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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