WEATHER

Conditions on the ice sheet as well as the sea ice in the Artic are influenced by the weather. Is it cold or warm? How much precipitation is there? Which direction is the wind coming from and how does it push the sea ice?

Shown here:

  1. Temperature and wind – the daily variations in temperature and wind conditions.
  2. Temperature anomaly and wind – how has the temperature developed from day to day compared with the period 2004-2013? That is to say: Has it been warmer or colder than in the previous decade? The temperatures can among other things give an indication of when the ice sheet and the sea ice can melt. The wind arrows show the current wind as in the tab “Temperature and wind”.
  3. Precipitation anomaly – it is shown here how much precipitation has fallen day to day compared with the period 2004-2013. It is through precipitation that new mass is added to the ice sheet.

In addition, the NAO index is shown. This is a measure of the strength of the westerly winds in the North Atlantic. When the index is negative, the flow is wavier, which increases the probability of transport of warm air to Greenland from the south (Read more below).

The animations show the development in wind, temperature and precipitation based on daily registrations of the past 50 days. All the quantities are shown as an average over the past 5 days.

Where does the data come from?

The numbers are based on data from the European Centre for Medium-Range Weather Forecasts (ECMWF) IFS forecast model. ECMWF is the European weather center, which is run as cooperation of European countries. Among other things ECMWF runs global weather models, from where each individual country can pull data to drive their local weather models.

Anomalies (deviations from the norm) are calculated in relation to ECMWF’s weather re-analysis, called ERA-Interim. A re-analysis is a review of observations and weather models run over a historical period that ensures consistent mapping of the state of the atmosphere over time.

The North Atlantic Oscillation (NAO)

Generally speaking, there is very often high pressure over the Azores and surrounding areas, whilst there is very often low pressure over Iceland. The pressure difference between the Azores and Iceland varies over time, and this variation is described by the so-called North Atlantic Oscillation (NAO). The NAO index is thus a measure of how strong westerly winds are over the East Atlantic and surrounding regions. If the pressure difference is great, strong westerly winds blow, and we say that the NAO index is positive; if the pressure difference is small, the westerly winds will be light, and the NAO negative. Occasionally, the pressure over Iceland may even be higher than that over the Azores. This will result in an easterly wind and a strongly negative NAO index.

In simple terms, a high NAO index means mild winters and cool summers in large parts of Europe, whilst a negative index results in cold winters and hot summers. It has also been a well-known phenomenon for more than 250 years that it is often cold in Greenland when it is warm in Denmark, and vice versa. When the NAO index is negative, the weak westerly air currents have a tendency to exhibit greater curvature, which increases the probability of warmer air from the south flowing up towards Greenland.

The NAO index can be determined in different ways. It can, for example, be seen directly from measurements of air pressure on Iceland and the Azores or Gibraltar. Re-analyses, however, are performed on a grid, and it is therefore more accurate to use a so-called EOF analysis, which provides more or less the same result, although based on the distribution of pressure throughout the Atlantic region.

The NAO index presented here on this page is calculated by the Climate Prediction Center at NOAA/ National Weather Service, and the calculation is described here.

Daily NAO data is obtained here.