River plumes are a dominant forcing agent in the coastal ocean, transporting tracers and nutrients offshore and interacting with coastal circulation. In this study we characterize the novel ‘cross-shelf’ regime of freshwater river plumes. Rather than remaining coastally-trapped (a well-established regime), a wind-driven cross-shelf plume propagates for tens to over one hundred kilometers offshore of the river mouth while remaining coherent. We perform a suite of high-resolution idealized numerical experiments that offer insight into how the cross-shelf regime comes about and the parameter space it occupies. The wind-driven shelf flow comprising the geostrophic along-shelf and the Ekman cross-shelf transport advects the plume momentum and precludes geostrophic adjustment within the plume, leading to continuous generation of internal solitons in the offshore and upstream segment of the plume. The solitons propagate into the plume interior, transporting mass within the plume and suppressing plume widening. We examine an additional ultra-high resolution case that resolves submesoscale dynamics. This case is dynamically consistent with the lower resolution simulations, but additionally captures vigorous inertial-symmetric instability leading to frontal erosion and lateral mixing. We support these findings with observations of the Winyah Bay plume, where the cross-shelf regime is observed under analogous forcing conditions to the model. The study offers an in-depth introduction to the cross-shelf plume regime and a look into the submesoscale mixing phenomena arising in estuarine plumes.
{"title":"The Cross-Shelf Regime of a Wind-Driven Supercritical River Plume","authors":"Elizabeth Yankovsky, A. Yankovsky","doi":"10.1175/jpo-d-23-0012.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0012.1","url":null,"abstract":"River plumes are a dominant forcing agent in the coastal ocean, transporting tracers and nutrients offshore and interacting with coastal circulation. In this study we characterize the novel ‘cross-shelf’ regime of freshwater river plumes. Rather than remaining coastally-trapped (a well-established regime), a wind-driven cross-shelf plume propagates for tens to over one hundred kilometers offshore of the river mouth while remaining coherent. We perform a suite of high-resolution idealized numerical experiments that offer insight into how the cross-shelf regime comes about and the parameter space it occupies. The wind-driven shelf flow comprising the geostrophic along-shelf and the Ekman cross-shelf transport advects the plume momentum and precludes geostrophic adjustment within the plume, leading to continuous generation of internal solitons in the offshore and upstream segment of the plume. The solitons propagate into the plume interior, transporting mass within the plume and suppressing plume widening. We examine an additional ultra-high resolution case that resolves submesoscale dynamics. This case is dynamically consistent with the lower resolution simulations, but additionally captures vigorous inertial-symmetric instability leading to frontal erosion and lateral mixing. We support these findings with observations of the Winyah Bay plume, where the cross-shelf regime is observed under analogous forcing conditions to the model. The study offers an in-depth introduction to the cross-shelf plume regime and a look into the submesoscale mixing phenomena arising in estuarine plumes.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-12-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139174011","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Peter R. Oke, T. Rykova, Bernadette M. Sloyan, Ken R. Ridgway
The East Australian Current (EAC) system includes a poleward jet that flows adjacent to the continental shelf, a southward and eastward extension, and a complex eddy field. The EAC jet is often observed to be subsurface-intensified. Here, we explain that there are two factors that cause the EAC to develop a subsurface maximum. First, the EAC flows as a narrow current, carrying low-density water from the Coral Sea into the denser waters of the Tasman Sea. This results in horizontal density gradients with a different sign on either side of the jet, negative onshore and positive offshore. According to the thermal wind relation, this produces vertical gradients in southward current that are surface-intensified onshore, and subsurface-intensified offshore. Second, we show that the winds over the shelf are mostly downwelling favourable, drawing the surface EAC waters onshore. This aligns the region of positive horizontal density gradients with the EAC core, producing a subsurface velocity maximum. The presence of a subsurface maximum may produce baroclinic instabilities that play a role in eddy formation and EAC separation from the coast.
{"title":"What causes the subsurface velocity maximum of the East Australian Current?","authors":"Peter R. Oke, T. Rykova, Bernadette M. Sloyan, Ken R. Ridgway","doi":"10.1175/jpo-d-23-0128.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0128.1","url":null,"abstract":"\u0000The East Australian Current (EAC) system includes a poleward jet that flows adjacent to the continental shelf, a southward and eastward extension, and a complex eddy field. The EAC jet is often observed to be subsurface-intensified. Here, we explain that there are two factors that cause the EAC to develop a subsurface maximum. First, the EAC flows as a narrow current, carrying low-density water from the Coral Sea into the denser waters of the Tasman Sea. This results in horizontal density gradients with a different sign on either side of the jet, negative onshore and positive offshore. According to the thermal wind relation, this produces vertical gradients in southward current that are surface-intensified onshore, and subsurface-intensified offshore. Second, we show that the winds over the shelf are mostly downwelling favourable, drawing the surface EAC waters onshore. This aligns the region of positive horizontal density gradients with the EAC core, producing a subsurface velocity maximum. The presence of a subsurface maximum may produce baroclinic instabilities that play a role in eddy formation and EAC separation from the coast.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138998585","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This study uses a simple diffusivity formulation to examine flow regime transition and jet-eddy energy partitioning in two-layer quasi-geostrophic turbulence. Guided by simulations, the formulation is empirically constructed so that the diffusivity is bounded by a f-plane asymptote (Df) in the limit of vanishing β (termed drag-controlled) while reduced to a drag-independent scaling (Dβ) of Lapeyre and Held (2003) toward large β (termed β-controlled). Good agreement is found for diffusivities diagnosed from simulations with both quadratic and linear drag and in 2D turbulence. From the formulation, a regime diagram is readily constructed, with Df /Dβ = 1 separating the drag- and β-controlled regimes. The diagram also sets the parameter range where an eddy velocity scaling is applicable. The quantitative representations of eddy variables then enable a reasonably skillful theory for zonal jet speed to be developed from energy balance. It is shown that, using Df /Dβ ≥ 10, a state where eddy statistics are approximately drag insensitive could be identified and interpreted using wave-damping competitions in slowing an inverse cascade. However, contrary to an existing hypothesis, the energy dissipation in such a state is not dominated by zonal jets. A modest revision for a way to maintain balance while keeping eddies drag insensitive is proposed. In the regime diagram, a subspace of zonostrophic condition, defined as jet dissipation surpassing eddy, is further quantified. It is demonstrated that a rough scaling could help interpret how the relative importance of jet and eddy dissipation varies across the parameter space.
{"title":"Application of a simple diffusivity formation to examine regime transition and jet-eddy energy partitioning in quasi-geostrophic turbulence","authors":"Shih-Nan Chen","doi":"10.1175/jpo-d-23-0110.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0110.1","url":null,"abstract":"\u0000This study uses a simple diffusivity formulation to examine flow regime transition and jet-eddy energy partitioning in two-layer quasi-geostrophic turbulence. Guided by simulations, the formulation is empirically constructed so that the diffusivity is bounded by a f-plane asymptote (Df) in the limit of vanishing β (termed drag-controlled) while reduced to a drag-independent scaling (Dβ) of Lapeyre and Held (2003) toward large β (termed β-controlled). Good agreement is found for diffusivities diagnosed from simulations with both quadratic and linear drag and in 2D turbulence. From the formulation, a regime diagram is readily constructed, with Df /Dβ = 1 separating the drag- and β-controlled regimes. The diagram also sets the parameter range where an eddy velocity scaling is applicable. The quantitative representations of eddy variables then enable a reasonably skillful theory for zonal jet speed to be developed from energy balance. It is shown that, using Df /Dβ ≥ 10, a state where eddy statistics are approximately drag insensitive could be identified and interpreted using wave-damping competitions in slowing an inverse cascade. However, contrary to an existing hypothesis, the energy dissipation in such a state is not dominated by zonal jets. A modest revision for a way to maintain balance while keeping eddies drag insensitive is proposed. In the regime diagram, a subspace of zonostrophic condition, defined as jet dissipation surpassing eddy, is further quantified. It is demonstrated that a rough scaling could help interpret how the relative importance of jet and eddy dissipation varies across the parameter space.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-12-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139003813","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Seth F. Zippel, James B. Edson, M. E. Scully, Oaklin R. Keefe
Surface waves grow through a mechanism in which atmospheric pressure is offset in phase from the wavy surface. A pattern of low atmospheric pressure over upward wave orbital motions (leeward side) and high pressure over downward wave orbital motions (windward side) travels with the water wave, leading to a pumping of kinetic energy from the atmospheric boundary layer into the waves. This pressure pattern persists above the air/water interface, modifying the turbulent kinetic energy in the atmospheric wave-affected boundary layer. Here, we present field measurements of wave-coherent atmospheric pressure and velocity to elucidate the transfer of energy from the atmospheric turbulence budget into waves through wave-coherent atmospheric pressure work. Measurements show that the phase between wave-coherent pressure and velocity is shifted slightly above 90° when wind speed exceeds the wave phase speed, allowing for a downwards energy flux via pressure work. Although previous studies have reported wave-coherent pressure, to the authors’ knowledge, these are the first reported field measurements of wave-coherent pressure work. Measured pressure work cospectra are consistent with an existing model for atmospheric pressure work. The implications for these measurements and their importance to the turbulent kinetic energy budget are discussed.
{"title":"Direct Observation of Wave-coherent Pressure Work in the Atmospheric Boundary Layer","authors":"Seth F. Zippel, James B. Edson, M. E. Scully, Oaklin R. Keefe","doi":"10.1175/jpo-d-23-0097.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0097.1","url":null,"abstract":"\u0000Surface waves grow through a mechanism in which atmospheric pressure is offset in phase from the wavy surface. A pattern of low atmospheric pressure over upward wave orbital motions (leeward side) and high pressure over downward wave orbital motions (windward side) travels with the water wave, leading to a pumping of kinetic energy from the atmospheric boundary layer into the waves. This pressure pattern persists above the air/water interface, modifying the turbulent kinetic energy in the atmospheric wave-affected boundary layer. Here, we present field measurements of wave-coherent atmospheric pressure and velocity to elucidate the transfer of energy from the atmospheric turbulence budget into waves through wave-coherent atmospheric pressure work. Measurements show that the phase between wave-coherent pressure and velocity is shifted slightly above 90° when wind speed exceeds the wave phase speed, allowing for a downwards energy flux via pressure work. Although previous studies have reported wave-coherent pressure, to the authors’ knowledge, these are the first reported field measurements of wave-coherent pressure work. Measured pressure work cospectra are consistent with an existing model for atmospheric pressure work. The implications for these measurements and their importance to the turbulent kinetic energy budget are discussed.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-12-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138588176","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Anne Takahashi, R. Lien, Eric Kunze, Barry Ma, Hirohiko Nakamura, A. Nishina, E. Tsutsumi, R. Inoue, T. Nagai, T. Endoh
Generating mechanisms and parameterizations for enhanced turbulence in the wake of a seamount in the path of the Kuroshio are investigated. Full-depth profiles of finescale temperature, salinity, horizontal velocity and microscale thermal-variance dissipation rate up- and downstream of the ∼ 10-km wide seamount were measured with EM-APEX profiling floats and ADCP moorings. Energetic turbulent kinetic energy dissipation rates ε ∼ О(10−7 – 10−6 W kg−1) and diapycnal diffusivities K ∼ О(10−2 m2 s−1) above the seamount flanks extend at least 20 km downstream. This extended turbulent wake length is inconsistent with isotropic turbulence which is expected to decay in less than 100mbased on turbulence decay time of N−1 ∼ 100 s and the 0.5m s−1 Kuroshio flowspeed. Thus, the turbulentwake must be maintained by continuous replenishment which might arise from (i) nonlinear instability of a marginally unstable vortexwake, (ii) anisotropic stratified turbulence with expected downstream decay scales of 10–100 km, and/or (iii) lee-wave critical-layer trapping at the base of the Kuroshio. Three turbulence parameterizations operating on different scales, (i) finescale, (ii) large-eddy and (iii) reduced-shear, are tested. Average ε vertical profiles are well-reproduced by all three parameterizations. Vertical wavenumber spectra for shear and strain are saturated over 10–100 m vertical wavelengths comparable to water depth with spectral levels independent of ε and spectral slopes of −1, indicating that the wake flows are strongly nonlinear. In contrast, vertical divergence spectral levels increase with ε.
{"title":"Energetic stratified turbulence generated by Kuroshio-seamount interactions in Tokara Strait","authors":"Anne Takahashi, R. Lien, Eric Kunze, Barry Ma, Hirohiko Nakamura, A. Nishina, E. Tsutsumi, R. Inoue, T. Nagai, T. Endoh","doi":"10.1175/jpo-d-22-0242.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0242.1","url":null,"abstract":"\u0000Generating mechanisms and parameterizations for enhanced turbulence in the wake of a seamount in the path of the Kuroshio are investigated. Full-depth profiles of finescale temperature, salinity, horizontal velocity and microscale thermal-variance dissipation rate up- and downstream of the ∼ 10-km wide seamount were measured with EM-APEX profiling floats and ADCP moorings. Energetic turbulent kinetic energy dissipation rates ε ∼ О(10−7 – 10−6 W kg−1) and diapycnal diffusivities K ∼ О(10−2 m2 s−1) above the seamount flanks extend at least 20 km downstream. This extended turbulent wake length is inconsistent with isotropic turbulence which is expected to decay in less than 100mbased on turbulence decay time of N−1 ∼ 100 s and the 0.5m s−1 Kuroshio flowspeed. Thus, the turbulentwake must be maintained by continuous replenishment which might arise from (i) nonlinear instability of a marginally unstable vortexwake, (ii) anisotropic stratified turbulence with expected downstream decay scales of 10–100 km, and/or (iii) lee-wave critical-layer trapping at the base of the Kuroshio. Three turbulence parameterizations operating on different scales, (i) finescale, (ii) large-eddy and (iii) reduced-shear, are tested. Average ε vertical profiles are well-reproduced by all three parameterizations. Vertical wavenumber spectra for shear and strain are saturated over 10–100 m vertical wavelengths comparable to water depth with spectral levels independent of ε and spectral slopes of −1, indicating that the wake flows are strongly nonlinear. In contrast, vertical divergence spectral levels increase with ε.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-12-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138597967","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Large-amplitude internal solitary waves were recently observed in a coastal plain estuary and were hypothesized to evolve from an internal lee wave generated at the channel-shoal interface. To test this mechanism, a 3D nonhydrostatic model with nested domains and adaptive grids was used to investigate the generation of the internal solitary waves and their subsequent nonlinear evolution. A complex sequence of wave propagation and transformation was documented and interpreted using the nonlinear wave theory based on the Korteweg-de Vries equation. During the ebb tide a mode-2 internal lee wave is generated by the interaction between lateral flows and channel-shoal topography. This mode-2 lee wave subsequently propagates onto the shallow shoal and transforms into a mode-1 wave of elevation as strong mixing on the flood tide erases stratification in the bottom boundary layer and the lower branch of the mode-2 wave. The mode-1 wave of elevation evolves into an internal solitary wave due to nonlinear steepening and spatial changes in the wave phase speed. As the solitary wave of elevation continues to propagate over the shoaling bottom, the leading edge moves ahead as a rarefaction wave while the trailing edge steepens and disintegrates into a train of rank-ordered internal solitary waves, due to the combined effects of shoaling and dispersion. Strong turbulence in the bottom boundary layer dissipates wave energy and causes the eventual destruction of the solitary waves. In the meantime, the internal solitary waves can generate elevated shear and dissipation rate in local regions.
{"title":"Generation and Evolution of Internal Solitary Waves in a Coastal Plain Estuary","authors":"Renjian Li, Ming Li","doi":"10.1175/jpo-d-23-0151.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0151.1","url":null,"abstract":"\u0000Large-amplitude internal solitary waves were recently observed in a coastal plain estuary and were hypothesized to evolve from an internal lee wave generated at the channel-shoal interface. To test this mechanism, a 3D nonhydrostatic model with nested domains and adaptive grids was used to investigate the generation of the internal solitary waves and their subsequent nonlinear evolution. A complex sequence of wave propagation and transformation was documented and interpreted using the nonlinear wave theory based on the Korteweg-de Vries equation. During the ebb tide a mode-2 internal lee wave is generated by the interaction between lateral flows and channel-shoal topography. This mode-2 lee wave subsequently propagates onto the shallow shoal and transforms into a mode-1 wave of elevation as strong mixing on the flood tide erases stratification in the bottom boundary layer and the lower branch of the mode-2 wave. The mode-1 wave of elevation evolves into an internal solitary wave due to nonlinear steepening and spatial changes in the wave phase speed. As the solitary wave of elevation continues to propagate over the shoaling bottom, the leading edge moves ahead as a rarefaction wave while the trailing edge steepens and disintegrates into a train of rank-ordered internal solitary waves, due to the combined effects of shoaling and dispersion. Strong turbulence in the bottom boundary layer dissipates wave energy and causes the eventual destruction of the solitary waves. In the meantime, the internal solitary waves can generate elevated shear and dissipation rate in local regions.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-12-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138603077","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. Neme, M. England, A. Hogg, Hemant Khatri, S. Griffies
The Weddell Gyre is one of the dominant features of the Southern Ocean circulation and its dynamics have been linked to processes of climatic relevance. Variability in the strength of the gyre’s horizontal transport has been linked to heat transport towards the Antarctic margins and changes in the properties and rates of export of bottom waters from the Weddell Sea region to the abyssal global ocean. However, the precise physical mechanisms that force variability in the Weddell’s lateral circulation across different timescales remain unknown. In this study, we use a barotropic vorticity budget from a mesoscale eddy active model simulation to attribute changes in gyre strength to variability in possible driving processes. We find that the Weddell Gyre’s circulation is sensitive to bottom friction associated with the overflowing dense waters at its western boundary. In particular, an increase in the production of dense waters at the southwestern continental shelf strengthens the bottom flow at the gyre’s western boundary, yet this drives a weakening of the depth-integrated barotropic circulation via increased bottom friction. Strengthening surface winds initially accelerates the gyre, but within a few years the response reverses once dense water production and export increases. These results reveal that the gyre can weaken in response to stronger surface winds, putting into question the traditional assumption of a direct relationship between surface stress and gyre strength in regions where overflowing dense water forms part of the depth-integrated flow.
{"title":"The role of bottom friction in mediating the response of the Weddell Gyre circulation to changes in surface stress and buoyancy fluxes","authors":"J. Neme, M. England, A. Hogg, Hemant Khatri, S. Griffies","doi":"10.1175/jpo-d-23-0165.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0165.1","url":null,"abstract":"\u0000The Weddell Gyre is one of the dominant features of the Southern Ocean circulation and its dynamics have been linked to processes of climatic relevance. Variability in the strength of the gyre’s horizontal transport has been linked to heat transport towards the Antarctic margins and changes in the properties and rates of export of bottom waters from the Weddell Sea region to the abyssal global ocean. However, the precise physical mechanisms that force variability in the Weddell’s lateral circulation across different timescales remain unknown. In this study, we use a barotropic vorticity budget from a mesoscale eddy active model simulation to attribute changes in gyre strength to variability in possible driving processes. We find that the Weddell Gyre’s circulation is sensitive to bottom friction associated with the overflowing dense waters at its western boundary. In particular, an increase in the production of dense waters at the southwestern continental shelf strengthens the bottom flow at the gyre’s western boundary, yet this drives a weakening of the depth-integrated barotropic circulation via increased bottom friction. Strengthening surface winds initially accelerates the gyre, but within a few years the response reverses once dense water production and export increases. These results reveal that the gyre can weaken in response to stronger surface winds, putting into question the traditional assumption of a direct relationship between surface stress and gyre strength in regions where overflowing dense water forms part of the depth-integrated flow.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-12-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138603522","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Hao-Ran Zhang, Yi Yu, Zhibin Gao, Yanwei Zhang, Wentao Ma, Dezhou Yang, Baoshu Yin, Yuntao Wang
The spatiotemporal variability of oceanic fronts in the Indonesian seas was investigated using high-resolution satellite observations. The study aimed to understand the underlying mechanism driving these fronts and their impact on chlorophyll-a variability. A high value of frontal probability was found near the coasts of major islands, exhibiting a distinct seasonal cycle with peaks occurrences during austral winter. The distribution variability of chlorophyll-a was generally consistent with the presence of active frontal zones, although a significantly positive relationship between fronts and chlorophyll-a was limited to only some specific areas, e.g., south Java Island and the Celebes Sea. Wind-driven upwelling played a major role in front generation in the Java upwelling region and enhanced frontal activity can promote the growth of phytoplankton, leading to higher chlorophyll-a. Furthermore, the study demonstrated that wind patterns preceded variations in front probability and chlorophyll-a by approximately two months. This lag suggests that the spatiotemporal variability of fronts and chlorophyll-a in this region is primarily influenced by the monsoon system. In addition, the sea surface temperature (SST) simultaneously modulated the chlorophyll-a variability. Negative SST anomalies were typically associated with positive anomalies in front probability the chlorophyll-a in most areas. Notably, the interannual variability of fronts and chlorophyll-a are prominent in the Java upwelling region. During El Niño years, this region experienced an enhanced monsoon, resulting in a negative SST anomaly alongside positive anomalies in front probability and chlorophyll-a. A comprehensive description and underlying dynamics of frontal activity in the Indonesian seas are provided by this study. The findings are helpful to delineate the variability in chlorophyll-a, thereby facilitating the future understanding of local primary production and the carbon cycle. As typical mesoscale processes, oceanic fronts have significant impacts on biological processes and fisheries in marginal seas. The complex spatiotemporal variability of fronts and their effects on biological processes in the Indonesian seas remain poorly understood. This study aimed to address this knowledge gap by investigating the seasonal and interannual variability of fronts and their influence on chlorophyll-a, a key indicator of phytoplankton biomass and primary productivity. The study identified a high frontal probability in south Java Island during austral winter and El Niño years. Wind-driven upwelling was found to be a major factor in front generation and promoting phytoplankton growth. The findings of this study will improve the theoretical knowledge of regional dynamics, local primary production, and the carbon cycle in the Indonesian seas, benefiting fisheries management and ecosystem conservation efforts.
{"title":"Seasonal and Interannual Variability of Fronts and Their Impact on Chlorophyll-a in the Indonesian Seas","authors":"Hao-Ran Zhang, Yi Yu, Zhibin Gao, Yanwei Zhang, Wentao Ma, Dezhou Yang, Baoshu Yin, Yuntao Wang","doi":"10.1175/jpo-d-23-0041.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0041.1","url":null,"abstract":"\u0000The spatiotemporal variability of oceanic fronts in the Indonesian seas was investigated using high-resolution satellite observations. The study aimed to understand the underlying mechanism driving these fronts and their impact on chlorophyll-a variability. A high value of frontal probability was found near the coasts of major islands, exhibiting a distinct seasonal cycle with peaks occurrences during austral winter. The distribution variability of chlorophyll-a was generally consistent with the presence of active frontal zones, although a significantly positive relationship between fronts and chlorophyll-a was limited to only some specific areas, e.g., south Java Island and the Celebes Sea. Wind-driven upwelling played a major role in front generation in the Java upwelling region and enhanced frontal activity can promote the growth of phytoplankton, leading to higher chlorophyll-a. Furthermore, the study demonstrated that wind patterns preceded variations in front probability and chlorophyll-a by approximately two months. This lag suggests that the spatiotemporal variability of fronts and chlorophyll-a in this region is primarily influenced by the monsoon system. In addition, the sea surface temperature (SST) simultaneously modulated the chlorophyll-a variability. Negative SST anomalies were typically associated with positive anomalies in front probability the chlorophyll-a in most areas. Notably, the interannual variability of fronts and chlorophyll-a are prominent in the Java upwelling region. During El Niño years, this region experienced an enhanced monsoon, resulting in a negative SST anomaly alongside positive anomalies in front probability and chlorophyll-a. A comprehensive description and underlying dynamics of frontal activity in the Indonesian seas are provided by this study. The findings are helpful to delineate the variability in chlorophyll-a, thereby facilitating the future understanding of local primary production and the carbon cycle.\u0000\u0000\u0000As typical mesoscale processes, oceanic fronts have significant impacts on biological processes and fisheries in marginal seas. The complex spatiotemporal variability of fronts and their effects on biological processes in the Indonesian seas remain poorly understood. This study aimed to address this knowledge gap by investigating the seasonal and interannual variability of fronts and their influence on chlorophyll-a, a key indicator of phytoplankton biomass and primary productivity. The study identified a high frontal probability in south Java Island during austral winter and El Niño years. Wind-driven upwelling was found to be a major factor in front generation and promoting phytoplankton growth. The findings of this study will improve the theoretical knowledge of regional dynamics, local primary production, and the carbon cycle in the Indonesian seas, benefiting fisheries management and ecosystem conservation efforts.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138621948","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The influence of meridional shift of the oceanic subtropical front (STF) on the Agulhas Current (AC) regime shifts is studied using satellite altimeter data and a 1.5-layer ocean model. The satellite observations suggest the northward shift of the STF leads to the AC leaping across the gap with little Agulhas leakage, and the southward shift of the STF mainly results in the AC intruding into the Atlantic Ocean in the forms of a loop current and an eddy-shedding path, while there are three flow patterns of AC for moderate latitude of the STF. The ocean model results suggest no hysteresis (associated with multiple equilibrium states) exists in the AC system. The model reproduces similar AC regimes depending on different gap widths as in the observations, and model results can be used to explain the observed Agulhas leakage well. We also present the parameter space of the critical AC strength that results in different AC flow patterns as a function of the gap width. The vorticity dynamics of the AC regime shift suggests that the β term is mainly balanced by the viscosity term for the AC in the leaping and loop current paths, while the β and instantaneous vorticity terms are mainly balanced by the advection and viscosity terms for the AC in the eddy-shedding path. These findings help explain the dynamics of the AC flowing across the gateway beyond the tip of Africa affected by the north–south shift of the STF in the leaping regime or penetrating regime.
{"title":"Dynamics of the Agulhas Current Influenced by the North–South Shift of Subtropical Front","authors":"Huan Mei, Jianxin Dong, Xiangbai Wu","doi":"10.1175/jpo-d-23-0078.1","DOIUrl":"https://doi.org/10.1175/jpo-d-23-0078.1","url":null,"abstract":"\u0000The influence of meridional shift of the oceanic subtropical front (STF) on the Agulhas Current (AC) regime shifts is studied using satellite altimeter data and a 1.5-layer ocean model. The satellite observations suggest the northward shift of the STF leads to the AC leaping across the gap with little Agulhas leakage, and the southward shift of the STF mainly results in the AC intruding into the Atlantic Ocean in the forms of a loop current and an eddy-shedding path, while there are three flow patterns of AC for moderate latitude of the STF. The ocean model results suggest no hysteresis (associated with multiple equilibrium states) exists in the AC system. The model reproduces similar AC regimes depending on different gap widths as in the observations, and model results can be used to explain the observed Agulhas leakage well. We also present the parameter space of the critical AC strength that results in different AC flow patterns as a function of the gap width. The vorticity dynamics of the AC regime shift suggests that the β term is mainly balanced by the viscosity term for the AC in the leaping and loop current paths, while the β and instantaneous vorticity terms are mainly balanced by the advection and viscosity terms for the AC in the eddy-shedding path. These findings help explain the dynamics of the AC flowing across the gateway beyond the tip of Africa affected by the north–south shift of the STF in the leaping regime or penetrating regime.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138625727","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
While mesoscale eddy-induced temperature and salinity (T and S) variations at depth levels were widely reported, those on isopycnal surfaces have been largely unexplored so far. This study investigates temperature and salinity anomalies (T′ and S′; dubbed “spiciness anomalies”) on isopycnal surfaces induced by mesoscale eddies in the Kuroshio Extension (KET) region, with a focus on the North Pacific Intermediate Water (NPIW) layer of 26.3–26.7σθ. Cyclonic eddies (CEs) and anticyclonic eddies (AEs) tend to cluster on the northern and southern flanks of the KET jet, respectively. These eddies are characterized by a large radius (CEs: 61.94 km; AEs: 68.05 km), limited zonal movement, and a tendency of meridional movement (CEs: 0.35 cm s−1 southward; AEs: 0.66 cm s−1 northward). The average eddy-induced T′ and S′ are −0.25°C (0.06°C) and −0.05 psu (0.01 psu) for CEs (AEs) in the 26.3–26.7σθ layer. We propose two mechanisms for the generation of subsurface spiciness anomalies, respectively, for moving eddies that travel over long distances with trapped waters and quasi-stationary meander eddies that are generated by the meanders of the KET front. The T′ and S′ induced by moving eddies cumulatively drive cross-front water exchanges. Meander eddies shift the position of the front and induce redistribution of properties. However, these anomalies do not contribute to heat and salt exchanges between water masses. This work provides a useful benchmark for model simulations of mesoscale isopycnal variability in subsurface waters.
{"title":"Eddy-Induced Subsurface Spiciness Anomalies in the Kuroshio Extension Region","authors":"Mingkun Lv, Fan Wang, Yuanlong Li","doi":"10.1175/jpo-d-22-0254.1","DOIUrl":"https://doi.org/10.1175/jpo-d-22-0254.1","url":null,"abstract":"\u0000While mesoscale eddy-induced temperature and salinity (T and S) variations at depth levels were widely reported, those on isopycnal surfaces have been largely unexplored so far. This study investigates temperature and salinity anomalies (T′ and S′; dubbed “spiciness anomalies”) on isopycnal surfaces induced by mesoscale eddies in the Kuroshio Extension (KET) region, with a focus on the North Pacific Intermediate Water (NPIW) layer of 26.3–26.7σθ. Cyclonic eddies (CEs) and anticyclonic eddies (AEs) tend to cluster on the northern and southern flanks of the KET jet, respectively. These eddies are characterized by a large radius (CEs: 61.94 km; AEs: 68.05 km), limited zonal movement, and a tendency of meridional movement (CEs: 0.35 cm s−1 southward; AEs: 0.66 cm s−1 northward). The average eddy-induced T′ and S′ are −0.25°C (0.06°C) and −0.05 psu (0.01 psu) for CEs (AEs) in the 26.3–26.7σθ layer. We propose two mechanisms for the generation of subsurface spiciness anomalies, respectively, for moving eddies that travel over long distances with trapped waters and quasi-stationary meander eddies that are generated by the meanders of the KET front. The T′ and S′ induced by moving eddies cumulatively drive cross-front water exchanges. Meander eddies shift the position of the front and induce redistribution of properties. However, these anomalies do not contribute to heat and salt exchanges between water masses. This work provides a useful benchmark for model simulations of mesoscale isopycnal variability in subsurface waters.","PeriodicalId":56115,"journal":{"name":"Journal of Physical Oceanography","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138613096","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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