On Earth, the surface winds in the tropics are dominated by weak easterlies referred to as trade winds. These winds have been known to sailors for several centuries, as they determined the commercial routes in the age of sail. However, on many planetary atmospheres, the circulation in the equatorial region is very different: strong westerlies blow on planets such as Venus, Jupiter, Saturn or Titan. Over the past few years, astrophysicists have come to the conclusion that similar patterns must be found in a large class of exoplanets. This state of the atmospheric circulation is referred to as superrotation, because it means that the tropical atmosphere carries larger angular momentum than solid body rotation.
A transition from a conventional circulation to a superrotating one would have considerable consequences on the climate. Although a lot of work has been done recently to better understand the dynamical mechanisms maintaining an equatorial jet, it remains unclear whether the transition to superrotation is smooth or abrupt. Is it a new kind of tipping point that could lead to abrupt climate change?
With Rodrigo Caballero (Stockholm University) and Freddy Bouchet (ENS de Lyon), we recently published a preprint where we study two positive feedback mechanisms which could lead to abrupt transitions to superrotation: the Hadley cell feedback, and a wave-jet resonance mechanism. As had been suggested by Karen Shell and Isaac Held (JAS 2004), the damping effect of the Hadley cell on the eastward flow in the upper troposphere suddenly collapses when the vertical shear reaches a certain threshold. On the other hand, Nathan Arnold, Eli Tzipermann and Brian Farrell (JAS 2012), as well as other authors, pointed out that the emission of waves in the tropics due to diabatic heating (e.g. by convection), which could accelerate the flow towards the east in a manner similar to mid-latitude jets, depended on the background flow in a resonant manner. In our manuscript, we show how this wave-jet coupling leads to a positive feedback, and how it competes with the Hadley cell feedback. We study these mechanisms in detail in a simplified model, and show numerically that the results carry over to an axisymmetric model of the atmospheric circulation. In particular, we obtain abrupt transitions to superrotation due to the wave-jet resonance, and argue that the Hadley cell feedback is a more fragile mechanism in a realistic framework.
Atmospheric bistability and abrupt transitions to superrotation: wave-jet resonance and Hadley cell feedbacks
Corentin Herbert, Rodrigo Caballero, Freddy Bouchet