Problems and Prospects in Large-Scale Ocean Circulation Models

Abstract

We overview problems and prospects in ocean circulation models, with emphasis on certain developments aiming to enhance the physical integrity and flexibility of large-scale models used to study global climate. We also consider elements of observational measures rendering information to help evaluate simulations and to guide development priorities. SCOPE OF THIS PAPERNumerical ocean circulation models support oceanography and climate science by providing tools to mechanistically interpret ocean observations, to experimentally investigate hypotheses for ocean phenemona, to consider future scenarios such as those associated with human-induced climate warming, and to forecast ocean conditions on weekly to decadal time scales using dynamical modeling systems. We anticipate that the already significant role models play in ocean and climate science will increase in prominence as models improve, observational datasets grow, and the impacts of climate change become more tangible.The Ocean Obs 2009 workshop focused on developing a framework for designing and sustaining world ocean observing and information systems that support societal needs concerning ocean weather, climate, ecosystems, carbon and chemistry. Many of the Community White Papers contributed to Ocean Obs 2009 directly discuss topics where ocean models play a central role in generating information, in conjunction with observations, appropriate for ocean forecasting/prediction, state estimation, data assimilation, sensitivity analysis, and other forms of ocean information on both short (days) and long (decades to centuries) time scales ( [1-9]). The central purpose of the present paper is to highlight important research that forms the scientific basis for ocean circulation models and their continued evolution. We provide examples and recommendations where observations support the evolution of ocean models. The above listed White Papers, those from [10] and [11], and others, provide further discussions and recommendations of measurements that support the development and use of ocean models. OCEAN MODELS AND MODELINGThe ocean is a forced-dissipative system, with forcing largely at the boundaries and dissipation at the molecular scale. It is contained by complex land-sea boundaries with motions also constrained by rotation and stratification. Flow exhibits boundary currents, large-scale gyres and jets, boundary layers, linear and nonlinear waves, and quasi-geostrophic and three dimensional turbulence. Water mass tracer properties are preserved over thousands of mesoscale eddy turnover time scales. These characteristics of the ocean circulation pose significant difficulties for simulations. Indeed, ocean climate modeling is an application of a very different nature to those found in other areas of computational fluid dynamics (CFD). The time-scales of interest are decades to millennia, yet simulations require resolution or parameterization of phenomena whose time scales are minutes to hours. Furthermore, the most energetic spatial scales are of order 10 k...

Upwelling in the Southern Ocean

“…But thanks to faster, more powerful computers, climate simulations, such as the ones shown in figure 2, can now resolve features smaller than 10 km in that part of the world. 15,17 Accurate predictions for how the upwelling is likely to respond to climate change and changes in wind speeds are crucial for quantifying heat uptake, carbon uptake, and changes in nutrient supply. The next generation of climate models is likely to be greatly improved in that regard.…”

Prospects for improving the representation of coastal and shelf seas in global ocean models

“…For example, Griffies et al (2009) note (in the context of mesoscale eddies) "There is no obvious place where grid resolution is unimportant". The refinement criteria we have considered here, while chosen for coastal ocean processes, have been applied globally.…”

Synthesis and Assimilation Systems - Essential Adjuncts to the Global Ocean Observing System

“…With the former, the severe under-sampling of the water column and of most regions of the ocean in the early periods, the issues of biases/uncertainties in air-sea fluxes (e.g., [43], [44]) and model biases [45] take on greater importance.…”

“…Another area of concern is the biases in coupled models (e.g., [45]) that will have a detrimental impact on state estimates during periods that are observation-challenged. Many aspects of ocean circulation simulation are improved with resolution in ocean-only models (e.g., [85]), however resolution does not necessarily ameliorate important biases in coupled models.…”

“…Reference [45] discusses origins of biases and differences between model simulations and between models and observations: shortcomings in grid resolution, both horizontal and vertical; poor numerical algorithms, subgridscale parameterizations, or representation of other climate components such as the atmosphere, cryosphere, ocean biogeochemistry, and land runoff; and possibly other reasons not yet identified.…”