Somali Current System Research Articles

Persistence of Cold Wedges in the Somali Current System

AbstractThe Somali Current system in the western Arabian Sea reverses seasonally with the South Asian Monsoon and is associated with localized upwelling cells or cold wedges during the summer southwest monsoon. Drifter trajectories in boreal summer and fall 2014 provide rare observational evidence that the northward Somali Current and associated cold wedges can persist into the boreal fall intermonsoon period. The near‐surface circulation and sea surface temperatures further suggest that the wedge‐like surface signatures may intermittently be capped and then reappear at a later time. Our observations show that the northward Somali Current system rapidly decayed within 1 week after the onset of the winter northeast monsoon and the arrival of a cyclonic eddy at the coast in early November 2014. This eddy may not only have affected regional ocean‐atmosphere interactions but also biogeochemical processes and the marine ecosystem through the transport of water properties and locally induced upwelling.

Eddy Tracking in the Northwestern Indian Ocean During Southwest Monsoon Regimes

AbstractThe northwestern Indian Ocean exhibits a relatively highly energetic eddy field during the southwest monsoon season between June and September. This study analyzes the seasonal and interannual variability of the eddy characteristics and their trajectories in the northwestern Indian Ocean using altimetric sea surface height observations from 1993 to 2014. Although the highest number of eddies is found in the Arabian Peninsula coastal region, the strongest eddies, characterized by large radii, amplitudes, and eddy kinetic energies are found along the Somali Current. Trajectories of anticyclonic and cyclonic eddies are investigated to provide insight on the generation and propagation of eddies of varying amplitudes. The largest annual eddy in the Somali Current system corresponds to the Great Whirl, for which the year‐to‐year variability with respect to shape, size, generation, and propagation was examined, as was the development of these characteristics over the Great Whirl's lifetime.

Remote Ocean Response to the Madden–Julian Oscillation during the DYNAMO Field Campaign: Impact on Somali Current System and the Seychelles–Chagos Thermocline Ridge

During the CINDY/DYNAMO field campaign, exceptionally large upper ocean responses to strong westerly wind events associated with the Madden–Julian oscillation (MJO) were observed in the central equatorial Indian Ocean. Strong eastward equatorial currents in the upper ocean lasted more than one month from late November 2011 to early January 2012. The remote ocean response to these unique MJO events are investigated using a high resolution (1/25°) global ocean general circulation model along with the satellite altimeter data. The local ocean response to the MJO events are realistically simulated by the global model based on the comparison with the data collected during the field campaign. The satellite altimeter data show that anomalous sea surface height (SSH) associated with the strong eastward jets propagated eastward as an equatorial Kelvin wave. The positive SSH anomalies then partly propagate westward as a reflected Rossby wave. The SSH anomalies associated with the reflected Rossby wave in the southern hemisphere propagate all the way to the western boundary. These remote ocean responses are well simulated by the global model. The analysis of the model simulation indicates the significant influence of reflected Rossby waves on sub-seasonal variability of Somali current system near the equator. The analysis further suggests that the reflected Rossby wave causes a substantial change in the structure of the Seychelles–Chagos thermocline ridge, which contributes to significant SST anomalies.

Northern Arabian Sea Circulation-Autonomous Research (NASCar): A Research Initiative Based on Autonomous Sensors

The Arabian Sea circulation is forced by strong monsoonal winds and is characterized by vigorous seasonally reversing currents, extreme differences in sea surface salinity, localized substantial upwelling, and widespread submesoscale thermohaline structures. Its complicated sea surface temperature patterns are important for the onset and evolution of the Asian monsoon. This article describes a program that aims to elucidate the role of upper-ocean processes and atmospheric feedbacks in setting the sea surface temperature properties of the region. The wide range of spatial and temporal scales and the difficulty of accessing much of the region with ships due to piracy motivated a novel approach based on state-of-the-art autonomous ocean sensors and platforms. The extensive data set that is being collected, combined with numerical models and remote sensing data, confirms the role of planetary waves in the reversal of the Somali Current system. These data also document the fast response of the upper equatorial ocean to monsoon winds through changes in temperature and salinity and the connectivity of the surface currents across the northern Indian Ocean. New observations of thermohaline interleaving structures and mixing in setting the surface temperature properties of the northern Arabian Sea are also discussed.

A numerical investigation of the Southern Gyre using ROMS

A numerical model (The Regional Ocean Modelling System—ROMS), configured over the western Indian Ocean and driven by monthly climatology winds and heat fluxes, is applied to examine the Southern Gyre in the Somali Current system during the Southwest Monsoon. Despite the Southern Gyre playing a role in transporting water masses and other properties northwards across the equator, it has not been much studied. The model results indicate that the Southern Gyre appears in early June in the upper ocean as a result of instability in the northward flowing Somali Current. The arrival of downwelling Rossby wave energy at the East African coast intensifies the recirculation of the Southern Gyre and causes its northward movement. The Southern Gyre is characterized as a shallow feature which deepens from 100m in June to 300m in July–August. The average spatial scale of the gyre is about 400km with subsequent development of positive vorticity bursts which are identified as potential contributors to the decay of the Southern Gyre. Cool and fresh waters observed in the gyre resulted from advection via the South Equatorial Current and then through the Somali Current (SC).

Interactions between the Somali Current eddies during the summer monsoon: insights from a numerical study

Abstract. Three hindcast simulations of the global ocean circulation differing by resolution (1/4 or 1/12°) or parametrization or atmospheric forcing are used to describe the interactions between the large anticyclonic eddies generated by the Somali Current system during the Southwest Monsoon. The present investigation of the Somalian coherent eddy structures allows us to identify the origin and the subsequent development of the cyclones flanked upon the Great Whirl (GW) previously identified by Beal and Donohue (2013) in satellite observations and to establish that similar cyclones are also flanked upon the Southern Gyre (SG). These cyclones are identified as potential actors in mixing water masses within the large eddies and offshore the coast of Somalia. All three simulations bring to light that during the period when the Southwest Monsoon is well established, the SG moves northward along the Somali coast and encounters the GW. The interaction between the SG and the GW is a collision without merging, in a way that has not been described in observations up to now. During the collision the GW is pushed to the east of Socotra Island, sheds several smaller patches of anticyclonic vorticity, and often reforms into the Socotra Eddy, thus proposing a formation mechanism for that eddy. During this process the GW gives up its place to the SG. This process is robust throughout the three simulations.

The North Indian Ocean circulation and its variability as seen in a numerical hindcast of the years 1993–2004

A 12-year hindcast of the physical state of the North Indian Ocean has been carried out for the period 1993–2004, using a data-assimilative, primitive equation, multi-level circulation model that assimilated altimetric sea surface height anomalies and weekly MCSST, and was driven by 6-h ECMWF winds. This period encompasses the anomalous wind events in 1994 and 1997–1998 that led to anomalous oceanic state including anomalous sea surface heights and sea surface temperatures, especially in the equatorial regions. Since the in situ database in the Indian Ocean is rather sparse, the hindcast provides an alternative means of examining the state of the ocean, including its interior, during these anomalous years, as well as normal years during this period. By comparison with observations available during this period, it is shown that the model possesses reasonable skill to be useful in the description of various events in the North Indian Ocean. In this paper, we examine the circulation and its variability over the 12 years of the hindcast. We discuss equatorial events, as well as events in the Bay of Bengal and the Arabian Sea, including the Somali Current system. The heat and mass fluxes are examined. Finally, the hindcast is repeated with QuikSCAT wind stress fields available from July 1999 onwards. Comparison of the 2000–2004 hindcasts forced by ECMWF winds and QS wind stresses shows that in the former, the currents and fluxes are underestimated by 20–30%, but the circulation patterns are roughly similar.

Somali Current rings in the eastern Gulf of Aden

New satellite‐based observations reveal that westward translating anticyclonic rings are generated as a portion of the Somali Current accelerates northward through the Socotra Passage near the mouth of the Gulf of Aden. Rings thus formed exhibit azimuthal geostrophic velocities exceeding 50 cm/s, are comparable in overall diameter to the width of the Gulf of Aden (250 km), and translate westward into the gulf at 5–8 cm/s. Ring generation is most notable in satellite ocean color imagery in November immediately following the transition between southwest (boreal summer) and northeast (winter) monsoon regimes. The observed rings contain anomalous fluid within their core which reflects their origin in the equator‐crossing Somali Current system. Estimates of Socotra Passage flow variability derived from satellite altimetry provide evidence for a similar ring generation process in May following the winter‐to‐summer monsoon transition. Cyclonic recirculation eddies are observed to spin up on the eastern flank of newly formed rings with the resulting vortex pair translating westward together. Recent shipboard and Lagrangian observations indicate that vortices of both sign have substantial vertical extent and may dominate the lateral circulation at all depths in the eastern Gulf of Aden.

New data on the sea current system in the Somali region, Indian Ocean

Direct measurements of current velocities conducted during the International World Ocean Circulation Experiment (WOCE) yielded new data on spatiotemporal characteristics of a current system off the Somali coast. It is shown that background circulation within the domain of the western boundary current in the Indian Ocean forms a regional anticyclonic gyre similar in general patterns to its counterparts in the Atlantic and Pacific oceans. However, the Indian Ocean is distinguished from other oceans by superimposition of the alternating monsoon signal with the prevalent annual harmonic on the background dynamic structure. It is established that abnormally high velocities in the central part of the Somali Current (up to 2 m/s) during the southwestern monsoon period is related to superimposition of unidirectional background and summer monsoon components of this powerful western boundary current. The Indian Ocean segment adjacent to the northeastern coast of the African continent is one of the most active regions of the planet with respect to climatic perturbations. The vast region of a cold surface layer in the southern Arabian Sea related to upwelling in the Somali Current system [2] substantially influences atmospheric processes and weather conditions in the spacious maritime region of southwestern Asia. There are also grounds to believe that inversion heat transfer from the atmosphere to the ocean observed during the summer monsoon in the Somali temperature anomaly zone acts as a damper in the generation of mobile cyclonic whirlwinds, preventing them from transforming into catastrophic hurricanes. All these and other climatically important peculiarities in the thermodynamic behavior of the ocean and atmosphere near the African Horn attract the constant attention of experts in meteorology and oceanography to this region [3‐7]. Nevertheless, despite a long history of investigations, many problems related particularly to variability in the Somali segment of the oceanic circulation system remain poorly studied so far. The hypothesis of reverse behavior of the Somali Current is one of the most intriguing scenarios of this variability [4, 8, 9]. It is known that the monsoon type of atmospheric circulation that prevails over the northwestern segment of the Indian Ocean influences the system of sea currents and determines the unique characteristic of the powerful jet stream that flows along the northeastern coast of Africa, i.e., annual periodicity in the alternating direction of the current velocity vector [9]. Most researchers believe that the intense western boundary transfer (the Somali Current) appears in the summer monsoon period (approximately from June to September), when a strong favorable southwestern wind accelerates the northeastern alongshore flow of surface waters up to velocities exceeding 300 cm/s. After the onset of the winter phase of the monsoon (November), the Somali Current presumably disappears [10] and gives way to a flow with the southern velocity component under influence of the northeastern wind. It is unclear, however, whether or not this scenario is real. Published materials [11‐13] provide evidence that a countercurrent (opposite to the Somali Current) appears in the subsurface layer at the end of the summer climatic season. The countercurrent also incorporates the surface layer water at the beginning of the winter monsoon period.

The volume transport of the Somali Current during the 1995 southwest monsoon

Abstract During the summer monsoon of 1995, two hydrographic and direct velocity sections were occupied over the continental shelf and slope off the coast of Somalia, in order to capture the flux of the western boundary current. The first section was in early June, just after the onset of the monsoon winds, when no current was observed. Instead there was an anticyclonic eddy at the boundary with a small net northward transport of 3.5±1.5 Sv . The second section was occupied in mid-September when a strong Somali Current was evident. The resulting snapshot represents the only direct velocity section of the jet structure as it flows along the western boundary. The strength of the Somali Current was 37±5 Sv . This is in general agreement with previous observations, given the temporal variability of the Current, and represents a peak value according to contemporary model simulations. The Somali Current of September extended far into shallow coastal waters of less than 200 m depth, with speeds over 150 cm s −1 , resulting in a considerable transport of 14 Sv over the shelf. Northward flow of more than 10 cm s −1 penetrated to over 2000 m depth along the continental slope, so that the subthermocline flux contributes another 13 Sv to the total volume transport. Model simulations and satellite images of sea-surface height illustrate that the Somali Current System exhibits large spatial variability on short timescales and thus differences in measured transports owe as much (maybe more) to the changing path of the Current as to variability in its strength.

Gulf of Aden eddies and their impact on Red Sea Water

New oceanographic observations in the Gulf of Aden in the northwestern Indian Ocean have revealed large, energetic, deep‐reaching mesoscale eddies that fundamentally influence the spreading rates and pathways of intermediate‐depth Red Sea Water (RSW). Three eddies were sampled in February 2001, two cyclonic and one anticyclonic, with diameters 150–250 km. Both cyclones had surface‐intensified velocity structure with maxima ∼0.5 m s−1, while the equally‐energetic anticyclone appeared to be decoupled from the surface circulation. All three eddies reached nearly to the 1000–2000 m deep sea floor, with speeds as high as 0.2–0.3 m s−1 extending through the depth range of RSW. Comparison of salinity and direct velocity measurements indicates that the eddies advect and stir RSW through the Gulf of Aden. Anomalous water properties in the center of the anticyclonic eddy point to a possible formation site in the Somali Current System.

Gyres off the Somali coast and western boundary currents in the Bay of Bengal during the south-west monsoon

Abstract Satellite remotely-sensed sea surface temperature (SST) data were obtained during the initial phase of the onset of the south-west monsoon in the north-western Indian Ocean for the years 1987and 1988. Large wedge-like areas of upwelled water during 1988 were observed at 5°Nand 10°N after the Somali current spinup, indicative of a two-gyre circulation in the Somali current system. Satellite infrared observations of the Bay of Bengal during the early February 1990revealed the existenceof two bands of warm water that resembled a western boundary current (WBC) along the east coast of India. A warm core eddy, with a major axis of nearly 120km, appeared at 89°E and 19°N at the end of the axis of the current. The existence of the two gyre system ofT the Somali coast and the presence or absence of the WBC in the Bay of Bengal in response to the onset of the summer monsoon has been discussed in detail.

Modeling the seasonal undercurrents in the Somali Current system

Seasonal changes in the Somali Current system are studied using a four‐layer model where the lowest layer is at rest. The results suggest that barotropic instability is likely to cause the generation of the Great Whirl in early June. We find a very good agreement between the observed undercurrents and the simulations in the model. Equatorial onshore flow below the thermocline in June is associated with the disappearance of the undercurrent below the Somali Current. The early return of the southward undercurrent in the fall is caused by baroclinic instability of the Great Whirl followed by a reversal to eastward undercurrents along the equator. Remote winds are suggested to control the intermediate depth flow along the Somali coast acting through equatorial waves and Rossby waves generated along the west coast of India. Experiments where the duration of the summer monsoon is extended show that the initial decrease in the magnitude of the Great Whirl is due to eastward and downward energy transfer rather than due to relaxation of the wind. The model solutions suggest that baroclinic instability plays an important role in the decay of the Great Whirl.

The Somali current at the equator: annual cycle of currents and transports in the upper 1000 m and connection to neighbouring latitudes

Current measurements were obtained with moored stations during October 1984 to October 1986 in two consecutive deployments across the Somali Current on the equator. For the transport calculations the deficiency of conventional subsurface moorings, i.e. no data from close to the surface, had to be overcome using the historical ship drift climatology. While the current structure during the summer monsoon is that typical of a western boundary current, the profile in winter is far from being a weaker southward reverse of the summer situation. Below a thin surface layer of southward flow, there is a northward undercurrent between about 120 and 400 m depth. Below that, the flow reverses again to southward. This results in drastic differences in cross-equatorial monsoon season transports. While the summer mean transport is 21 Sv for the upper 500 m, the winter monsoon mean for that depth range is close to zero. The annual mean transport in the upper 500 m is 10 Sv northward. Very little transport is measured in the 500–1000 m depth range in either season or the annual mean. The almost closed mass budget of the boundary current system during the winter circulation allows a calculation of cross-equatorial heat transport, which comes out to −3 × 10 14 W (southward) for the northeast monsoon season mean. The heat flux associated with the annually varying part of the boundary current is small, only about −0.3 × 10 14 W or about 5% of the total cross-equatorial heat flux as estimated by other methods. By combining the equatorial measurements with earlier off-equatorial current observations, particularly at 2°−4°S and 5°N, and with property distributions (salinity and oxygen) on isopycnal surfaces, analysed from the historical data file, a synopsis of the seasonal circulation changes of the entire Somali Current system between about 4°S and 12°N is then derived.

Western boundary circulation driven by an alongshore wind: with application to the Somali Current system : McCreary, J.P. and P.K. Kundu, 1985. J. mar. Res., 43(3):493–516

Western boundary circulation driven by an alongshore wind: with application to the Somali Current system : McCreary, J.P. and P.K. Kundu, 1985. J. mar. Res., 43(3):493–516

Western boundary circulation driven by an alongshore wind: With application to the Somali Current system

Western boundary circulation driven by an alongshore wind: With application to the Somali Current system

A model of the seasonal circulation in the Arabian Sea forced by observed winds

Results of a numerical model of the wind driven seasonal circulation in the Arabian Sea are presented, with particular emphasis on the ocean's response to the monsoon winds. The model equations are the fully nonlinear reduced gravity transport equations in spherical coordinates. The model resolution is 1/8° in the east-west direction and 1/4° in the north-south direction. The model basin geometry corresponds as closely as possible to that of the north-west Indian Ocean from 40°E to 73°E and from 10°S to 25°N, and includes the gulfs of Aden and Oman, and the islands of Socotra and the Seychelles. The southern boundary and a portion of the eastern boundary, from the equator to 6°S, representing the opening between the Maldives and the Chagos Archipelago, are open boundaries. At other boundaries, the no-slip condition is applied. The wind stress data used to force the model comes from the NOAA National Climate Center's TD-9757 Global Marine Sums data, which consists of monthly mean winds compiled on 1° squares from over 60 years of ship observations. These data are interpolated in time using the mean and first five Fourier harmonics at each point, and then interpolated linearly to the model grid. The model equations are integrated in time using centered finite differences in time and space (a leap-frog scheme), with lateral friction treated by a Dufort-Frankel scheme. After a one year spin up, the model settles into a regular periodic seasonal cycle, even though the solution to the model equations is locally highly nonlinear, with large nonlinear eddies developing in the same location at the same time of year from one year to the next. The development of the model Somali Current system with the onset of the (northern hemisphere) summer monsoon is consistent with the available observations in the region. The model reproduces many of the observed features in this region, such as the two-gyre circulation pattern, and the timing and movement of these features corresponds well with their real world counterparts. The model also shows an eastward jet forming in late June to early July at 13°N, just to the east of Socotra. This jet is fed by flow coming out of the great whirl. The break down of the two-gyre pattern occurs in mid to late August, when the southern gyre breaks up into several smaller eddies, the northern-most of which coalesces with the great whirl. Numerous small cyclonic eddies develop along the Arabian coast, from the Gulf of Oman into the Gulf of Aden, in early to mid August, and persist well into the winter monsoon. The model shows that it is possible to simulate very complicated flows, if one has sufficient wind data, using fairly simple models with a realistic basin geometry.

Variability of the Somali Current System during the Onset of the Southwest Monsoon, 1979

An array of six current-meter moorings and several coastal temperature recorders was deployed on the shelf and continental slope off northern Somalia from March to July 1979; a seventh mooring was placed near 2°S. In addition, four deep-sea moorings were deployed for a period of one month in May–June farther offshore. Already during the late northeast monsoon in March the Somali Current north of 5°N was flowing northeastward in the top 150 m. Underneath, in the depth range 150–400 m, a narrow southward undercurrent was observed from March to June. After the first onset of the southwest monsoon, which occurred around 5 May when winds shifted from easterly to southwesterly parallel to the coast, the near-surface temperatures on the shelf decreased immediately with no detectable phase difference between 6 and 10°N, but no change was observed in the offshore circulation pattern. The final monsoon onset around 10 June was characterized by a drastic increase in wind speeds and the establishment of a strong anticyclonic wind-stress curl over the northern Somali Basin. The current measurements showed that within a few days after this onset the northern Somali gyre spun up over the deep sea and then propagated northwestward toward the coast with a speed of 12 cm s−1. These findings are in good agreement with results of satellite infrared imagery. The observed gyre kinematics can be explained by locally generated non-equatorial Rossby waves. When the onset reaches the coast the shallow coastal undercurrent is extinguished. Superimposed on the gyre-scale variability were fluctuations in the period ranges of weeks to months and of 3–5 days. There is evidence that the energies of the latter were related to the development of the Somali Current. Significant differences were found in a comparison of the 1979 current measurements south of the equator with observations obtained there during the monsoon onset of 1976.

Propagation of thermal fronts in the somali current system

Surface ship-of-opportunity and satellite remotely sensed sea-surface temperature (SST) data were obtained during the mature phase of the Southwest Monsoon in the north-western Indian Ocean for the years 1976–1979. Large wedge-like areas of upwelled water are observed at 5°N and 10°N after Somali Current spinup, indicative of a two-gyre circulation in the Somali Current system. Several months later the southern separation region is observed to translate poleward (5–75 cm/s) in three of the four years. This translation occurs in several distinct phases over a one-month interval with the southern wedge eventually coalescing with the northern wedge between 8 and 10°N (1976, 1979). (In 1978 the northern wedge translated past Socotra and dissipated in the Arabian Sea). The thermal signature of the “new” northern wedge becomes indistinct over a period of two to three months later.

A Numerical Model of the Somali Current

Abstract We have sought to simulate and understand consistently observed features of the Somali Current system during the southwest monsoon using a two-layer, nonlinear numerical ocean model driven from rest by a uniform south wind in a flat bottom, rectangular geometry. High spatial resolution in both equatorial and coastal boundary regions was retained in this free-surface model. The model Somali Current is best classed as a time-dependent, baroclinic inertial boundary current. Analytical solutions to a quasi-steady linear model of the Somali Current are shown to be self-inconsistent with the linear approximation. While linear theory predicts perfect symmetry about the equator, the nonlinear numerical solutions exhibit marked asymmetries in less than a month as the model Somali Current becomes strongly inertial. By day 30 the current has attained its maximum value (140 cm s−1) within a few degrees of the equator, in accord with observations. In this time-dependent case, boundary layer separation occurs ...