Mean Eddy Research Articles

Evaluation of synoptic eddy activities and their feedback onto the midlatitude jet in five atmospheric reanalyses with coarse versus fine model resolutions

Interaction between synoptic eddy and mean flow plays a crucial role in maintaining midlatitude westerly jet. In this study, climatologies of synoptic eddy activities and their feedback onto midlatitude jet for 1980–2016 are evaluated and compared through analyzing daily data from five atmospheric reanalyses with different resolutions including one coarse-resolution reanalysis (NCEP2) and four fine-resolution reanalyses (ERA-Interim, JRA-55, MERRA-2, and CFSR). Horizontal resolutions of the atmospheric models generating those reanalyses are approximately equivalent to 210, 79, 60, 50, and 38 km, respectively. Results show that the eddy activities and their feedback onto the midlatitude jet in those fine-resolution reanalyses are consistently and significantly stronger than those in the coarse-resolution reanalysis (NCEP2). The maximal relative increases that are found to occur primarily in the midlatitudes of the Southern Hemisphere are estimated to be up to 55% for the baroclinicity, 53% for the eddy energetics, 59% for the eddy forcing, and even 85% for the eddy feedback onto the mean flow. Those increases are reasonably conjectured to be related to increased model resolutions, since the synoptic eddy genesis is proportional to the low-level atmospheric meridional temperature gradient which is sensitive to the meridional resolution of atmospheric models. Although the coarse-resolution reanalysis resolves synoptic eddies insufficiently and thus underestimates their feedback onto the mean flow, the magnitudes of eddy-driven jets are almost the same among five reanalyses, implying a mismatch between the eddy feedback and the eddy-driven jet in the coarse-resolution reanalysis. Therefore, the results of this study imply the importance of using fine-resolution reanalyses in accurately understanding the midlatitude synoptic eddy–mean flow interaction.

Forced flow response analysis of a turbulent swirling annular jet flame

This study of an externally forced, amplifier-type turbulent reacting swirling annular jet presents a low-order model for the flow response to transverse acoustic excitation and compares the model's predictions with experimental measurements. The model is formulated based on linear stability calculations about the turbulent mean flow and eddy viscosity fields obtained from separate measurements of the unforced flow. The stability calculations yield weakly global spatial modes associated with the forcing frequency, which serve as a basis upon which to project the forcing input. Thus, the model constitutes a hydrodynamic transfer function connecting the input forcing to the output coherent flow response through the linearized low Mach number compressible Navier–Stokes equations. Following a detailed presentation of the stability analysis underlying the model, the response predictions are evaluated against previously reported experiments where the jet was transversely excited at both an acoustic pressure node and an antinode. The results reveal excellent agreement between the predicted response and the measured fluctuating fields, suggesting that the low-order linear model based on the turbulent mean flow field captures the essential physics of the mode selection process in this forced configuration. This work provides further evidence that linear hydrodynamics govern the growth and decay of spatiotemporally coherent vortical structures in the swirling, turbulent jet flame, and, in particular, explains the dominance of co-rotating spiral structures.

Impact of the Antarctic topography on meridional energy transport and its consequential effect in the monsoon circulation

AbstractAntarctica has been losing ice in the past, and its ice loss is projected to further enhance in the future due to global warming, causing orographic changes over the region. Such orographic changes are likely to profoundly influence the global climate systems through energy transport over the Southern Hemisphere. The South Asian monsoon system, an anomalous climate system (a transient eddy regionally occurring rigorously during JJAS) originating in the Southern Hemisphere and transporting anomalous energy towards higher latitudes, is also expected to get influenced by its orography. A coupled global climate modelling framework is therefore used in this study to investigate the potential influence of the Antarctic orography on the South Asian monsoon system. The investigation reveals that a reduction in the Antarctic elevation would cause warming over the continent, which will cause changes in the energy budget, with more outgoing long‐wave radiation over Antarctica due to a warmer surface and moist atmosphere aloft. The global climate responds to this perturbation by enhancing the southward energy transport primarily by the atmospheric component via changes in mean flow and eddies from the equatorial region all the way to the South Pole. The monsoon system supports this southward energy transport demand in the Southern Hemisphere by strengthening the mean meridional circulation in the three cells over the Indian Ocean sector. Due to this, the low‐level cross‐equatorial flow enhances over the south equatorial Indian Ocean, which increases the evaporation, and causes changes in precipitation and stationary wave patterns. The sea‐level pressure and surface temperature changes adjust accordingly in response to changes in the circulation. However, rainfall and circulation over the Asian landmass do not change much as the southward energy transport is very much less and is mostly supported by the oceanic components and changes in latent energy over these latitudes.

Energetics of Eddy-Mean Flow Interactions in the Amery Ice Shelf Cavity

Previous studies demonstrated that eddy processes play an important role in ice shelf basal melting and the water mass properties of ice shelf cavities. However, the eddy energy generation and dissipation mechanisms in ice shelf cavities have not been studied systematically. The dynamic processes of the ocean circulation in the Amery Ice Shelf cavity are studied quantitatively through a Lorenz energy cycle approach for the first time by using the outputs of a high-resolution coupled regional ocean-sea ice-ice shelf model. Over the entire sub-ice-shelf cavity, mean available potential energy (MAPE) is the largest energy reservoir (112 TJ), followed by the mean kinetic energy (MKE, 70 TJ) and eddy available potential energy (EAPE, 10 TJ). The eddy kinetic energy (EKE) is the smallest pool (5.5 TJ), which is roughly 8% of the MKE, indicating significantly suppressed eddy activities by the drag stresses at ice shelf base and bottom topography. The total generation rate of available potential energy is about 1.0 GW, almost all of which is generated by basal melting and seawater refreezing, i.e., the so-called “ice pump.” The energy generated by ice pump is mainly dissipated by the ocean-ice shelf and ocean-bottom drag stresses, amounting to 0.3 GW and 0.2 GW, respectively. The EKE is generated through two pathways: the barotropic pathway MAPE→MKE→EKE (0.03 GW) and the baroclinic pathway MAPE→EAPE→EKE (0.2 GW). In addition to directly supplying the EAPE through baroclinic pathway (0.2 GW), MAPE also provides 0.5 GW of power to MKE to facilitate the barotropic pathway.

Lateral redistribution of heat and salt in the Nordic Seas

The locations, times, and mechanisms by which heat and salt are transported through and within the Nordic Seas are discussed. The analysis is based on a regional, high resolution coupled sea ice-ocean numerical model, a climatological hydrographic data set, and atmospheric reanalysis. The model and climatology are broadly consistent in terms of heat loss, water masses, and mean geostrophic currents. The model fields are used to demonstrate that the dominant exchange between basins is an export of warm, salty water from the Norwegian Sea into the Greenland and Iceland Seas, with both the mean cyclonic boundary current system and eddy fluxes playing important roles. In both the model and the climatology, approximately 2/3 of the heat loss to the atmosphere over the Nordic Seas is found over the mean cyclonic flow and 1/3 takes place within the closed recirculations in the interior of each of the basin gyres, with the Norwegian Sea having the largest heat loss. The seasonal cycle is dominated by local air-sea heat flux within the gyres while it is dominated by lateral advection in the cyclonic boundary current, particularly in the northern Norwegian and Greenland Seas. The freshwater flux off the east Greenland shelf is correlated with the local winds such that in winter, when winds are generally towards the southwest, freshwater is advected onto the shelf and in summer, when winds are weak or towards the northeast, freshwater is advected into the Greenland Sea, which leads to salinification in winter and freshening in summer.

Eddy Length Scale Response to Static Stability Change in an Idealized Dry Atmosphere: A Linear Response Function Approach*

AbstractThe response of mid-latitude equilibrated eddy length scale to static stability has long been questioned but not investigated in well-controlled experiments with unchanged mean zonal wind and meridional temperature gradient. With iterative use of the linear response function of an idealized dry atmosphere, we obtain a time-invariant and zonally-uniform forcing to decrease the near-surface temperature by over 2 K while keeping the change in zonal wind negligible (within 0.2m s−1). In such experiments of increased static stability, energy-containing zonal scale decreases by 3–4%, which matches with Rhines scale decrease near the jet core. Changes in Rossby radius (+2%), maximum baroclinic growth scale (-1%) and Kuo scale (0%) fail to match this change in zonal scale. These findings and well-controlled experiments help with better understanding of eddy–mean flow interactions and hence the mid-latitude circulation and its response to climate change.

Diagnosing surfzone impacts on inner-shelf flow spatial variability using realistic model experiments with and without surface gravity waves

AbstractRip currents are generated by surfzone wave breaking and are ejected offshore inducing inner-shelf flow spatial variability (eddies). However, surfzone effects on the inner-shelf flow spatial variability have not been studied in realistic models that include both shelf and surfzone processes. Here, these effects are diagnosed with two nearly identical twin realistic simulations of the San Diego Bight over summer to fall where one simulation includes surface gravity waves (WW) and the other that does not (NW). The simulations include tides, weak to moderate winds, internal waves, submesoscale processes, and have surfzone width Lsz of 96(±41) m (≈ 1 m significant wave height). Flow spatial variability metrics, alongshore root mean square vorticity, divergence, and eddy cross-shore velocity, are analyzed in a Lsz normalized cross-shore coordinate. At the surface, the metrics are consistently (> 70%) elevated in the WW run relative to NW out to 5Lsz offshore. At 4Lsz offshore, WW metrics are enhanced over the entire water column. In a fixed coordinate appropriate for eddy transport, the eddy cross-shore velocity squared correlation betweenWWand NW runs is < 0.5 out to 1.2 km offshore or 12 time-averaged Lsz. The results indicate that the eddy tracer (e.g., larvae) transport and dispersion across the inner-shelf will be significantly different in the WW and NW runs. The WW model neglects specific surfzone vorticity generation mechanisms. Thus, these inner-shelf impacts are likely underestimated. In other regions with larger waves, impacts will extend farther offshore.

Contourite and mixed turbidite-contourite systems in the Mozambique Channel (SW Indian Ocean): Link between geometry, sediment characteristics and modelled bottom currents

Oceanic currents can profoundly reshape the seafloor and even modify the characteristics of turbidite systems. Multiple erosional and depositional features directly formed by bottom currents (i.e. contourites), as well as by the interaction between bottom currents and turbidity currents or turbidite systems (i.e. mixed turbidite-contourite systems) have been identified in the Mozambique Channel (SW Indian Ocean) in multibeam bathymetry, seismic reflection data, sub-bottom profiler images and sediment cores. In this study, we characterise the morphology, stacking pattern and sedimentary characteristics of these sedimentary systems, and analysed the properties of bottom currents at these systems using a hydrodynamic numerical model. Modelled bottom currents are the highest at abraded surfaces and moats, but they also display a relatively high variability, suggesting that the observed erosion is not the result of a constant or persistent current but rather of episodes of intense circulation. Modelled bottom currents at contourite terraces are not significantly different from currents at related plastered drifts, where accumulation is enhanced. The formation of contourite terraces can thus not solely be explained by the mean oceanic circulation and eddies, implying that other processes such as internal waves may play a relevant role in their formation. Three different types of mixed turbidite-contourite systems were observed: one characterised by asymmetric channel-levee systems formed by the synchronous interaction of bottom currents and turbidity currents, one characterised by a phased interaction that resulted in the erosion of the channel flanks by bottom currents, and another one in which both synchronous and phased interaction played a relevant role in the evolution of the system. Finally, we propose a simplified classification of contourites that can be applied to any contourite system worldwide, and that comprises erosional and depositional features, including muddy and sandy contourite deposits.

Dynamics of the Layered Circulation Inferred from Kinetic Energy Pathway in the South China Sea

AbstractWe investigated the mean kinetic energy (MKE) and eddy kinetic energy (EKE) in the South China Sea to illustrate the dynamics of the vertically rotating cyclonic–anticyclonic–cyclonic (CAC) circulation in the upper, middle, and deep layers. We found that strong MKE along the basin slope and the associated EKE arising from the vertical shear and stratification of the mean current characterize the circulation. In the upper layer, the external MKE input from the Kuroshio intrusion and wind forcing drive the cyclonic circulation, with the wind forcing providing most of the EKE. External forcing, however, does not directly provide the MKE and EKE of the CAC circulation in the semi-enclosed middle and deep layers, where the internal pressure work near Luzon Strait and the vertical buoyancy flux (VBF) in the southern basin and along the western slope maintain the MKE and EKE. The internal pressure work is formed by ageostrophic motion and pressure gradient field associated with circulation. The VBF is generated by vertical motion induced by the geostrophic cross-isobath transport along the slope where variable density field is maintained by the external flow and the internal mixing. The kinetic energy pathway in the CAC circulation indicates that the external forcing dominates upper-layer circulation and the coupling between internal and external dynamics is crucial for maintaining the circulation in the middle and deep layers. This study provides a new interpretation to the maintenance of CAC circulation from energy prospect.

Investigating mesoscale eddy characteristics in the Luzon Strait region using altimetry

The circulation in the northern South China Sea (SCS) strongly responds to anticyclonic eddies that shed from the Kuroshio intruding across Luzon Strait. An eddy tracking algorithm was applied to 26 years of altimetry data to better quantify the surface characteristics of eddies in and near the Luzon Strait with an emphasis on eddies with significant westward propagation across the northern SCS. Spatial mean eddy characteristics revealed that the most robust anticyclonic eddies are in the region of 20–22° N and 118–120° E (where eddies have been observed to shed from the Kuroshio into the northern SCS) and in the North Pacific Gyre east of the Luzon Strait (18–22° N, 121–124° E). The composite structure of mesoscale eddies revealed stronger, more high-amplitude eddies east of the Kuroshio for both circulation types, which was true in all seasons but the starkest contrast was during the summer monsoon. We then isolated the ten farthest westward-propagating eddies that generated in the Luzon Strait region. The majority of these long-propagating eddies occurred during the boreal fall and winter when the cyclonic basin-scale circulation was conducive to eddy propagation along the Chinese shelf break. Using the Kuroshio South China Sea Index (KSI), we find that a highly viable condition for generation of North Pacific Gyre eddies that propagated westward across the Luzon Strait was found when the Kuroshio bifurcates with one branch into the northern SCS and the other northward along eastern Taiwan. In this state, an anticyclonic eddy can generate east of the Kuroshio and be advected into the SCS.

Modified View of Energy Budget Diagram and Its Application to the Kuroshio Extension Region

AbstractThe nonlocality of eddy–mean flow interactions, which appears explicitly in the modified Lorentz diagram as a form of the interaction energy, and its link to other estimation methods are revisited, and a new formulation for the potential enstrophy is proposed. The application of these methods to the Kuroshio Extension region suggests that the combined use of energy analysis with other methods, including the potential enstrophy diagram, provides more comprehensive understandings for the eddy–mean flow interactions in the limited region. It is shown that the interaction energy is transported from the nearshore and upstream regions to the downstream region in the form of the interaction energy flux, causing acceleration of the Kuroshio Extension jet in the downstream region. The potential enstrophy diagram indicates that the eddy field decelerates (accelerates) the jet in the nearshore (downstream) region, which is a consistent result with the energy analysis. It turns out that the interaction potential enstrophy flux is radiated from a region of the eddy kinetic energy maximum toward the upstream region, which is the opposite direction from the interaction energy flux. The interaction potential enstrophy flux that originated from this eddy kinetic energy maximum region also convergences near the center of the northern recirculation gyre of the Kuroshio Extension region and tends to stabilize the structures of the recirculation gyre. Together with the energy analysis that indicates the eddy field accelerates the northeastern part of the recirculation gyre through the local interactions, the present analyses support the arguments on the eddy-driven northern recirculation gyre.

Eddy memory as an explanation of intraseasonal periodic behaviour in baroclinic eddies

AbstractThe baroclinic annular mode (BAM) is a leading‐order mode of the eddy kinetic energy in the Southern Hemisphere exhibiting oscillatory behaviour at intraseasonal time‐scales. The oscillation mechanism has been linked to transient eddy–mean flow interactions which remain poorly understood. Here we demonstrate that the finite memory effect in eddy‐heat flux dependence on the large‐scale flow can explain the origin of the BAM's oscillatory behaviour. We represent the eddy memory effect by a delayed integral kernel that leads to a generalized Langevin equation for the planetary‐scale heat equation. Using a mathematical framework for the interactions between planetary‐ and synoptic‐scale motions, we derive a reduced dynamical model of the BAM – a stochastically forced oscillator with a period proportional to the geometric mean between the eddy memory time‐scale and the diffusive eddy equilibration time‐scale. Our model provides a formal justification for the previously proposed phenomenological model of the BAM and could be used to explicitly diagnose the memory kernel and improve our understanding of transient eddy–mean flow interactions in the atmosphere.

Effects of Stationary and Transient Transport of Ozone on the Ozone Valley Over the Tibetan Plateau in Summer

We studied the effects of the stationary and transient transport of ozone in the upper troposphere and lower stratosphere (UTLS) on the ozone valley over the Tibetan Plateau (OVTP) in summer using the daily ERA-Interim reanalysis dataset for the time period 1979–2016. We used the Lorenz circulation decomposition method to separate the stationary and transient transport of ozone into terms related to either the mean flow or eddies. The decrease in the total ozone concentration in summer is associated with the transport of ozone, which, in turn, reinforces the OVTP. The zonal (meridional) transport of ozone, which combines stationary and transient transport, strengthens (weakens) the ozone valley. The stationary zonal (meridional) transport of ozone strengthens (weakens) the ozone valley. The transient zonal (meridional) transport of ozone weakens (strengthens) the ozone valley, but this effect is weaker than that of stationary transport. The mean flow has the dominant role, especially in the stationary component. The effect of eddies on the zonal transient transport of ozone is as strong as that of the mean flow. For stationary transport, the zonal deviation of ozone transported by the zonal mean flow in the zonal (meridional) direction C(O3∗¯ [u]¯)(C(O3∗¯ [v]¯)) dominates total zonal (meridional) change of ozone C(O3¯ u¯)(C(O3¯ v¯)), which strengthens (weakens) the ozone valley. The transient transport of the zonal mean ozone by eddies (C([O3]′u∗′¯)), the zonal deviation of ozone by the zonal mean flow (C(O3∗′[u]′¯)) and the zonal deviation of ozone by eddies (C(O3∗′u∗′¯)) all have a strong effect on the ozone valley. By contrast, the transient transport of the zonal mean ozone by eddies in the meridional direction (C([O3]′v∗′¯)) has a much weaker and the smallest effect. Both the zonal deviation of ozone by the zonal mean flow and by eddies in the meridional direction (C(O3∗′[v]′¯) and C(O3∗′v∗′¯)) have major roles in transient meridional transport, but their roles are the opposite of each other. The contributions of stationary and transient transport to zonal transport are consistent, whereas their contributions to meridional transport are the opposite of each other. The influence of transient transport on the formation and maintenance of OVTP is not negligible.

Dynamics of zonally elongated transient flows

Abstract

The geometry of mesoscale eddies in the South China Sea: characteristics and implications

ABSTRACT The symmetrical circular shape of mesoscale eddies has been widely used in their scientific researches. Recently, an elliptical average eddy shape has been confirmed for eddies in the global ocean using multi-satellite altimeter data. As a regional extension of a previous study on the geometry of global eddies, a mean eddy shape in the South China Sea (SCS) has been derived by averaging a large number of orientational eddy boundaries. The mean shape is approximately a mathematic ellipse with a semimajor axis of 101.3 km and a semiminor axis of 61.3 km. Its size is larger than the global one. The principal eddy orientation in the SCS is 74°/254° (nearly northeast-southwest), different from that of eddies in the global ocean (171°/351°, nearly east–west). Composite analyses of chlorophyll (CHL) concentrations and sea surface temperature anomalies (SSTA) indicate a dipole structure for circular eddies in the non-rotated coordinate system. While a monopole structure for elliptical eddies in the eddy-centric coordinate system is obtained. The results demonstrate that the elliptical shape of eddies affects oceanographical variables. The findings provide a new approach for exploring the role of air–sea interactions on oceanic eddies.

The Importance of Including Sea Surface Current when Estimating Air–Sea Turbulent Heat Fluxes and Wind Stress in the Gulf Stream Region

AbstractUsing buoy observations from 2004 to 2010 and newly released atmospheric reanalysis and satellite altimetry-derived geostrophic currents from 1993 to 2017, the quantitative contribution of daily mean surface currents to air–sea turbulent heat flux and wind stress uncertainties in the Gulf Stream (GS) region is investigated based on bulk formulas. At four buoy stations, the daily mean latent (sensible) heat flux difference between the estimates with and without surface currents range from −18 (−4) to 20 (4) W m−2, while the daily mean wind stress difference ranges from −0.04 to 0.02 N m−2. The positive values indicate higher estimates with opposite directions between surface currents and absolute winds. The transition between positive and negative differences is significantly associated with synoptic-scale weather variations. The uncertainties based on buoy observations are approximately 7% and 3% for wind stress and turbulent heat fluxes, respectively. The new reanalysis and satellite geostrophic currents confirm the uncertainties identified by buoy observations with acceptable discrepancies and provide a spatial view of the uncertainty fields. The mean geostrophic currents are aligned with the surface wind along the GS; therefore, the turbulent heat fluxes and wind stress will be “underestimated” with surface currents included. However, on both sides of the GS, the surface flow can be upwind due to possible mechanisms of eddy–mean flow interactions and recirculations, resulting in higher turbulent heat flux estimations. The wind stress and turbulent heat flux uncertainties experience significant seasonal variations and show long-term trends.

Impact of the March Arctic Oscillation on the South China Sea summer monsoon onset

AbstractThis study reveals that the South China Sea summer monsoon (SCSSM) onset is closely linked to the preceding March Arctic oscillation (AO), the dominant mode of atmospheric circulation over extratropical Northern Hemisphere. The physical processes for the impact of the March AO on the SCSSM onset are further examined. March AO leads to an anomalous low‐level cyclone over subtropical western North Pacific (WNP) via eddy–mean flow interaction. The southwesterly wind anomalies to the southeastern side of the anomalous cyclone weaken the climatological trade winds, which contribute to warm sea surface temperature (SST) anomalies over tropical western‐central North Pacific via reduction of surface evaporation and latent heat flux. These warm SST anomalies excite an atmospheric Rossby wave response and reinforce the anomalous cyclone over the subtropical WNP. Via the above positive atmosphere–ocean interaction, the anomalous cyclone induced by the March AO could persist through the whole spring and shift equatorward. This anomalous cyclone creates favourable environment for the SCSSM onset via weakening the WNP subtropical high and is conducive to the transition of the low‐level zonal wind. Further analysis indicates that the March AO‐SCSSM onset relationship is independent of El Niño‐Southern Oscillation (ENSO). This result suggests that, besides the dominant tropical system (i.e., ENSO), interannual variation of the SCSSM onset also has a close relation with the dominant extratropical atmospheric system (i.e., AO), which may provide additional source for the seasonal prediction of the monsoon onset.

An Eddy–Zonal Flow Feedback Model for Propagating Annular Modes

Abstract The variability of the zonal-mean large-scale extratropical circulation is often studied using individual modes obtained from empirical orthogonal function (EOF) analyses. The prevailing reduced-order model of the leading EOF (EOF1) of zonal-mean zonal wind, called the annular mode, consists of an eddy–mean flow interaction mechanism that results in a positive feedback of EOF1 onto itself. However, a few studies have pointed out that under some circumstances in observations and GCMs, strong couplings exist between EOF1 and EOF2 at some lag times, resulting in decaying-oscillatory, or propagating, annular modes. Here, we introduce a reduced-order model for coupled EOF1 and EOF2 that accounts for potential cross-EOF eddy–zonal flow feedbacks. Using the analytical solution of this model, we derive conditions for the existence of the propagating regime based on the feedback strengths. Using this model, and idealized GCMs and stochastic prototypes, we show that cross-EOF feedbacks play an important role in controlling the persistence of the annular modes by setting the frequency of the oscillation. We find that stronger cross-EOF feedbacks lead to less persistent annular modes. Applying the coupled-EOF model to the Southern Hemisphere reanalysis data shows the existence of strong cross-EOF feedbacks. The results highlight the importance of considering the coupling of EOFs and cross-EOF feedbacks to fully understand the natural and forced variability of the zonal-mean large-scale circulation.

Comparative oceanographic eddy variability during climate change in the Agulhas Current and Somali Coastal Current Large Marine Ecosystems

Twenty years (1993–2012) of satellite altimetry dataset, through maps of absolute dynamic topography and their derived geostrophic velocities, as well as monthly climatological maps of windstress and windstress-curl have been used to assess the oceanic variability in the western Indian Ocean region (WIO). We have performed a comparative study between the Agulhas Current Large Marine Ecosystem (ACLME) and the Somali Coastal Current Large Marine Ecosystem (SCCLME) during the current time period when significant levels of climate change related variability have been documented. In the WIO region, climate variability, and/or change are impacting the atmospheric and oceanic circulation, e.g: excessive sea surface warming and consequent air-sea heat flux exchanges in the SCCLME; and broadening and weakening of the Agulhas Current, with potential negative implication on Agulhas leakage, in the ACLME. The latter being caused by an acceleration of the mesoscale eddy field in response to changing atmospheric wind forcing conditions. Therefore, eddies need to be properly quantified, characterized and monitored if their contribution on climate is to be assessed. While four hotspot regions of enhanced mesoscale variability have been identified in the combined Agulhas Somali Currents Large Marine Ecosystem (ASCLME), the main focus of this study has been in the ACLME and in the SCCLME. The results suggest that the levels of variability in these two LMEs are controlled by different physical processes: seasonality appears to be important in the SCCLME, but conversely less important in the Agulhas LME. The census conducted on the eddy field revealed that more eddies have been formed in the ACLME than in Somali Coastal Current. The eddy mean properties have shown that the eddies in the ACLME are smaller in diameter, and higher in amplitude than those in the SCCLME. In addition, the eddies in the ACLME are maintained as coherent structures longer; have been more strongly nonlinear; and have propagated longer distances from their origins than those in the SCCLME. These indicate that the eddies in the ACLME have relatively much stronger implication on the overall retention and transport of hydrographic properties (e.g. heat and salt). Therefore, they could have stronger influence on oceanic lateral fluxes and ocean-atmosphere flux exchanges, thus may influence more strongly the environmental conditions (both in the ocean and atmosphere) in the combined ASCLME, than those in the SCCLME. Time-evolution of the eddies mean properties suggests that inter-annual changes, may have influenced the eddy field differently in the ACLME and the SCCLME: moderate correlation r = 0.6, p = 0.05 has been found between time and eddy mean radius in the ACLME, and also r = 0.6, p = 0.05 was found between time and EKE in the SCCLME. Therefore, eddy size (for the Agulhas LME) and energy content (for Somali LME) could serve as potential indicators of change in oceanographic conditions over time in these LMEs.

The Turbulent Structure of the Azores Current System: A Statistical Analysis

AbstractTwenty‐five years of satellite altimetry Sea Level Anomaly (SLA) and SLA‐derived eddy trajectories are used for a thorough characterization of the Azores Current (AzC) eddies' kinematic properties (i.e., amplitude, radius, and swirl velocity) and evolution above the Mid Atlantic Ridge (MAR) and the Seewarte seamount chain (around 28°W). Changes in the mean flow kinetic energy and eddy kinetic energy (EKE) along the AzC main axis (around 34.2°N) allow to divide its zonal structure into three sectors: The western one, between the MAR and 28°W, the central, until 24°W and the eastern, between 24°W and 20°W. A clear intensification of the western sector is visible in both distribution maps, as well in the median, Skewness, and Kurtosis of the SLA field. Two main patterns of the SLA turbulent field also emerge from the statistical analysis: a meridional asymmetry around the current mean axis and a zonal one. The kinematic properties of the detected eddies clearly demonstrate that cyclones are more intense and non‐linear than anticyclones, mainly in the western region, exhibiting higher values of EKE. High amplitude cyclones reported in several papers of the area (with a time‐mean amplitude above 16 cm) are rarities, with only six cyclones detected in the 25 years of data.