Inertially Induced Connections Between Subgyres in the South Indian Ocean

FEBRUARY 2009 N O T E S A N D C O R R E S P O N D E N C E 465 Inertially Induced Connections between Subgyres in the South Indian Ocean V. PALASTANGA,H.A.DIJKSTRA, AND W. P. M. DE RUIJTER Institute for Marine and Atmospheric Research, Utrecht, Utrecht, Netherlands (Manuscript received 3 July 2007, in final form 18 August 2008) ABSTRACT A barotropic shallow-water model and continuation techniques are used to investigate steady solutions in an idealized South Indian Ocean basin containing Madagascar. The aim is to study the role of inertia in a possible connection between two subgyres in the South Indian Ocean. By increasing inertial effects in the model, two different circulation regimes are found. In the weakly nonlinear regime, the subtropical gyre presents a recirculation cell in the southwestern basin, with two boundary currents flowing westward from the southern and northern tips of Madagascar toward Africa. In the highly nonlinear regime, the inertial recirculation of the subtropical gyre is found to the east of Madagascar, while the East Madagascar Current overshoots the island’s southern boundary and connects through a southwestward jet with the current off South Africa. 1. Introduction (2003) calculated 20 Sv southward. The recirculation, the EMC, and the flow from the Mozambique Channel form The presence of Madagascar in the South Indian the sources of the AC. A recent analysis of climatological Ocean presents unique characteristics to the subtropical data revealed a surface anticyclonic recirculation to the gyre circulation. This large-scale island blocks the wind- east of Madagascar that is composed of an eastward cur- driven circulation between 128 and 258S. As a conse- rent in the upper 300 m around 258S, the South Indian quence, the South Equatorial Current (SEC) bifurcates Ocean Countercurrent (SICC) and, between 108 and 208S, around 178S into the North Madagascar Current (NMC) the westward flow of the SEC (Palastanga et al. 2007). to the north and the East Madagascar Current (EMC) The signature of these currents can be seen in the to the south (Swallow et al. 1988). The fate of the EMC mean dynamic topography (Fig. 1) of the South Indian at its termination point is still not fully clear. It either Ocean as presented in Rio and Hernandez (2004). The undergoes an eastward retroflection with subsequent plot suggests that there may be two subgyres that are eddy shedding (Lutjeharms 1988) or it continues west- connected in the region around south Madagascar. ward as a free jet toward the African coast (Quartly While the southwestern recirculation might be related to and Srokosz 2004). The main subtropical gyre western bottom topography (Stramma and Lutjeharms 1997), the boundary current, the Agulhas Current (AC), originates dynamical connection between the subgyres has not been around 278S along the African coast. Hydrographic data addressed as far as we know. Quick inspection of the of the Indian Ocean subtropical gyre (integrated over the structure of the wind stress curl in the South Indian Ocean upper 1000 m) indicate a broad westward flow between suggests that the recirculation east of Madagascar is not 108 and 308S and a recirculation in the southwestern part caused by linear Sverdrup dynamics (Pedlosky et al. of the gyre (Stramma and Lutjeharms 1997). 1997), so inertia likely is important for this connection. Recent estimates of the Mozambique Channel trans- Nonlinear effects on the circulation around Madagascar port showed a highly variable flow, with an annual mean are expected to be important due to western bound- transportof14Sv(1Sv[ 106 m3 s21; Ridderinkhof and de ary current separation at the island tips and the signifi- Ruijter 2003), while for the EMC, Donohue and Toole cant local mesoscale eddy activity (e.g., Schouten et al. 2003). Hydrographic observations in the Mozambique Corresponding author address: H. A. Dijkstra, Princetonplein 5, Channel showed a flow dominated by the southward 3584 CC, Utrecht, Netherlands. propagation of anticyclonic eddies (de Ruijter et al. E-mail: dijkstra@phys.uu.nl 2002), with a frequency of 4 times per year related to the DOI: 10.1175/2008JPO3872.1 Ó 2009 American Meteorological Society Unauthenticated | Downloaded 10/01/21 01:27 PM UTC 466 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 39 FIG. 1. Mean dynamic topography of the South Indian Ocean computed from hydrographic data, surface drifters, and altimetry (Rio and Hernandez 2004). Units in m2 s2. It shows the anticyclonic subtropical gyre circulation at the surface between 128 and 408S, with two separate recirculations: one to the east of Madagascar and another east of South Africa. pinching off of anticyclonic eddies from the northern the Mozambique Channel. Therefore, we use a baro- Channel anticyclonic loop current (see, e.g., Fig. 1). tropic shallow-water model (described in section 2) for Eddies from the Mozambique Channel and from the South Indian Ocean, including Madagascar, and around southern Madagascar have been traced with determine steady solutions for different degrees of satellite altimetry migrating south (-westward) into the nonlinearity using the wind stress amplitude as the main AC system (Schouten et al. 2002; Quartly and Srokosz control parameter. A comparison between the model 2004; de Ruijter et al. 2004). Ultimately, they may in- flows in the steady nonlinear regime with key charac- fluence the variability of the AC retroflection and/or the teristics of the observed flow in this region is made in formation of Agulhas rings (Schouten et al. 2002; de section 3. Section 4 offers a summary and discussion of Ruijter et al. 2004), constituting an important link in the the results. global ocean circulation. Only a few modeling studies were devoted to inves- 2. The model tigate the large-scale South Indian Ocean circulation. We use the same barotropic shallow-water model as Using a primitive equation model between 208 and 508S, in Dijkstra and de Ruijter (2001a), but here we will ig- Matano et al. (1999) reproduced the AC and the gyre’s nore bottom topography. The model consists of the southwestern recirculation in a baroclinic experiment, shallow-water equations in spherical coordinates f, u, whereas in a barotropic experiment, the mean circula- and z; and it has a single layer with constant density r tion was constrained to the central Indian Ocean due to and equilibrium thickness H. The flow is driven at the the blocking effect of bottom topography. Woodberry f u surface by a wind stress field, tðf; uÞ 5 t0ðt ; t Þ; where et al. (1989) used a 1.5-layer model to simulate the 22 f u t0 is the amplitude (Nm ) and (t , t ) provides the monsoonal changes in the tropical Indian Ocean cur- spatial pattern. Lateral Laplacian friction, with lateral rents. Because their model has a southern boundary at friction coefficient AH, is the only dissipative mecha- 258S, the flows in the Mozambique Channel and of the nism in the model. The model domain covers the South EMC were not fully analyzed. Other studies have used Indian Ocean from 208 to 908E and from 418 to 58S, with eddy-resolving numerical models but focused on the realistic geometry. In the south, a zonal channel of influence of Madagascar eddies on the AC system constant depth is present that extends from the southern (Biastoch and Krauss 1999; Penven et al. 2006). wall to 368S. The channel prevents nonlinearities asso- The main motivation for this work is to investigate ciated with the return of the western boundary current whether time-independent inertial processes can gen- into the ocean’s interior to dominate the solution for erate a connection between the two subgyres while large amplitudes of the wind stress (Dijkstra and de remaining consistent with the mean transport through Ruijter 2001a). Unauthenticated | Downloaded 10/01/21 01:27 PM UTC FEBRUARY 2009 N O T E S A N D C O R R E S P O N D E N C E 467 The model equations are nondimensionalized using TABLE 1. Standard values of parameters used in the numerical calculations. In the value of a, we have taken a 5 1. typical scales r0, H, U, r0/U, and t0 for length, layer 0 depth, velocity, time, and wind stress amplitude, re- 24 21 6 2V51.46 3 10 (s ) r0 5 6.37 3 10 (m) 3 23 2 spectively, where r0 is the radius of the earth. The r0 5 1.0 3 10 (kg m ) H 5 5.0 3 10 (m) nondimensional equations are then U 5 1.0 3 1021 (m s21) G 5 9.8 (m s22) 3 2 21 21 AH 5 1.0 3 10 (m s ) t0 5 1.0 3 10 (Pa) E 5 1.0 3 1024 () E 5 1.6 3 1027 () ›u u ›u ›u 22 5 e 1 1 y À uy tan u À y sin u a 5 2.2 3 10 () F 5 5.9 3 10 () ›t cos u ›f ›u eF ›h u 2 sin u ›y tf 5 À 1 E =2u À À 1 a ; is varied by varying a from 1 to 5. Steady solutions cos u ›f cos2 u cos2 u ›f h 0 using the Ekman number E as a control parameter (i.e., ð1aÞ varying E one order of magnitude) are also computed. Standard values of all the parameters used in the model ›y u ›y ›y 2 e 1 1 y 1 u tan u 1 u sin u are listed in Table 1. ›t cos u ›f ›u The model has been forced by the momentum flux ›h y 2 sin u ›u tu 5 e 1 =2 1 1 a ; fields obtained from the National Centers for Environ- À F E v À 2 2 ›u cos u cos u ›f h mental Prediction (NCEP) reanalysis data for the pe- ð1bÞ riod 1948–2003 (Kalnay et al.

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