Change of the Tropical Hadley Cell and Tropical Oceanic Warming

Abstract

The change in the Hadley circulation since 1950 is examined within the context of the long-term warming in global surface temperatures. The study involves analyses of observations, including various metrics of the Hadley Cell, and ensemble 50-year simulations by multiple atmospheric general circulation models (AGCM) forced with the observed sea surface temperature (SST) evolution and a hierarchy of idealized anomalous SST patterns.

Consistent evidence is found for an intensification of the Northern Hemisphere winter Hadley cell since 1950. This is shown to be an atmospheric response to the observed tropical warming trend, together with the intensification in El Ninos interannual flucuations, including larger amplitude and increased frequency after 1976. The intensification is shown to be associated with an intensified hydrological cycle consisting of increased equatorial oceanic rainfall and a general drying of tropical/subtropical landmasses. The Hadley cell change is consistent with previously documented dynamic changes in the extratropics, including a strengthening of westerly atmospheric flow and an intensification of mid-latitude cyclones.

An interesting question is why the intensification has only occurred in the Northern Hemispheric winter Hadley cell? Results of the model experiments indicate that the seasonality in the change of the tropical Hadley cell is naturally determined by the dynamic structure of the tropical ocean-atmosphere system.

Part of the presentation can be found in this book chapter:

Quan, X.W., H.F. Diaz, and M.P. Hoerling, 2004: Change of the Hadley circulation since 1950. The Hadley Circulation: Past, Present, and Future edited by H.F. Diaz and R.S. Bradley, Kluwer Academic Publishers, 85-120. (available here )