Transient responses of the CSIRO climate model to two different rates of CO2 increase

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Article

  Climate Dynamics
Publisher: Springer-Verlag GmbH
ISSN: 0930-7575 (Paper) 1432-0894 (Online)
DOI: 10.1007/s003820050237
Issue:  Volume 14, Numbers 7-8

Date:  June 1998
Pages: 503 - 516  
Transient responses of the CSIRO climate model to two different rates of CO2 increase


W. Cai A1 and H. B. Gordon A1

A1 CSIRO, Division of Atmospheric Research, Aspendale, Victoria 3195, Australia


Abstract:


Abstract The responses of the CSIRO coupled atmosphere-ocean-sea ice model to two greenhouse gas induced warming scenarios are described and compared to a control run with the current CO2 level. In one scenario, denoted IS92a, the atmospheric CO2 increases such that it reaches doubling after 128 years. In the other, the CO2 increases at 1% per year compounding (doubling after 70ry). As the CO2 increases in both scenarios, the top-of-atmosphere outgoing longwave radiation increases giving enhanced cooling of the coupled system, while the outgoing short wave radiation decreases contributing to a warming of the system. The latter overcompensates the former leading to a global mean net radiative heat gain. The distribution of this heat gain produces the well-known interhemispheric asymmetry in warming, despite a decrease in the sea ice around Antarctica in this model. It is found that the volume mean temperature response over the southern ocean is greater than that over the northern hemispheric oceans, and a maximum warming takes place at the subsurface rather at the surface of the ocean in the southern mid-to-high latitude region. The enhanced high-latitude freshening associated with the strengthened hydrological cycle significantly affects the latitudinal distribution of warming and other responses. It enhances the warming immediately equatorward of the deep water formation regions while produces a reduced warming, even a cooling, in these regions. In both runs, there is a decrease in the large-scale oceanic currents which have a significant thermohaline-driven component. The reduction in these currents reduces the poleward transport of salt out of the tropical and subtropical regions of these oceans. This and the enhanced evaporation contribute to considerable increases in surface salinity in the tropical and subtropical regions. In IS92a, the warming rate before doubling is smaller than that in 1% scenario, but the cumulative effects of the two experiments at the time of doubling are similar. Nevertheless, significant contrasts exist. For example, at the time of doubling in IS92a, the warming of the upper ocean is greater because a more developed temperature-albedo feedback occurs. In addition, a longer time is allowed for heat anomalies to spread downward, and so the effective heat penetration depth is greater than that in the 1% scenario. Thus the oceanic response is influenced by the CO2 increase scenario used.