Southeast Indian Ocean Research Articles

Rainfall asymmetry in the southeast Indian Ocean between positive and negative IODs and its local impact

AbstractThe analysis of Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) satellite observations and European Centre for Medium‐Range Weather Forecasts (ECMWF) reanalysis data identify an asymmetric rainfall pattern in the southeast Indian Ocean (SEIO) between the positive and negative Indian Ocean Dipoles (IODs). During positive (negative) IODs, the precipitation anomaly is more confined to equatorial (off‐equatorial) regions, even though their associated sea surface temperature anomalies almost reside in the same region. It is argued that the rainfall pattern asymmetry is attributed to the effect of the rainfall mean state. While the negative rainfall anomalies associated with positive IODs are confined in the region of mean precipitation, the positive rainfall anomalies associated with negative IODs do not have such a constraint. The so‐induced asymmetric rainfall pattern exerts a great impact on the upper ocean salinity and bushfires over the western maritime continent.

Front variability and surface ocean features of the presumed southern bluefin tuna spawning grounds in the tropical southeast Indian Ocean

The southern bluefin tuna (SBT, Thunnus maccoyii) is an ecologically and economically valuable fish. However, surprisingly little is known about its critical early life history, a period when mortality is several orders of magnitude higher than at any other life stage, and when larvae are highly sensitive to environmental conditions. Ocean fronts can be important in creating favourable spawning conditions, as they are a convergence of water masses with different properties that can concentrate planktonic particles and lead to enhanced productivity. In this study, we examine the front activity within the only region where SBT have been observed to spawn: the tropical southeast Indian Ocean between Indonesia and Australia (10°S–20°S, 105°E–125°E). We investigate front activity and its relationship to ocean dynamics and surface features of the region. Results are also presented for the entire Indian Ocean (30°N–45°S, 20°E–140°E) to provide a background context. We use an extension of the Cayula and Cornillon algorithm to detect ocean fronts from satellite images of sea surface temperature (SST) and chlorophyll-a concentration (chl-a). Front occurrence represents the probability of occurrence of a front at each pixel of an image. Front intensity represents the magnitude of the difference between the two water masses that make up a front. Relative to the rest of the Indian Ocean, both SST and chl-a fronts in the offshore spawning region are persistent in occurrence and weak in intensity. Front occurrence and intensity along the Australian coast are high, with persistent and intense fronts found along the northwest and west coasts. Fronts in the tropical southeast Indian Ocean are shown to have strong annual variability and some moderate interannual variability. SST front occurrence is found to lead the Southern Oscillation Index by one year, potentially linked to warming and wind anomalies in the Indian Ocean. The surface ocean characteristics of the offshore SBT spawning region are found to be particularly stable compared to the rest of the Indian Ocean in terms of stable SST, low eddy kinetic energy, i.e., low mesoscale eddy activity, and low chl-a. However, this region has high front occurrence, but low front intensity of both SST and chl-a fronts. The potential impact of these oceanic features for SBT spawning is discussed.

Evidence of submarine groundwater nutrient supply to an oligotrophic barrier reef

Detailed hydrographic survey revealed patches of high nutrients and low salinity in the surface waters over a submerged limestone barrier reef in the oligotrophic coastal waters of the southeast Indian Ocean. The nutrient patches exist throughout the year and represent concentrations that are more than 10 times those in the surrounding water. Numerical model results show that the nutrients originate from the reef and that the patchiness is caused by changes in water circulation. The limestone in the region is known to be solution riddled, and submarine seeps of nutrient‐rich groundwater on the reef are a likely source. Underground conduits represent major routes by which groundwater nutrients can be transported over several kilometers and supplied to barrier reef ecosystems, resulting in “islands” of high productivity.

Seasonal prediction for tropical cyclone frequency around Taiwan using teleconnection patterns

In this study, a statistical model is developed to predict the frequency of tropical cyclones (TCs) that influence Taiwan in boreal summer. Predictors are derived from large-scale environments from February to May in six regions, including four atmospheric circulation predictors over the western sea and eastern sea of Australia, the subtropical western North Pacific (SWNP), and the eastern sea of North America, and two sea surface temperature predictors in the Southeast Indian Ocean and the North Atlantic. The statistical model is verified based on statistical cross-validation tests and by contrasting the differences in the large-scale environments between high and low TC frequency years hindcasted by the model. The results show the relationships of two atmospheric circulation predictors and one SST predictor around Australia with Antarctic Oscillation (AAO) pattern, as well as the relationships of those in the SWNP and around eastern sea of North America with Pacific/North American teleconnection (PNA) pattern. When the anomalous anticyclone around Australia (positive AAO pattern) and the one over the region from eastern sea of North America and the Aleutian Islands to the SWNP (negative PNA pattern) are both strengthened from February, the trade wind in the equatorial Pacific is intensified and consequently plays an important role in steering TCs towards Taiwan during boreal summer.

Progress of Chinese research in physical oceanography of the Southern Ocean

Oceanographic surveying has been one of the key missions of the Chinese National Antarctic Research Expeditionsince 1984. Using the field data obtained in these surveys and the results from remote sensing and numerical models, Chinesephysical oceanographers have investigated the water masses, fronts and circulation patterns in the Southern Ocean. The results ofnearly 30 years of research are summarized in this paper. Most oceanographic observations by Chinese researchers have been conductedin Prydz Bay and the adjacent seas. CTD (Conductivity Temperature and Depth) data, collected during the past 20 years,have been applied to study several features of the water masses in this region: The spatial variation of warm summer surface water,the northward extension of shelf water, the flow of ice shelf water from the cavity beneath the Amery Ice Shelf, the upwelling ofthe Circumpolar Deep Water, and the formation of the Antarctic Bottom Water. The circulation and its dynamic factors have beenanalyzed with dynamic heights calculated from CTD data as well as by numerical models. The structure and strength of the frontsin the southeast Indian Ocean and the Drake Passage were investigated with underway XBT/XCTD (Expendable Bathythermograph/Expendable CTD) and ADCP (Acoustic Doppler Current Profiler) data. Their interannual variations have been determinedand the factors of influence, especially the atmospheric forcing and mesoscale oceanic processes, were studied using remote sensingdata. The dynamic mechanism of the Antarctic Circumpolar Current (ACC) was analyzed by theoretical models. The transportand pattern of the ACC have been well reproduced by coupled sea ice-ocean models. Additional details of ACC variability wereidentified based on satellite altimeter data. The response of the ACC to climate change was studied using reanalysis data. Prospectsfor future research are presented at the end of this paper.

Which environmental factors predict seasonal variation in the coral health of Acropora digitifera and Acropora spicifera at Ningaloo Reef?

The impact of physico-chemical factors on percent coral cover and coral health was examined on a spatial basis for two dominant Acropora species, A. digitifera and A. spicifera, at Ningaloo Reef (north-western Australia) in the southeast Indian Ocean. Coral health was investigated by measuring metabolic indices (RNA/DNA ratio and protein concentration), energy levels (lipid ratio) and autotrophic indices (chlorophyll a (chl a) and zooxanthellae density) at six stations during typical seasons (austral autumn 2010 (March and April), austral winter 2010 (August)) and during an extreme La Niña event in summer 2011 (February). These indices were correlated with 15 physico-chemical factors (measured immediately following coral sampling) to identify predictors for health indices. Variations in metabolic indices (protein concentration and RNA/DNA ratio) for A. spicifera were mainly explained by nitrogen, temperature and zooplankton concentrations under typical conditions, while for A. digitifera, light as well as phytoplankton, in particular picoeukaryotes, were important, possibly due to higher energy requirement for lipid synthesis and storage in A. digitifera. Optimum metabolic values occurred for both Acropora species at 26–28°C when autotrophic indices (chl a and zooxanthellae density) were lowest. The extreme temperature during the La Niña event resulted in a shift of feeding modes, with an increased importance of water column plankton concentrations for metabolic rates of A. digitifera and light and plankton for A. spicifera. Our results suggest that impacts of high sea surface temperatures during extreme events such as La Niña may be mitigated via reduction on metabolic rates in coral host. The high water column plankton concentrations and associated low light levels resulted in a shift towards high symbiont densities, with lower metabolic rates and energy levels than the seasonal norm for the coral host.

Correlation between Heat Flux over the Indian Ocean and Rainfalls in Coastal Thailand by Using the MM5 Numerical Model

Correlation between heat flux over the Indian Ocean and rainfalls in coastal Thailand by using the Pennsylvania State University/National Center for Atmospheric Research mesoscale model: MM5 is the method to study monsoon onset in Thailand. The study areas are over the Indian Ocean from Latitude 30°S to 30°N and Longitude 40°E to 120°E. The used data were during May to October since the years 1996 to 2000, and also covered the years where El Niño and La Niña were present. When the heat flux over the Southeast Indian Ocean was increased about100 w/m2, this persisted for approximately five consecutive days before the average onset date of the 15th May in Thailand. Once the recorded increase in heat flux of five days had passed, it was then observed that after a period of eleven days this caused the onset of rainfalls at nine Thai meteorological stations. From the observations recorded the results showed a correlation between the development of heat flux and rainfalls. During the normal years (1996, 1999 and 2000) the correlation of the heat flux with the rainfalls data at the twelve primary meteorological stations in Thailand were computed by linear regression analysis and the best of these resulted in coefficient of determination(R2)= 0.782. The results (correlation between heat flux which obtained from MM5 model and amounts of rainfalls from weather observation stations) of this study is possible to be utilize by the Thai Meteorological Department to predict the onset day of southwest monsoon rainfall, mainly for agricultural activities more accurately procedure.

Fronts, baroclinic transport, and mesoscale variability of the Antarctic Circumpolar Current in the southeast Indian Ocean

Fronts, baroclinic transport, and mesoscale variability of the Antarctic Circumpolar Current (ACC) along 115°E are examined on the basis of CTD data from two hydrographic cruises occupied in 1995 as a part of the World Ocean Circulation Experiment (WOCE cruise I9S) and in 2004 as a part of CLIVAR/CO2 repeat hydrography program. The integrated baroclinic transport across I9S section is (97.2×106±2.2×106) m3/s relative to the deepest common level (DCL). The net transport at the north end of I9S, determined by the south Australian circulation system, is about 16.5×106 m3/s westward. Relying on a consistent set of water mass criteria and transport maxima, the ACC baroclinic transport, (117×106±6.7×106) m3/s to the east, is carried along three fronts: the Subantarctic Front (SAF) at a mean latitude of 44°–49°S carries (50.6×106±13.4×106) m3/s; the Polar Front (PF), with the northern branch (PF-N) at 50.5°S and the southern branch (PFS) at 58°S, carries (51.3×106±8.7×106) m3/s; finally, the southern ACC front (SACCF) and the southern boundary of the ACC (SB) consist of three cores between 59°S and 65°S that combined carry (15.2×106±1.8×106) m3/s. Mesoscale eddy features are identifiable in the CTD sections and tracked in concurrent maps of altimetric sea level anomalies (SLA) between 44°–48°S and 53°–57°S. Because of the remarkable mesoscale eddy features within the SAF observed in both the tracks of the cruises, the eastward transport of the SAF occurs at two latitude bands separating by 1°. Both the CTD and the altimetric data suggest that the mesoscale variability is concentrated around the Antarctic Polar Frontal Zone (APFZ) and causes the ACC fronts to merge, diverge, and to fluctuate in intensity and position along their paths.

Extreme spatial variability in sessile assemblage development in subtidal habitats off southwest Australia (southeast Indian Ocean)

The development of marine benthic communities is strongly influenced by patterns of settlement, recruitment and survival, which may vary across multiple spatial scales in concordance with the scale-dependent processes that drive them. The temperate subtidal reefs off southwest Australia support highly diverse assemblages of macroalgae and sessile invertebrates, yet little is known about spatial variability in the structure of developing assemblages compared with established assemblages. Here, settlement panel arrays were deployed adjacent to subtidal rocky reefs, in 13–15m depth, at 3 locations spanning 400km of temperate coastline in Western Australia. Panel assemblages were allowed to develop for ~14months before they were harvested. Variability in ecological pattern was analyzed at 4 spatial scales, spanning centimeters to 100s of kilometers. The structure of sessile assemblages was vastly different between the 3 locations, in that one location (Geographe Bay) supported an impoverished assemblage comprising a single macrofaunal species whereas assemblages at the other two locations (Jurien Bay and Marmion Lagoon) supported fairly rich assemblages of macroalgae and sessile invertebrates. Multivariate assemblage structure, total richness and total cover varied significantly between the locations, although variability at the smallest spatial scale (centimeters) was consistently pronounced. Variability patterns for key taxa were less consistent across spatial scales. While explanations for the extreme between-location variability remain unclear, there was some evidence to suggest that herbivory by demersal fish may inhibit assemblage development at Geographe Bay, although local hydrodynamic factors (i.e. relatively lower water movement and influence of the dominant regional-scale oceanic current) could also be important.

Vertical structure of low-level atmosphere over the southeast Indian Ocean fronts

During the 25th Chinese National Antarctic Research Expedition, GPS radiosondes were launched to detect the atmospheric vertical structure over the southeast Indian Ocean frontal region. Some low-level characteristics along the cruise are studiedbased on in-situ observation. The observations reveal that vertical distributions of the low-level wind field and air temperature fieldon both sides of the Subantarctic Front are very different. A stronger (weaker) vertical gradient is on the cold (warm) side, whichdemonstrates that the mid-latitude ocean-atmosphere interaction is active in the southeast Indian Ocean frontal region. A low-leveljet is observed over the Subantarctic Front, with speed up to 14 m·s -1 . For the Antarctic polar front, low-level wind speed near thesea surface is greater than that aloft, in contrast with the situation of the Subantarctic Front. Comparing satellite remote sensingdata and widely-used reanalysis datasets with our in-situ observations, differences of varying magnitudes are found. Air temperature from Atmospheric Infrared Sounder (AIRS) data has a limited difference. The European Center for Medium Range WeatherForecasts Interim Re-Analysis (ERA Interim) dataset is much more consistent with the observations than the National Centers forEnvironmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis 1 in the southeast Indian Oceanfrontal region.

BOOK REVIEW: "Review of Fisheries Exploitation in the Indian Ocean — Threats and Opportunities" Edited by Dennis Rumley, Sanjay Chaturvedi and Vijay Sakhuja

The Singapore Economic ReviewVol. 57, No. 03, 1280009 (2012) Book ReviewsNo AccessBOOK REVIEW: "Review of Fisheries Exploitation in the Indian Ocean — Threats and Opportunities" Edited by Dennis Rumley, Sanjay Chaturvedi and Vijay SakhujaCLEM TISDELLCLEM TISDELLSchool of Economics, The University of Queensland, Australiahttps://doi.org/10.1142/S0217590812800094Cited by:0 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail Singapore Institute of Southeast Asian Studies, and Southeast Indian Ocean Research Group, 2009, pp. xvi+364, [ISBN: 978-981-230-986-0]. FiguresReferencesRelatedDetails Recommended Vol. 57, No. 03 Metrics History Published: 11 September 2012 PDF download

Punctuated global tropical cyclone activity over the past 5,000 years

There are now a substantial number of millennial scale records of tropical cyclones from a variety of locations globally. Some of these, such as those in the Atlantic and Gulf of Mexico, show patterns of long‐term, generally intense, tropical cyclone (TC) behaviour that have been suggested to be due to either variations in ENSO or shifts in the position of the Atlantic High pressure system and the jet stream. Comparisons are made here of the sedimentary overwash records from the Gulf of Mexico and Atlantic Ocean Basin with the overwash records from the north‐west Pacific and beach ridge records from the south‐west Pacific and south east Indian Ocean basins. There is a substantial degree of synchroneity in global intense TC behaviour over the past 3,000 to 5,000 years. One of the most striking aspects of these records is they all display extended alternating periods (centuries to millennia) of relative quiescence and heightened intense TC activity irrespective of both the resolution and type of long‐term TC record. The cause(s) of this punctuated activity are at present difficult to identify but are likely exogenic in origin rather than due to an intrinsic aspect of the records or the methods used to derive them. The identification of punctuated global long‐term TC behaviour is important for understanding future TC activity and for assessing risk to coastal communities.

The influence of Indian Ocean sea surface temperature on the variability of monsoon rainfall over India

This study focuses on the spatial and temporal variability of southwest monsoon (SWM) and northeast monsoon (NEM) rainfall over India, after the 1976–1977 regime shift. Through wavelet analysis the dominant mode of variability in the SWM and NEM rainfall is found to be in the 2–8-year scale (ENSO band). Therefore, scale-averaged wavelet power (SAP) of SWM (SAPSWM) and NEM (SAPNEM) rainfall obtained from a high-resolution dataset for the period 1977–2001 was subjected to empirical orthogonal function analysis, in order to understand the homogenous regions of rainfall variability. As the study utilizes SAP, the decadal variability is observed in the spatial and temporal patterns of rainfall. The maximum variability of SAPSWM rainfall is found to be in the 2–4-year period, whereas, for the SAPNEM rainfall, the biennial mode explains the variability over the core region of NEM rainfall. Significant power is also observed in the 4-year period. The evolution of the principal components (PCs) is consistent with the above/below normal rainfall received over the region. Similar analysis has been carried out also for the SAP of monthly sea surface temperature (SAPSST) over Indian Ocean for the months from January to September. Compared with any other month, the decadal changes of the PCs of SAPSWM and SAPNEM rainfall are highly correlated with the decadal variations of PC1 of SAPSST for the month of March. Therefore, the relation between the rainfall and SST was explained using SAPSST for the month of March. Significant biennial power is also observed in March SST over southeast Indian Ocean (85–110°E and 5–40°S) after the climate shift of 1976. In order to identify any change in the relation between NEM rainfall and SAPSST (March) before and after 1976, a correlation between NEM rainfall and PC1 of SAPSST (March) was found for the period 1951–1975 and 1977–2001. Results show that correlation has increased after 1976. Therefore, this PC can be made use of in the prediction of NEM rainfall. Copyright © 2012 Royal Meteorological Society

Vertical structure of the low level atmosphere around the Southeast Indian Ocean fronts

Vertical structure of the low level atmosphere around the Southeast Indian Ocean fronts

Wave energy potential along the northern coasts of the Gulf of Oman, Iran

This study aims to investigate wave power along the northern coasts of the Gulf of Oman. To simulate wave parameters the third generation spectral SWAN model was utilized, and the results were validated with buoy and ADCP data. First, annual energy was calculated in the study region with the hindcast data set covering 23 years (1985–2007). The areas with the highest wave resource were determined and the area proximity to the port of Chabahar is suggested as the best site for the installation of a wave farm. Second, the average monthly wave energy in this area was investigated. The most energetic waves are provided by the southeast Indian Ocean monsoon from June to August. Finally, the wave energy resource was characterized in terms of sea state parameters. It was found that the bulk of annual wave energy occurs for significant wave heights between 1 and 3 m and energy periods between 4 and 8 s in the direction of SSE.

Interannual variability of convective activity over the tropical Indian Ocean during the El Niño/La Niña events

Convection over the tropical Indian Ocean is important to the global and regional climate. This study presents the monthly climatology of convection, inferred from the outgoing longwave radiation (OLR), over the tropical Indian Ocean. We also examine the impact of El Niño/La Niña events on the convection pattern and how variations in convection over the domain influence the spatial rainfall distribution over India. We used 35 recent years (1974–2008) of satellite-derived OLR over the area, the occurrence of El Niño/La Niña events and high resolution grid point rainfall data over India. The most prominent feature of the annual cycle of OLR over the domain is the movements of convection from south-east to north and north-west during the winter to the summer monsoon season. This feature represents the movement of the inter-tropical convergence zone (ITCZ). The climatology of OLR during the winter months (December–February) over the domain is characterized by high subsidence over central India with a decrease of OLR values towards the north and south. Moderate convection is also seen over the Himalayan Range and the south-east Indian Ocean. In contrast, during the summer (June–September) the OLR pattern indicates deep convection along the monsoon trough and over central India, with subsidence over the extreme north-west desert region. The annual march of convection over the Arabian Sea and Bay of Bengal sector shows that the Arabian Sea has a limited role, compared to the Bay of Bengal, in the annual cycle of the convection over the tropical Indian Ocean. The composite OLR anomalies for the El Niño cases during the summer monsoon season show suppressed convection over all of India and moderate convection over the central equatorial Indian Ocean and over the northern part of the Bay of Bengal. Meanwhile in La Niña events the OLR pattern is nearly opposite to the El Niño case, with deep convection over entire Indian region and adjoining seas and subsidence over the northern Bay of Bengal and extreme north-west region. The spatial variability of the 1° × 1° summer monsoon rainfall data over India is also examined during El Niño/La Niña events. The results show that rainfall of the summer monsoon season over the southern peninsular of India and some parts of central India are badly affected during El Niño cases, while the region lying along the monsoon trough and the west coast of India have received good amounts of rainfall. This spatial seasonal summer monsoon rainfall distribution pattern seems to average out the influence of El Niño events on total summer monsoon rainfall over India. It seems that, in El Niño events, the convection pattern over the Bay of Bengal remains unaffected during summer monsoon months and thus this region plays an important role in giving good summer monsoon rainfall over the northern part of India, which dilutes the influence of El Niño on seasonal scale summer monsoon rainfall over India. These results are also confirmed by using a monthly bias-corrected OLR dataset.

New age for ferromanganese crust 109D-C and implications for isotopic records of lead, neodymium, hafnium, and thallium in the Pliocene Indian Ocean

[1] This study presents a high-resolution record of osmium and thallium isotopes in a ferro-manganese (Fe-Mn) crust from the Indian Ocean, Antipode 109D-C. These results, when combined with additional new Os isotope data from ODP Hole 756B in the southeast Indian Ocean, define a new best estimate for the age at the base of this crust of ∼6.5 Ma, which is significantly different from a previous estimate of ∼15 Ma based on Co-flux modeling. The Tl isotope record obtained for the Indian Ocean resembles that for the Pacific Ocean with a small but well-defined increase occurring over the last ∼5 Myr. This contrasts with two records from the Atlantic Ocean which do not have resolvable variations. Ocean basin–scale Tl isotope variation may be inconsistent with the inferred modern marine residence time for Tl of ∼20 kyr but could be explained by an increase in ocean crust production rates in the Pacific and Indian oceans since ∼10 Ma. The improved age model for 109D-C reveals that the Hf isotope composition of Indian Ocean bottom waters has remained homogenous over the last ∼6 Myr. Thus, this isotope system does not bear any evidence that the influence of North Atlantic Deep Water in the formation of Indian Ocean bottom waters has changed during that time. However, because of the lack of knowledge about Hf isotopes as a tracer of ocean circulation, we cannot conclude that export of NADW decreased over the last 6 Myr.

Seasonal Modulation of Eddy Kinetic Energy and Its Formation Mechanism in the Southeast Indian Ocean

Abstract Mesoscale eddy activity in the southeast Indian Ocean (15°–30°S, 60°–110°E) is investigated based on available satellite altimetry observations. The observed sea level anomaly data show that this region is the only eastern basin among the global oceans where strong eddy activity exists. Furthermore, the eddy kinetic energy (EKE) level in this region displays a distinct seasonal cycle with the maximum in austral summer and minimum in austral winter. It is found that this seasonal modulation of EKE is mediated by baroclinic instability associated with the surface-intensified South Indian Countercurrent (SICC) and the underlying South Equatorial Current (SEC) system. In austral spring and summer the enhanced flux forcing of combined meridional Ekman and geostrophic convergence strengthens the upper-ocean meridional temperature gradient, intensifying the SICC front and its vertical velocity shear. Modulation of the vertical velocity shear results in the seasonal changes in the strength of baroclinic instability, leading to the seasonal EKE variations in the southeast Indian Ocean.

Interannual modulation of eddy kinetic energy in the southeast Indian Ocean by Southern Annular Mode

[1] Interannual variability in the mesoscale eddy field over the southeast Indian Ocean of 15°S–35°S and 60°E–110°E is investigated on the basis of 16 year satellite altimetry observations. Eddy kinetic energy (EKE) in this region appears stronger in 2000–2004 and weaker in 1993–1996, 1998–2000, and 2007. It is found that this interannual modulation of EKE is mediated by the Southern Annular Mode (SAM), with a positive (negative) SAM corresponding to weak (strong) eddy activity in this region. The interannual modulation of the EKE by the SAM is through modulating the baroclinic instability associated with the surface-intensified South Indian Countercurrent (SICC) and the underlying South Equator Current (SEC) system. In the positive phase of the SAM, the southeastern subtropical Indian Ocean is dominated by an anomalous Ekman upwelling, which slackens the southward tilt of the isotherms and thus reduces the SICC. This shall reduce the vertical velocity shear of the SICC-SEC current system, leading to a weak instability and thus a weak eddy activity.

Arterial ocean circulation of the southeast Indian Ocean

[1] The time mean ocean circulation for the southeast Indian Ocean (SEIO) is analyzed using a colorwheel to reveal an “arterial-like” structure. Although quasi-zonal, the mean circulation in this region is distinctly more complex than previously presented in recent literature. The mean flow is derived from an eddy-resolving ocean reanalysis of the past 14 years. The ocean state is constrained through data assimilation with observed sea level anomaly, sea surface temperature and in-situ profiles. Geostrophic currents derived from the reanalyzed mean dynamic topography (MDT) are validated against two other MDT analyses to show a comparable arterial structure and distribution of scales. In particular, the broad scale eastward flow is regularly interspersed by narrow bands of the near westward flow. Meridional sections from the ocean reanalysis reveal a net eastward transport in the surface layer and a net westward flow at mid-depth. The seasonality of the surface layer flow in the SEIO appears to have increased (decreased) eastward mean transport that coincide with seasons of stronger (weaker) Leeuwin Current (LC). Whether the narrow bands of mean current and mass flux are simply the average of the eddies or represent an underlying weak jet remains a topic of current debate.