Variability Of Eddy Kinetic Energy Research Articles

Sezonnaya izmenchivost' kineticheskoy energii vikhrey v tsentral'noy chasti Indiyskogo okeana: analiz eksperimenta «Poligon-67» s sovremennykh pozitsiy

Purpose. The main goal of this study is to analyze seasonal variability of the meso-scale eddy activity in the north tropical Indian Ocean. The selected area coincides with the POLYGON-67 experiment location where the meso-scale eddies were first discovered in the open ocean. Methods and results. Variability of the meso-scale eddy kinetic energy in the ocean surface layer, enstrophy of the larger scale circulation, spatial and temporal patterns of the surface currents and the surface winds are jointly analyzed using a 20-year long daily time series, containing both the eddy-resolving ocean reanalysis data obtained from the EU Copernicus Marine Environment Monitoring Service, and the climatic wind data – from the US National Oceanographic and Atmospheric Administration. The fast meso-scale and slow large-scale processes are separated using the Savitsky – Golay filter with the cut-off time 103 days that corresponds to the local minimum in the full kinetic energy power spectrum. In contrast to the other parts of the tropical ocean, seasonal variability of the eddy kinetic energy exhibits 2 maxima (the largest – in April and the secondary – in October), which are related to the enstrophy maxima of the larger scale currents. Conclusions. The double variability peak in the eddy kinetic energy corresponds to seasonal variability of the large-scale currents enstrophy and the wind monsoon circulation. This supports the hypothesis that the main mechanism of the eddy kinetic energy generation is barotropic instability of the larger scale currents. The eddy kinetic energy variability within the POLYGON-67 is mostly controlled by energy advection from the neighbouring areas, and to a lesser extent – by local generation.

Seasonal Variability of Eddy Kinetic Energy in the Central Indian Ocean: POLYGON-67 Revised

Seasonal Variability of Eddy Kinetic Energy in the Central Indian Ocean: POLYGON-67 Revised

Response of the Pacific Sector of the Southern Ocean to Wind Stress Variability From 1995 to 2017

AbstractDrifter, satellite, expendable bathythermograph (XBT), and Argo float data are used to study the response of the Pacific Sector of the Southern Ocean (PSSO) to the wind stress field in the period 1995–2017, in terms of eddy field, water mass transport, and heat fluxes at large and regional scales. Increasing wind stress over the PSSO in those two decades led to a growth of the Eddy Kinetic Energy (EKE) in the region of the Antarctic Circumpolar Current (ACC). Increases of the EKE occur with delays of 1–4 years with respect to peaks in the zonal component of the wind stress. The persistent ACC meander located south of New Zealand (between 150°E and 180°W and 50°S to 66°S) responds to the interannual wind variations earlier than the entire ACC branch in the PSSO. In the same area, an estimate of the ACC transport based on in situ data shows interannual variability but no significant decadal trend over the study period. The effects of the EKE variability on the meridional eddy heat fluxes are significant on interannual scales. The strengthening of the EKE field leads to a local increase in the poleward meridional eddy heat fluxes in the PSSO, especially in the ACC band. The weakening of the EKE field defines an area of equatorward meridional eddy heat fluxes in the middle of the PSSO (south of 40°S, between 130°W and 160°W) and prevalent poleward fluxes in the further western and eastern regions.

Eddy Kinetic Energy in the Arctic Ocean From a Global Simulation With a 1‐km Arctic

AbstractSimulating Arctic Ocean mesoscale eddies in ocean circulation models presents a great challenge because of their small size. This study employs an unstructured‐mesh ocean‐sea ice model to conduct a decadal‐scale global simulation with a 1‐km Arctic. It provides a basinwide overview of Arctic eddy energetics. Increasing model resolution from 4 to 1 km increases Arctic eddy kinetic energy (EKE) and total kinetic energy (TKE) by about 40% and 15%, respectively. EKE is the highest along main currents over topography slopes, where strong conversion from available potential energy to EKE takes place. It is high in halocline with a maximum typically centered in the depth range of 70–110 m, and in the Atlantic Water layer of the Eurasian Basin as well. The seasonal variability of EKE along the continental slopes of southern Canada and eastern Eurasian basins is similar, stronger in fall and weaker in spring.

Interannual to Decadal Variations of Submesoscale Motions around the North Pacific Subtropical Countercurrent

The outputs from a submesoscale permitting hindcast simulation from 1990 to 2016 are used to investigate the interannual to decadal variations of submesoscale motions. The region we focus on is the subtropical Northwestern Pacific including the subtropical countercurrent. The submesoscale kinetic energy (KE) is characterized by strong interannual and decadal variability, displaying larger magnitudes in 1996, 2003, and 2015, and smaller magnitudes in 1999, 2009, 2010, and 2016. These variations are partially explained by those of the available potential energy (APE) release at submesoscale driven by mixed layer instability in winter. Indeed, this APE release depends on the mixed layer depth and horizontal buoyancy gradient, both of them modulated with the Pacific Decadal Oscillation (PDO). As a result of the inverse KE cascade, the submesoscale KE variability possibly leads to interannual to decadal variations of the mesoscale KE (eddy KE (EKE)). These results show that submesoscale motions are a possible pathway to explain the impact associated with the PDO on the decadal EKE variability. The winter APE release estimated from the Argo float observations varies synchronously with that in the simulation on the interannual time scales, which suggests the observation capability to diagnose the submesoscale KE variability.

Annual versus semi-annual eddy kinetic energy variability in the Celebes Sea

Eddy kinetic energy (EKE) in the Celebes Sea (CS) is investigated using a global, eddy-resolving, ocean general circulation model (OGCM) output. The OGCM simulation shows that a strong EKE is confined in the upper 300-m layer. The period of EKE variability varies with depth. In the 0–100-m layer, EKE has a distinct annual cycle that is strong in winter (December–February) and weak in summer (June–August). However, in the 100–300-m layer, semi-annual variation is dominant, which shows stronger EKE in spring and fall and weaker EKE in summer and winter. An eddy energy budget analysis reveals that the barotropic eddy energy conversion rate has a vertical structure similar to that of EKE. Compared to the barotropic eddy energy conversion rate, the baroclinic eddy energy conversion rate is much smaller and does not match the EKE vertical pattern. The budget analysis indicates that the variation in EKE in the CS is governed by barotropic instability of the background circulation. Further analysis reveals that the variation in the regional background circulation with depth is due to a combination of the forcing of the local monsoon and the Mindanao Current (MC) in the western tropical Pacific.

Dynamics on Seasonal Variability of EKE Associated with TIWs in the Eastern Equatorial Pacific Ocean

AbstractEnergetic mesoscale eddies (vortices) associated with tropical instability waves (TIWs) exist in the eastern equatorial Pacific Ocean between 0° and 8°N. This study examines the seasonal variations in eddy kinetic energy (EKE) of TIWs using in situ and satellite observations and elucidates the underlying dynamical mechanisms. The results reveal that the cross-equatorial southerly winds are key to sustaining the high-level EKE (up to ~600 cm2 s−2) from boreal summer to winter in 0°–6°N and 155°–110°W. Because of the β effect and the surface wind divergence, the southerly winds generate anticyclonic wind curls north of the equator that intensify the sea surface temperature (SST) fronts and force the downwelling annual Rossby waves. The resultant sea surface height ridge induces strong horizontal current shears between 0° and 5°N. The intensified current shears and SST fronts generate EKE via barotropic and baroclinic instabilities, respectively. To the extent that the seasonal migration of a northward-displaced intertropical convergence zone intensifies the southerly winds north of, but not south of, the equator, our study suggests that the climatic asymmetry is important for the oceanic eddy generations in the eastern equatorial Pacific Ocean—a result with important implications for coupled climate simulation/prediction.

Interannual to Multidecadal Forcing of Mesoscale Eddy Kinetic Energy in the Subtropical Southern Indian Ocean

AbstractA region of elevated mesoscale eddy activity spans the subtropical southern Indian Ocean (SSIO) between Madagascar and Australia. The interannual and decadal changes in eddy activity in the SSIO eddy band, as represented by the variability of eddy kinetic energy (EKE), have implications for the large‐scale circulation, mixed‐layer budgets, and biological activity. An analysis of nearly two and a half decades of sea level anomaly (SLA) data from merged satellite altimetry shows that, in the southeast Indian Ocean east of 90°E, the variations of EKE and SLA are positively correlated on interannual and decadal time scales. Moreover, EKE exhibits a multidecadal increasing linear trend that corresponds to an increasing trend of SLA in the region. The EKE‐SLA covariability in the southeast Indian Ocean does not appear to be associated with a preference for anticyclonic over cyclonic eddy activity; rather, it can be attributed to the common remote forcing from the tropical Pacific associated with the El Niño–Southern Oscillation and the Pacific Decadal Oscillation. In the west central SSIO, wind stress curl just south of the eddy band forces potential vorticity anomalies that affect conditions for instability in the west central SSIO; potential density and potential vorticity gradient anomalies also suggest a remote forcing mechanism originating in the region southwest of Australia. The interannual to multidecadal variability of EKE in the SSIO and its relationship with large‐scale SLA has implications for mixed‐layer dynamics and biogeochemistry and provides a basis for assessment of model simulations of eddy activity in the region.

Decadal Variability of Eddy Characteristics and Energetics in the Kuroshio Extension: Unstable Versus Stable States

AbstractUsing the Estimating Circulation and Climate of the Ocean Phase II product, this study investigates the eddy characteristics and energetics in two dynamical regimes within the Kuroshio Extension (KE) region. Based on the empirical orthogonal function analysis, it is found that the decadal evolution of eddy kinetic energy in the KE region is characterized by a delayed negative correlation between the upstream and downstream. Besides the out‐of‐phase change in eddy activity levels, eddy characteristics and energetics in the upstream KE are also different from those in the downstream. In the upstream region, eddies are stretched in the zonal direction and the unstable state is characterized by strong eddy‐shedding processes. During these processes, cyclonic eddies are found to be formed in the first quasi‐stationary meander and finally pinched off from the KE jet. Energy budget illustrates that the eddy shedding processes are triggered by baroclinic instability, while barotropic instability becomes the dominated energy source after the meander is fully developed. Accompanied by the generation of cyclonic eddies, significant inverse energy cascade is detected. When the upstream KE is stable, the eddy activity is dominated by baroclinic instability and the inverse energy cascade occurs similarly. Distinct from the upstream, eddies in the downstream KE tend to be stretched in the meridional direction and the decadal variability of eddy kinetic energy in this region is mainly regulated by the upstream through lateral advection.

A Coupled Ocean Physics‐Biology Modeling Study on Tropical Instability Wave‐Induced Chlorophyll Impacts in the Pacific

AbstractTropical instability waves (TIWs) play an important role in modulating the physical variability (e.g., sea surface temperature [SST] and heat flux) and biological activity (e.g., phytoplankton and chlorophyll (Chl)) in the eastern tropical Pacific. The impacts of TIW‐induced Chl perturbations (ChlTIW) are examined using a coupled ocean physics‐biology model in the tropical Pacific. TIW‐scale biological and physical perturbations are extracted using a zonal high‐passed filter. The TIW‐induced perturbations of biological and physical fields (e.g., Chl, nitrate, iron, and SST.) exhibit their close relationships, with negative correlation between SST and Chl at TIW scale. Processes responsible for the impacts of ChlTIW are diagnosed. ChlTIW acts to modulate the incoming solar radiation that penetrates the bottom of the mixed layer (Qpen). In addition, two experiments are conducted with the impacts of ChlTIW on the ocean included or not. The results show that the ChlTIW effects tend to substantially increase seasonal variability of eddy kinetic energy (EKE) and slightly decrease SST in the eastern equatorial Pacific. At large scale, ChlTIW leads to an increase in mean Chl concentration, which induces a SST cooling and an increase in EKE in the boreal winter. At TIW scale, ChlTIW leads to the change in Qpen and a decrease in baroclinic conversion term and eventually decreases the EKE in the boreal spring and summer. So the ChlTIW acts to have influences both on the intensity of TIW and large‐scale SST in the tropical Pacific, involving multiscale interactions and feedbacks between ocean physics and biology.

Decadal Variability of Eddy Kinetic Energy in the South Pacific Subtropical Countercurrent in an Ocean General Circulation Model

AbstractThe eddy kinetic energy (EKE) associated with the Subtropical Countercurrent (STCC) in the western subtropical South Pacific is known to exhibit substantial seasonal and decadal variability. Using an eddy-permitting ocean general circulation model, which is able to reproduce the observed, salient features of the seasonal cycles of shear, stratification, baroclinic production, and the associated EKE, the authors investigate the decadal changes of EKE. The authors show that the STCC region exhibits, uniquely among the subtropical gyres of the world’s oceans, significant, atmospherically forced, decadal EKE variability. The decadal variations are driven by changing vertical shear between the STCC in the upper 300 m and the South Equatorial Current below, predominantly caused by variations in STCC strength associated with a changing meridional density gradient. In the 1970s, an increased meridional density gradient results in EKE twice as large as in later decades in the model. Utilizing sensitivity experiments, decadal variations in the wind field are shown to be the essential driver. Local wind stress curl anomalies associated with the interdecadal Pacific oscillation (IPO) lead to upwelling and downwelling of the thermocline, inducing strengthening or weakening of the STCC and the associated EKE. Additionally, remote wind stress curl anomalies in the eastern subtropical South Pacific, which are not related to the IPO, generate density anomalies that propagate westward as Rossby waves and can account for up to 30%–40% of the density anomalies in the investigated region.

Interannual Variability of Eddy Kinetic Energy in the Subtropical Southeast Indian Ocean Associated With the El Niño‐Southern Oscillation

AbstractInterannual variability of eddy kinetic energy (EKE) in the subtropical southeast Indian Ocean (SEIO) is investigated using satellite observations in three regions in the 20°S–35°S latitude band: R1 (108°E–115°E), R2 (100°E–108°E), and R3 (60°E–100°E). The El Niño‐Southern Oscillation (ENSO) plays an important role in modulating the interannual variability of EKE in the SEIO. EKE in the three regions shows negative correlations with the Nino3.4, lagging Nino3.4 by 2, 14, and 22 months, respectively. In R1, the ENSO modulates the interannual variability of EKE through influencing baroclinic instability of meridional velocity shear between the upper poleward Leeuwin Current (LC) and the underlying equatorward Leeuwin Undercurrent (LUC). During La Niña (El Niño) year, both the poleward LC and equatorward LUC strengthened (weakened) due to the high (low) sea level anomaly (SLA) propagating from the Pacific Ocean as Rossby wave under geostrophic equilibrium, and baroclinic instability of vertical shear enhanced (slackened), further induced the strong (weak) EKE in the eastern boundary. In R2, the ENSO modulates the interannual variability of EKE through influencing baroclinic instability of zonal velocity shear between the central and southern branches of the upper eastward South Indian Countercurrent (SICC) and the lower extending westward South Equatorial Current (SEC). In R3, both the ENSO and the Southern Annual Mode modulate the interannual variability of EKE through influencing baroclinic instability of zonal velocity shear between the SICC and SEC. The interannual variability of EKE in the interior SEIO could be influenced by westward propagation of EKE originated from eastern boundary.

Modeling the Wake of the Marquesas Archipelago

AbstractIn this study, a high‐resolution (∼2.5 km) numerical model was set up to investigate the fine‐scale activity within the region of the Marquesas archipelago. This has never been performed before. The robustness of the model results is assessed by comparison with remote sensing and in situ observations. Our results highlight regions of warm waters leeward of the different islands with high eddy kinetic energy (EKE) on their sides. The analysis of energy conversion terms reveals contributions to EKE variability by wind, baroclinic, and barotropic instabilities. The use of a geometry‐based eddy detection algorithm reveals the generation of cyclonic and anticyclonic eddies in the wake of the largest islands, with both an inshore and offshore effect. Maximum eddy activity occurs in austral winter following the seasonality of both wind stress and EKE intensity. Most eddies have a radius between 20 and 30 km and are generally cyclonic rather than anticyclonic. Significant vertical velocities are observed in the proximity of the islands, associated with topographically induced flow separation. Eddy trapping inshore waters are advected offshore in the wake of the islands. The overall influence of these fine‐scale dynamics could explain the strong biological enhancement of the archipelago.

Variability of the Labrador Sea Surface Eddy Kinetic Energy Observed by Altimeter From 1993 to 2012

AbstractA merged along track altimeter data set is used to study the variability of eddy kinetic energy (EKE) in the Labrador Sea from 1993 to 2012. The EKE near the west Greenland current (WGC) has strong interannual variability without long‐term trend from 1993 to 2012. The propagation direction of the Irminger Rings (IRs) originating from the WGC can be inferred from the EKE derived from altimeter, and the southward propagation of the IRs varies interannually. The central Labrador Sea EKE increases significantly from 1993 to 2012. The central Labrador Sea temperature difference between the end and the beginning of the winter convections is defined as restratification index to measure the restratification strengths. The relation between the central Labrador Sea EKE and the restratification index shows that the enhanced eddy activity originating from the west of the central Labrador Sea may cool the central Labrador Sea significantly. The interannual variability of the WGC EKE is likely to be driven by the large scale Subpolar Gyre (SPG) circulation variability and the North Atlantic Oscillation (NAO). The NAO also affects the central Labrador Sea EKE through its fingerprint in the local wind stress and surface heat flux. The NAO affects the WGC EKE by changing the SPG circulation strength, which will subsequently affect the WGC EKE through unknown physical processes.

On the Decadal Variability of the Eddy Kinetic Energy in the Kuroshio Extension

AbstractPrevious studies have found that the decadal variability of eddy kinetic energy (EKE) in the upstream Kuroshio Extension is negatively correlated with the jet strength, which seems counterintuitive at first glance because linear stability analysis usually suggests that a stronger jet would favor baroclinic instability and thus lead to stronger eddy activities. Using a time-varying energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), and the MS-EVA-based nonlinear instability theory, this study investigates the physical mechanism responsible for such variations with the state estimate from the Estimating the Circulation and Climate of the Ocean (ECCO), Phase II. For the first time, it is found that the decadal modulation of EKE is mainly controlled by the barotropic instability of the background flow. During the high-EKE state, violent meanderings efficiently induce strong barotropic energy transfer from mean kinetic energy (MKE) to EKE despite the rather weak jet strength. The reverse is true in the low-EKE state. Although the enhanced meander in the high-EKE state also transfers a significant portion of energy from mean available potential energy (MAPE) to eddy available potential energy (EAPE) through baroclinic instability, the EAPE is not efficiently converted to EKE as the two processes are not well correlated at low frequencies revealed in the time-varying energetics. The decadal modulation of barotropic instability is found to be in pace with the North Pacific Gyre Oscillation but with a time lag of approximately 2 years.

Mechanism of seasonal eddy kinetic energy variability in the eastern equatorial Pacific Ocean

AbstractEnhanced mesoscale eddy activities or tropical instability waves (TIWs) exist along the northern front of the cold tongue in the eastern equatorial Pacific Ocean. In this study, we investigate seasonal variability of eddy kinetic energy (EKE) over this region and its associated dynamic mechanism using a global, eddy‐resolving ocean general circulation model (OGCM) simulation, the equatorial mooring data, and satellite altimeter observations. The seasonal‐varying enhanced EKE signals are found to expand westward from 100°W in June to 180°W in December between 0°N and 6°N. This westward expansion in EKE is closely connected to the barotropically‐baroclinically unstable zonal flows that are in thermal‐wind balance with the seasonal‐varying thermocline trough along 4°N. By adopting an 1½‐layer reduced‐gravity model, we confirm that the seasonal perturbation of the thermocline trough is dominated by the anticyclonic wind stress curl forcing, which develops due to southerly winds along 4°N from June to December.

Variability and trends in Southern Ocean eddy activity in 1/12° ocean model simulations

AbstractThe response of eddy kinetic energy (EKE) to the strengthening of Southern Hemisphere winds occurring since the 1950s is investigated with a global ocean model having a resolution of 1/12° in the Antarctic Circumpolar Current domain. The simulations expose regional differences in the relative importance of stochastic and wind‐related contributions to interannual EKE changes. In the Pacific and Indian sectors the model captures the EKE variability observed since 1993 and confirms previous hypotheses of a lagged response to regional wind stress anomalies. Here the multidecadal trend in wind stress is reflected in an increase in EKE typically exceeding 5 cm2 s−2 decade−1. In the western Atlantic, EKE variability is mostly stochastic, is weakly correlated with wind fluctuations, and its multidecadal trends are close to zero. The nonuniform distribution of wind‐related changes in the eddy activity could affect the regional patterns of ocean circulation and biogeochemical responses to future climate change.

Interannual modulation of eddy kinetic energy in the northeastern South China Sea as revealed by an eddy‐resolving OGCM

AbstractInterannual modulation of eddy kinetic energy (EKE) in the northeastern South China Sea (NE‐SCS) is investigated based on outputs of an eddy‐resolving oceanic general circulation model between 1980 and 2014. The EKE displays distinct interannual modulations with periods between 1.5 and 7 years. The maximum peak‐to‐trough amplitude of the interannual modulation occurred during period 2004–2005, which was about 1.5‐fold the time‐mean EKE level. Further analysis suggested that interannual variability of EKE in the NE‐SCS is primarily modulated by the Luzon Strait transport (LST). During high‐EKE years, the LST increases corresponding to a strengthened Kuroshio intrusion. The strengthened Kuroshio intrusion enhances the baroclinic instability of current in the NE‐SCS and thus leads to a strong EKE. The reverse is true during low‐EKE years when LST is smaller. Influences of ENSO and Pacific mesoscale eddies on the interannual modulation of LST are also discussed in this study.

Seasonal variability of eddy kinetic energy in a global high‐resolution ocean model

AbstractA global ocean model with 1/12° horizontal resolution is used to assess the seasonal cycle of surface eddy kinetic energy (EKE). The model reproduces the salient features of the observed mean surface EKE, including amplitude and phase of its seasonal cycle in most parts of the ocean. In all subtropical gyres of the Pacific and Atlantic, EKE peaks in summer down to a depth of ∼350 m, below which the seasonal cycle is weak. Investigation of the possible driving mechanisms reveals the seasonal changes in the thermal interactions with the atmosphere to be the most likely cause of the summer maximum of EKE. The development of the seasonal thermocline in spring and summer is accompanied by stronger mesoscale variations in the horizontal temperature gradients near the surface which corresponds, by thermal wind balance, to an intensification of mesoscale velocity anomalies toward the surface.

On the modulation of the periodicity of the Faroe Bank Channel overflow instabilities

Abstract. The Faroe Bank Channel (FBC) is one of the major pathways where dense, cold water formed in the Nordic Seas flows southward as a bottom-attached energetic plume towards the North Atlantic. The plume region downstream of the FBC sill is characterized by high mesoscale variability, quasi-regular oscillations and intense mixing. Here, 1 year long time series of velocity and temperature from ten moorings deployed in May 2012 in the plume region are analysed to describe variability in the strength and period of the oscillations. The eddy kinetic energy (EKE) associated with the oscillations changes by a factor of 10 during the year and the dominant period of the oscillations is modulated and varies between 3 to 4 and 6 days, where the shorter-period oscillations are more energetic. The dense water is observed on a wider portion of the slope (both deeper and shallower) during periods with energetic, short-period oscillations. The observations are complemented by results from a regional, high-resolution model that shows a similar variability in EKE and a gradual change in oscillation period of between 3 and 4 days. The observed variability in oscillation period is directly linked to changes in the volume transport across the sill: the oscillation period increases from approximately 3 days to about 6 days when the transport decreases from 2.4 to 1.9 Sv. A similar relation is obtained from the model. This is in agreement with results from a linear baroclinic instability analysis, which suggests that the period increases while the growth rate decreases for decreased plume thickness. Advective effects, caused by the variable background current, further modulate the observed periodicity by up to 1 day. In addition, it is shown that about 50 % of the transport variability across the sill is explained by changes in the local sea surface height gradient.