Schedule May 23, 2018
Role of BL dynamics in large-scale Hadley circulations & convergence zones
Simona Bordoni (Caltech)

Hadley circulation describes the large-scale north-south movement of air in the atmosphere that encircles the globe, forming two closed loops (cells) that straddle the equator in both hemispheres. Warm, moist air converges near the equator, rising and moving poleward before descending at mid-latitudes and turning back towards the equator. This circulation helps redistribute the sun's heat from the tropics to the poles, and is essential for maintaining climates at different latitudes. The band of rising air due to the convergence of the two Hadley cells near the equator, called the inter tropical convergence zone (ITCZ), is known for its strong storms and rain. Its strength and north-south location vary depending on the time of year. It is intuitive to expect that its average position over the course of a year would be at the equator; however, it is shifted slightly north without a complete explanation. This study seeks to understand how the north-south ITCZ location is affected by boundary layers (slabs of air neighboring the ocean surface whose movement is affected by it) by using a simple numerical model of atmosphere circulation coupled to the upper ocean. Because the Hadley cells must redistribute heat and balance energy, the transfer (flux) of energy across the equator most often points away from the ITCZ. The study found that the seasonal variation of the ITCZ location lags that of the energy flux across the equator; consequently, the ITCZ is occasionally (and paradoxically) on the receiving end of energy flux. It is suggested that dynamics within the boundary layer could help explain these results, and may be significant to our understanding of the average northward ITCZ shift.

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