The role of breaking synoptic scale Rossby waves for the North Atlantic oscillation and its coupling with the stratosphere

Abstract

This thesis investigates, from a modeling and observational perspective, the role of breaking synoptic scale Rossby waves for the North Atlantic oscillation (NAO) and its coupling with the stratosphere. The issue is addressed from three different approaches: (i) Forced-dissipative simulations with a simplified general circulation model are carried out to investigate both, the evolution of baroclinic wave packets in a mid-latitude eddy-driven jet which result in either anticyclonic (AB) or cyclonic wave breaking (CB) during their saturation stage, and its sensitivity to stratospheric flow conditions. (ii) Adiabatic and frictionless simulations of individual baroclinic wave life cycles are performed, using different initial stratospheric flow conditions to study its influence on baroclinic wave breaking. (iii) Observational data are used to analyze AB and CB events in the North Atlantic sector. Their impact on the NAO and its connection to the stratosphere is investigated by a composite analysis, based either on wave breaking events in the troposphere or on episodes of negative stratospheric northern annular mode (NAM).

From the synthesis of the results of these approaches, the following picture is obtained: (1) Stratospheric perturbations (characterized by a negative NAM) appear first in at upper stratospheric levels and propagate downward into the lower stratosphere where they persist for about two months. (2) As a response, the behavior of tropospheric baroclinic waves is changed through a non-linear wave-mean flow interaction, with the consequence of more frequent CB than AB events. (3) This change in wave breaking frequencies is intimately linked --- by a positive feedback --- with a concurrent negative NAO anomaly at the Earth's surface. (4) Baroclinic wave packets of AB- (CB-) like behavior drive meridional circulation dipoles resembling the positive (negative) phase of the NAO. (5) A distinct asymmetry is found between the two kinds of synoptic scale wave breaking: Major AB events emerge from eastward and equatorward propagating wave packets as the center of the packet reaches the equatorward side of the jet, and are typically preceded by a minor AB event that occurs immediately upstream within the same wave group, due to downstream development of the dispersive Rossby waves. Furthermore, the interaction of these two AB events leads to an asymmetry in the vertical in the sense that the resulting positive phase NAO-like dipole is much stronger at the surface than at upper tropospheric levels. On the other hand, CB events emerge from eastward propagating wave packets within a zonal wave guide, and are triggered preferably on the poleward flank of the jet as the center of the packet reaches the diffluent flow field to the west of a preexisting blocking pattern. The resulting negative phase NAO-like dipole is more pronounced at upper levels than at the surface. (6) Despite the asymmetry in the vertical, an equivalent barotropic NAO-like variability pattern may arise from the successive occurrence of AB and CB events within the zonally confined North Atlantic storm-track region.