The Role of Large-Scale Atmospheric Circulation in the Relationship between Tropical Convection and Sea Surface Temperature

In this paper, the authors study the influence of the large-scale atmospheric circulation on the relationship between sea surface temperature (SST) and tropical convection inferred from outgoing longwave radiation (OLR). They find that under subsidence and clear sky conditions there is an increase in OLR with respect to SST at a rate of 1.8–2.5 Wm−2 (°C)−1. In regions of large-scale ascending motions, which is correlated to, but not always collocated with, regions of warm water, there is a large reduction of OLR with respect to SST associated with increase in deep convection. The rate of OLR reduction is found to be a strong function of the large-scale motion field. The authors find an intrinsic OLR sensitivity to SST of approximately −4 to −5 Wm−2 (°C)−1 in the SST range of 27°–28°C, under conditions of weak large-scale circulation. Under the influence of strong ascending motion, the rate can be increased to −15 to −20 Wm−2 (°C)−1 for the same SST range. The above OLR–SST relationships are strongly dependent on geographic locations. On the other hand, deep convection and large-scale circulation exhibit a nearly linear relationship that is less dependent on SST and geographic locations. The above results are supported by regression analyses. In addition, they find that on interannual timescales, the relationship between OLR and SST is dominated by the large-scale circulation and SST changes associated with the El Niño–Southern Oscillation. The relationship between anomalous convection and local SST is generally weak everywhere except in the equatorial central Pacific, where large-scale circulation and local SST appear to work together to produce the observed OLR–SST sensitivity. Over the equatorial central Pacific, approximately 45%–55% of the OLR variance can be explained by the large-scale circulation and 15%–20% by the local SST. Their results also show that there is no fundamental microphysical or thermodynamical significance to the so-called SST threshold at approximately 27°C, except that it represents a transitional SST between clear-sky/subsiding and convective/ascending atmospheric conditions. Depending on the ambient large-scale motion associated with basin-scale SST distribution, this transitional SST can occur in a range from 25.5° to 28°C. Similarly, there is no magic to the 29.5°C SST, beyond which convection appears to decrease with SST. The authors find that under the influence of strong large-scale rising motion, convection does not decrease but increases monotonically with SST even at SST higher than 29.5°C. The reduction in convection is likely to be influenced by large-scale subsidence forced by nearby or remotely generated deep convection.