Circulation around La Réunion and Mauritius islands in the south-western Indian Ocean: A modeling perspective

Abstract

The objective of this study is to document the circulation in the vicinity of La R eunion and Mauritius islands, i.e., within 500 km offshore, on the intraseasonal time scale, using a high-resolution realistic modeling strategy. The simulated sea level anomalies, water mass properties, and large-scale circulation compare favorably with satellite and in situ observations. Our high-resolution simulation suggests that the currents around the islands are maximal locally, oriented southwestward, to the southeast of both islands which is not visible in low-resolution satellite observations. It also highlights the high degree of variability of the circulation, which is dominated by westward propagating features. The predominant time scale of variability is 60 days. This coincides with the period of a barotropic mode of variability confined to the Mascarene Basin. The characteristics of the westward propagating anomalies are related to baroclinic Rossby waves crossing the Indian Ocean but only in the long-wave resting ocean limit. Tracking those anomalies as eddies shows that they also have a meridional tendency in their trajectory, northward for cyclones and southward for anticyclones, which is consistent with previous studies. Sensitivity experiments suggest that they are predominantly advected from the east, but there is also local generation in the lee of the islands, due to interaction between the circulation and topography.

Population differentiation or species formation across the Indian and the Pacific Oceans? An example from the brooding marine hydrozoan Macrorhynchia phoenicea

“…Oceanic circulation models of the studied regions are still under development and we could not meaningfully adjust our dispersal distance estimates. However, for the Western Indian Ocean where oceanic models are more developed (Pous et al., ; Schott, Xie, & McCreary, ), our findings are consistent with those of other genetic connectivity studies (Bourjea et al., ; Muths et al., ; Postaire et al., ; Ridgway et al ., ; Ridgway & Sampayo, ).…”

Superclone Expansion, Long-Distance Clonal Dispersal and Local Genetic Structuring in the Coral Pocillopora damicornis Type β in Reunion Island, South Western Indian Ocean

“…Thus the fact that REU4, the southernmost population, appeared as a group genetically distinct from the three other populations could therefore result (1) from a particular connection of the southern part of Reunion Island with other reefs from Mauritius or Rodrigues through the SEC and (2) from the flushing out of larvae broadcasted from REU4 to the west (towards Madagascar), without going to the north of Reunion Island. Besides, on the west coast of Reunion Island (REU1, REU2 and REU3), oceanic currents are complex, due to local scale eddies created by the SEC residues [37]. Then, larvae from elsewhere might have some issues to reach the northern reefs and larvae produced on the west coast might be retained by local currents.…”

“…The channel between REU1 and REU2 is symbolized with a dash line and the direction of the South Equatorial Current around Reunion Island is symbolized with black arrows (from [37]). (Open access data: http://sextant.ifremer.fr/fr/web/ocean_indien/geoportail/sextant#/metadata/f3587329-b5b7-4ee1-822d-5cda9cef4d09).…”

One species for one island? Unexpected diversity and weak connectivity in a widely distributed tropical hydrozoan

“…Marine circulation models at even smaller scales (that is, around islands and coastal areas) are still under development for most of the sampled islands. Nevertheless, a model of oceanic circulation around Reunion Island indicates high heterogeneity in the direction of currents across seasons and years (Pous et al, 2014), partly explaining the absence of IBD at this geographic scale, because reproduction has been observed throughout the year (BP, HM and CAF Bourmaud, personal observation). To our knowledge, no detailed population genetic study of a marine hydrozoan species presenting biphasic life cycle (that is, medusa and fixed colonial stages) using microsatellites exists yet to compare our findings.…”