Equatorial beams

A linear stratified model is used to study the response of the equatorial ocean to forcing by the wind at periods from one month to one year. Solutions are represented as double sums of vertical modes (designated by the index n) and of the various types of waves associated with each mode (designated by the index l). Waves associated with a considerable number of vertical modes contribute to the solutions. They superpose in such a way that energy and phase propagate vertically as well as horizontally. It is useful to isolate the individual contributions of various pieces of the complete solution. One way to do this isolates the response of individual vertical modes (that is, specifies a value for n, but carries out the summation over l). Pieces of the solution defined in this way tend to focus energy at specific points on the equator. These focal points, however, are not at all visible in the complete solution. Another way isolates the response due to waves of a particular type (that is, fixes a value of l, but carries out the summation over n). These pieces of the solution form well-defined beams that carry energy into the deep ocean at slopes predicted by inviscid ray theory, and they are visible in the complete solution. Solutions for zonal winds are complicated. A Kelvin beam directly forced by the wind reflects from the eastern boundary of the ocean as a set of l = 1,3,5,... Rossby beams. These beams, in turn, reflect from the western boundary as Kelvin beams. All of them reflect efficiently from the ocean surface and bottom. It is the multiple reflection of these beams from basin boundaries that makes the response so complicated. The most visible beams in the solutions are the wind-driven Kelvin beam and a reflected l = 1 Rossby beam. The response is strong at frequencies of the order of 2π years–1 (or lower), and weakens considerably at higher frequencies. Solutions for meridional winds are much simpler. At a frequency of 2π months–1 a beam of Rossby-gravity waves directly forced by the wind reflects entirely poleward along the eastern boundary as a packet of coastal Kelvin waves, since there are no Rossby waves available for this reflection. The response is strong at frequencies of the order of 2π months–1, and weakens markedly at lower frequencies

Western boundary circulation driven by an alongshore wind: With application to the Somali Current system

The linear, continuously stratified, eastern-boundary model of McCreary (1981) is extended to apply to a western ocean boundary and to wind fields with curl. The model has vertical and horizontal mixing, and both types of mixing are important in its dynamics. Solutions are represented as expansions in vertical modes, and the dynamics of low-order and high-order modes are very different. Low-order modes tend to be in Sverdrup balance in the interior ocean, and to form a Munk layer at the coast. High-order modes are in Ekman balance in the interior ocean, and the coastal balance is two dimensional.The model is forced by steady northward winds, both with and without curl. Solutions at the western boundary differ fundamentally from comparable ones at the eastern boundary. For winds without curl, the surface jet is stronger, and there is essentially no coastal undercurrent. This difference is due to the fact that at an eastern boundary the currents associated with low-order modes leak offshore. For winds with curl, a sizeable undercurrent develops, but only south of the region of the wind. A strong onshore current, located near the southern edge of the wind, is generated by offshore wind curl. Part of this current turns southward at the coast, thereby generating the undercurrent in the south.The existence of this undercurrent is in accord with observations off Somalia, where a southward undercurrent has been observed at 5N during the Southwest Monsoon. The wind at this time is oriented alongshore, reaches maximum strength well to the north of 5N, and is associated with a region of large, negative wind curl off the coast

Three-dimensional shelf circulation along an eastern ocean boundary

A linear, three-dimensional, continuously stratified model is used to study wind-driven ocean circulation near an eastern coast in the presence of a continental shelf. A simplifying assumption is that the alongshore flow field is in geostrophic balance. This assumption allows steady solutions to be obtained numerically with a very efficient scheme. As a result, it is possible to find solutions for a wide variety of model parameters and shelf profiles.A band of equatorward wind forces the ocean, and the resulting solutions have many features in common with observations at eastern boundaries. They all have a surface equatorward jet, but do not always have a coastal undercurrent. When the shelf depth is sufficiently shallow or vertical mixing is sufficiently strong, the speed of the undercurrent, if it exists, is usually weak; in that case, only when there is positive wind curl near the coast does its speed reach commonly observed values. Solutions are sensitive to the choice of bottom topographic profile. A general result is that the continental shelf always acts to strengthen the equatorward jet and to weaken or eliminate the undercurrent. The reason is that the shelf induces an equatorward barotropic component to the shelf currents, a component that is not present in flat-bottom solutions

A numerical investigation of the Somali Current during the Southwest Monsoon

The dynamics of the Somali Current system during the Southwest Monsoon are investigated using a 2½-layer numerical model that includes entrainment of cool water into the upper layer. Entrainment cools the upper layer, provides interfacial drag, and prevents the interface from surfacing in regions of strong coastal upwelling. Solutions are forced by a variety of wind stress fields in ocean basins with western boundaries oriented either meridionally or at a 45° angle. Solutions forced by southern hemisphere easterlies develop a strong coastal current south of the equator. When the western boundary is slanted, this current bends offshore at the equator and meanders back into the ocean interior. No cold wedge forms on the Somali Coast. These solutions suggest that the southern hemisphere trades are not an important forcing mechanism of the Somali Current circulation. Solutions forced by northward alongshore winds differ considerably depending on the orientation of the western boundary and the horizontal structure of the wind. When the boundary is meridional and the wind is uniform (a curl-free wind field), solutions continuously shed eddies which propagate northward along the coast and weaken. When the boundary is meridional and the wind weakens offshore, they reach a completely steady, eddy-free state with no coastal upwelling. If the boundary is slanted and the wind does not vary alongshore, solutions reach a steady state that now contains stationary gyres and cold wedges. If the boundary is slanted and the forcing is a strong wind patch confined north of the equator, the flow field slowly vacillates between single-gyre and double-gyre states. Solutions are also forced by an idealized representation of the observed alongshore wind field, consisting of two components: a moderate background field (∼1 dyn/cm2) turned on in May, and a Findlater jet (∼4 dyn/cm2) turned on gradually in June. A single gyre, the Southern Gyre, initially develops south of 4N due to the background wind, and a second gyre, the Great Whirl, develops later between 4N–9N in response to the Findlater jet. Cold wedges form on the northern flanks of both gyres. In some of the solutions, the Southern Gyre moves northward and coalesces with the Great Whirl in early September, before the monsoon begins to weaken. Thus the collapse of the two-gyre system is part of the adjustment of the model to the peak phase of the Southwest Monsoon, and is not due to a relaxation of the wind

Thermohaline forcing of eastern boundary currents: with application to the circulation off the west coast of Australia

The linear, viscid, continuously stratified model of McCreary (1981) is extended to allow for thermohaline forcing by a specified, longitudinally independent, surface density field ρs. When the ocean is unbounded and ρs is steady, the density field is altered throughout the water column by vertical diffusion. If ρs increases poleward, the resulting pressure field slopes downward toward the pole in the upper water column, and there is an associated eastward geostrophic current. This interior current forces downwelling at an eastern ocean boundary, and generates a poleward surface coastal current and an equatorward undercurrent. For realistic choices of model parameters the coastal circulation is as strong as, and opposite in direction to, that caused by a typical equatorward wind stress τy. When ρs oscillates at the annual cycle, the unbounded flowis confined to a surface boundary layer. The coastal circulation is qualitatively quasi-steady, but also has characteristics of a vertically propagating Kelvin wave. One solution is forced by an idealized representation of the observed ρs and τy fields off the west coast of Australia. This solution compares well with observations of the oceanic circulation there. In particular, there is a surface coastal jet (the model Leeuwin Current) that flows against the wind. This successful comparison suggests that the mean circulation in the region is significantly forced by ρs, whereas the annual variability is strongly forced by τy

Effects of remote annual forcing in the eastern tropical Atlantic Ocean

An ocean model is used to study the effects of remote annual forcing in the eastern tropical Atlantic. The model is linear, viscid and continuously stratified. The ocean basin is an idealized version of that of the tropical Atlantic, and the wind stress forcing the model is an idealized representation of the annual variation of the equatorial trades in the western Atlantic. Solutions are represented as expansions of the baroclinic modes of the system. The response of each mode is found numerically, not by integrating the equations of motion forward in time, but at a fixed frequency (2π year–1) using techniques that are typically used in models of the tides. Prominent features of the solution are the following. When the remote trades strengthen, sea level drops and the pycnocline rises markedly throughout the Gulf of Guinea. At 4W the annual response is tightly trapped to the equator and to the coast of Africa near 5N. In contrast, the response propagates offshore along the southern coast of Africa near 10E. Events propagate upward everywhere in the Gulf of Guinea and poleward (nearly) everywhere along the coast of Africa. These features compare favorably with observations. A single baroclinic mode does not dominate the response. Instead, waves associated with several modes superpose to form beams that propagate energy vertically as well as horizontally (McCreary, 1984). Along the equator the response is predominantly a combination of a beam of equatorial Kelvin waves and a lowest order (l – 1) Rossby beam. Along the coast of Africa at 5N it is primarily a beam of coastal Kelvin waves

The response of the coastal ocean to strong offshore winds: With application to circulations in the Gulfs of Tehuantepec and Papagayo

Two ocean models are used to investigate the response of the coastal ocean to strong offshore winds: a linear 1½-layer model, and a nonlinear 1½-layer model that allows entrainment of cool water into the surface layer. The models are forced by wind stress fields similar in structure to the intense winter-time, mountain-pass jets (∼20 dyne/cm2) that appear in the Gulfs of Tehuantepec and Papagayo for periods of 3–10 days. Solutions are arranged in a hierarchy of increasing dynamical complexity, in order to illustrate the important physical processes. They compare favorably with observations in several ways. Some properties of solutions are the following. While the wind strengthens there is an ageostrophic current (not Ekman drift) that is directed offshore. This offshore drift forces coastal upwelling, thereby lowering the local sea level and sea surface temperature (SST). Although the drop in sea level at the coast can be large and rapid (of the order of 20 cm at the peak of a wind event), none of this signal propagates poleward as a coastally trapped wave. While the wind weakens the ageostrophic current is directed onshore, and consequently the coastal ocean readjusts toward its initial state. Throughout the wind event, cyclonic and anticyclonic gyres spin up offshore on either side of the jet axis due to Ekman pumping. Entrainment cools SST offshore, on and to the right (looking onshore) of the jet axis, and virtually eliminates the cyclonic gyre. The advection terms intensify the anticyclonic gyre and give it a more circular shape. After a wind event, the anticyclonic gyre propagates westward due to β. Its propagation speed is enhanced over that of a linear Rossby wave due to the nonlinear terms associated with the increased layer thickness at the center of the gyre and with the divergence of momentum flux

On the dynamics of the California Current system

The dynamics of the California Current system are studied using two ocean models, one with a shelf and one without. Both models are viscid and linearized about a background density field Pb(Z), Solutions are forced by steady and annually periodic winds with and without curl, and by an idealization of the observed wind field off California. Solutions forced by a steady, equatorward, curl-free wind τy all have an equatorward surface coastal jet and a poleward undercurrent. Due to the β-effect and horizontal mixing, the circulation is not necessarily confined within a Rossby radius of the coast. The strength and structure of the currents vary considerably with parameters, the currents being stronger and broader when the forcing includes remote winds to the south and when ρb has a near-surface pycnocline. If τy oscillates at the annual cycle the response is qualitatively quasi-steady, but it also involves a poleward, offshore and vertical propagation of waves, and the maximum coastal current leads τy by several weeks.Solutions forced by a steady, positive wind curl τyx develop a deep, broad, poleward surface current near the coast, consistent with Sverdrup theory. Interestingly, there is also an equatorward surface flow located farther offshore, which exists because of the vertical mixing in the model. Solutions are not very dependent on model parameters, because they are primarily interior currents directly in balance with the wind curl and do not require the coast for their existence. If τyx oscillates at the annual cycle, the response is not at all quasi-steady, and the maximum coastal current lags τyx by 1–2 months. Solutions forced by an idealization of the observed wind field off California compare favorably with observations, but only if Pb has a realistic pycnocline and the forcing includes remote winds off Baja California. Forcing by positive τyx accounts for both the poleward Davidson Current during the winter and the equatorward flow located more than 100 km offshore throughout the year. The coastal jet forced by τy provides summertime equatorward flow within 100 km of the coast that is strong enough to reverse the poleward flow driven by τyx

The deep equatorial ocean circulation in wind-forced numerical solutions

We perform eddy-resolving and high-vertical-resolution numerical simulations of the circulation in an idealized equatorial Atlantic Ocean in order to explore the formation of the deep equatorial circulation (DEC) in this basin. Unlike in previous studies, the deep equatorial intraseasonal variability (DEIV) that is believed to be the source of the DEC is generated internally by instabilities of the upper ocean currents. Two main simulations are discussed: Solution 1, configured with a rectangular basin and with wind forcing that is zonally and temporally uniform; and Solution 2, with realistic coastlines and with an annual cycle of wind forcing varying zonally. Somewhat surprisingly, Solution 1 produces the more realistic DEC: The large-vertical-scale currents (Equatorial Intermediate Currents or EICs) are found over a large zonal portion of the basin, and the small-vertical-scale equatorial currents (Equatorial Deep Jets or EDJs) form low-frequency, quasi-resonant, baroclinic equatorial basin modes with phase propagating mostly downward, consistent with observations. We demonstrate that both types of currents arise from the rectification of DEIV, consistent with previous theories. We also find that the EDJs contribute to maintaining the EICs, suggesting that the nonlinear energy transfer is more complex than previously thought. In Solution 2, the DEC is unrealistically weak and less spatially coherent than in the first simulation probably because of its weaker DEIV. Using intermediate solutions, we find that the main reason for this weaker DEIV is the use of realistic coastlines in Solution 2. It remains to be determined, what needs to be modified or included to obtain a realistic DEC in the more realistic configuration

Heat content of the Arabian Sea Mini Warm Pool is increasing

© The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Atmospheric Science Letters 17 (2016): 39-42, doi:10.1002/asl.596.Sea surface temperature in the Arabian Sea Mini Warm Pool has been suggested to be one of the factors that affects the Indian summer monsoon. In this paper, we analyze the annual ocean heat content (OHC) of this region during 1993–2010, using in situ data, satellite observations, and a model simulation. We find that OHC increases significantly in the region during this period relative to the north Indian Ocean, and propose that this increase could have caused the decrease in Indian Summer Monsoon Rainfall that occurred at the same time