Elsevier

Continental Shelf Research

Volume 215, 15 February 2021, 104355
Continental Shelf Research

Absence of the annual cycle in shelf current inshore of the East Indian Coastal Current

https://doi.org/10.1016/j.csr.2021.104355Get rights and content

Highlights

  • Annual cycle dominates North Indian Ocean (NIO) monsoon winds.

  • The cycle appears in virtually all NIO wind-driven processes reported so far.

  • Coastal radar data show absence of the cycle in shelf current off Indian east coast.

  • The cycle is present in shelf winds and slope current about 50 km offshore.

  • Data support theory: bottom friction makes shelf a high pass filter (Brink, 2006).

Abstract

A ubiquitous feature of the winds over the North Indian Ocean (NIO), which are dominated by monsoons, is the occurrence of variability with the annual period. It is equally pervasive in the ocean's wind-driven circulation. Here we report observations from the shelf off the east coast of India where this periodicity is absent even though local alongshore wind stress has it prominently, and so does the East India Coastal Current (EICC) that flows along the slope off the shelf only about 40 km away. Our observations are based on a high-frequency coastal radar (HF-R) installed at approximately 11.7°N on the east coast of India. It provided surface currents up to 200 km offshore. We use hourly data from two years, January 2017 to December 2018, to compare the alongshore current over the depth contour 50 m (taken to represent the shelf current, Sh-C) with that over the depth contour 1700 m (taken to represent the slope current, Sl-C). Wavelet analysis shows that Sh-C did not have the annual cycle and had periods primarily less than about 50 days. In contrast, Sl-C, i.e., the EICC, shows the annual period prominently and other lower periods from days to months. The two time-series when low-passed with a 100-day filter are uncorrelated. Theoretical models (Brink (2006), for example) attribute the absence of long periods on the shelf to finite friction on the shelf. It prompts longer-period shelf-wave modes to be weak near the coastline and stronger in deeper waters, making the shelf a high pass filter. Most marine processes (including biogeochemistry and fishery) in the NIO have been assumed to have an annual cycle due to a monsoon driven annual cycle in large-scale physical processes. Our observations show that this need not be the case on the shelf. Hence, a re-evaluation of existing ideas on shelf processes is needed.

Introduction

A distinguishing feature of the large scale wind-driven near-surface circulation of the North Indian Ocean (NIO, approximately north of about 10°N) is its seasonality (Schott and McCreary Jr (2001); Peng et al. (2015)). The feature arises because of the monsoons (seasons) that dominate the basin's winds. They reverse direction twice a year. During summer (approximately June–September), the winds are predominantly from the southwest (southeast) north (south) of the equator. During winter (November–February), they are northeasterly (northwesterly) north (south) of the equator. During fall (October) and spring (March–May) inter-monsoons, the winds are weak and variable. On the equator, there are westerly wind bursts during the two inter-monsoons. The seasonal cycle has two prominent periods associated with it. The most important of these is the annual period (365 days). The second significant period is semi-annual (~180 days). It arises primarily because the summer monsoon winds are significantly stronger than those in winter and because of equatorial wind bursts during inter-monsoons. Superimposed on the seasonal winds with these two prominent periodicities are winds with periods that range from days to weeks: quasi weekly (Krishnamurti and Bhalme (1976); Murakami (1976); Yasunaga et al. (2010)), quasi-biweekly (Krishnamurti and Bhalme (1976); Wang et al. (2017)), and intra-seasonal, approximately 30–90 days (Annamalai and Slingo (2001); Goswami and Mohan (2001); Yasunari (1981)). The winds force the NIO circulation through four driving mechanisms (McCreary Jr et al. (1993)). Wind-driven equatorial Kelvin waves drive circulation north and south of the equator on reflection from the eastern boundary. Alongshore wind stress along the boundaries of the NIO basin drive circulations locally and remotely. In the latter case, the drivers of energy are westward propagating Rossby waves or coastally trapped waves. Finally, Ekman suction and resulting Rossby waves drive circulation in the open sea and basins' western boundaries. These mechanisms' net result is a seasonally reversing baroclinic circulation in which the annual cycle is invariably the most prominent (Frankignoul and Müller (1979); Meyers (1979)).

Boundary currents of the NIO circulation provide the best signatures of the seasonal circulation. The most celebrated of these currents is the Somali Current, which flows northward from March to September and southward from December to February (Quadfasel and Schott (1983); Schott et al. (2002)). Other well-known boundary currents are East Arabia Coastal Current (Elliott and Savidge (1990)), West India Coastal Current (Amol et al. (2014); Shetye et al. (1990, 1991)), East India Coastal Current (Durand et al. (2009); Mukherjee et al. (2014)), and Sumatra Java Coastal Current (Iskandar et al. (2005); Quadfasel and Cresswell (1992); Susanto et al. (2001)). All these boundary currents reverse direction twice a year.

The width of the shelf in the NIO's boundary region varies from tens to hundreds of kilometers (refer Fig. 1(n)). The shelf depth is less than about 200 m, and the abyssal plain is two to four thousand kilometers deep (Sindhu et al. (2007)). The structure of boundary currents is expected to be related to topography's nature in a boundary region (Beardsley and Lentz (1987)). The study of this relationship is in its infancy in the NIO. It has been assumed that the core of boundary flows occurs on the slope. Some empirical evidence can be found for it. For example, hydrographic data show that the core of the West Indian Coastal Current during winter at a latitude of about 20°N occurs on the slope off approximately 500 km wide shelf (Shetye et al. (1991)). Simple numerical models with no topography variation tend to assume that the basin's boundary coincides with the slope (Chatterjee et al. (2017); McCreary et al. (1996)). There are no Ocean General Circulation Model (OGCM) based studies that examine the relationship between the core of a boundary current and underlying bathymetry.

The velocity field variation inshore of a boundary current's core has not received much attention in the NIO. Studies show the presence of shelf waves adjacent to the coastline (see, for example, Amol et al. (2012)), but they do not go far enough offshore to compare and contrast the shelf flow with the slope flow. A significant reason for this has been the inability to make direct current measurements on the shelf because of many maritime activities, including fishing, shipping, offshore petroleum extraction, etc. The lack of empirical data has prevented addressing several important questions. Does the circulation on the shelf mimic that on the slope? Do currents on the shelf and slope show the presence of similar periods? Most notably, does the seasonal cycle, with its annual periodicity, ubiquitous in the winds over the NIO and surrounding lands, appear in shelf circulation? If yes, what are its dynamics?

Long term measurements in the NIO have been mostly in the open ocean, prominently on the equator (Han et al. (2014); Masumoto et al. (2010); McPhaden et al. (2009)) during the last two decades. In the last decade or so, there have also been long term ADCP moorings deployed along the east and west coasts of India (Amol et al. (2014); Mukherjee et al. (2014)). These observations show the presence of the annual cycle on the slope along both coasts. The data also show other periodicities like seasonal (120 days) and intraseasonal (30–90 days). Arunraj et al. (2018) compared the slope measurements with recently deployed high-frequency coastal radars (HF-R). They inferred that small-scale variability (mesoscale eddies) associated with East India Coastal Current (EICC) formed along-shelf break, possibly because of local winds, bottom topography, and coastline orientation. Mukhopadhyay et al. (2017) compared HF-R data with ADCP data. However, neither of the two studies that used HF-R data address the questions we have raised in the last paragraph. In particular, they did not attempt distinguishing between the velocity field on the shelf and that on the slope.

In this paper, we use HF-R data on the shelf and slope off approximately 12°N on the east coast of India to study similarities and differences between the circulation on the shelf and that on the slope. The EICC, the western boundary current of the Bay of Bengal, is located on the continental slope. The availability of data has prompted the location of this study. It is one of the few locations in the NIO where HF-R data are available and found consistent with long term ADCP measurements in the region (Mukhopadhyay et al. (2017)). It is also a location with a narrow shelf, only about 35 km wide. Hence, the slope current has perhaps a more significant chance of overwhelming the shelf circulation. If the shelf shows distinct differences with the slope flow despite its narrowness, it is quite likely that the prominent shelf processes that we identify here will prevail in other shelves of the NIO.

Our analysis showed that while the annual cycle is very much present on the slope (EICC), it is absent on the shallow shelf near the coast despite local wind stress having a distinct annual cycle together with other periods. Though unexpected, this result has support from models of shelf circulation, which is usually dominated by shelf waves or coastally trapped waves (Gill (1982)). Brink (2006) has pointed out that increasing bottom drag leads to a change in the structure of the free wave modes and that the change depends on the period of the waves. Longer period waves tend to have their amplitude maxima for alongshore current move offshore to the slope's deeper region. As a result, bottom friction causes the alongshore current variance to occur primarily on the slope for longer periods. Shorter period waves are not affected as much. In essence, this behavior makes the shallow shelf a high-pass filter, making it a region that supports only high-frequency motions, while the slope anchors motions with lower frequency.

The paper is organized as follows. Section 2 describes the EICC. Section 3 discusses the data that we use. Section 4 examines the surface currents at points along a line perpendicular to the coastline. Section 5 discusses the data that were filtered to retain only the seasonal variability. The seasonal data structure allows us to conclude that the core of the boundary current is located on the slope. In Section 5 compares characteristics of the current on the shelf with that on the slope. We find that circulations with periods longer than about 50 days are absent on the shelf though they occur on the slope. In Section 6, we examine possible mechanisms that contribute to the absence of low frequencies from the shelf. Section 7 summarizes our findings and looks at their implications to other regions of the NIO.

Section snippets

East India coastal current

EICC is one of the better-studied currents of the NIO (Fig. 1). Shetye et al. (1991, 1996) and Sanilkumar et al. (1997) have noted the annual cycle of EICC in hydrographic data. Mukherjee et al. (2014) have observed the EICC in ADCPs over the continental slope. Durand et al. (2009) have described the spatio-temporal structure of EICC with the help of altimeter data. Arunraj et al. (2018) and Mukhopadhyay et al. (2017) have observed the EICC in HF-R data. McCreary Jr et al. (1993); McCreary et

Data

The HF-R data used here provided surface currents up to 200 km offshore. Over this distance, the bottom topography increases from zero at the coastline to about 3500 m (see Fig. 3). Given that the shelf width varies from 35 to 120 km along the east coast of India (Faruque et al. (2014)), HF-R data is well-suited to observe the surface current simultaneously over the shelf and the slope. Our region of study is located approximately at (80°E, 11.7°N), where the HF-R is installed (Fig. 2(b)). The

Surface currents

Ocean dynamics respond significantly to the ocean's underlying topography (Beardsley and Lentz (1987)) - particularly at regions close to the coast where the bathymetry is shallow. The analysis of these currents, using the conventional zonal (eastward) and meridional (northward) component, obscures the interpretation. Instead, the conventional approach is to orient the components to the underlying topography. Note that rotation, being a linear operator, does not alter the frequency spectrum of

The slope current (Sl-C)

The location of the Sl-C is in the region of the large-scale seasonal EICC. We expect the current here to show variability from days to months superimposed on the seasonal current. We first examine only the seasonal part of the current by applying a 100-day low-pass Butterworth filter to the alongshore and crossshore currents computed on the red line in Fig. 2(a). The high-velocity core of the current is found mostly between depth contours 1000–2500 m (Fig. 6). However, it meanders both onshore

Shelf dynamics

In this section we explore the dynamical link between bottom friction and the tendency of the shelf to serve as a high pass filter. Sub-inertial shelf circulation is commonly dominated by wind generated barotropic shelf waves or baroclinic coastally trapped waves that propagate with the coast on their right. Variation of bottom topography in the cross-shore direction plays an important role in determining the structure of the waves perpendicular to the coastline. Power et al. (1989) have

Summary and discussion

A hallmark of large scale wind-driven circulation of the NIO is its seasonal cycle. It arises because of the impact of monsoons, that reverse twice a year. This reversal makes the annual cycle a significant period present in circulation anywhere in the ocean. There are, however, other periods, from days to months, also present in the monsoon winds. It is now reasonably well accepted that the NIO's basin-wide circulation can be understood as the largely baroclinic linear inviscid response to the

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors wish to thank NIOT, Chennai and Ocean Data Management Division, INCOIS for proving the HF-R data. The authors also wish to thank Dr. S.S.C. Shenoi, Former Director, INCOIS for encouraging them for this research. The authors acknowledge the facility and support provided by Dr. T. Srinivasa Kumar, Director, INCOIS. Authors thank anonymous reviewers for their valuable suggestions and comments, which improved the quality of the manuscript. This is INCOIS contribution number 400.

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