Internal solitary waves on the Saya de Malha bank of the Mascarene Plateau: SAR observations and interpretation

Highlights

• Saya de Malha, the largest underwater bank in the world, is a hot-spot for Internal Solitary Waves (ISWs).

• Two-dimensional horizontal structure and generation sites are revealed with Synthetic Aperture Radar (SAR).

• SAR/SAR image synergy is used for studying internal wave generation.

• Shallow bathymetry drives ISW breaking processes and local chlorophyll production.

Abstract

Energetic Internal Solitary Waves (ISWs) were recently discovered radiating from the central region of the Mascarene Plateau in the south-western Indian Ocean (da Silva et al., 2011). SAR imagery revealed the two-dimensional structure of the waves which propagated for several hundred kilometres in deep water both to the east and west of a sill, located near 12.5°S, 61°E between the Saya de Malha and Nazareth banks. These waves were presumed to originate from the disintegration of a large lee wave formed on the western side of the sill at the time of maximum barotropic flow to the west. In the present paper we focus instead on ISWs propagating in the shallow water above the Saya da Malha (SM) bank (to the north of the sill), rather than on those propagating in deep water (here denominated as type-I or -II waves if propagating to the west or east respectively). Analysis of an extended SAR image dataset reveals strong sea surface signatures of complex patterns of ISWs propagating over the SM bank arising from different sources. We identify three distinct types of waves, and propose suitable generation mechanisms for them using synergy from different remotely sensed datasets, together with analyses of linear phase speeds (resulting from local stratification and bathymetry). In particular, we find a family of ISWs (termed here A-type waves) which results from the disintegration of a lee wave which forms on the western slopes of SM. We also identify two further wave trains (B- and C-type waves) which we suggest result from refraction of the deep water type-I and -II waves onto the SM bank. Therefore, both B- and C-type waves can be considered to result from the same generation source as the type-I and -II waves. Finally, we consider the implications of the ISWs for mixing and biological production over the SM bank, and provide direct evidence, from ocean colour satellite images, of enhanced surface chlorophyll over a shallow topographic feature on the bank, which is consistent with the breaking of the ISWs.

Introduction

Satellite imagery (mainly from Synthetic Aperture Radar, SAR) has been playing a key role in revealing Internal Wave (IW) hotspots in the world's oceans, and in documenting their main characteristics (see e.g. New and da Silva, 2002, Jackson, 2004, da Silva and Helfrich, 2008, Jackson et al., 2012, Magalhaes and da Silva, 2012). Indeed, the two-dimensional horizontal wavecrest coherence of IWs can be easily revealed in SAR images as bright and dark patterns, which essentially result from the hydrodynamic modulation of Bragg waves by surface current gradients (Alpers, 1985), surface wave breaking (Kudryavtsev et al., 2005) and/or wave damping due to surface films (da Silva et al., 1998, Ermakov et al., 1998). In this way satellite SAR systems are able to image internal waves over wide areas, and are particularly valuable for studying remote regions where in situ data are difficult to obtain or even non-existent. In particular, the wide field of view of the WS (Wide Swath) acquisitions from Envisat (spanning 400×400 km²) has been extensively used to detect and reveal the main characteristics of many different IW hotspot regions. These include the Bay of Biscay (New and da Silva, 2002, Azevedo et al., 2006), the South China Sea (Zhao et al., 2004, Jackson, 2009), the west Iberian Coast (da Silva et al., 2007, Magalhaes and da Silva, 2012), the Red Sea (da Silva et al., 2012), the Mozambique Channel (da Silva et al., 2009), and the Mascarene Ridge (da Silva et al., 2011).

These efforts represent a major contribution to assessing the key characteristics of the global IW field (e.g. their spatial and temporal variability, and their source regions and generation mechanisms), which in turn contributes to our understanding of the redistribution of barotropic tidal energy available for ocean mixing (Alford, 2003). In fact, such studies have revealed the existence of large barotropic tidal sinks, where tidal energy is converted to Internal Tides (ITs) and subsequently to shorter scale Internal Solitary Waves (ISWs) through nonlinear disintegration (see also Gerkema, 1996). These internal oscillations can subsequently dissipate locally, but may also propagate away from the source region for considerable distances, giving rise to the possibility of interfering patterns from local and remote generation (Kelly and Nash, 2010, Nash et al., 2012). These ISWs and ITs may also significantly affect across-shelf transport and mixing (Shroyer et al., 2010).

Recent attention has been devoted to the study of the Mascarene Plateau, one of the major tidal sinks in the Indian Ocean (New et al., 2007, da Silva et al., 2011). The Mascarene Plateau (or Ridge) is located to the northeast of Madagascar in the western portion of the southern Indian Ocean (Fig. 1), and consists of a series of ridges separated by shallow banks. These banks extend between the Seychelles in the north (near 4°S, 56°E) and the Mauritius in the south (near 20°S, 57°E). Two large shallow banks stand out in this region, namely the Saya de Malha (SM) bank and the Nazareth bank. The former is in fact the largest underwater bank in the world's ocean. Between these submarine banks, a deep passage (of the order of 70 km width) exists between 12°S and 13°S, which is oriented in the NNE–SSW direction (New et al., 2007). The passage consists of a sill that is some 400 m deep, and a narrow deeper channel centred approximately at 12.5°S and 60.9°E that is 1100 m deep (see Fig. 1). We also note that New et al. (2007), in a ship survey of the region, found significant differences in the bathymetric description of this region when comparing the Smith and Sandwell (1997) global bathymetry with the ship's bottom topographic measurements. This led da Silva et al. (2011) to propose a new bathymetric chart for the sill in question and the deep channel between the two banks (see their Fig. 6 and the present Fig. 11a), which had an overall shape defined from the Smith and Sandwell (1997) dataset, but with depths matching those observed along the ship's track in New et al. (2007).

Internal waves in this region were first studied by Sabinin et al. (1992) from a survey undertaken by the R/V Akademic Nikolay Andreyev (17–19 March, 1987). The authors reported several ISW packets on the eastern side of the Plateau propagating towards the east and also suggested that these might be generated by a large lee wave forming to the west of the sill (between the Saya de Malha and Nazareth banks) at the time of maximum westward tidal flow (see also Konyaev et al. (1995)). Furthermore, Sabinin et al. (1992) also recorded an instance of a large (45 m) ISW packet above the eastern side of the SM bank near 12.33°S, 61.33°E.

However, neither of these ship-based studies was able to document the full two dimensional spatial structure of the ISW field in this region of the world's ocean. Motivated by the observational campaign undertaken by the R.R.S. Charles Darwin (cruise 141, New, 2003, New et al., 2005, New et al., 2007) between 1 June and 11 July 2002, da Silva et al. (2011) were able to utilise a large collection of SAR imagery to undertake this task. In their initial study, da Silva et al. (2011) revealed for the first time that large-scale ISWs (amongst the longest yet recorded anywhere in the world's oceans) radiate into deep water both to the west and to the east of a central sill between the SM and Nazareth banks—hereafter referred to as type-I and -II waves, respectively (see Fig. 1 for locations). Using a large SAR dataset and composite maps, they concluded that the waves appeared in tidally (semi-diurnal) generated packets and appeared to be generated from the western side of the sill at the predicted time of maximum tidal flow to the west. The linear splitting of a large lee wave (forming near X′ in the present Fig. 1) was therefore proposed as the most plausible generation mechanism. The existence of such a lee wave was then confirmed by the in situ observations of current and temperature structures recorded by the R.R.S. Charles Darwin. Furthermore, the local in situ stratification (based on CTD stations) and ADCP current measurements from the survey were used to infer the linear modal phase speeds of the internal waves, which agreed with the propagation speeds of the waves derived from the SAR imagery. We note here that the tidal forcing in this study region is indeed primarily modulated by a strong semi-diurnal signal (unlike other low latitude regions where diurnal tides can be significant, e.g. Ramp et al. (2004)). Fig. 2, for instance, illustrates that the tidal flow at position X′ (60.610°E, 12.822°S, see Fig. 1) computed with the M₂, S₂ and K₁ tidal constituents (used hereafter as representative of the tidal forcing) can be used as a good fit to the complete regional solution for the OTIS (Oregon Tidal Inversion Software) model (Egbert and Erofeeva, 2002). Again, this is consistent with the information drawn from the satellite imagery, where the inter-packet separations (and hence average propagation speeds) observed in this study region are consistent with ITs of semi-diurnal period.

Further analysis of an extended version of the SAR dataset used by da Silva et al. (2011), as well as new satellite acquisitions requested in tandem mode (using Envisat and TerraSAR-X), has now also revealed different sets of ISWs that have not been discussed before. These ISWs propagate over the SM region and appear to originate from multiple sources on the western and southern slopes of the bank. The wave trains from these different sources form intricate patterns as they travel across the bank. In the present study, we build on the description made by da Silva et al. (2011) of the type-I and -II waves propagating in deep water, by focussing instead on a detailed description of the structure, propagation and generation of the ISWs propagating in the shallower water over the SM bank.

The remainder of this paper is organised as follows: Section 2 investigates the SAR satellite imagery showing typical sea surface signatures of ISWs propagating over the SM bank. In Section 3, the corresponding generation mechanisms for these waves are addressed using synergy from different remotely sensed datasets, together with linear analyses of phase speeds (resulting primarily from local stratification and bathymetry derived from the R.R.S. Charles Darwin survey). Finally, Section 4 summarises our results, provides further evidence for the generation mechanisms proposed, and investigates the implications of the ISWs for mixing and biological production over the SM bank.