Abstract. We review what is known about the convec-tive process in the open ocean, in which the properties of large volumes of water are changed by intermittent, deep-reaching convection, triggered by winter storms. Observational, laboratory, and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales, the large-scale circulation of the ocean, and the prevailing meteorology. Two aspects make ocean convection interesting from a theoretical point of view. First, the timescales of the convective process in the ocean are sufficiently long that it may be modified by the Earth’s rotation; second, the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability. Moreover, the convec-

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High resolution numerical experiments of a circumpolar current are diagnosed to study how lateral and vertical transfer of buoyancy by geostrophic eddies balances advection by a meridional circulation driven by surface wind stresses and buoyancy fluxes. A theory is developed in the framework of the ‘‘residual circulation’ ’ to relate the vertical and horizontal stratification set up to the transfer properties of eddies and the patterns of imposed wind and buoyancy forcing. Simple expressions are found for the depth of penetration, stratification, baroclinic transport, and residual circulation of the current. Finally, the ideas are applied to the Antarctic Circumpolar Current (ACC) and yield predictions for how its properties depend on wind and buoyancy forcing. 1.

We use an ocean circulation, biogeochemistry, and ecosystem model to explore the interactions between ocean circulation, macro- and micro-nutrient supply to the euphotic layer, and biological productivity. The model suggests a tight coupling between the degree of iron limitation in the upwelling subpolar and tropical oceans and the productivity of the adjacent subtropical gyres. This coupling is facilitated by lateral Ekman transfer of macro-nutrients in the surface ocean. We describe a coarse resolution configuration of the MIT ocean circulation and biogeochemistry model in which there are fully prognostic representations of the oceanic cycles of phosphorus, iron, and silicon. The pelagic ecosystem is represented using two functional groups of phytoplankton and a single grazer. Using present-day forcing, the model qualitatively captures the observed basin and gyre scale patterns of nutrient distributions and productivity. In a suite of sensitivity studies we find significant regional variations in response to changes in the aeolian iron supply. In a dustier (model) world, the Southern Ocean and Indo-Pacific upwelling regions are more productive, but there is a decrease in productivity in the subtropical gyres and throughout the Atlantic basin. These results can be described most

Mixing in the abyssal ocean is known to play an important role in controlling the large-scale ocean circulation. In the search for sources of mechanical energy for mixing, internal tides generated by the interaction of the barotropic tide with bottom topography (mode conversion) have been implicated. However, estimates of the rate at which barotropic tidal energy is converted into the internal wave field are quite uncertain. Here, I present analytical and numerical calculations of internal tide generation in a fluid layer of finite depth to better understand the energetics of the wave generation process. Previous theoretical models of wave generation have assumed an upper radiation boundary condition (BC) appropriate for an ocean of infinite depth. But recent observations of internal tides at significant distances from their generation region indicate that this BC is not always valid, and that reflection from the upper surface is important. I show that the presence of an upper free-surface reduces the rate at which energy is fed into the internal wave field (the power) and thus the energy available for mixing. This reduction increases with the horizontal extent of the topography (relative to the wavelength of a mode-1 internal wave). Fully nonhydrostatic, nonlinear numerical calculations are used to both test the theory and to explore more realistic parameters for which linear theories are formally invalid. As bottom topography becomes steeper, linear theory underestimates mode conversion by an increasing amount, although even at critical slope the difference is quite small (O(20%)). An important finding of this

An analysis of ocean volume, heat and freshwater transports from a fully con-strained general circulation model is described. Output from a data synthesis, or state estimation, method is used by which the model was forced to a large-scale, time varying global ocean data set over six years. Time-mean uxes estimated from this fully time-dependent circulation have converged with independent time-independent estimates from box inversions over most parts of the world ocean but especially in the southern hemisphere. However, heat transport estimates dier substantially in the North Atlantic where our estimates result in only 1/2 previous heat transports. The estimated mean circulation around Australia involves a net volume ux of 14 Sv through the Indonesian Through ow and the Mozambique Channel. In addition we show that this ow regime exist on all time scales above one month rendering the variability in the South Pacic strongly coupled to the Indian Ocean. Moreover, the dynamically consistent variations in the model show temporal variability of oceanic heat uxes, heat storage and atmospheric exchanges that are complex and with a strong dependence upon location, depth, and time-scale. Results presented demonstrate the great potential of an ocean /state estimation system to provide a dynamical description of the time-dependent observed heat transport and heat content changes and their relation to air-sea interactions.

The restratification of the oceanic surface mixed layer that results from lateral gradients in the surface density field is studied. The lateral gradients are shown to be unstable to ageostrophic baroclinic instabilities and slump from the horizontal to the vertical. These instabilities, which are referred to as mixed layer instabilities (MLIs), differ from instabilities in the ocean interior because of the weak surface stratification. Spatial scales are O(1–10) km, and growth time scales are on the order of a day. Linear stability analysis and fully nonlinear simulations are used to study MLIs and their impact on mixed layer restratification. The main result is that MLIs are a leading-order process in the ML heat budget acting to constantly restratify the surface ocean. Climate and regional ocean models do not resolve the scales associated with MLIs and are likely to underestimate the rate of ML restratification and consequently suffer from a bias in sea surface temperatures and ML depths. In a forthcoming paper, the authors discuss a parameterization scheme to include the effect of MLIs in ocean models. 1.

This paper details a free surface method using an explicit time stepping scheme for use in z-coordinate ocean models. One key property that makes the method especially suitable for climate simulations is its very stable numerical time stepping scheme, which allows for the use of a long density time step, as commonly employed with coarse-resolution rigid-lid models. Additionally, the effects of the undulating free surface height are directly incorporated into the baroclinic momentum and tracer equations. The novel issues related to local and global tracer conservation when allowing for the top cell to undulate are the focus of this work. The method presented here is quasi-conservative locally and globally of tracer when the baroclinic and tracer time steps are equal. Important issues relevant for using this method in regional as well as large-scale climate models are discussed and illustrated, and examples of scaling achieved on parallel computers provided.

Eight earth system models of intermediate complexity (EMICs) are used to project climate change commit-ments for the recent Intergovernmental Panel on Climate Change’s (IPCC’s) Fourth Assessment Report (AR4). Simulations are run until the year 3000 A.D. and extend substantially farther into the future than conceptually similar simulations with atmosphere–ocean general circulation models (AOGCMs) coupled to carbon cycle models. In this paper the following are investigated: 1) the climate change commitment in response to stabilized greenhouse gases and stabilized total radiative forcing, 2) the climate change commitment in response to earlier CO2 emissions, and 3) emission trajectories for profiles leading to the stabilization of atmospheric CO2 and their uncertainties due to carbon cycle processes. Results over the twenty-first century compare reasonably well with results from AOGCMs, and the suite of EMICs proves well suited to complement more complex models. Substantial climate change commitments for sea level rise and global mean surface temperature increase after

[1] We formulate a mechanistic model of the coupled oceanic iron and phosphorus cycles. The iron parameterization includes scavenging onto sinking particles, complexation with an organic ligand, and a prescribed aeolian source. Export production is limited by the availability of light, phosphate, and iron. We implement this biogeochemical scheme in a coarse resolution ocean general circulation model using scavenging rates and conditional stability constants guided by laboratory studies and a suite of box model sensitivity studies. The model is able to reproduce the broad regional patterns of iron and phosphorus. In particular, the high macronutrient concentrations of the Southern Ocean, tropical Pacific, and subarctic Pacific emerge from the explicit iron limitation of the model. In addition, the model also qualitatively reproduces the observed interbasin gradients of deep, dissolved iron with the lowest values in the Southern Ocean. The ubiquitous presence of significant amounts of free ligand is also explicitly captured. We define a tracer, Fe * which quantifies the degree to which a water mass is iron limited, relative to phosphorus. Surface waters in high-nutrient, low-chlorophyll regions have negative Fe * values, indicating Fe limitation. The extent of the decoupling of iron and phosphorus is determined by the availability and binding strength of the ligand relative to the scavenging by particulate. Global iron concentrations are sensitive to changes in scavenging rate and physical forcing. Decreasing the scavenging rate 40 % results in 0.1 nM increase in dissolved iron in deep waters. Forcing the model with weaker wind stresses leads to a decrease in surface [PO4] and [Fe] in the Southern Ocean due to a reduction in the upwelling strength.