Equatorial Mountain Torques and Cold Surge Preconditioning

Abstract

The evolution of the two components of the equatorial mountain torque (EMT) applied by mountains on the atmosphere is analyzed in the NCEP reanalysis. A strong lagged relationship between the EMT component along the Greenwich axis T M1 and the EMT component along the 908E axis T M2 is found, with a pronounced signal on T M1 followed by a signal of opposite sign on T M2 . It is shown that this result holds for the major massifs (Antarctica, the Tibetan Plateau, the Rockies, and the Andes) if a suitable axis system is used for each of them. For the midlatitude mountains, this relationship is in part associated with the development of cold surges.Following these results, two hypotheses are made: (i) the mountain forcing on the atmosphere is well measured by the regional EMTs and (ii) this forcing partly drives the cold surges. To support these, a purely dynamical linear model is proposed: it is written on the sphere, uses an f-plane quasigeostrophic approximation, and includes the mountain forcings. In this model, a positive (negative) peak in T M1 produced by a mountain massif in the Northern (Southern) Hemisphere is due to a large-scale high surface pressure anomaly poleward of the massif. At a later stage, high pressure and low temperature anomalies develop in the lower troposphere east of the mountain, explaining the signal on T M2 and providing the favorable conditions for the cold surge development.It is concluded that the EMT is a good measure of the dynamical forcing of the atmospheric flow by the mountains and that the poleward forces exerted by mountains on the atmosphere are substantial drivers of the cold surges, at least in their early stage. Therefore, the EMT time series can be an important diagnostic to assess the representation of mountains in general circulation models.

Cold surge episodes over southeastern Brazil – a potential vorticity perspective

“…Furthermore, in addition to the mechanisms leading to friagen presented in this study, i.e. the amplification of wave trains upstream of South America, several other mechanisms have been shown to be important: Reason (1994) investigate the equatorward propagation of orographically trapped disturbances (Kelvin waves), which are excited during the passage of synoptic-scale wave trains over South America; Mailler and Lott (2010) consider the mountain forcing and build-up of a cold dome which then is advected equatorward; Mysak (1980) and Pedlosky (1987) look at topographic Rossby waves, which are the atmospheric pendant to oceanic shelf waves and might propagate along the Andes; Colle and Mass (1995) and Garreaud (2000) focus on nonlinear advection; and finally, Takaya and Nakamura (2005) looked at cold air advection driven by upper-level PV anomalies, which -using a PV inversion -they found to be important for Asian cold surges. A good overview of different cold surge mechanisms, although for the Asia, is given in Compo et al (1999).…”

Review of Tropical‐Extratropical Teleconnections on Intraseasonal Time Scales

“…Although cold surges have regional dependencies, they all share common features such as a shallow layer of cold, dry air in the lower troposphere with a horizontal scale of 500–1000 km (Garreaud, ) and a time scale varying from 2 days to a few weeks. The short lived cold surges result from the interaction of synoptic‐scale flow with the Earth's major mountain ranges (Garreaud, ; Mailler & Lott, ) and are accompanied by a hydrostatically induced ridge of surface pressure (Garreaud, ) and strong meridional low‐level winds (Vizy & Cook, ). The longer time scale cold surge cases are associated with a number of mechanisms: (1) the development of a short wave train triggered by the expansion of the semipermanent core‐pressure system, the Siberian high, over South East China (e.g., Ding, ; Lau et al, ; Martin et al, ; Ryoo et al, ; Wu & Chan, ; Yen & Chen, ); (2) a Rossby wave train emanating from the contractions/extensions of the westerly polar jet located over the northeastern Atlantic and propagating along the subtropical westerly jet waveguide across the Eurasia (e.g., Chauvin et al, ; Vizy & Cook, ); (3) variations in the position of the Pacific High (Schultz et al, ); and (4) Rossby waves emanating from the high latitudes of the Southern Ocean (e.g., Fukutomi & Yasunari, ; Perrin & Simmonds, ).…”

Impact of Subgrid-Scale Orography on Equatorial Angular Momentum Budget and the Cold Surges in a General Circulation Model

“…Following Lott (1999), this is represented by applying tendencies on the wind essentially perpendicular to the flow, and we use the term SSO lift. We know from this paper, but also from Mailler and Lott (2009), that it is almost equivalent to rising the lowest model level, at least in the quasigeostrophic framework. It has been shown by Lott (1999) that the inclusion of a SSO lift in LMDz improves the representation of the steady planetary waves in the model by enhancing vortex stretching above the major mountain massifs.…”

“…In eastern Asia, this meridional propagation toward the tropics affects the winter monsoon convection over the South China Sea (Mailler and Lott 2009). In North America, the most intense cold surges produce freezing temperatures as far south as Florida and eastern Mexico (Pérez García 1996;Schultz et al 1997).…”