Equatorial mountain torques and cold surges preconditionning

The purpose of this study is to investigate the dynamical causes of the cold surges, and especially to test the hypothesis that the preconditionning of the cold surges is essentially due to dynamical forcings by the relevant mountain range (Andes, Rockies, Tibetan Plateau) on the atmospheres, and that the equatorial mountain torques (EMT) are a good measure of these forcings. This is done through statistical analysis and theoretical modelling.

Cold surges are orographically induced phenomena that occur east of the Rockies, the Tibetan Plateau and the Andes. Unlike many other topographic phenomena, the space and time scale of the cold surges is almost synoptic. They have major impacts on agriculture in Florida, Mexico, central America, and Brazil, as well as on the transportation systems of China. Further into the tropics, they can contribute to strengthening the east-asian winter monsoon circulation and trigger deep tropical convection contributing to extreme events in, e.g., Malaysia. It has also been shown that the cold surges, by bringing unusually cold temperatures in mild subtropical places, have strong implications for public health, increasing significantly the cardiovascular disease mortality rates.

The evolution of the two components of the EMT applied by mountains on the atmosphere is analyzed in the NCEP reanalysis data. A strong lagged relationship between the EMT component along the Greenwich axis (T_M1) and the EMT component along the 90^oE 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, it is shown that this relationship is in part associated to the development of cold surges, in the sense that cold surges tend to follow with a 2-4 day lag the strongest positive (negative) events on T_M1 in the northern (southern) hemisphere.

Following these results, two hypotheses are made: that the mountain forcing on the atmosphere is well measured by the regional EMTs and that this forcing partly drives the cold surges. To support these, a purely dynamical linear model is proposed: it is written on the sphere, uses a 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 temperatures develop in the lower troposphere east of the mountains, explaining the signal on T_M2 and providing the favorable conditions for the cold surge development: the model is able to show how the dynamical forcing by the mountain on the atmosphere when an anticyclone is located poleward the mountain can rapidly produce a shallow dome of cold air at the eastern side of the mountain, which is typical of the early phase of the cold surges.

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.