A numerical investigation of the Southern Gyre using ROMS

Gamoyo, Majambo; Reason, C. J. C.; Collins, Charine

doi:10.1016/j.jmarsys.2017.01.002

Cited by 8 publications

(16 citation statements)

References 67 publications

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“…Offshore advection of larvae particularly off the Kenya‐Somali coast is evident from November to August where some larvae disperse toward the Seychelles Archipelago from June to August. This corresponds to the seasonal strengthening of the EACC/Somali Current northward from June, which then turns offshore after crossing the equator to form the Southern Gyre (Gamoyo et al, ). The offshore advection of larvae is absent in September and October.…”

Section: Resultsmentioning

confidence: 99%

Estimating Connectivity Through Larval Dispersal in the Western Indian Ocean

2019

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The health and functioning of coral reef ecosystems in the Western Indian Ocean are in decline due to anthropogenic stress, and the rate of decline is set to accelerate. Marine reserves have become important tools in mitigating these pressures, and one of the most critical factors in determining their spatial design is the degree of connectivity among populations. However, the spatial scale and strength at which populations are potentially connected by dispersal remain uncertain. In this paper, the potential connectivity of corals was assessed through a Lagrangian particle-tracking algorithm coupled offline with an ocean circulation numerical model. Connectivity metrics and graph theory were used to analyze the networks and highlight those reefs that are critical for maintaining the connectivity of corals across the region. Western Indian Ocean coral reefs formed about 13 clusters for larvae with short pelagic larval duration (PLD; 5 days) but grouped into stable larger clusters at 15, 30, and 60 days PLDs, with strong westward dispersal from the northern tip of Madagascar and Comoro Islands toward the East African coast. Reefs at Mafia-Latham Island (Tanzania), Angoche-Nacala and Pemba-Mucufi (Mozambique), Anjouan and Ngazidja (Comoros), and Glorieuses (France) were the most significant for multigenerational connectivity, as measured by betweenness centrality. The small islands Agalega and Tromelin were completely disconnected from other sites at all PLDs. These findings identify priority conservation areas and actions to help support coral reef resilience through a connected network of marine protected areas.Plain Language Summary Given that the health of coral reefs have declined due to a combination of global and local stressors, managing this ecosystem and associated resources is crucial. The establishment of marine protected areas is one widely used conservation instrument. This study assesses the potential connectivity of coral reefs within the Western Indian Ocean region using a biophysical model (i.e., a Lagrangian particle-tracking algorithm coupled with an ocean circulation numerical model). The results show that coral reefs in the Western Indian Ocean form a relatively well connected network, with reefs at Mafia-Latham Island (Tanzania), Angoche-Nacala and Pemba-Mucufi (Mozambique), Anjouan and Ngazidja (Comoros), and Glorieuses (France) being the most significant for multigenerational connectivity. This indicates the imperative need to include connectivity information when considering setting up marine protected areas.The modeling approach used in this study has three components: (1) a physical oceanographic component containing daily 3-D current velocities from a hydrodynamic model, (2) a Lagrangian module that takes the This work has demonstrated how a biophysical model and graph-theoretic framework aids in the understanding and identification of levels of connectivity between reefs that are relevant to coral populations. Key nodes that act as gateways to dispersal, and clusters of connected reefs,...

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Section: Resultsmentioning

confidence: 99%

Estimating Connectivity Through Larval Dispersal in the Western Indian Ocean

2019

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“…The ability of ROMS to accurately simulate the flow field in the region of interest has been demonstrated successfully in previous studies (Manyilizu et al, 2014(Manyilizu et al, , 2016Gamoio et al, 2017)…”

Section: Model Performancementioning

confidence: 88%

“…Furthermore, both products also reproduce vigorous inter-annual (frequency < 1 cycle year -1 ) and intra-seasonal events (frequency > 8 cycles year -1 ), with a consistent decreasing slope towards lower energy levels. There is also a relatively strong variability at frequency of 4 -5 cycles year -1 , which could possibly suggest mesoscale activity identified by Gamoio et al (2017).…”

Section: Eacc Volume Transport At 6°smentioning

confidence: 91%

Biophysical modelling of coastal upwelling variability and circulation along the Tanzanian and Kenyan coasts

Halo

Sagero²,

Manyilizu³

et al. 2021

West Ind. Oc. J Mar. Sci.

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Ocean circulation, upwelling phenomena and chlorophyll-a concentrations were investigated within the framework of numerical model simulations with 1/12° nested horizontal grid-size, in the tropical western Indian Ocean, along the coasts of Tanzania and Kenya. Ekman driven upwelling exhibited high levels of spatial and temporal variability in the region, characterized by a more vigorous occurrence/intensification during the Northeast than the Southwest Monsoon season. A similar trend was observed for chlorophyll-a distribution, but with an additional strong contribution during the inter-monsoon period from March to April. Trend analysis of a SST-derived coastal upwelling index (CUI) computed over the Pemba Channel and offshore of the East African Coastal Current (EACC), for 24 years (1990 - 2013), revealed a general linear relation of the form CUI(yr) = 2.4x10-7yr – 285, with a steady small annual increase of the upwelling phenomena by 0.0024/year ≃ 4% during the whole period of the simulation, which could be attributed to documented increasing trends of wind intensity and water volume transport in the region. The CUI exhibited the two most dominant peaks of variabilities on the range of annual and semi-annual timescales. The wind-stress southward component and the easting/westing veering of the northward EACC at 6°S revealed that these parameters were moderate and significantly correlated with the CUI (r = - 0.53 and 0.52, p<0.05) respectively, further suggesting its intensification during the Northeast Monsoon season.

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“…The largest difference is observed from July to October, when η we remains negative and fairly constant while η tot increases from June to July and decreases from August to October. South of the equator, several studies have described annual downwelling and upwelling westward propagating Rossby waves associated with anomalous wind stress curl (Gamoyo et al, 2017; CASTILLO-TRUJILLO ET AL.…”

Section: (A) (B)mentioning

confidence: 99%

Observations and Modeling of Ocean Circulation in the Seychelles Plateau Region

et al. 2021

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The ocean circulation around and over the Seychelles Plateau (SP) is characterized using 35 months of temperature and velocity measurements along with a numerical model. The results here provide the first documented description of the ocean circulation atop the SP. The SP is an unusually broad (∼200 km), shallow (∼50 m) plateau, dropping off steeply to the abyss. It is situated in a dynamic location (3.5–5.5°S, 54‐57°E) in the south‐western tropical Indian Ocean where northwesterly winds are present during austral summer and become southeasterly in austral winter, following the reversal of the western Indian ocean monsoon winds. Measurements around the Inner Islands, on the SP, have been carried out since 2015. Velocity measurements show that most of the depth‐averaged current variance on the SP arises from near‐inertial oscillations and lower‐frequency variability. Lower‐frequency variability encompasses seasonal and intraseasonal variability, the latter of which includes the effects of mixed Rossby‐gravity waves and mesoscale eddies. A global 0.1° numerical ocean simulation is used in conjunction with these observations to describe the regional circulation around the SP. Atop the SP, circulation is dominated by ageostrophic processes consistent with Ekman dynamics, while around the SP, both geostrophic and ageostrophic processes are important and vary seasonally. Stratification responds to the sea surface height semiannual signal which is due to Ekman pumping‐driven upwelling (related to the Seychelles‐Chagos Thermocline Ridge) and the arrival of an annual downwelling Rossby wave.

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A numerical investigation of the Southern Gyre using ROMS

Cited by 8 publications

References 67 publications

Estimating Connectivity Through Larval Dispersal in the Western Indian Ocean

Estimating Connectivity Through Larval Dispersal in the Western Indian Ocean

Biophysical modelling of coastal upwelling variability and circulation along the Tanzanian and Kenyan coasts

Observations and Modeling of Ocean Circulation in the Seychelles Plateau Region

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