Title: Dependence of climate response on latitudinal position of thermal forcing

Abstract: A variety of recent studies have shown that extratropical heating anomalies can be remarkably effective at causing meridional shifts in the intertropical convergence zone (ITCZ). But what latitudinal location of forcing is most effective at shifting the ITCZ? In a series of aquaplanet simulations with the GFDL AM2 model, we show that high latitude forcing actually causes a larger shift in the ITCZ than when equivalent surface forcing is applied in the tropics. We run equivalent simulations with an idealized general circulation model (GCM) without cloud- or water vapor-feedbacks, where the ITCZ response instead becomes weaker the farther the forcing is from the equator, indicating radiative feedbacks must be important in AM2. In the absence of radiative feedbacks, the tendency for anomalies to decrease in importance the farther away they are from the equator is due to the quasi-diffusive nature of energy transports. Cloud shortwave responses in AM2 act to strengthen the ITCZ response to extratropical forcing, amplifying the response as it propagates towards the equator. These results emphasize the great importance of the extratropics in determining the position of the ITCZ.
We also show that the global surface warming greatly depends on the meridional distribution of external thermal forcing. The heating is distributed uniformly globally or confined to narrow tropical or polar bands, and the amplitude is adjusted to ensure that the global-mean remains the same for all cases. Since the tropical temperature is close to a moist adiabat, the prescribed heating leads to a stronger warming in the upper troposphere than at the surface, whereas the polar warming is trapped near the surface due to strong atmospheric stability. Hence, surface warming is more effectively damped by radiation in the tropics than in the polar region. As a result, the global surface temperature increase is weak (strong) when the given amount of heating is confined to the tropical (polar) band. The degree of this contrast is shown to depend on water vapor- and cloud-radiative feedbacks that alter the effective strength of prescribed thermal forcing.

Speaker's website: https://sites.google.com/site/cdlunist/