Distribution Of Eddy Kinetic Energy Research Articles

Characteristics of mesoscale eddies in the Mozambique Channel.

The mesoscale eddy characteristics of the Mozambique Warm Current were investigated by detecting and tracking satellite altimetry data from 2010 to 2019. A total of 1,086 eddies were identified in the Mozambique Channel, comprising 509 cyclonic eddies and 577 anticyclonic eddies. The results revealed that the bay area on the northwest coast of Madagascar was the main hotspot of eddy generation, and the mean amplitude and radius of the anticyclonic eddies in the Mozambique Channel were 24.23 cm and 82.7 km, respectively, which are larger than those of the cyclonic eddies. Local wind forcing had a significant impact on the formation of mesoscale eddies in the Mozambique Channel. In winter, the wind stress in the northern and southern areas of the Mozambique Channel exhibited a strong correlation with the distribution of eddy kinetic energy (EKE), where both monsoonal winds in the north and trade winds in the south could facilitate mesoscale anticyclonic eddy formation. In addition, the variability in the number of anticyclonic and cyclonic eddies in the Mozambique Channel may have exerted a significant influence on the seasonal anomalous fluctuations in local sea surface temperatures (SSTs). This study presented a novel analysis of the mesoscale eddy characteristics in the Mozambique Channel.

Energetics of eddy-mean flow interactions in the western tropical Pacific Ocean

The eddy-mean flow interactions are critical processes in the ocean energy cycle. This study investigated the energetics of eddy-mean flow interactions in the western tropical Pacific Ocean using the multiscale window transform (MWT) and MWT-based canonical transfer theory, based on 16-year numerical outputs from the Ocean General Circulation Model for the Earth Simulator and two mooring measurements. Energy analysis revealed that prominent eddy kinetic energy (EKE) appears in the upper layer of the Sulawesi Sea and the source region of North Equatorial Countercurrent (NECC), and in the subsurface layer along the Philippine coast. The kinetic energy in the upper layer is mainly transferred from the mean flow to the eddy field through barotropic instability. Below the thermocline, the prominent subsurface EKE is not only caused by barotropic conversion, but also by baroclinic one. Subsurface mesoscale eddies east of the Mindanao Island extract energy from mean flow through barotropic instability, and those further north seem to release their kinetic energy to the mean flow of North Equatorial Undercurrent, indicating inverse energy cascades. Our results highlight that the advection effect plays a key role in the horizontal distribution of EKE, which generally shifts the high EKE area a few degrees downstream from the eddy formation region. Particularly, in the source region of NECC, the advection term shifts the EKE center 5° eastward from ∼132°E to ∼137°E.

The role of surface potential vorticity in the vertical structure of mesoscale eddies in wind-driven ocean circulations

Abstract The vertical structure of ocean eddies is generally surface-intensified, commonly attributed to the dominant baroclinic modes arising from the boundary conditions (BCs). Conventional BC considerations mostly focus on either flat- or rough-bottom conditions. The impact of surface buoyancy anomalies—often represented by surface potential vorticity (PV) anomalies—has not been fully explored. Here, we study the role of the surface PV in setting the vertical distribution of eddy kinetic energy (EKE) in an idealized adiabatic ocean model driven by wind stress. The simulated EKE profile in the extra-tropical ocean tends to peak at the surface and have an e-folding depth typically smaller than half of the ocean depth. This vertical structure can be reasonably represented by a single surface quasi-geostrophic (SQG) mode at the energy-containing scale resulting from the large-scale PV structure. Due to isopycnal outcropping and interior PV homogenization, the surface meridional PV gradient is substantially stronger than the interior PV gradient, yielding surface-trapped baroclinically unstable modes with horizontal scales comparable to or smaller than the deformation radius. These surface-trapped eddies then grow in size both horizontally and vertically through an inverse energy cascade up to the energy-containing scale, which dominates the vertical distribution of EKE. As for smaller horizontal scales, the EKE distribution decays faster with depth. Guided by this interpretation, an SQG-based scale-aware parameterization of the EKE profile is proposed. Preliminary offline diagnosis of a high-resolution simulation shows the proposed scheme successfully reproducing the dependence of the vertical structure of EKE on the horizontal grid resolution.

Exploring the ocean mesoscale at reduced computational cost with FESOM 2.5: efficient modeling strategies applied to the Southern Ocean

Abstract. Modeled projections of climate change typically do not include a well-resolved ocean mesoscale due to the high computational cost of running high-resolution models for long time periods. This challenge is addressed using efficiency-maximizing modeling strategies applied to 3 km simulations of the Southern Ocean in past, present, and future climates. The model setup exploits reduced-resolution spin-up and transient simulations to initialize a regionally refined, high-resolution ocean model during short time periods. The results are compared with satellite altimetry data and more traditional eddy-present simulations and evaluated based on their ability to reproduce observed mesoscale activity and to reveal a response to climate change distinct from natural variability. The high-resolution simulations reproduce the observed magnitude of Southern Ocean eddy kinetic energy (EKE) well, but differences remain in local magnitudes and the distribution of EKE. The coarser, eddy-permitting ensemble simulates a similar pattern of EKE but underrepresents observed levels by 55 %. At approximately 1 ∘C of warming, the high-resolution simulations produce no change in overall EKE, in contrast to full ensemble agreement regarding EKE rise within the eddy-permitting simulations. At approximately 4 ∘C of warming, both datasets produce consistent levels of EKE rise in relative terms, although not absolute magnitudes, as well as an increase in EKE variability. Simulated EKE rise is concentrated where flow interacts with bathymetric features in regions already known to be eddy-rich. Regional EKE change in the high-resolution simulations is consistent with changes seen in at least four of five eddy-permitting ensemble members at 1 ∘C of warming and all ensemble members at 4 ∘C. However, substantial noise would make these changes difficult to distinguish from natural variability without an ensemble.

Vertical distribution of intraseasonal variation signals in the Kuroshio Current source area during 2018–2020

In the Kuroshio Current (KC) source area, intraseasonal variation (ISV) plays a significant role in dynamic oceanic processes. This study used data collected from three moorings (122.7°E, 123°E, 123.3°E) along 18°N from January 2018 to May 2020 to investigate the ISVs of meridional velocities. Notably, our findings reveal that the ISV above 200 m has a period of approximately 56 days and its intensity exhibits a gradual increase toward the west. For the 500–800 m depth interval, the ISV period is 73 days at 122.7°E/18°N and 60 days at 123.3°E/18°N. This discrepancy indicates that the ISVs have different vertical structures and frequencies at 122.7°E and 123.3°E along 18°N. In particular, at 122.7°E/18°N, the distinctiveness of two different periods of ISVs in surface and subsurface layers was more pronounced in 2018 than in 2019. The analyses of eddy kinetic energy distribution and eddy tracking indicate a connection between ISV in stratification and locally generated mesoscale eddies in the KC source area. Specifically, the stronger eddy activity in 2018, in contrast with that in 2019, correlates with a more pronounced ISV. Energy analysis demonstrates a distinct positivity in the baroclinic conversion rate (BC) in the surface layer (upper 200 m) of the KC source region, surpassing the absolute value of the barotropic conversion rate (BT). This finding indicates a notable shift of energy from eddy available potential energy to eddy kinetic energy, strengthening the high-frequency ISV signals in this area. In the subsurface layer, a strong negative BT is observed west of 122.8°E, with its absolute value exceeding the BC. This finding indicates that the energy is converted from eddy kinetic energy into mean kinetic energy, resulting in the appearance of the Luzon Undercurrent (LUC) at mooring station 122.7°E/18°N, characterized by a low frequency of ISV. Contrastingly, a positive BT plays a dominant role at 123.3°E/18°N, leading to the disappearance of the LUC amid an apparent presence of high-frequency ISV.

The Sensitivity of an Idealized Weddell Gyre to Horizontal Resolution

AbstractEstimates of the Weddell Gyre transport vary widely between climate simulations. Here, we investigate if inter‐model variability can originate from differences in the horizontal resolution of the ocean model. We run an idealized model of the Weddell Gyre at eddy‐parameterized, eddy‐permitting, and eddy‐rich resolutions and find that the gyre is strongly sensitive to horizontal resolution. The gyre transport is largest at eddy‐permitting resolutions (45 Sv with a noisy bathymetry) and smallest at eddy‐parameterized resolutions (12 Sv). The eddy‐permitting simulations have the largest horizontal density gradients and the weakest stratification over the gyre basin. The large horizontal density gradients induce a significant thermal wind transport and increase the mean available potential energy for mesoscale eddies. The distribution of eddy kinetic energy indicates that explicit eddies in simulations intensify the bottom circulation of the gyre via non‐linear dynamics. If climate models adopt horizontal resolutions that the Weddell Gyre is most sensitive to, then simulations of the Weddell Gyre could become more disparate.

The role of eddy-wind interaction in the eddy kinetic energy budget of the Agulhas retroflection region

The Agulhas retroflection (AR) region possesses the highest eddy kinetic energy (EKE) level in the Indian Ocean. However, mechanisms regulating EKE of the AR remain uncertain. Here, by analyzing an eddy-resolving coupled model simulation with improved EKE representation, we show that the upper-ocean EKE of the AR is mainly generated through barotropic instability in its upstream and leakage zones and is by nonlocal transport in its downstream zone. The interaction between mesoscale eddies and local winds plays a key role in EKE dissipation. The lack of eddy-wind interaction results in flawed EKE budget in the leakage zone in ocean-alone models, leading to severe biases in EKE distribution with overestimation and over-strong penetration into the South Atlantic. Our results highlight the essence of mesoscale air-sea interaction in the dynamics of the AR, with implications for understanding the inter-basin transport of the Agulhas leakage.

Impact of the interannual variability of the South China Sea monsoon trough on tropical cyclone activity over the western North Pacific

AbstractThe present study reveals the impact of the interannual variability of the South China Sea monsoon trough (SCSMT) on tropical cyclone (TC) activity over the South China Sea (SCS) and the western North Pacific (WNP) from June to August during the period of 1958–2019. The SCSMT can influence TC genesis and occurrence through affecting the large‐scale environmental conditions and barotropic energy conversion. The intensification of the SCSMT can lead to enhanced low‐level vorticity and upper‐level divergence, decreased vertical wind shear, and increased midlevel humidity over the northern SCS and subtropical WNP. The strengthening of the SCSMT is accompanied by an intensification and eastward extension of the WNP monsoon trough and eastward retreat of the WNP subtropical high. All these changes associated with the SCSMT are favourable for TC genesis and occurrence over the north SCS and WNP. More TCs form over the WNP around 20° N, while higher TC occurrence frequency appears over the north SCS and WNP north of 20° N. Diagnostic results of eddy kinetic energy distribution and barotropic energy conversion provide a possible explanation for the impact of the SCSMT on TC activity. The synoptic‐scale disturbances are more active in the WNP region north of 20° N which might lead to an increase in TC occurrence frequency. While the barotropic energy conversion is intensified in the northern SCS and east of Philippines, more eddy energy converted from the mean flow is favourable for TC genesis. The meridional shear of mean zonal wind and the meridional convergence of mean meridional wind, which are related with the interannual variability of the SCSMT, primarily contribute to the barotropic energy conversion.

Eddies and the Distribution of Eddy Kinetic Energy in the Arctic Ocean

Mesoscale eddies are important to many aspects of the dynamics of the Arctic Ocean. Among others, they maintain the halocline and interact with the Atlantic Water circumpolar boundary current through lateral eddy fluxes and shelf-basin exchanges. Mesoscale eddies are also important for transporting biological material and for modifying sea ice distribution. Here, we review what is known about eddies and their impacts in the Arctic Ocean in the context of rapid climate change. Eddy kinetic energy (EKE) is a proxy for mesoscale variability in the ocean due to eddies. We present the first quantification of EKE from moored observations across the entire Arctic Ocean and compare those results to output from an eddy resolving numerical model. We show that EKE is largest in the northern Nordic Seas/Fram Strait and it is also elevated along the shelf break of the Arctic Circumpolar Boundary Current, especially in the Beaufort Sea. In the central basins, EKE is 100–1,000 times lower. Generally, EKE is stronger when sea ice concentration is low versus times of dense ice cover. As sea ice declines, we anticipate that areas in the Arctic Ocean where conditions typical of the North Atlantic and North Pacific prevail will increase. We conclude that the future Arctic Ocean will feature more energetic mesoscale variability.

Response of ocean dynamics to multiple equilibria of the Kuroshio path South of Japan

Variability of the Kuroshio path to the south of Japan plays a central role in the local climate change and exerts tremendous influences on the local atmosphere and ocean. In this study, the response of ocean dynamics, in terms of the eddy kinetic energy (EKE), potential vorticity (PV), relative vorticity, and eddy-mean flow interaction, to the Kuroshio path change is discussed. Kuroshio path south of Japan includes the near-shore non-large meander (nNLM), the off-shore non-large meander (oNLM), and the typical large meander (tLM). Analyses reveal that the distribution of EKE, PV, relative vorticity, and energy exchange between the eddy field and the mean flow respectively varies with the Kuroshio path: (1) The tLM has the maximum EKE along the path; (2) The positive and negative PV are located at the onshore and offshore side of Kuroshio axis, respevetively; (3) The distributions of anomalous relative voritcity of nNLM, oNLM, and tLM are consistent with sea surface height anomalies (SSHAs); (4) The tLM has the largest energy exchange between the eddy field and the mean flow in terms of the rate of barotropic energy conversion. On the other hand, the stability analysis of ocean currents suggests that the three Kuroshio paths south of Japan have their own intrinsic properties of the instability.

Mean circulation and EKE distribution in the Labrador Sea Water level of the subpolar North Atlantic

Abstract. A long-term mean flow field for the subpolar North Atlantic region with a horizontal resolution of approximately 25 km is created by gridding Argo-derived velocity vectors using two different topography-following interpolation schemes. The 10-day float displacements in the typical drift depths of 1000 to 1500 m represent the flow in the Labrador Sea Water density range. Both mapping algorithms separate the flow field into potential vorticity (PV) conserving, i.e., topography-following contribution and a deviating part, which we define as the eddy contribution. To verify the significance of the separation, we compare the mean flow and the eddy kinetic energy (EKE), derived from both mapping algorithms, with those obtained from multiyear mooring observations. The PV-conserving mean flow is characterized by stable boundary currents along all major topographic features including shelf breaks and basin-interior topographic ridges such as the Reykjanes Ridge or the Rockall Plateau. Mid-basin northward advection pathways from the northeastern Labrador Sea into the Irminger Sea and from the Mid-Atlantic Ridge region into the Iceland Basin are well-resolved. An eastward flow is present across the southern boundary of the subpolar gyre near 52∘ N, the latitude of the Charlie Gibbs Fracture Zone (CGFZ). The mid-depth EKE field resembles most of the satellite-derived surface EKE field. However, noticeable differences exist along the northward advection pathways in the Irminger Sea and the Iceland Basin, where the deep EKE exceeds the surface EKE field. Further, the ratio between mean flow and the square root of the EKE, the Peclet number, reveals distinct advection-dominated regions as well as basin-interior regimes in which mixing is prevailing.

Impacts of the QBO on the North Atlantic and Mediterranean storm tracks: An energetic perspective

AbstractImpacts of the quasi‐biennial oscillation (QBO) on the energetics of the North Atlantic and Mediterranean storm tracks are discussed. The daily National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data for the period 1953–2015 are used to evaluate the spatial distribution of eddy kinetic energy (EKE) and the main factors involved in generation, conversion, and propagation of the EKE in the westerly and easterly phases of the QBO. Results are presented for both early and late winter to uncover intraseasonal variations of the impact. It is shown that the QBO exerts a marked influence on the Mediterranean storm track in early winter, which is of the same order of its impact on the North Atlantic storm track. Further, the impact on the Mediterranean storm track is rather local in early winter but more influenced by variations in the North Atlantic storm track in late winter.

Subtropical surface circulation in the southern Indian Ocean derived from surface drifters

Using trajectory tracing method from 308 long-lived surface drifters, the subtropical surface circulations in the southern Indian Ocean (SIO) are investigated in the views of both Lagrangian and Euler.Analysis of the results from the Lagrangian view confirm the two pathways of seawater transport from high to low latitude based on different types of drifter trajectory. One pathway is that the seawater formed in high latitude flows northward along the eastern boundary of SIO, joints the westward South Equatorial Current (SEC), and reaches east of the Mozambique along the north side of the Madagascar Island, then bifurcates into northward and southward branches: the northward branch cross the equator and finally joints the north Indian Ocean circulation system; the southward branch meanders along the Mozambique Channel and forms a series of mesoscale eddies. The other pathway is that the seawater flows northward through the central SIO, then turns to west following the SEC, and flows back to the south when reaches the east of the Madagascar Island. Based on the trajectory of drifters, the mean life cycle of subtropical circulation is about 43±20 months. In Euler view, zonal and meridional velocity, mean kinetic energy (MKE), and eddy kinetic energy (EKE) are calculated in 0.5°×0.5° grid using Poulain’s method. The high MKE reaches over 1000 (cm/s)2 in the regions of South Equatorial Current, Agulhas Current/Agulhas Return Current, Eastern Madagascar Current, and Eastern African Coastal Current, the two latter of which are western boundary currents. The EKE has similar spatial distribution with the MKE in the region west of Australia. The spatial variations of current in several areas are significantly modulated by mesoscale eddies, such as in the regions of Agulhas Retroflection, western basin of SIO, and west of Australia. Agulhas system, especially in the area of Agulhas Retroflection, has highest MKE and EKE, due to the interaction with the bottom topography and the wind stress curl induced instability. West of Australia, the EKE is remarkably larger than the MKE due to active mesoscale eddies. In particular, Leeuwin Current can be detected by drifters in the region. For the South Indian Ocean Countercurrent (SICC), although previous studies suggested an eastward current near 25°S, this study does not find drifters following eastward along this proposed current. However, similar result in EKE distribution as previous studies suggest that the eddy activities are much more significant that the mean current in this proposed SICC region. Ocean surface drifters have important role in track the missing crafts. The long-lived drifters passing around the Reunion Island are collected and traced back to their origins and to rebuild the track of missing MH370’s wrecks. Three possible pathways are rebuilt and two possible crash sites are identified around 35°S, 95°E and 38°S, 92°E, which are consistent with former results from satellite but show more evidences and details. The latest news said that some wrecks were found in the coast of Mozambique, which can be further validated by analysis of drifter trajectories in this study. Along both pathways, passing by north or south of Madagascar Island, surface drifters can reach the coast of Mozambique in different landing time. Both the currents/circulations in the SIO basin and Mozambique Channel contribute to the routes of the wrecks. In this study, the subtropical surface circulation in the SIO is investigated from surface drifters, which help us to understand the mass transport between high and low latitude. A few puzzles still remain, such as there is none of trajectory supporting the SICC at the sea surface. The use of surface drifters on tracing the MH370’s wrecks provides a good try to search missing craft in the ocean, and thus suggests more possible applications in the future in this isolated ocean.

Characterizing the surface circulation in Ebro Delta (NW Mediterranean) with HF radar and modeled current data

Quality-controlled current observations from a High Frequency radar (HFR) network deployed in the Ebro River Delta (NW Mediterranean) were combined with outputs from IBI operational ocean forecasting system in order to comprehensively portray the ocean state and its variability during 2014. Accurate HFR data were used as benchmark for a rigorous validation of the Iberia-Biscay-Ireland (IBI) regional system, routinely operated in the frame of the Copernicus Marine Environment Monitoring Service (CMEMS). The analysis of skill metrics and monthly averaged current maps showed that IBI reasonably captured the prevailing dynamic features of the coastal circulation previously observed by the HFR, according to the moderate resemblance found in circulation patterns and the spatial distribution of eddy kinetic energy. The model skill assessment was completed with an exploration of dominant modes of spatiotemporal variability. The EOF analysis confirmed that the modeled surface current field evolved both in space and time according to three significantly dominant modes of variability which accounted for the 49.2% of the total variance, in close agreement with the results obtained for HFR (46.1%). The response of the subtidal surface current field to prevailing wind regime in the study area was examined in terms of induced circulation structures and immediacy of reaction by performing a conditional averaging approach and a time-lagged vector correlation analysis, respectively. This observations-model synergistic strategy has proved to be valid to operationally monitor the complex coastal circulation in Ebro Delta despite the observed model drawbacks in terms of reduced energy content in surface currents and some inaccuracies in the wind-driven low frequency response. This integrated methodology aids to improve the prognostic capabilities of IBI ocean forecasting system and also to facilitate high-stakes decision-making for coastal management in the Ebro River Delta marine protected area.

Near-surface structure of the Antarctic Circumpolar Current derived from World Ocean Circulation Experiment drifter data

[1] In this work, the dynamics of the Antarctic Circumpolar Currrent (ACC) are investigated on the basis of near-surface Lagrangian observations carried out under the framework of the World Ocean Circulation Experiment (WOCE) and contained within the WOCE drifter data set. Approximately 440 drifters that were deployed in the Southern Ocean, between 1995 and 2002, were considered during the study. The mean surface current field was derived using natural bicubic splines, whose application yielded a high-resolution mean flow which allowed a detailed description for ACC meridional shear as well as its jets. ACC volume transport was estimated using a novel approach that was based upon the surface transport that was computed on the basis of drifter data and by assuming an exponential form for vertical current shear. The volume transport was determined to be 133 Sv for an e folding depth of 1000 m. Lagrangian diffusivities and integral parameters were derived by assuming a broadly homogenous eddy field. The zonal and meridional diffusivity components were determined to be 9 × 107 and 2 × 107 cm2 s−1, respectively. The eddy field is described in terms of the eddy kinetic energy distribution.

Impact of partial steps and momentum advection schemes in a global ocean circulation model at eddy-permitting resolution

Series of sensitivity tests were performed with a z-coordinate, global eddy-permitting (1/4°) ocean/sea-ice model (the ORCA-R025 model configuration developed for the DRAKKAR project) to carefully evaluate the impact of recent state-of-the-art numerical schemes on model solutions. The combination of an energy–enstrophy conserving (EEN) scheme for momentum advection with a partial step (PS) representation of the bottom topography yields significant improvements in the mean circulation. Well known biases in the representation of western boundary currents, such as in the Atlantic the detachment of the Gulf Stream, the path of the North Atlantic Current, the location of the Confluence, and the strength of the Zapiola Eddy in the south Atlantic, are partly corrected. Similar improvements are found in the Pacific, Indian, and Southern Oceans, and characteristics of the mean flow are generally much closer to observations. Comparisons with other state-of-the-art models show that the ORCA-R025 configuration generally performs better at similar resolution. In addition, the model solution is often comparable to solutions obtained at 1/6 or 1/10° resolution in some aspects concerning mean flow patterns and distribution of eddy kinetic energy. Although the reasons for these improvements are not analyzed in detail in this paper, evidence is shown that the combination of EEN with PS reduces numerical noise near the bottom, which is likely to affect current–topography interactions in a systematic way. We conclude that significant corrections of the mean biases presently seen in general circulation model solutions at eddy-permitting resolution can still be expected from the development of numerical methods, which represent an alternative to increasing resolution.

Mediterranean sea eddy kinetic energy variability from 11 years of altimetric data

The eddy kinetic energy (EKE) variability of the Mediterranean Sea over eleven years (1993–2003) was studied using merged (T/P, Jason, ERS and Envisat) altimetric data. Special efforts were made to improve the data selection and the mapping procedure. The introduction of the Mean Dynamic Topography [Rio M.-H., Poulain P.-M., Pascual A., Mauri E., Larnico G., 2005, “A mean dynamic topography of the Mediterranean Sea computed from altimetric data and in situ measurements”, submitted to J. Mar. Syst. (Liège Colloquium special issue).] made it possible to derive the absolute dynamic topography. Thus, an improved and more complete data set was obtained, which enabled previous studies based on altimetric data ([Iudicone D., Santoreli R., Marullo S., Gerosa P., 1998, “Sea level variability and surface eddy statistics in the Mediterranean Sea from TOPEX/POSEIDON data”, J. Geoph. Res. 103 (C2), 2995–3011.]; [Ayoub N., Le Traon P.-Y., De Mey P., 1998, “A description of the Mediterranean surface circulation from combined ERS-1 and TOPEX/POSEIDON altimetric data”, J. Mar. Syst. 18 (1–3), 3–40.]; [Larnicol G., Le Traon P.-Y., Ayoub N., De Mey P., 1995, “Mean sea level and surface circulation variability of the Mediterranean Sea from 2 years of TOPEX/POSEIDON altimetry ”, J. Geoph. Res. 100 (C12), 25,163–25,177.]; [Larnicol G., Ayoub N., Le Traon P.-Y., 2002, “Major changes in Mediterranean Sea level variability from 7 years of TOPES/Poseidon and ERS-1/2 data”, J. Mar. Syst. 33–34, 63–89.] to be enhanced. In particular, for the first time a relatively accurate description of the mean EKE distribution in the Mediterranean Sea could be provided. The mean EKE structure is the consequence of the superposition of different variability components. As previously described in other studies, mesoscale, seasonal and interannual components play a key role in this variability, but the contribution of long-term decadal signals, as well as sporadic events, are also important. In addition, a specific variability pattern is observed in each sub-basin of the Mediterranean Sea (Western, Ionian and Levantine). The observed permanent/recurrent structures, as well as the transient ones, are subject to annual variations that occurred with different phases: permanent/recurrent structures (such as the Alboran Gyres and the Ierapetra Eddy — IE) are at maximum intensity in late summer (October to January) whereas areas of mesoscale activity (southwest of Sardinia, central Ionian and Mersa-Matruh areas) display maximum intensity in winter (January to April). Both annual and interannual variations are more pronounced in the eastern part of the basin. On the western side, mesoscale activity is relatively stable. The appearance of sporadic energetic structures leads to a positive trend in the Alboran Sea and north of the Tyrrhenian, while a negative trend is observed south of Majorca. The seasonal EKE variations are mainly concentrated southwest of Sardinia. They are found to be modulated by interannual variability, with a weakening of both the mean EKE and the amplitude of the seasonal cycle between 1997 and 2000 and between 2001 and 2003. In the Ionian Sea, the EKE variability is affected by the reversal of the circulation in 1997. This phenomenon suggests that the decadal scale of variability should not be overlooked in this basin. A strong positive EKE trend, coupled with an increase in the seasonal cycle amplitude, is observed in 1997 in the central Ionian. The high correlation of wind stress variations and high-frequency EKE in the Ionian Sea, suggests that the seasonal variations are partly due to wind-induced mesoscale activity. In the Levantine Sea, an even stronger positive EKE trend is observed, partly influenced by strong activity around the IE and in the Mersa-Matruh and Shikmona areas. Seasonal variations are present all over the studied period and are either largely influenced by the IE activity at the beginning and end of the period, or are concentrated at the entrance of the basin and in the Mersa-Matruh area. In the latter, larger amplitude are observed after 1997, as in the Ionian Sea. Also, different processes of variability are observed, confirming that the Mersa-Matruh area must be considered as the site of intense mesoscale activity.

Dynamical Response of the Oceanic Eddy Field to the North Atlantic Oscillation: A Model–Data Comparison

Abstract Observational studies have shown that in many regions of the World Ocean the eddy kinetic energy (EKE) significantly varies on interannual time scales. Comparing altimeter-derived EKE maps for 1993 and 1996, Stammer and Wunsch have mentioned a significant meridional redistribution of EKE in the North Atlantic Ocean and suggested the possible influence of the North Atlantic Oscillation (NAO) cycle. This hypothesis is examined using 7 yr of Ocean Topography Experiment (TOPEX)/Poseidon altimeter data and three ⅙°-resolution Atlantic Ocean model simulations performed over the period 1979–2000 during the French “CLIPPER” experiment. The subpolar–subtropical meridional contrast of EKE in the real ocean appears to vary on interannual time scales, and the model reproduces it realistically. The NAO cycle forces the meridional contrast of energy input by the wind. The analysis in this paper suggests that after 1993 the large amplitude of the NAO cycle induces changes in the transport of the baroclinically unstable large-scale circulation (Gulf Stream/North Atlantic Current) and, thus, changes in the EKE distribution. Model results suggest that NAO-like fluctuations were not followed by EKE redistributions before 1994, probably because NAO oscillations were weaker. Strong NAO events after 1994 were followed by gyre-scale EKE fluctuations with a 4–12-month lag, suggesting that complex, nonlinear adjustment processes are involved in this oceanic adjustment.

Effects of lateral mixing on the mean state and eddy activity of an equatorial ocean

Numerical experiments are performed using an ocean general circulation model (OGCM) to assess the impact of the form of lateral mixing of momentum and tracers on the state of an equatorial ocean. It is found that the large‐scale structure of both the velocity and temperature fields are very sensitive to the imposed parameterization of lateral mixing in the model. With uniform values for the viscosity and diffusion coefficient across the domain, a decrease in these coefficients increases the activity of tropical instability waves (TIW), resulting in an increase in the sea surface temperature of the cold tongue, and thus overcoming the cold bias often found in ocean models of the equatorial Pacific. However, there is an associated increase in the strength of the Equatorial Under‐Current (EUC) to unrealistic levels. It is found that the strength of the EUC can be limited, while little affecting the TIW activity, by applying an enhanced level of mixing in the vicinity of the equator. This enhanced mixing is used to model the effect of the observed interleaving of water masses across the equator. Both the level and distribution of eddy kinetic energy (associated with TIWs) varies with the level and orientation of the lateral mixing. An examination of the main energy conversion terms of mean to eddy energy reveals that both the barotropic and baroclinic energy conversion terms are important for the production of TIWs. The relative role of each of these conversion terms changes as the lateral mixing is decreased. At low levels of lateral mixing, barotropic instability is found to dominate the production of total eddy energy. Including enhanced mixing at the equator decreases this barotropic production, but surprisingly increases the baroclinic production away from the equator.

How Topographic Smoothing Contributes to Differences between the Eddy Flows Simulated by Sigma- and Geopotential-Coordinate Models

The characteristics of the mesoscale turbulence simulated at a resolution of ⅓° by a sigma-coordinate model (SPEM) and a geopotential-coordinate model (OPA) of the South Atlantic differ significantly. These two types of models differ with respect to not only their numerical formulation, but also their topography (smoothed in SPEM, as in every sigma-coordinate application). In this paper, the authors examine how these topographic differences result in eddy flows that are different in the two models. When the topography of the Agulhas region is smoothed locally in OPA, as is done routinely in SPEM, the production mechanism of the Agulhas rings, their characteristics, and their subsequent drift in the subtropical gyre, are found to converge toward those in SPEM. Furthermore, the vertical distribution of eddy kinetic energy (EKE) everywhere in the basin interior becomes similar in SPEM and OPA and, according to some current meter data, becomes more realistic when mesoscale topographic roughness is removed from the OPA bathymetry (as in SPEM). As expected from previous process studies, this treatment also makes the sensitivity of the Agulhas rings to the Walvis Ridge become similar in SPEM and OPA. These findings demonstrate that many properties of the eddies produced by sigma- and geopotential-coordinate models are, to a significant extent, due to the use of different topographies, and are not intrinsic to the use of different vertical coordinates. Other dynamical differences, such as the separation of western boundary currents from the shelf or the interaction of the flow with the Zapiola Ridge, are attributed to intrinsic differences between both models. More generally, it is believed that, in the absence of a correct parameterization of current–topography interactions, a certain amount of topographic smoothing may have a beneficial impact on geopotential coordinate model solutions.