Mid-Latitude Atmospheric Regimes, Subtropical Jet, and ENSOPrimary Author: Lucarini, Valerio
Additional Authors: Sandro Calmanti, Alessandro Dell\'Aquila, Paolo M. Ruti, Antonio Speranza
Mid-Latitude Atmospheric Regimes, Subtropical Jet, and ENSO
Valerio Lucarini, Sandro Calmanti, Alessandro Dell'Aquila, Paolo M. Ruti, Antonio Speranza
email: valerio.lucarini@unicam.it
Understanding the atmospheric low-frequency variability is of crucial importance in fields such as climate studies, climate change detection, and extended range weather forecast. The Northern Hemisphere climate features the planetary waves as a relevant ingredient of the atmospheric variability. Several observations and theoretical arguments seem to support the idea that winter planetary waves indicator obey a non-Gaussian statistics and may present a multimodal probability density function, thus characterizing the low-frequency portion of the climate system. We show that the upper tropospheric jet strength is a critical parameter in determining whether the planetary waves indicator exhibits a uni- or bimodal behavior, and we determine the relevant threshold value of the jet. These results are obtained by considering the data of the NCEP/NCAR and ECMWF reanalyses for the overlapping period. Our results agree with the non-linear orographic theory, which explains the statistical non-normality of the low-frequency variability of the atmosphere and its possible bimodality. Moreover, since the intensity of the jet is related to the ENSO phase, these results show a connection between the tropical and the mid-latitude climate. Data coming from the 1961-2000 simulations performed by two very high-resolution global climate models, which have been previously shown to represent very well the overall properties of the Northern Hemisphere mid-latitude winter atmospheric variability, match only partially this picture, thus providing some hints on models inaccuracy in representing large scale features.
|
