Abstract
Abstract. Recent studies demonstrate that the Hadley Circulation has intensified and expanded for the past three decades, which has important implications for subtropical societies and may lead to profound changes in global climate. However, the robustness of this intensification and expansion that should be considered when interpreting long-term changes of the Hadley Circulation is still a matter of debate. It also remains largely unknown how the Hadley Circulation has evolved over longer periods. Here, we present long-term variability of the Hadley Circulation using the 20th Century Reanalysis. It shows a slight strengthening and widening of the Hadley Circulation since the late 1970s, which is not inconsistent with recent assessments. However, over centennial timescales (1871–2008), the Hadley Circulation shows a tendency towards a more intense and narrower state. More importantly, the width of the Hadley Circulation might have not yet completed a life-cycle since 1871. The strength and width of the Hadley Circulation during the late 19th to early 20th century show strong natural variability, exceeding variability that coincides with global warming in recent decades. These findings raise the question of whether the recent change in the Hadley Circulation is primarily attributed to greenhouse warming or to a long-period oscillation of the Hadley Circulation – substantially longer than that observed in previous studies.
Highlights
The Hadley Circulation is a fundamental regulator of the Earth’s energy budget, i.e. redistribution of energy from tropics to higher latitudes
Various widely used reanalyses show that the winter northern Hadley Circulation has strengthened over the past few decades (Diaz and Bradley, 2004; Mitas and Clement, 2005; Song and Zhang, 2007), no discernible trend is found in the global radiosonde observations for 1959–1989 (Mitas and Clement, 2004)
We conclude that the 20th Century Reanalysis Version 2 (20CR2) does indicate a slight strengthening and widening of the Hadley Circulation for the past three decades, corroborated by the results of recent analyses
Summary
The Hadley Circulation (ascent near the equator and subsidence in the subtropics) is a fundamental regulator of the Earth’s energy budget, i.e. redistribution of energy from tropics to higher latitudes. Satellite observations provided evidence of an increase of thermal radiation emitted by Earth and a decrease of reflected solar radiation in the tropics from the mid-1980s to the 1990s (Chen et al, 2002; Wielicki et al, 2002) Further analysis suggested such change might be attributed to a decadal strengthening of the Hadley Circulation (Chen et al, 2002). The widening appears to be nearly an order of magnitude smaller in the IPCC AR4 simulations than those in the observations (Johanson and Fu, 2009) This suggests that further studies are necessary to confirm long-term changes in the meridional extent of the Hadley Circulation.
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