Zonal Asymmetries in the Widening of the Tropics Under Climate Change

The changes in the region of the atmosphere that might be considered “tropical” – based on upper tropospheric humidity – over the last three decades are investigated using the High-resolution Infra-Red Sounder brightness temperature as a proxy for upper level relative humidity. The trends in the annual mean brightness temperatures are calculated. We observe that the tropics are widening, but the widening is highly asymmetric in the zonal direction. There is an expansion of the moist tropical upper tropospheric signal that is seen over the Indian Ocean and Indonesian region northwards above India, South-east Asia, the Bay of Bengal and the Arabian Sea. The dry region associated with subtropical subsidence over Northern India and the Arabian region appears to by moving to the North-East. Similarly, the subsidence region over West/Central Australia is moving to the south.

Similar analysis of upper level relative humidity trends in the ERA-interim reanalysis show qualitative similarities to the observations.

Analysis of upper tropospheric humidity in the the 20th century climate reconstructions within the CMIP5 results shows trends that differ widely from the satellite observations. Analysis of various AMIP and historical experiments within the data set allow us to investigate some of the factors involved in these observations.