Southwest Monsoon

Indian Ocean Circulation

Figure 5.12 Surface currents in the northern Indian Ocean, as deduced from ships' drift data. The North-East Monsoon is most fully established from January to March ((a),(b)), and the South-West Monsoon is most fully established during July-September ((d),(e)). The thicknesses of the lines indicate the relative intensities of the flows. For example, current speeds in the Equatorial Jet ((c),(f)) may reach 1.0-1.3ms"1, but are mostly 0.3-0.7ms~1. The South Equatorial Current (SEC) is spread over the area between the two flow lines. The westward flow along the Equator within the South-West Monsoon Current, shown in (d), Is discussed in Section 5.3.1. EAC is the East Arabian Current.

Figure 5.12 Surface currents in the northern Indian Ocean, as deduced from ships' drift data. The North-East Monsoon is most fully established from January to March ((a),(b)), and the South-West Monsoon is most fully established during July-September ((d),(e)). The thicknesses of the lines indicate the relative intensities of the flows. For example, current speeds in the Equatorial Jet ((c),(f)) may reach 1.0-1.3ms"1, but are mostly 0.3-0.7ms~1. The South Equatorial Current (SEC) is spread over the area between the two flow lines. The westward flow along the Equator within the South-West Monsoon Current, shown in (d), Is discussed in Section 5.3.1. EAC is the East Arabian Current.

The mosi spectacular seasonal change is the reversal of the Somali Current, oi l'cast Africa This reversal has been known about for centuries In the ninth century. Ibn Khorda/heh noted (in sailing instructions for masters of dhows carrying slaves from east Africa to Oman) thai "the sea flows during (he summer months to the north-east', and 'during the w inter months to the south-west*. So. during the North-East Monsoon the Somali Current flows to the south west, but for the rest of the year it flows to the north-east. During the South-We si Monsoon n becomes a major western boundary current (Figure 5.12<dt.le)> - its surface velocity may reach 3.7 ms"1. and the volume transport in the upper 200 m of its flow is about 60 x 1()f'mV.

The low-level atmospheric jet mentioned in the previous Section (Figure 5.1 b is thought to play an important role in the generation of the intense coastal upwelling ihut occurs off Somalia during the South-West Monsoon. At this time of year, upwelling also occurs off Arabia, both along the coast where the north-easterly current diverges from it. and offshore in response to local cyclonic w inds. The Somali arid Arabian regions of upwelling are the most vigorous in (he Indian Ocean (Figure 5.131.

Figure 5 13 Satellite images stiowmg chlorophyll concentrations in surface waters of the northern Indian Ocean in (a) February and |b) September. Note the high chlorophyll concentrations (orange/yellow) alt Somalia and Oman in SeplernOer, resulting trom upwelling associated with Ihe South-West Monsoon. (The high chlorophyll concentrations close to the coasts pf India and Bangladesh are a result of nutfient-rich rivet outflows.)

Figure 5 13 Satellite images stiowmg chlorophyll concentrations in surface waters of the northern Indian Ocean in (a) February and |b) September. Note the high chlorophyll concentrations (orange/yellow) alt Somalia and Oman in SeplernOer, resulting trom upwelling associated with Ihe South-West Monsoon. (The high chlorophyll concentrations close to the coasts pf India and Bangladesh are a result of nutfient-rich rivet outflows.)

Would you expect open ocean upwelling to occur in ihe region of ihe Equator. as ii does in the Pacific and Atlantic 1

No. because these regions nfupwelling occur us u resuli of the South-East Trade Winds hlowing across the Equator and causing a surface divergence just to the south of il (Figure 5.1 taf). Figure 2.3 shows that these wind conditions are not typical of either season of the year,

Would you expect there to be an Equatorial Undercurrent in the Indian Ocean?

Only for part of the year, not as a permanent feature. The existence of an Undercurrent depends on there being wind stress towards the west, to drive a westward surface current and hence cause a sea-surface slope up to the west; this would provide an eastward horizontal pressure gradient force to drive an eastward current below the wind-driven layer. In the equatorial Indian Ocean, the direction of the wind varies seasonally, but best fulfils the necessary conditions for an Undercurrent during the first few months of the year. The first direct observations of the Undercurrent were made during the International Indian Ocean Expedition (1962-65) at the end of the NorthEast Monsoon, when it was seen to be an ocean-wide feature; but this may not have been typical. In general, in the Indian Ocean the Undercurrent seems to be a stronger and more persistent flow in the western part of the ocean than in the central or eastern parts.

The changing pattern of surface currents in Figure 5.12 was deduced from ships' drift data collected by the UK Meteorological Office from log books of merchant vessels. Such information can provide reasonably accurate estimates of surface flow velocities in the Indian Ocean because, for most of the year, currents here are generally stronger than those in the Pacific and Atlantic Oceans.

What intense surface current feature appears between the monsoons (Figure 5.12(c) and il l)?

It is an eastward Equatorial Jet, which is driven by westerly winds over the central equatorial ocean. Although the jet is detected in ships' drift data between April and June, and in October/November, it is possible that it is in fact a brief event, lasting perhaps only a month at a time.

Generally speaking, how would you expect the direction of the slopes of the sea-surf ace and of the thermocline to change in response to the changing directions of the vund and surface current along the Equator?

When winds and currents along the Equator become westward, the sea-surface will (eventually) slope up to the west, and the thermocline slope down to the west; when winds and currents along the Equator are eastward, the sea-surface will (eventually) slope up to the east, and the thermocline slope down to the east.

The complexity of the surface circulation of the Indian Ocean, which contrasts with the relative simplicity of the gyral systems of the Pacific and Atlantic Oceans, is a result of the frequency and rapidity with which the overlying wind system changes. Wind speeds and directions change so fast that there is not always time for the upper ocean to adjust so that it is in equilibrium with the wind - as a result, during the relatively short inter-monsoon period, while the prevailing wind directions are changing dramatically, there is an increase in the large number of eddies, both cyclonic and anticyclonic and ranging in size from 100 to 1000 km across. The ocean's response time - or. looked at another way, its 'memory' - is many times longer than that of the atmosphere, and one of the most interesting questions that can be asked about the ocean circulation is: How is it possible for the ocean to react as fast as it does?

We will address this question in the next Section, but we cannot leave the Indian Ocean without considering a western boundary current which is even more powerful than the Somali Current during the South-West Monsoon. This is the Agulhas Current, second only to the Gulf Stream in its volume transport. Flowing polewards along the coast of Africa from south of20°S and the island of Madagascar (the Malagasy Republic!, (he Agulhas Current seems mostly to derive not from the Mozambique Current (as might be expected), but from the East Madagascar Current, which is itself quite a strong western boundary current (both are labelled on Figure 5 12(c)). Unlike other western boundary currents, the Agulhas Current shows little seasonal variation, averaging speeds of 1,6 ms'1 throughout the year, and exceeding 2.5 in s1 in mosl months. Near 3PS, transport in the current is estimated to be 70 x lO^m-V. but it increases by approximately 6 x lO^m's"1 for every 100km travelled, so by the time it approaches the shallow Agulhas Bank at about 35°S its volume transport has grown to between x lO^m's-1 and 135 x lO^m-'s-1.

Off the tip of South Africa the Agulhas Current loops back on itself, forming a feature known as (he "Agjlhas retroflexion'. As shown in Figure 5.14, the retroflexion is a source of 'ring-like' eddies (cf. Section 4.3.6), which form when the westerly part of the loop "pinches off, Mosl of the eddies, which are rings of Agulhas Current water encircling Indian Ocean Water, are injected into the Benguela Currenr and are carried away northwestwards into the Atlantic. They are highly energetic and are thought to have life-spans of many years.

Figure 5.14 Map showing the changing patti of the Agulhas Current and its current loop or retroflexion off South Africa. The blue line in (act corresponds to different positions of the temperature front on the coastal side of the (warm) current over a 12-month period during 1984-85. The tings are eddies which form from the loop when it extends westwards towards the Atlantic: when an eddy pinches off. the loop retreats lo its mosl easterly position, and the cycle repeats,

Figure 5.14 Map showing the changing patti of the Agulhas Current and its current loop or retroflexion off South Africa. The blue line in (act corresponds to different positions of the temperature front on the coastal side of the (warm) current over a 12-month period during 1984-85. The tings are eddies which form from the loop when it extends westwards towards the Atlantic: when an eddy pinches off. the loop retreats lo its mosl easterly position, and the cycle repeats,

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