Monsoons over an Idealized Zonally-Symmetric Continent

Idealized modeling studies have provided the basis for significant progress on our conceptual and theoretical understanding of the fundamental dynamics of monsoons. Here, we study monsoons over an idealized zonally symmetric, fully-saturated continent north of 10N. Our results show that adding a hemispheric asymmetry in surface heat capacity between land and ocean is sufficient to cause symmetric breaking in both the spatial and temporal distribution of precipitation. The spatial symmetry breaking is manifest in a poleward displaced ITCZ over the continent during NH summer. Interestingly, while the ITCZ is further displaced poleward during NH summer, the cross-equatorial energy transport maximizes in the opposite season, when the ITCZ remains closer to the equator over the ocean. This seems at odd with previous work that has correlated the ITCZ position with the cross-equatorial energy transport, but arises because the energy input into the equatorial atmosphere, which acts as a sensitivity factor of the ITCZ position to cross-equatorial energy transport, is smaller in JJA than it is in DJF. The temporal symmetric breaking appears in an asymmetry between a rapid NH monsoon onset and a much more gradual retreat. This asymmetry results from the tropical overturning circulation being in different dynamical regimes at the beginning and end of the NH summer, which causes a fundamentally different response to the insolation forcing. Interestingly, the seasonal cycle of this idealized monsoon bears resemblance to that of the Indian monsoon, suggesting that fundamental mechanisms emerging from this study might be indeed relevant for observed monsoons.