Polar lows influence ocean circulation

The strong winds and large ocean-atmosphere temperature differences associated with polar lows result in significant local impacts on the surface of the ocean. Indeed in some cases polar lows can generate heat fluxes of a similar magnitude to those seen in category one hurricanes. Althougth the sensitivity of the intensification of polar lows to sea surface conditions has been studied extensively, up to now the extent to which polar lows themselves influence large-scale ocean circulation has not been clearly demonstrated (“large-scale” here refers to the spatial scale of seas such as the Norwegian Sea). This is a difficult problem to study because ocean circulation varies on longer time scales and responds slowly to the cumulative effect of many polar lows. However, a study published in Nature Geosciences this month by Alan Condron and Ian Renfrew (C&R) is a major advance in understanding the effect of polar lows on ocean circulation. In particular their study demonstrates that polar lows probably have a significant influence on the strength of ocean currents in the northern North Atlantic. Their approach was to run a state-of-the-art ocean model both with and without the effects of small-scale (less than approximately 500 km) atmospheric cyclones (see Figure below). C&R note that they don’t exclusively assess polar lows in their study, but the combined effect of all detected small-scale cyclones. It is likely however that polar lows are the most important subset of these small-scale cyclones in generating the ocean response, although it would be interesting to confirm this in a future study since the wind drag of the other small-scale cyclones may be important.

Example case (b) with and (a) without the surface wind associated with a polar low. In more detail: (a) The near-surface wind speed without the polar low resolved, (b) as in (a) but with the addition of an estimated wind field from the polar low and (c) best estimates of the actual wind field. For details of the methodology see supplementary material of Condron and Renfrew, from which Figure S1 is reproduced here by permission from Macmillan Publishers Ltd: Nature Geoscience, copyright (2013).

C&R demonstrate that small-scale cyclones have a significant effect on simulated northeast Atlantic Ocean circulation. Most significantly, the current that flows westward around Greenland and then southward down the Labrador Sea (the North Atlantic subpolar gyre) increases in strength by 5.5% when small-scale cyclones are taken into account. As a result the northward transport of heat to northern Europe and North America is increased. This effect is missing in most global climate models, which do not have a sufficiently high spatial resolution to capture small-scale cyclones. This motivates the use of higher-resolution climate models to capture this and other important processes in seasonal and climate forecasting.

Full reference

Condron, A. and I. A. Renfrew (2013). The impact of polar mesoscale storms on northeast Atlantic Ocean circulation. Nature Geoscience, 6, 34-37, doi:10.1038/ngeo1661.

Cold air outbreak in amazing detail

The current cold air outbreak, the first proper one this winter (see previous post), reveals incredible small-scale structure in the satellite image below. The land in the upper middle part of the picture is the Svalbard archipelago in the Northeast Atlantic.

The resolution of the original image (which you can see by clicking on the image above or go here) is 500 metres, which means that each pixel in the image is 500 metres wide. For maximum detail, check out this image at a resolution of 250 metres. There are so many things to see in the image, but here are some of the highlights.

In the upper left part of the picture, the white tendril-like features are sea ice. We also see some large floes. It’s all being blown southwards and at the same time broken up by the strong surface winds.

The north-south-oriented stripes that are made up of individual white dots are what we call cloud streets. They only occur when there’s a lot of convection (rising air) going on, and the dots themselves are cumulus clouds. Further south, away from the sea ice edge, the convection becomes less intense, and the cumulus clouds merge together into large patches of stratocumulus clouds. Cloud streets are a tell-tale sign of strong surface winds.

Downstream from Svalbard and in the lower part of the picture, the clouds get organized into a vortex. This is because there’s a small low-pressure system there, and the air starts to move in spiral-like, anti-clockwise patterns around the low (because of the earth’s rotation and the Coriolis force). The beautiful pattern we see in the image is, in other words, a polar low. Not a big one, but a polar low nonetheless.

Another interesting thing in the image are the wave clouds over and downstream from Svalbard. They form because the air moves over the mountains, much in the same way that waves form when water in a river hits rocks.

If you want to see more images from this region, take a look at NASA’s Hornsund subset.

First polar low this season

The satellite image on the left shows what I think must be the first polar low in the Northeast Atlantic this winter. At least it’s the first time snow is forecast for large parts of Norway, a sure sign that there’s a cold air outbreak in its way. The cold air outbreak itself is also clearly visible in the image, with its patchy or dotty structure. These are clouds that are organized into what we call “convective cells”.

A satellite image of the Northeast Atlantic, taken at 0940 on 23 October 2012. Downloaded from the Dundee Satellite Receiving Station in Scotland.

Convection is the meteorological term for rising air, and rising air is what creates all kinds of clouds. The convective cells in the picture are cumulus clouds which are formed because the cold air is heated from below by the (relatively) warm ocean surface, much the same as what happens when you blow on a bowl of hot soup. When air is heated, it gets lighter, and therefore it rises, letting colder air flow in from all sides to replace it.

So, the cold air outbreak in the picture sets up a lot of rising motion in the air masses. At the same time, there’s evaporation going on so that the air gets humid. And when humid air rises fast to create cumulus clouds, you get precipitation, in this case in the form of massive snow amounts forecast for Norway. Along the coast, people are going to witness tall cumulus and cumulonimbus clouds marching past them on their way south. Cold air outbreaks don’t always come as far south as Bergen, but I sometimes see these majestic clouds trotting along. It’s a very nice spectacle.

Arctic hurricane?

Do hurricanes form in the Arctic? Sometimes polar lows closely resemble hurricanes and a detailed study of one such case has just been published. The polar low (PL) in question occurred on 18-21 December 2002 over the Barents Sea, north of Norway, and is clearly visible at the top right of the image below. This shows spiral bands of cloud surrounding a clear eye at the centre of the storm, which are common features of hurricanes. The study, led by Ivan Fore, used a state-of-the-art weather computer model to answer the intriguing question of what caused this “hurricane-like”PL to intensify. Using this weather computer model they were able assess the details of the polar low dynamics that are not apparent from satellite imagery.

Infra-red satellite image of the polar low over the Barents Sea taken at 0204UTC, 20 December 2002. Source: NERC Dundee Satellite Receiving Station, http://www.sat.dundee.ac.uk

Very strong fluxes of thermal energy from the ocean to the atmosphere were found around the central eye, which are similar in magnitude to those observed in hurricanes. These large fluxes come mainly from the very large temperature difference between the winter atmosphere and open ocean, which can often be around 20 deg C. However, the maximum simulated surface winds of about 90 km/hour were not as strong as the threshold for category one hurricanes of 118 km/hour.

The results showed that the period of rapid intensification did not resemble a hurricane. The dynamics were closer to a standard mid-latitude cyclone (known as baroclinic). However a maintenance period followed the intensification, during which hurricane-like dynamics dominated. The polar low then decayed after moving over land, which cuts off the supply of energy from the ocean.

Would hurricane-like maintenance, or even intensification, have continued if the PL had not moved over land? Theory suggests that in winter the temperature difference between the atmosphere and ocean (and therefore potential for large energy fluxes) is large enough to support a genuine Arctic hurricane. One possible reason that PLs never develop into mature hurricanes is that there is simply not a large enough expanse of relatively warm ocean over the Arctic in winter. Hurricanes take of the order of five days to drift across the tropical Atlantic and slowly intensify, whereas most polar lows move over land or ice in one day.

Citation: Fore, I., J.E. Kristjansson, E.W. Kolstad, T.J. Bracegirdle, O. Saetra and B. Rosting (2012). A ‘hurricane-like’ polar low fuelled by sensible heat flux: High-resolution numerical simulations. Quarterly Journal of the Royal Meteorological Society, published online, doi:10.1002/qj.1876.

Polar low hits the UK

This picture shows a nice polar low when its centre was north of Scotland in the early hours of 6 December 2011. (Image downloaded from the Dundee Satellite Receiving Station.) It later moved towards the south-east and led to snowfall in western Norway – snow still on the ground outside my window. The same cold air outbreak, which covered large parts of the Nordic Seas, also left some snow in the UK. You can see the cloud streets stretching all the way up to the East Greenland coast in the same image. It’s quite rare that polar lows move this far south, and this is a particularly nice specimen, with the beautiful spiral form near the low centre. Let’s hope that we get more of these this winter…

Here’s a StormGeo surface wind speed forecast for 0800 in the morning on the same day (click on the picture for a larger version):

A band of pretty strong winds, up to 24 m/s, hit Shetland according to the forecast. Although I don’t know how correct the forecast was, it’s very interesting to look at the structure of the winds. The area with strong winds is quite small – this is typical for polar lows. Eyewitness accounts report that they see a wall of clouds coming towards them from the north-east in otherwise calm weather. This is not what would happen in a regular storm coming in from the south-west. Then you can see the storm advancing hours ahead – typically you see high, wispy cirrus clouds first, followed by a denser, more stratified layer of clouds, and then you get the rain, snow and/or hail in the cumulus clouds or thunderstorms. Polar lows just sneak up on you, and that’s exactly why they’re so dangerous.

I’ve read lots of accounts of shipwrecks in northern Norway, and one pattern seems to stand out. The fishermen would stay ashore while large storms rolled by from the south, and then when the weather cleared, they were understandably anxious to go out in their boats. Thr trouble is that polar lows almost always form in the cold air outbreaks that follow behind these storms, and then they would get surprised by very strong, sudden winds. This can still happen; polar lows are notoriously difficult to forecast, but it is encouraging to see that we at least got the general structure of this one right.

Polar low in the wake of storm

A terrific storm moved north along the coast of northern Norway on 25 and 26 November 2011, causing massive damage to houses and boats. It was well forecast, so one can only hope that no one was out at sea during its passage. A cold air outbreak in the wake of the storm was also well forecast, and even the development of a polar low in those cold air masses.

This is the forecast for midnight on Saturday, where the storm, which was called “Berit” by the way, is clearly visible just off the coast of Lofoten in northern Norway:

If you look at the isobars (the black curves) circling around the centre of the storm, you can tell that there were northerly winds coming down from the Svalbard region and the Barents Sea. This is because the wind blows roughly parallel to the isobars, and the direction is always counterclockwise around the low centre.

Now take a look at the “analysis”, which was computed after the fact, taking all observations into account, for midnight on Sunday:

There seems to be a dual feature in pretty much the same location as the storm was in the previous picture. (Also note that there seems to be a polar low in development west of Iceland.) These are polar lows that have formed as the icy cold winds blew down from the north and interacted with the much warmer ocean surface. The process is the same as what happens when you blow on a bowl of hot soup. The air is a lot colder than the soup, and therefore gets heated from below and starts rising and producing steam. Over the ocean, the steam turns into clouds, and nasty ones at that if the temperature difference between the water and the air is large enough. The perfect recipe for polar lows.

The picture on the left is a satellite image taken at 3 in the morning on Sunday, three hours after the analysis above. Click on the image for a larger version. You’ll have no trouble picking out the polar low here, but I’ve marked it in the picture all the same. You can also see the original storm, which has moved up into the Barents Sea now. There’s also a new storm coming in from the south, which is producing heavy rain here in Bergen as I write this.

The current forecast predicts that the PL will move towards the south-east and hit the coast of Norway later today. It might also merge with the new storm coming in.

The consequences will probably be thunderstorms, heavy snowfall, and a drop in temperatures. These are common features of polar lows, and is why they have always represented a major hazard for the coastal communities in northern Norway, Iceland and Japan, which are the regions that are most prone to polar lows. The odd polar low also strikes the UK and the island communities further north (Shetland, the Faeroes), sometimes causing panic on the roads.

I’ll try to get a good capture from the MODIS satellite as well.

First cold air outbreak 2011-2012

This satellite image was taken on 10 October 2011, when we got the first snows in the mountains here in western Norway. The first brush with winter, but as you can see from the image, no proper polar lows formed over the Atlantic. But they’re coming soon.

First satellite image of a polar low

For some historical background, here’s what is probably the first satellite image of a polar low.  It was taken from the NASA Nimbus 3 satellite on 5 January 1970. The clouds associated with the polar low can be clearly seen to the north-west of Scotland.

 

Up to this point observational studies had been based on other sources of data such as weather balloons. This polar low was the subject of a case study in Lyall (1972), who used this image as part of the study. At this time there was uncertainty over whether polar lows were driven directly by heat transfer from the ocean (i.e. a mechanism similar to hurricanes) or mainly from rebalancing of atmospheric horizontal temperature gradients (i.e. a smaller version of the majority of mid-latitude cyclones). This picture suggested the latter. However, as more polar lows were studied and observed from satellite imagery it was found that some appear more similar in structure to hurricanes.

Lyall, IT (1972). The polar low over Britain. Weather, 27, 378-390.

 

Barents Sea polar low 9 Feb 2011

There was a classic Barents Sea polar low yesterday. It’s apparent at the top right of this NOAA 16 image from 1740 on 9 Feb, downloaded from the NERC Dundee Satellite Receiving Station.

Polar low and von Karman vortices

Check out the incredible detail in this image, downloaded from NASA’s MODIS Rapid Response System. The island at the top is Spitsbergen, the biggest island in the Svalbard achipelago. South of it there’s a polar low. It’s also cool to see the wake to the west of Svalbard. There’s probably very weak winds in that area, but there’s a strong tip jet around Spitsbergen’s south cape. I sailed through this jet when I was onboard a coast guard icebreaker in 2008. Standing on the bridge, I could see that the wind speed increased rapidly from 22 knots to 44 knots inside the narrow jet, and then it dropped down to 22 again as we sailed out of it.

Near the bottom of the image, the small island of Jan Mayen is seen, and downstream of it, some amazing von Kármán vortices are very evident. I zoomed in on these vortices here: