The Response of the Surface Circulation of the Arabian Sea to Monsoonal Forcing

2013 ◽  
Vol 43 (9) ◽  
pp. 2008-2022 ◽  
Author(s):  
L. M. Beal ◽  
V. Hormann ◽  
R. Lumpkin ◽  
G. R. Foltz
Keyword(s):  
Arabian Sea ◽  
Southwest Monsoon ◽  
Western Boundary ◽  
Coastal Current ◽  
Ekman Pumping ◽  
East African ◽  
Wind Stress Curl ◽  
Surface Circulation ◽  
Monsoon Winds ◽  
Counter Current

Abstract Two decades of drifter and satellite data allow the authors to describe the seasonal evolution of the surface circulation of the Arabian Sea, which reverses annually with the Indian monsoon winds. This study finds several features that advance current understanding. Most significantly, northward flow appears along the length of the western boundary, together with a weak anticyclone at 6°N (a precursor to the Great Whirl) as early as March or April, one or two months before the southwest monsoon winds. This circulation is driven by planetary waves, which are initiated by wind curl forcing during the previous southwest monsoon, leading the authors to speculate that there is an oceanic mechanism through which one monsoon may precondition the next. Second, the authors find that the eastward South Equatorial Counter Current (SECC) is present year-round, fed by the northward East African Coastal Current (EACC). During the southwest monsoon the EACC overshoots the equator and splits, feeding both northward into the Somali Current and eastward into the SECC by looping back across the equator. This retroflection of the EACC is what was previously known as the southern gyre. At the surface, this circulation is obscured by strong, locally wind-driven, cross-equatorial transport. The semiannual variability of the SECC is governed by Ekman pumping over the equatorial gyre. Finally, there is broad, strong eastward flow at the mouth of the Gulf of Aden throughout the southwest monsoon, coincident with alongshore winds and a switch in sign of the wind stress curl along the axis of the atmospheric monsoon jet.

scholarly journals Controlling factors of the OMZ in the Arabian Sea

2012 ◽  
Vol 9 (5) ◽  
pp. 5509-5550
Author(s):  
L. Resplandy ◽  
M. Lévy ◽  
L. Bopp ◽  
V. Echevin ◽  
S. Pous ◽  
...  
Keyword(s):  
Spatial Distribution ◽  
Oxygen Concentration ◽  
Seasonal Cycle ◽  
Arabian Sea ◽  
Coastal Upwelling ◽  
Western Boundary ◽  
Ekman Pumping ◽  
Low Oxygen ◽  
Boundary Current ◽  
Ocean Dynamic

Abstract. In-situ observations indicate that the Arabian Sea oxygen minimum zone (OMZ) is only weakly influenced by the strong seasonal cycle of ocean dynamic and biogeochemistry forced by the asian monsoon system and it is spatially decorrelated from the coastal upwelling systems where the biological production is the strongest. In this study we examine the factors controlling the seasonality and the spatial distribution of the OMZ in the Arabian Sea using a coupled bio-physical model. We find that the oxygen concentration in the OMZ displays a seasonal cycle with an amplitude of 5–15 % of the annual mean oxygen concentration. The OMZ is ventilated by lateral ventilation along the western boundary current and in the coastal undercurrent along India during the summer monsoon and by coastal downwelling and negative Ekman pumping during the fall intermonsoon and winter monsoon. This ventilation is counterbalanced by strong coastal upwelling and positive Ekman pumping of low oxygen waters at the base of the OMZ during the spring intermonsoon. Although the factors controlling the OMZ seasonality are associated with the men circulation, we find that mesoscale dynamics modulates them by limiting the vertical ventilation during winter and enhancing it through lateral advection during the rest of the year. Processes explaining the establishment and spatial distribution of the OMZ were quantified using a perturbation experiment initialised with no OMZ. As expected, the oxygen depletion is triggered by strong biological activity in central Arabian Sea during winter and in western and eastern boundary coastal upwelling systems during summer. We find that the 3-D ocean dynamic largely controls the spatial distribution of the OMZ. The eastward shift ensues from the northward lateral transport of ventilated waters along the western and eastern coasts and the advection offshore of low oxygen waters formed in the upwelling system.

scholarly journals From monsoon to marine productivity in the Arabian Sea: insights from glacial and interglacial climates

2017 ◽  
Vol 13 (7) ◽  
pp. 759-778 ◽  
Author(s):  
Priscilla Le Mézo ◽  
Luc Beaufort ◽  
Laurent Bopp ◽  
Pascale Braconnot ◽  
Masa Kageyama
Keyword(s):  
Stress Intensity ◽  
Summer Monsoon ◽  
Wind Stress ◽  
Arabian Sea ◽  
Coastal Upwelling ◽  
Boreal Summer ◽  
Tropospheric Temperature ◽  
Ekman Pumping ◽  
Wind Stress Curl ◽  
Jet Axis

Abstract. The current-climate Indian monsoon is known to boost biological productivity in the Arabian Sea. This paradigm has been extensively used to reconstruct past monsoon variability from palaeo-proxies indicative of changes in surface productivity. Here, we test this paradigm by simulating changes in marine primary productivity for eight contrasted climates from the last glacial–interglacial cycle. We show that there is no straightforward correlation between boreal summer productivity of the Arabian Sea and summer monsoon strength across the different simulated climates. Locally, productivity is fuelled by nutrient supply driven by Ekman dynamics. Upward transport of nutrients is modulated by a combination of alongshore wind stress intensity, which drives coastal upwelling, and by a positive wind stress curl to the west of the jet axis resulting in upward Ekman pumping. To the east of the jet axis there is however a strong downward Ekman pumping due to a negative wind stress curl. Consequently, changes in coastal alongshore stress and/or curl depend on both the jet intensity and position. The jet position is constrained by the Indian summer monsoon pattern, which in turn is influenced by the astronomical parameters and the ice sheet cover. The astronomical parameters are indeed shown to impact wind stress intensity in the Arabian Sea through large-scale changes in the meridional gradient of upper-tropospheric temperature. However, both the astronomical parameters and the ice sheets affect the pattern of wind stress curl through the position of the sea level depression barycentre over the monsoon region (20–150° W, 30° S–60° N). The combined changes in monsoon intensity and pattern lead to some higher glacial productivity during the summer season, in agreement with some palaeo-productivity reconstructions.

scholarly journals Origins of Coupled Model Biases in the Arabian Sea Climatological State

2018 ◽  
Vol 31 (5) ◽  
pp. 2005-2029 ◽  
Author(s):  
Motoki Nagura ◽  
J. P. McCreary ◽  
H. Annamalai
Keyword(s):  
Mixed Layer ◽  
Persian Gulf ◽  
Arabian Sea ◽  
Coupled Model ◽  
Sea Surface Salinity ◽  
Western Boundary ◽  
Coastal Current ◽  
Surface Cooling ◽  
Surface Salinity ◽  
High Salinity Water

This study investigates biases of the climatological mean state of the northern Arabian Sea (NAS) in 31 coupled ocean–atmosphere models. The focus is to understand the cause of the large biases in the depth of the 20°C isotherm [Formula: see text] that occur in many of them. Other prominent biases are the depth [Formula: see text] and temperature [Formula: see text] of Persian Gulf water (PGW) and the wintertime mixed-layer thickness (MLT) along the northern boundary. For models that lack a Persian Gulf (group 1), [Formula: see text] is determined by the wintertime MLT bias [Formula: see text] through the formation of an Arabian Sea high-salinity water mass (ASHSW) that is too deep. For models with a Persian Gulf (group 2), if [Formula: see text] > MLT (group 2B), PGW remains mostly trapped to the western boundary and, again, [Formula: see text] directly controls [Formula: see text]. If [Formula: see text] MLT (group 2A), PGW spreads into the NAS and impacts [Formula: see text] because [Formula: see text] > 20°C; nevertheless [Formula: see text] still influences [Formula: see text] indirectly through its impact on [Formula: see text]. The thick wintertime mixed layer is driven primarily by surface cooling [Formula: see text] during the fall. Nevertheless, variations in ΔMLT among the models are more strongly linked to biases in the density stratification (jump) across the bottom of the mixed layer than to [Formula: see text] biases. The jump is in turn determined primarily by sea surface salinity biases (ΔSSS) advected into the NAS by the West India Coastal Current, and the source of ΔSSS is the rainfall deficit associated with the models’ weak summer monsoon. Ultimately, then, ΔD20 is linked to this deficit.

scholarly journals Employing multivariate analysis to determine the drivers of productivity on the North Kenya Bank and in Kenyan territorial waters

2021 ◽  
pp. 33-41
Author(s):  
Joseph Kamau ◽  
Oliver Ochola ◽  
Boaz Ohowa ◽  
Charles Mitto ◽  
Charles Magori ◽  
...  
Keyword(s):  
Chlorophyll A ◽  
Marine Ecosystem ◽  
Interdisciplinary Studies ◽  
Coastal Current ◽  
East African ◽  
African Region ◽  
Natural Processes ◽  
The North ◽  
The Difference ◽  
Monsoon Winds

A complex mix of natural processes exist in nearshore and offshore waters which influence coastal and marine ecosystem productivity. An understanding of the biogeochemical processes involved is a key element in interdisciplinary studies of primary production, oceanic flux and storage of carbon dioxide. Water circulation in the East African region is influenced by coastal currents driven by monsoon winds. There are four oceanic currents influencing Kenya’s coastal waters; namely the East African Coastal Current, the Somali Current, the Southern Equatorial Current and the Equatorial Counter Current. The Kenyan fishing industry is slowly embracing offshore fishing grounds, and the North Kenya Bank is emerging as the next fishery frontier. This study aims to provide insight on the processes driving the productivity of Kenya’s territorial waters. The variable Si* (the difference between available silicate [Si(OH)4] and nitrate [NO3-]) was employed as a proxy of upwelling. It was highly positively correlated to chlorophyll-a, indicating that upwelling is a major phenomenon driving productivity in Kenyan territorial waters. Particulate Organic Carbon (POC) and Dissolved Oxygen (DO) exhibited a lesser positive correlation with chlorophyll-a, implying that remineralization also has some influence in the productivity of the area.

scholarly journals Latitude of Eastward Jet Prematurely Separated from the Western Boundary in a Two-Layer Quasigeostrophic Model

2015 ◽  
Vol 45 (3) ◽  
pp. 737-754 ◽  
Author(s):  
Yasunori Sue ◽  
Atsushi Kubokawa
Keyword(s):  
Lower Layer ◽  
Kuroshio Extension ◽  
Integral Condition ◽  
Subtropical Gyre ◽  
Partial Slip ◽  
Western Boundary ◽  
Ekman Pumping ◽  
Wind Stress Curl ◽  
Northern Boundary ◽  
Layer Width

AbstractThis paper investigates the formation of eastward jets extended from western boundary currents, using a simple two-layer quasigeostrophic (QG) model forced by a wind stress curl consistent with the formation of a subtropical gyre. The study investigated the dependency of the latitude of the eastward jet on various parameters and on the meridional distribution of the Ekman pumping velocity. The parameters considered in the present study included the viscous and inertial western boundary layer width, the parameter representing the degree of the partial-slip boundary condition, the ratio of the upper- to lower-layer depth, and the bottom friction. With the parameters used, two types of stable structures are found in the time-mean field. One type of structure represented the “prematurely separated jet case,” in which the eastward extension jet was located far south of the northern boundary of the subtropical gyre, as is the Kuroshio Extension; the other type was the “gyre boundary jet case,” in which the eastward jet occurred along the northern boundary. The initial condition decides which type of structure would occur. When the prematurely separated jet case occurred, the authors found that the latitude of the eastward jet depended very little on the parameters. In addition, this study also observed that the latitude was determined by the meridional distribution of the Ekman pumping velocity. The eastward extension jet was usually located near the latitude that was half of the maximum value of the Sverdrup streamfunction and satisfied an integral condition derived from the QG potential vorticity equation.

scholarly journals Interannual Variations of Subsurface Spiciness in the Philippine Sea: Observations and Mechanism

2012 ◽  
Vol 42 (6) ◽  
pp. 1022-1038 ◽  
Author(s):  
Yuanlong Li ◽  
Fan Wang ◽  
Fangguo Zhai
Keyword(s):  
North Pacific ◽  
Low Frequency ◽  
Phase Changes ◽  
Enso Cycle ◽  
Western Boundary ◽  
Ekman Pumping ◽  
Wind Stress Curl ◽  
Central Pacific ◽  
Philippine Sea ◽  
Salinity Changes

Abstract The Philippine Sea (PS) is a key region connecting North Pacific subtropics to the equator via western boundary currents. Using available measurements from Argo profiling floats, satellite altimeters, and research surveys, the authors investigate the characteristics and mechanism of subsurface spiciness variability (represented by salinity changes between 23.5 and 24.5 σθ) in the PS. During the past decade, low-frequency salinity variability was dominated by interannual signals characterized by out-of-phase changes between the southern and northern PS with peak-to-peak amplitudes exceeding 0.1 psu. These salinity anomalies are mainly generated locally by anomalous cross-front geostrophic advections. In 2003, an anomalous cyclonic circulation developed in the PS, which transported greater (less) than normal high-salinity North Pacific Tropical Water to the northern (southern) PS and produced positive (negative) salinity anomalies there. In 2009, an anomalous anticyclone emerged, which produced negative (positive) salinity anomalies in the northern (southern) PS. These year-to-year variations are closely associated with ENSO cycle. During strong El Niño (La Niña) episodes, positive (negative) wind stress curl anomalies between 8° and 18°N evoke westward-propagating upwelling (downwelling) Rossby waves in the central Pacific and positive (negative) anomalous Ekman pumping in the western Pacific, resulting in the observed current and salinity changes in the PS. Further analysis suggests that these locally generated spiciness anomalies disperse quickly while propagating to the equatorial Pacific in the Mindanao Current (MC). In the meantime, anomalies advected from higher latitudes are nearly diminished upon reaching the PS. The western boundary of the North Pacific seems quite efficient in damping extratropical signals.

scholarly journals Interannual Variability of the Mindanao Current/Undercurrent in Direct Observations and Numerical Simulations

2016 ◽  
Vol 46 (2) ◽  
pp. 483-499 ◽  
Author(s):  
Shijian Hu ◽  
Dunxin Hu ◽  
Cong Guan ◽  
Fan Wang ◽  
Linlin Zhang ◽  
...  
Keyword(s):  
Pacific Ocean ◽  
Interannual Variability ◽  
Western Pacific ◽  
Western Boundary ◽  
Western Pacific Ocean ◽  
Ekman Pumping ◽  
Wind Stress Curl ◽  
Boundary Currents ◽  
Mindanao Current ◽  
The Western Pacific

AbstractThe interannual variability of the boundary currents east of the Mindanao Island, including the Mindanao Current/Undercurrent (MC/MUC), is investigated using moored acoustic Doppler current profiler (ADCP) measurements combined with a series of numerical experiments. The ADCP mooring system was deployed east of the Mindanao Island at 7°59′N, 127°3′E during December 2010–August 2014. Depth-dependent interannual variability is detected in the two western boundary currents: strong and lower-frequency variability dominates the upper-layer MC, while weaker and higher-frequency fluctuation controls the subsurface MUC. Throughout the duration of mooring measurements, the weakest MC was observed in June 2012, in contrast to the maximum peaks in December 2010 and June 2014, while in the deeper layer the MUC shows speed peaks circa December 2010, January 2011, April 2013, and July 2014 and valleys circa June 2011, August 2012, and November 2013. Diagnostic analysis and numerical sensitivity experiments using a 2.5-layer reduced-gravity model indicate that wind forcing in the western Pacific Ocean is a driving agent in conditioning the interannual variability of MC and MUC. Results suggest that westward-propagating Rossby waves that generate in the western Pacific Ocean (roughly 150°–180°E) are of much significance in the interannual variability of the two boundary currents. Fluctuation of Ekman pumping due to local wind stress curl anomaly in the far western Pacific Ocean (roughly 120°–150°E) also plays a role in the interannual variability of the MC. The relationship between the MC/MUC and El Niño is discussed.

scholarly journals The Annual Cycle of Circulation of the Southwest Subtropical Pacific, Analyzed in an Ocean GCM*

2007 ◽  
Vol 37 (6) ◽  
pp. 1610-1627 ◽  
Author(s):  
William S. Kessler ◽  
Lionel Gourdeau
Keyword(s):  
Annual Cycle ◽  
Linear Models ◽  
Current System ◽  
Subtropical Gyre ◽  
Western Boundary ◽  
Coastal Current ◽  
Thermocline Depth ◽  
Boundary Current ◽  
Wind Stress Curl ◽  
Equatorial Current

Abstract An ocean GCM, interpreted in light of linear models and sparse observations, is used to diagnose the dynamics of the annual cycle of circulation in the western boundary current system of the southwest Pacific Ocean. The simple structure of annual wind stress curl over the South Pacific produces a large region of uniformly phased, stationary thermocline depth anomalies such that the western subtropical gyre spins up and down during the year, directing flow anomalies alternately toward and away from the boundary at its northern end, near 10°S. The response of the western boundary currents is to redistribute these anomalies northward toward the equator and southward to the subtropical gyre, a redistribution that is determined principally by linear Rossby processes, not boundary dynamics. When the subtropical gyre and South Equatorial Current (SEC) are strong (in the second half of the year), the result is both increased equatorward transport of the New Guinea Coastal Current and poleward transport anomalies along the entire Australian coast. Because of this opposite phasing of boundary current anomalies across 10°S, annual migration of the bifurcation point of the total SEC, near 18°S in the mean, has no significance regarding variability of transport from subtropics to equator.

scholarly journals Distinct Influence of Air–Sea Interactions Mediated by Mesoscale Sea Surface Temperature and Surface Current in the Arabian Sea

2017 ◽  
Vol 30 (20) ◽  
pp. 8061-8080 ◽  
Author(s):  
Hyodae Seo
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Wind Stress ◽  
Arabian Sea ◽  
Spatial Scales ◽  
Surface Current ◽  
Sea Surface ◽  
Surface Currents ◽  
Ekman Pumping ◽  
Wind Stress Curl

Abstract During the southwest monsoons, the Arabian Sea (AS) develops highly energetic mesoscale variability associated with the Somali Current (SC), Great Whirl (GW), and cold filaments (CF). The resultant high-amplitude anomalies and gradients of sea surface temperature (SST) and surface currents modify the wind stress, triggering the so-called mesoscale coupled feedbacks. This study uses a high-resolution regional coupled model with a novel coupling procedure that separates spatial scales of the air–sea coupling to show that SST and surface currents are coupled to the atmosphere at distinct spatial scales, exerting distinct dynamic influences. The effect of mesoscale SST–wind interaction is manifested most strongly in wind work and Ekman pumping over the GW, primarily affecting the position of GW and the separation latitude of the SC. If this effect is suppressed, enhanced wind work and a weakened Ekman pumping dipole cause the GW to extend northeastward, delaying the SC separation by 1°. Current–wind interaction, in contrast, is related to the amount of wind energy input. When it is suppressed, especially as a result of background-scale currents, depth-integrated kinetic energy, both the mean and eddy, is significantly enhanced. Ekman pumping velocity over the GW is overly negative because of a lack of vorticity that offsets the wind stress curl, further invigorating the GW. Moreover, significant changes in time-mean SST and evaporation are generated in response to the current–wind interaction, accompanied by a noticeable southward shift in the Findlater Jet. The significant increase in moisture transport in the central AS implies that air–sea interaction mediated by the surface current is a potentially important process for simulation and prediction of the monsoon rainfall.

scholarly journals Paleoproductivity Variations in the Equatorial Arabian Sea: Implications for East African and Indian Summer Rainfalls and the El Niño Frequency

Radiocarbon ◽  
2006 ◽  
Vol 48 (1) ◽  
pp. 17-29 ◽  
Author(s):  
Manish Tiwari ◽  
Rengaswamy Ramesh ◽  
Ravi Bhushan ◽  
B L K Somayajulu ◽  
A J Timothy Jull ◽  
...  
Keyword(s):  
El Niño ◽  
Arabian Sea ◽  
El Nino ◽  
Southern Oscillation ◽  
Southwest Monsoon ◽  
East African ◽  
Radiocarbon Dates ◽  
Last Glacial ◽  
Productivity Data ◽  
The Last Glacial

We analyzed a sediment core from the equatorial Arabian Sea, chronologically constrained by accurate accelerator mass spectrometry (AMS) radiocarbon dates on selected planktonic foraminiferal species, for paleoproductivity variations corresponding to the variations in the Indian Ocean Equatorial Westerlies (IEW). The IEW in turn are positively correlated to the Southern Oscillation Index (SOI), which is a measure of El Niño, Southwest monsoon (SWM), and east African rainfall (EAR). The productivity data show that Indian and east African rainfalls declined from 35,000 calendar yr BP up to the last glacial maximum (LGM), with the maximum El Niño frequency during the last glacial period. From ∼14,500 to ∼2000 calendar yr BP (i.e. core top), we find strengthening SWM and EAR along with declining El Niño frequency.

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