Does the NAO index represent zonal flow? The influence of the NAO on North Atlantic surface temperature (original) (raw)
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The North Atlantic Oscillation and its relationship with near surface temperature
Geophysical Research Letters, 2001
A new NAO index is presented here using homogenized surface pressure data from Reykjavik and Gibraltar (for November to March) and Reykjavik and Ponta Delgada (for April to October). This index suggests that the positive trend in recent years is not unprecedented, as the early 20th century was also a period of persistently positive NAO values. The relationship between the secular warming trend and the influence of the NAO on near-surface temperatures over the North Atlantic region and surrounding land masses is examined on a seasonal basis using standardized temperature anomalies since 1900. The nearsurface temperature field separates into two independent modes, which we designate a "warming" mode and dynamic ("NAO") mode, with distinct seasonal cycles.
Climatic variability over the North Atlantic
International Geophysics, 2002
This paper summarizes an invited presentation given at the historic "Meteorology at the Millennium" Conference in July 2000, which marked the 150 th anniversary of the Royal Meteorological Society. It begins with a broad review of the North Atlantic Oscillation (NAO) and the mechanisms that might influence its phase and amplitude on decadal and longer time scales. New results are presented which suggest an important role for tropical ocean forcing of the unprecedented trend in the wintertime NAO index over the past several decades. We conclude with a brief discussion of a recent significant change in the pattern of the summertime atmospheric circulation over the North Atlantic.
Variability of the North Atlantic Oscillation over the past 5,200 years
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
Meteorologische Zeitschrift, 2001
The ocean-atmosphere interactions of sea surface temperature and the North Atlantic Oscillation as an atmospheric phenomenon are examined in an ensemble of climate change simulations. The principal task concentrates on the common climate change signals, the lead-lag relationship, the time-space covariability, and the aspect of predictability. The study is based on Monte Carlo experiments of the German coupled climate model ECHAM-3/LSG with increasing greenhouse-gas concentrations according to IPCC scenario A. The Atlantic SST variability is mainly determined by the greenhouse-gas induced warming signal with maximum temperature rise in the low latitudes whereas in the Icelandic region the CO 2 in uence is blotted out by intense natural variability. On a monthly time scale, the North Atlantic SST eld is regionally affected by the NAO. An oceanic impact cannot be observed on this short time scale. The main oceanic response occurs in the Sargasso Sea, the Irminger Sea, and less pronounced in the subtropical North Atlantic building a tripole structure. A climatological index is de ned indicating the main response of North Atlantic SST to the atmospheric forcing. This index is largely coherent with the index of the North Atlantic Oscillation on the interannual and decadal time scale. The covariability of sea surface temperature and sea level pressure in the North Atlantic sector amounts to 26 % of total variance. Zusammenfassung Die vorliegende Studie untersucht die Wechselwirkungen zwischen den Meeresober ächentemperaturen (SST) im Nordatlantik und dem atmosphärischen Phänomen der Nordatlantik-Oszillation (NAO). Die Datengrundlage bildet ein Ensemble von vier Simulationen des gekoppelten Klimamodelles ECHAM-3/LSG mit externem Treibhausgasantrieb gemäß IPCC-Szenario A. Neben dem Aspekt des gemeinsamen Klimasignales werden die Richtung der Ein ussnahme im ozeanisch-atmosphärischen System, die raumzeitlichen Ausprägungen der Kovariabilität und die Möglichkeiten zur Vorhersage betrachtet. Die SST-Variabilität im Atlantik ist nach 1970 eindeutig von einem CO 2-induzierten Erwärmungssignal geprägt. Dieses Signal tritt zuerst in den niederen Breiten auf und propagiert von dort aus polwärts, ist hingegen in den Regionen häu ger Zyklonenaktivität nicht nachweisbar. Auf der Monatszeitskala beein usst die NAO die Variabilität des nordatlantischen SST-Feldes, wohingegen eine andersgerichtete Ein ußnahme nicht zu verzeichnen ist. Die Zentren des Kreuzkorrelationsmusters liegen in der Sargasso-See und der Irminger-See sowie in schwächerer Ausprägung imöstlichen subtropischen Atlantik, so dass ein dreipoliges Muster vorliegt. Aus diesem Muster wird ein Indexwert de niert, welcher die wesentliche ozeanische Reaktion auf den atmosphärischen Antrieb beschreibt. Dieser Index ist auf der interannuellen bis dekadischen Zeitskala weitgehend kohärent mit dem Index der NAO. Die raumzeitliche Kovariabilität von nordatlantischem SSTund Bodendruckfeld beläuft sich im Modell auf 26 % der Gesamtvarianz.
Global and Planetary Change, 2012
Within the last Millennium, the transition between the Medieval Climate Anomaly (MCA; ca. 1000-1300 CE) and the Little Ice Age (LIA; ca. 1400-1800 CE) has been recorded in a global array of climatic and oceanographic proxies. In this study, we review proxy evidence for two alternative hypotheses for the effects of this shift in the North Atlantic region. One hypothesis postulates that the MCA/LIA transition included a weakening of the Atlantic Meridional Overturning Circulation (AMOC) and a transition to more negative North Atlantic Oscillation (NAO) conditions, resulting in a strong cooling of the North Atlantic region. The alternative hypothesis proposes a MCA/LIA shift to an increased number of storms over the North Atlantic linked to increased mid-latitude cyclogenesis and hence a pervasive positive NAO state. The two sets of proxy records and thus of the two competing hypotheses are then reconciled based on available results from climate model simulations of the last Millennium. While an increase in storm frequency implicates positive NAO, increased intensity would be consistent with negative NAO during the LIA. Such an increase in cyclone intensity could have resulted from the steepening of the meridional temperature gradient as the poles cooled more strongly than the Tropics from the MCA into the LIA.
INTERANNUAL VARIABILITY OF THE LOCATION OF THE MAIN ATLANTIC PRESSURE SYSTEMS AND THE NAO INDEX
The North Atlantic Oscillation (NAO) is a major feature of the Northern Hemisphere climate system. The unprecedented trends it exhibited after the early 60's led some authors to propose human induced climate forcing as a possible cause. The various NAO indices are not able to distinguish the variability associated to the magnitude strengthening/weakening of the action centers and the variability associated with the movement of these centers. Using the NCEP/NCAR reanalysis for the years 1958 till 1998 the positions of the Iceland low-pressure and of the Azores high-pressure systems were tracked down. With the geographic position of these two action centers and the corresponding pressure values, an
Complex North Atlantic Oscillation (NAO) Index signal of historic North Atlantic storm-track changes
Holocene, 2002
Previous reports have suggested a link between increased storminess in the North Atlantic during recent years with a period of time during which the North Atlantic Oscillation Index has been strongly positive. New analyses of late nineteenth-century gale-day data for meteorological stations in northern Scotland and western Ireland indicate that the relatively high storminess that characterized this period was associated with monthly NAO Index values that rarely exceeded +2 and, on several occasions, were strongly negative. It is speculated that this difference may re ect the in uence of an expanded sea-ice cover in the Greenland Sea that caused a considerable southward displacement of the North Atlantic storm track during the late nineteenth century. Such changes imply that the polar atmospheric and oceanic fronts in the North Atlantic were displaced southward during the late nineteenth century.
Dynamics of Atmospheres and Oceans, 2008
In this paper we discuss the atmospheric dynamics of the North Atlantic Oscillation (NAO), the zonal index, and annular patterns of variability (also known as annular modes). Our goal is to give a unified treatment of these related phenomena, to make explicit how they are connected and how they differ, and to illustrate their dynamics with the aid of an idealized primitive equation model. Our focus is on tropospheric dynamics.
Decadal Variations in Climate Associated with the North Atlantic Oscillation
Climatic Change at High Elevation Sites, 1997
Large changes in the wintertime atmospheric circulation have occurred over the past two decades over the ocean basins of the Northern Hemisphere, and these changes have had a profound effect on regional distributions of surface temperature and precipitation. The changes over the North Pacific have been well documented and have contributed to increases in temperatures across Alaska and much of western North America and to decreases in sea surface temperatures over the central North Pacific. The variations over the North Atlantic are related to changes in the North Atlantic Oscillation (NAO). Over the past 130 years, the NAO has exhibited considerable variability at quasibiennial and quasi-decadal time scales, and the latter have become especially pronounced the second half of this century. Since 1980, the NAO has tended to remain in one extreme phase and has accounted for a substantial part of the observed wintertime surface warming over Europe and downstream over Eurasia and cooling in the northwest Atlantic. Anomalies in precipitation, including dry wintertime conditions over southern Europe and the Mediterranean and wetter-than-normal conditions over northern Europe and Scandinavia since 1980, are also linked to the behavior of the NAO. Changes in the monthly mean flow over the Atlantic are accompanied by a northward shift in the storm tracks and associated synoptic eddy activity, and these changes help to reinforce and maintain the anomalous mean circulation in the upper troposphere. It is important that studies of trends in local climate records, such as those from high elevation sites, recognize the presence of strong regional patterns of change associated with phenomena like the NAO.