Diagnosing topographic forcing in an atmospheric dataset: The case of the North American Cordillera

Kerstin Hartung,John Methven,Gunilla Svensson,Theodore G Shepherd,Brian J Hoskins

doi:10.1002/qj.3677

Abstract

AbstractIt is well known from modelling studies that surface topography influences the large‐scale atmospheric circulation and that several model biases are associated with incorrect representation of topography. The textbook explanation of topographic effects on large‐scale circulation appeals to the theoretical relationship between surface forcing and vortex stretching along trajectories in single‐layer models. The goal of this study is to design and use a simple diagnostic of the large‐scale forcing on the atmosphere when air is passing over topography, directly from atmospheric fields, based on this theoretical relationship. The study examines the interaction of the atmosphere with the North American Cordillera and samples the flow by means of trajectories during Northern Hemisphere winter. We detect a signal of topographic forcing in the atmospheric dataset, which, although much less distinct than in the theoretical relationship, nevertheless exhibits a number of expected properties. Namely, the signal increases with latitude, is usually stronger upslope than downslope, and is enhanced if the flow is more orthogonal to the mountain ridge, for example during periods of positive Pacific–North American index (PNA). Furthermore, a connection is found between an enhanced signal of topographic forcing downslope of the North American Cordillera and periods of more frequent downstream European blocking.

Highlights

The zonal asymmetry of the climatological atmospheric circulation on Earth is primarily induced by zonal asymmetries of the lower boundary conditions
It is well known from modelling studies that surface topography influences the large-scale atmospheric circulation and that several model biases are associated with incorrect representation of topography
The signal increases with latitude, is usually stronger upslope than downslope, and is enhanced if the flow is more orthogonal to the mountain ridge, for example during periods of positive Pacific–North American index (PNA)

Summary

Introduction

The zonal asymmetry of the climatological atmospheric circulation on Earth is primarily induced by zonal asymmetries of the lower boundary conditions. Topography induces flow anomalies because air is forced either to ascend or to flow around an obstacle, depending on atmospheric stability and the large-scale structure of the topography (Valdes and Hoskins, 1991; Lott and Miller, 1997). Hoskins and Karoly, 1981; Held, 1983; Held et al, 2002) In atmospheric models, both resolved and unresolved topographic processes need to be captured to ensure correct representation of the complete effect of topography on the system. It is known that the representation of topography is crucial for model fidelity of phenomena downstream, for example the North American Cordillera and the Rocky Mountains for the Atlantic storm track (Brayshaw et al, 2009; Pithan et al, 2016). The representation of topography, and potentially of the Rocky Mountains, modulates atmospheric blocking over the Atlantic and Europe (Jung et al, 2012; Berckmans et al, 2013; Pithan et al, 2016)

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