The Role of Tropospheric Rossby Wave Breaking in the Pacific Decadal Oscillation

Courtenay Strong,Gudrun Magnusdottir

doi:10.1175/2008jcli2593.1

Courtenay Strong, Gudrun Magnusdottir

Open Access

https://doi.org/10.1175/2008jcli2593.1

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Abstract

Abstract The leading pattern of extratropical Pacific sea surface temperature variability [the Pacific decadal oscillation (PDO)] is shown to depend on observed variability in the spatiotemporal distribution of tropospheric Rossby wave breaking (RWB), where RWB is the irreversible overturning of potential vorticity on isentropic surfaces. Composite analyses based on hundreds of RWB cases show that anticyclonic (cyclonic) RWB is associated with a warm, moist (cool, dry) column that extends down to a surface anticyclonic (cyclonic) circulation, and that the moisture and temperature advection associated with the surface circulation patterns force turbulent heat flux anomalies that project onto the spatial pattern of the PDO. The RWB patterns that are relevant to the PDO are closely tied to El Niño–Southern Oscillation, the Pacific–North American pattern, and the northern annular mode. These results explain the free troposphere-to-surface segment of the atmospheric bridge concept wherein El Niño anomalies emerge in summer and modify circulation patterns that act over several months to force sea surface temperature anomalies in the extratropical Pacific during late winter or early spring. Leading patterns of RWB account for a significant fraction of PDO interannual variability for any month of the year. A multilinear model is developed in which the January mean PDO index for 1958–2006 is regressed upon the leading principal components of cyclonic and anticyclonic RWB from the immediately preceding winter and summer months (four indexes in all), accounting for more than two-thirds of the variance.

Highlights

The Pacific decadal oscillation (PDO) is the leading pattern of extratropical Pacific sea surface temperature (SST) variability (Mantua et al 1997)
We presented evidence in support of the hypothesis that Rossby wave breaking (RWB) variability is the physical mechanism by which ENSOand PNA-related changes in the background flow project surface heat flux anomalies onto the spatial pattern of the PDO
Leading patterns of cyclonic and anticyclonic RWB variability can be used to account for a significant fraction of the PDO index (PDOI) for any month of the year, and four empirical orthogonal function (EOF)-based RWB indices from July to August and December to January account for more than two-thirds of the January mean PDOI for 1958–2006

Summary

Introduction

Modeling studies indicate that approximately 1⁄4 to 1⁄2 of the variance of the dominant pattern of decadal, extratropical Pacific SST variability is associated with the atmospheric bridge from ENSO, with changes in net surface heat flux being the dominant factor (Alexander et al 2002; Liu and Alexander 2007). We found two months to be a useful window for this analysis, balancing degrees of freedom against the sharpness with which we resolve seasonally migrating regions of RWB These relative frequencies ga and gc are functions of three independent variables: the year T for T 5 1958, 1959, . Given the 12 values of m and 19 values of ‘, we have 228 linear regression models of the form

Lagged relationships between RWB and PDO

Composite analyses

Regions where RWB influences the PDO

Findings

Summary and discussion