestward and upward. The annual frequency dominates, but the positive and negative phases of the undercurrent are not symmetric in the yearly cycle because of the contributions from the semi-annual and 1/3-annual components. We propose a hypothesis that this undercurrent is a beam of coastal trapped wave (CTW). As an initial attempt to assess the plausibility of this hypothesis, we construct an idealized linear coastal-trapped wave (CTW) solution driven by an idealized harmonic meridional winds at the annual frequency. The solution takes the form of a beam originating from the forcing region on the continental shelf and propagating offshore and southward. When it emerges on to the continental slope, it takes the form of an undercurrent. This idealized solution shares several properties with the undercurrent in the OGCM despite several discrepancies.

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Section: Introductionmentioning

confidence: 99%

A Seasonal Undercurrent Along the Northwest Coast of Australia

Furue

2022

Front. Mar. Sci.

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“…In the NIO, the western boundary of the Arabian Sea along the coast of Somalia and Arabia shows strong variability in seasonal and interannual timescale driven by the summer monsoon winds and its climate variability (Beal and Donohue, 2013;Chatterjee et al, 2019;Lakshmi et al, 2020;Vinayachandran et al, 2021). Also, coastal waveguide along the perimeter of the Bay of Bengal exhibits significant variability driven by coastal propagation of planetary waves forced by the equatorial winds (Vialard et al, 2009;Suresh et al, 2012;Chatterjee et al, 2017;Mukherjee et al, 2017;Phillips et al, 2021). The historical simulations of CMIP6 models are evaluated for the Indian Ocean variability for the common period of data availability from altimeter and CMIP6 models i.e.…”

Section: Variabilitymentioning

confidence: 99%

“…The sea level over the Indian Ocean shows strong variability in a large spectrum of timescale. While the variability in the seasonal timescale is most prominent due to the dominance of the seasonally reversing monsoon winds in the north Indian Ocean (Chatterjee et al, 2012), the variability in the interannual and decadal timescale is of key importance for the Indian Ocean climate variability and its impact on the global climate (Schott et al, 2009;Phillips et al, 2021). Observed variability of the Indian Ocean shows maxima along the front of the ACC in the SIO (Figure 4a) owing to the interannual meridional movement of the ACC current regime (Figure 4g) driven by ENSO and SAM climate modes (Kim and Orsi, 2014).…”

Section: Variabilitymentioning

confidence: 99%

“…The internal variability in the Indian ocean is associated with coupled climate modes such as El Nino Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), Pacific decadal Oscillation (PDO), Southern Annular Mode (SAM), etc. (Phillips et al, (2021) and the references therein). Therefore, at a local level, for the policymakers, the global mean assessments are of little use.…”

Section: Introductionmentioning

confidence: 99%

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Indian Ocean Dynamic Sea Level, Variability And Projections In CMIP6 Models

Chatterjee

Sajidh

2022

Preprint

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The regional sea level variability and its projection amidst the global sea level rise is one of the major concerns for coastal communities. The dynamic sea level plays a major role in the observed spatial deviations in regional sea level rise from the global mean. The present study evaluates 27 climate model simulations from the sixth phase of the coupled model intercomparison project (CMIP6) for their representation of the historical mean states, variability and future projections for the Indian Ocean. Most models reproduce the observed mean state of the dynamic sea level realistically, however consistent positive bias is evident across the latitudinal range of the Indian Ocean. The strongest sea level bias is seen along the Antarctic Circumpolar Current (ACC) regime owing to the stronger than observed south Indian Ocean westerlies and its equatorward bias. Further, this equatorward shift of the wind field resulted in stronger positive windstress curl across the southeasterly trade winds in the southern tropical basin and easterly wind bias along the equatorial waveguide. These anomalous easterly equatorial winds cause upwelling in the eastern part of the basin and keeps the thermocline shallower in the model than observed, resulted in enhanced variability for the dipole zonal mode or Indian Ocean dipole in the tropics. In the north Indian Ocean, the summer monsoon winds are weak in the model causing weaker upwelling and positive sea level bias along the western Arabian Sea. The high-resolution models compare better in simulating the sea level variability, particularly in the eddy dominated regions like the ACC regime in interannual timescale. However, these improved variabilities do not necessarily produce a better mean state likely due to the enhanced mixing driven by parametrizations set in these high-resolution models. Finally, the overall pattern of the projected dynamic sea level rise is found to be similar for the mid (SSP2-4.5) and high-end (SSP5-8.5) scenarios, except that the magnitude is higher under the high emission situation. Notably, the projected dynamic sea level change is found to be milder when only the best performing models are used compared to the full ensemble.

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“…Therefore, it is considered that the island mass effect, a phenomenon of increased chlorophyll near an island [21], and wind-driven Ekman pumping are thought to determine the existence and strength of the SLD, respectively. The dissipation of the SLD is attributed to the westward propagation of Rossby (downwelling) waves, which are associated with the reflection of the spring Wyrtki jet at the eastern boundary of the Indian Ocean [18,22,23].…”

Section: Introductionmentioning

confidence: 99%

Seasonal Variability in Chlorophyll and Air-Sea CO2 Flux in the Sri Lanka Dome: Hydrodynamic Implications

Wang

Bai

et al. 2022

Remote Sensing

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Multiple upwelling systems develop in the Indian Ocean during the summer monsoon. The Sri Lanka dome (SLD), which occurs in the open ocean off the east coast of Sri Lanka from June to September, is distinct from those near the coast. The SLD is characterized by uplifted thermocline and increased chlorophyll concentration. Mechanisms of the upwelling and its biogeochemical response are not well understood. Here, we explored the dynamics of the chlorophyll and sea-to-air CO2 flux in the SLD using ocean color and altimetry remote sensing data, together with other reanalysis products. We found that the occurrence of high chlorophyll concentration and sea-to-air CO2 flux happens along the pathway of the southwest monsoon current (SMC). The annual cycle of chlorophyll in the SLD has a one-month lag relative to that in the southern coast of Sri Lanka. The positive wind stress curl that forms in the SLD during the summer does not fully explain the seasonal chlorophyll maximum. Transport of the SMC, eddy activity, and associated frontal processes also play an important role in regulating the variability in chlorophyll. In the SLD, upwelled subsurface water has excess dissolved inorganic carbon (DIC) relative to the conventional Redfield ratio between DIC and nutrients; thus, upwelling and sub-mesoscale processes determine this region to be a net carbon source to the atmosphere.

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Progress in understanding of Indian Ocean circulation, variability, air–sea exchange, and impacts on biogeochemistry

Cited by 69 publications

References 708 publications

A Seasonal Undercurrent Along the Northwest Coast of Australia

A Seasonal Undercurrent Along the Northwest Coast of Australia

Indian Ocean Dynamic Sea Level, Variability And Projections In CMIP6 Models

Seasonal Variability in Chlorophyll and Air-Sea CO2 Flux in the Sri Lanka Dome: Hydrodynamic Implications

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