Cyclone-Anticyclone asymmetry of eddy detection on gridded altimetry product in the Mediterranean Sea

<p>Thanks to a Observing System Simulation Experiment (OSSE) that simulate the along-track satellite measuring process on the sea surface of the high resolution model CROCO-MED60v40-15-16 we investigate how the reliability and the accuracy of the detected eddies are influenced by the satellite sampling and the mapping procedure. The main result of this study is that there is that there is a strong cyclone-anticyclone asymmetry of the eddy detection on the altimetry products AVISO/CMEMS in the Mediterranean Sea. Large scale cyclones having a characteristic radius larger than the local deformation radius are much less reliable than large scale anticyclones. We estimate, that less than 60% of these cyclones detected on gridded altimetry product are reliable, while more than 85% of mesoscale anticyclones are reliable. Besides, both the barycenter and the size of these mesoscale anticyclones are relatively accurate. This asymmetry comes from the difference of stability between cyclones and anticyclones. Large mesoscale cyclones often splits into smaller sub mesoscale structures hav ing a rapid dynamical evolution. The high resolution model CROCO-MED60v40 shows that this complex dynamic is too fast and too small to be accurately captured by the gridded altimetry products based on a strong spatio-temporal  interpolation. The later smooth out this sub mesoscale dynamics and tend to generate an excessive number of unrealistic (i.e. unreliable) mesoscale cyclones in comparison with the reference field. On the other hand, large mesoscale anticyclones,  which are more robust and that evolve more slowly, can be spatially resolved and accurately tracked by standard altimetry products.<span>  </span>However, we confirm that gridded altimetry products have a systematic bias on the eddy intensity and especially for anticyclones. The azimuthal geostrophic velocities are always underestimated on the AVISO/CMEMS products even for large mesoscale anticyclones.<span> </span></p>

Characteristics of Cold Air Outbreak events and associated Polar Mesoscale Cyclogenesis over the North Atlantic region

Equatorward excursions of cold polar air masses into ice-free regions, so-called Cold Air Outbreaks (CAO), are frequently accompanied by the development of severe mesoscale weather features. Focusing on two key regions, the Labrador Sea and the Greenland/Norwegian Sea, we apply objective detection for both CAO events and polar mesoscale cyclones to outline the temporal evolution of CAO events and quantify associated mesoscale cyclogenesis. We introduce a novel metric, the CAO-depth, which incorporates both the static stability and the temperature of the air mass. The large-scale atmospheric conditions during the onset of CAO events comprise a very cold upper level trough over the CAO region and a surface cyclone downstream. As the CAO matures, the cold air mass extends southeastward, accompanied by lower static stability and enhanced surface fluxes. Despite the nearly 20 degrees difference in latitude, CAO events over both regions exhibit similar evolution and characteristics including surface fluxes and thermodynamic structure. About 2/3rd of the identified CAO events are accompanied by polar mesoscale cyclogenesis, with the majority of mesoscale cyclones originating inside the cold air masses. Neither the duration nor the maturity of the CAO event seems relevant for mesoscale cyclogenesis. Mesoscale cyclogenesis conditions during CAO events over the Labrador Sea are warmer, moister, and exhibit stronger surface latent heat fluxes than their Norwegian Sea counterparts.

Polar lows – moist-baroclinic cyclones developing in four different vertical wind shear environments

Polar lows are intense mesoscale cyclones that develop in polar marine air masses. Motivated by the large variety of their proposed intensification mechanisms, cloud structure, and ambient sub-synoptic environment, we use self-organising maps to classify polar lows. The method is applied to 370 polar lows in the north-eastern Atlantic, which were obtained by matching mesoscale cyclones from the ERA-5 reanalysis to polar lows registered in the STARS dataset by the Norwegian Meteorological Institute. ERA-5 reproduces most of the STARS polar lows. We identify five different polar-low configurations which are characterised by the vertical wind shear vector, the change in the horizontal-wind vector with height, relative to the propagation direction. Four categories feature a strong shear with different orientations of the shear vector, whereas the fifth category contains conditions with weak shear. This confirms the relevance of a previously identified categorisation into forward- and reverse-shear polar lows. We expand the categorisation with right- and left-shear polar lows that propagate towards colder and warmer environments, respectively. For the strong-shear categories, the shear vector organises the moist-baroclinic dynamics of the systems. This is apparent in the low-pressure anomaly tilting with height against the shear vector and the main updrafts occurring along the warm front located in the forward-left direction relative to the shear vector. These main updrafts contribute to the intensification through latent heat release and are typically associated with comma-shaped clouds. Polar-low situations with a weak shear, which often feature spirali-form clouds, occur mainly at decaying stages of the development. We thus find no evidence for hurricane-like intensification of polar lows and propose instead that spirali-form clouds are associated with a warm seclusion process.

ALEUTIAN EDDIES AND THEIR IMPACT ON TEMPERATURE AND DISSOLVED OXYGEN IN THE WESTERN SUBARCTIC PACIFIC

Aleutian eddies are anticyclonic vortices on scale of 100–200 kilometers that transport the Alaskan Stream warm water originating in the Gulf of Alaska to the western Subarctic Pacific (WSP). We present an analysis of satellite, Argo floats and ship borne observations of Aleutian eddies, revealing the influence of the wind forcing on the eddy formation and shedding. We demonstrate that the eastern winds promote the Aleutian eddy formation south of Near Islands. Positive (cyclonic) wind stress curl in late fall- winter results in the SSH decrease and the mesoscale cyclones formation at the Aleutian eddy boundaries and thereby leads to the eddy shedding from the Alaskan Stream. An appearance of the Aleutian eddies in the central part of the WSP is accompanied by the deepening of the isopycnals and an increase of the temperature and dissolved oxygen (DO) concentration in 150–400 m layer. The eddies decrease the DO concentration on 26.7σθ and 26.8σθ surfaces in the upper intermediate layer but enrich the 27.0σθ surface by DO. The extent to which eddies modulate the temperature and DO concentration in the WSP is related to the SSH amplitude of eddies.

Polar Lows – Moist Baroclinic Cyclones Developing in Four Different Vertical Wind Shear Environments

Polar lows are intense mesoscale cyclones that develop in polar marine air masses. Motivated by the large variety in their proposed intensification mechanisms, cloud structure, and ambient sub-synoptic environment, we use self-organising maps to classify polar lows. The method is applied to 370 polar lows in the North-East Atlantic, which were obtained by matching mesoscale cyclones from the ERA-5 reanalysis to polar lows registered by the Norwegian Meteorological Institute in the STARS dataset. ERA-5 reproduces 93 % of the STARS polar lows. We identify five different polar-low configurations, which are characterised by the vertical wind shear vector relative to the propagation direction. Four categories feature a strong shear with different orientations of the shear vector, whereas the fifth category contains conditions with weak shear. The orientation of the vertical-shear vector for the strong shear categories determines the dynamics of the systems, confirming the relevance of the previously identified categorisation into forward and reverse-shear polar lows. In addition, we expand the categorisation with right and left-shear polar lows that propagate towards colder and warmer environments, respectively. Polar lows in the four strong shear categories feature an up-shear tilt in the vertical, typical for the intensification through moist baroclinic processes. As weak-shear conditions mainly occur at the mature or lysis stage of polar lows, we find no evidence for hurricane-like development and propose that spirali-form PLs are most likely associated with a warm seclusion process.

Polar Lows

Polar lows are intense maritime mesoscale cyclones developing in both hemispheres poleward of the main polar front. These rapidly developing severe storms are accompanied by strong winds, heavy precipitation (hail and snow), and rough sea states. Polar lows can have significant socio-economic impact by disrupting human activities in the maritime polar regions, such as tourism, fisheries, transportation, research activities, and exploration of natural resources. Upon landfall, they quickly decay, but their blustery winds and substantial snowfall affect the local communities in coastal regions, resulting in airport-closure, transportation breakdown and increased avalanche risk. Polar lows are primarily a winter phenomenon and tend to develop during excursions of polar air masses, originating from ice-covered areas, over the adjacent open ocean. These so-called cold-air outbreaks are driven by the synoptic scale atmospheric configuration, and polar lows usually develop along air-mass boundaries associated with these cold-air outbreaks. Local orographic features and the sea-ice configuration also play prominent roles in pre-conditioning the environment for polar low development. Proposed dynamical pathways for polar low development include moist baroclinic instability, symmetric convective instability, and frontal instability, but verification of these mechanisms is limited due to sparse observations and insufficient resolution of reanalysis data. Maritime areas with a frequent polar low presence are climatologically important regions for the global ocean circulation, hence local changes in energy exchange between the atmosphere and ocean in these regions potentially impacts the global climate system. Recent research indicates that the enhanced heat and momentum exchange by mesoscale cyclones likely has a pronounced impact on ocean heat transport by triggering deep water formation in the ocean and by modifying horizontal mixing in the atmosphere. Since the beginning of the satellite-era a steady decline of sea-ice cover in the Northern Hemisphere has expanded the ice-free polar regions, and thus the areas for polar low development, yet the number of polar lows is projected to decline under future climate scenarios.

Characteristics of Cold Air Outbreaks and associated Polar Mesoscale Cyclones in the North-Atlantic region

<p>Geographically confined, equatorward excursions of cold air masses into ice-free regions account for the majority of oceanic heat loss in key regions for deepwater formation in the North Atlantic. These cold-air outbreaks (CAO) are frequently accompanied by the development of severe mesoscale weather features, such as intense low-level jets and polar lows. Exchange of heat, moisture and momentum between the ocean and atmosphere in response to mesoscale features, either directly, or indirectly via modulating the longevity and intensity of the cold air mass modulates the wind-driven oceanic gyres. Yet, it remains unclear how often mesoscale cyclones accompany cold-air outbreaks, and how mesoscale features modify the air-sea interactions. </p><p>Focusing on two key regions, the Labrador Sea and the Greenland/Norwegian Sea, we outline the temporal evolution of CAO events and associated mesoscale cyclogenesis. We apply objective detection to both CAO events and mesoscale cyclones and introduce an alternative metric to characterize the cold air mass. Despite the nearly 20 degrees difference in latitude, CAOs over both regions exhibit rather similar evolution, surface fluxes, and thermodynamic structure. The large scale configuration during CAO onset comprises a very cold upper level through over the CAO region and a surface cyclone downstream. As the CAO matures the cold air mass extends towards the south-east, accompanied by enhanced surface fluxes and destabilization of the CAO airmass. About 2/3 of the CAO events are accompanied by mesoscale cyclogenesis, with the majority of mesoscale cyclones originating inside the cold air masses. Neither the duration nor the maturity of the CAO event is relevant for the initiation of mesoscale cyclogenesis. Genesis conditions for mesoscale cyclogenesis during CAOs over the Labrador Sea are moister and exhibit stronger surface fluxes compared to their Norwegian Sea counterparts.</p>

ВИХРЕВАЯ СТРУКТУРА ВОСТОЧНО-КАМЧАТСКОГО ТЕЧЕНИЯ ПО СПУТНИКОВЫМ НАБЛЮДЕНИЯМ

На основе анализа архива спутниковых мультисенсорных данных исследована динамика ледяного покрова в области ВосточноКамчатского течения. При развитом зимнем муссоне плавучий лёд смещается вдоль побережья Камчатки на юг, трассируя разномасштабные вихри, которые возникают при взаимодействии потока ВосточноКамчатского течения с неоднородностями береговой черты и континентального склона. Средняя скорость дрейфа, рассчитанная по изображениям, полученным в марте 2016 г. в один день с разницей в 110 мин, составила 0,25 м/с, что в два с половиной раза превысило суточное значение. На севере и на юге района значения скорости дрейфа превышали скорости в центральной части. При ослаблении зимнего муссона в поле дрейфующего льда в западной части Берингова моря образуются хорошо выраженные грибовидные структуры, горизонтальные размеры которых могут превышать 200 км. Наряду с известными ранее синоптическими вихревыми образованиями синоптического масштаба спутниковые данные высокого (10 15 м) и среднего (250 м) пространственного разрешения позволили зарегистрировать многочисленные мезомасштабные циклоны диаметром 10 25 км и временем жизни от 1 до 14 суток.On the basis of the analysis of the satellite multisensor data archive, the dynamics of the ice cover in the region of the East Kamchatka Current is investigated. With the developed winter monsoon, floating ice shifts along the Kamchatka coast to the south, tracing the multiscale eddies that appear when the East Kamchatka stream interacts with the inhomogeneities of the coastline and the continental slope. The average drift speed calculated from images taken in March 2016 on the same day with a difference of 110 min was 0.25 m / s, which was two and a half times higher than the daily value. On the north and south of the region, the drift velocity values exceeded the velocities in the central part. With the attenuation of the winter monsoon in the field of drifting ice in the western part of the Bering Sea, welldefined mushroomlike structures are formed, the horizontal dimensions of which can exceed 200 km. Together previously known synoptic eddy formations of a synoptic scale, satellite data of high (10 15 m) and moderate (250 m) spatial resolution made it possible to register numerous mesoscale cyclones with a diameter of 10 25 km and a life time of 1 to 14 days.

Modification of Polar Low Development by Orography and Sea Ice

ABSTRACTThe life cycles of intense high-latitude mesoscale cyclones and polar lows are strongly shaped by their ambient environments. This study focuses on the influence of the orography of Svalbard and the sea ice cover in the Norwegian and Barents Seas on polar low development. We investigate two typical polar lows that formed near Svalbard during northerly cold-air outbreaks. Each case is simulated using the Met Office Unified Model with convection-permitting grid spacing. A series of sensitivity experiments is conducted with an artificially changed land mask, orography, and sea ice distribution. We find that Svalbard acts to block stably stratified air from the ice-covered Arctic Ocean, and as an additional source of low-level cyclonic vorticity aiding polar low genesis and intensification. A decrease in sea ice cover west of Svalbard results in a moderate intensification of the polar lows, particularly for the more convectively driven case, while an increase in the sea ice cover significantly hinders their development. These experiments exemplify that polar mesoscale cyclones in the northeast Atlantic can withstand large perturbations in the surface conditions (such as the removal of Svalbard) and still develop to sufficient intensity to be labeled as polar lows. However, there is a sensitivity to Svalbard’s orography and surrounding sea ice cover, illustrated by a clear modulation of polar low genesis and development.