Variability of the impact of El Nino-southern oscillation on sea-level pressure anomalies over the North Atlantic in January to March (1874-1996)

Sea-level pressure (SLP) anomalies over the North Atlantic and European (NAE) sector (25-70degreesN, 100degreesW-50degreesE) and over a larger domain encompassing the entire North Pacific domain are studied to demonstrate that SLP anomalies (SLPAs) during boreal winter (January-March) vary widely between years characterized by the same El Nino-southern oscillation (ENSO) phase. The typical cold ENSO signal tends to be more stable than the warm one during the 1874-1996 period. The typical cold ENSO pattern (e.g. positive SLPA south of 55degreesN across the North Atlantic and negative SLPA in the northern North Atlantic) is similar to the positive phase of: the North Atlantic oscillation (NAO) and occurs throughout the 20th century, except during the 1950s and 1960s when the basinwide westerlies are particularly slow. On the contrary, the typical warm ENSO pattern (e.g. positive SLPA from central Canada to Scandinavia and negative SLPA from the southeastern USA to central Europe, corresponding to the negative phase of the NAO) occurs mainly from 1930 to 1970. Another robust warm ENSO pattern is associated with a large positive (negative) SLPA between Newfoundland and western Europe (between Greenland and Scandinavia), and occurs mainly at the beginning and the end of the 20th century when the basinwide North Atlantic westerlies are strengthened. All these patterns stay statistically significant when the multi-decadal variability is removed from the North Atlantic SLPA. It is shown that the low-frequency variability of the north tropical Atlantic sea-surface temperature anomalies could exert a modulating effect on the ENSO teleconnection. NAE SLPAs tend to be strong during warm (cold) ENSO winters and consistent with a negative (positive) phase of the NAO when the north tropical Atlantic is anomalously warm (cold). Lastly, the magnitude of the SLPA patterns over the NAE sector appears poorly related to the intensity of sea-surface temperature anomalies in the central and eastern equatorial Pacific. Copyright (C) 2003 Royal Meteorological Society.