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- | ====== Background ====== | ||
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- | Intense meso-scale cyclones known as polar lows are frequently observed in the Arctic sector | ||
- | of the North Atlantic Ocean. During winter, cold-air outbreaks may be triggered by the large | ||
- | scale atmospheric flow exposing dry and very cold air to the relatively warm ocean surface. | ||
- | One of the ocean areas mostly favoured for polar low development is between the Norwegian | ||
- | mainland and the Svalbard island. Due to the presence of the North Atlantic Current (NAC) | ||
- | the waters in this area may be warmer than 6o C in January. The large air-sea temperature | ||
- | differences experienced during cold-air outbreaks result in the formations of atmospheric | ||
- | fronts, large heat fluxes and sometimes the development of polar lows. As polar lows are | ||
- | almost exclusively observed over open waters and are characterised by deep convective | ||
- | clouds, it is usually assumed that atmosphere-ocean heat and moist exchange are important | ||
- | contributions to the energetics of polar lows. The striking similarities to Tropical hurricanes | ||
- | displayed by the satellite images, such as clear eye and spiral bands of deep-convective | ||
- | clouds, has lead several authors to suggest the release of latent heat as one major energy | ||
- | source for at least a class of polar lows. studied a polar low south of the Bear island and | ||
- | concluded that in the final phase of its development some polar lows could act as Carnot | ||
- | engines, working between a warm (the ocean) and a cold (the tropopause) reservoir, and | ||
- | referred to these as Arctic hurricanes. | ||
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- | Still, it is important to keep in mind the baroclinic nature of cold-air outbreaks. Often, polar | ||
- | lows develop from disturbances on Arctic fronts that are formed during cold-air outbreaks | ||
- | when dry Arctic air is exposed to oceanic heat fluxes. The Arctic fronts are characterised by | ||
- | strong horizontal temperature gradients in the lower troposphere and an intense low level jet. In such cases the presence of the front suggests that baroclinic instability plays a role in the | ||
- | polar low formation. Nonetheless, | ||
- | satellite images suggests that large amounts of latent heat must be released in the troposphere. | ||
- | Possibly, both baroclinic and diabatic processes are important during the early stages of polar | ||
- | low development. Pure Arctic hurricanes as described by Emanuel and Rotunno (1989) may | ||
- | represent the final stage for vertically aligned symmetric cyclones with no baroclinic potential | ||
- | left. | ||
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- | Despite the fact that polar lows are almost exclusively marine phenomena, surprisingly few | ||
- | investigations of the interaction process between polar lows and the ocean. A noteworthy | ||
- | exception is PhD thesis by Linders (2009) who investigated the role of ocean surface | ||
- | temperature and air-sea fluxes on the development and intensity of polar lows. Saetra et al. | ||
- | (2008) demonstrated that during winter, the NAC sub-ducts under colder and less saline | ||
- | waters, leading to a warm subsurface core under cold upper-level waters. During strong wind | ||
- | events such warm sub-surface waters may entrain the surface by intense turbulent mixing. In | ||
- | the paper by Saetra et al. (2008), microwave satellite images revealing rapid surface warming | ||
- | during a polar low event outside the coast of Norway is presented. These finding raises some | ||
- | interesting questions. How may a rapid surface warming affect the life-cycle and intensity of | ||
- | polar lows? Also, a surface warming by entrainment of warm waters to the surface represents | ||
- | a cooling of the ocean by increased surface fluxes of latent and sensible heat. Is this a | ||
- | significant contribution to the cooling of the NAC and subsequent ocean overturning? | ||
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- | The oceanic response to hurricanes has long been recognised (Price, 1983; Sanford et al., | ||
- | 1987; Brink, 1989). Strong turbulent mixing entrainment of cold waters from deep layers | ||
- | leads to a cooling of the sea-surface. This rapid surface cooling reduces the surface fluxes and | ||
- | inhibits further hurricane intensification. When hurricanes moves over deep cores of warm | ||
- | waters, such as the Loop Current in the Gulf of Mexico, or warm core rings this surface | ||
- | cooling is strongly reduced. The warm water will then act to insulate the entrainment of cold | ||
- | waters form even deeper layers (Hong et al., 2000; Shay et al., 2000). In such cases, strong | ||
- | hurricane intensification has been observed. In 2005, Katrina intensified into a category 5 | ||
- | hurricane as it entered the warm Gulf of Mexico (Kafatos et al., 2006). | ||
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- | The ocean surface warming reported by Saetra et al. (2008) has only been observed by | ||
- | microwave satellite data. During could air outbreaks the ubiquitous cumulus convection | ||
- | prevents the sea-surface to be observed by infrared sensors (IR) such as AATSR, AVHRR | ||
- | and MODIS. However, verification of such ocean response to polar lows is urgent. Here, we | ||
- | propose to use altimeter combined with SST products from both microwave and infrared | ||
- | sensors to investigate possible surface warming in connection with polar lows. As the | ||
- | altimeter measures the surface anomaly (SLA) this can be related to the ocean heat content. | ||
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- | The main scientific questions to be addressed are: | ||
- | * Can satellite IR observations in combination with altimeter be used to detect possible | ||
- | sea-surface warming caused by strong winds under polar low events? | ||
- | * Can we identify a Polar Low Indicator based on satellite data that could be a useful | ||
- | tool for polar low forecasting? | ||
- | * Can the forecasting of polar lows be improved by introducing coupled atmosphere | ||
- | ocean models? | ||
- | * Does the strong turbulent mixing induced by polar lows have an anomalously strong | ||
- | impact on the cooling of the North Atlantic Current? | ||