Sea ice swirls in the Skagerrak Strait

By Jochen Kerkmann, Juergen Zimmermann (EUMETSAT) and Steinar Eastwood (met.no)

In the Skagerrak, the last winter with major ice formation and obstructions to shipping was 1995/96.

Daily temperature anomalies in Oslo, source: Climate Prediction Center/NCEP).

The image below shows a Metop-A AVHRR RGB composite over southern Norway, the Skagerrak and western Sweden, taken on 10 February 2010. Snow and ice appear with a magenta colour, open sea with a dark blue colour and low-level water clouds with a white colour. Southern Norway, western Sweden and northern Denmark appear fully snow covered. The sea ice in the Skagerrak and Kattegat (streaks of magenta colour) stands out clearly against the dark blue-coloured open sea. The cloud streets in the southern parts of the Skagerrak indicate a relatively cold situation with northeasterly winds (Oslo reported -14°C this morning).

The Metop-A RGB composite shown below is part of the Nowcasting SAF product generation pipeline of SMHI, which produces cloud products (cloud mask, cloud type, cloud height, precipitating clouds) from polar orbiting satellites (NOAA and Metop). These products have been fine-tuned in the last years to achieve the highest possible quality, but sometimes they contain errors (especially in cold winter situations).

Therefore, it is highly recommended not to blindly trust the cloud products but to always compare the products with the original images and RGB products. In this case, the cloud type product (source: SMHI) corresponding to the RGB image shows two major errors: 1) it does not detect the snow in forest areas of southern Norway and 2) it indicates thin cirrus clouds instead of cloud free in the sea ice area over the Skagerrak (see colour scale ). Obviously, the radiative properties of broken sea ice are similar to those of thin cirrus clouds (relatively cold, medium reflectivity).

According to a report from BSH (Bundesamt fuer Seeschifffahrt und Hydrographie) , in the winter of 2009/10, some small fjords on the Norwegian coast were completely covered with ice from early January to late March, and a few of them were closed to shipping. Left undisturbed, the ice cover in places reached thicknesses over 50 cm. Major quantities of ice occurred temporarily in the navigation channel to Kristiansand and in the Oslofjord.

In February, ice belts through which only high-powered ice-class vessels were able to navigate floated off the coast. In smaller harbours and fjords on the Danish coast in Kattegat, the ice reached 10–30cm thick. In mid-February, 20–45cm fast ice was observed in the skerries and sheltered bays of the Swedish coast north of Gothenburg, and 10–25cm south of it. Areas with thin ice or new ice occurred in the offshore waters, and in the Belts and Sounds.

Another way to study the sea ice in the Skagerrak is with high resolution MODIS images, which can be downloaded from the MODIS Rapid Response web page . The image below shows an Aqua MODIS image taken about two-and-a-half hours after the Metop-A image. In this true colour RGB composite, snow and ice appear with a white colour (but low level clouds are also white).

The MODIS image shows greater details of the sea ice. The form of the ice streaks (with a kind of spiral arms) gives the impression of a clockwise rotation in the Skagerrak area. To confirm this, we used all available cloud-free Aqua and Terra MODIS images to create the following animation (25 Jan–10 Mar 2010, animated image source: NASA, loop prepared by Steinar Eastwood). Note that this loop consists of irregular time intervals, due to irregular cloud coverage and satellite overpass times.

Despite this difficulty, one can easily discern the clockwise swirl of the sea ice in the Skagerrak Strait, most easily seen in the few totally cloud free periods. "I cannot remember seeing anything like this" oceanographer Lars Petter Røed said. Normally, the waters in the Skagerrak Strait rotate in a counter-clockwise sense, but in the period January-February 2010 a lot of wind from the northeast has simply reversed the circulation of ocean waters in the Skagerrak.

Moving to larger scales, the image below shows the sea ice concentration product for the Northern Hemisphere produced by the Ocean and Sea Ice SAF. The colour scale is given in a link below the image. The product is based on passive microwave SSM/I data from the DMSP F15 satellite. It is a daily product that combines all 14–15 swaths during the day. Four different frequencies and polarizations are used for each satellite observation to calculate the fraction of the area covered by sea ice.

The product shows the sea ice in the Skagerrak with yellow colour, which indicates an ice concentration (at this scale) of 20–30%. It also shows the ice situation in the Baltic Sea, with 100% ice coverage in the Gulfs of Bothnia, Finland and Riga. However, at this resolution it is not possible to see the ice situation around the Danish, Swedish and German coastline.

For this application, higher resolution products are needed, which serve as input to detailed daily ice maps provided by weather and oceanographic services (see e.g. ice map on 15 February (source: SMHI) or ice map on 17 February (source: BSH)). Finally, you may have a look at the 2009/10 ice situation in the North Atlantic/Baltic Sea by clicking on one of the animations (see links below the image).