Australia: The Land Where Time Began

A biography of the Australian continent 

Mid-Cretaceous Supergreenhouse - Drastic Shrinking of the Hadley Circulation

One of the major themes of Palaeoclimatology is understanding the behaviour of the global climate system during periods that are extremely warm. The temperature gradient from pole-to-pole has been demonstrated by proxy data to have been much lower during the "supergreenhouse" period of the mid-Cretaceous than at present, which implied that meridional heat transport by atmospheric and/or oceanic circulation was much higher. A lack of datasets that are based on reliable proxies have made it more difficult to reconstruct the atmospheric circulation of the Cretaceous. The position of the high-pressure belt is reflected directly by the distribution of deserts, and desert deposits, and the exact position of its divergence axis, marking the poleward edge of the Hadley circulation, as shown by the prevailing surface wind pattern, that has been preserved in the desert deposits. Temporal changes in the latitude of the subtropical high-pressure belt and the divergence axis during the Cretaceous were reconstructed by the authors¹ based on the spatio-temporal changes of the latitudinal desert distribution and patterns of prevailing surface wind in the interior of Asia. Their results suggested there was a poleward shift of the subtropical high-pressure belt that occurred in the Early and Late Cretaceous which suggested the Hadley circulation had expanded towards the pole, though during the "supergreenhouse" period of the mid-Cretaceous there was a shift of the belt towards the equator, which suggested shrinkage of the Hadley circulation. A threshold in the atmospheric CO₂ level and/or global temperature, which when exceeded leads to drastic shrinkage of the Hadley circulation, is suggested by their results and recent observations.

See Sourhttp://www.clim-past.net/8/1323/2012/cp-8-1323-2012.pdfce 1 for more information

The authiors¹ studied spatio-temporal distribution of deserts and the patterns of prevailing winds in the interior of Asia in order to reconstruct the temporal changes of latitude of the subtropical pressure belt and its divergence axis in the Late Cretaceous. They found the high-pressure belt had shifted to latitudes between about 33^o N ± 4.2^o N in the Early Cretaceous and in the Late Cretaceous, between about 31^o N and 37^o N (36.8^o N ± 4.20^o N and 33.9^o N ± 3.1^o N, respectively. During the mid-Cretaceous their results showed the belt shifted towards the Equator to latitudes between 22^o N and 30^o N, 25.7^o N ± 3.9^o N (in the mid-Cretaceous). Between the Early and Middle Cretaceous the magnitude of the latitudinal shifts were found to be 11.1^o  ± 8.1^o and between the Middle Cretaceous and the Late Cretaceous 8.2^o ± 7^o, respectively.

The author¹ suggests the results, together with climate observations from the present, suggest the latitudinal shifts of the subtropical high-pressure belt appear to be associated with changes of the width of the Hadley circulation, which could be linked to global temperature changes and/or atmospheric CO₂ levels in the Cretaceous, also there appears to be a threshold beyond which global temperatures and atmospheric CO₂ levels increases lead to drastic shrinkage of the Hadley circulation towards the Equator.

Sources & Further reading

1. Hasegawa, H., Tada, R., Jiang, X., Suganuma, Y., Imsamut, S., Charusiri, P., Ichinnorov, N., and Khand, Y.: Drastic shrinking of the Hadley circulation during the mid-Cretaceous supergreenhouse, Clim. Past Discuss., 7, 119-151, doi:10.5194/cpd-7-119-2011, 2011