Southeast Indian Ocean Research Articles

The JADE and WOCE I10/IR6 Throughflow sections in the southeast Indian Ocean. Part 1: water mass distribution and variability

Abstract The distribution and configuration of water properties are described along five hydrographic sections that cross the Indonesian Throughflow at two longitudes not far from the outflow straits. Properties are further analyzed through the use of concentration anomalies along neutral surfaces. The analysis confirms the two core structure of the Indonesian Throughflow, which has distinct thermocline and intermediate water components. The latter occupies a larger area than the former on all five sections. Westward increase in the salinity of the South Equatorial Current, which carries the Indonesian Throughflow, implies that lateral entrainment and recirculation of saltier thermocline waters of Indian Ocean origin must occur in the region. Relatively salty North Indian Ocean water is advected eastward south of Java within and below the South Java Current. While the upper portion of this flow appears ‘blocked’ at Lombok Strait by the Throughflow, the deeper portions penetrate further east below the Lombok sill depth (350 m). A rather complex intermediate water circulation in the Southeast Indian Ocean is found involving the recirculation of waters of Indian Ocean origin between a set of zonal currents crossed by the sections. An intermediate water countercurrent (counter to the sense of the Sverdrup flow above it) is described, flowing eastwards between the edge of the subtropical gyre and the Indonesian Throughflow.

The JADE and WOCE I10/IR6 Throughflow sections in the southeast Indian Ocean. Part 2: velocity and transports

Abstract Velocity and transports are estimated for five recent hydrographic sections and one full-depth velocity section that cross the Indonesian Throughflow in the southeast Indian Ocean between Australia and Indonesia. The sections are: JADE August 1989 and JADE February 1992, both between northwestern Australia and Bali; WOCE repeat sections IR6 April 1995, IR6 September 1995 and I10 November 1995, all between Western Australia to near Sunda Strait, Java. The three WOCE surveys include direct velocity measurements from shipboard and lowered (I10 only) acoustic Doppler current profilers. None of the WOCE hydrographic surveys measured the Indonesian Exclusive Economic Zone between ∼9°S and the Java coast. Concurrent XBT and XCTD observations taken as part of the regular co-located IX1-XBT transect were used to complete the sections. Both the velocity fields and the transport estimates vary widely in magnitude and direction, and reflect the seasonal and the shorter-term variability from the eddy field at the time of each survey. The eddies are found during each of the WOCE surveys in the South Equatorial Current between 14°S–12°S, and have a significant impact on the velocity structure: the mass transport tends to recirculate within the features. The direct current estimates support the finding that the strongest flow across all sections is found within the eddy structures, as well as in the South Java Current that flows along the Java coastline. The semi-annually reversing South Java Current plays an important role in distributing freshwater into and out of the southeast Indian Ocean. The errors and uncertainties of the transport calculations are assessed in terms of salinity variability in the region using different available salinity fields. For the shallow-layer transport, a new mean historical salinity estimate does a good job of reproducing the transports determined from synoptically available fields. However, for deeper reference layers, the presence of the eddies significantly affects the salinity and therefore the transport estimates in the region.

Intraseasonal Variability in the South Equatorial Current of the East Indian Ocean

Abstract Satellite altimeter data reveal that the strongest intraseasonal variability in the southeast Indian Ocean occurs in the South Equatorial Current (SEC) during the second half of the year. The length scale of the variability is 100–150 km, with a westward phase speed of 15–19 cm s−1 and dominant periods between 40 and 80 days. A continuously stratified quasigeostrophic model is solved to analyze the baroclinic stability of the climatological SEC. Large growth rate of the instability waves (with e-folding timescale of less than 50 days) can be found east of the active region of intraseasonal variability during the July–September season, when the SEC and the Pacific to Indian Ocean throughflow are at their strongest. Geostrophic current shear in the upper 200-m ocean is crucial for the growth of the instability. The results suggest that the baroclinic instability draws most of its energy from the available potential energy associated with the throughflow, and, to a lesser degree, by local Ekman pump...

On the interpretation of linear relationships between seafloor subsidence rate and the height of the ridge

SUMMARY Several studies have found that the ‘subsidence rate’ of the oceanic lithosphere measured along flow lines is a linear function of the height of the ridge. This might have very important implications concerning our understanding of the cooling of the lithosphere. Here we study the relationship between the subsidence rate and the height of the ridge in the South Atlantic and East Indian Oceans. From the regression slopes of this relationship, it has been inferred that lithosphere created at shallower ridges subsides faster and to deeper levels than that generated at deeper ridges. We show however, that the topography at young ages for various seafloor segments stays positively correlated with the topography at older ages. Synthetic data sets are obtained by adding random synthetic signals with the same spectrum as the residual topography to the average depth versus age relationship. They are also characterized by the same linear relationship between ‘subsidence rates’ and the height of the ridge. This relationship is therefore not proof that lithosphere formed at shallow ridges cools faster than lithosphere formed at deeper ridges. Random signals linked with crustal thickness variations, hotspot bulges and perhaps large wavelength ‘dynamical topography’ would also yield such a relationship.

Source of the baroclinic waves in the southeast Indian Ocean

The dynamics of the seasonal and interannual sea level variability in the southeast Indian Ocean are investigated using a simple model of low‐frequency quasi‐geostrophic thermocline variability in order to determine whether the observed variability responds primarily to local or remote forcing. This region is important for climate studies, in relation to the interannual variations of the tropical oceans and atmosphere. The eastern Indian Ocean is directly forced by the strong seasonal monsoons as well as by a remote ocean forcing from the tropical Indian Ocean and by the western Pacific via the Indonesian Throughflow. As a result, the dynamics of the southeast Indian Ocean are unique, with unusually large variability and bands of energetic Rossby waves. The annual wave signal around 10°S is clearly marked and a band of propagating mesoscale variability between 20° and 35°S extends across the entire Indian Ocean, with characteristic timescales between 100 and 200 days. There is also strong interannual variability. To investigate the origin of the observed long baroclinic waves, we use a simple reduced gravity model allowing the radiation of long waves due to Ekman pumping and the radiation of long waves from the eastern boundary. Eastern boundary conditions are given by expendable bathythermograph data. In the band 10°–15°S the thermocline depth oscillation corresponds mainly to waves radiating from wind forcing in the east. Their amplitude is strongly damped west of 90°E. In the southeastern tropical Indian Ocean (STIO) the influence of free waves emanating from the eastern boundary is small but significant. In addition, a strong interannual signal appears to originate from Lombok Strait to the north and propagates southwestward into the STIO. In the band 20°–35°S the observed waves appear to be free waves generated by eastern boundary processes.

Geochemical tracing of Pacific-to-Atlantic upper-mantle flow through the Drake passage.

The Earth's convecting upper mantle can be viewed as comprising three main reservoirs, beneath the Pacific, Atlantic and Indian oceans. Because of the uneven global distribution and migration of ridges and subduction zones, the surface area of the Pacific reservoir is at present contracting at about 0.6 km2 x y(r-1), while the Atlantic and Indian reservoirs are growing at about 0.45 km2 x yr(-1) and 0.15 km2 x yr(-1), respectively. Garfunkel and others have argued that there must accordingly be net mantle flow from the Pacific to the Atlantic and Indian reservoirs (in order to maintain mass balance), and Alvarez further predicted that this flow should be restricted to the few parts of the Pacific rim (here termed 'gateways') where there are no continental roots or subduction zones that might act as barriers to shallow mantle flow. The main Pacific gateways are, according to Alvarez, the southeast Indian Ocean, the Caribbean Sea and the Drake passage. Here we report geochemical data which confirm that there has been some outflow of Pacific mantle into the Drake passage--but probably in response to regional tectonic constraints, rather than global mass-balance requirements. We also show that a mantle domain boundary, equivalent to the Australian-Antarctic discordance, must lie between the Drake passage and the east Scotia Sea.

The impact of snowmelt on the late Cenozoic landscape of the southern Rocky Mountains, USA

At Elan Bank of the Kerguelen Plateau in the southeast Indian Ocean, Leg 183 of the Ocean Drilling Program recovered clasts of garnet-biotite gneiss in a fluvial conglomerate intercalated with basalt flows. U-Pb and Pb-Pb dates of zircons and monazites in these clasts and an overlying sandstone range from 534 to 2547 Ma, which is much older than the surrounding Indian Ocean seafloor. These dates show that old continental crust resides in the shallow crust of the oceanic Kerguelen Plateau and that the breakup of Gondwana dispersed continental fragments into the nascent Indian Ocean lithosphere.

The Kerguelen Province revisited: Additional constraints on the early development of the Southeast Indian Ocean

The Kerguelen Province, consisting of two oceanic plateaus (Kerguelen, Broken Ridge) and three basins (Enderby, Labuan and Diamantina), covers a large area of ocean floor in the southeast Indian Ocean. As very few magnetic anomalies have been identified in this area and only a few basement ages from the Kerguelen Plateau are known, reconstruction models of the Kerguelen Province are not well constrained. In an effort to gain more understanding about the evolution of this area, we have used satellite gravity to identify additional fracture zones. As they are likely to be associated with high frequency and low amplitude gravity anomalies, we have computed the vertical derivative map instead of the regular satellite gravity map. Using this approach, we have identified a series of fracture zones in the Enderby Basin, which are aligned with the Mesozoic fracture zones in the Perth Basin and converge to the Kerguelen Fracture Zone. In the conjugate Bay of Bengal, we traced an equivalent pattern of fracture zones which, together, better constrain the early evolution of this part of the Indian Ocean. Synthesis of these images and the other available data from the Kerguelen Province, suggests that the spreading of India from both Australia and Antarctica is closely related. Spreading between the three continents appears to have begun about the same time, in the early Cretaceous and thus, the accretion of some parts of the Kerguelen Province must have occurred before the onset of the quiet magnetic period at 118 Ma. At about 96–99 Ma, when the spreading direction in the Indian Ocean had changed into a N-S direction, it also took place throughout the Kerguelen Province. We find that previously proposed slow spreading in the Diamantina Zone and Labuan Basins, between 96–99 Ma and the initiation of the Southeast Indian Ridge at 43 Ma, could not have taken place. Furthermore, we suggest that there is growing evidence that the same is true for spreading in the eastward continuation of the Diamantina Zone and Labuan Basin, between Australia and Antarctica. Initiation of spreading in this area is likely to be contemporaneous with the spreading in the Kerguelen Province and, thus, older than 96–99 Ma.

Variability in the tropical southeast Indian Ocean and links with southeast Australian winter rainfall

Variability in the southeast (SE) Indian Ocean is examined in relation to SE Australian winter rainfall. Analyses of sea surface temperature (SST), sea surface height (SSH), thermocline depth, wind, outgoing longwave radiation (OLR) and moisture flux data suggest, firstly, that SE Australian winter rainfall is influenced by SST in the tropical SE Indian Ocean and, secondly, that SST anomalies in this region are associated with local atmosphere‐ocean coupling. Based on these analyses, a mechanism for SE Australian interannual winter rainfall variability is proposed.

The paleoceanography for the last 30,000 years in the southeastern Indian Ocean by means of calcareous nannofossils

The paleoceanography during the latest Quaternary (last ∼30 kyr), based on calcareous nannofossils, was studied in the southeastern Indian Ocean through the analysis of 202 sediment samples taken from three deep-sea cores taken along a north–south transect between 12°S and 25°S offshore western Australia. A total of 24 taxonomic groups of calcareos nannofossil was identified by light microscope observation. Two biotic proxies were applied: Gephyrocapsa morphotypes as a sea-surface temperature (SST) indicator; and the ratio of small (<2.5 μm) placoliths of Reticulofenestra and Gephyrocapsa against the lower-photic species indicating the nutrient condition. The relative abundance of Gephyrocapsa types, defined by the angle between the bridge and the long axis of the coccolith, corresponds well with the modern SST. Small placoliths commonly occur in the northern and central cores, while they are rare in the southern core during the Holocene period. The Java upwelling system (JUS) operates above the northern core site and induces higher counts of small placoliths. In the central and southern core sites, located outside of the JUS, the nutrient levels and primary productivity are usually low. The difference in the small placolith abundances between the latter two core sites was explained by an appearance of a small scale local coastal upwelling which induces a common occurrence of the small forms at the central core site; the continuous presence through time of Gephyrocapsa low-angle type supports evidence for upwelling. An intensification of upwelling offshore southern Java during the last glacial period has been inferred from the foraminiferal record (e.g. Martı́nez et al., 1999. Palaeogeogr., Palaeoclimatol., Palaeoecol. 147, 73–99), and the high abundance of small placoliths, indicative of eutrophication as well as low SST values deduced from the nannoflora recognized here are concordant with the postulated upwelling. Low SSTs at the central core site prior to the middle of marine isotope stage 2 indicate a reduction of the Leeuwin Current (LC). Cooler temperatures at the southern site interpreted from the nannoflora indicate that the LC did not reach this site during stage 3 and early stage 2; instead, the South Indian Current dominated there during this period. Small placoliths commonly occurred not only at the central core site but also at the southern core site during stage 3, but disappeared from the southern one for the entire stages 2 and 1. A similar temporal change was also observed in the Tasman Sea (Hiramatsu and De Deckker, 1997. Palaeogeogr., Palaeoclimatol., Palaeoecol. 131, 391–412) and the southeast Indian Ocean (Okada and Wells, 1997. Palaeogeogr., Palaeoclimatol., Palaeoecol. 131, 413–432).

Interannual variability in the Southern Ocean from an ocean model forced by European Centre for Medium‐Range Weather Forecasts reanalysis fluxes

Anomaly patterns in the Southern Ocean in response to variability in the atmospheric forcing are investigated. To this end we forced the Hamburg large‐scale geostrophic ocean general circulation model with surface fluxes from the European Centre for Medium‐Range Weather Forecasts reanalysis (ERA). ERA covers the period January 1979 through February 1994. First, the atmospheric variability of sea level pressure and the associated wind stress anomalies within the ERA data set are analyzed. An Antarctic Circumpolar Wave type pattern is identified. In the ocean response, sea water temperature and salinity anomalies are found to vary with similar periods. The anomalies advect with the Antarctic Circumpolar Current. High amplitudes occur in the southeast Indian and Pacific Oceans. Sensitivity studies are made, pinning down wind stress and heat flux as the dominant factors generating these anomalies. The oceanic interannual variations are explained in terms of enhanced oceanic convection resulting from anomalous Ekman pumping and anomalous heat fluxes, both operating in a standing pattern dominated by wavenumbers 2 and 3.

A study of the south-east Indian Ocean from altimetric, wind and XBT data

The dynamics of the seasonal and interannual sea level variability in the south-east Indian Ocean are investigated using Topex/Poseidon altimetric data and a combination of ERS-1 and ERS-2 winds and a simple dynamic model. This region is important for climate studies, in relation to the interannual variations of the tropical oceans and atmosphere. The eastern Indian region is directly affected by the strong seasonal monsoons as well as a remote ocean forcing from the tropical Indian Ocean and the western Pacific via the Indonesian Throughflow. As a result, the dynamics of the south-east Indian Ocean are unique, with a very large mesoscale variability off western Australia. This eastern boundary is the likely generating region for the westward propagating Rossby waves observed in the subtropical band. The signature of these waves is evident in both the ocean dynamic height and sea surface temperature. To investigate the origin of the observed sea surface variability, we use the vorticity equation with the low-frequency quasi-geostrophic assumption to calculate the sea level response to local wind forcing or remote forcing from the eastern boundary. We use ERS-1 scatterometry for the wind forcing; eastern boundary conditions are given by XBT data. First results indicate that a significant correlation exists between the wind forced model's results and the sea level variability observed by T/P around 15 °S, emphasising the importance of the local wind on the eastern Indian Ocean tropical dynamics. In contrast, the subtropical variability seem to be associated with remote forcing.

The effects of the Southwest Indian Ocean tropical cyclones on Ethiopian drought

Tropical cyclones are one of the prominent weather systems that are generated over the tropical oceans. The cyclones that develop in the Southwest Indian Ocean (SWIO) usually travel west then southwest and finally recurve to southeast, generally before reaching the East African coast. However, it is shown in this study how SWIO-tropical cyclones/depressions can indirectly affect Ethiopian weather. Using correlation and composite analyses, interannual and intraseasonal rainfall variations in Ethiopia were compared with the frequency and time of occurrence of the tropical cyclones. Years showing the consecutive occurrence of several tropical depressions over the SWIO coincide with the drought years of Ethiopia. Reciprocally, years of abnormally low frequency of tropical cyclones are associated with heavy rainfall in Ethiopia. Belg rainfall (‘small rains’, February–May) is much more influenced by the cyclonic activity than Kiremt rainfall (‘big rains’, June–September), which occurs outside the cyclonic season of the Southeast Indian Ocean. On a daily basis, rainfall activity during the Belg period is significantly reduced when a tropical depression is observed in the SWIO, before picking up again a few days later. No systematic time-lag was found between the cyclone occurrence and the decrease in daily rainfall amounts. However, at interannual time-scales, a higher (lower) frequency of tropical depressions during the months of November–January tends to be followed by abnormally low (high) Belg rainfall. One of the mechanisms suggested to explain this seasonal persistence in the frequency of tropical depressions involves sea-surface temperatures in the SWIO, from where many of the depressions originate. The atmospheric processes accounting for the teleconnection between Ethiopia rainfall and tropical depressions were examined. Monthly anomalies of upper air winds for years showing frequent occurrence of tropical depressions proved significant not only over the SWIO, but also the Northern Hemisphere above Northeast Africa. In addition to a slightly enhanced cross-equatorial flow, diverting moisture advection towards the active centre of the system, much stronger than usual equatorial easterlies in the upper troposphere and a reduced southward excursion of the subtropical jet stream result in weaker convective activity over Ethiopia. © 1998 Royal Meteorological Society

Age determination of mixed water masses using CFC and oxygen data

We present a new method based on a combination of optimum multiparameter analysis and CFC/oxygen mixing analysis to determine the ages of water masses in regions of mixing. It enables us to follow water mass movements in greater detail than with other methods, which give only the combined pseudoage of a water mass mixture. We define the age of a water mass as the time a water parcel needs to spread from its source region, where it received its individual tracer characteristics, to the point of observation. The age distribution allows us to determine pathways of water masses, which differ from simple advection trajectories because the age is determined by a combination of advective and diffusive processes. We apply the method to hydrographic data along World Ocean Circulation Experiment section 15 in the south east Indian Ocean. In the thermocline, Indian Central Water (ICW) and Subantarctic Mode Water (SAMW) meet and mix. These distinct water masses have different formation mechanisms but similar temperature/salinity characteristics. It is shown that the convective formation of SAMW is a major ventilation mechanism for the lower Indian thermocline. In the eastern part of the south Indian Ocean, SAMW dominates the oceanic thermocline and is found to be about 5 years old. Pure ICW is present only in the thermocline of the region 48°–55°E, with increasing age with depth, confirming the subduction theory. While most SAMW joins the equatorward gyre movement of the southeastern Indian Ocean, some of it propagates westward through turbulent diffusive mixing, reaching 55°E after 15–20 years. It takes ICW some 25–30 years to reach 110°E.

Variability in the southeast Indian Ocean from altimetry: Forcing mechanisms for the Leeuwin Current

Seasonal and interannual variability in the southeastern Indian Ocean is investigated with the aid of Topex/Poseidon (T/P) altimeter data for the 3‐year period 1993–1995. The annual Rossby wave signal around 10°S is clearly marked, and consistent with modelling and Geosat results from earlier periods. A band of higher mesoscale variability between 20° and 35°S extends across the entire Indian Ocean, with characteristic timescales between 120 and 180 days and length scales of order 500 km. Sea level anomalies are shown to propagate at around 1.5 to 2 times the theoretical linear Rossby wave speed, with an associated signal in sea surface temperature (SST) anomalies, and the propagation suggests that the variability is not locally forced but originates near the eastern boundary. Altimeter data is also used to examine variations in the alongshore pressure gradient, thought to be the principal mechanism forcing the Leeuwin Current poleward against the prevailing equatorward winds. The T/P data confirms that the alongshore pressure gradient is maximum in May when the Leeuwin Current is strongest, but we find a consistent secondary peak in November which is not evident in the climatological data. There is also significant interannual variation, related to large interannual variations on the Australian Northwest Shelf. The seasonal and interannual variations also influence the thermal structure of the instabilities associated with the Leeuwin Current, which may be the source of the westward propagating anomalies between 20°S and 35°S. The results compare well with satellite SST and in situ XBT data at the eastern boundary and suggest that satellite data can be used to monitor the variability of the Leeuwin Current.

Ventilation processes and water mass ages in the thermocline of the southeast Indian Ocean

Indian Central Water (ICW) and Subantarctic Mode Water (SAMW) are the two major sources for the ventilation of the permanent thermocline in the Indian Ocean. ICW is formed by subduction in the region of negative wind stress curl, while SAMW is formed by convective overturning at the Subantarctic Front. SAMW contributes to the depth range of ICW, but is not easily identified, because most hydrographic properties (temperature, salinity and nutrients) of SAMW do not differ much from those of ICW. This study identifies ICW and SAMW in a zonal section near 32°S and evaluates the relative importance of convection vs. subduction for the ventilation process. Oxygen and nutrient data from the eastern part (50–114°E) of WOCE section 15 are used with temperature and salinity to determine water mass fractions of subducted ICW and of SAMW from water mass mixing analysis. The individual age fields of the two water mass components are then derived from a combination of the fractions obtained with a linear oxygen/CFC mixing model. Unlike earlier studies, which derive an uncalibrated apparent age, our results express water mass age in true units of time (years). The core of the SAMW near 114°E is about 5 years old, while the core of the subducted ICW (at 60–80°E) shows an increase of age with depth, in agreement with the subduction process. ICW moves eastward with the South Indian Current, reaching an age of 35 years at 114°E. SAMW spreads westward against the mean flow through turbulent diffusion, reaching an age of 25 years at 50°E.

Late Quaternary palaeoceanography near Tasmania, southern Australia

Abstract This study of the palaeoceanographic record south of Australia is based on two cores taken from lower Circumpolar Deep Water depths: core E27-30, located south of Tasmania in water depth of 3552 m, to the north of the present location of the Subtropical Convergence, and core E55-6 from the continental margin off Victoria at 2346 m depth, in a region of very low surface productivity. A preliminary examination of planktonic foraminiferal faunas in core E27-30, located south of Tasmania, provides a record of movement of the Subtropical Convergence through the late Quaternary that is consistent with records elsewhere in the Southeast Indian Ocean. The Convergence has been located equatorward of its present position through much of the late Quaternary, with poleward excursions in the Holocene and at oxygen-isotope stage 5e. Benthic foraminiferal faunas contrast strongly in the two cores, despite their location at similar depths within the same water mass. Core E55-6 is largely dominated by deep infaunal taxa, while core E27-30 has a high proportion throughout of epifaunal taxa. The benthic foraminiferal fauna in core E27-30 is dominated by Epistominella exigua, known to feed directly on phytoplankton detritus in the modern deep-sea. Abundances of that species coincide with the presence of the Subtropical Convergence at or near the site, indicating that surface phytoplankton production has had a major influence on benthic productivity. Dissolution records in the two cores also differ markedly. Core E55-6 records a strong dissolution event in oxygen-isotope stage 4. The cause of the event is not clear, although it appears to be associated with increased productivity levels. This pronounced dissolution event is not evident in core E27-30, suggesting either the event is localised or that the record in core E27-30 has been overprinted by the Convergence. The extent to which the records in the two cores vary emphasises the influence of frontal structures on core records in this region and the need to take this into account when investigating Southern Ocean history.

Effect of the Missing Indonesian Throughflow in the Fine Resolution Antarctic Model

Abstract South of Tasmania, the Fine Resolution Antarctic Model (FRAM) shows a narrow band of westward flow in the surface layer. The authors argue that this is caused by the closure of the Indonesian passage in FRAM. This is supported by numerical experiments carried out by Hirst and Godfrey and by recent radiocarbon observations in the Great Australian Bight (Ribbe et al.). The FRAM surface temperature and salinity distribution exhibits distinct anomalies in the southeast Indian Ocean, in good agreement with anomalies observed in the Hirst and Godfrey model. Indonesian Throughflow water advects heat into the Indian Ocean; its absence in FRAM results in a lack of thermal energy to warm the Indian Ocean in the model. The surface salinity anomaly is most likely caused by an overestimated Ekman transport. The weakened heat and salinity transport in FRAM restricts surface convection in the midlatitude region to approximately 350 m. The effect of the Indonesian Throughflow closure in FRAM is even more dramati...

Analysis of the global relationship of biennial variation of sea surface temperature and air-sea heat flux using satellite data

We investigated the phase difference and the cross correlation coefficient between the band-pass filtered biennial variations of sea surface temperature (SST) and air-sea heat flux estimated by the monthly mean 2°×2° satellite data of Advanced Very High Resolution Radiometer (AVHRR) and Special Sensor Microwave/Imager (SSM/I) from July 1987 to June 1991. Judging from the phase difference, it can be determined whether the biennial variation of SST is controlled by local thermal air-sea interaction or oceanic processes of horizontal transport. When the local air-sea heat flux controls the biennial variation of SST, the phase of SST advances π/2 (∼6 months) against that of the air-sea heat flux. In contrast, when the biennial variation of SST is controlled by the oceanic process, the phase difference between the SST and the air-sea heat flux becomes 0 or π(∼12 months). In this case, two types of the phase differences are determined, depending on which variability of SST and air-sea heat flux is larger. The close thermal air-sea interaction is noticeable in the tropics and in the western boundary current region. The phase difference of π/2 appears mainly in the north Pacific, the southeast Indian Ocean, and the western tropical Pacific; zero in the eastern tropical Pacific and the northeast and equatorial Atlantic; and that of π in the central equatorial Pacific and north of the intertropical convergence zone (ITCZ) of the Atlantic. Phase differences of 0, π, or π/2 are possible in the western boundary current regions. This fact indicates that each current plays a different role to the biennial variation of SST. It is inferred that SST anomalies in the tropics are mutually correlated, and the process in which marked SST anomalies in the tropics are transferred to the remote area was probed. In the equatorial Pacific, the SST anomaly is transferred by the long planetary wave. On the other hand, it is found from the phase relationship and the horizontal correlation of SST that the SST anomaly in the central and western equatorial Pacific is connected through atmospheric mediation. It is suggested that the biennial variation of SST in the eastern Indian Ocean is affected by heat transport due to the Indonesian throughflow from the western tropical Pacific. It is found that the mentioned pattern of the interannual variation of SST in the tropical Atlantic as a dipole is not tenable.

Geochemical characteristics of lavas from Broken Ridge, the Naturaliste Plateau and southernmost Kerguelen Plateau: Cretaceous plateau volcanism in the southeast Indian Ocean

Lavas from several major bathymetric highs in the eastern Indian Ocean that are likely to have formed as Early to Middle Cretaceous manifestations of the Kerguelen hotspot are predominantly tholeiitic; so too are glass shards from Eocene to Paleocene volcanic ash layers on Broken Ridge, which are believed to have come from eruptions on the Ninetyeast Ridge. The early dominance of tholeiitic compositions contrasts with the more recent intraplate, alkalic volcanism of the Kerguelen Archipelago. Isotopic and incompatible-element ratios of the plateau lavas are distinct from those of Indian mid-ocean ridge basalts; their Nd, Sr, 207Pb 204Pb and 208Pb 204Pb isotopic ratios overlap with but cover a much wider range than measured for more recent oceanic products of the Kerguelen hotspot (including the Ninetyeast Ridge) or, indeed, oceanic lavas from any other hotspot in the world. Samples from the Naturaliste Plateau and ODP Site 738 on the southern tip of the Kerguelen Plateau are particularly noteworthy, with ϵ Nd( T) = − 13 to −7, ( 87 Sr 86 Sr) T=0.7090 to 0.7130 and high 207 Pb 204 Pb relative to 206 Pb 204 Pb . In addition, the low- ϵ Nd(T) Naturaliste Plateau samples are elevated in SiO 2 (> 54 wt%). In contrast to “DUPAL” oceanic islands such as the Kerguelen Archipelago, Pitcairn and Tristan da Cunha, the plateau lavas with extreme isotopic characteristics also have relative depletions in Nb and Ta (e.g., Th Ta, La Nb > primitive mantle values); the lowest ϵ Nd( T) and highest Th Ta and La Nb values occur at sites located closest to rifted continental margins. Accepting a Kerguelen plume origin for the plateau lavas, these characteristics probably reflect the shallow-level incorporation of continental lithosphere in either the head of the early Kerguelen plume or in plume-derived magmas, and suggest that the influence of such material diminished after the period of plateau construction. Contamination of asthenosphere with the type of material affecting Naturaliste Plateau and Site 738 magmatism appears unlikely to be the cause of low- 206 Pb 04 Pb Indian mid-ocean ridge basalts. Finally, because isotopic data for the plateaus do not cluster or form converging arrays in isotope-ratio plots, they provide no evidence for either a quickly evolving, positive ϵ Nd, relatively high- 206 Pb 204 Pb plume composition, or a plume source dominated by mantle with ϵ Nd of −3 to ∼ 0.