CMIP Subproject:

The Correlation between Oceanic Structure, Ocean Circulation and Heat Transport in Coupled Models
Yanli Jia and David J. Webb
Southampton Oceanography Centre
Empress Dock
Southampton SO14 3ZH
United Kingdom
e-mail: Yanli.Jia@soc.soton.ac.uk and David.J.Webb@soc.soton.ac.uk 

Introduction

The ocean is an important component of the climate system. It, together with the atmosphere, is responsible for the redistribution of the global incoming solar radiation and therefore the maintenance of the climate. While the ocean heat transport is crucial to the climate and climate change, its characteristics are only poorly known. Indirect estimates based on atmospheric fluxes are subject to many uncertainties (Trenberth and Solomon 1994) and direct estimates using oceanographic observations are few. Modelling studies (Cohen-Solal and Le Treut 1997) show that uncertainties in the ocean heat transport have a large impact on the atmospheric circulation. Often the ocean heat transport computed from an ocean model (or the ocean component of a coupled model) differs from that derived from observed atmospheric fluxes or implied by atmospheric models. In a coupled ocean-atmosphere general circulation model, such a discrepancy will lead to a climate drift which introduces great uncertainties to the understanding of anthropogenic climate change. Recent advances in both atmospheric and ocean general circulation models have resulted in much improved coupled models in which the ocean heat transport from the ocean component is the same as that implied by the atmospheric component for integrations of a few centuries (e.g. Boville and Gent 1998). This compatibility generally improves the simulation of sea surface temperature, but the sea surface salinity and the ocean interior may still experience significant drift, which is expected to affect the climate system on a longer time scale.

Studies with ocean general circulation models (Boning et al. 1996) indicate that in the North Atlantic, the ocean heat transport is highly correlated to the strength of the meridional overturning circulation. The latter, in turn, depends on how well the model represents the formation and transport of the North Atlantic Deep Water. In a similar way the heat transport in the southern hemisphere is expected to be influenced by the formation and transport of the Antarctic Bottom Water. We expect similar relationships to hold in coupled models, but then any errors can have widespread effects on the total climate response.

Because of the potential importance of these effects, we propose using the CMIP results to investigate the sensitivity of heat transport to the oceanic structure and ocean circulation in a coupled system. The CMIP study has the advantage that a number of different models and parameterisations are involved so this allows us to see beyond the specific behaviour of an individual model.

Objectives

  1. To assess how well the coupled models reproduce the observed ocean heat transport.
  2. To examine the relative roles of the meridional overturning circulation and gyre circulation in determining the ocean heat transport in coupled models.
  3. To identify factors (e.g. air-sea fluxes, the mean temperature and salinity distributions, the formation and properties of deep and bottom water masses, boundary currents) which contribute to the discrepancies in heat transport between models and between models and observations.

Methodology

The ocean heat transport in the coupled models will be compared and validated against indirect observational estimates based on atmospheric fluxes (e.g. Trenberth and Solomon 1994) and direct estimates using oceanographic observations (e.g. Hall and Bryden 1982; Bryden et al. 1991). In particular, recent estimates from the WOCE database will be used as they become available.

The temperature field (zonally averaged) combined with the meridional overturning circulation will be used to estimate the mass transport at different temperature classes which will be compared with other estimates (e.g. Macdonald and Wunsch 1996). This calculation gives an estimate of the contribution to the heat transport by the meridional overturning. Similarly the contribution of the gyre circulation to the heat transport will be estimated using the vertically integrated mass transport streamfunction and the temperature field (vertically averaged).

In order to establish the correlation between the ocean heat transport and the oceanic structure and ocean circulation, we will carry out a detailed examination of the temperature and salinity distributions in each of the coupled models. Comparisons will be made with the available climatologies (e.g. Levitus, 1982), recent observational datasets (e.g. WOCE) and output from high resolution ocean models (e.g. CME and OCCAM).

The analysis will be based on the output from CMIP2 control experiments (20-year means) and the annual mean fields from CMIP1.

Data Requirements

  1. CMIP1 ocean
  2. CMIP2 atmosphere: control case, 80-year runs (four 20-year means)
  3. CMIP2 ocean: control case, 80-year runs (four 20-year means)
  4. bathymetry

References

Boning, C. W., F. O. Bryan, W. R. Holland, and R. Doscher, 1996: Deep-water formation and meridional overturning in a high-resolution model of the North Atlantic. J. Phys. Oceanogr., 26, 1142-1164.

Boville, B. A., and P. R. Gent, 1998: The NCAR Climate Systems Model, Version One. J. Climate, 11, 1455-1471.

Bryden, H. L., D. H. Roemmich, and J. A. Church, 1991: Ocean heat transport across 24N in the Pacific. Deep-Sea Res., 38, 297-324.

Cohen-Solal, E., and H. Le Treut, 1997: Role of the oceanic heat transport in climate dynamics: A sensitivity study with an atmospheric general circulation model. Tellus, 49A, 371-387.

Hall, M. M., H. L. Bryden, 1982: Direct estimates and mechanisms of ocean heat transport. Deep-Sea Res., 29, 339-359.

Levitus, S., 1982: Climatological Atlas of the World Ocean. NOAA Prof. Paper 13, U.S. Govt. Printing Office, 173pp.

Macdonald, A. M., and C. Wunsch, 1996: An estimate of global ocean circulation and heat fluxes. Nature, 382, 436-439.

Trenberth, K. E., and A. Solomon, 1994: The global heat balance: heat transports in the atmosphere and ocean. Climate Dyn., 10, 107-134.