Atmospheric Ocean System

Upwelling -Diffusion Climate Model: Model description

The different chemical compounds along with the determination of their radiative forcing specified, are listed below

UDCM is an upwelling-diffusion, energy-balance model of which the basis is presented by Wigley and Schlesinger (1985). The model consists of an atmosphere box, two land and two ocean boxes (representing the northern and southern hemisphere). The two ocean boxes are divided into 40 layers each, with a mixed layer on top which absorbs the energy of solar radiation. It is assumed that no energy is adsorbed above land. The energy balance of the climate system can be described as:

∆Q = λ∆T + ∆F

with:
∆Q  = radiative forcing (J yr^-1 m^-2)
∆F  = net heat flux into the ocean (J yr^-1 m^-2)
λ = feedback parameter (J yr^-1 m^-2 K^-1)

∆Q  and  ∆F  are both averaged over the entire world area. The term λ∆T  is the change in the rate of heat loss to space from the climate system. The feedback parameter is the inverse of the climate sensitivity parameter. The climate sensitivity parameter is defined as the global-mean surface temperature response to the radiative forcing. The climate sensitivity parameter is determined by the eventual global-mean temperature change for a CO2 doubling (∆T_2x) divided by the radiative forcing that accompanies such a doubling (∆Q_2x). ∆T_2x is the most uncertain input of UDCM. According to TAR (IPCC, 2001), three values are used: 1.5ºC as the lowest, 2.5ºC as best guess and 4.5ºC as the highest. Note that instead of using a global mean value for the feedback parameter λ, UDCM uses a land/ocean sensitivity ratio of 1.2:1 to account for a difference in response of land and ocean to changes in CO₂ concentrations (Raper et al., 1996).

On time scales relevant to climate change, the atmosphere may be assumed to be in equilibrium with the underlying oceanic mixed layer:

With: d∆ T/dt = temperature change per year for the oceanic upper layer (K yr^-1)
Cm = the effective bulk heat capacity of the oceanic mixed layer (J yr^-1 m-2 K^-1).

The absorbed heat is transported within each ocean box by diffusion and upwelling. The upwelling decreases at increasing temperatures of the ocean, with a maximum decrease of 1.2 m per year (from 4.0 to 2.8 m per year) to simulate the slowing down of the thermohaline circulation of the ocean (Raper et al., 2000).

The output of UDCM is the global-mean surface temperature change and the global temperature change of the 40 oceanic layers. Since large areas are ice-covered the temperature change in the oceanic mixed layer is corrected to account for a slower response of the upper layer .

• to  Oceanic Carbon Model (OCM)
• to  external references
• to  MNP references