Orbital forcing and climate model response

The effect of orbital forcing on sea-surface temperatures (SSTs) at 6 ka compared with pre-industrial conditions has been studied in the Paleoclimate Modeling Intercomparison Project PMIP2 with coupled ocean-atmosphere climate models (AOGCMs) (see Crucifix this volume, Figure 4.3). Similar simulations have also been performed with other models, including models of intermediate complexity (e.g. Liu et al. 2003; Rimbu et al. 2004; Renssen et al. 2005). A consistent feature of these simulations is the positive summertime SST anomaly in the North Atlantic-Nordic Seas area. The SST response of the MIT-EMIC (Michigan Institute of Technology - Earth System Model of Intermediate Complexity) to the 6 ka orbital forcing compared with the response of the model to pre-industrial forcing, i.e. the response solely due to changing orbital parameters, is shown in Figure 5.4. The response is strongest in high latitudes, with very little response in the tropics, and the most pronounced warming occurs near the Polar Front in the North Atlantic-Nordic Seas. This poleward amplification in the Atlantic sector is consistent with observational evidence (Figure 5.3; data from Kim et al. 2004; Kim and Schneider 2004a,b). The degree to which the models are able to separate surface and thermocline temperatures and the seasonal response is variable. The

Figure 5.3 6-0 ka alkenone sea-surface temperature differences for selected North Atlantic and Mediterranean cores. (Data from Kim and Schneider 2004a,b.)

Figure 5.3 6-0 ka alkenone sea-surface temperature differences for selected North Atlantic and Mediterranean cores. (Data from Kim and Schneider 2004a,b.)

imprint of the seasonal forcing on the ocean sub-surface is clearly different between models and observations, as noted in the following section. This implies that for climate models to represent the true ocean sub-surface and thermocline variability in response to the forcing, a better representation of high-latitude surface ocean stratification is required.

0 0

Post a comment