Baroclinic Energy Research Articles

A Global Diagnosis of Eddy Potential Energy Budget in an Eddy-Permitting Ocean Model

Abstract We use an interannually forced version of the Parallel Ocean Program, configured to resolve mesoscale eddies, to close the global eddy potential energy (EPE) budget associated with temperature variability. By closing the EPE budget, we are able to properly investigate the role of diabatic processes in modulating mesoscale energetics in the context of other processes driving eddy–mean flow interactions. A Helmholtz decomposition of the eddy heat flux field into divergent and rotational components is applied to estimate the baroclinic conversion from mean to eddy potential energy. In doing so, an approximate two-way balance between the “divergent” baroclinic conversion and upgradient vertical eddy heat fluxes in the ocean interior is revealed, in accordance with baroclinic instability and the relaxation of isopycnal slopes. However, in the mixed layer, the EPE budget is greatly modulated by diabatic mixing, with air–sea interactions and interior diffusion playing comparable roles. Globally, this accounts for ∼60% of EPE converted to EKE (eddy kinetic energy), with the remainder being dissipated by air–sea interactions and interior mixing. A seasonal composite of baroclinic energy conversions shows that the strongest EPE to EKE conversion occurs during the summer in both hemispheres. The seasonally varying diabatic processes in the upper ocean are further shown to be closely linked to this EPE–EKE conversion seasonality, but with a lead. The peak energy dissipation through vertical mixing occurs ahead of the minimum EKE generation by 1–2 months.

Topographic Hotspots of Southern Ocean Eddy Upwelling

The upwelling of cold water from the depths of the Southern Ocean to its surface closes the global overturning circulation and facilitates uptake of anthropogenic heat and carbon. Upwelling is often conceptualised in a zonally averaged framework as the result of isopycnal flattening via baroclinic eddies. However, upwelling is zonally non-uniform and occurs in discrete hotspots near topographic features. The mechanisms that facilitate topographically confined eddy upwelling remain poorly understood and thus limit the accuracy of parameterisations in coarse-resolution climate models.Using a high-resolution global ocean sea-ice model, we calculate spatial distributions of upwelling transport and energy conversions associated with barotropic and baroclinic instability, derived from a thickness-weighted energetics framework. We find that five major topographic hotspots of upwelling, covering less than 30% of the circumpolar longitude range, account for up to 76% of the southward eddy upwelling transport. The conversion of energy into eddies via baroclinic instability is highly spatially correlated with upwelling transport, unlike the barotropic energy conversion, which is also an order of magnitude smaller than the baroclinic conversion. This result suggests that eddy parameterisations that quantify baroclinic energy conversions could be used to improve the simulation of upwelling hotspots in climate models. We also find that eddy kinetic energy maxima are found on average 110 km downstream of upwelling hotspots in accordance with sparse observations. Our findings demonstrate the importance of localised mechanisms to Southern Ocean dynamics.

Effect of scale interaction on the development of typhoon intensity in summer and autumn

<p>本研究利用綜觀尺度擾動(SSE)動能方程式，定量分析尺度交互作用對颱風強度發展的影響。研究結果發現，強烈颱風(C3-C5等級，最大強度≥96 knots)的移動速度(發展時間)較弱颱風(C1-C2等級，最大強度64~95 knots)慢(長)，增強率較大。分析強颱移動速度較慢的原因，發現弱颱風與強烈颱風主要受到大尺度副高環流場的導引而移動。然而，強颱伴隨的駛流場較弱，其可能與副高環流場減弱及季風槽和季內震盪(ISO)氣旋式環流的增強有關。此外，強(弱)颱往西北方向(往西以及往北轉向)移動。而強颱增強率較強的原因為其東南-西北向的移動路徑，經過西北太平洋海溫最高、對流層頂溫度最低、垂直風切場最弱，以及ISO擾動動能最大的區域，有利強颱的強度發展。</p> <p>SSE動能方程式的研究結果顯示，在颱風從生成增強至最大強度的過程中，正壓能量轉換(CK項)與斜壓能量轉換(CE項)均是颱風強度增強的能量來源。CK項中的CKS-M 和CKS-ISO兩項均有正貢獻，即季節平均環流與ISO均傳送能量給颱風發展。然而，在颱風發展後期，強颱與弱颱CKS-ISO差異大，強颱自ISO 獲得較多能量。CKS-ISO差異主要來源為CKS-ISO中的-(〖u_s^’ v〗_s^’ (∂u_I^’)/∂y) ̅ 項，該項與強颱伴隨ISO氣旋式環流(-(∂u_I^’)/∂y>0)的增強及強颱 〖u_s^’ v〗_s^’向北傳送動量較多有關。因此，當ISO與颱風增強，強颱將自ISO 獲得更多能量。此外，因強颱與伴隨強颱的ISO氣旋式環流移速較慢，在颱風發展後期，強颱與ISO氣旋式環流仍位在暖洋面上。而ISO氣旋式環流南側西南氣流所提供的水氣，亦有利強颱的潛熱釋放，將強颱可用位能轉換成更多的強颱動能。此正回饋效應，使強颱得到較多能量而強度較強。因此，尺度交互作用是颱風強度發展的重要機制。</p> <p> </p><p>This study quantitatively analyzed the influence of scale interaction on typhoon intensity by using the synoptic-scale eddy (SSE) kinetic energy equation. Our research showed that speed of movement, development time, and intensification rate of intense typhoons (categories 3–5 with maximum sustained wind speeds greater than 96 knots) are slower, longer, and larger, respectively, than those of weak typhoons (categories 1–2 with maximum sustained wind speeds between 64 and 95 knots). By analyzing the reasons for the slower speed of movement of intense typhoons, we discovered that weak and intense typhoons are mainly steered by large-scale subtropical high circulation during the intensification process. However, the steering flow of intense typhoons are much weaker which may result from the weakened subtropical high circulation, the enhanced monsoon trough and strengthened intraseasonal oscillation (ISO) cyclonic circulation. In addition, intense typhoons were steered northwestward, while weak typhoons moved westward or northward recurving. The northwestward propagation of intense typhoons experienced the highest sea surface temperature (SST), lowest tropopause temperature, weakest vertical wind shear, and largest ISO kinetic energy region over the western North Pacific (WNP). These large-scale environments were favorable for the growth (a larger intensification rate) of intense typhoons.</p> <p>Further diagnosis of the SSE kinetic energy budget suggested that the enhancement of typhoon intensity is contributed by both barotropic (CK) and baroclinic (CE) energy conversions during the intensification process. The positive contributions of CKS-M and CKS-ISO in the CK term indicate that both seasonal mean circulation and ISO flow provide energy to typhoons. However, intense typhoons gain more eddy kinetic energy from the ISO flow during the late period of typhoon development. This CKS-ISO difference is dominated by the -(〖u_s^’ v〗_s^’ (∂u_I^’)/∂y) ̅ term associated with the strengthened ISO cyclonic circulation (-(∂u_I^’)/∂y>0) and the greater momentum transport (〖u_s^’ v〗_s^’) of intense typhoons. Thus, as ISO and typhoons intensified, intense typhoons gain more energy from ISO. In addition, both the intense typhoons and their accompanying ISO cyclonic circulation are still located over the warm ocean due to their slower speed of movement. The moisture provided by the southwesterly flows at the southern flank of this ISO cyclonic circulation was also favorable for the latent heat release of intense typhoons and converted more typhoon available potential energy into typhoon kinetic energy. This positive feedback causes intense typhoons to receive more energy and further results in more intensification of intense typhoons. This research indicates that scale interaction plays an important role in the development of typhoon intensity.</p> <p> </p>

Diurnal Coastal Trapped Waves and Associated Deep‐Ocean Mixing Near the Head of Suruga Trough

AbstractCoastal submarine canyons are expected to be sites of enhanced turbulent mixing which create productive fishing areas in coastal waters. Recent studies suggested the potential role of diurnal coastal trapped waves (CTWs) in inducing significant turbulent mixing in coastal submarine canyons poleward of 30° where the diurnal tide becomes subinertial. Nevertheless, the detailed physical processes responsible for the generation and dissipation of CTWs in such submarine canyons have hardly been investigated so far. In the present study, to investigate the turbulent mixing processes associated with diurnal CTWs in submarine canyons, we carry out high‐resolution (Δx, Δy = 1/240°) non‐hydrostatic three‐dimensional numerical experiments focusing on Suruga Bay located at the north end of Suruga Trough, Japan. The numerical experiments show that diurnal (subinertial) tidal currents over the Izu Ridge generate CTWs which propagate cyclonically along the coast of Suruga Bay. In the middle of Suruga Bay, bottom‐intensified flow due to the diurnal CTWs interacts with rough seafloor topography to excite upward propagating internal lee waves, which create mixing hotspots extending high above the seafloor. The resulting diurnal (subinertial) tidal energy dissipation in Suruga Bay is found to be much larger than that associated with semidiurnal (superinertial) internal tides. Furthermore, it is found that baroclinic energy flux based on a simple barotropic‐baroclinic decomposition severely underestimates the wave energy flux in areas with high CTW activity. These results strongly indicate that the existence of CTWs might be a key factor to clarify the dissipation/mixing processes in submarine canyons.

Mechanical and Thermal Impacts of the Tibetan–Iranian Plateau on the North Pacific Storm Track: Numerical Experiments by FGOALS‐f3‐L

AbstractBy employing a state‐of‐the‐art global atmospheric general circulation model from the Global Monsoons Model Inter‐comparison Project, this study investigates the mechanical and thermal impacts of the Tibetan‐Iranian Plateau (TIP) on the North Pacific storm track (NPST). When the TIP‐associated mechanical forcing is removed, the winter NPST strengthens remarkably on the northern flank of its climatological maximum and becomes inconspicuously separated from storm track activities over the Asian continent. The midwinter suppression of the NPST is almost inapparent but still exists. In contrast, when TIP‐associated thermal forcing is absent, the winter NPST amplitudes weaken prominently on the north and west sides, and the midwinter suppression remains obvious. The mechanical and thermal impact of the TIP on the NPST are robust not only in winter but also in other seasons. Without the TIP‐associated mechanical forcing, the intensified winter NPST may be dependent on the weakened and widened upper‐level jet, strengthened atmospheric baroclinicity on the north side of climatological winter upper‐level jet, enhanced baroclinic energy conversion, and attenuated East Asian trough. In contrast, without the TIP‐associated thermal forcing, the weakened winter NPST may be determined by the strengthened and narrowed upper‐level jet, reduced atmospheric baroclinicity on the northern flank of upper‐level jet, decreased baroclinic energy conversion, and intensified East Asian trough.

A new subseasonal atmospheric teleconnection bridging tropical deep convection over the western North Pacific and Antarctic weather

AbstractPrevious studies indicate that convective heating variability of the western North Pacific summer monsoon (WNPSM) influences strongly weather and climate over East Asia. Based on daily reanalysis data and interpolated outgoing longwave radiation (OLR) data, this study demonstrates that the WNPSM convection can also cause severe weather events remotely over the Antarctic through the exciting of the Australia‐South Pacific‐Atlantic wave train (ASPA pattern). Surface air temperature (SAT) rises over the Ross Sea‐Mawson‐Dumont d'Urville Seas sector and over the Weddell Sea, while the SAT drops over the Amundsen–Bellingshausen Seas. Concurrently, sea ice concentration (SIC) is reduced over the Ross Sea and enhanced over the Amundsen–Bellingshausen Seas. The result suggests that the newly found ASPA pattern may serve as an important bridge linking the WNPSM convection and the weather over the Antarctic region.The dynamics of ASPA's formation and propagation are also investigated comprehensively. Day‐to‐day energy budget analysis suggests that after its initiation by WNPSM convection, the ASPA pattern is driven by the baroclinic energy conversion from the climatological flow and nonlinear term. The barotropic energy conversion from the climatological flow contributes to positive KE tendency before day +1 and negative KE tendency after day +1. It is therefore extremely important to improve the representations of the climatological‐mean sea ice and jet stream, wave‐mean flow interaction and wave‐wave interaction in the mid‐ and high‐latitudes of the Southern Hemisphere, as well as the convection over the WNPSM region of the Northern Hemisphere in numerical model for a better weather prediction for the Antarctic.

Distinct intensity of 10–30-day intraseasonal waves over the North Pacific between early and late summers

The authors’ previous study identified the wave trains of intraseasonal oscillations, which are mainly in the band of 10–30 days, over the North Pacific during summer. The wave trains are zonally oriented and trapped along the upper-tropospheric westerly jet, and accordingly gain energy mainly through baroclinic energy conversion. In this study, the authors investigate the distinct features of the wave trains between early summer (1 June to 7 July) and late summer (8 July to 31 August), considering that the westerly jet experiences a remarkable subseasonal variation over the North Pacific during summer—that is, the jet is much stronger in early summer than late summer. The results indicate that the wave trains are stronger in early summer compared with late summer. Further analysis suggests that, in early summer, the wave trains can obtain energy more efficiently from the basic flow; or more exactly, stronger westerlies through baroclinic energy conversion.摘要我们之前的研究工作表明, 夏季北太平洋上空存在主导周期为10-30天的季节内波列, 波列纬向分布于上层西风急流带中, 并通过斜压能量转换从基本气流获取能量得到发展和维持. 由于西风急流在前夏(6月1日–7月7日)明显强于后夏(7月8日–8月31日), 因而, 在本研究中, 我们着重研究了波列在前, 后夏的不同特征. 研究结果表明, 波列强度在前夏明显强于后夏, 其原因在于波列在前夏能够通过斜压能量转换从更强的西风中获取更多的能量.

Energetics of Transient Eddies Related to the Midwinter Minimum of the North Pacific Storm-Track Activity

Abstract Storm-track activity over the North Pacific (NP) climatologically exhibits a clear minimum in midwinter, when the westerly jet speed sharply maximizes. This counterintuitive phenomenon, referred to as the “midwinter minimum (MWM),” has been investigated from various perspectives, but the mechanisms are still to be unrevealed. Toward better understanding of this phenomenon, the present study delineates the detailed seasonal evolution of climatological-mean Eulerian statistics and energetics of migratory eddies along the NP storm track over 60 years. As a comprehensive investigation of the mechanisms for the MWM, this study has revealed that the net eddy conversion/generation rate normalized by the eddy total energy, which is independent of eddy amplitude, is indeed reduced in midwinter. The reduction from early winter occurs mainly due to the decreased effectiveness of the baroclinic energy conversion through seasonally weakened temperature fluctuations and the resultant poleward eddy heat flux. The reduced net normalized conversion/generation rate in midwinter is also found to arise in part from the seasonally enhanced kinetic energy conversion from eddies into the strongly diffluent Pacific jet around its exit. The seasonality of the net energy influx also contributes especially to the spring recovery of the net normalized conversion/generation rate. The midwinter reduction in the normalized rates of both the net energy conversion/generation and baroclinic energy conversion was more pronounced in the period before the late 1980s, during which the MWM of the storm-track activity was climatologically more prominent.

Transient Eddy Kinetic Energetics on Mars in Three Reanalysis Datasets

Abstract The ability of Martian reanalysis datasets to represent the growth and decay of short-period (1.5 < P < 8 sol) transient eddies is compared across the Mars Analysis Correction Data Assimilation (MACDA), Open access to Mars Assimilated Remote Soundings (OpenMARS), and Ensemble Mars Atmosphere Reanalysis System (EMARS). Short-period eddies are predominantly surface based, have the largest amplitudes in the Northern Hemisphere, and are found, in order of decreasing eddy kinetic energy amplitude, in Utopia, Acidalia, and Arcadia Planitae in the Northern Hemisphere, and south of the Tharsis Plateau and between Argyre and Hellas basins in the Southern Hemisphere. Short-period eddies grow on the upstream (western) sides of basins via baroclinic energy conversion and by extracting energy from the mean flow and long-period (P > 8 sol) eddies when interacting with high relief. Overall, the combined impact of barotropic energy conversion is a net loss of eddy kinetic energy, which rectifies previous conflicting results. When Thermal Emission Spectrometer observations are assimilated (Mars years 24–27), all three reanalyses agree on eddy amplitude and timing, but during the Mars Climate Sounder (MCS) observational era (Mars years 28–33), eddies are less constrained. The EMARS ensemble member has considerably higher eddy generation than the ensemble mean, and bulk eddy amplitudes in the deterministic OpenMARS reanalysis agree with the EMARS ensemble rather than the EMARS member. Thus, analysis of individual eddies during the MCS era should only be performed when eddy amplitudes are large and when there is agreement across reanalyses. Significance Statement Dust storms on Mars are initiated by traveling atmospheric waves, so understanding the relationships between waves and dust is critical to surface spacecraft safety. The growth and decay of waves are compared in three datasets to evaluate whether waves behave consistently across datasets and are represented similarly across different eras of instrumentation. Waves grow by instabilities caused by horizontal and vertical temperature gradients and lose energy to slower-traveling waves at higher altitudes, but agreement across datasets declines using more recent observations because of problems measuring temperatures near the surface. Regardless, combining dust storm observations and descriptions of traveling waves provides a new avenue for explaining dust storm variability on Mars.

Effects of an along-shelf current on the generation of internal tides near the critical latitude

The effects of along-shelf barotropic geostrophic currents on internal wave generation by the $K_1$ tide interacting with a shelf at near-critical latitudes are investigated. The horizontal shear of the background current results in a spatially varying effective Coriolis frequency which modifies the slope criticality and potentially creates blocking regions where freely propagating internal tides cannot exist. This paper is focused on the barotropic to baroclinic energy conversion rate, which is affected by a combination of three factors: slope criticality, size and location of the blocking region where the conversion rate is extremely small and the internal tide (IT) beam patterns. All of these are sensitive to the current parameters. In our parameter space, the current can increase the conversion rate up to 10 times.

Modulation of Internal Tides by Turbulent Mixing in the South China Sea

Modulations of internal tides (ITs) including the baroclinic tidal energy budget, the incoherency, and the nonlinear interactions among different tidal components by turbulent mixing in the South China Sea (SCS) are investigated through numerical simulations. The baroclinic tidal energy budget can hardly be affected by the structure of mixing. Meanwhile, change in the mixing intensity in a reasonable range also cannot obviously modulate the baroclinic tidal energy budget in the SCS. Compared to the baroclinic energy budget, the distributions of conversion and dissipation are more sensitive to the change of mixing. Turbulent mixing also modulates the incoherency of ITs by changing the horizontal density in the ocean. The horizontal variation of density adds incoherence to ITs largely by affecting the internal tidal amplitudes. Furthermore, nonlinear interactions among different components of ITs are generally modulated by the mixing intensity, whereas the variation of the mixing structure can hardly influence the nonlinear interactions. Therefore, the diapycnal diffusivity can set to be horizontally and vertically homogeneous in most of the internal tidal simulations, except for those in which the incoherency of ITs needs to be simulated. However, excessive strong mixing will destroy the stratification. Thus, the optimum range for IT simulations in the SCS is from O (10–5) to O (10–3) m2s–1.

Energetics of Interactions between African Easterly Waves and Convectively Coupled Kelvin Waves

Abstract Perturbation kinetic and available energy budgets are used to explore how convectively coupled equatorial Kelvin waves (KWs) impact African easterly wave (AEW) activity. The convective phase of the Kelvin wave increases the African easterly jet’s meridional shear, thus enhancing the barotropic energy conversions, leading to intensification of southern track AEWs perturbation kinetic energy. In contrast, the barotropic energy conversion is reduced in the suppressed phase of KW. Baroclinic energy conversion of the southern track AEWs is not significantly different between Kelvin waves’ convective and suppressed phases. AEWs in the convective phase of a Kelvin wave have stronger perturbation available potential energy generation by diabatic heating and stronger baroclinic overturning circulations than in the suppressed phase of a Kelvin wave. These differences suggest that southern track AEWs within the convective phase of Kelvin waves have more vigorous convection than in the suppressed phase of Kelvin waves. Barotropic energy conversion of the northern track AEWs is not significantly different between Kelvin waves’ convective and suppressed phases. The convective phase of the Kelvin wave increases the lower-tropospheric meridional temperature gradient north of the African easterly jet, thus enhancing the baroclinic energy conversion, leading to intensification of northern track AEWs perturbation kinetic energy. In contrast, the baroclinic energy conversion is reduced in the suppressed phase of KW. These results provide a physical basis for the modulation of AEWs by Kelvin waves arriving from upstream.

Contrasting interannual impacts of European and Greenland blockings on the winter North Atlantic storm track

Based on the daily reanalysis, the present study examines the interannual influences of the European blocking (EB) and Greenland blocking (GB) frequencies on the winter North Atlantic storm track (WNAST) intensity. There are contrasting relationships between the two types of atmospheric blockings and the WNAST. The EB and GB frequencies are significantly positively and negatively related to the WNAST intensity, respectively. Composite analysis shows a meridional dipole of the WNAST anomalies associated with both frequent EB and GB. The area of EB-related positive WNAST anomalies is obviously larger than that of the negative anomalies, which directly results in their in-phase relationship. However, the amplitudes of the GB-related negative WNAST anomalies that are larger than those of the positive anomalies directly lead to their out-of-phase relationship. The EB-related intensified WNAST is dependent on the enhanced baroclinic energy conversion and the strengthened atmospheric baroclinicity induced by the anomalous westerly winds on the northern side of the EB-related anticyclone. In contrast, the GB-related weakened WNAST is determined by the attenuated atmospheric baroclinicity and baroclinic energy conversion on the southern flank of the WNAST climatology.

Wave trains of 10–30-day meridional wind variations over the North Pacific during summer

AbstractThe present study investigates the intraseasonal oscillations over the North Pacific during summer based on the ERA-Interim reanalysis dataset. It is shown that the main component of intraseasonal variations in meridional wind is dominated by 10–30-day variability. Zonally-oriented wave trains are identified over the North Pacific at this band, with a zonal wavenumber 6. The wave trains exhibit an equivalent-barotropic structure, with the maximum amplitude in the upper troposphere, and are manifested as quasi-stationary Rossby waves with the energy dispersing eastward. The wave trains do not show a phase-locking feature, that is, they have no preferred geographical locations in the zonal direction. Furthermore, energy analyses suggest that the intraseasonal waves gain energy through baroclinic energy conversion, while the barotropic energy conversion plays a negligible role. The present results have implications for better understanding and forecasting weather and climate in North America, since the intraseasonal waves over the North Pacific may act as precursory signals for extreme events occurring over North America.

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.

Formation and Maintenance Mechanisms of the Subseasonal Eastern Pacific Pattern: Energetics Analysis

AbstractBased on daily data from Japanese 55‐year Reanalysis (JRA‐55), this study examines the formation and maintenance mechanisms of the subseasonal eastern Pacific (EP) pattern from an energetics perspective. Results show that the baroclinic energy conversion (CPB) acts as an important energy source to overcome the thermal damping caused by the transient eddies and as a major kinetic energy (KE) source. The feedback forcing of transient eddies (CKE) acts as a major KE source during the growing stage and a major KE sink during the decay of the EP pattern. At the same time, the barotropic energy conversion (CKB) also makes substantial positive KE contribution. The results also suggest that the EP dynamics differ among different tropospheric layers. In the upper layer, CPB, CKB, and the anomalous geopotential height flux divergence (AGHFD) term all act as major KE source, while CKE acts as a major KE source (sink) during the growth (decay) stage of the EP pattern. The middle layer is mainly driven by CPB and AGHFD: CPB acts as a KE source while AGHFD as a sink. In the lower layer, CPB and AGHFD act as major KE sources with the later larger than the former, while the friction term damps the EP pattern heavily. It is found that the vertical component of the divergence of geopotential height flux anomaly plays a crucial role in the dynamic coupling of the different vertical parts of the EP pattern.

Impacts of ocean currents on the South Indian Ocean extratropical storm track through the relative wind effect

AbstractThis study examines the role of the relative wind (RW) effect (wind relative to ocean current) in the regional ocean circulation and extratropical storm track in the South Indian Ocean. Comparison of two high-resolution regional coupled model simulations with/without the RW effect reveals that the most conspicuous ocean circulation response is the significant weakening of the overly energetic anticyclonic standing eddy off Port Elizabeth, South Africa, a biased feature ascribed to upstream retroflection of the Agulhas Current (AC). This opens a pathway through which the AC transports the warm and salty water mass from the subtropics, yielding marked increases in sea surface temperature (SST), upward turbulent heat flux (THF), and meridional SST gradient in the Agulhas retroflection region. These thermodynamic and dynamic changes are accompanied by the robust strengthening of the local low-tropospheric baroclinicity and the baroclinic wave activity in the atmosphere. Examination of the composite lifecycle of synoptic-scale storms subjected to the high THF events indicates a robust strengthening of the extratropical storms far downstream. Energetics calculations for the atmosphere suggest that the baroclinic energy conversion from the basic flow is the chief source of increased eddy available potential energy, which is subsequently converted to eddy kinetic energy, providing for the growth of transient baroclinic waves. Overall, the results suggest that the mechanical and thermal air-sea interactions are inherently and inextricably linked together to substantially influence the extratropical storm tracks in the South Indian Ocean.

Laboratory experiments on the influence of stratification and a bottom sill on seiche damping

Abstract. The damping of water surface standing waves (seiche modes) and the associated excitation of baroclinic internal waves are studied experimentally in a quasi-two-layer laboratory setting with a topographic obstacle at the bottom representing a seabed sill. We find that topography-induced baroclinic wave drag contributes markedly to seiche damping in such systems. Two major pathways of barotropic–baroclinic energy conversions were observed: the stronger one – involving short-wavelength internal modes of large amplitudes – may occur when the node of the surface seiche is situated above the close vicinity of the sill. The weaker, less significant other pathway is the excitation of long waves or internal seiches along the pycnocline that may resonate with the low-frequency components of the decaying surface forcing.

Surface wind and vertical extent features of the explosive cyclones in the Northern Hemisphere based on the ERA‐I reanalysis data

AbstractAlthough explosive cyclones (ECs) have long been a focus of research, there remains a lack of knowledge of the statistical characteristics of their associated maximum surface winds and vertical extents. This study fills this gap by conducting a targeted statistical analysis of ECs in the Northern Hemisphere using the ERA‐I reanalysis data during a 40‐year period. Some new findings are obtained: (a) The average location of formation of ECs undergoes a notable westward and equatorward shift from September to April in the next year which is consistent with the location variations of sea surface temperature's strong gradients in subtropical regions. (b) Extreme ECs with a deepening rate more than 2.0 Bergeron or with a longer lifespan more than 10 days tend to have a larger occurrence number over the Northern Atlantic Ocean than over the Northern Pacific. (c) The maximum surface wind associated with an EC tends to appear between the EC reaching its maximum deepening rate and reaching its minimum central pressure. (d) The northeastern quadrant of ECs accounts for the highest proportion of maximum surface wind and strongest wind speed, as the baroclinic energy conversion is generally strongest in this quadrant. (e) Over 60% of ECs belong to a type of vertically deep cyclone (ECs' top levels show close relationship to ascending motions within their central regions), and they tend to reach their maximum vertical extent around the time when they reach their minimum central pressures. (f) The highest top levels of ECs exhibit an overall upward extending trend as their minimum central pressure decreases or their maximum deepening rate increases.

The Impact of Fortnightly Stratification Variability on the Generation of Baroclinic Tides in the Luzon Strait

The impact of fortnightly stratification variability induced by tide–topography interaction on the generation of baroclinic tides in the Luzon Strait is numerically investigated using the MIT general circulation model. The simulation shows that advection of buoyancy by baroclinic flows results in daily oscillations and a fortnightly variability in the stratification at the main generation site of internal tides. As the stratification for the whole Luzon Strait is periodically redistributed by these flows, the energy analysis indicates that the fortnightly stratification variability can significantly affect the energy transfer between barotropic and baroclinic tides. Due to this effect on stratification variability by the baroclinic flows, the phases of baroclinic potential energy variability do not match the phase of barotropic forcing in the fortnight time scale. This phenomenon leads to the fact that the maximum baroclinic tides may not be generated during the maximum barotropic forcing. Therefore, a significant impact of stratification variability on the generation of baroclinic tides is demonstrated by our modeling study, which suggests a lead–lag relation between barotropic tidal forcing and maximum baroclinic response in the Luzon Strait within the fortnightly tidal cycle.