Dept. of Geosciences Colloquium: Thermodynamic and dynamic constraints on tropical modes

Prof. Ori Adam, HUJI

10 June 2024, 11:00 
Ornstein Building, Room 111 
Dept. of Geosciences Colloquium




It is not clear how systematic model biases in tropical precipitation and evaporation affect projections under global warming. In the first part of the talk I will show how thermodynamic considerations allow us to constrain the effects of these biases and estimate the increased projection uncertainty associated with these biases, through an extension of the well-known wet-gets-wetter/dry-gets-drier scaling. In the second part of the talk I will argue that the variation of the tropical rain belt between unimodal and bimodal meridional precipitation distributions is a fundamental characteristic of tropical climate, which I define as ’Tropical Modality’ and quantify using equatorial cooling. Specifically, increased tropical modality (i.e., a bi-modal distribution) is related to increased width of the tropical rain belt, wider and weaker meridional overturning circulation, colder equatorial cold tongues, and more severe double intertropical convergence zone (ITCZ) bias in modern climate models. In addition, the seasonal cycle of the tropical rain belt varies with Tropical Modality, from being monsoonal for unimodal climates, to introducing additional degrees of freedom for bimodal climates. A surprising result of the study is that for bimodal climates, ITCZs do not necessarily follow the Sun in their seasonal cycle. To elucidate the relation of tropical modality and the large-scale circulation, axisymmetric theory of atmospheric circulation is extended for the case of concentrated equatorial cooling and annually averaged heating. The solutions are derived in a 1.5-layer shallow water model on the spherical Earth, which includes vertical mixing with diagnostic surface momentum. The axisymmetric solutions capture the sensitivity of the large-scale circulation to equatorial cooling seen in observations and in eddy-permitting models, namely, (i) weakening and widening of the meridional overturning circulation (MOC) and (ii) weakening and poleward shift of the subtropical jet. For sufficiently large equatorial cooling, a tropical anti-Hadley cell emerges that transports energy equatorward, balancing the equatorial energetic sink. The analytic solutions predict the critical cooling required for the emergence of the anti-Hadley cell and provide a simple mechanism for the response of the MOC to equatorial cooling, with implications for ENSO and global warming.


  • Adam O., M. Shourky, C.I. Garfinkel, and M. Byrne, 2023: Increased uncertainty in projections of precipitation and evaporation due to wet-get-wetter/dry-get-drier biases.  Geophysical Research Letters, 50, e2023GL106365

  • Adam O., A. Farnsworth, and D. J. Lunt, 2023: Modality and seasonal variation of the tropical rain belt across climates and models, Journal of Climate, 36(5), 1331-1345

  • Adam O., 2023: The influence of equatorial cooling on axially symmetric atmospheric circulation forced by annually averaged heating. Journal Of Climate, 36, 7451–7464



Event Organizer: Dr. Roy Barkan



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