The Meridional Mode in an Idealized Aquaplanet Model: Dependence on the Mean State

The meridional mode provides a source of predictability for the tropical climate variability and change on seasonal and longer time scales by transporting extratropical climate signals into the tropics. Previous research shows that the tropical imprint of the meridional mode is constrained by the interhemispheric asymmetry of the tropical mean climate state. In this study the constraint of the zonal asymmetry is investigated in an AGCM thermodynamically coupled with an aquaplanet slab oceanmodel. The strategy is to modify the zonal asymmetry of the mean climate state and examine the response of the meridional mode. Presented here are two simulations of different zonal asymmetries in the mean state. In the zonally symmetric case, the meridional mode operates throughout the subtropics but only becomes evident after removing a dominant global-scale eastward-propagating mode. In the zonally asymmetric case, the meridional mode operates only in regions where trade winds converge onto the equator and has an enlarged spatial scale due to the modified mean climate including cold sea surface and weak trade winds. In both simulations, the tropical imprint of the meridional mode is constrained by the north-south seasonal migration of the intertropical convergence zone. These results suggest that the meridional mode does not require the zonal asymmetry of the mean state but is intrinsic to the subtropical ocean-atmosphere coupled system with its characteristics subject to the mean climate state. The implication is that the internal climate variability needs to be assessed in the context of the mean climate state.