Climatic variability over the North Atlantic

2003

Climate Dynamics, 2006

Global North Atlantic Oscillation (NAO) oceanic precipitation features in the latter half of the twentieth century are documented based on the intercomparison of multiple state-of-the-art precipitation datasets and the analysis of the NAO atmospheric circulation and SST anomalies. Most prominent precipitation anomalies occur over the ocean in the North Atlantic, where in winter a ''quadrupole-like'' pattern is found with centers in the western tropical Atlantic, sub-tropical Atlantic, high-latitude eastern Atlantic and over the Labrador Sea. The extent of the subtropical and high-latitude center and the amount of explained variance (over 50%) are quite remarkable. However, the tropical Atlantic center is probably the most intriguing feature of this pattern apparently linking the NAO with ITCZ variability. In summer, the pattern is ''tripole-like'' with centers in the eastern Mediterranean Sea, the North Sea/Baltic Sea and in the sub-polar Atlantic. In the eastern Indian Ocean, the correlation is positive in winter and negative in summer, with some link to ENSO variability. The sensitivity of these patterns to the choice of the NAO index is minor in winter while quite important in summer. Interannual NAO precipitation anomalies have driven similar fresh water variations in these ''key'' regions. In the sub-tropical and high-latitude Atlantic in winter precipitation anomalies have been roughly 15 and 10% of climatology per unit change of the NAO, respectively. Decadal changes of the NAO during the last 50 years have also influenced precipitation and fresh water flux at these time-scales, with values lower (higher) than usual in the high-latitude eastern North Atlantic (Labrador Sea) in the 1960s and the late 1970s, and an opposite situation since the early 1980s; in summer the North Sea/Baltic region has been drier than usual during the period 1965-1975 when the NAO was generally positive.

2001

This paper aims to provide a comprehensive review of previous studies and concepts concerning the North Atlantic Oscillation. The North Atlantic Oscillation (NAO) and its recent homologue, the Arctic Oscillation/Northern Hemisphere annular mode (AO/NAM), are the most prominent modes of variability in the Northern Hemisphere winter climate. The NAO teleconnection is characterised by a meridional displacement of atmospheric mass over the North Atlantic area. Its state is usually expressed by the standardised air pressure difference between the Azores High and the Iceland Low. This NAO index is a measure of the strength of the westerly flow (positive with strong westerlies, and vice versa). Together with the El Niño/Southern Oscillation (ENSO) phenomenon, the NAO is a major source of seasonal to interdecadal variability in the global atmosphere. On interannual and shorter time scales, the NAO dynamics can be explained as a purely internal mode of variability of the atmospheric circulation. Interdecadal variability may be influenced, however, by ocean and sea-ice processes.

Geophysical Research Letters, 2000

The North Atlantic Oscillation (NAO) exhibits variations at interannual to multidecadal time scales and is associated with climate variations over eastern North America, the North Atlantic, Europe, and North Africa. Therefore, it is very important to understand causes of these NAO variations and assess their predictability. It has been hypothesized, based on observations, that sea surface temperature (SST) and sea-ice variations in the North Atlantic Ocean influence the NAO. We describe results of an ensemble of sixteen experiments with an atmospheric general circulation model in which we used observed SST and sea-ice boundary conditions globally during 1949-1993. We show that multiyear NAO and associated climate variations can be simulated reasonably accurately if results from a large number of experiments are averaged. We also show that the ambiguous results of previous NAO modeling studies were strongly influenced by the ensemble size, which was much smaller than that in the present study. The implications of these results for understanding and predictability of the NAO are discussed.

Journal of Climate, 2009

Summer climate in the North Atlantic-European sector possesses a principal pattern of year-to-year variability that is the parallel to the well-known North Atlantic Oscillation in winter. This summer North Atlantic Oscillation (SNAO) is defined here as the first empirical orthogonal function (EOF) of observed summertime extratropical North Atlantic pressure at mean sea level. It is shown to be characterized by a more northerly location and smaller spatial scale than its winter counterpart. The SNAO is also detected by cluster analysis and has a near-equivalent barotropic structure on daily and monthly time scales. Although of lesser amplitude than its wintertime counterpart, the SNAO exerts a strong influence on northern European rainfall, temperature, and cloudiness through changes in the position of the North Atlantic storm track. It is, therefore, of key importance in generating summer climate extremes, including flooding, drought, and heat stress in northwestern Europe. The El Niñ o-Southern Oscillation (ENSO) phenomenon is known to influence summertime European climate; however, interannual variations of the SNAO are only weakly influenced by ENSO. On interdecadal time scales, both modeling and observational results indicate that SNAO variations are partly related to the Atlantic multidecadal oscillation. It is shown that SNAO variations extend far back in time, as evidenced by reconstructions of SNAO variations back to 1706 using tree-ring records. Very long instrumental records, such as central England temperature, are used to validate the reconstruction. Finally, two climate models are shown to simulate the present-day SNAO and predict a trend toward a more positive index phase in the future under increasing greenhouse gas concentrations. This implies the long-term likelihood of increased summer drought for northwestern Europe.

We review the role of ocean-atmosphere interactions over the Atlantic sector in North Atlantic Oscillation (NAO) variability. The emphasis is on physical mechanisms, which are illustrated in simple models and analyzed in observations and numerical models. Some directions of research are proposed to better assess the relevance of Atlantic air-sea interactions to observed and simulated NAO variability. Figure 4. Intraseasonal (right thick curve) and interannual (left thick curve) spectrum of an observed NAO index. The red noise or AR(1) spectrum (lowest thin line), and its a priori (middle thin line) and a posteriori (upper thin line) 95% confidence levels are also shown. From Feldstein [2000; his Figure 4].

Journal of Climate, 2004

The observed low-frequency winter atmospheric variability of the North Atlantic-European region and its relationship with global surface oceanic conditions is investigated based on the climate and weather regimes paradigm. Asymmetries between the two phases of the North Atlantic Oscillation (NAO) are found in the position of the Azores high and, to a weaker extent, the Icelandic low. There is a significant eastward displacement or expansion toward Europe for the NAOϩ climate regime compared to the NAOϪ regime. This barotropic signal is found in different datasets and for two quasi-independent periods of record (1900-60 and 1950-2001); hence, it appears to be intrinsic to the NAOϩ phase. Strong spatial similarities between weather and climate regimes suggest that the latter, representing long time scale variability, can be interpreted as the time-averaging signature of much shorter time scale processes. Model results from the ARPEGE atmospheric general circulation model are used to validate observed findings. They confirm in particular the eastward shift of the Atlantic centers of action for the NAOϩ phase and strongly suggest a synoptic origin as it can be extracted from daily analyses. These results bring together present-day climate variability and scenario studies where such an NAO shift was suggested, as it is shown that the last three decades are clearly dominated by the occurrence of NAOϩ regimes when concentrations of greenhouse gases are rapidly increasing. These findings highlight that the displacement of the North Atlantic centers of action should be treated as a dynamical property of the North Atlantic atmosphere and not as a mean longitudinal shift of climatological entities in response to anthropogenic forcings. The nonstationarity with time of the atmospheric variability is documented. Late-century decades differ from early ones by the predominance of NAO climate regimes versus others. In such a context, comments on the relevance of the station-based NAO index is provided. Both tropical and extratropical sea surface temperature (SST) anomalies alter the frequency distribution of the North Atlantic regimes. Evidence is presented that the so-called ridge regime is preferably excited during La Nin ˜a events, while the NAO regimes are associated with the North Atlantic SST tripole. The ARPEGE model results indicate that the tropical branch of the SST tripole affects the NAO regimes occurrence. Warm tropical SST anomalies are more efficient at exciting NAOϪ regimes than cold anomalies are at forcing NAOϩ regimes. The extratropical portion of the North Atlantic SST tripole also seems to play a significant role in the model, tending to counteract the dominant influence of the tropical Atlantic basin on NAO regimes.

International Journal of Climatology, 2001

Variability of the North Atlantic Oscillation and the Tropical Atlantic dominate the climate of the North Atlantic sector, the underlying ocean and surrounding continents on interannual to decadal time scales. Here we review these phenomena, their climatic impacts and our present state of understanding of their underlying cause. Copyright

Atmospheric Science Letters, 2000

The extent to which the North Atlantic Oscillation (NAO) is in¯uenced by changes in the ocean state is an issue that has attracted much recent attention. Although there have been counter claims, the weight of evidence clearly suggests that forcing by the ocean of year-to-year changes in the NAO is a weak in¯uence by comparison with atmospheric internal variability. The NAO is thus very different in character to the Southern Oscillation (SO), and its predictabilityÐat least on seasonal-to-interannual timescalesÐis almost certainly much lower.

Nature Geoscience, 2012

Climate in the Arctic region and northwestern Europe is strongly affected by the North Atlantic Oscillation 1,2 (NAO), the dominant mode of atmospheric variability at mid-latitudes in the North Atlantic region. The NAO index is an indicator of atmospheric circulation and weather patterns: when the index is positive, Europe and the eastern US are mild and wet, whereas Greenland and northern Canada are cold and dry. A negative index is associated with the reverse pattern. Reconstructions of the NAO have so far been limited to the past 900 years 3. Here we analyse a 5,200-year-long, highresolution lake sediment record from southwestern Greenland to reconstruct lake hypolimnic anoxia, and link the results to an existing reconstruction of the NAO index from tree rings and speleothems 3. Using the relationship between the two records, we find that around 4,500 and 650 years ago-around the end of the Holocene Thermal Maximum and the beginning of the Little Ice Age, respectively-the NAO changed from generally positive to variable, intermittently negative conditions. We suggest that variability in the dominant state of the NAO tend to coincide with large-scale changes in Northern Hemisphere climate. However, the onset of the Medieval Climate Anomaly was not associated with any notable changes in the NAO. Future climate change is predicted to warm Arctic regions more than elsewhere, accelerating melting of the Greenland ice sheet with a resultant increase in global sea level of ∼0.5-1.0 m (ref. 4). Injection of freshwater into the North Atlantic Ocean may disrupt the thermohaline circulation and weaken meridional energy exchange. A better delimitation of natural Arctic climate variability is therefore essential to obtain a more complete understanding of the mechanisms driving climate change in this area 5,6. At mid-latitudes in the North Atlantic region, the NAO represents the dominant mode of atmospheric circulation variability. The NAO, defined as the difference in atmospheric pressure at sea level between the Icelandic low and the Azores high, controls the strength and direction of westerly winds and storm tracks across the North Atlantic. This phenomenon exerts a major influence on temperature and precipitation patterns in the region bounding the northern North Atlantic, and affects the oceanic meridional overturning circulation and Arctic sea-ice distribution 7-9. Moreover, it has been demonstrated that the NAO influences ecosystem functioning and, in particular, the length of the growing season, which has a significant impact on biological productivity in the Arctic 1. The NAO is strongly linked to the Arctic Oscillation 10-12 , which represents the leading mode of variability for the whole Northern Hemisphere circulation. At Kangerlussuaq in southwestern Greenland, the NAO is highly correlated to regional