Northern hemisphere circulation regimes during the extremes of the El Ni±o/Southern Oscillation

Abstract

Mid-latitude circulation regimes affected by the El Ni±o/Southern Oscillation (ENSO) warm and cold extremes are analysed using a daily data catalogue (after Dzerdzeevskii) of 36 different hemispheric circulation patterns per season. These patterns are combined to form a bimodal structure of predominantly zonal and meridional steering of the cyclones and anticyclones. The largest ENSO response of these modes is observed in winter seasons at the end of the year of the event with enhanced zonality (meridionality) responding to warm (cold) episodes. Individual circulation patterns show that a warm event forces the storm track across the North Pacific into a more zonal direction, in particular its more sensitive tail in the eastern region. Furthermore, the distribution of residence times of the circulation regimes reveals an enhanced warm event response of short period (4–8 days) disturbances associated with a larger zonal wavenumber.