This then leads onto a debate

This then leads onto a debate as to whether the NAO is distinct from the AO/NAM, and if not, which of the two is to be considered the most physically based expression of atmospheric structure (as opposed to the one that most clearly falls out of mathematical expression). Westerly winds blowing across the Atlantic bring moist air into Europe. Sony VAIO VPCCW21FX/B Battery In years when westerlies are strong, summers are cool, winters are mild and rain is frequent. If westerlies are suppressed, the temperature is more extreme in summer and winter leading to heatwaves, deep freezes and reduced rainfall. A permanent low-pressure system over Iceland (the Icelandic Low) Sony VAIO VPCCW21FX/L Battery and a permanent high-pressure system over theAzores (the Azores High) control the direction and strength of westerly winds into Europe. The relative strengths and positions of these systems vary from year to year and this variation is known as the NAO. A large difference in the pressure at the two stations (a high index year, denoted NAO+)Sony VAIO VPCCW21FX/R Battery leads to increased westerlies and, consequently, cool summers and mild and wet winters in Central Europe and its Atlantic facade. In contrast, if the index is low (NAO-), westerlies are suppressed, these areas suffer cold winters and storms track southerly toward the Mediterranean Sea. Sony VAIO VPCCW21FX/W Battery This brings increased storm activity and rainfall to southern Europe and North Africa.Especially during the months of November to April, the NAO is responsible for much of the variability of weather in the North Atlantic region, affecting wind speed and wind direction changes, changes in temperature and moisture distribution and the intensity, number and track of storms. Sony VAIO VPCCW26EC Battery Although having a less direct influence than for Western Europe, the NAO is also believed to have an impact on the weather over much of eastern North America. During the winter, when the index is high (NAO+),Sony VAIO VPCCW26FX/B Battery the Icelandic low draws a stronger south-westerly circulation over the eastern half of the North American continent which prevents Arctic air from plunging southward. In combination with the El Niño, this effect can produce significantly warmer winters over the northeastern United States and southeastern Canada. Sony VAIO VPCCW28EC Battery Conversely, when the NAO index is low (NAO-), the eastern seaboard and southeastern United States can incur winter cold outbreaks more than the norm with associated snowstorms and sub-freezing conditions into Florida, in summer, a strong NAO- is thought to contribute to a weakened jet stream that normally pulls zonal systems into the Atlantic Basin, thus contributing to heat waves. Sony VAIO VPCCW28FJ/P Battery Under a positive NAO index (NAO+), regional reduction in atmospheric pressure results in a regional rise in sea level due to the 'inverse barometer effect'. This effect is important to both the interpretation of historic sea level records and predictions of future sea level trends, Sony VAIO VPCCW28FJ/R Battery as mean pressure fluctuations of the order of milibars can lead to sea level fluctuations of the order of centimeters. By controlling the position of the Azores high, the NAO also influences the direction of general storm paths for major North Atlantic tropical cyclones: Sony VAIO VPCCW28FJ/W Battery a position of the Azores high farther to the south tends to force storms into the Gulf of Mexico, whereas a northern position allows them to track up the North American Atlantic Coast.^[4] As paleotempestological research has shown, few major hurricanes struck the Gulf coast during 3000–1400 BC and again during the most recent millennium. Sony VAIO VPCCW29FJ/W Battery These quiescent intervals were separated by a hyperactive period during 1400 BC and 1000 AD, when the Gulf coast was struck frequently by catastrophic hurricanes and their landfall probabilities increased by 3–5 times. Sony VAIO VPCCW2AFJ Battery Until recently, the NAO had been in an overall more positive regime since the late 1970s, bringing colder conditions to the North-West Atlantic, which has been linked with the thriving populations of Labrador Sea snow crabs, which have a low temperature optimum. Sony VAIO VPCCW2AHJ Battery The NAO+ warming of the North Sea reduces survival of cod larvae which are at the upper limits of their temperature tolerance, as does the cooling in the Labrador Sea, where the cod larvae are at their lower temperature limits.Though not the critical factor, the NAO+ peak in the early 1990s may have contributed to the collapse of the Newfoundland cod fishery. Sony VAIO VPCCW2S1E Battery On the East Coast of the United States an NAO+ causes warmer temperatures and increased rainfall, and thus warmer, less saline surface water. This prevents nutrient-rich upwelling which has reduced productivity. Georges Bank and the Gulf of Maine are affected by this reduced cod catch. Sony VAIO VPCCW2S1E/B Battery The strength of the NAO is also a determinant in the population fluctuations of the intensively studied Soay sheep.The winter of 2009-10 in Europe was unusually cold. It is theorized that this may be due to solar activity.The Met Office reported that the UK, for example, had experienced its coldest winter for 30 years. Sony VAIO VPCCW2S1E/L Battery This coincided with an exceptionally negative phase of the NAO.^[11] Analysis published in mid-2010 suggests it was caused by a freak combination of an 'El Niño' event and the rare occurrence of an extremely negative NAO. However, during the winter of 2010-11 in Northern and Western Europe, Sony VAIO VPCCW2S1E/P Battery the Icelandic low, typically positioned west of Iceland and east of Greenland, appeared regularly to the east of Iceland and so allowed exceptionally cold air into Europe from the Arctic. A strong area of high pressure was initially situated overGreenland, reversing the normal wind pattern in the northwestern Atlantic, Sony VAIO VPCCW2S1E/R Battery creating a blocking pattern driving warm air into northeastern Canada and cold air into Western Europe, as was the case during the previous winter. This occurred during a La Niña season, and is connected to the rare Arctic dipole anomaly.On the other side of the Atlantic both of these winters were mild, especially 2009-2010, which was the record warmest in Canada. Sony VAIO VPCCW2S1E/W Battery The winter of 2010-2011 was particularly above normal in the northern Arctic regions of the country. The probability of cold winters with much snow in Central Europe rises when the Arctic is covered by less sea ice in summer. Sony VAIO VPCCW2S5C CN1 Battery Scientists of the Research Unit Potsdam of the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association have decrypted a mechanism in which a shrinking summertime sea ice cover changes the air pressure zones in the Arctic atmosphere and impacts on European winter weather. Sony VAIO VPCF112FX/B Battery If there is a particularly large-scale melt of Arctic sea ice in summer, as observed in recent years, two important effects are intensified. Firstly, the retreat of the light ice surface reveals the darker ocean, causing it to warm up more in summer from the solar radiation (ice-albedo feedback mechanism). Sony VAIO VPCF115FG/B Battery Secondly, the diminished ice cover can no longer prevent the heat stored in the ocean being released into the atmosphere (lid effect). As a result of the decreased sea ice cover the air is warmed more greatly than it used to be particularly in autumn and winter because during this period the ocean is warmer than the atmosphere. Sony VAIO VPCF116FGBI Battery The warming of the air near to the ground leads to rising movements and the atmosphere becomes less stable. One of these patterns is the air pressure difference between the Arctic and mid-latitudes: the so-called Arctic oscillation with the Azores highs and Iceland lows known from the weather reports. Sony VAIO VPCF117FJ/W Battery If this difference is high, a strong westerly wind will result which in winter carries warm and humid Atlantic air masses right down to Europe. In the negative phase when pressure differences are low, cold Arctic air can then easily penetrate southward through Europe without being interrupted by the usual westerlies, Sony VAIO VPCF117HG/BI Battery as has been the case frequently over the last three winters. Model calculations show that the air pressure difference with decreased sea ice cover in the Arctic summer is weakened in the following winter, enabling Arctic cold to push down to mid-latitudes. Sony VAIO VPCF118FJ/W Battery The Pacific Decadal Oscillation (PDO) is a pattern of Pacific climate variability that shifts phases on at least inter-decadal time scale, usually about 20 to 30 years. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20° N. During a "warm", or "positive", phase, the west Pacific becomes cool and part of the eastern ocean warms; Sony VAIO VPCF119FC Battery during a "cool" or "negative" phase, the opposite pattern occurs. The Pacific (inter-)decadal oscillation was named by Steven R. Hare, who noticed it while studying salmonproduction pattern results in 1997.^[1] The prevailing hypothesis is that the PDO is caused by a "reddening" of the El Niño-Southern Oscillation (ENSO) combined with stochastic atmospheric forcing.^[2] Sony VAIO VPCF119FC/BI Battery A PDO signal has been reconstructed to 1661 through tree-ring chronologies in the Baja California area.^[3]The interdecadal Pacific oscillation (IPO or ID) display similar sea-surface temperature (SST) and sea-level pressure (SLP) patterns, with a cycle of 15–30 years, but affects both the north and south Pacific. Sony VAIO VPCF119FJ/BI Battery In the tropical Pacific, maximum SST anomalies are found away from the equator. This is quite different from the quasi-decadal oscillation (QDO) with a period of 8-to-12 years and maximum SST anomalies straddling the equator, thus resembling the ENSO. Sony VAIO VPCF11AFJ Battery Several studies have indicated that the PDO index can be reconstructed as the superimposition of tropical forcing and extra-tropical processes. Thus, unlike ENSO, the PDO is not a single physical mode of ocean variability, but rather the sum of several processes with different dynamic origins. Sony VAIO VPCF11AGJ Battery At inter-annual time scales the PDO index is reconstructed as the sum of random and ENSO induced variability in the Aleutian low, on decadal timescales ENSO teleconnections, stochastic atmospheric forcing and changes in the North Pacific oceanic gyre circulation contribute approximately equally, Sony VAIO VPCF11AHJ Battery additionally sea surface temperature anomalies have some winter to winter persistence due to the reemergence mechanism. ENSO can influence the global circulation pattern thousands of kilometers away from the equatorial Pacific through the "atmospheric bridge". Sony VAIO VPCF11JFX/B Battery During El Nino events deep convection and heat transfer to the troposphere is enhanced over the anomalously warm sea surface temperature, this ENSO related tropical forcing generates Rossby waves that propagates poleward and eastward and are subsequently refracted back from the pole to the tropics. Sony VAIO VPCF11M1E Battery The planetary waves forms at preferred locations both in the North and South Pacific Ocean and the teleconnection pattern is established within 2–6 weeks.^[8] ENSO driven patterns modify surface temperature,humidity, wind and the distribution of cloud over the North Pacific that alter surface heat, momentum and freshwater fluxes and thus induce sea surface temperature,salinity andmixed layer depth (MLD) anomalies. Sony VAIO VPCF11M1E/H Battery The atmospheric bridge is more effective during boreal winter when the deepened Aleutian low results in stronger and cold northwesterly winds over the central Pacific and warm/humid southerly winds along the North American west coast, the associated changes in the surface heat fluxes and to a lesser extentSony VAIO VPCF11MFX/B Battery Ekman transport creates negative sea surface temperature anomalies and a deepened MLD in the central pacific and warm the ocean from the Hawaii to the Bering Sea. Midlatitude SST anomaly patterns tend to recur from one winter to the next but not during the intervening summer, this process occurs because of the strong mixed layer seasonal cycle. Sony VAIO VPCF11S1E Battery The mixed layer depth over the North Pacific is deeper, typically 100-200m, in winter than it is in summer and thus SST anomalies that forms during winter and extend to the base of the mixed layer are sequestered beneath the shallow summer mixed layer when it reforms in late spring and are effectively insulated from the air-sea heat flux. Sony VAIO VPCF11S1E/B Battery When the mixed layer deepens again in the following autumn/early winter the anomalies may influence again the surface. This process has been named "reemergence mechanism" by Alexander and Deser^[10] and is observed over much of the North Pacific Ocean although is more effective in the west where the winter mixed layer is deeper and the seasonal cycle greater. Sony VAIO VPCF11Z1E Battery Long term sea surface temperature variation may be induced by random atmospheric forcings that are integrated and reddened into the ocean mixed layer. The stochastic climate model paradigm was proposed by Frankignoul and Hasselmann,^[12] in this model a stochastic forcing Sony VAIO VPCF11Z1E/BI Battery represented by the passage of storms alter the ocean mixed layer temperature via surface energy fluxes and Ekman currents and the system is damped due to the enhanced (reduced) heat loss to the atmosphere over the anomalously warm (cold) SST via turbulent energy and longwave radiative fluxes, in the simple case of a linear negative feedback the model can be written as: Sony VAIO VPCF11ZHJ Battery where v is the random atmospheric forcing, λ is the damping rate (positive and constant) and y is the response.