Tuesday, 11 February 2003: 4:30 PM
Surface evaporation and the greenhouse effect over the intertropical oceans
In the intertropical belt, the sources of moisture are located over the subtropical oceans. Water evaporated in the subtropics is then transported by the low level branches of the Hadley/Walker circulation towards the Inter Tropical Convergence Zone. In the ITCZ, deep convection further forms upper level cloudiness and rainfall and transports water vapor upwards where its radiative effect are the strongest, yielding to strong greenhouse effect. This dynamical loop hence remotely links the moisture source and evaporative cooling regions to the moisture sinks and greenhouse warming regions (Gershunov, 1998). In this paper, we present a quantitative analysis of this coupled process using satellite observations and investigate its representation in the CCM coupled ocean-atmosphere General Circulation Model. The strong coupling between these two parameters is established using a Canonical Correlation Analysis (CCA) technique. First, at the seasonal scale, the analysis shows that the winter hemisphere ITCZ greenhouse effect is fed from the evaporation in the summer hemisphere, confirming previous observational studies and the remote dynamical link between GH and LHF. It further reveals that the leading coupled mode of the CCA explains most of the seasonal variability in each of the variables. The modeled seasonal cycle captures the main features of the observed remote link. At the inter annual scale, the observed CCA mode extends the previous interpretation with the source and sink regions located around the Equator. But the analysis indicates that the model fails in reproducing this dynamical link between GH and LHF. Instead in the GCM, local sources of moisture feed the local greenhouse effect at the inter annual scale. Implications for climate change will be discussed at the conference. Preliminary results of this diagnostic applied onto the IPSL coupled ocean-atmosphere model will also be shown.