High-Resolution Image

The question of remote versus local influence of sea-surface temperatures (SSTs) on the precipitation was researched using data from a recent 16-year long simulation with the fv-GCM using Goddard cloud physics parameterization. In this simulation, SSTs were prescribed from NCEP analysis of SST-observations. The figure shows that when precipitation and vertical velocity were binned by SST, the influence of SSTs could be expected to dominate the scene, since merging data from different geographical locations and seasons largely eliminates the remote effects. However, because the SST distribution is not random, we can isolate some influences of the residual mean vertical velocity on the circulation. Consequently, mid-latitude SSTs, under the sinking branch of the Hadley circulation, produce less precipitation due to near-surface moisture flux divergence, even though the SSTs are higher than those of high latitudes. The figure also shows that the model does a good job of simulating SST-binned mean precipitation and vertical velocity, but the correlations among model fields were larger than that of the corresponding analyzed fields. Now we need to find out if the model is flawed, or if the relationships between the observed precipitation and analyzed vertical velocity are inconsistent. Overall, our analysis shows that useful discoveries can be made by focusing on circulation and rainfall.

(submitted by Yogesh Sud and Greg Walker)