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

doi:10.1016/S0399-1784(00)01108-7

Oceanologica Acta

Volume 23, Issue 5, October 2000, Pages 635-640

https://doi.org/10.1016/S0399-1784(00)01108-7 Get rights and content

Abstract

Satellite-tracked surface drifters were used to analyze the characteristics of inertial currents in the tropical Indian Ocean. The drifters were drogued at 15 m depth and had wind-produced slips less than 0.1 % of the wind speed. The rotary spectra of surface velocity components indicated the significance of inertial currents. They are circular (rotary coefficient >0.5), highly intermittent and contribute up to 46 % to the total kinetic energy of the surface flow field. Events of inertial activity, either triggered by the passage of atmospheric disturbances or by the local fluctuations in the atmospheric pressure (winds), did not last for more than 4 to 5 inertial cycles. The observed inertial frequency exhibited a shift towards the red end of the spectrum by 12 %. Cyclonic storm induced inertial events even at a location 300 km away from it.

Résumé

Courants d'inertie dans l'océan Indien estimés à partir de flotteurs de surface suivis par satellite. Des flotteurs de surface suivis par satellite ont été utilisés pour analyser les caractéristiques des courants d'inertie dans l'océan Indien tropical. Largués à 15 mètres de profondeur, ces flotteurs présentent une vitesse de dérive inférieure à 0,1 % de celle du vent. Le spectre de rotation des composantes de la vitesse superficielle souligne l'importance des courants d'inertie. Ceux-ci sont circulaires (coefficient de rotation >0.5), très intermittents et ils contribuent environ à 46 % de l'énergie cinétique totale du champ de courants superficiels. Les événements de l'activité inertielle, qu'ils soient provoqués par le passage de perturbations atmosphériques ou par des fluctuations locales de pression atmosphérique (vents), ne durent pas plus de 4 à 5 cycles d'inertie. La fréquence d'inertie observée est déplacée vers les basses fréquences du spectre d'environ 12 %. Un cyclone génère des événements inertiels jusqu'à 300 kilomètres du lieu où il se situe.

Section snippets

INTRODUCTION

Inertial currents in the ocean rotate clockwise (anticlockwise) in the northern (southern) hemisphere with period T=2π/2Ω sin θ; where Ω is the angular velocity of earth and θ the local latitude of observation (Perkins, 1972). The inertial currents were observed in the oceans and large lakes at all depths with velocities ranging from 10–80 cm·s⁻¹ (Webster, 1968) and are believed to be forced primarily by the winds (Pollard, 1970, Poulain, 1990). It has been estimated that the energy in the

MATERIALS AND METHODS

The data from 29 drifting buoys, extracted from the archives of Atlantic Oceanographic and Meteorological Laboratory (AOML), National Institute of Oceanography (NIO) (Shenoi et al., 1997) and the data personally obtained from Dr Robert Molinari are used in this analysis. All the drifters had a 7 m long holey-sock drogue centered at a nominal depth of 15 m to ensure the water particle following property of the drifter and a drogue ON/OFF sensor to continuously monitor its presence. The data are

Spectral characteristics

In order to determine the distribution of the kinetic energy due to horizontal currents as a function of frequency of motion, the average rotary spectra were determined for each latitude band following Gonella (1972). The significant peaks of velocity spectra in the inertial band indicate the substantial presence of inertial currents in the tropical Indian Ocean (figure 1). The energy in the inertial band is well above the tidal and higher frequency oscillations. As expected, the inertial

Acknowledgements

The authors would like to thank the Global Drifter Centre at AOML, Miami and Dr Robert Molinari for providing the drifter data from their archives. This work was supported by the grants from Department of Ocean Development, New Delhi. The graphics package GMT was used extensively. Comments from the anonymous referees helped in improving the manuscript. This is NIO contribution 3546.

References (18)

J. Gonella
A rotary component method for analysing meteorological and oceanographic vector time series
Deep Sea Res.
(1972)
P.P. Niiler et al.
Measurements of the water-following characteristics of Tristar and Holeysock drifters
Deep Sea Res.
(1995)
H. Perkins
Inertial oscillations in the Mediterranean
Deep Sea Res.
(1972)
R.T. Pollard
On the generation by winds of inertial waves in the ocean
Deep Sea Res.
(1970)
S.C. Shenoi et al.
Current measurements over the western continental shelf of India
Con. Shelf Res.
(1991)
Anonymous, 1990. ARGOS User manual, Chapter 3,...
E.A. D'Asaro
Upper ocean temperature structure, inertial currents and Richardson numbers observed during strong meteorological forcing
J. Phys. Oceanogr.
(1985)
N.P. Fofonoff
Spectral characteristics of internal waves in the ocean
Deep Sea Res.
(1969)
D.V. Hansen et al.
Processing of WOCE/TOGA drifter data
J. Atm. Ocean. Tech.
(1996)

There are more references available in the full text version of this article.

Cited by (10)

A review of ocean-atmosphere interactions during tropical cyclones in the north Indian Ocean
2022, Earth-Science Reviews
Citation Excerpt :
The vertical profile of the temperature of the ocean up to 100 m is significantly altered by these inertial oscillations (Rao et al., 2010). During a cyclone in 1995, inertial currents are observed as far as 300 km from the cyclone center, attaining a maximum speed of 0.75 m s−1 (Saji et al., 2000). For very intense cyclones such as Cyclone Phailin, very intense currents of velocity 1.29 m s−1 are observed (Venkatesan et al., 2014).
The north Indian Ocean accounts for 6% of the global tropical cyclones annually. Despite the small fraction, some of the most devastating cyclones have formed in this basin, causing extensive damage to the life and property in the north Indian Ocean rim countries. In this review article, we highlight the advancement in research in terms of ocean-atmosphere interaction during cyclones in the north Indian Ocean and identify the gap areas where our understanding is still lacking.
There is a two-way ocean-atmosphere interaction during cyclones in the north Indian Ocean. High sea surface temperatures (SSTs) of magnitude 28–29 °C and above provide favorable conditions for the genesis and evolution of cyclones in the Arabian Sea and the Bay of Bengal. On the other hand, cyclones induce cold and salty wakes. Cyclone induced cooling depends on the translation speed of the cyclone, wind power input, ocean stratification, and the subsurface conditions dictated by the ocean eddies, mixed layer, and the barrier layer in the north Indian Ocean. The average cyclone-induced SST cooling is 2–3 °C during the pre-monsoon season and 0.5–1 °C during the post-monsoon season. This varying ocean response to cyclones in the two seasons in the Bay of Bengal is due to the difference in the ocean haline stratification, whereas, in the Arabian Sea, it is due to the difference in cyclone wind power input and ocean thermal stratification. The oceanic response to cyclone is asymmetric due to the asymmetry in the cyclone wind stress, cyclone induced rainfall and the dynamics of the ocean inertial currents. The cyclone induced wake is salty and is the saltiest in the Bay of Bengal among all the ocean basins. The physical response of the ocean to the cyclone is accompanied by a biological response also, as cyclones induce large chlorophyll blooms in the north Indian Ocean that last from several days to weeks.
SSTs leading to cyclogenesis in the Arabian Sea are 1.2–1.4 °C higher in recent decades, compared to SSTs four decades ago. Rapid warming in the north Indian Ocean, associated with global warming, tends to enhance the heat flux from the ocean to the atmosphere and favor rapid intensification of cyclones. Monitoring and forecasting rapid intensification is a challenge, particularly due to gaps in in-situ ocean observations. Changes in ocean-cyclone interactions are emerging in recent decades in response to Indian Ocean warming, and are to be closely monitored with improved observations since future climate projections demonstrate continued warming of the Indian Ocean at a rapid pace along with an increase in the intensity of cyclones in this basin.
Near-inertial currents off the east coast of India
2013, Continental Shelf Research
Citation Excerpt :
In the Indian Ocean, however, the dearth of current measurements implies that studies of NICs are limited. They have been observed in drifting-buoy data in the South Indian Ocean (Shetye and Michael, 1988) and over most of the Indian Ocean (Saji et al., 2000). NICs have been observed in current-meter data from the Bay of Bengal (Joseph et al., 2007) and the Arabian Sea (Rao et al., 1996), and on the continental shelf off the Indian west coast (Shenoi and Antony, 1991).
We use data from moorings equipped with Acoustic Doppler Current Profilers (ADCPs) and deployed in the Bay of Bengal off the east coast of India from May 2009 to February 2012 to study the near-inertial currents (NICs) on the continental shelf and slope. The data show that the NICs are much weaker at the shelf break than on the slope. Inertial energy is weak all along the east coast during January–April. It is high during the summer monsoon (May–September) in the northern Bay of Bengal and early during the winter monsoon (October–December) in the southern bay; at locations in the central bay, the inertial energy does not show this seasonality. This difference between the northern and southern bay is due to the seasonality in the occurrence of storms, which tend to occur in the north (south) during the summer (winter) monsoon. Variability across years is evident in the three-year record, with the NICs being weaker during 2010–2011 compared to 2009. Upward phase propagation is evident in the data, indicating downward propagation of energy. During severe cyclones, the data suggest that the strong NICs extend below the thin surface mixed layer in the bay. A comparison of the NICs amplitude with that of the detided (residual) current shows that the NICs make a significant contribution to the observed current on the east-coast slope: the magnitude of the NICs exceeds that of the residual current on the slope in the northern and southern Bay of Bengal on over 10 days in a year.
Upper ocean variability in the Bay of Bengal during the tropical cyclones Nargis and Laila
2012, Progress in Oceanography
Citation Excerpt :
Chintalu et al. (2001) reported the measurements of inertial oscillations at 2.5 m depth from a surface buoy in the central BoB due to the passage of a tropical cyclone (No name was given to this), with the magnitude of inertial currents reaching 0.80–0.90 ms−1. Using data from surface drifter buoys drogued at 15 m depth in the central BoB, Saji et al. (2000) also reported the occurrence of inertial currents with clockwise rotation in the wake of a storm. Fig. 3a and b show the RAMA buoy (15°N, 90°E) measured currents at 10 m depth during 1 April–31 May, 2008 covering the Nargis period, and the generation of inertial oscillations of 2 day period due to Nargis.
Upper ocean variability at different stages in the evolution of the tropical cyclones Nargis and Laila is evaluated over the Bay of Bengal (BoB) during May 2008 and May 2010 respectively. Nargis initially developed on 24 April 2008; intensified twice on 27–28 April and 1 May, and eventually made landfall at Myanmar on 2 May 2008. Laila developed over the western BoB in May 2010 and moved westward towards the east coast of India. Data from the Argo Profiling floats, the Research Moored Array for African–Asian–Australian Monsoon Analysis and prediction (RAMA), and various satellite products are analyzed to evaluate upper ocean variability due to Nargis and Laila. The analysis reveals pre-conditioning of the central BoB prior to Nargis with warm (>30 °C) Sea Surface Temperature (SST), low (<33 psu) Sea Surface Salinity (SSS) and shallow (<30 m) mixed layer depths during March–April 2008. Enhanced ocean response to the right of the storm track due to Nargis includes a large SST drop by ∼1.76 °C, SSS increase up to 0.74 psu, mixed layer deepening of 32 m, shoaling of the 26 °C isotherm by 36 m and high net heat loss at the sea surface. During Nargis, strong inertial currents (up to 0.9 ms⁻¹) were generated to the right of storm track as measured at a RAMA buoy located at 15 °N, 90 °E, producing strong turbulent mixing that lead to the deepening of mixed layer. This mixing facilitated entrainment of cold waters from as deep as 75 m and, together with net heat loss at sea surface and cyclone-induced subsurface upwelling, contributed to the observed SST cooling in the wake of the storm. A similar upper ocean response occurs during Laila, though it was a significantly weaker storm than Nargis.
Estimation of the surface and mid-depth currents from Argo floats in the Pacific and error analysis
2008, Journal of Marine Systems
Citation Excerpt :
For low wind speed, it is designed to follow water parcels well (e.g., K = ρaCDaAa / ρsCDsAs ∼ 10− 4, cf. Kirwan et al. (1975)) as both drifting on the surface and parking on mid-depth. With more than 2000 floats in the global oceans at present, there has been a unique opportunity to simultaneously measure surface and mid-depth currents (Schmid et al., 2001; Ichikawa et al., 2001; Iwao et al., 2003; Park et al., 2005a) in the global scale at near real time. However, the accuracy of surface current estimated from Argo float is constrained by several factors.
With the large deployment, the Array for Real-time Geostrophic Oceanography program has great potential for measuring the ocean currents both on the surface and at mid-depth. However the positioning error of fixes in a trajectory varies from 150 m to 1000 m, and thus created difficulty for accurate estimations of the surface and mid-depth currents. Also the reliability of the estimated surface and mid-depth currents requires accurate error estimations.
In this study a new sequential method of Argo float surface trajectory tracking and extrapolating is proposed based on Kalman Filter (KF), under the presumption that a surface trajectory of Argo float is dominated by a constant current plus inertial oscillation. This trajectory tracking and extrapolating method is able to reduce the positioning uncertainties of Argo surface trajectories and provides error estimations. When this method was applied to extrapolate the positions when float resurfacing and descending, the estimation error of the mid-depth currents can be reduced. Utilizing this method in the Pacific, surface and mid-depth currents were estimated from surface trajectories of Argo floats from 2001 to 2004, along with their detailed error estimations. The average error for surface currents is about 4.4 cm s^− 1 which is equivalent to the accuracy order (5 cm s^− 1) of the Surface Velocity Program drifters. The estimation error of the mid-depth currents at 1000 db is reduced to about 0.21 cm s^− 1 without considering the effect of vertical shear.
This study shows that the surface trajectory from Argo float provides a new means to measure surface circulations in the global ocean at real time, and that the estimated mid-depth current could be one of the important sources to improve the understanding for ocean dynamic.
Characteristics of Wind-Generated Near-Inertial Waves in the Southeast Indian Ocean
2022, Journal of Physical Oceanography
A review of the ocean-atmosphere interactions during tropical cyclones in the north Indian Ocean
2020, arXiv

View all citing articles on Scopus

View full text

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

Abstract

Résumé

Section snippets

INTRODUCTION

MATERIALS AND METHODS

Spectral characteristics

Acknowledgements

Deep Sea Res.

Deep Sea Res.

Deep Sea Res.

Deep Sea Res.

Con. Shelf Res.

Upper ocean temperature structure, inertial currents and Richardson numbers observed during strong meteorological forcing

J. Phys. Oceanogr.

Spectral characteristics of internal waves in the ocean

Deep Sea Res.

Processing of WOCE/TOGA drifter data

J. Atm. Ocean. Tech.