SPELLS OF LOW-FREQUENCY OSCILLATIONS AND WEATHER REGIMES IN THE NORTHERN-HEMISPHERE

The low-frequency variability in the midlatitudes is described through an analysis of the oscillatory phenomena. In order to isolate nearly periodic components of the atmospheric flow, the multichannel version of the singular spectrum analysis (M-SSA) is developed and applied to an NMC 32-year long set of 700-hPa geopotential heights. In the same w ay that principal component analysis identifies the spatial patterns d ominating the variability, M-SSA identifies dynamically relevant space -time patterns and provides an adaptive filtering technique. Three maj or low-frequency oscillations (LFOs) are found, with periods of 70 day s, 40-45 days, and 30-35 days. The 70-day oscillation consists of fluc tuations in both position and amplitude of the Atlantic jet, with a po leward-propagating anomaly pattern. The 40-45-day oscillation is speci fic to the Pacific sector and has a pronounced Pacific/North American (PNA) structure in its high-amplitude phase. The 30-35-day mode is con fined over the Atlantic region, and consists of the retrogression of a dipole pattern. All these oscillations are shown to be intermittently excited, and M-SSA allows the localization of their spells. The two A tlantic oscillations turn out to be frequently phase locked, so that t he 30-35-day mode is likely to be a harmonic of the 70-day mode. The p hase locking of the Pacific 40-45-day with the Atlantic 30-35-day osci llations is also studied. Next, the relationships between LFOs and wea ther regimes are studied. It is shown in particular that the occurrenc e of the Euro-Atlantic blocking regime is strongly favored, although n ot systematically caused, by particular phases of the 30-35-day mode. The LFOs themselves are able to produce high-amplitude persistent anom alies by interfering with each other. The transition from a zonal regi me to a blocking regime is also shown to be highly connected to the li fe cycle of the 30-35-day mode, indicating that regime transitions do not result only from the random occurrence of particular transient edd y forcing. There are preferred paths between weather regimes. This res ult leaves us with the hope that at least the large-scale environment- favoring weather regimes may be forecast in the long range. Conditiona l probability of occurrence of blocking 30 days ahead, is enhanced, re lative to climatological probability, by a factor of 2 if the phase of the 30-35-day oscillation is known. This also emphasizes the necessit y of operational models to represent correctly the extratropical LFOs in order to produce skillful long-range and even medium-range forecast s of weather regimes.