The sensitivity of climate response to the wintertime Niño3.4 sea surface temperature anomalies of 1855-2002

Abstract

The sensitivity of winter atmosphere to ENSO forcing is analysed from an ensemble of integrations forced with observed monthly sea-surface temperatures (SSTs) for the period 1854–2002 with particular interest on the North Atlantic European (NAE) region. A categorisation into warm and cold events, according to the strength of Niño3.4 index, is performed providing main features of simulated atmospheric response to ENSO forcing. A symmetry in a spatial pattern of the atmospheric response to warm and cold phase is found, but with stronger amplitude for warm events. An additional experiment with SST forcing restricted to the tropical Pacific confirmed that atmospheric variability over the Northern Hemisphere is considerably affected by ENSO, even over the distant NAE region. Signals calculated over the Niño3.4, Pacific-North American (PNA) and NAE regions are analysed as functions of the amplitude of the winter (JFM) SST anomalies in the Niño3.4 region. Both the precipitation and large-scale circulation signals over the Niño3.4 and PNA regions gradually increase with the intensity of SST forcing. However, an asymmetry in amplitude of atmospheric response respecting the polarity of the tropical SST anomalies is found. The tropical and PNA signals associated with warm ENSO events are found to be stronger than the signals related to the cold events. A saturation effect is noticed for the cold events. No such evident characteristics are obtained for the NAE region, but the results imply discernible ENSO impact on atmospheric signals over that region. Using a correlation coefficient approach, it is shown that the JFM Niño3.4 SST anomaly affects not only the atmospheric response in the simultaneous season, but also during the next few seasons. Significant correlations between the strength of ENSO and the signals are found for the Niño3.4 and PNA regions. Although with smaller absolute values, statistically significant correlations are also found for the NAE region. Copyright © 2010 Royal Meteorological Society