High North Atlantic Oscillation Research Articles

Decadal and longer changes of the winter sea level pressure fields and related synoptic activity over the North Atlantic

Long-term intensity changes in the winter synoptic activity over the North Atlantic are studied in relation to changes of the winter mean sea level pressure (SLP) fields. Analysis of linear trends has revealed a good agreement between long-term (interdecadal) changes in the intensity of synoptic processes and variations of the winter SLP. On the contrary, no such agreement was found between detrended and low-pass filtered anomalies. There are periods, that are characterised by the enhanced (reduced) synoptic activity attributed to the low (high) index of the North Atlantic Oscillation (NAO). It appears that low-passed anomalies of intramonthly root mean square deviations (RMSD) of SLP are negatively correlated with NAO and East Atlantic (EA) teleconnection patterns over most of the North Atlantic. The low-passed winter SLP anomalies demonstrate both propagating and standing patterns. The latter have a period of about 8 years. While meridional dipole-like structures formed by the winter SLP anomalies are shifted to the west (east) of the North Atlantic, the related anomalies of synoptic activity tend to be located in the eastern (western) part of the region. When decadally averaged, anomalies in the intensity of synoptic activity are strongly linked to the North Atlantic storm track position. The exception is the 1980–1990 decade, characterised by a very high NAO index. During this decade, enhanced synoptic activity is observed to the south of the North Atlantic storm track. Copyright © 1999 Royal Meteorological Society

Variability of Fram Strait ice flux and North Atlantic Oscillation

We estimate the winter sea ice export through the Fram Strait using ice motion from satellite passive microwave data. Sea ice motion (October‐May) is obtained by tracking the displacement of common features in sequential 85 and 37 GHz brightness temperature fields. The average winter area flux over the 18‐year record (1978–1996) is 670,000 km2, ∼7% of the area of the Arctic Ocean. The winter area flux ranges from a minimum of 450,000 km2 in 1984 to a maximum of 906,000 km2 in 1995. The daily, monthly, and interannual variabilities of the ice area flux are high. There is an upward trend in the ice area flux over the 18 year record. The average winter volume flux over the winters of October 1990 through May 1995 is 1745 km3 ranging from a low of 1375 km3 in the 1990 flux to a high of 2791 km3 in 1994. The sea level pressure gradient across the Fram Strait explains more than 80% of the variance in the ice flux over the 18 year record. We use the coefficients from the regression of the time series of area flux versus pressure gradient across the Fram Strait and ice thickness data to estimate the summer area and volume flux. The average 12 month area flux and volume flux are 919,000 km2 and 2366 km3. We find a significant correlation (R = 0.86) between the area flux and positive phases of the North Atlantic Oscillation (NAO) index over the months of December‐March. Correlation between our 5 years of volume flux estimates and the NAO index gives R = 0.56. During the high NAO years a more intense Icelandic low increases the gradient in the sea level pressure by almost 1 mbar across the Fram Strait, thus increasing the atmospheric forcing on ice transport. Correlation is reduced during the negative NAO years because of decreased dominance of this large‐scale atmospheric pattern on the sea level pressure gradient across the Fram Strait.

El Niño–Southern Oscillation and North Atlantic Oscillation Control of Climate in Puerto Rico

Abstract Many studies have shown that the El Nino–Southern Oscillation (ENSO) has a significant influence on climate in many parts of the globe, mostly in the Pacific Basin. The objective of this study is to examine the possible impact of ENSO on climatic patterns on the island of Puerto Rico in the Caribbean. The authors find that annual mean air temperatures are controlled by ENSO since 1914. El Nino years are associated with warm air temperatures, whereas El Viejo (La Nina) years, which are the opposite of El Nino, are cooler. On the other hand, since 1911 fluctuations in annual rainfall amounts are synchronous with variations in the North Atlantic Oscillation (NAO) during the winter and are not controlled by ENSO. During years of a high winter NAO index, when the axis of moisture transport in the North Atlantic changes to a more southwesterly–northeasterly orientation, annual precipitation in Puerto Rico is lower than average.