SIDDIES Corridor: A Major East‐West Pathway of Long‐Lived Surface and Subsurface Eddies Crossing the Subtropical South Indian Ocean

Abstract

South Indian Ocean eddies (SIDDIES), originating from a high evaporation region in the eastern Indian Ocean, are investigated by tracking individual eddies from satellite data and co‐located Argo floats. A subsurface‐eddy identification method, based on its steric dynamic height anomaly, is devised to assign Argo profiles to surface eddies (surfSIDDIES) or subsurface eddies (subSIDDIES). These westward‐propagating, long‐lived features (>3 months) prevail over a preferential latitudinal band, forming a permanent structure linking the eastern to the western Indian Ocean, that we call the 'SIDDIES Corridor’. Key features have been revealed in the mean thermohaline vertical structure of these eddies. Anticyclonic SIDDIES are characterized by positive subsurface salinity anomalies, with subSIDDIES not exhibiting negative surface anomalies, as opposed to surfSIDDIES. Cyclonic subSIDDIES also occur, but their related salinity anomalies are weaker. SubSIDDIES exhibit two cores of different temperature polarities in their surface and subsurface levels. Cyclonic subSIDDIES have their cores at around 150‐200 m depth along the 25.4‐25.8 kg m−3 potential density layer with anticyclonic subSIDDIES having their cores at 250‐300 m along the 26‐26.4 kg m−3 density layer. The SIDDIES corridor acts as a zonal pathway for both eddy‐types to advect water masses and biogeochemical properties across the basin. This study provides a new insight on heat/salt fluxes, showing that 58% (32%) of the total heat eddy‐flux is ascribed to cyclonic (anticyclonic) subSIDDIES, respectively, in the eastern South Indian Ocean. Anticyclonic subSIDDIES have also been found to be the sole high‐saline water eddy‐conveyor toward the western Indian Ocean.

Role of Indian Ocean Dynamics on Accumulation of Buoyant Debris

“…Interaction between these two accumulation regions has already been shown (van Sebille et al, ), but it still needs to be confirmed if this is enough to explain the emptying of the subtropical SIO accumulation region. Second, the drogued simulations show a persistent accumulation forming in a region that is associated with high eddy kinetic energy (Dilmahamod et al, ). It is not yet clear why this accumulation is so stable.…”

“…The SICC flows across the entire width of the SIO and splits into three separate branches between the southern tip of Madagascar and the western coast of Australia (Lambert et al, 2016;Menezes et al, 2014). The SICC is also associated with high activity of westward propagating eddies (Dilmahamod et al, 2018).…”

Lagrangian Evolution of Two Madagascar Cyclonic Eddies: Geometric Properties, Vertical Structure, and Fluxes

“…Previous results suggested Madagascar cyclonic eddies have the ability to advect mass, heat, and freshwater in distinct depth layers, depending on the ratio between their rotational and propagation speeds. We take the estimates of time‐varying trapping depths for the model April and July eddies and combine them with the temperature and salinity anomalies shown in Figures and through the growth, mature, and decay phases to calculate the available heat and salt contents (AHA, ASA) following (Barceló‐Llull et al, ; Dilmahamod et al, ): where z is the vertical coordinate (depth, m), z trap is the trapping depth (m), ρ 0 is the mean upper ocean density (1,026 kg m −3 ), C p is the specific heat capacity (4,000 J·kg −1 ·°C −1 ), and r is the eddy radius (m). Then temperature and salinity anomalies (Θ ′ and , respectively) are calculated comparing model daily fields of the selected dates of study against the corresponding multiyear‐averaged monthly field over the period 1993–2015.…”

“…The mean vertically integrated AHA (ASA) of the April eddy was −58.8 × 10 18 ± 65.1 × 10 18 J (−145.5 × 10 10 ± 158.8 × 10 10 kg), and the July eddy was −151.7 × 10 18 ± 83.7 × 10 18 J (−336.7 × 10 10 ± 189.6 × 10 10 kg). We find these averaged estimates of total AHA and ASA (Table ) larger than those found for cyclonic eddies in the Peru‐Chile Current System (−5.9 × 10 18 J and −14.7 × 10 10 kg, respectively; Chaigneau et al ) and in the Western and Eastern Indian Ocean for subsurface cyclonic eddies (−5.1 × 10 18 J and −25.6 × 10 10 kg; Dilmahamod et al ). However, we must also note that estimates in these previous works are based on smoother vertical structures of the eddies since they were constructed from averaging Argo profiles over large areas to provide climatological views.…”

“…Interestingly, estimates of heat fluxes driven by Madagascar cyclonic eddies approached values reported for Agulhas Rings (anticyclonic eddies) using Argo floats and satellite data reaching up to 0.07 PW (Souza et al, ); whereas freshwater fluxes driven by the April and July eddies fall within the same order of magnitude as those reported for surface and subsurface cyclonic eddies in the South Indian Ocean, which ranged along −0.4, −0.02, and 0.8 Sv (Dilmahamod et al, ).…”