Abstract
Climate interaction between the tropics and extratropics provides a global perspective for understanding and predicting regional climate variability and change. Previous studies on this topic have focused extensively on the impacts of tropical climate variability such as the impacts of El Niño Southern Oscillation (ENSO) on the extratropics. This dissertation investigates the opposite direction—the impacts of extratropical climate variability on the tropics—by focusing on the Meridional Mode. Identified first in the subtropical North Atlantic and North Pacific more than a decade ago, the Meridional Mode transports atmospheric intrinsic variability signal on seasonal and longer time scales from the northern extratropics to the tropics via the wind-evaporation-sea surface temperature (WES) feedback. Here in observations and multiple climate models of varying degrees of ocean-atmosphere coupling, the Meridional Mode is shown to operate in the subtropical South Pacific. Compared to its North Pacific counterpart, the South Pacific Meridional Mode (SPMM) has a larger equatorial impact, leads to ENSO-like variability in the absence of active ocean dynamics, and can likely trigger ENSO events in both the fully coupled climate models and real world. In addition, modeling evidence is presented for the existence of the Meridional Mode in the subtropical South Indian and Atlantic. Together with previous observational studies, these results suggest that the Meridional Mode operates in all global subtropical oceans as an extratropics-to-tropics climate linkage. The different equatorial impacts of the two Pacific Meridional Modes arise from the interhemispherically asymmetric distribution of mean trade winds in the tropical Pacific. Mean surface winds determine the spatial propagation of the WES feedback—the fundamental mechanism for the Meridional Mode, therefore stronger cross-equatorial mean southeasterly trades in the central to eastern Pacific favor the larger equatorial impact from the SPMM. With support from both numerical experiments and analytical solutions, this interpretation suggests that the equatorial imprint of the Meridional Mode is strongly constrained by the interhemispheric asymmetry of tropical mean climate state. The zonal asymmetry of the tropical mean climate state is not required for the Meridional Mode, but can significantly modulate its characteristics. In an AGCM coupled with an idealized aquaplanet slab ocean model of zonally symmetric mean climate, the Meridional Mode operates throughout the subtropical oceans, but only becomes evident when a dominant global scale eastward-propagating mode is removed. With an idealized zonal asymmetry of realistic magnitude added to the mean climate, the Meridional Mode impacts the equator only in regions where mean subtropical trade winds reach the equator, highlighting the strong constraint from the distribution of mean surface winds; in addition, the spatial scale of the Meridional Mode is enlarged owing to the weakened damping rate (thus enhanced persistence) of SST variability associated with the mean climate change. These results suggest that the Meridional Mode, intrinsic to the subtropical ocean-atmosphere system, is dependent on the mean climate state, particularly the distribution of mean surface winds. This dissertation generalizes the Meridional Mode from the northern hemisphere to the global ocean. Its findings provide crucial insights from a global perspective for understanding and predicting the tropical climate variability and change on seasonal and longer time scales. The dependence of the Meridional Mode on the mean climate state highlights the important role of the mean climate in regulating internal climate variability and suggests that the internal climate variability needs to be assessed in the context of the mean climate state.