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Why does the tropical wet-dry climate have a longer dry season farther from the Equator?

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Köppen climate classification map — Köppen climate classification The major climatic types are based on patterns of average precipitation, average temperature, and natural vegetation. This map depicts the world distribution of climate types based on the classification originally invented by Wladimir Köppen in 1900.

The tropical wet-dry climate (Aw) is characterized by distinct seasonal cycles driven by the annual migration of the intertropical convergence zone (ITCZ) and the subtropical anticyclone.1

During the high-sun season, the ITCZ moves poleward, bringing convergent, ascending air that stimulates convective rainfall. Conversely, during the low-sun season, the ITCZ shifts toward the summer hemisphere, and the region comes under the influence of the subtropical anticyclone. This high-pressure system is part of the descending branch of the Hadley cell, which produces stable, subsiding air that results in dry, clear weather.12

The length of the dry season increases as one moves poleward because these locations are situated farther from the equatorial belt where the ITCZ resides for a longer duration. As a location moves toward higher latitudes, it remains under the influence of the dry, subsiding air of the subtropical anticyclone for a greater portion of the year, while the period during which the ITCZ provides moisture becomes progressively shorter.13