Significant change of extratropical natural variability and potential predictability associated with the El Niño/Southern Oscillation

Abstract

Beyond the deterministic limit where the initial value sensitive predictability can hardly befound, the boundary condition dependent potential predictability is examined. The tropicalanomalies of the opposite phases of El Niño/Southern Oscillation (ENSO) can significantlychange the extratropical natural variability on a wide range of spatial and temporal scales. Weaddress the impact on the low-frequency variability, such as the persistent blocking flows, andthe high-frequency variability, represented primarily by the storm tracks. The NCEP/NCARreanalyses are used for this investigation. Several diagnostics tools help to reveal the dynamicalprocesses leading to the large change of the natural variability and the potential predictabilityin the extratropical latitudes between these two phases of the ENSO cycle. During El Niño winters, the principal storm tracks are steered more into the southern and Baja Californiaregion, by the much eastward extended subtropical jets. On the other hand, the storms arediverted more into the higher latitudes (Aleutians and Gulf of Alaska) during La Niña winters, when jetstreams are much weaker east of 160°W. Although being passively steered to widelydifferent regions, the high-frequency transients do feed back actively to strengthen and maintainthe subtropical jet across the central North Pacific and also act to slow down the equatorwardflank of the jet. The feedback by the transients is stronger during the El Niño than the La Niñawinters, helping in maintaining stronger signals from the tropics for the El Niño winters. Thereis also a large change of low-frequency variability: much larger magnitude of kinetic energyand height variance during La Niña than El Niño winters. The local barotropic energy diagnosisreveals that, on average, the low-frequency components extract more energy from time-meanflows during La Niña than El Niño winters, helping in explaining the presence of much largerlow-frequency variability during the La Niña winters. With stronger ENSO signals and weakernatural variability during El Niño winters, the potential predictability in the north Pacific sectoris significantly higher, on these two counts, than during the La Niña winters.