Structure and Seasonal Variation of the Indian Ocean Tropical Gyre Based on Surface Drifters

<p>Using the Gauss&#8211;Markov decomposition method, this study investigates the mean structure and seasonal variation of the tropical gyre in the Indian Ocean based on the observations of surface drifters. In the climatological mean, the clockwise tropical gyre consists of the equatorial Wyrtki Jets (WJs), the South Equatorial Current (SEC), and the eastern and western boundary currents. This gyre system redistributes the water mass over the entire tropical Indian Ocean basin. Its variations are associated with the monsoon transitions, featuring a typical clockwise pattern in the boreal spring and fall seasons. The relative importance of the geostrophic and Ekman components of the surface currents as well as the role of eddy activity were further examined. It was found that the geostrophic component dominates the overall features of the tropical gyre, including the SEC meandering, the broad eastern boundary current, and the axes of the WJs in boreal spring and fall, whereas the Ekman component strengthens the intensity of the WJs and SEC. Eddies are active over the southeastern tropical Indian Ocean and transport a warm and fresh water mass westward, with direct impact on the southern branch of the tropical gyre. In particular, the trajectories of drifters reveal that during strong Indian Ocean Dipole or El Ni&#241;o-Southern Oscillation events, long-lived eddies were able to reach the southwestern Indian Ocean with a moving speed close to that of the first baroclinic Rossby waves.</p>

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Seasonal variation in the long-term warming trend in water temperature off the Western Australian coast

Marine and Freshwater Research ◽

10.1071/mf08199 ◽

2009 ◽

Vol 60 (2) ◽

pp. 129 ◽

Cited By ~ 29

Author(s):

N. Caputi ◽

S. de Lestang ◽

M. Feng ◽

A. Pearce

Keyword(s):

Seasonal Variation ◽

Indian Ocean ◽

Water Temperature ◽

West Coast ◽

Climate Models ◽

Warming Trend ◽

Temperature Cycle ◽

Rock Lobster ◽

Leeuwin Current ◽

The Indian Ocean

Previous studies have demonstrated that one area of greatest increase in surface sea temperatures (SST) (0.02°C per year) in the Indian Ocean over the last 50 years occurs off the lower west coast of Australia, an area dominated by the Leeuwin Current. The present paper examines water temperature trends at several coastal sites since the early 1970s: two rock lobster puerulus monitoring sites in shallow water (<5 m); four sites from a monitoring program onboard rock lobster vessels that provide bottom water temperature (<36 m); and an environmental monitoring site at Rottnest (0–50 m depth). Two global SST datasets are also examined. These data show that there was a strong seasonal variation in the historic increases in temperature off the lower west coast of Australia, with most of the increases (0.02–0.035°C per year) only focussed on 4–6 months over the austral autumn–winter with little or no increase (<0.01°C per year) apparent in the austral spring–summer period. These increases are also apparent after taking into account the interannual variation in the strength of the Leeuwin Current. The warming trend results in a change to the seasonal temperature cycle over the decades, with a delay in the peak in the temperature cycle during autumn between the 1950s and 2000s of ~10–20 days. A delay in the timing of the minimum temperature is also apparent at Rottnest from August–September to October. This seasonal variation in water temperature increases and its effect on the annual temperature cycle should be examined in climate models because it provides the potential to better understand the specific processes through which climate change and global warming are affecting this region of the Indian Ocean. It also provides an opportunity to further test the climate models to see whether this aspect is predicted in the future projections of how increases will be manifest. Any seasonal variation in water temperature increase has important implications for fisheries and the marine ecosystem because it may affect many aspects of the annual life cycle such as timing of growth, moulting, mating, spawning and recruitment, which have to be taken into account in the stock assessment and management of fisheries.

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High occurrence of new particle formation events at the Maïdo high-altitude observatory (2150 m), Réunion (Indian Ocean)

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-9243-2018 ◽

2018 ◽

Vol 18 (13) ◽

pp. 9243-9261 ◽

Cited By ~ 9

Author(s):

Brice Foucart ◽

Karine Sellegri ◽

Pierre Tulet ◽

Clémence Rose ◽

Jean-Marc Metzger ◽

...

Keyword(s):

Seasonal Variation ◽

Indian Ocean ◽

Particle Growth ◽

Threshold Value ◽

Particle Formation ◽

New Particle Formation ◽

The Indian Ocean ◽

Median Particle ◽

Particle Sizer ◽

The One

Abstract. This study aims to report and characterise the frequent new particle formation (NPF) events observed at the Maïdo observatory, Réunion, a Southern Hemisphere site located at 2150 m (a.s.l.) and surrounded by the Indian Ocean. From May 2014 to December 2015, continuous aerosol measurements were made using both a differential mobility particle sizer (DMPS) and an air ion spectrometer (AIS) to characterise the NPF events down to the lowest particle-size scale. Carbon monoxide (CO) and black carbon (BC) concentrations were monitored, as well as meteorological parameters, in order to identify the conditions that were favourable to the occurrence of nucleation in this specific environment. We point out that the annual NPF frequency average (65 %) is one of the highest reported so far. Monthly averages show a bimodal variation in the NPF frequency, with a maximum observed during transition periods (autumn and spring). A high yearly median particle growth rate (GR) of 15.16 nm h−1 is also measured showing a bimodal seasonal variation with maxima observed in July and November. Yearly medians of 2 and 12 nm particle formation rates (J2 and J12) are 0.858 and 0.508 cm−3 s−1, respectively, with a seasonal variation showing a maximum during winter, that correspond to low temperature and RH typical of the dry season, but also to high BC concentrations. We show that the condensation sink exceeds a threshold value (1.04×10−3 s−1) with a similar seasonal variation than the one of the NPF event frequency, suggesting that the occurrence of the NPF process might be determined by the availability of condensable vapours, which are likely to be transported together with pre-existing particles from lower altitudes.

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Migrant waders at Aldabra, September 1967—March 1968

Philosophical Transactions of the Royal Society of London Series B Biological Sciences ◽

10.1098/rstb.1971.0023 ◽

1971 ◽

Vol 260 (836) ◽

pp. 549-559 ◽

Cited By ~ 2

Keyword(s):

Seasonal Variation ◽

Indian Ocean ◽

Royal Society ◽

Safety Net ◽

The Other ◽

Check List ◽

Firm Conclusion ◽

Dromas Ardeola ◽

The Indian Ocean ◽

Wintering Ground

Observations of palaearctic waders during the first three phases of the Royal Society Expedition to Aldabra, between 1 September 1967 and 14 March 1968 are listed and compared with those from other places in the area. Seventeen species are included in the check list, of which four are considered vagrants, and the rest more or less regular migrants. Counts made during the expedition with the intention of finding the seasonal variation in numbers of the birds have proved statistically invalid, and only a few tentative conclusions may be drawn from them. The Turnstone, Arenaria i. inter pres , may be a winter resident in fair numbers, as may the Whimbrel, Numenius p. phaeopns . Although no firm conclusion can be drawn, it is suggested that most of the other birds are passage migrants, with few staying at Aldabra for the winter. Thus the island is not an important wintering ground for any species, though it probably has some value as a resting place for the birds during their migratory flights. For the Crab Plover, Dromas ardeola , however, it may be one of the few wintering grounds where the species occurs in any numbers, and as such important. The suggestion is made that the other islands in the Indian Ocean, east of the axis of Madagascar, may function not as a destination but as a ‘safety net’ for vagrant birds or flocks.

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Inertial currents in the Indian Ocean derived from satellite tracked surface drifters

Oceanologica Acta ◽

10.1016/s0399-1784(00)01108-7 ◽

2000 ◽

Vol 23 (5) ◽

pp. 635-640 ◽

Cited By ~ 5

Author(s):

P.Kuttan SAJI ◽

Satheesh C. SHENOI ◽

Anselm ALMEIDA ◽

Gangadhara RAO

Keyword(s):

Indian Ocean ◽

The Indian Ocean ◽

Surface Drifters ◽

Inertial Currents

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Characteristics of the Near-Surface Currents in the Indian Ocean as Deduced from Satellite-Tracked Surface Drifters. Part I: Pseudo-Eulerian Statistics

Journal of Physical Oceanography ◽

10.1175/jpo-d-14-0050.1 ◽

2015 ◽

Vol 45 (2) ◽

pp. 441-458 ◽

Cited By ~ 15

Author(s):

Shiqiu Peng ◽

Yu-Kun Qian ◽

Rick Lumpkin ◽

Yan Du ◽

Dongxiao Wang ◽

...

Keyword(s):

Indian Ocean ◽

Kinetic Energy ◽

Eastern Coast ◽

Western Boundary ◽

Surface Currents ◽

Mean Velocity ◽

Near Surface ◽

The Indian Ocean ◽

Central Eastern ◽

Surface Drifters

AbstractUsing the 1985–2013 record of near-surface currents from satellite-tracked drifters, the pseudo-Eulerian statistics of the near-surface circulation in the Indian Ocean (IO) are analyzed. It is found that the distributions of the current velocities and mean kinetic energy (MKE) in the IO are extremely inhomogeneous in space and nonstationary in time. The most energetic regions with climatologic mean velocity over 50 cm s−1 and MKE over 500 cm2 s−2 are found off the eastern coast of Somalia (with maxima of over 100 cm s−1 and 1500 cm2 s−2) and the equatorial IO, associated with the strong, annually reversing Somalia Current and the twice-a-year eastward equatorial jets. High eddy kinetic energy (EKE) is found in regions of the equatorial IO, western boundary currents, and Agulhas Return Current, with a maximum of over 3000 cm2 s−2 off the eastern coast of Somalia. The lowest EKE (<500 cm2 s−2) occurs in the south subtropical gyre between 30° and 40°S and the central-eastern Arabian Sea. Annual and semiannual variability is a significant fraction of the total EKE off the eastern coast of Somalia and in the central-eastern equatorial IO. In general, both the MKE and EKE estimated in the present study are qualitatively in agreement with, but quantitatively larger than, estimates from previous studies. These pseudo-Eulerian MKE and EKE fields, based on the most extensive drifter dataset to date, are the most precise in situ estimates to date and can be used to validate satellite and numerical results.

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