[DLS] Kerry Emanuel (EAPS)

“A Unified Framework for Surface Flux-Driven Cyclones Outside the Tropics”

Cyclonic storms closely resembling tropical cyclones are occasionally observed well outside the tropics, at places and times in which sea surface temperatures are well below the conventional threshold of 26 C. These include medicanes, polar lows, some subtropical cyclones, and Kona storms. Like their tropical cousins, these storms have concentrated inner cores, including eyes and eyewalls, and spiral bands at larger radii. Previous work demonstrated that, like tropical cyclones, they are driven mostly or entirely by enthalpy fluxes from the underlying ocean, rather than by baroclinic instability, which is powered by isobaric density gradients. But they form in places and at times where the climatological thermodynamic potential (called the potential intensity) for flux-driven cyclones is either absent or greatly insufficient.

In this talk I will present observational evidence that that these variously named storms are indeed tropical cyclones but arise from potential intensity that is locally generated by breaking Rossby waves that massively convert the kinetic energy of jet streams to potential energy through lifting and cooling of air masses.  As such, we refer to them as CYclones Originating from Locally Originating Potential intensity (CYCLOPs). These conditions are transient, curtailing the lifetimes of CYCLOPs, but nevertheless permitting dangerously high winds. As a practical matter, CYCLOps are generally smaller, and intensify more rapidly, than baroclinic cyclones and, as such present a challenge to forecasters and numerical forecast models.

EAPS Department Lecture Series —

Weekly talks aimed to bring together the entire EAPS community, given by leading thinkers in the areas of geology, geophysics, geobiology, geochemistry, atmospheric science, oceanography, climatology, and planetary science. Runs concurrently with class 12.S501.

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