States of Tropical Convection and their Relationship to SST Variability

An analysis of data from the International Satellite Cloud Climatology Project (ISCCP) merged with data from the Tropical Rainfall Measurement Mission (TRMM) reveals that on a scale of a few hundred kilometers, tropical deep convection can be classed into 3 main categories or regimes. These are i) an unstructured field of deep convection with some mid-level top clouds; ii) a field of isolated deep convective clouds with extensive but relatively thin cirrus coverage; and iii) deep convective clouds embedded in an extensive field of thick stratiform anvil clouds likely with mesoscale organization. It is shown that the vertical distribution of latent heating varies significantly across the regimes with only one of the regimes exhibiting a strong stratiform heating component often associated with organized convection. It is also shown that the regimes have a unique association with large-scale tropical phenomena, such as the major monsoon systems and the Madden-Julian Oscillation (MJO).

Having identified the three major states of tropical convection, their relationship to SST variability on various time-scales is investigated. It is shown that on seasonal and interannual timescales convection strength and organization show the expected relationship with warmer SST often associated with stronger convection. However, this relationship breaks down on longer timescales, where a positive trend in SST is not usually associated with stronger convection. It is demonstrated that while the area of warm SST (> 28 C) has expanded the area of strong convection has not, explaining the relationship between convection strength and SST trends. Instead convection over the already warm parts of the ocean is shown to intensify. These findings support the recent hypothesis of a Dynamic Warm Pool (Hoyos and Webster, pers. communication).