RESTRICTED only accessable in CEN/MPI net or via CliSAP login What does that mean?
Data access via file system: /data/icdc/climate_indices/ild_index/
The weather and climate in the North Atlantic has a strong influence on Europe's weather. Especially the position of the controlling lows is of great interest. In the long-term average of atmospheric pressure in the North Atlantic there are two regions with increased activity. These lows have a significant impact on the meridional heat flows to Europe. The two activity centers are located on the one hand over the Irminger Sea near Iceland and on the other over the Norwegian Sea near the Lofoten Islands. There is also a primary and secondary minimum in the long-term mean air pressure field at sea level.
To study these minima, a climate index of the Iceland-Lofoten pressure difference (ILD index) was defined [Jahnke-Bornemann 2008, 2010]. The index was calculated analogously to the station-based NAO index [Hurrell, 1995] as the difference between the normalized atmospheric pressure anomalies of a defined Icelandic and Lofoten region [Formula 1].
For this purpose, the average air pressure at sea level for the Lofoten and Iceland regions is determined spatially and temporally for each time point t. The anomaly Δp (t) is then calculated as the difference between the mean air pressure value at time t and the long-time average air pressure in the respective area. These pressure anomalies are normalized with the respective standard deviation σpL and σpI of the normalization period. The difference of these normalized pressure anomalies gives a time series and is the index of the Iceland-Lofoten pressure difference:
The Icelandic and Lofoten composites of winter atmospheric pressure shown in Figure 2 were calculated using this index. Figure 3 shows the difference between the Lofoten composite and the Iceland composite with a significant ILD pattern of air pressure, which in addition to the pressure centers over Iceland and Lofoten also shows anomalies in the area of the Azores high, the Siberian high and the Aleut low. The ILD pattern accounts for 8% of the northern hemisphere air pressure variance and up to 27% of the variance in an area of the Northeast Atlantic that includes the centers of Iceland and Lofoten. Further investigations showed that the signal of the ILD pattern extended through the entire troposphere into the stratosphere.
The standardization period plays a role in the calculation of the ILD. For the version offered here, the index calculated from ERA40 described in [Jahnke-Bornemann, 2010] for the period 09 / 1957-08 / 2002 was recalculated from the ERA interim data in order to obtain a longer time series until 01/2017. For the new calculation, ECMWF's ERA interim MSLP data (6 hourly, 0 o'clock) with a resolution of approximately 1,125 degrees was used to calculate the monthly mean pressure (with CDO). The corners of the Icelandic area where the monthly mean air pressure was calculated were at coordinates 60 ° N 322 ° E to 63 ° N 325 ° E and the corners of the Lofoten area were 70 ° N 13.5 ° E to 72.5 ° N 15.75 ° O. The reference period was January 1979 to December 2000. To extend the time series, the ILD index from ERA-40 was calculated using the new reference period and this time series was extended from 01/1979 with ERA interim.
- Coverage: 70N to 72.5N, 13.5E to 15.75E for Lofoten box, and 60N to 63N, 35W to 38W for Island box
- spatially: point
- time: monthly, 09/1957 -01/2017
The ILD index was calculated from re-analysis data of the ECMWF. The quality of this reanalysis data is considered in the Re-analysis data atmosphere section. The choice of version (ERA-40, ERA interim) has a small impact on the result (Figure 4). The result also depends to a small extent on the choice of the areas for Iceland and Lofoten used for air pressure calculation (see Figure 5.17 in Jahnke-Bornemann, Annika (2010)). The result is also slightly dependent on the choice of the standardization period (Figure 5); exact numbers can be seen from the results of the 3 indices calculated here as a text file.
- Annika Jahnke-Bornemann & Burghard Brümmer (2008) The Iceland—Lofotes pressure difference: different states of the North Atlantic low-pressure zone, Tellus A: Dynamic Meteorology and Oceanography, 61:4, 466-475, DOI: 10.1111/j.1600-0870.2009.00401.x
- Jahnke-Bornemann, Annika (2010) Zyklonen im Europäischen Nordmeer und Zusammenhänge mit der nordatlantischen Tiefdruckzone JAHNKE-BORNEMANN, Annika, Dissertation, URN: urn:nbn:de:gbv:18-44949
- Hurrell, J.W., 1995: Decadal Trends in the North Atlantic Oscillation: Regional Temperatures and Precipitation. Science: Vol. 269, pp.676-679, http://dx.doi.org/10.1126/science.269.5224.676