Transitions Between Circulation Regimes: The Role of Tropical Heating

Abstract

Persistent weather patterns are responsible for much of the extreme weather experienced around the world. These persistent patterns have also been found to reoccur over certain regions including the Eastern Pacic, North America, Europe, and the North Atlantic. We term these persistent, reoccurring patterns "circulation regimes". In this thesis we focus on regimes in the Euro-Atlantic region. Circulation Regimes provide a single consistent framework to describe large-scale patterns in the Euro-Atlantic (EA) region such as the North Atlantic Oscillation (NAO) and European Blocking. Each individual state of the atmosphere is uniquely associated with one regime, and each regime is accompanied by distinct weather patterns, and likelihoods of extreme events. This thesis focuses on transitions between four EA regimes and precursors of such transitions. We examine the signature of tropical heating preceding transitions between each type of regime transition and identify transitions for which this forcing plays a role. We further examine the relationship of heating to the Madden-Julian Oscillation (MJO), the El-Nino Southern Oscillation, and shifts in the intertropical Convergence Zone (ITCZ). Mid-latitude diabatic heating is examined to determine shifts in the storm tracks. We use the ERA-Interim reanalysis (at full horizontal resolution, 6 hourly) to estimate the diabatic heating as a residual in the thermodynamic equation. The same reanalysis is used to perform a k-means cluster analysis of 500 hPa geopotential height in the EA region during boreal winter, yielding the regimes of NAO+, NAO-, Scandinavian Blocking and Atlantic Ridge. We composite the vertically integrated, planetary wave diabatic heating, 300 hPa streamfunction, and Rossby wave activity. We nd that the effects of Indian Ocean tropical heating are to enhance the transition from the Scandinavian Blocking EA circulation regime to the NAO+ regime, while western Pacic heating seems to force transitions from all other regimes into the NAO- regime. The flux of Rossby wave activity shows that for six transitions, mid-latitudes play a role in forcing the tropical heating. It was also found that six of the 10 transitions examined show indications of being tropically forced. Only four showed evidence that mid-latitude dynamics were the primary cause to the transition. Only four of the transitions appeared to be related to phases of the MJO. In fact, intensification of eastern equatorial Pacic and equatorial Atlantic heating (intensification of the ITCZ) plays a role in some transitions. The role of the El-Nino Southern Oscillation is found to be only a modest factor.