Eddy Available Potential Energy Research Articles

Phase dependence of growth mechanisms in the daily energetics of the North Atlantic Oscillation

AbstractThe North Atlantic Oscillation (NAO) is a dominant atmospheric variability in the Northern Hemisphere that has a substantial impact on weather and climate on various time scales. Here, we investigate the role of energy conversion terms day‐by‐day during the NAO life cycle. The relative timing and contribution of the energy conversion terms are quantified by projecting the terms onto the eddy total energy, eddy kinetic energy (EKE), and eddy available potential energy (EAPE) patterns associated with the NAO. The results show a remarkable phase dependence in the growth and maintenance mechanisms. For positive NAOs, the initial growth is driven by the barotropic interactions between the eddies propagating over North America. This suggests that a non‐local growth mechanism plays an important role in the initiation of positive NAOs. In contrast, it is the baroclinic processes that initiate negative NAOs. The conversion of the mean APE into the EAPE and then into the EKE, processes which include eddy heat fluxes and vertical motions, results in the relatively local growth of negative NAOs. During the mature phase, the largest source of energy is the conversion of the mean APE into EAPE. Part of this is converted to EKE, making a substantial contribution to the maintenance of the EKE. This process is particularly important for negative NAOs.

Eddy‐Freshwater Interaction Using Regional Ocean Modeling System in the Bay of Bengal

AbstractEddy‐freshwater interaction is studied in the north Bay of Bengal (BoB) with a high‐resolution simulation using the Regional Ocean Modeling System. Following observations, the model simulates the trapping and homogenization of river water by a cyclonic mesoscale eddy on a submonthly time scale from October to November 2015. As fresh river water is trapped in the eddy, it is characterized by strong vertical and lateral gradients in salinity. Within a few weeks, these gradients relax along with the progressive homogenization of freshwater within the eddy. A mixed‐layer salinity budget shows the importance of ageostrophic vertical advection in addition to lateral advection during the evolution of salinity within the eddy. An analysis of the eddy kinetic energy (EKE) budget in the upper ocean indicates the development of barotropic and baroclinic instabilities. The vertical profiles of EKE conversion terms reveal that the surface freshwater was involved in the evolution of baroclinic instability within the mixed layer. In addition, an eddy available potential energy (EPE) budget shows that the entrainment of the river water raises the EPE, which is due to an increase in lateral salinity gradients across the eddy during the trapping event. Subsequently, the salinity homogenization leads to a decrease in the EPE, and its rate of decay is modulated by a correlation between surface buoyancy fluxes and density anomalies. Finally, reanalysis data show similar trapping and homogenization of freshwater events across multiple years, highlighting the importance of this mechanism on the subseasonal time scale in the BoB.

Representation of the Convectively Coupled Kelvin Waves in Modern Reanalysis Products

Abstract This study aims to deepen our understanding of the destabilization mechanisms and the mean-state modulation of the convectively coupled Kelvin waves (CCKWs) while testing simple models for CCKWs. We examine CCKW precipitation, vertical structure, and energetics in four modern reanalyses: the fifth version of ECMWF Reanalysis (ERA5), NASA Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), the second version of the NCEP Climate Forecast System Reanalysis (CFSR), and the Japanese 55-year Reanalysis (JRA-55). The CCKW precipitation signal strength in the wavenumber–frequency domain and the geographical distribution of CCKW precipitation variability are reasonably represented in all reanalyses, although they commonly underestimate the amplitude of CCKW precipitation. Despite considerable interreanalysis differences in the vertical structure of temperature and diabatic heating anomalies, the eddy available potential energy (EAPE) generation within the CCKWs is found to be associated with the second baroclinic mode whereas the first baroclinic mode damps CCKW EAPE in three out of four reanalyses. Geographically, strong CCKW activity occurs in the areas of high mean-state sea surface temperature (SST), where the second mode EAPE generation is higher, mainly due to a stronger stratiform heating and a tighter wave–convection coupling. Our results are supportive of the simple models for CCKWs in which CCKWs are destabilized within the second baroclinic mode component.

A Mesoscale Ocean–Atmosphere Coupled Pathway for Decadal Variability of the Kuroshio Extension System

Abstract The Kuroshio Extension (KE) system has been observed to experience a decadal cycle between dynamically stable and unstable states. However, divergent conclusions on its interaction with the atmosphere obfuscate the understanding of its oscillatory nature at the preferred decadal time scale. Here, using satellite observations and the ERA-Interim reanalysis in 2002–16, physical process-oriented diagnoses suggest that the wintertime finer-scale thermodynamic response to mesoscale oceanic surface conditions and slow oceanic Rossby wave adjustment frame a coupled ocean–atmosphere delayed oscillator for the decadal KE variability. During the stable state of the KE system, the downstream KE transition region is rich in mesoscale oceanic warming associated with warm eddies, which induces surface wind convergence and upward motion, probably via the enhanced turbulent mixing. Meanwhile, increased finer-scale diabatic heating in the lower troposphere with abundant moisture supply from warmer water likely facilitates the deep-reaching updraft that adiabatically cools the middle troposphere. The background northwesterly wind helps to spread out the cooling, leading to southward deflection of local atmospheric eddy available potential energy (EAPE) production by baroclinic conversion. Consequently, the synoptic eddy activity displaces southward across the basin with additional energy supply from the increased diabatic production of EAPE downstream. Anomalous synoptic eddy thermal and vorticity forcing eventually fosters the basin-scale equivalent-barotropic cyclonic circulation anomaly, which is further maintained by energy conversion from the background state. The resultant wind-driven negative sea surface height anomalies propagate westward into the upstream KE region with a delay of ∼4 years and can trigger the unstable state of the KE system.

A Model of the Convectively Coupled Equatorial Rossby Wave over the Indo-Pacific Warm Pool

Abstract The convectively coupled equatorial Rossby (CCER) wave can significantly affect tropical and extratropical weather, yet its dynamics is not fully understood. Here, a linear two-layer model is proposed for the n = 1 CCER wave over the Indo-Pacific warm pool. The physical processes include moisture feedback (i.e., a prognostic moisture variable), cloud–radiation feedback, moist convection that depends on column moisture, effect of background zonal flow, and wind-induced surface flux exchange (WISHE) that links enhanced surface evaporation to low-level zonal westerly anomaly based on observation. The emerging CCER mode possesses many features consistent with the observations, including the horizontal structures, a broad range of frequency, and the amplification at both planetary and synoptic scales. This CCER mode can be viewed as a westward-propagating moisture mode, which is driven westward by the Doppler shifting effect of background easterly flow and the pre-moistening effect of WISHE. This CCER mode is destabilized by WISHE and background easterly shear. The WISHE shifts the enhanced convection into warm zone at planetary scales (wavenumbers 1–5), therefore, inducing planetary-scale instability through generating the eddy available potential energy (EAPE). The background easterly shear stimulates the interaction between the barotropic and baroclinic components of the circulation, amplifying the CCER wave at synoptic scales (wavenumbers 6–15) by increasing the EAPE generation through modifying the phase relation between low-level moisture convergence and temperature.

Role of Frictional Processes in Mesoscale Eddy Available Potential Energy Budget in the Global Ocean

AbstractMesoscale eddies in the global ocean are thought to be powered primarily through transfer from available potential energy stored in mean flows to eddy available potential energy (EAPE). Part of the EAPE is further converted to eddy kinetic energy (EKE) with the residue dissipated through diabatic processes. Currently, the effect of frictional processes on the EAPE budget has not been thoroughly analyzed. Using an eddy‐resolving (1/10°) global climate simulation, we demonstrate that frictionally driven mesoscale eddy flow plays an important role in the EAPE budget, accounting for more than 30% of the EAPE‐to‐EKE conversion in the global ocean and dominating its seasonal cycle. This role can be understood based on the turbulent thermal wind balance under which the frictional force induces an ageostrophic secondary circulation producing a prominent EAPE‐to‐EKE conversion in the winter surface boundary layer.

Storm track response to uniform global warming downstream of an idealized sea surface temperature front

Abstract. The future evolution of storm tracks, their intensity, shape, and location, is an important driver of regional precipitation changes, cyclone-associated weather extremes, and regional climate patterns. For the North Atlantic storm track, Coupled Model Intercomparison Project (CMIP) data indicate a tripole pattern of change under the RCP8.5 scenario. In this study, the tripole pattern is qualitatively reproduced by simulating the change of a storm track generated downstream of an idealized sea surface temperature (SST) front under uniform warming on an aquaplanet. The simulated tripole pattern consists of reduced eddy kinetic energy (EKE) upstream and equatorward of the SST front, extended and poleward shifted enhanced EKE downstream of the SST front, and a regionally reduced EKE increase at polar latitudes. In the absence of the idealized SST front, in contrast, the storm track exhibits a poleward shift but no tripole pattern. A detailed analysis of the EKE and eddy available potential energy (EAPE) sources and sinks reveals that the changes are locally driven by changes in baroclinic conversion rather than diabatic processes. However, globally the change in baroclinic conversion averages to zero; thus the observed global EAPE increase results from diabatic generation. In particular, resolved-scale condensation plus parameterized cloud physics dominate the global EAPE increase followed by longwave radiation. Amplified stationary waves affect EKE and EAPE advection, which contributes to the local EKE and EAPE minimum at polar latitudes. Feature-based tracking provides further insight into cyclone life cycle changes downstream of the SST front. Moderately deepening cyclones deepen less in a warmer climate, while strongly deepening cyclones deepen more. Similarly, the average cyclone becomes less intense in a warmer climate, while the extremely intense cyclones become more intense. Both results hold true for cyclones with genesis in the vicinity of the SST front and elsewhere. The mean cyclone lifetime decreases, while it increases for those cyclones downstream of the SST front. The mean poleward displacement between genesis and maximum intensity increases for the most intense cyclones, while averaged over all cyclones there is a mild reduction and the result depends on the definition of the displacement. Finally, the number of cyclones decreases by approximately 15 %. Aquaplanet simulations with a localized SST front thus provide an enriched picture of storm track dynamics and associated changes with warming.

Energetics Analysis of the Eddy–Kuroshio Interaction East of Taiwan

Abstract The three-dimensional energetics evolution during eddy–Kuroshio interactions east of Taiwan is systematically investigated in a time-dependent theoretical framework using outputs from an eddy-resolving ocean general circulation model. Composite analyses are conducted based on 17 anticyclonic eddies (AEs) and 19 cyclonic eddies (CEs). These westward propagating mesoscale eddies impinge on the Kuroshio at ∼22°N, ∼124.5°E and interact with the Kuroshio with a mean duration of ∼70 days. During the interaction, all the eddy energy reservoirs and eddy–mean flow energy conversions exhibit complex spatial–temporal variations. In particular, during the strong interaction period (days 18–54), both AEs and CEs are deformed into an elliptic shape, with the major axis in the northeast–southwest direction due to the squeeze of surrounding eddies, and obtain kinetic energy from the mean flow. Overall, the eddies are weakened gradually after encountering the Kuroshio, with the energy of CEs decreased more rapidly than that of AEs. The eddies decay through two pathways: transferring ∼8% of eddy available potential energy (EPE) to the mean flow, and converting ∼64% of EPE to eddy kinetic energy (EKE) via the baroclinic instability with the majority of the EKE finally dissipated. The results suggest that although the time-dependent energy conversion terms vanish upon time averaging, they play important but opposite roles in the evolution of AEs and CEs. The analysis in this work is on the synoptic and intraseasonal time scales; hence, it provides a basis for understanding the long-term variations of the eddy–Kuroshio interaction and associated climate change.

Influence of Atmospheric Blocking on Storm Track Activity Over the North Pacific During Boreal Winter

AbstractThe observed interannual impact of the North Pacific atmospheric blocking (PB) on the North Pacific storm track (PST) during boreal winter is investigated. There is a significant out‐of‐phase relationship between the intensities of PB and PST. The strong PB is accompanied by prominent geopotential height anomalies exhibiting a meridional dipole. PB‐related anomalous easterlies south of the anticyclonic anomaly reduce the low‐level atmospheric baroclinicity by weakening the upper‐level zonal wind, which may be responsible for the attenuated PST. The situations are opposite when PB is weak but with relatively weak magnitudes. However, the intensified PST associated with weak PB may not be determined by the strengthened baroclinicity. Additionally, by diagnosing the energy budget, the contribution of energy conversion from background available potential energy to the altered eddy available potential energy (EAPE) can be elucidated. PB‐related anomalous baroclinic energy conversion from EAPE to eddy kinetic energy (EKE) largely explains the altered EKE.

Geographical variation and controlling mechanism of eddy-induced vertical temperature anomalies and eddy available potential energy in the South China Sea

Using observational temperature profiles, this study reveals that eddy-induced oceanic vertical temperature anomalies are geographically varying in the South China Sea (SCS). The maximum anomalies are more intensified in the central and south SCS. Due to ocean stratification, the eddy available potential energy (EAPE) can more comprehensively reflect the eddy effect on the ocean vertical structure. The EAPE within a composite eddy is the highest in the northern region and lowest in the southeastern region. The geographical variation of vertical anomalies induced by eddies is because of the combination effect of the ocean stratification, the background energy (available potential energy and barotropic kinetic energy), and the energy conversion rate. The anomalies induced by cyclonic and anticyclonic eddies might be asymmetrical in the same region. For example, in the regions to the west of Luzon Island and in the central west SCS, the temperature anomalies and EAPE within a composite cyclonic eddy are significantly larger than those within an anticyclonic eddy. This variation is owing to the different oceanic stratification along the tracks of cyclonic and anticyclonic eddies.

The Global Sink of Available Potential Energy by Mesoscale Air-Sea Interaction.

The thermal component of oceanic eddy available potential energy (EPE) generation due to air‐sea interaction is proportional to the product of anomalous sea surface temperature (SST) and net air‐sea heat flux (SHF). In this study we assess EPE generation and its timescale and space‐scale dependence from observations and a high‐resolution coupled climate model. A dichotomy exists in the literature with respect to the sign of this term, that is, whether it is a source or a sink of EPE. We resolve this dichotomy by partitioning the SST and net heat flux into climatological mean, climatological seasonal cycle, and remaining transient contributions, thereby separating the mesoscale eddy variability from the forced seasonal cycle. In this decomposition the mesoscale air‐sea SST‐SHF feedbacks act as a 0.1 TW global sink of EPE. In regions of the ocean with a large seasonal cycle, for example, midlatitudes of the Northern Hemisphere, the EPE generation by the forced seasonal cycle exceeds the mesoscale variability sink, such that the global generation by seasonal plus eddy variability acts as a 0.8 TW source. EPE destruction is largest in the midlatitude western boundary currents due to mesoscale air‐sea interaction and in the tropical Pacific where SST variability is due mainly to the El Niño–Southern Oscillation. The EPE sink in western boundary currents is spatially aligned with SST gradients and offset to the poleward side of currents, while the mean and seasonal generation are aligned with the warm core of the current. By successively smoothing the data in space and time we find that half of the EPE sink is confined to timescales less than annual and length scales less than 2°, within the oceanic mesoscale band.

Generation mechanism of an observed submesoscale eddy in the Chukchi Sea

A submesoscale eddy was detected by satellite in the summer Chukchi Sea, which was generated by the salinity front slump. Multiple reanalysis data suggested that the front was generated as a result of denser water transport northward. The whole story can be separated into two stages: front genesis and eddy genesis (front slump). The front genesis event started with denser water located in southern of the Chukchi Sea. Wind-driven Ekman pumping lifted the salty water upward and increased the salinity in the upper layers. Later, the current transported the denser water northward until it arrived in the northeastern Chukchi Sea, where the fronts and submesoscale eddy were detected. The denser (lighter) water was located in the south (north) of the front, covering over 100 km and 80 m in the horizontal and vertical directions, respectively. The first stage was completed in 15 days. The front contained a great amount of mean available potential energy (MAPE), which was converted into eddy available potential energy (EAPE) via the EAPE production term when the salinity front slumped. EAPE was in turn converted into eddy kinetic energy (EKE) via the buoyancy flux work, and submesoscale eddies appeared. As the salinity front diminished, the salinity field became more homogenous in both the horizontal and vertical directions. The second stage was completed in 18 days. Numerical simulations suggested that the eddy activity was determined by the total depth of the fronts instead of the mixed layer depth. However, if a strong pycnocline was added between the fronts, the vertical buoyancy gradient largely increased there, which reduced the growth rate of unstable energy and produced weaker eddy activities on the denser side of the front.

On scale interactions between the MJO and synoptic scale

DYNAMO was a field experiment conducted over the near‐equatorial Indian Ocean during 2011 and 2012 for a better understanding of the mechanisms of the Madden–Julian Oscillation (MJO). This study uses DYNAMO observations to study the organization of convection and maintenance of the MJO within the framework of synoptic‐scale weather systems. Lorenz box energetics is used over the limited DYNAMO domain to study the in‐scale energetics (i.e. the exchange of energy within a single scale). Formal scale energetics in the frequency domain is used to study the in‐scale as well as out‐of‐scale interactions (i.e. the exchange of energy among diverse scales) of the kinetic energy (KE) and available potential energy (APE) over limited DYNAMO domain and a zonal belt around the Earth extending 20° north and south of the Equator. The synoptic‐scale precipitation distributions, generation of eddy APE (from the covariance of convective heating and temperature) and its disposition to eddy KE (from the covariance of vertical velocity and temperature) confirm the importance of in‐scale energetics for the maintenance of the synoptic scale. The in‐scale energetics in the frequency domain suggest that conversion of APE to KE on the MJO scale is crucial for its maintenance. Out‐of‐scale energetics suggest that over both domains, MJO loses KE to the synoptic time‐scale (2–7 days), but a reverse situation is encountered in the vicinity of the summer subtropical jet. Overall, MJO is found weakly modulating synoptic scales via transferring eddy KE. These results also imply that out‐of‐scale KE interactions between MJO and synoptic scales are not crucial in understanding the maintenance of the MJO over the DYNAMO domain.

Use of large-scale atmospheric energetics for understanding the dynamics of contrasting Indian summer monsoon rainfall in different years

An attempt has been made to understand the dynamics of contrasting Indian summer monsoon rainfall (ISMR) in different years during 1979–2017, from large-scale atmospheric energetics aspects. Daily values of eddy and zonal available potential energy (APE), their generation, eddy and zonal kinetic energy (KE), conversions of zonal KE and eddy APE to eddy KE, and conversions of zonal APE to zonal KE and eddy APE were computed over the region bounded by 65°E–95°E and 5°N–35°N during the period 1 May to 30 September for 39 years (1979–2017), using daily ECMWF reanalyzed atmospheric data at 0.125° × 0.125° resolution (3 components of wind and temperature). ISMR was classified into three categories, viz., deficient and below normal, normal and above normal and excess. The daily anomaly of these energetics parameters in each of these years was computed using jackknife method and then the composite of the daily anomalies of these parameters constructed for the years with the above-mentioned three categories of ISMR. The following salient features emerge from this study: Analysis of composite anomaly shows that in case of excess and above normal (below normal and deficient) ISMR, C(AZ, KZ) was less (more) than normal. In case of excess and above normal (below normal and deficient) ISMR, C(AE, KE) was more (less) than normal. Broadly, C(AZ, AE) was more than normal in the years with deficient and below normal ISMR, whereas it was less than normal for years with excess and above normal ISMR. Broadly, G(AZ) was below normal for the years with above normal and excess ISMR, whereas it was above normal for the years with below normal and deficient ISMR. Total kinetic energy and total conversion to eddy kinetic energy was above normal for the years with above normal and excess ISMR.

Influence of the Madden–Julian Oscillation and Caribbean Low-Level Jet on East Pacific Easterly Wave Dynamics

Abstract The east Pacific warm pool exhibits basic-state variability associated with the Madden–Julian oscillation (MJO) and Caribbean low-level jet (CLLJ), which affects the development of easterly waves (EWs). This study compares and contrasts composite changes in the background environment, eddy kinetic energy (EKE) budgets, and EW tracks during MJO and CLLJ events. While previous studies have shown that the MJO influences jet activity in the east Pacific, the influence of the MJO and CLLJ on the east Pacific and EWs is not synonymous. The CLLJ is a stronger modulator of the ITCZ than the MJO, while the MJO has a more expansive influence on the northeastern portion of the basin. Anomalous low-level westerly MJO and CLLJ periods are associated with favorable conditions for EW development paralleling the Central American coast, contrary to previous findings about the relationship of the CLLJ to EWs. Easterly MJO and CLLJ periods support enhanced ITCZ EW development, although the CLLJ is a greater modulator of EW tracks in this region, which is likely associated with stronger moisture and convection variations and their subsequent influence on the EKE budget. ITCZ EW growth during easterly MJO periods is more reliant on barotropic conversion than during strong CLLJ periods, when eddy available potential energy (EAPE)-to-EKE conversion associated with ITCZ convection is more important. Thus, the influence of these phenomena on east Pacific EWs should be considered distinct.

The Global Mesoscale Eddy Available Potential Energy Field in Models and Observations

AbstractGlobal maps of the mesoscale eddy available potential energy (EAPE) field at a depth of 500 m are created using potential density anomalies in a high‐resolution 1/12.5° global ocean model. Maps made from both a free‐running simulation and a data‐assimilative reanalysis of the HYbrid Coordinate Ocean Model (HYCOM) are compared with maps made by other researchers from density anomalies in Argo profiles. The HYCOM and Argo maps display similar features, especially in the dominance of western boundary currents. The reanalysis maps match the Argo maps more closely, demonstrating the added value of data assimilation. Global averages of the simulation, reanalysis, and Argo EAPE all agree to within about 10%. The model and Argo EAPE fields are compared to EAPE computed from temperature anomalies in a data set of “moored historical observations” (MHO) in conjunction with buoyancy frequencies computed from a global climatology. The MHO data set allows for an estimate of the EAPE in high‐frequency motions that is aliased into the Argo EAPE values. At MHO locations, 15–32% of the EAPE in the Argo estimates is due to aliased motions having periods of 10 days or less. Spatial averages of EAPE in HYCOM, Argo, and MHO data agree to within 50% at MHO locations, with both model estimates lying within error bars observations. Analysis of the EAPE field in an idealized model, in conjunction with published theory, suggests that much of the scatter seen in comparisons of different EAPE estimates is to be expected given the chaotic, unpredictable nature of mesoscale eddies.

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.

On the response of the Lorenz energy cycle for the Southern Ocean to intensified westerlies

AbstractThe potential impact of intensified westerlies on the Lorenz energy cycle for the Southern Ocean is examined by employing a global eddy‐permitting ocean‐sea ice model. Two idealized sensitivity experiments are designed for this purpose: one is driven by 1992 forcing with weaker westerlies and the other driven by 1998 forcing with stronger westerlies. The intensified westerlies lead to the most significant increase of about 30% in the eddy kinetic energy (EKE) reservoir, followed by the mean kinetic energy (MKE) reservoir increase (17.9%), eddy available potential energy (EAPE) reservoir increase (8.6%), and mean available potential energy (MAPE) reservoir increase (6.5%). In contrast, the increases in the generations of kinetic energy and available potential energy are quite similar, ranging from 21% for EAPE generation to 26% for MKE generation. There are considerably increased energy transfers from MKE to MAPE (about 75%) and from MAPE to EAPE (about 78%), reflecting greatly enhanced baroclinic instability pathway. The conversion rates are strongly influenced by large topography; in particular, a relatively large energy conversion from EKE to MKE exists in the regions associated with large topography, in contrast to the energy flow from MKE to EKE over the broad Southern Ocean. Under stronger wind forcing, all energy conversions are enhanced, and the increases in the conversion rates from EAPE to EKE and from EKE to MKE are more prominent than the increases from MKE to MAPE and from MAPE to EAPE near large topography.

Saharan Dust and the Nonlinear Evolution of the African Easterly Jet–African Easterly Wave System

Abstract The direct radiative effects of Saharan mineral dust (SMD) aerosols on the nonlinear evolution of the African easterly jet–African easterly wave (AEJ–AEW) system is examined using the Weather Research and Forecasting Model coupled to an online dust model. The SMD-modified AEW life cycles are characterized by four stages: enhanced linear growth, weakened nonlinear stabilization, larger peak amplitude, and smaller long-time amplitude. During the linear growth and nonlinear stabilization stages, the SMD increases the generation of eddy available potential energy (APE); this occurs where the maximum in the mean meridional SMD gradient is coincident with the critical surface. As the AEWs evolve beyond the nonlinear stabilization stage, the discrimination between SMD particle sizes due to sedimentation becomes more pronounced; the finer particles meridionally expand, while the coarser particles settle to the surface. The result is a reduction in the eddy APE at the base and the top of the plume. The SMD enhances the Eliassen–Palm (EP) flux divergence and residual-mean meridional circulation, which generally oppose each other throughout the AEW life cycle. The SMD-modified residual-mean meridional circulation initially dominates to accelerate the flow but quickly surrenders to the EP flux divergence, which causes an SMD-enhanced deceleration of the AEJ during the linear growth and nonlinear stabilization stages. Throughout the AEW life cycle, the SMD-modified AEJ is elevated and the peak winds are larger than without SMD. During the first (second) half of the AEW life cycle, the SMD-modified wave fluxes shift the AEJ axis farther equatorward (poleward) of its original SMD-free position.

The eddy kinetic energy budget in the Red Sea

AbstractThe budget of eddy kinetic energy (EKE) in the Red Sea, including the sources, redistributions, and sink, is examined using a high'resolution eddy‐resolving ocean circulation model. A pronounced seasonally varying EKE is identified, with its maximum intensity occurring in winter, and the strongest EKE is captured mainly in the central and northern basins within the upper 200 m. Eddies acquire kinetic energy from conversion of eddy available potential energy (EPE), from transfer of mean kinetic energy (MKE), and from direct generation due to time‐varying (turbulent) wind stress, the first of which contributes predominantly to the majority of the EKE. The EPE‐to‐EKE conversion occurs almost in the entire basin, while the MKE‐to‐EKE transfer appears mainly along the shelf boundary of the basin (200 m isobath) where high horizontal shear interacts with topography. The EKE generated by the turbulent wind stress is relatively small and limited to the southern basin. All these processes are intensified during winter, when the rate of energy conversion is about 4–5 times larger than that in summer. The EKE is redistributed by the vertical and horizontal divergence of energy flux and the advection of the mean flow. As a main sink of EKE, dissipation processes is ubiquitously found in the basin. The seasonal variability of these energy conversion terms can explain the significant seasonality of eddy activities in the Red Sea.