M5 Tropical Circulation and Disturbances

Topic

Tropical circulation is dominated by the Intertropical Convergence Zone (ITCZ)—a band of maximum convection that closely tracks the highest sea-surface temperatures at any given time and shifts seasonally in tandem with those temperatures. This zone also serves as the ascending branch of the Hadley cell and is responsible for a significant portion of the planet's total tropical precipitation. Ultimately, the energy balance of each hemisphere determines the ITCZ's location at any given moment; this same balance drives the convective band to shift toward the hemisphere receiving the most energy during a particular season.

Over the tropical Atlantic, the disturbances that most frequently disrupt this circulation pattern are African easterly waves—vorticity disturbances that propagate westward with periods of three to five days and wavelengths ranging from 2,500 to 4,000 kilometers. These waves originate from the barotropic-baroclinic instability of the African Easterly Jet; this same instability causes the jet stream to generate a succession of westward-propagating waves over the ocean.

These African easterly waves are the primary precursors to Atlantic tropical cyclones, although the mere arrival of a wave over the ocean is insufficient for cyclone formation. Tropical cyclogenesis requires that, in addition to the initial disturbance, three other conditions be met simultaneously: sea-surface temperatures must exceed 26°C, vertical wind shear must be low, and tropospheric humidity must be high. Only when these three conditions align with the initial disturbance provided by an African easterly wave does the process of tropical cyclogenesis culminate in the formation of a tropical cyclone.

Once a tropical cyclone has formed, the theory of wind-induced surface heat exchange—proposed by Emanuel—describes its intensification by likening the cyclone to a Carnot engine: an ideal heat engine that extracts heat from the warm ocean beneath it (via evaporation enhanced by its own winds) and expels that heat aloft at the cold tropopause. The greater the temperature difference between the warm ocean and the cold tropopause, the higher the thermodynamic intensity limit that this Carnot engine allows the tropical cyclone to reach; indeed, it is this temperature difference that causes two cyclones originating over different oceans to reach vastly different thermodynamic limits.

Thus, under favorable conditions, a tropical cyclone can reach its thermodynamic intensity limit within three to five days of formation. From the Intertropical Convergence Zone—which organizes tropical convection on a seasonal scale—to the African easterly waves providing the initial disturbance, and finally the Carnot engine driving the ultimate intensification, the entire tropical circulation system converges on a single outcome: it is the successive fulfillment of each of these conditions, stage by stage, that determines the ultimate intensity and rapid development of a given tropical cyclone.

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