How Far Poleward Can the Monsoonal Precipitation Maxima over Land Extend under High Obliquity?

Abstract:

The seasonal response of the poleward extent of continental precipitation maxima to high obliquity and its controlling mechanisms are examined using the NCAR-CESM and Budyko-Sellers energy balance model. In CESM, the latitude of the poleward-most continental precipitation maximum (denoted as Φ_mp) migrates poleward with increased obliquity, but is limited to equatorward of the latitude of maximum daily insolation. Some insight is gained on the controlling mechanism of the migration of Φ_mp in the Budyko-Sellers model, assuming Φ_mp coinciding with the position of maximum surface moist static energy (denoted as Θ_mp) as in the present-day observation and CESM pre-industrial simulation. Heat capacity exerts a primary control on Φ_mp such that Φ_mp is determined by the position of maxima of accumulated insolation over the radiative relaxation time and therefore lies equatorward of the maximum daily insolation. Variations of surface albedo and horizontal heat transport, as secondary effects, further shift the Φ_mp. This understanding from the Budyko-Sellers model, however, fails in CESM in the case of high obliquity of 40°, with Φ_mp now lying equatorward of Θ_mp. This separation between Φ_mp and Θ_mp is explained by the constraint of rotation on the latitudinal range of weak temperature gradients, which is the prerequisite for the prediction of Φ_mp from Θ_mp. When the rotation rate is reduced, the latitudinal range of weak temperature gradients expands poleward roughly following the change of the equatorial Rossby radius, allowing Φ_mp moving close to Θ_mp.