The Oceanography and the General physical and geographical characteristics of the underwater Ridges of the Southern Indian Ocean

Median ridges of the Indian Ocean form three branches diverging from the area of the Rodriguez Island north-westwards (the Northwest Indian Ocean Ridge), south-westwards (the Southwest Indian Ridge) and south-eastwards (South-Eastern Ridge). The first two branches represent typical middle oceanic ridges, and the last – a lower, wide middle oceanic ridge. The whole system of the ridges is generally called the Middle Indian Ocean Ridge.

Each branch of the Middle Indian Ocean Ridge divides into two parts: the north-western one – into Middle Aden Ridge and Arabian-Indian Ridge; the south-western one – into Southwest Indian and African-Antarctic Ridges and the south-eastern – into Mid-Indian Ridge and Australian-Antarctic Elevation. The border between two parts of each branch is the Owen Fracture Zone, Prince Edward Island and Amsterdam and Saint-Paul Islands (Kanaev et al., 1975). The principal scheme of the underwater ridges in the southern Indian Ocean shown in the following Figures.

The Southwest Indian Ocean Ridge extends from the Central Massive south-westwards conjugating with the Mid-Atlantic Ridge. In the area of the Prince Edward Islands this branch is divided into two ridges: the Southwest-Indian and African-Antarctic Ridges. The latter ends in the Atlantic Ocean in close proximity of Bouvet Island. Madagascar Ridge is located southwards from the Madagascar. Breaks and seamounts are characteristic of the Southwest Indian and Madagascar Ridges, being found even at the bottom of the basins.  The South-Eastern Middle Ocean Ridge is represented by the Mid-Indian Ridge and Australian-Antarctic Elevation. The greater part of the ridge belongs to the Australian-Antarctic Elevation. The border between them is the south-eastern foot of the mountain system Amsterdam.

The Southwest Indian Ridgeis a large underwater mountain system with highly cut terrain, extending 1200 miles from the area of the Rodriguez Island to the Prince Edward Islands; its width is 200-300 miles (Shcherbachev et al., 1989). The relative height of the base of the north-western part of the Ridge is equal to 2500-3000 m, and the south-western – 4000-3000 m over the bottom of the Agulhas and Mozambique Basins and totally 500-2000 m over the bottom of the narrow gully extending between the middle ridge and the foot of the Prince Edward and Crozet Islands. The depths vary in the range from 1000 to 4000 m in the ridge zone, reaching the maximum height (5300 m) on 37° S (Kanaev et al., 1975). The minimum depths over the peaks of seamounts, found between 32° and 42°S, are 102, 150, 250, 422, 690, 710 m, etc. The slopes of the mounts are steep, rocky, bottom sediments at the ridge are distributed patchy. Principal sediments are foraminifer silt. The thickness of the sediments increases along the ridge from north-eastwards to south-westwards in connection with the increased income of the sediments from the water due to the high biological productivity of the Antarctic waters. However, in the axis part of the ridge, including the bottom of the rift canyon, sediments generally absent.

The following image illustrates the bathymetry in the south-west corner of the SIOFA area.  The Madagascar Ridge extends south from the land mass, rising to a depth of 18 m on the Walters Shoal.  Many fisheries occur around on the underwater topographic seafloor features around Walters Shoal.Around 46 degree the Prince Edward Islands are to the west, Crozet Islands to the east and the Kerguelen Ridge further to the east.  SIODFA vessels ony fish in high seas waters so these islands are never seen.

The structure of the active layer of waters over the ridge is represented by the transitional zone of the interaction of two surface water masses: southern tropical and subtropical ones. The southern tropical water mass lays northwards from the parallel 30-32°S from the surface down to 500-600 m: its distinctive fluctuations of the temperature make 16-27°С, and those of salinity are 34.8-35.3‰. The surface southern subtropical water mass is located between the parallel 30°S and the Subantarctic front (located at 45-50°S), it is characterized by the temperature 17-24°С and salinity 35.4-35.8‰. This water mass has also subsurface structure zone with temperatures 12-17°С, salinity 35.0-35.5‰, separated from the surface water with the layer of the increased vertical gradients of the oceanographic parameters. These three water structures in different seasons have from 3 to 8 modifications when mixing with each other and intermediary Antarctic water mass. A particularly great variety of modifications of surface, subsurface and intermediary mater masses are distinguished at seamounts of the Western Indian Ridge (seamounts “150”, “251”, “415” and “102”) due to the specific oceanographic conditions (upwelling, orographic gyres), generated in the zone of these seamounts. The intermediary Subantarctic water mass in the area of the ridge positioning in the layer deeper than 800 m. Its distinctive temperature is 3-5°С, salinity 34.3-34.6‰. In the area of seamounts due to the waters moving up, this water mass may be found in the horizon 500-600 m (Burlenko, 2001).

The Madagascar Ridgerepresents the massive elevation of the bottom, extending between micro-continent of Madagascar Island and the Western Indian Ridge for almost 700 miles; the crest of the ridge is wide, with depths from 1000 to 2500 m (at seamounts up to 567 m); the minimum depth falls on the Walters shoals (15 m). The shoals is a cone with the flat top, its foundation is outlined with isobath of 800 m. The flat top is covered by coral reefs, having the cut relief, especially at the outer edge. The slopes of the shoal are steep; the angle of the incline is 6-12°. The sediments are represented with foraminifer mud, enriched with sand in the crest (Kanaev et al., 1975). The tops of other mounts rising over the ridge are located in depth from 84 down to 567 m and deeper. In the southern part of the ridge between 31° and 35°S depths 900-1100 m are predominant. Down to the depth 1500 m the ridge has mainly the flattened relief with the predominance of sloping-wave and stepped valleys. On the surface of these valleys hollows with depth down to 20 m are occur, as well as hills and furrows. At the eastern slope of the ridge there are several troughs and crest with sharp peaked mounts of up to 250 m high and more. The surface of the ridge down to depth 1400-1900 m is covered with sand, silty sand, deeper – with foraminifer silt (Kanaev et al., 1975, Shcherbachev et al., 1989).

The structure of waters over the ridge is represented by the surface southern subtropical water mass and its modifications down to depth 300-350 m, and intermediary Subantarctic water mass down to depth 1700 m, layer 1200-1300 m (their hydrological parameters are described above).

Mid-Indian Ridgediffers from the other middle ridges of the Indian Ocean: it is lower, slightly cut, and has no deep rift valley. The length of the ridge makes 1200 miles, the width – 400-500 miles. The highest mounts are Amsterdam and Saint-Paul – near the south-eastern extremity of the ridge they form the volcanic islands with the same names. The tops of the highest underwater mounts in the ridge are located in depth 2000-2500 m. Sediments are represented with foraminifer mud (Kanaev et al., 1975, Shcherbachev et al., 1989).

The structure of water masses in the Mid-Indian Ridge is similar to that of the Western Indian Ridge as ridges are located in the same climate zone.

Climate of the Southwest Indian, Madagascar and Mid-Indian Ridges depends upon their location in two climate zones: tropical and subtropical. In the tropical zone during austral winter air masses of the temperate latitudes are predominant, in summer – tropical ones. Precipitation exceeds slightly the evaporation. The distinctive features of the weather and climate reveal themselves at interaction of the Southern Indian Ocean subtropical zone of high pressure, which is situated between 25° and 35°S, intra-tropical zone of convergence, located northwards from 10°S, and southern polar front, which is located between 44° and 46°S. In austral winter the Southern Indian Ocean subtropical zone of high pressure extends along the parallel 30°S with maximum pressure 1024 gPa at the eastern border of the area, and in summer it shifts to 35-36°S with simultaneous reduction in pressure down to 1016 gPa. The air temperature reduces from north to south. The minimum temperatures are recorded in July-August and made on the northern border of the area 18-26°С, on the southern border 4-18°С.

The highest warming of the air takes place in February, when temperature increases up to 30°С in north, and up to 22°С in south. The daily change of temperature is not large and makes 0.5-1.0°С. Short-term air temperature decrease is recorded during the cyclones of pressure trough moving over the area. Since July till September, anticyclone type of the weather is predominant over the greater part of the area. Since October till June northwards to the parallel 35°S slightly cloudy weather is observed, southwards – cloudy with cloudiness of 6-8 points. As a whole for a year 1135 mm of precipitation falls, their minimum quantity (10-20 mm) falls on July, and maximum (140 mm) – on February-March. Fogs are often in winter. By the nature of the wind regime the area of ridges are provisionally divided into three parts: northern (20-30°S), central (30-38°S) and southern (38-42°S).

In the northern part of the area winds of the eastern direction are predominant all the year round with speed 6-10 m/s and recurrence – 46-52% in winter period, and 32% – in summer.  In the central part the wind direction is not stable. Since November till March winds of all directions with equal recurrence are feasible. Since June till October the northern, north-western, southern and south-western winds are observed most of all, their recurrence is 18-20%.  In the southern part winds of western directions (20-30%) are predominant all the year round; also northern winds are often (25%).

The mean height of waves is about 1.5 m. In June-August southwards 30° they can reach 15 m (recurrence 1%), the rest time of the year the sea roughness does not exceed 10 m (Anon., 1986).

Water circulation over ridges makes extensive rotation eastwards and southwards from Madagascar Island, where shift of warm waters from north is made by the southern branch of the South Equatorial Current, Mozambique Current and its continuation as Cape Agulhas current. The greater part of the latter one is turns to south and east, forming the Agulhas Retroflection. One part of this current comes back to the system of the Cape Agulhas current, the other moves to east. This chain of the southern subtropical rotation called Southern Indian Ocean Current, moves water to east with speed 0.6-1.0 knots. As far as moving eastwards streams divide from the South-Indian current, which make whirlwind flows at the greater part of the area. In the southern part of the area together with the Agulhas Retroflection and South Indian Ocean Current are confluent with the Antarctic Circumpolar Current, moving with the speed of 0.4-0.8 knots in November- March and in May-September up to 0.5-1.3 knots. As a result of three currents confluence the Subantarctic zone which extends from the surface down to depths 80-1200 m is formed. Under the impact of the seasonal and synoptic variability of currents the frontal zone is subject to great latitudinal and longitudinal fluctuations. In general this scheme of circulation preserves down to 200 m (Kanaev et al., 1975, Shcherbachev et al., 1989). Over the seamounts of the ridges local gyres are formed, being subject to the changes under the impact of the synoptic changes in water movement. The area of the intense upwelling is greater in May-September than in November-March. As a result of this process the enrichment of the active layer with nutrients and oxygen takes place, which explains increased biological productivity of the seamounts.

The Ninety East Indian Ocean Ridge is the largest elevation of volcanic origin () of the Indian Ocean bed extending along of the meridian 90°E from the Bengal Bay almost to the western extremity of the Australian-Antarctic Elevation. Its length is more 4700 km. The ridge is narrow, approximately 120 miles width, consisting of a large amount of elevations as seamounts at one base. The least depths over seamounts discovered in YugNIRO expeditions in the southern part of the ridge, is about – 400, 540 and 620 m. At the crest of the ridge foraminifer silt are predominant (Kanaev et al., 1975).

Water masses over the ridge northwards from the equator are represented by the surface northern tropical water mass (0-400 m), Bengal intermediary (400-1500 m) and north-Indian deep water masses (1800-3500 m). Southwards to the equator, up to 12°S the transitional zone of the interaction of the southern tropical and equatorial surface waters, intermediary waters of the Banda sea and Subantarctic waters (down to the depth 1600 m). From 14°S and to its southern tip, the Eastern Indian Ridge is occupied by the surface tropical and subtropical (the interaction zone is about 23-25°S.) – down to the depth 350-400 m. Tropical surface waters have higher temperature (in summer – 22-26°С, in winter – 20-22°С) and lower salinity (35.0-35.4‰) than subtropical surface waters. The latter occupy the area from 25° S to 38-39° S and have temperature characteristics 20-22°С in summer and 15-10°С in winter with salinity 35.2-35.9‰, and are distinguished by the presence of the intermediary salinity maximum (35.9‰), and oxygen minimum within 25-200 m depth. The intermediary structure is separated by thermocline from the surface one at the depths 300-700 m. The core of the intermediary Subantarctic waters (down to 1700 m) distinguished by the salinity minimum (34.4‰) and temperature 3-8°С, is located in depths 200-1000 m in the southern part and 700-800 – in the northern. The increased salinity and decreased depth of the location of the core of the Subantarctic intermediary waters are caused by underflowing of the Bandа Sea waters, having the same temperature, higher salinity (35.6‰). The deep structure is represented with the central Indian deep water mass (from 2000 down to 4000 m) (Anon., 1982a, 1982b, 1982c).

The Broken Ridgeis located approximately at 600 miles from the underwater extremity of the Western Australia and morphologically related with Australian continent. Broken Ridge is bordering with the Eastern Indian Ridge at 30°S. This latitudinal massive mountain system is about 800 miles long and up to 300 miles wide, elevating from the ocean bed at the highest sites for 4000 m, sharply asymmetrically from south to north. The least known depths over the mounts are 600-800 m. The seamounts with lesser depth are not excluded to be present, where as a result of the topogenic effect like at the Southwest Indian Ridge seamounts, formation of fish commercial aggregations is possible (Anon, 1979, Anon. 1982a). Sediments of the crest and the northern slope are represented with the foraminifer silt up to 800 m wide, on the southern slope sediments are developed on the gentle slopes, and on the steep stairs bedrock is predominant (Kanaev et al., 1975, Shcherbachev et al., 1989).

The water structure on the ridge is generated by the southern subtropical water mass (0-300 m), intermediary Subantarctic (500-1700 m), the Banda Sea (1000-2000 m) and deep central Indian (2000-4000 m) water masses. Water characteristics were described above.

The weather conditions in the south-eastern Indian Ocean are determined by the general type of the atmospheric circulation (depending on the season), location and capacity of the South Indian maximum of the atmospheric pressure. The geographical location of its center is found between coordinates 25°-30°S, 70-80°E. Southwards from 35°S highly gradient baric field is located with the pressure trough in the area of 50-60°E, often transforming to the local cyclones with pressure in the center down to 1000 Mb. The shift of the center of the South-Indian maximum, breaks-through of the low pressure beyond 30°S with formation of the local cyclones stipulate the unstable weather and changeability of the wind regime in the area. The winds of the eastern directions are predominant.

In the area of ridges the meridianal type of the atmospheric circulation reigns during winter (June-August). The stratocumulus cloudiness about 6-8 points is predominant, with low precipitation. Air temperature decreasing by 5-6°С is recorded during atmosphere fronts passing. The air temperature varies from 22°С in north to 10°С in south. The unstableness of the baric field in this season results in that direction of swell does not always correspond to wind direction. Waves of southern directions are predominant.

The wind regime in the area of ridges differs in summer (December-February). The winds of eastern directions are predominant at southern seamounts of the Ninety East Ridge located along the periphery of the centre with high pressure (occurrence 34%), and at seamounts of the Broken Ridge, where weather conditions in this season forming under the impact of the central part of the Southern Indian maximum – winds of southern directions (occurrence 22%). The range of air temperature vary greatly from the north to the south: from 27°С at 20°S and to 12°С at 40° S. The mean extent of the sky coverage with clouds reduces from 6-7 points southwards from 33°S to 3-4 points at 18-24° S. Occurrence of days with precipitation is less than 5%. Visibility 8-9 points (Anon, 1979, Anon, 1982a, Anon, 1982b, Anon, 1982c).

The water circulation in the area is determined by the South Indian Ocean Current which is the southern periphery of the pertinent ant-cyclonic subtropical water gyre. Reaching the western coast of the Australia southwards of 19°S, it turns northwards, giving the beginning for the Western Australian current, that in its instance, shifts cold Subantarctic waters into the area of the southern tropics and closes up in north the anti-cyclonic gyre (Sukhovej, Baskaran, 1995).

The oceanographic conditions of the south-eastern parts of the Eastern-Indian and Western-Australian Ridges are characterized by the active gyre-formation and the availability of the quasi-stationary frontal zones in the area of seabed elevations. The zones of the vertical lifting shifts and falling of waters are peculiar for the area between 25° and 31°S. The general declining of the subtropical waters during their movement northwards is destroyed by the raising movements in the zone between 27 and 29°S (Anon, 1982a).