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gfdl on-line bibliography > forthcoming citations
Characteristics of the Atlantic storm-track Eddy activity and its relation with the North American Oscillation
| Rivière, G. and I. Orlanski, 2006: Characteristics of the Atlantic storm-track Eddy activity and its relation with the North American Oscillation. Journal of the Atmospheric Sciences. 2/06. |
| Abstract: Our study focuses on feedbacks of the high-frequency eddy activity onto the quasi-stationary circulation, particularly with regard to the North Atlantic Oscillation (NAO). The methodology consists in analyzing observations from NCEP reanalysis data and sensitivity runs from a high resolution non hydrostatic regional model. |
| Consistent with recent studies, results show that the jet displacement characteristic of the NAO phenomenon depends strongly on the dynamics of the synoptic-scale waves and the way they break. Positive and negative phases of the NAO are closely related to anticyclonic and cyclonic wave breaking respectively. Indeed, the monthly average of the high-frequency momentum fluxes whose sign is directly related to the type of wave breaking is shown to be correlated with the monthly NAO index over the Atlantic. Furthermore, a daily analysis indicates that the signal associated with the high-frequency fluxes precedes that of the NAO by a few days suggesting that wave breaking is triggering NAO events. Two examples illustrate the significant impact of single storms, in particular those occurring in the East coast of the US. The wave breaking at the end of their life cycle can suddenly change the NAO index in a few days and as the return to equilibrium takes a much longer time, it can even affect the sign of the NAO during an entire month. |
| An important issue determining the type of wave breaking and the phase of the NAO is related to upstream effects. By considering a domain extending from Eastern Pacific to Western Europe and by forcing the regional model with real data at the western boundary, sensitivity runs show that the right sign of the NAO index can be recovered.These studies further indicate that waves coming from Eastern Pacific and North America are crucial for determining the NAO phase. According to their spatial scales and frequencies when they reach the Atlantic domain, they can break one way or another and push the Atlantic jet equatorward or poleward. Synoptic waves with periods between 5 days and 12 days break anticyclonically whereas those with periods between 2 and 5 days break both anticyclonically and cyclonically with a predominance for cyclonic wave breaking. A similar conclusion exists in terms of spatial scale; the larger (smaller) the spatial scale of the wave is the stronger the chance is for the wave to break anticyclonically (cyclonically). |
| Another crucial factor concerns surface effects. Cyclonic wave breaking in the upper levels is strongly connected with an explosive cyclonic development at the surface accompanied by strong surface moisture fluxes whereas such an explosive growth is not present in the anticyclonic wave breaking case. |
| Finally, we present evidence that our results are not only useful for explaining the intraseasonal variations of the NAO but would serve also as a basis for understanding its interannual and interdecadal variations. |