The North Atlantic Oscillation
The term 'North Atlantic Oscillation' (NAO) is used by meteorologists to refer to a naturally-occurring feature of the large-scale surface pressure variability in the North Atlantic region.
In the average state of the atmosphere, the North Atlantic surface pressure is relatively high in the subtropics at latitudes 20°N to 40°N ('the Azores High'), and lower further north at latitudes 50°N to 70°N (the 'Icelandic Low'). This state extends through higher levels in the atmosphere, and affects the north-south pressure difference, which determines the strength of the westerly winds directed from North America towards Europe. More can be found on this under the Met Office learning section North Atlantic Oscillation.
The NAO is basically a name for fluctuations in these north-south differences in pressure. When the difference is larger (for example, the High is higher and the Low is lower than usual) the westerly winds are stronger, and vice versa. As well as wind changes, the NAO is associated with changes in temperature and rainfall in regions such as Europe and North America, so it is an important atmospheric feature. In winter, storms from the North Atlantic tend to be stronger and more frequent. When the pressure difference is larger, these storms tend to travel across north-west Europe whereas, when the pressure difference is smaller, they tend to head across southern Europe.
The fluctuations in the NAO occur on a wide range of time-scales. There are day-to-day changes associated with weather systems, and slower changes associated with seasonal and longer term variability in other climate system components such as ocean temperature. Due to these slower changes, the NAO is thought to be, to some extent, predictable from November for the coming winter.
To monitor the state of the NAO it is convenient to use a single number, or index. Several different versions of NAO indices are available, mostly based on measurements of surface pressure.
A commonly used index for the NAO during winter makes use of surface pressure data for meteorological stations in Stykkisholmur in Iceland and Ponta Delgada in the Azores. This index is positive when the difference in pressure between these locations is larger than its long-term average, and negative when the pressure difference is smaller.
The long-term-average pressure patterns vary with season, and for summer a different index has better associations with summer climatic fluctuations. The summer version is referred to as the Summer North Atlantic Oscillation index (SNAO). The SNAO index used here represents the strength of a spatial pattern of pressure variability in July-August whose main feature is located over northern Europe and the northern North Atlantic ocean. This index tends to be positive when pressure in this area is higher than usual (for more details see Figure 1 in Folland et al., 2009).
Temperature and precipitation conditions associated with the NAO
From historical data, the seasonal conditions associated with high and low values of the NAO indices can be analysed. Schematic maps showing these precipitation and near-surface land temperature effects are provided below. These maps indicate the approximate areas where seasonally-averaged temperatures and rainfall are likely to be in the top or bottom one third of observed values, given that the seasonal NAO index is in the top one quarter of observed values (and likewise for the NAO index in the bottom one quarter). For example, in a region indicated as 'warmer when the NAO index is well above zero' there is approximately a greater than 50% chance that the temperature will be in the top one-third of observed seasonal temperatures, given that the NAO index is in the top one-quarter of index values.
These maps indicate the large-scale effects, while details on a small scale may not be significant. Areas outside the coloured regions may also be influenced by the NAO, but with lower odds of a substantial effect. In general the odds for temperature impacts are higher than those for precipitation.
Winter (December-January-February) conditions
When the NAO index is well above normal, there is an increased chance that seasonal temperatures will be higher than normal in northern Europe, northern Asia and south east North America, and lower than normal in North Africa, north east Canada and southern Greenland. The patterns for precipitation (rainfall, snowfall) are more localised, with an increased chance of higher rainfall in northwest Europe and lower rainfall in southern Europe. When the NAO index is well below normal, the tendencies are generally opposite.
Summer (July-August) NAO conditions
When the summer NAO index is well above normal there is an increased chance that seasonal temperatures will be higher than normal over north-west Europe, central Canada, north central USA, and parts of North Africa, and lower than normal over the Mediterranean region, southern USA, north Mexico, and the Greenland region. For precipitation this increased chance is for lower rainfall in north-west Europe, north-east Canada, and the Sahel region of Africa, and higher rainfall in the Mediterranean, and parts of southern Greenland and Iceland. When the SNAO index is well below normal, conditions are mainly opposite in the European sector, with an increased likelihood of north-west Europe being cooler and wetter.
- The maps are schematic and the areas marked are not geographically precise.
- The NAO in winter is different to the NAO in summer and has different associated impacts.
- While the maps for positive NAO and negative NAO have many areas in common, usually with opposite impacts, there are differences for example, some regions have an associated impact for positive NAO but not for negative NAO.
- These summary maps do not indicate the severity of impacts and the probabilities and magnitudes vary from place to place.
- The maps are based on categorised information. Maps produced using different criteria will have somewhat different features.
- Conditions vary from year to year, and changes in the NAO occur as a part of other climatic events, so the maps do not represent any particular year. These maps provide an indication of regions where impacts are likely to occur based on historical evidence.
For winter the NAO data are from the Hurrell station-based index (monthly values) provided by the Climate Analysis Section, National Center for Atmospheric Research, USA.
The summer NAO index data are provided by the Met Office.
The analyses of temperature and precipitation impacts made use of gridded monthly-average data: temperature and precipitation from CRU TS (Harris et al., 2014) , temperature from the National Centers for Environmental Prediction atmospheric re-analysis, and precipitation from the Global Precipitation Climatology Project.
The NAO fluctuates on time-scales from days to decades, reflecting changing weather and climatic patterns.
This diagram illustrates values for the NAO index for eight past winters, showing both individual months (December, January and February: thin red bars) and the winter season average of those monthly values. The index can change somewhat from month to month, while the winter average (thick bars, values indicated below the year labels) is an indicator of conditions for the season as a whole.
Partly through the influence of slowly evolving components of the climate system, the NAO index may have the same sign for several successive years, as evident in the 50-year sample of winter values below. In the 100 years from 1915/16 to 2014/15 the highest winter average value on record occurred in 2014/15, and the lowest value in 2009/10 .