The near-surface circulation of the northwestern Indian Ocean during the summer monsoon is the siege of large and strong anticyclonic eddies produced by recirculation cells of the Somali Current system. In the schematic representation of the typical surface current patterns proposed in the comprehensive review of  and updated in their Fig. 3, see also Fig. , the South Equatorial Current (SEC) and the East African Coastal Current (EACC) are supplying the Somali Current (SC), a low-latitude western boundary current flowing northward along the coast of Somalia. A large branch of the EACC turns offshore after crossing the Equator at about 2 or 3∘ N and forms the so-called Southern Gyre (SG), a large anticyclonic retroflection cell with a well-marked wedge of cold upwelled water attached to its northern flank (the southern cold wedge driven by the upwelling favourable winds). According to , this cell re-circulates southward across the Equator to feed into the South Equatorial Counter Current (SECC). In the north, located between the SG and the Island of Socotra (i.e. between 5 and 10∘ N) is the Great Whirl (GW), a large anticyclone which exhibits very intense swirling currents. Its generation mechanism involves the arrival of remote Rossby waves in spring (see ) and an amplification in summer by the monsoon winds via an intensification and a retroflection of the Somali Current. The GW also exhibits an upwelling wedge at its northern flank (the northern cold wedge). A third anticyclonic eddy named the Socotra Eddy (SE) is reported to be frequently present to the east of the Island of Socotra. The regional numerical model study of of the Arabian Sea points out the importance of the regional wind stress curl in the growth and maintenance of the GW, and the influence of basin-scale Rossby waves in its generation and its interannual variability.

Investigating the time evolution of these large eddies is possible since satellite altimeter measurements provide a dense enough mapping of the sea surface height (SSH). The analysis of 18 years of weekly AVISO SSH fields by revealed a chaotic evolution of the GW, variations in size, shape and location of the eddy being greatly influenced by strong eddy–eddy interactions. Such behaviour was suggested by the idealized numerical model studies of and . This contrasts with the previous studies which convey the view of a GW that is slowly varying in response to the wind forcing (e.g. ). observed that one to three cyclonic eddies of smaller size flank the GW most of the time, appearing in late June and circulating clockwise around the GW. They also observed the Socotra Eddy, but found that its variability in shape, size, and position was even greater than the one seen for the GW, making the SE often difficult to identify. Nevertheless, they report its frequent merging with the GW. At the best of our knowledge there is still no clear formation mechanism proposed for this eddy. The analysis of does not report any interaction between the SG and the GW, possibly because they limited the domain of study to 3–15∘ N. However, the northward migration of the SG and its possible merging with the GW is mentioned in several observational studies . Based on the observation of a merging of the southern cold wedge with the northern one, those studies suggest that the SG and the GW could coalesce. Such merging was also observed in the numerical model study of , but was not reported in the observations collected during the WOCE cruises of 1995–1996 .

The lack of understanding of the processes governing the dynamics of the large circulation features that are the SG, the GW, and the Socotra Eddy largely resides in the lack of dense in space and time observations. The objective of the present study is to gain insights into the nature of the interactions between these anticyclonic eddies. The paper addresses the following questions:

What is the generating mechanism of the cyclones flanking the GW?

We address the above questions through realistic numerical eddying model simulations, since they can provide dense spatio-temporal information required for a synoptic description of the mesoscale circulation.

The numerical model experiments that are the basis of our analysis are described in Sect. . The ability of the model to reproduce the upper-layer circulation in the Arabian Sea during the summer monsoon is assessed in Sect. . In Sect.  we perform a description and an analysis of the dynamics of the Somali Current eddies as simulated by the model experiments. In Sect.  we discuss the results put forward by our analysis of the eddy–eddy interactions in the western Arabian Sea and summarize the main findings.

Three global ocean hindcast simulations, one at 1/4∘ resolution and two at 1/12∘ resolution, made available by the DRAKKAR consortium are used here to study the Somali eddies during the Southwest Monsoon.

The model configurations used to produce these hindcasts are based on the NEMO ocean/sea-ice general circulation numerical model and utilize specifications developed by the DRAKKAR consortium. Among them is the ORCA025 eddy-permitting configuration which has a nominal resolution at the Equator of 0.25∘ and 75 vertical levels. ORCA025 is extensively described in and has been widely used to address scientific questions in physical oceanography, biogeochemistry, and marine biology (see refereed publications at www.drakkar-ocean.eu). Although ORCA025 is only eddy-permitting, even at these low latitudes, it is widely used to perform ocean reanalyses and is the ocean component of several Earth System Models in Europe . Therefore, we find it useful to report any assessment of its solution. As is shown in this study, the eddy-permitting 1/4∘ solution behaves in a way that is qualitatively comparable to the eddy-resolving 1/12∘ solution in term of large-scale circulation of the Indian Ocean and in its representation of the main features of the Somali Current eddies.

The other model configuration is the ORCA12 eddy-resolving configuration with a resolution of 1/12∘ and 46 vertical levels. ORCA12 is the most recent and the highest-resolution global configuration of the DRAKKAR hierarchy, and its effective horizontal resolution ranges between 9.25 km at the Equator, 7 km at Cape Hatteras (mid-latitudes), and 1.8 km in the Ross and Weddell seas. Models of that resolution have been shown to drastically improve the representation of western boundary currents . Driven by atmospheric forcing derived from atmospheric reanalysis , ORCA12 simulations are good tools to investigate global dynamical and thermo-dynamical balances .

The specificities of the three simulations used here are presented in Table 1. Main differences between the simulations lie: in the horizontal resolution (1/4∘ or 1/12∘); or in the vertical resolution (75 or 46 levels); or in the atmospheric forcing used which can be ERA-interim or the DFS4.3 atmospheric forcing (building on ERA40, ); or in the length of integration which can vary from 15 to 30 years; or in the side wall boundary conditions which can be free slip or partial slip. The differences in lateral friction (free-slip versus partial-slip boundary conditions) may have an impact on the mean profile of the boundary current, and consequently an impact on its stability. However, there are too many differences between the free-slip and the partial-slip experiments (differences in spin-up time, forcing, and period of integration, see Table 1) to assess in a significant manner the impact of the friction parameter on the eddy–eddy interactions described here.

The above discussion of the lateral friction parameter introduces our strategy of using those three different simulations. These global simulations were not initially designed to be sensitivity studies of the Arabian Sea to various processes or parameters – as were the regional simulations of for example – and are not suited to be used for such purpose. The use of several simulations rather than a single one is motivated by the fact that the solutions provided by present state-of-the-art eddy-resolving OGCMs still show some dependency on parameter choices and are subject to a chaotic behaviour specific to turbulent flows . Identifying parts of the solution that are robust through all simulations contributes to building confidence in the results. A similar paradigm was used in the past for model inter-comparison studies (e.g. ). In the present case, because the focus is on highly non-linear turbulent processes, many occurrences of these processes are necessary to assess their significance. This is why we have chosen three simulations, each providing 10 years of data, such that in total 30 realizations of the SG/GW annual interaction cycle are available, from which scenarios are drawn.

The analysis of model results presented here is performed on the last 10 years of every simulation using model output every 5 days and focuses on features that are robust through all simulations. Because the model output provides a dense space and time sampling of the ocean variables, the tracking of the Somali Current eddies described here (mainly the GW and the SG) was simply made by looking carefully at individual snapshots.

The ability of the above model configurations to realistically simulate the large-scale and mesoscale features of the global ocean circulation has been demonstrated in several studies (as mentioned in the previous section). We present here a short validation of the surface ocean circulation in the Arabian Sea during the Southwest Monsoon for the 1/4∘ resolution ORCA025 simulation S4-1. The two ORCA12 simulations (S12-1 and S12-2) have been validated in the same way. The results described in this section, although illustrated with ORCA025, hold for the two ORCA12 simulations (small differences being the appearance of structures of smaller scale and some spatial and temporal lags expected from the turbulent nature of the flow), and are very consistent with the circulation schemes proposed in the literature (e.g. ). A more thorough validation of the 1/12∘ model simulations can be found in .

The model surface circulation during the summer monsoon is compared with an observational reference in Fig. . The climatological monthly means of the sea surface height (SSH) and of the currents at 15 m depth are displayed for June and September. Following what was done in , the observational reference for the SSH is a combination of the AVISO sea level anomalies (SLA) data with the annual climatological mean SSH of . The observational currents are the drifter-derived currents from which are a good proxy of the 15 m depth currents. The agreement between the model and the observations is qualitatively good. Drifter-derived currents are not as smooth as model currents (as expected due to their respective processing). Ekman currents are quite important at this depth, masking in some places the surface signature of the geostrophic currents.

In the early phase of the monsoon, both the model and the observations exhibit large similarities in their representation of the large-scale circulation patterns (June, Fig. a). The EACC is well established as a northward continuous coastal current that stretches across the Equator from 10∘ S to 2∘ N. A large part of the current turns offshore at around 2∘ N to form the SG, a retroflection loop that crosses back the Equator and joins the eastward flow of the SECC at about 4∘ S. Note that the surface (15 m depth) signature of the SECC, the core of which is at 100 m depth, is weak and partly masked by the Ekman currents. Part of the EACC continues flowing northward along the coast and joins the SC at 3∘ N. The SC flows along the coast of Somalia up to 10–12∘ N where it loops offshore to join an intense Great Whirl easily identified in Fig. a and b by the strong SSH high off the Horn of Africa at 6–11∘ N. The SC continues to flow northward beyond the GW, a branch passing through the Socotra Passage as described in and , and another branch flowing around the Socotra Island by the east as reported in and . Both branches join again north of the island to cross the mouth of the Gulf of Aden and flow north along the Omani coast.

Great similarities are also found between the model and observations in the late phase of the Southwest Monsoon (September, Fig. b). The EACC is fully connected with the SC to the point that it is not possible to distinguish between each current. The GW, identified by the SSH high off Somalia, has grown considerably. A relative SSH high (anticyclonic circulation) is observed to the northeast of the GW (and east of the Socotra Island) – it is the Socotra Eddy (SE) which forms at the end of July or at the beginning of August. The intensity of the GW and the SE begins to decrease in September, and both eddies disappear completely in November (not shown). The SG persists until the end of December (not shown).

At the scale of the whole Indian Ocean, the year-round large circulation patterns and the planetary wave dynamics (not shown, ) simulated by the 1/4∘ and 1/12∘ models are in qualitatively good agreement with the analysis of surface drifter and satellite altimetry performed by .

Previous efforts to understand the Somali Current and the East African Coastal Current retroflection have focused primarily on the large-scale dynamics governing the seasonal establishment of the large anticyclones, i.e. the Great Whirl and the Southern Gyre, rather than on their fine-scale structures (e.g. the sharp currents and vorticity fronts, smaller flanking cyclones) and their local interactions with the other coherent structures generated within the Somali Current system. This is likely due to the lack of dense in space and time observations. High-resolution model hindcast simulations used herein allowed us to go beyond the view of the large anticyclones only and identify small-scale coherent structures which allows us to some extent to shed light on the dynamics of the Somali Current eddies.

Three eddying global model simulations provided by the DRAKKAR consortium have been used, differing by their resolution (1/4 or 1/12∘) or by their parametrization (e.g. free-slip or partial-slip boundary condition), or duration (from 15 to 30 years), or atmospheric forcing. The analysis of the last 10 years of these simulations first demonstrated the model ability to reproduce with a fairly good realism the major circulation patterns of the circulation in the Northwest Indian Ocean during the Southwest Monsoon. The analysis of 5-day snapshots (over a total of 30 years for all three simulations together) that focused on the generation mechanism of the cyclones flanking the GW and on the nature of interaction between the SG and the GW permits one to follow the time evolution of the dynamics of the Somali eddies.

The Somali eddies described herein appear to be quite similar to eddies shed by low-latitude western boundary currents elsewhere in the global ocean, even though they are among the lowest-latitude and most topographically constrained eddies. For example, the generation mechanism and ultimate structure of the Somali eddies have large similarities with the anticyclonic eddies formed in the western tropical Atlantic by the retroflection of the North Brazil Current . Although the basic formation mechanism and physical characteristics of these low-latitude eddies are similar, their interactions with the general circulation and regional topography differ substantially. Indeed the GW is unique as it is first generated by remote Rossby waves (see ) and later amplified by the monsoon winds via an intensification and a retroflection of the Somali Current. Somewhat differently, the SG is the result of pure retroflection of the East African Coastal Current, several retroflection events of that type eventually occurring during the monsoon cycle.

The main findings of this study can be summarized as follows.

The smaller cyclones flanking the GW identified by in satellite observations are also found around the SG. They are due to the tearing off from the boundary current of intense patches of positive vorticity called bursts that later get organized in coherent cyclones (see ). The cyclones often pair with the large anticyclones. In that case they circulate around before finally being sucked up into the anticyclones contributing to their decay or colliding with the boundary current where they are absorbed. They are sometimes detached from the anticyclone to later collapse in the open ocean. These flanking cyclones are likely important drivers in the mixing that occurs offshore of Somalia and into the large anticyclones. They are also largely responsible for the short-time variability of the large anticyclones (as suggested by ) and imprint a strong chaotic character to the flow field.

It is worth mentioning that within a single simulation, the scenarios described above show a significant year-to-year variability (e.g. Fig. ). It is quite possible that interannual variations within a given scenario could be influenced by the planetary waves in the Arabian Sea. It is also possible (even likely) that the occurrence of one scenario rather than another in a given year be selected or influenced by Rossby waves or interannual variations of the monsoon winds. These are new issues raised by our study that could not be addressed with the simulations made available to us. Our results also highlighted other phenomena, like the complete disintegration of the GW into small vorticity patches or the merging of part of the GW into the SG, that are too rare or too specific to a given simulation to assess their robustness to the model parameters that were used in the simulations (e.g. side wall friction).

Model results produced by a single numerical code must be interpreted with caution as they are, to a degree that is often not possible to assess, influenced by specificities of the numerical code used, and the scenarios described here are no exceptions. Therefore it cannot be ruled out that the numerical model used here (i.e. NEMO), although describing the various possible scenarios in a rather robust way, may unrealistically favour one specific scenario (i.e. the collision without merging of the GW and the SG) rather than the others. Indeed, it is somewhat puzzling that our most frequent scenario is not being frequently mentioned in the literature. A reason could be that this event is not as frequent in the real ocean as the model shows, and that the northward motion of the SG is more often limited below 5∘ N. Our models would therefore be biased toward one specific scenario for reasons that still have to be determined but might well be related to the fundamentals of the numerical code (e.g. vertical coordinate system, order of the numerical schemes, etc.) rather than configuration settings (all three configurations used here are favouring the same scenario). But studies of the SG are rare (attention is usually given to the GW) and to our knowledge satellite altimetry has not been applied to the dynamics of this circulation feature. Looking at the model 5-day snapshots of Sea Surface Height (SSH) we found that the SG begins to be detectable in this variable only after it reaches latitudes of 4 to 5∘ N (not shown).

To further investigate this issue, we compared our model output with satellite altimetry data (Figs.  and ). The SSH observational reference is the combination of the AVISO SLA with the annual mean SSH of already used in Sect. . For the comparison, the SSH of the 1/12∘ simulation S12-1 has been co-located (in space and time) onto the AVISO data (see ), thus degrading its grid resolution to 1/3∘ and its time sampling to 7 days. Both data sets have been processed over the same period (12 years from 1993 to 2004).

The comparison of SSH 7-day snapshots over the full period shows that model eddies are generally not in phase with the observations, which strongly suggests that the turbulent dynamics of this region is largely intrinsic and generated by non-linear instabilities of the large-scale circulation. We found a number of events in the AVISO data that are consistent with the collision scenario because they present a strong pattern that is very similar to that of the S12-1 data. An example is shown in Fig.  which compares two “look alike” sequences in the simulation and in AVISO.

The model sequence (from 7 July to 4 August 1999) corresponds to a collision between the SG and the GW during which the SG takes the place of the GW. The latter is almost dislocated and ends as a small-size eddy east of Socotra Island (Fig. a). The course of this scenario has been clearly established from the fields of vorticity and spiciness of the S12-1 simulation at full resolution. However, the evolution of the model SSH sampled like AVISO could be (falsely) interpreted as a merging of the two eddies: the SSH highs representing the SG and the GW, which are well separated on 7 July, seem to merge into a very intense SSH high between 14 and 27 July at the usual location of the GW. The collision scenario is however confirmed by the sequence shown in Fig. a which displays the evolution of the adimensionalized Laplacian of the SSH (a proxy for the relative vorticity of the geostrophic currents). Therefore, the SSH sampled like AVISO alone is not able to discriminate between a merging or a collision. Additional variables (e.g. vorticity and spiciness) are necessary.

The AVISO sequence (28 July to 25 August 2004, Fig. b) qualitatively resembles the model sequence, with a lag of 20 days. It could thus be interpreted as a collision. The corresponding vorticity patterns (Fig. b) largely reflect the correlation scales used to project the satellite data onto the regular grid. If they do not allow us to distinguish clearly between a merging or a collision, they do not exclude the possibility of a collision event such as the one suggested by the arrows in Fig. b. Nevertheless, the AVISO data clearly show a northward move of the SG which finally occupies the place of the GW, but the resolution of the data does not allow us to be certain that the GW was pushed north-eastward in a collision with no merging, or that the two eddies merged. The 7-day sampling of the AVISO data does not seem to be fine enough to distinguish between merging or collision.

We also found a few sequences in the AVISO data that are clearly consistent with a SG that does not migrate northward (again by analogy with the model, not shown). Therefore, the eddy–eddy interaction events seen in the model simulations do show some consistency with the AVISO observation data.

A simple analogy with a model sequence is certainly not accurate enough to reach a clear conclusion on the process described by the satellite data. It is therefore possible that the present nadir altimetry, which provides heavily filtered/extrapolated maps of SSH every 7 days, does not have the adequate sampling to follow these circulation features with the required level of detail, and that additional observations or different processing taking into account the time evolution of the signal are necessary.

It would be necessary to perform longer simulations, as well as simulations using other ocean models (e.g. other than NEMO) or a broader range of parameters (e.g. advection schemes or subgridscale parametrizations) in order to assess the robustness of our conclusions.

The conclusions of the present study should also be challenged by future studies that may use sufficiently dense (in space and time) satellite observations (e.g. SWOT) or eddy-resolving ocean reanalysis, thus giving opportunities to consolidate our findings or to suggest alternative explanations.