No abstract

Recently it has been shown that anticyclonic eddies are generated in the Comoros Basin contesting the long-held notion of a single large anticyclonic cell, the Comoros Gyre. Limited knowledge exists about the mesoscale activity within the basin, a potential key source of variability for the Mozambique Channel and subsequently the Agulhas Current. In this paper an automated eddy tracking scheme, applied to satellite altimetry data and a high-resolution model simulation, is used to determine the characteristics of the anticyclonic eddies generated in the Comoros Basin. The generation and characteristics of cyclonic eddies are also investigated. The eddy tracking scheme revealed that anticyclonic eddies are primarily generated west of the tip of Madagascar due to barotropic instabilities whereas cyclonic eddies are mainly generated along the northwest coast of Madagascar as a result of baroclinic instabilities. Anticyclonic eddies, with a mean lifespan of about 3 months, reside in the basin for half their lifespan before propagating into the Mozambique Channel. On the other hand, the majority of cyclonic eddies, with a similar mean lifespan, dissipate within the basin. Initially, the anticyclones, with translation speeds of 6-8 km d21 and mean radii of 80-100 km, follow the trajectory of the North East Madagascar Current and turn south upon reaching the African coast. The cyclonic eddies tend to be smaller (60 km) and have slower translation speeds (2.5-3.5 km d 21 ) than their anticyclonic counterparts.

Accurate knowledge of the wind stress field is fundamental for understanding the ocean dynamics of a particular region. Ocean circulation models typically require this as forcing field and to be specified as one of the boundary conditions. Despite its importance, there are still substantial uncertainties in obtaining sufficiently accurate wind stress fields for many regions as well as sizeable differences between wind stress products for the same region. In this study, an assessment is made of three different wind products (one satellite scatterometer and two reanalyses) for the western tropical Indian Ocean, an area of complex ocean dynamics which has important influences on regional climate. In addition to this assessment, an investigation of the local scale spatial variability in the winds is made. It is found that the wind fields over the western Indian Ocean have prominent semiannual and annual cycles, but there are significant differences in these cycles when comparing different products. For example, for ERA‐interim, the annual cycle of wind speed associated with the Somali Jet display a longer Southwest monsoon season compared to QuikSCAT and NCEP. Both the scatterometer and two reanalysis products display analogous large‐scale wind features; however, differences were noted in the strengths of wind speed and wind stress curl. It is also evident that the local scale wind features associated with orographic effects are better resolved by QuikSCAT and ERA‐interim than NCEP. These differences in wind products have important implications for regional ocean dynamics and modeling.

No abstract

The Comoros Basin, a region of eddies that propagate into the Mozambique Channel and subsequently the Agulhas Current, has not been well studied. This paper presents the findings from the first dedicated research cruise in the basin. ADCP data collected during the cruise show that the circulation in the Comoros Basin is dominated by the strong westward flowing North East Madagascar Current, which bifurcates twice: once upon encountering the Davie Ridge and again upon reaching the East African coast. One of the branches flows southward along the African coastline and appears to continue into the Mozambique Channel while the other branch turns northward to become the East African Coastal Current. The ADCP data also show evidence of a cyclonic eddy along the northwest coast of Madagascar. Water mass analysis indicates that all the major Indian Ocean water masses are present in the Comoros Basin. Surprisingly, North Atlantic Deep Water, a water mass previously assumed to only occur south of ∼16°S in the Mozambique Channel, was found to be present in the basin at depths below 2000 m. Antarctic Intermediate Water is found to enter the basin via two routes; from the west within the North East Madagascar Current and from the south within the northward flowing Mozambique Undercurrent.