scholarly journals Spatiotemporal Variability of Mesoscale Eddies in the Indonesian Seas

2021 ◽  
Vol 13 (5) ◽  
pp. 1017
Author(s):  
Zhanjiu Hao ◽  
Zhenhua Xu ◽  
Ming Feng ◽  
Qun Li ◽  
Baoshu Yin
Keyword(s):  
World Ocean ◽  
Mesoscale Eddies ◽  
Spatial Inhomogeneity ◽  
Spatiotemporal Variability ◽  
Identification Algorithm ◽  
Mean Flow ◽  
Sulu Sea ◽  
Indonesian Seas ◽  
Highly Nonlinear ◽  
Eddy Generation

Mesoscale eddies are ubiquitous in the world ocean and well researched both globally and regionally, while their properties and distributions across the whole Indonesian Seas are not yet fully understood. This study investigates for the first time the spatiotemporal variations and generation mechanisms of mesoscale eddies across the whole Indonesian Seas. Eddies are detected from altimetry sea level anomalies by an automatic identification algorithm. The Sulu Sea, Sulawesi Sea, Maluku Sea and Banda Sea are the main eddy generation regions. More than 80% of eddies are short-lived with a lifetime below 30 days. The properties of eddies exhibit high spatial inhomogeneity, with the typical amplitudes and radiuses of 2–6 cm and 50–160 km, respectively. The most energetic eddies are observed in the Sulawesi Sea and Seram Sea. Eddies feature different seasonal cycles between anticyclonic and cyclonic eddies in each basin, especially given that the average latitude of the eddy centroid has inverse seasonal variations. About 48% of eddies in the Sulawesi Sea are highly nonlinear, which is the case for less than 30% in the Sulu Sea and Banda Sea. Instability analysis is performed using high-resolution model outputs from Bluelink Reanalysis to assess mechanisms of eddy generation. Barotropic instability of the mean flow dominates eddy generation in the Sulu Sea and Sulawesi Sea, while baroclinic instability is slightly more in the Maluku Sea and Banda Sea.

scholarly journals On the Evaluation of Seasonal Variability of the Ocean Kinetic Energy

2017 ◽  
Vol 47 (7) ◽  
pp. 1675-1683 ◽  
Author(s):  
Dujuan Kang ◽  
Enrique N. Curchitser
Keyword(s):  
Kinetic Energy ◽  
Seasonal Variability ◽  
Moving Average ◽  
Spatial Inhomogeneity ◽  
Mean Flow ◽  
Annual Cycles ◽  
Residual Term ◽  
The Mean ◽  
Mean Differences ◽  
Average Filter

AbstractThe seasonal cycles of the mean kinetic energy (MKE) and eddy kinetic energy (EKE) are compared in an idealized flow as well as in a realistic simulation of the Gulf Stream (GS) region based on three commonly used definitions: orthogonal, nonorthogonal, and moving-average filtered decompositions of the kinetic energy (KE). It is shown that only the orthogonal KE decomposition can define the physically consistent MKE and EKE that precisely represents the KEs of the mean flow and eddies, respectively. The nonorthogonal KE decomposition gives rise to a residual term that contributes to the seasonal variability of the eddies, and therefore the obtained EKE is not precisely defined. The residual term is shown to exhibit more significant seasonal variability than EKE in both idealized and realistic GS flows. Neglecting its influence leads to an inaccurate evaluation of the seasonal variability of both the eddies and the total flow. The decomposition using a moving-average filter also results in a nonnegligible residual term in both idealized and realistic GS flows. This type of definition does not ensure conservation of the total KE, even if taking into account the residual term. Moreover, it is shown that the annual cycles of the three types of EKEs or MKEs have different phases and amplitudes. The local differences of the EKE cycles are very prominent in the GS off-coast domain; however, because of the spatial inhomogeneity, the area-mean differences may not be significant.

The structure of highly nonlinear vortices in curved channel flow

1992 ◽  
Vol 439 (1906) ◽  
pp. 317-336 ◽  
Keyword(s):  
Boundary Layer ◽  
Numerical Solutions ◽  
Wall Layer ◽  
Taylor Number ◽  
Neutral Stability ◽  
Mean Flow ◽  
Curved Channel ◽  
Fully Developed Flow ◽  
Highly Nonlinear ◽  
Curved Channel Flow

The structure of highly nonlinear, small wavelength Gӧrtler vortices is analysed for fully developed flow in a curved channel. The Taylor number is taken to be T 0 ϵ -5 , which is asymptotically larger than the neutral stability Taylor number for vortices of small wavelength 2 πϵ . Over most of the flow domain the motion is determined by the interaction of the mean flow and terms proportional to exp {i z/ϵ }, whereas adjacent to the outer channel wall there is a boundary layer of thickness O(ϵ) which requires all the harmonics of the disturbance to be considered. Numerical solutions of the wall layer equations are presented for T 0 up to 14000 with the main feature of these results being the development of a weak vortex on the boundary layer lengthscale.

scholarly journals Formation of Intrathermocline Lenses by Eddy–Wind Interaction

2015 ◽  
Vol 45 (2) ◽  
pp. 606-612 ◽  
Author(s):  
Dennis J. McGillicuddy
Keyword(s):  
Water Mass ◽  
World Ocean ◽  
Mesoscale Eddies ◽  
Water Layer ◽  
Alternative Mechanism ◽  
Mode Water ◽  
Concave Lens ◽  
Convex Lens ◽  
Local Generation

AbstractMesoscale intrathermocline lenses are observed throughout the World Ocean and are commonly attributed to water mass anomalies advected from a distant origin. An alternative mechanism of local generation is offered herein, in which eddy–wind interaction can create lens-shaped disturbances in the thermocline. Numerical simulations illustrate how eddy–wind-driven upwelling in anticyclones can yield a convex lens reminiscent of a mode water eddy, whereas eddy–wind-driven downwelling in cyclones produces a concave lens that thins the mode water layer (a cyclonic “thinny”). Such transformations should be observable with long-term time series in the interiors of mesoscale eddies.

scholarly journals Rossby waves and two-dimensional turbulence in a large-scale zonal jet

1987 ◽  
Vol 183 ◽  
pp. 467-509 ◽  
Author(s):  
Theodore G. Shepherd
Keyword(s):  
Large Scale ◽  
Zonal Flow ◽  
Order Effect ◽  
Spatial Inhomogeneity ◽  
Mean Flow ◽  
Scale Separation ◽  
Zonal Flows ◽  
Beta Plane ◽  
Zonal Wavenumber ◽  
Scale Shear

The theory of homogeneous barotropic beta-plane turbulence is here extended to include effects arising from spatial inhomogeneity in the form of a zonal shear flow. Attention is restricted to the geophysically important case of zonal flows that are barotropically stable and are of larger scale than the resulting transient eddy field.Because of the presumed scale separation, the disturbance enstrophy is approximately conserved in a fully nonlinear sense, and the (nonlinear) wave-mean-flow interaction may be characterized as a shear-induced spectral transfer of disturbance enstrophy along lines of constant zonal wavenumber k. In this transfer the disturbance energy is generally not conserved. The nonlinear interactions between different disturbance components are turbulent for scales smaller than the inverse of Rhines's cascade-arrest scale κβ≡ (β0/2urms)½ and in this regime their leading-order effect may be characterized as a tendency to spread the enstrophy (and energy) along contours of constant total wavenumber κ ≡ (k2 + l2)½. Insofar as this process of turbulent isotropization involves spectral transfer of disturbance enstrophy across lines of constant zonal wavenumber k, it can be readily distinguished from the shear-induced transfer which proceeds along them. However, an analysis in terms of total wavenumber K alone, which would be justified if the flow were homogeneous, would tend to mask the differences.The foregoing theoretical ideas are tested by performing direct numerical simulation experiments. It is found that the picture of classical beta-plane turbulence is altered, through the effect of the large-scale zonal flow, in the following ways: (i) while the turbulence is still confined to KKβ, the disturbance field penetrates to the largest scales of motion; (ii) the larger disturbance scales K < Kβ exhibit a tendency to meridional rather than zonal anisotropy, namely towards v2 > u2 rather than vice versa; (iii) the initial spectral transfer rate away from an isotropic intermediate-scale source is significantly enhanced by the shear-induced transfer associated with straining by the zonal flow. This last effect occurs even when the large-scale shear appears weak to the energy-containing eddies, in the sense that dU/dy [Lt ] κ for typical eddy length and velocity scales.

scholarly journals Identification of temperature anomalies in the western Caspian Sea in 2017 based on remote sensing data

2021 ◽  
Vol 15 (4) ◽  
pp. 63-74
Author(s):  
A. A. Bagomaev ◽  
N. O. Guseynova
Keyword(s):  
Remote Sensing ◽  
Water Temperature ◽  
Caspian Sea ◽  
World Ocean ◽  
Remote Sensing Data ◽  
Spatial Inhomogeneity ◽  
Science Data ◽  
Space Images ◽  
Sensing Data ◽  
Temperature Anomalies

Aim. The study of temperature anomalies in the western Caspian Sea based on space imagery materials in order to detect upwelling phenomena.Materials and Methods. We used temperature indicators of seawater for the summer season of 2017 when a sharp decrease by more than 2 °C in average daily temperature occurred. Space images were obtained from the specialized centres of Ocean Color NASA, Earth Science Data Systems NASA and SATIN. Remote sensing data were processed using SeaDAS and ArcGIS programs. Ground data were obtained from the resources of the Unified State System of Information about the Situation in the World Ocean (ESIMO). An ArcGIS database was created and maps compiled.Results. The first upwelling occurred on 9-17 June 2020. The minimum water temperature in the Makhachkala area was 14°C with an increase in salinity to 12%o over an area of 1,500 sq.km. An increase in the content of dissolved oxygen of up to 9.70 mg/l and pH 8.64 was recorded. A second upwelling of medium intensity occurred from 19 June-July 1 with a minimum temperature of 17.9C. The decrease in temperature was 2.8°C with an increase in salinity by 1%o. The surface area was 454 sq.km. A third case of upwelling was recorded from 26 August-September 1 and was characterised by a decrease in water temperature of 7.4C (near the coast, 17.1°C). The average salinity increase was 0.32%o while the 02 concentration was 8 mg/l over an area of 500 sq.km.Conclusion. Due to its large size, the Caspian Sea is characterised by spatial inhomogeneity of oceanological parameters, which can be recorded based on the results of processing satellite images and their verification using ground data. In the western part of the sea the upwelling is periodic and of different scales.

Strong long-lived anticyclonic mesoscale eddies in the Balearic Sea: formation and intensification mechanisms

2021 ◽  
Author(s):  
Eva Aguiar ◽  
Baptiste Mourre ◽  
Adèle Revélard ◽  
Mélanie Juza ◽  
Aida Alvera-Azcárate ◽  
...  
Keyword(s):  
Mesoscale Eddies ◽  
Mean Flow ◽  
Thermal Front ◽  
Warm Core ◽  
Salinity Gradients ◽  
Balearic Sea ◽  
Sensitivity Tests ◽  
Mass Properties ◽  
Flow Interactions ◽  
Wind Events

&lt;p&gt;&lt;span&gt;Anticyclonic mesoscale eddies are often formed in the Balearic Sea towards the end of summer and autumn. In some years, these eddies become strong and persistent, modifying the ocean currents and water mass properties in the area. The generation and intensification mechanisms of two long-lived events observed in 2010 and 2017 were studied by means of the energy conversion terms associated with eddy-mean flow interactions and through complementary model sensitivity tests.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Results show that these eddies were formed through mixed barotropic and baroclinic instabilities. The former was associated with weak meandering of the shelf current near the coast produced by northwesterly wind events, and the latter with the existence of the northward intrusions of relatively warm waters through the intense Pyrenees thermal front. &lt;/span&gt;&lt;span&gt;The intensification mechanism varied between the two&lt;/span&gt; &lt;span&gt;events. While in 2010 it was driven by intense salinity gradients in the Balearic Sea, in 2017 it resulted from an extra barotropic energy term fed by northwesterly winds.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;These eddies lasted more than two months with a radius varying between 30km and 90km and a vertical structure that reached 1500 m depth. Their presence resulted in a 3&amp;#186;C anomaly between the warm core waters and the outer parts of the eddies. &lt;/span&gt;&lt;/p&gt;

The Effects of Mesoscale Eddies on the Main Subtropical Thermocline

2004 ◽  
Vol 34 (11) ◽  
pp. 2428-2443 ◽  
Author(s):  
Cara C. Henning ◽  
Geoffrey K. Vallis
Keyword(s):  
Mixed Layer ◽  
Mesoscale Eddy ◽  
Mesoscale Eddies ◽  
Western Boundary ◽  
Residual Circulation ◽  
Mean Flow ◽  
Vertical Diffusion ◽  
Mode Water ◽  
Water Region ◽  
The Mean

Abstract The effects of mesoscale eddies on the main subtropical thermocline are explored using a simply configured wind- and buoyancy-driven primitive equation numerical model in conjunction with transformed Eulerian mean diagnostics and simple scaling ideas and closure schemes. If eddies are suppressed by a modest but nonnegligible horizontal diffusion and vertical diffusion is kept realistically small, the model thermocline exhibits a familiar two-regime structure with an upper, advectively dominated ventilated thermocline and a lower, advective– diffusive internal thermocline, and together these compose the main thermocline. If the horizontal resolution is sufficiently high and the horizontal diffusivity is sufficiently low, then a vigorous mesoscale eddy field emerges. In the mixed layer and upper-mode-water regions, the divergent eddy fluxes are manifestly across isopycnals and so have a diabatic effect. Beneath the mixed layer, the mean structure of the upper (i.e., ventilated) thermocline is still found to be dominated by mean advective terms, except in the “mode water” region and close to the western boundary current. The eddies are particularly strong in the mode-water region, and the low-potential-vorticity pool of the noneddying case is partially eroded away as the eddies try to flatten the isopycnals and reduce available potential energy. The intensity of the eddies decays with depth more slowly than does the mean flow, leading to a three-way balance among eddy flux convergence, mean flow advection, and diffusion in the internal thermocline. Eddies subduct water along isopycnals from the surface into the internal thermocline, replenishing its water masses and maintaining its thickness. Just as in the noneddying case, the dynamics of the internal thermocline can be usefully expressed as an advective–diffusive balance, but where advection is now by the residual (eddy-induced plus Eulerian mean) circulation. The eddy-induced advection partially balances the mean upwelling through the base of the thermocline, and this leads to a slightly thicker thermocline than in the noneddying case. The results suggest that as the diffusivity goes to zero, the residual circulation will go to zero but the thickness of the internal thermocline may remain finite, provided eddy activity persists.

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