Anticyclonic Ones Research Articles

Hurricane influence on the oceanic eddies in the Gulf Stream region

The Gulf Stream region (GSR) represents an area of robust oceanic eddies, active hurricanes, and more importantly, frequent encounters between the two phenomena. However, the direct impact of the intense storms on the eddy field has seldom been comprehensively examined. Here based on a multi-year analysis of eddy energy changing rate, we demonstrate that hurricanes enhance cyclonic eddies but weaken anticyclonic ones by injecting potential vorticity into the ocean. Such effects are not only pronounced immediately following hurricane-eddy encounters, but also retained for extended periods within large eddies that have long lifespans. Consequently, the variation of the annual mean energy and vorticity of the eddy field exhibits a high correlation with hurricane intensity. It can thus be argued that hurricanes over the GSR play an important role in driving the long-term variation of the underlying eddy field, thereby affecting ocean circulation and climate.

Fine-Scale Eddies Detected by SWOT in the Kuroshio Extension

Conventional altimetry has greatly advanced our understanding of mesoscale eddies but falls short in studying fine-scale eddies (<150 km). The newly launched Surface Water and Ocean Topography (SWOT) altimeter, however, with its unprecedented high-resolution capabilities, offers new opportunities to observe these fine-scale eddies. In this study, we use SWOT data to explore these previously elusive fine-scale eddies in the Kuroshio Extension. During SWOT’s fast sampling phase from 29 May 2023 to 10 July 2023, we identified an average of 4.5 fine-scale eddies within each 120 km wide swath. Cyclonic eddies, which are slightly more frequent than the anticyclonic ones (ratio of 1.16), have a similar mean radius of 23.4 km. However, cyclonic eddies exhibit higher amplitudes, averaging 3.5 cm compared to 2.8 cm for anticyclonic eddies. In contrast to the mesoscale eddies detected by conventional altimeters, the fine-scale eddies revealed by SWOT are characterized by smaller sizes and weaker amplitudes. This study offers a preliminary view of fine-scale eddy characteristics from space, highlighting SWOT’s potential to advance our understanding of these dynamic processes. Nonetheless, it also emphasizes the necessity for comprehensive analysis to fully exploit the satellite’s capabilities in monitoring and interpreting complex eddy behaviors.

Mesoscale eddy variability in the Caribbean Sea

The spatial distribution, and the monthly and seasonal variability of mesoscale eddy observations derived from the AVISO eddy atlas are assessed in the Caribbean Sea during 1993–2019. The average lifetime for the whole set of eddies is 62 ± 37 days, mean amplitude of 7 ± 4 cm for cyclonic and 7 ± 4 cm for anticyclonic and mean radius of 100 ± 31 km for cyclonic and 108 ± 32 km for anticyclonic. Cyclonic eddies are on average more nonlinear than anticyclonic ones. The spatio-temporal variability in the number of eddy observations is evaluated against the Mean Eddy Kinetic Energy (MEKE) derived from geostrophic currents as well as from seasonal winds. Spatial distribution of eddy observations is correlated with MEKE while the migration of the intertropical convergence zone explains the advection of eddies towards the southern part of the basin.

Divergence and Dispersion of Global Eddy Propagation from Satellite Altimetry

Abstract It is well understood that isolated eddies are presumed to propagate westward intrinsically at the speed of the annual baroclinic Rossby wave. This classic description, however, is known to be frequently violated in both propagation speed and its direction in the real ocean. Here, we present a systematic analysis on the divergence of eddy propagation direction (i.e., global pattern of departure from due west) and dispersion of eddy propagation speed (i.e., zonal pattern of departure from Rossby wave phase speed). Our main findings include the following: 1) A global climatological phase map (the first of its kind to our knowledge) indicating localized direction of most likely eddy propagation has been derived from 28 years (1993–2020) of satellite altimetry, leading to a leaf-like full-angle pattern in its overall divergence. 2) A meridional deflection map of eddy motion is created with prominent equatorward/poleward deflecting zones identified, revealing that it is more geographically correlated rather than polarity determined as previously thought (i.e., poleward for cyclonic eddies and equatorward for anticyclonic ones). 3) The eddy–Rossby wave relationship has a duality nature (waves riding by eddies) in five subtropical bands centered around 27°N and 26°S in the two hemispheres, outside which their relationship has a dispersive nature with dominant waves (eddies) propagating faster in the tropical (extratropical) oceans. Current, wind, and topographic effects are major external forcings responsible for the observed divergence and dispersion of eddy propagations. These results are expected to make a significant contribution to eddy trajectory prediction using physically based and/or data-driven models.

Impact of the Scandinavian Pattern on Long-Lived Cold Surges over the South China Sea

Abstract This study investigates the influence of the Scandinavian (SCA) pattern on long-lived cold surges over the South China Sea (SCS). The results show that, different from the short-lived ones, the majority of long-lived cold surges over the SCS are preceded by a negative phase of the quasi-stationary SCA pattern in the extratropics, which is characterized as a primary cyclonic center over the Scandinavian Peninsula and two anticyclonic ones over the North Atlantic and central Siberia. This connection is mainly conducted through a continuous amplification of the high pressure anomalies over East Asia. On the other hand, the SCA-related anomalies also reveal identical responses as an increase in sea level pressure over East Asia and northerly flows over the SCS. Besides, the SCA pattern may influence the long-lived cold surges over the SCS by facilitating blocking occurrences through the extensive and quasi-stationary anticyclone over central Siberia. The present results have an implication for the extended weather forecast: long-lasting circulation anomalies, such as the SCA pattern, can affect long-lasting weather phenomena in the regions that are located remotely in both the zonal and meridional directions, such as long-lived cold surges over the SCS.

Struktura i kinematika sinopticheskikh vikhrey v okeane: teoriya i sovremennyye nablyudeniya

Purpose . Assessment of general ideas about the character of synoptic processes in the ocean which came out from the results of the experiment at the POLYMODE polygon, and quantitative verification of the previously developed model of intense eddies from the viewpoint of new data on the eddy characteristics in the open ocean obtained from the satellite altimetry observations constitute the aim of the article. Methods and Results. Comparison of the characteristic features of synoptic variability in the ocean resulted from the observational data obtained at the POLYMODE polygon with the generalized analysis of modern satellite altimetry observations showed relevance of the previously formed notions implying that far from the jet currents, the eddies are relatively sparsely distributed over the ocean surface; at that they contain a significant portion of total energy on the synoptic scales. Quantitative verification of the previously constructed model of an intense synoptic eddy through its comparison with the eddy features revealed from the satellite altimetry observations showed, on the whole, satisfactory agreement between the theoretical characteristics and those observed. The altimetry-based median estimate of the eddy structure confirms existence of the eddy core including the main vorticity, and the trap zone in which the particles of the fluid surrounding the core are involved in the orbital motion. According to the observations, the cyclonic eddies drift mainly to the northwest, whereas the anticyclonic ones – to the southwest that is forecasted by the theoretical model. It is shown that the anomalous drift of the eddies along the meridian (the cyclonic eddies – to the south and the anticyclonic ones – to the north) is explained by the effect of mean currents. The distances, over which the eddies drift along the meridian, and the inclination angles of the eddy trajectories relative to the parallels are quantitatively close to those observed. Conclusions . The results of processing the satellite altimetry observations confirm adequacy of the previously developed general notions about the character of the synoptic processes in the ocean and the physical prerequisites constituting a foundation for the theoretical model of an intense eddy radiating the Rossby waves. At that, interaction of the baroclinic and barotropic modes should be necessarily taken into account for more accurate reproduction of the eddy lifetime.

Changing forces in midstream

Ocean Circulation The intensity and frequency of the strongest cyclones east of Taiwan have increased over the past several decades as the climate has warmed. Zhang et al. found that one result of this trend has been the strengthening of Kuroshio current transport off the coast of Japan. The Kuroshio, like its Atlantic counterpart the Gulf Stream, is a surface current that moves huge volumes of warm water from low latitudes to high ones. As strong Pacific cyclones have become stronger, they have increased the amount of energy contained in cyclonic mesoscale ocean eddies and decreased that of anticyclonic ones. This in turn has increased the transfer of energy to the Kuroshio as eddies move into the current, providing a feedback between climate warming and ocean heat transport. Science , this issue p. [988][1] [1]: /lookup/doi/10.1126/science.aax5758

The size and lifetime of organised eddies in a non-solid-body rotating turbulence experiment

Abstract This paper follows our previous study of forced non-solid-body rotating turbulence and summarises results on the eddy size and lifetime in the same flow. The experiment is conducted in a dodecagonal tank ( R t ≈ 1 m in radius and 2m in height) and the bulk rotation is onset by the co-rotation of two circular impellers installed on the top and the bottom of the tank. Turbulence is continuously injected to the central region mainly from the vertical baffles installed on the tank wall. As the impellers rotate in a steady manner, the global Reynolds number ( R e G ) and Rossby number ( R o G ) sustain in the range of 350 ∼ 5900 and 1 . 44 ∼ 3 . 35 respectively. Two-dimensional particle image velocimetry (2DPIV) measurements are carried out in a plane normal to the axis of the rotation and in the central region of the tank, where the flow is found Rayleigh stable. By developing an eddy detection and a lifetime estimation algorithm, we show that there seems to be a characteristic eddy size d ∗ , which is about 0 . 08 ∼ 0 . 1 R t and is weakly dependent on R e G . Among eddies which are smaller than d ∗ , anti-cyclonic ones are dominant, but only marginally, while for eddies larger than d ∗ , prevalence of cyclonic eddies is evident in both number density and lifetime. The demarcation eddy size d ∗ corresponds to approximately constant values of macro Rossby number R o ∗ ≈ 2 and micro Rossby number R o ω ∗ ≈ 8 at high R e . This is consistent with the case of solid-body rotating turbulence and indicates that the cyclonic prevalence is here also due to the centrifugal instability, which preferentially depletes anti-cyclones at R o ∼ 1 . On average, the longest lived eddies are found to be of d ≈ 1 ∼ 1 . 5 d ∗ and have life spans which are an order of magnitude larger than the turbulence eddy turn over time. The local R o , defined based on the local flow rotation rate and local averaged size of organised cyclonic eddies, is found to be self-adjusted to 2 . 75 ∼ 3 . 75 , inside which the maximum extent of cyclonic eddy predominance, measured by the vorticity skewness, is observed. This brings about that the average size of the cyclonic eddies at a given radius is 〈 d 〉 ∼ u r ∕ Ω r (where u r and Ω r are the local fluctuating velocity and local rotation rate), which is found to be an important scale in this problem.

Eddy surface properties and propagation at Southern Hemisphere western boundary current systems

Abstract. Oceanic eddies exist throughout the world oceans, but are more energetic when associated with western boundary currents (WBC) systems. In these regions, eddies play an important role in mixing and energy exchange. Therefore, it is important to quantify and qualify eddies associated with these systems. This is particularly true for the Southern Hemisphere WBC system where only few eddy censuses have been performed to date. In these systems, important aspects of the local eddy population are still unknown, like their spatial distribution and propagation patterns. Moreover, the understanding of these patterns helps to establish monitoring programs and to gain insight in how eddies would affect local mixing. Here, we use a global eddy data set to qualify eddies based on their surface characteristics in the Agulhas Current (AC), the Brazil Current (BC) and the East Australian Current (EAC) systems. The analyses reveal that eddy propagation within each system is highly forced by the local mean flow and bathymetry. Large values of eddy amplitude and temporal variability are associated with the BC and EAC retroflections, while small values occur in the centre of the Argentine Basin and in the Tasman Sea. In the AC system, eddy polarity dictates the propagation distance. BC system eddies do not propagate beyond the Argentine Basin, and are advected by the local ocean circulation. EAC system eddies from both polarities cross south of Tasmania but only the anticyclonic ones reach the Great Australian Bight. For all three WBC systems, both cyclonic and anticyclonic eddies present a geographical segregation according to radius size and amplitude. Regions of high eddy kinetic energy are associated with the eddies' mean amplitudes, and not with their densities.

The nonlinear decay of vortex flows in a rotating fluid

The manifestations of the cyclone-anticyclone asymmetry effect, which is observed in nature [1–3] and in laboratory experiments with a rotating fluid [4–6], have been studied in the scope of the vorticity evolution generalized equation. Under laboratory conditions, asymmetry is observed as a faster decay of cyclonic vortices as compared to anticyclonic ones. This effect has been analytically described simply based on the problem of the vortex spot decay and spatially periodic vortex lattice. It has been indicated that anticyclonic vortices compress and cyclonic ones expand when a lattice damps. Examples of exact solutions of the vorticity equation [7] (describing, specifically, collapsing of an axisymmetric vortex core with anticyclonic vorticity) have been constructed. The effect of the Ekman friction on the interaction between singular vortices is discussed.

Gulf Stream eddy characteristics in a high‐resolution ocean model

A detailed statistical study of the mesoscale eddy activity in the Gulf Stream (GS) region is performed based on a high‐resolution multidecadal regional ocean model hindcast. An eddy detection and tracking method that can be used to capture eddy features from large datasets is presented. This method is applied to the 50 year model hindcast within a domain with the most energetic eddy activity along the GS. Detection results are then analyzed to investigate the kinematic properties and temporal variability of GS mesoscale eddies. The studied kinematic properties include the eddy size, duration, intensity, propagation, and spatial distribution. On average, cyclonic eddies are smaller in size but more energetic and remain coherent longer than anticyclonic ones. Cyclonic eddies generally travel further from the generation sites and have a strong tendency for westward propagation with a small equatorward deflection. Anticyclonic eddies remain near their generation locations and tend to propagate northward. The temporal evolution of eddy properties for long‐lived eddies (lifetime >90 days) is also examined. For both cyclonic and anticyclonic eddies, the size increases rapidly to their maximum value within the first 20 days at which point they begin to slowly decay. In terms of intensity, cyclonic eddies show a quasi‐linear decay while the anticyclonic ones reach a quasi‐steady state after 3–4 months of a more rapid decay. Finally, the seasonal variability of the GS mesoscale eddies is explored. In autumn and winter, both types of eddies are more numerous and larger but less intense, while in spring they are more intense but less numerous and generally smaller. Several possible mechanisms, including the wind stress, thermal forcing, and topographic influence, are considered to explain the seasonal cycle of eddy variability.

A Resonance Mechanism Leading to Wind-Forced Motions with a 2f Frequency

Abstract This study revisits the mechanisms that spatially reorganize wind-forced inertial motions embedded in an oceanic mesoscale eddy field. Inertial motions are known to be affected by the eddy relative vorticity, being expelled from cyclonic structures and trapped within anticyclonic ones. Using shallow water equations (involving a single baroclinic mode), the authors show that nonlinear wave–wave interactions excite, through a resonance mechanism, motions with a 2f frequency (with f being the inertial frequency) and a specific length scale. In a more general situation involving several baroclinic modes, this resonance is found to be reinforced and principally efficient for the lowest baroclinic modes. Such characteristics make the energy of the wind-forced motions potentially available to small-scale mixing through parametric subharmonic instability (not taken into account in the present study).

A Generation Mechanism for Mesoscale Eddies in the Kuril Basin of the Okhotsk Sea: Baroclinic Instability Caused by Enhanced Tidal Mixing

Mesoscale eddies, particularly anticyclonic ones, are dominant features in the Kuril Basin of the Okhotsk Sea. In 1999, both surface drifter and hydrographic observations caught the same anticyclonic eddy northwest of Bussol’ Strait, which has a diameter of ∼100 km, typical surface velocity of 0.2–0.3 m s−1, and less dense core extending to a depth of ∼1200 m. Based on an idea that the generation of mesoscale eddies is caused by strong tidal mixing in and around Kuril Straits, we have conducted a series of three-dimensional numerical model experiments, in which strong tidal mixing is simply parameterized by increasing coefficients of vertical eddy viscosity and diffusivity along the eastern boundary. Initially, a regular series of disturbances with a wavelength of ∼70 km starts to develop. The disturbances can be clearly explained by a linear instability theory and regarded as the baroclinic instability associated with the near-surface front formed in the region between the enhanced mixing and offshore regions. In the mature phase, the disturbances grow large enough that some eddies pinch off and advect offshore (westward), with the scale of disturbances increasing gradually. Typical eddy scale and its westward propagation speed are ∼100 km and ∼0.6 km day−1, respectively, which are consistent with the observations by satellites. The westward propagation can be explained partly due to nonlinear effect of self-offshore advection and partly due to the β-effect. With the inclusion of the upper ocean restoring, the dominance of anticyclonic eddy, extending from surface to a depth of ∼1200 m, can be reproduced.

Modern sedimentary environments and dynamic depositional systems in the southern Yellow Sea

Based on analyses of more than 600 surface sediment samples together with large amounts of previous sedimentologic and hydrologic data, the characteristics of modern sedimentary environments and dynamic depositional systems in the southern Yellow Sea (SYS) are expounded, and the controversial formation mechanism of muddy sediments is also discussed. The southern Yellow Sea shelf can be divided into low-energy sedimentary environment and high-energy sedimentary environment; the low-energy sedimentary environment can be further divided into cyclonic and anticyclonic ones, and the high-energy environment is subdivided into high-energy depositional and eroded environments. In the shelf low-energy environments, there developed muddy depositional system. In the central part of the southern Yellow Sea, there deposited the cold eddy sediments under the actions of a meso-scale cyclonic eddy (cold eddy), and in the southeast of the southern Yellow Sea, an anticyclonic eddy muddy depositional system (warm eddy sediment) was formed. These two types of sediments showed evident differences in grain size, sedimentation rate, sediment thickness and mineralogical characteristics. The high-energy environments were covered with sandy sediments on seabed; they appeared mainly in the west, south and northeast of the southern Yellow Sea. In the high-energy eroded environment, large amounts of sandstone gravels were distributed on seabed. In the high-energy depositional environment, the originally deposited fine materials (including clay and fine silt) were gradually re-suspended and then transported to a low-energy area to deposit again. In this paper, the sedimentation model of cyclonic and anticyclonic types of muddy sediments is established, and a systematic interpretation for the formation cause of muddy depositional systems in the southern Yellow Sea is given.

Wave–vortex interaction in rotating shallow water. Part 1. One space dimension

Using a physical space (i.e. non-modal) approach, we investigate interactions between fast inertio-gravity (IG) waves and slow balanced flows in a shallow rotating fluid. Specifically, we consider a train of IG waves impinging on a steady, exactly balanced vortex. For simplicity, the one-dimensional problem is studied first; the limitations of one-dimensionality are offset by the ability to define balance in an exact way. An asymptotic analysis of the problem in the small-amplitude limit is performed to demonstrate the existence of interactions. It is shown that these interactions are not confined to the modification of the wave field by the vortex but, more importantly, that the waves are able to alter in a non-trivial way the potential vorticity associated with that vortex. Interestingly, in this one-dimensional problem, once the waves have traversed the vortex region and have propagated away, the vortex exactly recovers its initial shape and thus bears no signature of the interaction. Furthermore, we prove this last result in the case of arbitrary vortex and wave amplitudes. Numerical integrations of the full one-dimensional shallow-water equations in strongly nonlinear regimes are also performed: they confirm that time-dependent interactions exist and increase with wave amplitude, while at the final state the vortex bears no sign of the interaction. In addition, they reveal that cyclonic vortices interact more strongly with the wave field than anticyclonic ones.

Seasonal Variation of Geostrophic Current and Water Transport in the Central Part of The Red Sea

The data from meteor R-V cruise legs-5 are used to discuss the circulation pattern and geostrophic current in the area extending between 19? and 24?N during February and July 1987. During winter season, the distributions of temperature, salinity, density and geostrophic current show the existence of gyratory pattern as a result of convection or advection processes. The extension of these gyres differs from surface to 250 m depth. About 7 gyres can be detected at the surface level and 4 gyres at 250 m depth. The existence of these gyres causes a strong cross currents. On the other hand the cyclonic gyratory pattern help in the renewal of deep Red Sea water. The pattern of flow along the east-west section consists of 2 layers. The surface one flowing to the north with net volume transport through this layer of about 1.975 × 106 m3/sec. While the lower flows to the south and with net volume transport equal to 0.984 × 106 m3/sec. During the summer season, the geostrophic currents appear in southeast direction in the area extending between 19 and 22?N, while at the rest of the investigated area (22-24?N) 6 gyres in between cyclonic and anticyclonic ones are present. At 250 m depth, the current problem is changed. In the area between 19-21?N, the currents flow in northwest direction and in the rest of the investigated area the currents were mainly in southwest direction accompanied with 4 different gyres. The water transport at the same east-west section is through two layers. The upper one moves to the south and the net volume transport is about 2.621 106 m3/sec. While the second layer was to the north with net volume transport of about 2.1368 × 106 m3/sec. استخدمت البيانات التي تم الحصول عليها من الرحلة 5 لسفينة الأبحاث الألمانية ميتيور في البحر الأحمر في مناقشة نظام 19 إلى 24رجة شمال. تمت القياسات البحرية خلال شهري فبراير ويوليو 1987 بما يتوافق مع تواد الرياح الموسمية الشمالية الشرقية والجنوبية الغربية العالية على التوالي. أيضا تم اختيار مستوى سطح البحر و250متر عمق كمستويين لتمثيل كل من الطبقة السطحية والطبقة المتوسطة حيث نوقشت الخواص الفيزيائية وحسبت السرعات الجيوستروفية لكل من الطبقة السطحية والطبقة المتوسطة. أظهرت توزيعات درجة الحرارة ، الملوحة والكثافة خلال فصل الشتاء وجود نظام دوامي يختلف في السطح عنه في الطبقة المتوسطة. كما أكد نظام التيارات وجود العديد من الدوامات (7دوامات على السطح و 4 على عمق 250متر). ويساعد وجود هذا النظام الدوراني على إعادة تكوين المياه العميقة في البحر الأحمر. ولقد وجد أن الحجم الانتقالي للمياه خلال قطاع ممتد من الشرق إلى الغرب لكل من الطبقة السطحية والمتوسطة حوالي 1,975×610 متر3 في اتجاه الشمال و0,984 × 610 متر3 في اتجاه الجنوب على التوالي. خلال فصل الصيف أظهرت حسابات التيارات الجيوستروفية وجود تيار جنوبي شرقي ممتد من 19 إلى 22 درجةشمال بينما توجد 6 دوامات سكلونية وانتيسكلونية في بقية منطقة الدراسة. انحرف التيار الجنوبي الشرقي إلى اتجاه جنوبي غربي خلال الطبقة المتوسطة في نفس المنطقة الممتدة من 19 إلى 21 درجة شمال بينما يستمر في اتجاه الجنوب الغربي في بقة منطقة الدراسة أيضا وجد إن حجم المياه المنقول على نفس القطاع الغربي بلغت حوالي 2,621 ×610، 2,137 ×610 متر3 لكل من الطبقة السطحية والطبقة المتوسطة.

Nutritional condition of female Calanus euxinus from cyclonic and anticyclonic regions of the Black Sea

The content and composition of lipids in Calanus euxinus females were studied in cyclonic and anticyclonic regions in the southern Black Sea during the period of persistent density stratification (September 1996). The lipid content of C. euxinus from cyclonic regions was greater than of individuals from anticyclonic regions (average 101.9 and 58.8 μg ind. -1 , respectively). Total lipid content (mainly wax esters, 70 to 72 %, and triacylglycerols, 8 to 13 %) of female C. euxinus was correlated with chlorophyll a (chl a) (mean integrated concentration) (r = 0.92, p < 0.005). The high correlation with chl a was shown also for wax ester content, used as a long-term index of food supply (r = 0.89, p < 0.05). The correlation between triacylglycerol content (a short-term index) and chl a was not significant (r = 0.66, p < 0.2). In cyclonic regions the mean integrated chl a concentration (0.38 mg m -3 ) (as an indicator of phytoplankton food biomass) was 1.7-fold higher than in anticyclonic areas. The same trend occurred for mean integrated concentrations of nitrate and phosphate (0.996 and 0.299 μM in cyclonic and 0.438 and 0.061 μM in anticyclonic regions, respectively) in the chl a containing layer. Thus, conditions for total primary productivity in cyclonic regions were more favorable than in anticyclonic ones. This was consistent with lipid levels in female C. euxinus.

Long-lived planetary vortices and their evolution: Conservative intermediate geostrophic model.

Large, long-lived vortices, surviving during many turnaround times and far longer than the dispersive linear Rossby wave packets, are abundant in planetary atmospheres and oceans. Nonlinear effects which prevent dispersive decay of intense cyclones and anticyclones and provide their self-propelling propagation are revised here using shallow water equations and their balanced approximations. The main physical mechanism allowing vortical structures to be long-lived in planetary fluid is the quick fluid rotation inside their cores which prevents growth in the amplitude of asymmetric circulation arising due to the beta-effect. Intense vortices of both signs survive essentially longer than the linear Rossby wave packet if their azimuthal velocity is much larger than the Rossby wave speed. However, in the long-time evolution, cyclonic and anticyclonic vortices behave essentially differently that is illustrated by the conservative intermediate geostrophic model. Asymmetric circulation governing vortex propagation is described by the azimuthal mode m=1 for the initial value problem as well as for steadily propagating solutions. Cyclonic vortices move west-poleward decaying gradually due to Rossby wave radiation while anticyclonic ones adjust to non-radiating solitary vortices. Slow weakening of an intense cyclone with decreasing of its size and shrinking of the core is described assuming zero azimuthal velocity outside the core while drifting poleward. The poleward tendency of the cyclone motion relative to the stirring flow corresponds to characteristic trajectories of tropical cyclones in the Earth's atmosphere. The asymmetry in dispersion-nonlinear properties of cyclones and anticyclones is thought to be one of the essential reasons for the observed predominance of anticyclones among long-lived vortices in the atmospheres of the giant planets and also among intrathermoclinic eddies in the ocean.

The wind and thermally driven circulation of the eastern Mediterranean Sea. Part II: the Baroclinic case

Compared with other interesting parts of the World Ocean, little is known of the eastern Mediterranean and major issues of the Mediterranean circulation are still unsolved. Among them, the most crucial one is: what is the dominant driving mechanism of the eastern Mediterranean general circulation: (1) the wind stress; (2) the thermohaline surface fluxes; (3) the inflow forcing at the Sicily Straits? What is the relative importance of these three forcing functions? Is it the same in the different sub-basins comprising the eastern Mediterranean? What modelling factors are important for the simulation of the seasonal cycle and is the general circulation overall dominated by the annual mean or seasonal signal? To answer the above questions we have carried out an extensive and thorough series of numerical experiments using a multilevel model of the circulation, suitable for coarse-resolution studies but endowed with active thermodynamics and allowing for realistic geometry (coastlines, islands, bottom relief). The model is used in a three-level version as the minimum one capable of simulating the vertical superposition of different water masses observed in the eastern Mediterranean. The climatological monthly averages of wind-stress, thermal and evaporative fluxes and inflow at Sicily are used to drive the model. In Part I of the present study it was shown that the seasonal cycle present in the wind-stress curl induces a strongly seasonal barotropic circulation comprising the entire eastern Mediterranean. This seasonal gyre reverses from being cyclonic in winter to anticyclonic in summer. The inclusion of baroclinicity, however, profoundly modifies the purely wind-driven, barotropic circulation, eliminating the strong seasonality and the winter-to-summer reversal. The first important result is that the general circulation pattern now consists of a succession of sub-basin-scale gyres, with a seasonal modulation emphasizing the cyclonic centres in winter and the anticyclonic ones in summer, according to the vorticity input by the wind. When surface thermal forcing is included, the winter-to-summer differences become very small and the yearly pattern is dominant. The second important result is that the intercomparisons of the various numerical experiments in which each driving mechanism is studied in isolation from the others allows us to classify unambiguously the three forcing mechanisms in order of relative importance in driving the circulation in the different sub-basins of the eastern Mediterranean. Specifically, for the Ionian Sea and Sea of Crete the dominant forcing is the inflow at the Straits of Sicily while for the Levantine Sea thermohaline fluxes are the main driving function. The wind-stress forcing induces a seasonal fluctuation only in the meandering path of the Atlantic jet entering the Ionian Sea through the Sicily Straits. We finally carry out a ‘central experiment’, the most realistic one in which all forcing functions drive the circulation that we compare quantitatively with other model results and qualitatively with observations. Major features can be recognized and are shown to be persistent all year long. These features are also found in the dynamic heights of the general hydrographic surveys of the Physical Oceanography of the Eastern Mediterranean (POEM) programme. The only POEM feature not reproduced by the model, an intense anticyclonic region in the south eastern Levantine, may be attributed to errors and specifically underestimates, of the available thermal fluxes whose effect is partially overcome by the wind-stress forcing. This anticyclonic cell is in fact obtained when the model is driven by the thermal fluxes alone. Overall, the model results compare well with the observational evidence provided by the POEM surveys and the thermohaline vertical circulation cell reproduced by the model is consistent with the preliminary results of the transient-tracer survey POEM-V-87. Finally, many of the persistent features found in the model circulation patterns can be related to the strong control exerted by the ambient potential vorticity f/ H upon the eastern Mediterranean circulation.

Circulation Patterns Near the Tail of the Grand Banks

Dynamic topographies for the region surrounding the tail of the Grand Banks have been examined for the years 1922–65, in an attempt to define the characteristics of the eddies and meanders typical of the circulation pattern. The size distribution of the cyclonic eddies appears to be bimodal and thus suggests the possibility of two different modes of eddy formation. The shear of the mean current system is cyclonic, and it is therefore not surprising that cyclonic eddies dominate over anticyclonic ones by 2.5:1. However, the anticyclonic eddies appear faster; this may in part be associated with the choice of reference level. Other than a seasonal decrease in eddy strength from April-June, no secular trends could be found. The eddies were characteristically found in certain areas only, suggesting that topography plays a role in their formation. The meanders were markedly smaller and slower than those of the Kuroshio front. No secular trends could be found in the meander patterns. A simple vorticity analysis indicates the meander pattern to be bathymetrically controlled.