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In a numerical model of the equatorial Pacific Ocean, the ∼20-day period tropical instability waves, excited in the eastern half of the domain, are found to damp the strong zonal mean currents. The waves generate large, nonlinear, advection terms in the momentum balance, change the vorticity balance, and thus modulate the low-frequency state. The authors explore whether the effect of tropical instability waves on the background flow can instead be adequately parameterized by a constant-coefficient Laplacian friction scheme. On annual mean, a Laplacian friction coefficient that varies in space is required, for the coefficient is twice as large along the equator and a few degrees more to the north than elsewhere. In addition, wave activity varies in time. During active phases, such as the second half of the year and during La Niñas, the activity increases, which would require the Laplacian coefficient of friction to be at least twice as strong as during the inactive phases. Thus, a more sophisticated damping parameterization than simple Laplacian friction is required in ocean models that do not explicitly resolve tropical instability waves. [ABSTRACT FROM AUTHOR]

Brown et al. analyzed the kinematics of flow in the equatorial Pacific Ocean, along time-varying isopycnals in a three-dimensional eddy-permitting model. Here the dynamics of these flows is explored in the same model via the zonal momentum equation (ZME). Previous work has shown that the dominant terms of the ZME, on and near the equator, are the pressure gradient, wind stress, and Coriolis term. In one model study, the nonlinear and friction terms were significant but negated each other. In this study, with a higher-resolution model and more realistic friction scheme it is shown that the nonlinear term is important along and north of the equator, while the explicit friction term is negligible. The part of the nonlinear term derived from high-frequency eddy flows acts like a friction on the Equatorial Undercurrent, while the remaining part of the nonlinear term from smooth flows enhances it. In density coordinates, meridional tropical cells lie on either side of the equator in the first half of the year (January–June) as expected. In July–December, a continuous southward surface flow appears from 4°N into the Southern Hemisphere and arises from variations in the geostrophic flow and the nonlinear term. Variations in the geostrophic flow are due to both seasonal variability in the thermocline and a surface bolus effect arising from baroclinic instability. The nonlinear term increases in the surface layers at the same time assisting the southward flow, most likely because of tropical instability waves. [ABSTRACT FROM AUTHOR]

Present-day OGCMs give low values of annual mean net heat flux (AMNHF) in the tropical Indian Ocean, compared to climatologies. AMNHF generation is examined in an open-boundary model of this region with realistic coastlines. In the first two of three experiments only annual mean wind stresses were applied so that a modified form of the “minimum depth criterion” of the previous paper would be applicable. Area-integrated AMNHF was well below observed values, despite the fact that western boundary inflow was substantially deeper and colder than was expected from the modified minimum depth estimate. The model showed large “spikes” in the gradient of “depth-integrated steric height” (DISH) along the western boundary, coinciding with coastline steps (which were absent in the previous paper). Most diapycnal entrainment occurred next to the coast, near these steps. In a third experiment a seasonal cycle of wind stress was added to the same annual mean. Annual mean diapycnal mixing and entrainment increased and the western boundary inflow deepened, resulting in substantially greater AMNHF for the same annual mean Ekman transports. However, area-integrated AMNHF was still well below the mean of directly observed surface fluxes. The recirculation around the “Great Whirl” doubled, permitting more cold water crossing the equator in one year to mix with recirculated water generated in a previous year. Entrainment up to the surface thus went by stages, over more than one year. The increased Great Whirl was related to stronger annual mean curls of nonlinear terms in the momentum equation, while the deeper entrainment was caused by stronger annual mean diapycnal mixing. In all experiments, diapycnal mixing was primarily due to the “flux corrected transport” (FCT) advective scheme, which in effect replaces spurious convective overturn by numerical diffusion. More research is needed to solve such problems, but sensitivity of AMNHF in OGCMs to time-varying forcing—due to seasonal, intraseasonal, or baroclinic instability—may offer a new source of climate predictability. [ABSTRACT FROM AUTHOR]

Annual mean net heat fluxes from ocean general circulation models (OGCMs) are systematically too low in the tropical Indian Ocean, compared to observations. In the models, only some of the geostrophic inflow replacing southward Ekman outflow is colder than the minimum sea surface temperature (MINSST). Observed heat fluxes imply that much more inflow is colder than MINSST. Since inflow below MINSST can only join the surface Ekman transport after diathermal warming, the OGCMs must underestimate diathermal effects. A crude analog of the annual mean Indian Ocean heat budget was generated, using a rectangular box model with a deep “Indo–Pacific” gap at 7°–10°S in its eastern side. Wind stress was zonal and proportional to the Coriolis parameter, so Ekman transport was spatially constant and equaled Sverdrup transport. For three experiments, zonally integrated Ekman transport was steady and southward at 10 Sv (Sv ≡ 106 m3 s-1). In steady state, a 10 Sv “Indonesian Throughflow” fed a northward western boundary current of 10 Sv, which turned eastward along the northern boundary at 10°N to feed the southward Ekman transport. Most diathermal mixing occurred within an intense eddy in the northwest corner. Some of the geostrophic inflow was at temperatures colder than MINSST (found at the northeast corner of the eddy); it must warm to MINSST via diathermal mixing. Northern boundary upwelling exceeded the 10-Sv Ekman transport. The excess warms as it recirculates around the eddy, apparently supplying the heat to warm inflow below MINSST. In an experiment using the “flux-corrected transport” (FCT) scheme, diathermal mixing occurred in the strongly sheared currents around the eddy. However the Richardson number never became low enough to drive strong diathermal mixing, perhaps because (like that of other published models) the present model’s vertical resolution was too coarse. In three experiments, the dominant mixing was caused by horizontal diffusion, spurious convective overturn, and numerical mixing invoked by the FCT scheme, respectively. All three mixing mechanisms are physically suspect; such model problems (if widespread) must be resolved before the mismatch between observed and modeled heat fluxes can be addressed. However, the fact that the density profile at the western boundary must be hydrostatically stable places a lower limit on the area-integrated heat fluxes. Results from the three main experiments—and from many published OGCMs—are quite close to this lower limit. [ABSTRACT FROM AUTHOR]

An eddy-permitting global ocean model is used to interpret kinematics within the central and eastern equatorial Pacific Ocean, from 160°E to the coast of America. Because of high levels of variability in this region, observational studies of meridional flow are contradictory, in particular as to whether the net flow is northward or southward. Unlike most oceanographic datasets, model output can be analyzed at high temporal and spatial resolution, providing clues as to real ocean behavior. In the model, a net southward flow occurs across the equator east of 160°W, at most density layers throughout the year. In the central Pacific, from 160°E to 160°W, the net flow is northward but varies with season and occurs primarily in the mixed layer. This is a key region for the flow of Equatorial Undercurrent water into the Northern Hemisphere. The three-dimensional flow is very complex and seasonally dependent. It is vital that these flows are analyzed in an isopycnal framework, or else the pathways are very misleading. In the first half of the year, evidence is found of meridional tropical cells on either side of the equator out to ±5°. These cells appear to exist without any need for diapycnal downwelling. In the second half of the year, when tropical instability waves are active, the cells are overlaid by a strong surface southward flow that appears to be a bolus-type transport. This transport is not apparent unless the flow is calculated in the aforementioned manner. [ABSTRACT FROM AUTHOR]

Twenty years of monthly or more frequent repeat expendable bathythermograph data are used to estimate the mean geostrophic velocity and transport relative to 750 m of the Indonesian Throughflow (ITF) and its partitioning through the major outflow straits into the Indian Ocean. Ekman transports are estimated from satellite and atmospheric reanalysis wind climatologies. A subsurface maximum near 100 m characterizes the geostrophic ITF, but Ekman flows drive a warm near-surface component as well. A subsurface intensified fresh Makassar Jet feeds the Lombok Strait Throughflow (∼2 Sv; 1Sv ≡ 106 m3 s-1) and an eastward flow along the Nusa Tenggara island chain [the Nusa Tenggara Current (6 Sv)]. This flow feeds a relatively cold 3.0-Sv flow through the Ombai Strait and Savu Sea. About 4–5 Sv pass through Timor Passage, fed by both the Nusa Tenggara Current and likely warmer and saltier flow from the eastern Banda Sea. The Ombai and Timor Throughflow feature distinctly different shear profiles; Ombai has deep-reaching shear with a subsurface velocity maximum near 150 m and so is cold (∼15.5°–17.1°C), while Timor Passage has a surface intensified flow and is warm (∼21.6°–23°C). At the western end of Timor Passage the nascent South Equatorial Current is augmented by recirculation from a strong eastward shallow flow south of the passage. South of the western tip of Java are two mean eastward flows—the very shallow, warm, and fresh South Java Current and a cold salty South Java Undercurrent. These, along with the inflow of the Eastern Gyral Current, recirculate to augment the South Equatorial Current, and greatly increase its salinity compared to that at the outflow passages. The best estimate of the 20-yr-average geostrophic plus Ekman transport is 8.9 ± 1.7 Sv with a transport-weighted temperature of 21.2°C and transport-weighted salinity of 34.73 near 110°E. The warm temperatures of the flow can be reconciled with the much cooler estimates based on mooring data in Makassar Strait by accounting for an unmeasured barotropic and deep component, and local surface heat fluxes that warm the ITF by 2°–4°C during its passage through the region. [ABSTRACT FROM AUTHOR]