A novel approach to diagnosing Southern Hemisphere planetary wave activity and its influence on regional climate variability

Abstract

Southern Hemisphere mid-to-upper tropospheric planetary wave activity is characterized by the superposition of two zonally-oriented, quasi-stationary waveforms: zonal wavenumber one (ZW1) and zonal wavenumber three (ZW3). Previous studies have tended to consider these waveforms in isolation and with the exception of those studies relating to sea ice, little is known about their impact on regional climate variability. We take a novel approach to quantifying the combined influence of ZW1 and ZW3, using the strength of the hemispheric meridional flow as a proxy for zonal wave activity. Our methodology adapts the wave envelope construct routinely used in the identification of synoptic-scale Rossby wave packets and improves on existing approaches by allowing for variations in both wave phase and amplitude. While ZW1 and ZW3 are both prominent features of the climatological circulation, the defining feature of highly meridional hemispheric states is an enhancement of the ZW3 component. Composites of the mean surface conditions during these highly meridional, ZW3-like anomalous states (i.e. months of strong planetary wave activity) reveal large sea ice anomalies over the Amundsen and Bellingshausen Seas during autumn and along much of the East Antarctic coastline throughout the year. Large precipitation anomalies in regions of significant topography (e.g. New Zealand, Patagonia, coastal Antarctica) and anomalously warm temperatures over much of the Antarctic continent were also associated with strong planetary wave activity. The latter has potentially important implications for the interpretation of recent warming over West Antarctica and the Antarctic Peninsula.

Introduction

\label{s:introduction}

The relationship between mid-to-upper tropospheric planetary wave activity and regional climate variability in the Northern Hemisphere (NH) has received a great deal of attention in recent times, as researchers try to better understand the links between the Arctic Amplification and mid-latitude weather (e.g. Cohen et al., 2014; Screen et al., 2014). While the meridional temperature gradient has not undergone such dramatic changes in the Southern Hemisphere (SH), this flurry of research activity has highlighted the deficits in our understanding of SH planetary wave activity and its link to surface conditions.

In both hemispheres, large-scale topography and continent-ocean heating contrasts provide strong forcing for longitudinally asymmetric planetary scale time-mean motions. Such motions, usually referred to as stationary or planetary waves, are especially strong during winter and tend to have an equivalent barotropic structure, meaning the wave amplitude increases with height but phase lines tend to be vertical (Holton et al., 2013). In the context of weather and climate variability at the surface, these waves are important because they produce local regions of enhanced and diminished time-mean westerly winds, which strongly influence the development and propagation of transient weather disturbances. Persistent (or blocked) weather patterns, for instance, are typically associated with high-amplitude waves in the upper troposphere (e.g. Trenberth et al., 1985; Renwick, 2005). The meridional transport of heat and moisture associated with these waves also influences surface conditions.

It was van Loon et al. (1972) who first characterized SH planetary wave activity as the superposition of two zonally-oriented, quasi-stationary waveforms of wavenumber one (ZW1) and wavenumber three (ZW3). Based on Fourier decompositions of the mid-to-upper tropospheric circulation, they concluded that the net effect of the other wavenumbers was simply to modulate ZW1 and ZW3. Since that landmark study, the ZW1 and ZW3 patterns have been identified as dominant features of the mid-latitude circulation on daily (e.g. Kidson, 1988), seasonal (e.g. Mo et al., 1985) and interannual (e.g. Karoly, 1989) timescales. Corresponding metrics and climatologies have been developed (Raphael, 2004; Hobbs et al., 2007) and their relationship with circulation features including the Amundsen Sea Low (Turner et al., 2013) and two prominent quasi-stationary anticyclones in the sub-Antarctic western hemisphere (Hobbs et al., 2010) have been investigated.

While these climatologies and investigations reveal many of the basic characteristics of the ZW1 and ZW3 patterns (e.g. their variability and spatial pattern), with the exception of the ZW3 sea ice analyses of Raphael (2007) and Yuan et al. (2008) and the ZW1 sea surface temperature results of Hobbs et al. (2007), subsequent studies have not yet extended these climatologies to look at their influence on key variables such as surface temperature and precipitation. Related studies on topics such as Australian (Frederiksen et al., 2014) and Patagonian (Garreaud et al., 2013) precipitation variability sometimes mention a ZW3-like pattern in passing, but the literature lacks a broad, hemispheric perspective on the link between planetary wave activity and regional climate variability. One reason for this might be that the ZW1 and ZW3 patterns never really occur in isolation, which makes analyses of just one or the other somewhat problematic (Hobbs et al., 2010).

In this study we take a new approach to the analysis of quasi-stationary, monthly timescale (30-day running mean) zonal wave activity. Rather than focus on a specific wavenumber or stationary pattern, we use a signal processing technique based on the Hilbert transform to identify all data times of strong meridional hemispheric flow. The vast majority of these times are associated with a mixed ZW1 / ZW3 pattern, and thus strong meridional flow is used as a proxy for planetary wave activity that does not suffer many of the shortcomings of existing approaches. We apply this proxy in considering (a) the climatological characteristics of SH planetary wave activity, (b) its impact on surface temperature, precipitation and sea ice and (c) the implications of this new approach for interpreting existing analyses of SH planetary wave activity. This investigation is particularly timely given that the mid-to-high southern latitudes have exhibited significant circulation (and consequent temperature, precipitation and sea ice) changes over recent decades (e.g. Bromwich et al., 2013; Burgener et al., 2013; Simmonds, 2015).