Decomposing Atmospheric Versus Oceanic Contributions to the High-To-Low Latitude Teleconnection during a Freshwater Triggered Abrupt Climate Change

Abstract

In this dissertation we study the high-to-low latitude teleconnection during Younger Dryas-like abrupt climate events using models. The teleconnection considered here is between climate change induced by a freshwater input in high-latitude North Atlantic and global response over the Northern Hemisphere and in the tropics. We focus on three primary questions: (1) What is the relative importance of oceanic vs. atmospheric processes in the teleconnection? (2) What are the respective mechanisms of the atmospheric and oceanic controlled teleconnection? (3) How important is sea surface temperature to the teleconnection, particularly in tropical climate responses.

To answer these questions we performed a series of model experiments using an Atmospheric General Circulation Model coupled to a thermodynamic slab ocean model. Previous studies identified a teleconnection between the high-latitude fresh-waterforced abrupt climate change and the low-latitude climate response during a Younger Dryas-like abrupt climate change using coupled Atmosphere-Ocean General Circulation Models. In this study we attempt to separate and compare the atmospheric and oceanic contributions to this teleconnection. The results show that these contributions have comparable climate response magnitudes, but different spatial characteristics with the atmospheric contribution being more zonally symmetric than the oceanic counterpart.

Physical atmospheric and oceanic processes are also analyzed to address the second question. It is found that the equator ward propagation of the high-latitude surface cooling is induced by increasing surface sensible heat flux in northern mid-latitudes and subtropics and surface latent heat flux in northern equatorial region. The increase in sensible heat flux is due to cooling of near surface air temperatures, whereas latent heat response is caused by strengthening of the surface trade winds linked to an increase in meridional surface temperature gradient. The oceanic contribution in the North Atlantic is through changes in the oceanic circulation caused by freshwater forcing.

To address the third question, we performed additional modelling experiments with same high-latitude forcing but different oceanic mixed layer depths. It is found that change in sea surface temperature is necessary for the high-to-low latitude teleconnection and the tropical precipitation response. To determine the importance of sea surface temperature in Intertropical Convergence Zone (ITCZ) response, we further performed an experiment using a simple model developed by Lindzen and Nigam (1987) and found that the change in sea surface temperature, in concert with lower-tropospheric vertical mixing and surface drag, largely contributes to the simulated ITCZ shift.