The Cross Equatorial (CEC) and Subtropical (STC) Cells are the major shallow-overturning cells of the Indian Ocean, corresponding to the North and South STCs in the Pacific and Atlantic Oceans. Their meridional (overturning) structure is illustrated by two-dimensional (y, z) streamfunction plots from two solutions to ocean general circulation models (OGCMs). The plots show that the cells’ sinking branches are located in the southern hemisphere, whereas their upwelling branches are located in the southern hemisphere along the South Equatorial Thermocline Ridge for the STC, and in the northern hemisphere for the CEC. The cells’ three-dimensional (x, y, z) structures are determined by following pathways of model drifters in solutions to two layer models and an OGCM. Transports of the cell branches in each solution are compared to each other and to available observations. A suite of solutions to an idealized two-layer, reduced-gravity model (a 2 $$\frac{1}{2}$$ -layer model) is used to isolate the basic processes (wind forcing, upwelling, and detrainment) that generate the cells. In addition to the CEC and STC, the solutions have an equatorial “roll,” a small-scale overturning feature confined to the upper 50–100 m within a few degrees of the equator; its dynamics are discussed, and observational support for its existence noted.

Free-wave solutions for a mode of the LCS model are obtained that are valid at midlatitudes (away from the equator). The dispersion relation for Rossby waves is derived when the Coriolis parameter f is constant, and under a realistic restriction it is shown to be valid even when f varies. The concepts of critical frequency $$\sigma _{cr}$$ and critical latitude $$\theta _{cr}$$ are introduced. Kelvin waves are found along both zonal and meridional coasts. Along a meridional coast and when f varies, Kelvin waves exist only poleward of $$\theta _{cr}$$ ( $$\beta $$ -plane Kelvin waves); as for f-plane Kelvin waves, $$\beta $$ -plane Kelvin waves decay offshore, but they also have a weak westward propagation. Equatorward of $$\theta _{cr}$$ , Rossby waves radiate offshore along ray paths that are directed meridionally, as well as westward. Similar properties exist even if the coast if the coast is slanted (i.e., not directed precisely north-south), the major difference being that the value of $$\theta _{cr}$$ is decreased.

The ocean is forced by radiation from and to the atmosphere, fluxes across the air-sea interface, and by freshwater input from precipitation and rivers. They drive circulations in the surface mixed layer (ML) of the ocean, which in turn force deeper circulations. We review each of these forcings, and discuss their impacts on ML properties in both the real ocean and ML models. One impact of evaporation and freshwater input is that the ML thickness differs markedly in the northern areas of the Bay of Bengal and Arabian Sea. In the northern Bay, high freshwater input decreases near-surface salinity and density, and the resulting increase in near-surface stratification ensures that the ML remains relatively thin. Conversely, in the northern Arabian Sea high evaporation increases near-surface salinity and density, decreasing the near-surface stratification and allowing the ML to thicken to larger values. During the summer monsoon, the thinner ML in the northern Bay leads to sea-surface temperature being warm enough to support atmospheric convection, making the northern Bay one of the rainiest regions in the global tropics.

In his original paper, Sverdrup obtained a steady-state solution to the depth-integrated fluid equations without momentum advection and mixing. That fundamental response is now called a “Sverdrup flow” and the balance of terms that generates it a “Sverdrup balance.” A mode of the LCS model can also be in a state of Sverdrup balance, and it is useful to refer to that response as also being a Sverdrup flow. In response to forcing by zonal winds, Sverdrup flow extends west of the forcing region, owing to Rossby-wave radiation. In contrast, when forced by meridional winds Sverdrup flow is confined to the forcing region, because the total Ekman pumping cancels out across the region; however, when there is vertical diffusion, the cancellation isn’t complete, allowing the response to extend west of the region. Sverdrup flows that extend to the western boundary of the basin, are closed by western-boundary currents confined to narrow boundary layers. Solutions are obtained for the well-known, frictional, western-boundary layers obtained by Stommel and Munk. Dynamically similar boundary layers exist on eastern, northern, and southern basin boundaries, and in the interior ocean along edges of forcing regions.

A near-equatorial solution forced by periodic winds is obtained to the complete LCS equations. It is found assuming that the Coriolis parameter is $$f=\beta y$$ , allowing it to be represented as an expansion in Hermite functions. The same processes identified in the previous chapter for switched-on winds occur in the periodic one, except that they happen continuously rather then sequentially. When there is an eastern boundary, the wind-forced equatorial Kelvin wave reflects as a packet of Rossby and evanescent waves, with Rossby waves existing only equatorward of the critical latitude $$\theta _{cr}$$ , and evanescent ones superposing to form a $$\beta $$ -plane Kelvin wave north of $$\theta _{cr}$$ . With both boundaries, two types of resonant responses are possible: “equatorial basin resonance,” which is linked to the natural ringing time of the basin; and “zero-group-velocity” resonance that occurs when the ocean is forced at the critical frequency of an Rossby or gravity wave.

Near-equatorial solutions forced by switched-on winds are found to a simplified version of the LCS equations that neglects the acceleration and damping terms in the meridional momentum equation (the “long-wavelength” approximation). They are found under the assumption that the Coriolis parameter is given by $$f=\beta y$$ , allowing them to be represented as expansions in Hermite functions. In an unbounded ocean, Ekman drift and Ekman pumping quickly establish an accelerating jet, the Yoshida Jet. Subsequently, the radiation of equatorial Rossby and Kelvin waves adjusts the response to a Sverdrup-balanced state plus a zonally-independent, equatorial jet that does not accelerate, the “bounded” Yoshida Jet. With an eastern boundary, the equatorial Kelvin wave reflects as a packet of Rossby waves, which has a characteristic wedge-shaped pattern as it radiates offshore. With both boundaries, signals are present with a period P close to the time it takes an equatorial Kelvin wave to cross the basin and the lowest order ( $$j=1$$ ) Rossby wave to return; period P is a natural “ringing” time of the basin, and is the basis for equatorial basin resonance discussed in the next chapter.

This book provides a comprehensive description of circulations of the North Indian Ocean (NIO), their forcing functions, and their underlying dynamics

The complete solutions to the LCS model are superpositions of the responses to many modes. In these solutions and for periodic forcing, wave energy propagates both vertically and horizontally along ray paths determined by the wave’s dispersion relation. Provided the ray-path slopes are independent of wavenumbers, the responses have a beam-like structure. Solutions are found that illustrate beams associated with coastal and equatorial Kelvin waves, Yanai waves, and equatorial Rossby waves. For constant background stratification $$N_{b}$$ , the beams are very clear, but when $$N_{b}$$ varies with depth they are blurred by reflections of wave energy in regions where $$N_{bz}\not =0$$ . For switched-on forcing and when vertical mixing (damping) is sufficiently large, solutions adjust to steady-state, coastal and equatorial circulations that have realistic undercurrent structures.

Solutions that illustrate Ekman drift and inertial oscillations are obtained under a variety of settings: for a single mode of the LCS model; as a function of depth z both with and without a surface mixed layer; and for constant and variable f. At midlatitudes, the steady Ekman drift associated with a single mode of the LCS model is oriented to the right (left) of the wind in the northern (southern) hemisphere and, when the modes are summed, the solution converges to the classic Ekman spiral; however, if the Ekman drift is confined to a surface mixed layer as is commonly observed, the spiral structure is lost for sufficiently (realistically) strong vertical mixing. Ekman drift also exists near the equator, remaining finite there because pressure is involved in the dynamical balance. In response to zonal winds, Ekman drift diverges from the equator (equatorial Ekman pumping), generating a zonal jet that continuously accelerates (the Yoshida Jet). Inertial oscillations are generated whenever winds are switched on. When f varies, their energy propagates efficiently away from the latitude where they were generated along ray paths predicted by their dispersion relation, a process known as $$\beta $$ -dispersion.

Wind-forced solutions are found to a simplified version of the LCS equations that neglects the acceleration and damping terms in the zonal and meridional momentum equations. When the wind is switched on, the Coriolis parameter f is constant, and there is no vertical diffusion, Ekman flow continuously drains (piles up) water to the left (right) of the wind axis in the northern hemisphere, and vice versa in the southern hemisphere, a process known as open-ocean Ekman pumping. When the wind is switched on and f varies, Ekman pumping is stopped by the radiation of Rossby waves, and without mixing the response adjusts to a steady-state Sverdrup flow. When the wind is periodic, these processes vary continuously.

Wind-forced solutions along eastern and western coasts are found to a simplified version of the LCS equations that neglects the acceleration and damping terms in the zonal momentum equation. All are forced by a zonally-independent band of meridional wind stress $$\tau ^{y}$$ that is either switched-on or periodic. Most are discussed in terms of a one-layer, reduced-gravity model (a 1 $$\frac{1}{2}$$ -layer model) with layer-thickness h. For switched-on winds, solutions are obtained: i) in two dimensions (x, h) when the Coriolis parameter f is constant; and in three dimensions (x, y, h) when (ii) f is constant and (iii) f varies. In case (i) and without vertical mixing, h continuously thins at the coast, a process known as “coastal Ekman pumping.” In case (ii), the thinning is weakened or eliminated by Kelvin-wave radiation, which establishes an alongshore pressure gradient that balances $$\tau ^{y}$$ . In case (iii), the coastal response is further modified by Rossby-wave propagation, which: from an eastern coast carries the coastal currents completely offshore; and from a western coast continuously narrows the currents (without viscosity) or adjusts them to a Stommel or Munk layer (with viscosity). These solutions are modified to provide a simple representation of the coastal response forced by river outflow.

The West India Coastal Current (WICC) flows southward (northward) during summer (winter). We examine the nature of circulation in the region of WICC during an inter-monsoon period using hydrographic data collected during March 6–21, 1994, and archived 1994 daily altimeter data. The hydrographic data did not show any organized northward or southward flow, implying that the amplitudes of the Rossby and Kelvin waves that make the WICC were negligible. Instead, cyclonic and anticyclonic eddies, well recorded in altimeter data, dominated the circulation. Because eddies occur throughout the year, our analysis highlights the need to study their role in WICC all through the year.

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 degrees 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.

The Indian Ocean Dipole is a leading phenomenon of climate variability in the tropics, which affects the global climate. However, the best lead prediction skill for the Indian Ocean Dipole, until recently, has been limited to ~6 months before the occurrence of the event. Here, we show that multi-year prediction has made considerable advancement such that, for the first time, two general circulation models have significant prediction skills for the Indian Ocean Dipole for at least 2 years after initialization. This skill is present despite ENSO having a lead prediction skill of only 1 year. Our analysis of observed/reanalyzed ocean datasets shows that the source of this multi-year predictability lies in sub-surface signals that propagate from the Southern Ocean into the Indian Ocean. Prediction skill for a prominent climate driver like the Indian Ocean Dipole has wide-ranging benefits for climate science and society.

Observations in the Mandovi estuary, located on the central west coast of India, have shown that the salinity field in this estuary is remarkably time-dependent and passes through all possible states of stratification (riverine, highly-stratified, partially-mixed and well-mixed) during a year as the runoff into the estuary varies from high values (similar to 1000 m(3) s(-1)) in the wet season to negligible values (similar to 1 m(3) s(-1)) at end of the dry season. The time-dependence is forced by the Indian Summer Monsoon (ISM) and hence the estuary is referred to as a monsoonal estuary. In this paper, we use a three-dimensional, open source, hydrodynamic, numerical model to reproduce the observed annual salinity field in the Mandovi. We then analyse the model results to define characteristics of residual estuarine circulation in the Mandovi. Our motivation to study this aspect of the Mandovi's dynamics is derived from the following three considerations. First, residual circulation is important to long-term evolution of an estuary; second, we need to understand how this circulation responds to strongly time-dependent runoff forcing experienced by a monsoonal estuary; and third, Mandovi is among the best studied estuaries that come under the influence of ISM, and has observations that can be used to validate the model. Our analysis shows that the residual estuarine circulation in the Mandovi shows four distinct phases during a year: a river like flow that is oriented downstream throughout the estuary; a salt-wedge type circulation, with flow into the estuary near the bottom and out of the estuary near the surface restricted close to the mouth of the estuary; circulation associated with a partially-mixed estuary; and, the circulation associated with a well-mixed estuary. Dimensional analysis of the field of residual circulation helped us to establish the link between strength of residual circulation at a location and magnitude of river runoff and rate of mixing at the location. We then derive an analytical expression that approximates exchange velocity (bottom velocity minus near freshwater velocity at a location) as a function of freshwater velocity and rate of mixing. (C) 2016 Published by Elsevier Ltd.