Interannual variations of tropical upper tropospheric humidity and tropical rainy‐region SST: Comparisons between models, reanalyses, and observations

Abstract

Tropical deep convection plays a key role in the vertical transport of moisture to the upper troposphere. Here we investigate relations between the interannual anomalies of the sea surface temperature (SST) in the tropical strong precipitation regions (rainy‐region SST) and tropical upper tropospheric humidity (UTH), as seen from the ECMWF ERA‐40 and ERA‐interim reanalyses, AIRS observations, and four global climate model (GCM) simulations from the IPCC AR4 archive. Important differences are found between the two reanalyses and between models and observations/reanalyses. The mean correlation coefficients between interannual anomalies of UTH (250–400 mb) and rainy‐region SST are 0.46 for the ECMWF ERA‐40 but 0.86 for ERA‐interim and 0.87 for the 6‐year AIRS data, which are all higher than their counterparts when the tropical‐mean SST anomalies are used instead. All GCMs exhibit systematically stronger correlations (∼0.95) for both types of SST anomalies. Fractional changes of UTH anomalies with respect to the two types of SST anomalies are also examined. Using modeled and reanalyses tropical temperature profiles as input, an idealizing model that assumes deep convection for the only source of water vapor is used to compute the UTH variations. Except for the case of ECMWF ERA‐40, the idealized model reasonably reproduces the UTH profiles at 250 and 300 mb for all other cases, corroborating the dominate control of deep convection on the humidity in these layers. The UTH at 200 mb derived from the idealized model is 1.5 to 2 times more humid than those from the GCMs and reanalyses, suggesting the importance of drying mechanisms at this level and probably above. Possible causes for the outlier behavior of ERA‐40 are also discussed.