Thermocline Circulation and Ventilation in the Indian Ocean Derived from Water Mass Analysis
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Deep-Sea Re.~eurrh I. V,.)I .-~). No. 1. pp I.!-50, lta,J3 0(~7-,(~37/t)3 S6.IIO .*. O.(t() Pnnted m Great Brttaln. ~ tt~ " Pergamon Pres6 Ltd Thermocline circulation and ventilation in the Indian Ocean derived from water mass analysis Y. You** and M. TOMCZAKt.~ (Received 23 July 1991; in revised[otto 27 January 1992; accepted 18 February 1992) Abstract--A mixing model, which combines cluster analysis with optimum muitiparameter (OMP) analysis, is used to determine the spreading and mixing of water masses in the thermocline of the Indian Ocean. focusing on the ventilation process for the thermocline in the northern hemisphere. Mixing ratios are quantified and plotted on five isopycnal surfaces covedng the depth range 150- 800 m. two meridiona[ sections along 60°E and 90°E, and one zonal section along I0*S. Three water masses are identified in the thermocline by cluster analysis. Indian Central Water (ICW) is subducted at the Subtropical Front in the southern Indian Ocean and advected with the southern subtropical gyre. Australasian Mediterranean Water (AAMW) enters from the Indone- sian seas as the result of throughflow from the Pacific Ocean. Red Sea Water (RSW) combines with water from the Persian Gulf to provide a minor source. A fourth water mass identified by cluster analysis occurs in the Bay of Bengal; it is labelled North Indian Central Water (NICW) and interpreted :is aged ICW. Cluster analysis did not produce evidence for the existence of Equatorial Water. a water mass often referred to in the literature. Mixing ratios attd pathways of the thermoelinc water masses are established using OMP analysis. The input of RSW is insufficient to renew the thcrmoclinc witters of the northern Indian Ocean, which therefore has to be ventilated by ;tdvection front the south. The jet-like inflow of AAMW produces one of the strongest frontal systems of the world (~ce;.ln°s thcrmoclinc, suppressing mcridional motion across 10--15°S cast of 51)°E. This leaves the western boundary currents as the only rcgitm for advectivc transfer of thcrmoclinc water between the hemispheres. ICW is shown to advect along this path on the isopyenal surface ~ru = 26.7 (depth range 3(10-4(10 m). Above and below this surface ICW movement into the northern hemisphere is accompanied by significant diapycnal mixing. The annual mean distribution of ICW shows that it ages rapidly as it crosses the equator. This is interpreted as the result of seasonally varying advcction, producing very little mean net transport across the equator. As a consequence. ICW in the northern hemisphere is extremely low in oxygen and high in nutrients. AAMW can be traced to 70"E, but is mixed into the ICW background and no longer recognizable as AAMW by the time it leaves the Indian Ocean with Agulhas Current eddies. It is argued that the so-called Equatorial Water is the result of the mixing process, i.e, a mixture of two well-defined water masses that should not be considered a water mass in its own right. I. INTRODUCTION THIS paper studies the paths of water masses in the Indian Ocean north of 40°S between about 150 and 800 m depth (the permanent thermocline). SVERDRUP et al. (1942) identified "Marine Studies Center, The Univcrsity of Sydney. NSW 2006, Australia. *Ocean Sciences Institute, The University of Sydney, NSW 2006, Australia. *Present address: School of Earth Sciences, The Flindcrs University of South Australia, G.P.O. Box 21(10. Adelaide. S.A. 5(101, Australia. 13 14 Y. You and M. TOmCZAK three water masses in this depth range and discussed their formation mechanisms. According to their description. Red Sea Water (RSW) forms in the mediterranean basin of the Red Sea, Indian Central Water (ICW) is subducted in the Subtropical Convergence of the southern hemisphere, and Indian Equatorial Water (IEW) is formed in the western equatorial region through some unspecified mechanism. In temperature-salinity (T-S) diagrams ICW and IEW were displayed as nearly linear relationships over the entire thermocline range between these two conservative properties. MAMAYEV(1975) added two more water masses to the Indian Ocean north of 40°S: Bengal Bay Water, which is only found in the surface layer, and Timor Sea Water--in this paper referred to as Australasian Mediterranean Water (AAMW)---which originates from the deep basins of the Indonesian archipelago (the Australasian Mediterranean Sea). With respect to the processes of formation, Mamayev put strong emphasis on vertical mixing and did not consider subduction and spreading on isopycnal surfaces a possibility, so all his water mass definitions are based on single T-S points. A recent review by EMERY and MEINCKE (1986) returns to the ideas expressed in SVERDRUPet al. (1942) and presents the water masses of the Indian Ocean thermocline as T-S relationships rather than points. It also includes Arabian Sea Water [water influenced by outflow of Persian Gulf Water (PGW) through the Strait of Oman] as another water mass distinct from RSW, thus giving a total of six water masses for the upper kilometre of the northern Indian Ocean. Modern methods of water mass analysis enable us to expand the pioneering work of SVERDRUPet al. (1942) beyond the definition of water mass properties and derive the water mass distribution in space through objective methods. In the Indian Ocean this is particularly true, since the International Indian Ocean Expedition (IIOE) of 1960-1965, documented in the atlas by Wvrr~l (1971), created an invaluable database, allowing the application of inverse modelling techniques. This paper uses the IlOE dataset and additional data collected at the World Oceanographic Data Centre for an application of optimum multiparamctcr (OMP) analysis. The mcthod, first proposcd by TOMCZA~ (198 i) and further developed by MACKAS et al. (1987) and TOmCZAR and LARGE (1989), USCS nutrients and dissolved oxygen along with temperature and salinity to derive a distribution of water masses that matches the observed distribution of properties in an objectively definable best tit. The aim of the present study is to establish pathways for water masses in the permanent thermocline and contribute to our understanding of the upper circulation and thermocline ventilation in an ocean that has received surprisingly little attention, despite improve- ments in data density and its undeniably important role in climate variability. One example of the outstanding problems awaiting explanation is the origin of the so-called Indian Equatorial Water. SvErt)ruP et al. (1942) introduced this water mass but did not offer a formation mechanism, stating instead that IEW is not as well-defined as Indian Central Water. SHCIIEratNIN (1969) distinguished three main water masses in the equatorial area of the Indian Ocean; the central Indian; the equatorial transitional; and the north Indian; using maximum horizontal gradients of oceanographic parameters to define water mass boundaries, and again without reference to processes of formation. Using temperature- salinity frequency analysis, S~ARmA (1976) argued that IEW is a mixture of water masses from the northern and southern hemispheres and from the Pacific Ocean. This point of view is supported by Warren in a personal communication to QUADFASEL and ScHorr (1982), which suggests that IEW off the Somali Coast originates from AAMW (referred to as Banda Intermediate Water by Warren), and Antarctic Intermediate Water, which Thcrmodine circulation and ventilation in the Indian Ocean 15 enters the equatorial Indian Ocean from the Pacific Ocean and is carried westward by the South Equatorial Current. If IEW is indeed merely a mixture of other water masses (i.e. formed without a contribution of air-sea interaction), the Red Sea and Persian Gulf Waters are the only sources for thermocline ventilation north of the equator. But the volumes of both sources are much too small to have a significant impact on nutrient and oxygen levels in the thermocline. It seems fair to say, therefore, that the ventilation of the Indian Ocean thermocline in the northern hemisphere is not well understood. 2. DATA AND METHODS The historical hydrographic data archived by the World Oceanographic Data Center in Washington (obtained through the Australian Oceanographic Data Centre in Sydney) were used for the study. The data contain physical-chemical observations recorded at discrete depth levels. Most of the observations were made using multi-bottle Nansen casts or other types of water samplers. A small amount of the data was obtained using CTD or STD instrumentation with rosette samplers. In this type of data set, the parameters potentially useful for OMP analysis include temperature, T, salinity, S, oxygen, O2, phosphate, PO4-P, nitrate, NO3-N and silicate, Si. Unfortunately, few of the historical stations contain nitrate data, and inclusion of nitrate in the analysis reduces the numbers of useful stations significantly. We therefore decided to use only five variables (T, S, O,, PO~-P, Si) in return for an acceptable station coverage, and interpolated all data linearly for five selected density surfaces. Figure I shows the station distribution for the oo = 25.7 40° E 60 ° 80 ° 100° 120 ° 140° E I /I I 1 I I I I I '~ ~,~~ stations / ," oo_ I _ - ~ , ~ ; ! .:: :! .~" " 20 o_ ~20 ° " • ' '. " • . '. • - '" • • • 40*S ( 40 S I .I I I I I I I '1 I I 40° E 60 ° 80 ° 100° 120 ° 140° E Fig. l. The distribution of hydrographic stations on the o, = 25.7 isopycn;d surface. Each station has five parameters: temperature, salinity, oxygen, phosphate and silicate. 16 Y. You and M. TOMeZAK isopycnal surface that has the largest number of stations. We did not attempt to perform OMP analysis separately for the two monsoon seasons, which would have reduced the data density for each analysis beyond acceptable levels.