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Lewis M. Rothstein

Abstract

A linear, continuously stratified analytical model of the equatorial ocean is developed and used to simulate the major features of the equatorial sea surface temperature (SST) field and current system. The model is bounded by meridional barriers to flow in the east and west and is forced by an idealized southeast trade wind field. The steady state response is studied here, although the model is time periodic and capable of simulating the annual cycle or a more general time dependence. The surface heat flux forcing is set to zero so that the contribution of the equatorial upwelling field in maintaining the cool equatorial surface water can be isolated.

Many of the observed features of the equatorial SST field and current system are simulated. The model generates cool SST anomalies an the equator in the eastern half of the basin. The cool water core is located to the south of the equator in the easternmost regions of the basin, with the coldest water along the eastern boundary. The latitudinal and longitudinal scales of the model SST anomaly agree with observations. The latitudinal scales are established by a linear combination of the equatorial deformation radii of all model baroclinic modes and not just the first few modes. A southerly wind that crosses the equator along the eastern boundary is necessary for proper simulation of the equatorial SST field.

The model current structure is familiar. The Equatorial Undercurrent is geostrophic and symmetric about the equator in the center of the basin. It is situated between a surface westward flow and deep westward flow. The eastward volume transport associated with the Undercurrent moves southward in the eastern part of the basin and at some depths, the Equatorial and Coastal Undercurrents are continuous. Boundary effects in the eastern region force the Equatorial Undercurrent to bifurcate at a distance of approximately 100 km from the coast. The southerly core is continuous with the model generated Coastal Undercurrent. All current magnitudes agree reasonably well with observational estimates.

The solution techniques are the standard separation of variables but in order to incorporate a heat flux surface boundary condition, special methods are employed. We have termed these “inverse methods” as we need to eventually solve for a boundary condition (the SST anomaly field).

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Kate Qian Zhang
and
Lewis M. Rothstein

Abstract

A 3D tropical upper-ocean circulation model is employed to study the generation mechanism for subsurface reversing currents forced by strong westerly wind bursts in the western equatorial Pacific. The westerly wind bursts last from a few days to a few weeks and reverse the surface current from westward to eastward, setting up a zonal pressure gradient that generates a westward subsurface current. Although less affected by the local wind, the eastward Equatorial Undercurrent (EUC) decelerates. The authors verified the hypothesis that the subsurface reversal is a local response to westerly wind bursts. However, the model response is very sensitive to the wind fetch. The observed reversal of the subsurface current can only be reproduced by wind bursts with a zonal extent of less than 700 km. This is because the zonal extent determines the timescale on which the pressure gradient is set up by equatorial waves. This timescale must be shorter than the timescales of vertical mixing and downwelling in order for the pressure-driven subsurface westward acceleration to overtake the eastward acceleration due to downward transport of momentum. The results emphasize the importance of resolving spatial variations in simulating the upper ocean response to atmospheric forcing.

The influence of off-equator winds on equatorial currents is also investigated. It is found that the remote effect of an off-equator wind can be larger on subsurface currents than on surface currents. An off-equator westerly (easterly) wind decelerates (accelerates) the EUC but has little effect on the surface equatorial current. When a cyclone with a westerly wind at the equator and an easterly off-equator wind is present, the local response near the surface is dominated by the westerly wind. But the remote effect of the off-equator wind significantly modifies the local wind effect at the depth of the EUC.

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Qianqian Liu
,
Lewis M. Rothstein
, and
Yiyong Luo

Abstract

Buoyant discharge of freshwater from Long Island Sound (LIS) forms a seasonal buoyant plume outside Block Island Sound (BIS) between the coast of Long Island and the denser shelf waters. The plume’s seasonal variability and its response to tides, winds, and surface heating are investigated through a series of process-oriented experiments using the Regional Ocean Modeling System (ROMS). Results show the importance of river discharge, wind directions, and surface heating in the seasonal variation of the BIS buoyant plume. In winter and spring, the plume is intermediate with a large surface offshore extension detached from the bottom. From winter to spring, the river discharge increases; meanwhile, upwelling-favorable winds keep dominating. They compete with the increase of surface heating and generate a broader buoyant plume in spring than in winter. In summer, the plume is bottom advected with most of its width in contact with the bottom and is featured with the steepest isopycnals and narrowest plume, which is driven by a combination of strong insolation, weak buoyant discharge from LIS, and feeble winds. In fall, although the river discharge is comparable to that in winter, the upwelling-favorable wind is relatively weaker, corresponding to a narrower intermediate plume.

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Regina R. Rodrigues
,
Lewis M. Rothstein
, and
Mark Wimbush

Abstract

In this study, a reduced-gravity, primitive equation OGCM is used to investigate the seasonal variability of the bifurcation of the South Equatorial Current (SEC) into the Brazil Current (BC) to the south and the North Brazil Undercurrent/Current (NBUC/NBC) system to the north. Annual mean meridional velocity averaged within a 2° longitude band off the South American coast shows that the SEC bifurcation occurs at about 10°–14°S near the surface, shifting poleward with increasing depth, reaching 27°S at 1000 m, in both observations and model. The bifurcation latitude reaches its southernmost position in July (∼17°S in the top 200 m) and its northernmost position in November (∼13°S in the top 200 m). The model results show that most of the seasonal variability of the bifurcation latitude in the upper thermocline is associated with changes in the local wind stress curl due to the annual north–south excursion of the marine ITCZ complex. As the SEC bifurcation latitude moves south (north) the NBUC transport increases (decreases) and the BC transport decreases (increases). The remote forcing (i.e., westward propagation of anomalies) appears to have a smaller impact on the seasonal variability of the bifurcation in the upper thermocline.

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Raymond A. Richardson
,
Isaac Ginis
, and
Lewis M. Rothstein

Abstract

Numerical simulations of the local equatorial ocean response to idealized westerly wind burst (WWB) forcing are described. In particular, the authors examine the development and evolution of the subsurface westward jet (SSWJ) that has been observed to accompany these wind events. This westward current is interpreted as the signature of equatorial waves that accompany the downwelling and upwelling that occurs along the edges of the wind forcing region. Some important features of the SSWJ include maximum intensity toward the eastern edge of the forcing region, a time lag between the wind forcing and peak SSWJ development, and an eastward spreading of the SSWJ with time. The effect of wind burst zonal profile, magnitude, duration, and fetch on the SSWJ are explored. The response of an initially resting ocean to WWB forcing is compared with that for model oceans that are spun up with annual-mean surface fluxes and monthly varying fluxes. It is demonstrated that the gross features of the response for the spun up simulations can be well approximated by adding the background zonal current structure prior to the introduction of the wind burst to the initially resting ocean current response to the WWB. This result suggests that the zonal current structure that is present prior to the commencement of WWB forcing plays a key role in determining whether or not a SSWJ will develop.

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William S. Kessler
,
Lewis M. Rothstein
, and
Dake Chen

Abstract

The annual onset of the east Pacific cold tongue is diagnosed in an ocean GCM simulation of the tropical Pacific. The model uses a mixed-layer scheme that explicitly simulates the processes of vertical exchange of heat and momentum with the deeper layers of the ocean; comparison with observations of temperature and currents shows that many important aspects of the model fields are realistic. As previous studies have found, the heat balance in the eastern tropical Pacific is notoriously complicated, and virtually every term in the balance plays a significant role at one time or another. However, despite many complications, the three-dimensional ocean advection terms in the cold tongue region tend to cancel each other in the annual cycle and, to first order, the variation of SST can be described as simply following the variation of net solar radiation at the sea surface (sun minus clouds). The cancellation is primarily between cooling due to equatorial upwelling and warming due to tropical instability waves, both of which are strongest in the second half of the year (when the winds are stronger). Even near the equator, where the ocean advection is relatively intense, the terms associated with cloudiness variations are among the largest contributions to the SST balance. The annual cycle of cloudiness transforms the semiannual solar cycle at the top of the atmosphere into a largely 1 cycle yr−1 variation of insolation at the sea surface. However, the annual cycle of cloudiness appears closely tied to SST in coupled feedbacks (positive for low stratus decks and negative for deep cumulus convection), so the annual cycle of SST cannot be fully diagnosed in an ocean-only modeling context as in the present study. Zonal advection was found to be a relatively small influence on annual equatorial cold tongue variations; in particular, there was little direct (oceanic) connection between the Peru coastal upwelling and equatorial annual cycles. Meridional advection driven by cross-equatorial winds has been conjectured as a key factor leading to the onset of the cold tongue. The results suggest that the SST changes due to this mechanism are modest, and if meridional advection is in fact a major influence, then it must be through interaction with another process (such as a coupled feedback with stratus cloudiness). At present, it is not possible to evaluate this feedback quantitatively.

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Dake Chen
,
Lewis M. Rothstein
, and
Antonio J. Busalacchi

Abstract

A novel hybrid vertical mixing scheme, based jointly on the Kraus–Turner-type mixed layer model and Price's dynamic instability model, is introduced to aid in parameterization of vertical turbulent mixing in numerical ocean models. The scheme is computationally efficient and is capable of simulating the three major mechanisms of vertical turbulent mixing in the upper ocean, that is, wind stirring, shear instability, and convective overturning.

The hybrid scheme is first tested in a one-dimensional model against the Kraus–Turner-type bulk mixed layer model and the Mellor–Yamada level 2.5 (MY2.5) turbulence closure model. As compared with those two models, the hybrid model behaves more reasonably in both idealized experiments and realistic simulations. The improved behavior of the hybrid model can be attributed to its more complete physics. For example, the MY2.5 model underpredicts mixed layer depth at high latitudes due to its lack of wind stirring and penetrative convection, while the Kraus–Turner bulk model produces rather shallow mixed layers in the equatorial region because of its lack of shear-produced mixing. The hybrid model reproduces the good results of the MY2.5 model toward the equator and the bulk model toward high latitudes, thereby taking the advantages of those two models while avoiding their shortcomings.

The hybrid scheme is then implemented in a three-dimensional model of the tropical Pacific Ocean. This leads to an improved simulation of the large-scale equatorial circulation. Compared with the other two commonly used mixing schemes tested in this experiment, the hybrid scheme helps to produce more realistic velocity profiles in the eastern and central equatorial Pacific. This is mainly due to the improved parameterization of interior mixing related to the large shears of the Equatorial Undercurrent. Another feature in this model that is sensitive to the vertical mixing scheme is the equatorial instability waves; in the eastern Pacific Ocean these waves are most energetic when the hybrid scheme is used. The meridional heat flux associated with these waves can be locally important in the mixed layer heat budget.

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Dailin Wang
,
Dennis W. Moore
, and
Lewis M. Rothstein

Abstract

Exact solutions are found for Kawase's linear, two-layer model of mass-driven deep-ocean circulation. It is demonstrated that for strong damping, even though the deep western boundary current (DWBC) bifurcates at the equator as found in Kawase's perturbation solution, the equatorial flow is much weaker and has a much broader scale than the imposed DWBC. It is found that the presence of an eastern boundary is not necessary for the DWBC to cross the equator in the weak damping case.

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Lewis M. Rothstein
,
Michael J. McPhaden
, and
Jeffrey A. Proehl

Abstract

A numerical model is designed to study the effects of the strong, near-surface associated with the equatorial current system on energy transmission of time-periodic equatorial waves into the deep mean. The present paper is confined to long wavelength, low-frequency Kelvin waves forced by a longitudinally confined patch of zonal wind. Energy transmission into the deep ocean is investigated as a function of mean current shear amplitude and geometry and the forcing frequency.

Solutions form well-defined beams of energy that radiate energy eastward and vertically toward the deep ocean in the absence of mean flow. However, the presence of critical surfaces associated with mean currents inhibits low-frequency energy from reaching the deep ocean. For a given zonal wavenumber, longitudinal propagation through mean currents will be less inhibited as the frequency increases (phase speed increases). When the mean current amplitude is large enough, the beam encounters multiple critical surfaces (i.e., critical surfaces for different wavenumber components of the beam) where significant and momentum can take place with the men currents via Reynolds stress transfers. Work against the dominant vertical shear is the dominant wave energy loss for the case of a mean South Equatorial Current–Equatorial Undercurrent system, illustrating the need for high vertical resolution in equatorial ocean models.

The model also describes the possible induction of a mean zonal acceleration as well as a mean meridional circulation. Eliassen-Palm fluxes are used to diagnose these dynamics. The presence of critical surfaces result in mean field accelerations on the equator above the core of the Equatorial Undercurrent. Implications of these results with regard to observations in the equatorial waveguide are discussed.

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Lewis M. Rothstein
,
Dennis W. Moore
, and
Julian P. McCreary

Abstract

A fully three-dimensional, wind-forced equatorial model is used to study the effects of the strong near- surface equatorial pycnocline on energy transmission into the deep ocean. The equatorial Kelvin waves forced by a patch of zonal wind oscillating at the annual period are isolated from the complete response, and their energy transmission into the deep ocean is investigated as a function of forcing geometry, pycnocline structure, and the amplitude of deep-ocean mixing. Solutions form well-defined beams of energy that propagate through realistic pycnoclines with surprisingly little reflection. Vertical mixing damps the beams in the direction of their propagation and stretches their longitudinal extent. For sufficiently strong mixing the solutions low their beamline character and appear as surface-trapped signals. This result may help to resolve the differences between the solutions found in previous investigations.

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