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Horizontal fluxes of heat and other scalar quantities in the ocean are due to correlations between the horizontal velocity and tracer fields. However, the limited spatial resolution of ocean models means that these correlations are not fully resolved using the velocity and temperature evaluated on the model grid, due to the limited spatial resolution and the boxcar-averaged nature of the velocity and the scalar field. In this article, a method of estimating the horizontal flux due to unresolved spatial correlations is proposed, based on the depth-integrated horizontal transport from the seafloor to the density surface whose spatially averaged height is the height of the calculation. This depth-integrated horizontal transport takes into account the subgrid velocity and density variations to compensate the standard estimate of horizontal transport based on staircase-like velocity and density. It is not a parameterization of unresolved eddies, since it utilizes data available in ocean models without relying on any presumed parameter such as diffusivity. The method is termed the horizontal residual mean (HRM). The method is capable of estimating the spatial-correlation-induced water transport in a 1/48 global ocean model, using model data smoothed to 3/48. The HRMextra overturning has a peak in the Southern Ocean of about 1.5 Sv (1 Sv = 106 m² s-1). This indicates an extra heat transport of 0.015 PW on average in the same area. It is expected that implementing the scheme in a coarse-resolution ocean model will improve its representation of lateral heat fluxes. [ABSTRACT FROM AUTHOR]

We investigate the processes responsible for the intraseasonal displacements of the eastern edge of the western Pacific warm pool (WPEE), which appear to play a role in the onset and development of El Niño events. We use 25 years of output from an ocean general circulation model experiment that is able to accurately capture the observed displacements of the WPEE, sea level anomalies, and upper ocean zonal currents at intraseasonal time scales in the western and central Pacific Ocean. Our results confirm that WPEE displacements driven by westerly wind events (WWEs) are largely controlled by zonal advection. This paper has also two novel findings: first, the zonal current anomalies responsible for the WPEE advection are driven primarily by local wind stress anomalies and not by intraseasonal wind-forced Kelvin waves as has been shown in most previous studies. Second, we find that intraseasonal WPEE fluctuations that are not related to WWEs are generally caused by intraseasonal variations in net heat flux, in contrast to interannual WPEE displacements that are largely driven by zonal advection. This study hence raises an interesting question: can surface heat flux-induced zonal WPEE motions contribute to El Niño-Southern Oscillation evolution, as WWEs have been shown to be able to do? [ABSTRACT FROM AUTHOR]

Pathways of wind-power input into the ocean general circulation are analyzed using Ekman theory. Direct rates of wind work can be calculated through the wind stress acting on the surface geostrophic flow. However, because that energy is transported laterally in the Ekman layer, the injection into the geostrophic interior is actually controlled by Ekman pumping, with a pattern determined by the wind curl rather than the wind itself. Regions of power injection into the geostrophic interior are thus generally shifted poleward compared to regions of direct wind-power input, most notably in the Southern Ocean, where on average energy enters the interior 10° south of the Antarctic Circumpolar Current core. An interpretation of the wind-power input to the interior is proposed, expressed as a downward flux of pressure work. This energy flux is a measure of the work done by the Ekman pumping against the surface elevation pressure, helping to maintain the observed anomaly of sea surface height relative to the global-mean sea level. [ABSTRACT FROM AUTHOR]

The temperature variability of the Atlantic Ocean is investigated using an eddy-permitting (1/4°) global ocean model (ORCA-025) forced with historical surface meteorological fields from 1958 to 2001. The simulation of volume-averaged temperature and the vertical structure of the zonally averaged temperature trends are compared with those from observations. In regions with a high number of observations, in particular above a depth of 500 m and between 22° N and 65° N, the model simulation and the dataset are in good agreement. The relative contribution of variability in ocean heat transport (OHT) convergence and net surface heat flux to changes in ocean heat content is investigated with a focus on three regions: the subpolar and subtropical gyres and the tropics. The surface heat flux plays a relatively minor role in year-to-year changes in the subpolar and subtropical regions, but in the tropical North Atlantic, its role is of similar significance to the ocean heat transport convergence. The strongest signal during the study period is a cooling of the subpolar gyre between 1970 and 1990, which subsequently reversed as the mid-latitude OHT convergence transitioned from an anomalously weak to an anomalously strong state. We also explore whether model OHT anomalies can be linked to surface flux anomalies through a Hovmöller analysis of the Atlantic sector. At low latitudes, increased ocean heat gain coincides with anomalously strong northward transport, whereas at mid-high latitudes, reduced ocean heat loss is associated with anomalously weak heat transport. [ABSTRACT FROM AUTHOR]

The properties of the waters that move from the Pacific to the Indian Ocean via passages in the Indonesian archipelago are observed to vary with along-flow-path distance. We study an ocean model of the Indonesian Seas with reference to the observed water property distributions and diagnose the mechanisms and magnitude of the water mass transformations using a thermodynamical methodology. This model includes a key parameterization of mixing due to baroclinic tidal dissipation and simulates realistic water property distributions in all of the seas within the archipelago. A combination of air–sea forcing and mixing is found to significantly change the character of the Indonesian Throughflow (ITF). Around 6 Sv (approximately 1/3 the model net ITF transport) of the flow leaves the Indonesian Seas with reduced density. Mixing transforms both the intermediate depth waters (transforming 4.3 Sv to lighter density) and the surface waters (made denser despite the buoyancy input by air–sea exchange, net transformation = 2 Sv). The intermediate transformation to lighter waters suggests that the Indonesian transformation contributes significantly to the upwelling of cold water in the global conveyor belt. The mixing induced by the wind is not driving the transformation. In contrast, the baroclinic tides have a major role in this transformation. In particular, they are the only source of energy acting on the thermocline and are responsible for creating the homostad thermocline water, a characteristic of the Indonesian outflow water. Furthermore, they cool the sea surface temperature by between 0.6 and 1.5°C, and thus allow the ocean to absorb more heat from the atmosphere. The additional heat imprints its characteristics into the thermocline. The Indonesian Seas cannot only be seen as a region of water mass transformation (in the sense of only transforming water masses in its interior) but also as a region of water mass formation (as it modifies the heat flux and induced more buoyancy flux). This analysis is complemented with a series of companion numerical experiments using different representations of the mixing and advection schemes. All the different schemes diagnose a lack of significant lateral mixing in the transformation. [ABSTRACT FROM AUTHOR]

A global ocean general circulation model (OGCM) is used to investigate the mixed layer heat budget of the northern Indian Ocean (NIO). The model is validated against observations and shows fairly good agreement with mixed layer depth data in the NIO. The NIO has been separated into three subbasins: the western Arabian Sea (AS), the eastern AS, and the Bay of Bengal (BoB). This study reveals strong differences between the western and eastern AS heat budget, while the latter basin has similarities with the BoB. Interesting new results on seasonal time scales are shown. The penetration of solar heat flux needs to be taken into account for two reasons. First, an average of 28 W m-2 is lost beneath the mixed layer over the year. Second, the penetration of solar heat flux tends to reduce the effect of solar heat flux on the SST seasonal cycle in the AS because the seasons of strongest flux are also seasons with a thin mixed layer. This enhances the control of SST seasonal variability by latent heat flux. The impact of salinity on SST variability is demonstrated. Salinity stratification plays a clear role in maintaining a high winter SST in the BoB and eastern AS while not in the western AS. The presence of freshwater near the surface allows heat storage below the surface layer that can later be recovered by entrainment warming during winter cooling (with a winter contribution of +2.1°C in the BoB). On an interannual time scale, the eastern AS and BoB are strongly controlled by the winds through the latent heat flux anomalies. In the western AS, vertical processes, as well as horizontal advection, contribute significantly to SST interannual variability, and the wind is not the only factor controlling the heat flux forcing. [ABSTRACT FROM AUTHOR]

Highlights • Tides enhance simulated ice-shelf melting by 1–39% depending on the ice shelf. • Enhanced melting is mostly due to strengthened turbulence near the ice/ocean interface. • Tidal processes occurring seaward are less important than those occurring underneath ice shelves. • A methodology to prescribe tidal effects on ice shelf melt is proposed. Abstract The representation of tides in regional ocean simulations of the Amundsen Sea enhances ice-shelf melting, with weakest effects for Pine Island and Thwaites (< + 10 %) and strongest effects for Dotson, Cosgrove and Abbot (> + 30 %). Tides increase vertical mixing throughout the water column along the continental shelf break. Diurnal tides induce topographically trapped vorticity waves along the continental shelf break, likely underpinning the tidal rectification (residual circulation) simulated in the Dotson–Getz Trough. However, the primary effect by which tides affect ice-shelf melting is the increase of ice/ocean exchanges, rather than the modification of water masses on the continental shelf. Tide-induced velocities strengthen turbulent heat fluxes at the ice/ocean interface, thereby increasing melt rates. Approximately a third of this effect is counterbalanced by the resulting release of cold melt water that reduces melt downstream along the meltwater flow. The relatively weak tide-induced melting underneath Pine Island and Thwaites could be partly related to their particularly thick water column, which limits the presence of quarter wavelength tidal resonance. No sensitivity to the position of Pine Island and Thwaites with respect to the M 2 critical latitude is found. We refine and evaluate existing methodologies to prescribe the effect of tides on ice-shelf melt rates in ocean models that do not explicitely include tidal forcing. The best results are obtained by prescribing spatially-dependent tidal top-boundary-layer velocities in the melt equations. These velocities can be approximated as a linear function of existing barotropic tidal solutions. A correction factor needs to be applied to account for the additional melt-induced circulation associated with tides and to reproduce the relative importance of dynamical and thermodynamical processes. [ABSTRACT FROM AUTHOR]