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Julian P. McCreary Jr.

Abstract

A model is used to study ocean-atmosphere interaction in the tropics. The model ocean consists of the single baroclinic mode of a two-layer ocean. Thermodynamics in the upper layer is highly parameterized. If the interface is sufficiently shallow (deep), sea surface temperature is cool (warm). The model atmosphere consists of two wind states that interact with the ocean according to the ideas of Bjerknes. When the eastern ocean is cool, the trade winds expand equatorward in the central Pacific, simulating an enhanced Walker circulation (WC). When the eastern ocean is warm, the trade winds expand eastward, simulating an enhanced Walker circulation (WC) there. For reasonable choices of parameters, the model oscillates at all time scales associated with the Southern Oscillation.

The WC has positive feedback with the ocean. This interaction generates persistence, and thereby makes it possible for solutions to oscillate at long time scales. Interaction of the HC with the ocean prevents the model from ever reaching, an equilibrium state. Wind curl associated with the HC generates a Rossby wave in the subtropics. It is the travel time of this wave across the basin that sets the oscillation period of the model.

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Shenn-Yu Chao and Julian P. McCreary Jr.

Abstract

A numerical model is used to study the interaction of the Kuroshio with boundary constraints south of Japan. The model consists of two layers with the lower layer inert, and is both nonlinear and quasi-geostrophic. The boundary constraints are highly idealized. The Kyushu Peninsula and its adjacent continental shelf have the form of a V-shaped wedge on an otherwise rectilinear coastline; the Izu Ridge is represented as a square island. Solutions almost always form a meander downstream from Kyushu. The dynamics of meander formation involve the interaction of a Rossby lee wave generated by the Kyushu Peninsula and a westward disturbance forced by the Izu Ridge.

Three different quasi-steady paths have been identified. The first path has a meander between Kyushu and the Izu Ridge, passes to the north of the ridge, and is associated with a current of low-volume transport. The second path has a large meander between Kyushu and the Izu Ridge, passes to the south of the Izu Ridge, and always occurs when the volume transport is sufficiently large. The third path primarily meanders downstream of the Izu Ridge, and occurs for intermediate values of transport. Deceleration of the current decreases the wavelength of the meander and markedly increases its amplitude; conversely, acceleration increases. the meander wavelength and decreases its amplitude. The model does not possess a steady-state path without any meander; such a path only occurs during periods of intense acceleration.

For intermediate values of transport the quasi-steady path selected by the model does not depend on transport alone, but depends on the nature of the spin-up of the solution as well. For example, when the current is turned on abruptly, the second path develops; when the current is turned on gradually, the first path develops. This behavior indicates that the model possesses multiple states of equilibrium.

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Pijush K. Kundu and Julian P. McCreary Jr.

Abstract

The circulation forced by an inflow of water through an eastern ocean boundary is investigated using two linear, viscid, and continuously stratified models. One of the models has a flat bottom, and solutions are obtained analytically; the other has a continental shelf, and solutions are found numerically. Without vertical mixing all the inflow continues across the ocean. With vertical mixing, however, part of it bends poleward to generate a coastal circulation. The presence of a shelf displaces the coastal currents offshore, but otherwise changes their structure and magnitude very little. Solutions suggest that the southward bending of the throughflow from the Pacific into the Indian Ocean may contribute to the Leeuwin Current off western Australia, but that it is not the dominant mechanism for driving the circulation there.

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Julian P. McCreary Jr. and Peng Lu

Abstract

A 4½ layer model is used to study intermediate-water circulation in the Pacific Ocean. Solutions are forced by annual-mean winds. They are also driven by a prescribed inflow of water through the southwestern corner of the basin [12 Sv (Sv ≡ 106 m3 s−1)] and a compensating outflow in layers 1, 2, and 3 through the western boundary just north of the equator; this mass exchange simulates the Pacific interocean circulation (IOC), in which intermediate water enters the South Pacific, and the same amount of upper water exits via the Indonesian passages. The water in each subsurface layer is formed by specific processes, and hence can be interpreted as corresponding to a distinct water-mass type. The types are thermocline water generated by subtropical subduction (layer 2), upper-intermediate and lower-thermocline water generated by midlatitude subduction in the North and South Pacific (NPIW and SPLTW, respectively; layer 3), and lower-intermediate water that corresponds to Antarctic Intermediate Water (AAIW; layer 4).

Solutions are compared for different throughflow vertical structures and without the IOC. For all solutions, the amount of NPIW that moves into the Tropics is almost unchanged (∼4 Sv), indicating that it is remotely determined by midlatitude processes. If the layer 3 outflow is sufficiently large (≳4 Sv), most of the tropical NPIW exits the basin in the throughflow, and SPLTW fills the northern tropical ocean, consistent with the observed circulation. If it is small, however, most of the tropical NPIW recirculates in the northern Tropics, and no SPLTW enters this region. With the IOC, AAIW crosses into the Northern Hemisphere (3.8 Sv), and more than half eventually moves into subpolar ocean. Without the IOC, the transport of AAIW into the Northern Hemisphere is an order of magnitude less (0.37 Sv), and NPIW occupies most of the tropical ocean in both hemispheres, properties that differ markedly from observations. These results suggest that the Indonesian Throughflow is a possible reason why intermediate waters of Southern Hemisphere origin fill the tropical Pacific and spread to higher northern latitudes.

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Peng Lu and Julian P. McCreary Jr.

Abstract

The flow of thermocline water from the subtropical to the equatorial Pacific Ocean is investigated using a 2½-layer numerical model. In this system, the lower of the two active layers represents the thermocline region of the ocean, and the upper layer simulates the near-surface region including the mixed layer. Water is allowed to move between the layers via an across-interface velocity that parameterizes the processes of upwelling and subtropical subduction. Solutions are obtained in a basin that resembles the Pacific basin, and they are forced by Hellerman and Rosenstein winds.

The primary result is that the intertropical convergence zone (ITCZ) creates a potential vorticity barrier that inhibits the direct flow of lower-layer (thermocline) water from the subtropical North Pacific to the equator. Lower-layer water must first flow to the western boundary north of the ITCZ and only then can it move equatorward in a western boundary current to join the Equatorial Undercurrent. Another result is that there is a convergence of lower-layer water onto the equator in the central ocean; however, it is associated with a pair of lower-layer recirculation cells confined within 5° of the equator and is not part of the large-scale circulations that carry subtropical water to the equator (the north and south subtropical cells).

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Julian P. McCreary Jr. and Peng Lu

Abstract

Three versions of a 2 1/2-layer ocean model are used to study the subtropical cell (STC), a shallow, meridionalcirculation cell consisting of subtropical subduetion, equatorward advection of cool subsurface water into thetropics, upwelling at the equator, and poleward advection of warm surface water back to midlatitudes. Thethree versions are a steady-state analytic model, a numerical model with constant layer temperatures, and anumerical model with variable layer temperatures and active thermodynamics. Two different pammeterizationsof mixed-layer processes are utilized to determine how water moves between the lwo layers. In the simplerparameterization, entrainment and detrainment rates, we and wd, are specified so that the upper-layer thicknessh1 relaxes back to an externally prescribed thickness; in the other, they are related to the surface heat flux Q.in both versions detrainment is cut off at the latitude yd = 18° to prevent subduction from occurring in thetropics. Solutions are obtained in a rectangular basin that is symmetric about the equator. They are forced byidealized representations of observed zonal wind stress τx and Q fields, the latter used only for the thermodynamicmodel.

The analytic solution provides a comprehensive, three-dimensional description ofthe STC and illustrates itsfundamental dynamics. First, it indicates that the strength of the STC depends only on the wind stress τx andCoriolis force f at the latitude yd; it is not related to the Ekman pumping velocity (τx/f)y over the subtropicalocean or to the strength of the equatorial wind field. Thus, the amount of subtropical water that upwells in thetropics is remotely forced by processes outside the tropics (along yd). Second, two types of water contributeroughly equally to the STC: unventilated water from the lower-layer western boundary current and ventilatedwater subducted in the subtropical ocean. Third, an internally determined streamline xe(y) determines whetherthe subtropical water approaches the equator entirely in the western boundary current or partly through theinterior ocean. Fourth, another streamline xb(y) defines the western edge of the equatorward branch of the STCand thereby determines the latitude at which the westward lower-layer flow bifurcates at the western boundary. Solutions to the constant-temperature numerical model corroborate the analytic results and illustrate thenature of boundary layers. Among other things, they demonstrate that the equatorial circulation is sensitive tothe equatorial wind through its influence on the location of tropical upwelling field we; in our control run forcedwith equatorial easterly winds, we occurs on the equator in the eastern ocean, and the lower-layer flow fielddevelops an equatorial undercurrent (EUC); in test solutions forced without equatorial winds, we, exists in anoff-equatorial band across the interior ocean and there is no EUC. It follows that local forcing by equatorialwinds is required for the existence of equatorial upwelling and the EUC in the control run.

In the solution to, the thermodynamic model, the circulation is similar to that in the control run, except thath1 deepens markedly north of the line where Q changes sign to become negative. As a result, the total subductionin the subtropics increases by a factor of 2.1, and the source of all the water that contributes to the equatorwardbranch of the STC is subtropical subduction. In the tropics, the lower-layer temperature is maintained at a coolvalue by adveetion associated with the STC.

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Zuojun Yu, Julian P. McCreary Jr., and Jeffrey A. Proehl

Abstract

One of the striking features of tropical instability waves (TIWs) is that they appear to be more prominent north of the equator. A linearized, 2½-layer ocean model is used to investigate effects of various asymmetric background states on structures of equatorial, unstable waves. Our results suggest that the meridional asymmetry of TIWs is due to asymmetries of the two branches of the South Equatorial Current (SEC) and of the equatorial, sea surface temperature front; it is not due to the presence of the North Equatorial Countercurrent. Energetics analyses indicate that frontal instability associated with the equatorial, SST front, as well as barotropic instability due to shear associated with the SEC, are energy sources for the model TIWS.

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Masami Nonaka, Julian P. McCreary Jr., and Shang-Ping Xie

Abstract

The stratification of the equatorial thermocline is a key variable for tropical climate dynamics, through its influence on the temperature of the water that upwells in the eastern equatorial ocean. In this study, two types of ocean models are used, an ocean general circulation model (GCM) and a 1½-layer model, to investigate processes by which changes in the midlatitude winds affect the equatorial stratification. Specifically, the influences of anomalous mode-water formation, Ekman pumping, and entrainment in the subpolar ocean are examined. The effects of a “sponge layer” adjacent to the northern boundary of the basin are also assessed. Solutions are forced by idealized zonal winds with strong or weak midlatitude westerlies, and they are found in rectangular basins that extend from the equator to 36°N (small basin) or to 60°N (large basin). In the GCM solutions, a prominent response to reduced winds is the thinning of the mixed layer in the northwestern region of the subtropical gyre, leading to less subduction of low-potential-vorticity mode water and hence thinning of the upper thermocline in the central-to-eastern subtropics. Almost all of this thinning signal, however, recirculates within the subtropics, and does not extend to the equator. Another midlatitude response is shallowing (deepening) of the thermocline in the subtropical (subpolar) ocean in response to Ekman pumping. This, primarily, first-baroclinic-mode (n = 1) response has the most influence on the equatorial thermocline. First-baroclinic-mode Rossby waves propagate to the western boundary of the basin where they reflect as packets of coastal Kelvin and short-wavelength Rossby waves that carry the midlatitude signal to the equator. Subsequently, equatorial Kelvin waves spread it along the equator, leading to a shoaling and thinning of the equatorial thermocline. The layer-thickness field h in the 1½-layer model corresponds to thermocline depth in the GCM. Both the sponge layer and subpolar Ekman suction are important factors for the 1½-layer model solutions, requiring water upwelled in the interior ocean to be transported into the sponge layer via the western boundary layer. In the small basin, equatorial h thins in response to weakened westerlies when there is a sponge layer, but it thickens when there is not. In the large basin, equatorial h is unaffected by weakened westerlies when there is a sponge layer, but it thins when water is allowed to entrain into the layer in the subpolar gyre. It is concluded that the thinning of the equatorial thermocline in the GCM solutions is caused by the sponge layer in the small basin and by entrainment in the subpolar ocean in the large one.

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Ryo Furue, Julian P. McCreary Jr., and Zuojun Yu

Abstract

The Tsuchiya jets (TJs) are narrow eastward currents located along thermal fronts at the poleward edges of thermostad water in the Pacific Ocean. In this study, an oceanic general circulation model (OGCM) is used to explore the dynamics of the northern TJ. Solutions are found in a rectangular basin, extending 100° zonally and from 40°S to 40°N. They are forced by three idealized forcings: several patches of idealized wind fields, including one that simulates the strong Ekman pumping region in the vicinity of the Costa Rica Dome (CRD); surface heating that warms the ocean in the tropics; and a prescribed interocean circulation (IOC) that enters the basin through the southern boundary and exits through the western boundary from 2° to 6°N (the model’s Indonesian passages).

Solutions forced by all the aforementioned processes and with minimal diffusion resemble the observed flow field in the tropical North Pacific. A narrow eastward current, the model’s northern TJ, flows across the basin along the northern edge of a thick equatorial thermostad. Part of the TJ water upwells at the CRD upwelling region and the rest returns westward in the lower part of the North Equatorial Current. The deeper part of the TJ is supplied by water that leaves the western boundary current somewhat north of the equator. Its shallower part originates from water that diverges from the deep portion of the Equatorial Undercurrent (EUC); as a result, the TJ transport increases to the east and the TJ warms as it flows across the basin. These and other properties suggest that the dynamics of the model’s TJ are those of an arrested front, which in a 2½-layer model are generated when characteristics of the flow converge strongly or intersect.

Eddy form stress, due to instability waves generated at the CRD region, extends the TJ circulation to deeper levels. When diffusivity is increased to commonly used values, the thermostad is less well defined and the TJ is weak. In a solution without the IOC, the TJ is shifted to higher temperatures with its water supplied by the subtropical cell. When horizontal viscosity is reduced, the TJ becomes narrower and is flanked by a westward current on its equatorward side.

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David L. T. Anderson and Julian P. McCreary Jr.

Abstract

The coupled ocean-atmosphere model of Anderson and McCreary is extended to include two oceans. An advantage of the two-ocean system is that it is not necessary to specify externally convection over land.

For a basin geometry that most resembles the Indian and Pacific Oceans, strong permanent convection exists in the eastern Indian Ocean, and there is an oscillation in the Pacific Ocean with a period of about five years. Associated with this oscillation is a patch of convection that develops in the central and western ocean and propagates into the eastern ocean before dissipating.

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