15.6 Coupled Ocean Atmosphere Models
Coupled numerical models of the atmosphere and the ocean are used to study the climate system, its natural variability, and its response to external forcing. The most important use of the models has been to study how Earth's climate might respond to a doubling of CO2 in the atmosphere. Much of the literature on climate change is based on studies using such models. Other important uses of coupled models include studies of El Niño and the meridional overturning circulation. The former varies over periods of a few years, the latter varies over a period of a few centuries.
Development of the work tends to be coordinated through the World Climate Research Program of the World Meteorological Organization WCRP/WMO, and recent progress is summarized in Chapter 8 of the Climate Change 2001: The Scientific Basis report by the Intergovernmental Panel on Climate Change (McAvaney, et al, 2001).
Many coupled ocean and atmosphere models have been developed. Some include only physical processes in the ocean, atmosphere, and the ice-covered polar seas. Others add the influence of land and biological activity in the ocean. Let's look at the oceanic components of a few models.
Climate System Model
The model has been spun up and integrated for 300 years, the results are
realistic, and there is no need for a flux adjustment. (See the special issue
Princeton Coupled Model
Hadley Center Model
In contrast to most coupled models, this one is spun up as a coupled system with flux adjustments during spin up to keep sea surface temperature and salinity close to observed mean values. The coupled model was integrated from rest using Levitus values for temperature and salinity for September. The initial integration was from 1850 to 1940. The model was then integrated for another 1000 years. No flux adjustment was necessary after the initial 140-year integration because drift of global-averaged air temperature was ≤ 0.016 K/century.
Comments on Accuracy of Coupled Models
Because models must be simplified to run on existing computers, the models must be simpler than models that simulate flow for a few years (WCRP, 1995).
In addition, the coupled model must be integrated for many years for the ocean and atmosphere to approach equilibrium. As the integration proceeds, the coupled system tends to drift away from reality due to errors in calculating fluxes of heat and momentum between the ocean and atmosphere. For example, very small errors in precipitation over the Antarctic Circumpolar Current leads to small changes the salinity of the current, which leads to large changes in deep convection in the Weddell Sea, which greatly influences the volume of deep water masses.
Some modelers allow the system to drift, others adjust sea-surface temperature and the calculated fluxes between the ocean and atmosphere. Returning to the example, the flux of fresh water in the circumpolar current could be adjusted to keep salinity close to the observed value in the current. There is no good scientific basis for the adjustments except the desire to produce a "good" coupled model. Hence, the adjustments are ad hoc and controversial. Such adjustments are called flux adjustments or flux corrections.
Fortunately, as models have improved, the need for adjustment or the magnitude of the adjustment has been reduced. For example, using the Gent-McWilliams scheme for mixing along constant-density surfaces in a coupled ocean-atmosphere model greatly reduced climate drift in a coupled ocean-atmosphere model because the mixing scheme reduced deep convection in the Antarctic Circumpolar Current and elsewhere (Hirst, O'Farrell, and Gordon, 2000).
Grassl (2000) lists four capabilities of a credible coupled general circulation model:
McAvaney et al. (2001) compared the oceanic component of twenty-four coupled models, including models with and without flux adjustments. They found substantial differences among the models. For example, only five models calculated a meridional overturning circulation within 10% the observed value of 20 Sv. Some had values as low as 3 Sv, others had values as large as 36 Sv. Most models could not calculate a realistic transport for the Antarctic Circumpolar Current.
Grassl (2000) found four years later that many models, including models with and without flux adjustment, meet the first criterion. Some models meet the second criterion, but external solar forcing is still not well known and more work is needed. And a few models are starting to reproduce some aspects of the warm event of 6,000 years ago.
|Department of Oceanography, Texas A&M University
Robert H. Stewart, firstname.lastname@example.org
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Updated on November 10, 2006