Jiping Xie, R. Raj, Laurent Bertino, Justino Martínez, C. Gabarró, R. Catany
Abstract. In the Arctic, the sea surface salinity (SSS) plays a key role in processes related to water mixing and sea ice. However, the lack of salinity observations causes large uncertainties in Arctic Ocean forecasts and reanalysis. Recently the Soil Moisture and Ocean Salinity (SMOS) satellite mission was used by the Barcelona Expert Centre to develop an Arctic SSS product. In this study, we evaluate the impact of assimilating this data in a coupled ocean–ice data assimilation system. Using the deterministic ensemble Kalman filter from July to December 2016, two assimilation runs respectively assimilated two successive versions of the SMOS SSS product on top of a pre-existing reanalysis run. The runs were validated against independent in situ salinity profiles in the Arctic. The results show that the biases and the root-mean-squared differences (RMSD) of SSS are reduced by 10 % to 50 % depending on the area and highlight the importance of assimilating satellite salinity data. The time series of freshwater content (FWC) further shows that its seasonal cycle can be adjusted by assimilation of the SSS products, which is encouraging of the assimilation of SSS in a long-time reanalysis to better reproduce the Arctic water cycle.
{"title":"Assimilation of sea surface salinities from SMOS in an Arctic coupled ocean and sea ice reanalysis","authors":"Jiping Xie, R. Raj, Laurent Bertino, Justino Martínez, C. Gabarró, R. Catany","doi":"10.5194/os-19-269-2023","DOIUrl":"https://doi.org/10.5194/os-19-269-2023","url":null,"abstract":"Abstract. In the Arctic, the sea surface salinity (SSS) plays a key role in processes related to water mixing and sea ice. However, the lack of salinity\u0000observations causes large uncertainties in Arctic Ocean forecasts and reanalysis. Recently the Soil Moisture and Ocean Salinity (SMOS) satellite\u0000mission was used by the Barcelona Expert Centre to develop an Arctic SSS product. In this study, we evaluate the impact of assimilating this data in\u0000a coupled ocean–ice data assimilation system. Using the deterministic ensemble Kalman filter from July to December 2016, two assimilation runs\u0000respectively assimilated two successive versions of the SMOS SSS product on top of a pre-existing reanalysis run. The runs were validated against\u0000independent in situ salinity profiles in the Arctic. The results show that the biases and the root-mean-squared differences (RMSD) of SSS are\u0000reduced by 10 % to 50 % depending on the area and highlight the importance of assimilating satellite salinity data. The time series of\u0000freshwater content (FWC) further shows that its seasonal cycle can be adjusted by assimilation of the SSS products, which is encouraging of the\u0000assimilation of SSS in a long-time reanalysis to better reproduce the Arctic water cycle.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"50 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-03-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85207051","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Alexandre Barboni, Solange Coadou-Chaventon, A. Stegner, B. Le Vu, F. Dumas
Abstract. The mixed layer is the uppermost layer of the ocean, connecting the atmosphere to the subsurface ocean through atmospheric fluxes. It is subject to pronounced seasonal variations: it deepens in winter due to buoyancy loss and shallows in spring while heat flux increases and restratifies the water column. A mixed-layer depth (MLD) modulation over this seasonal cycle has been observed within mesoscale eddies. Taking advantage of the numerous Argo floats deployed and trapped within large Mediterranean anticyclones over the last decades, we reveal for the first time this modulation at a 10 d temporal scale, free of the smoothing effect of composite approaches. The analysis of 16 continuous MLD time series inside 13 long-lived anticyclones at a fine temporal scale brings to light the importance of the eddy pre-existing vertical structure in setting the MLD modulation by mesoscale eddies. Extreme MLD anomalies of up to 330 m are observed when the winter mixed layer connects with a pre-existing subsurface anticyclonic core, greatly accelerating mixed-layer deepening. The winter MLD sometimes does not achieve such connection but homogenizes another subsurface layer, then forming a multi-core anticyclone with spring restratification. An MLD restratification delay is always observed, reaching more than 2 months in 3 out the 16 MLD time series. The water column starts to restratify outside anticyclones, while the mixed layer keeps deepening and cooling at the eddy core for a longer time. These new elements provide new keys for understanding anticyclone vertical-structure formation and evolution.
{"title":"How subsurface and double-core anticyclones intensify the winter mixed-layer deepening in the Mediterranean Sea","authors":"Alexandre Barboni, Solange Coadou-Chaventon, A. Stegner, B. Le Vu, F. Dumas","doi":"10.5194/os-19-229-2023","DOIUrl":"https://doi.org/10.5194/os-19-229-2023","url":null,"abstract":"Abstract. The mixed layer is the uppermost layer of the ocean, connecting the atmosphere to the subsurface ocean through atmospheric fluxes. It is subject to pronounced seasonal variations: it deepens in winter due to buoyancy loss and shallows in spring while heat flux increases and restratifies the water column. A mixed-layer depth (MLD) modulation over this seasonal cycle has been observed within mesoscale eddies. Taking advantage of the numerous Argo floats deployed and trapped within large Mediterranean anticyclones over the last decades, we reveal for the first time this modulation at a 10 d temporal scale, free of the smoothing effect of composite approaches. The analysis of 16 continuous MLD time series inside 13 long-lived anticyclones at a fine temporal scale brings to light the importance of the eddy pre-existing vertical structure in setting the MLD modulation by mesoscale eddies. Extreme MLD anomalies of up to 330 m are observed when the winter mixed layer connects with a pre-existing subsurface anticyclonic core, greatly accelerating mixed-layer deepening. The winter MLD sometimes does not achieve such connection but homogenizes another subsurface layer, then forming a multi-core anticyclone with spring restratification. An MLD restratification delay is always observed, reaching more than 2 months in 3 out the 16 MLD time series. The water column starts to restratify outside anticyclones, while the mixed layer keeps deepening and cooling at the eddy core for a longer time. These new elements provide new keys for understanding anticyclone vertical-structure formation and evolution.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"40 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-03-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76148576","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
D. M. Dimoune, F. Birol, F. Hernandez, F. Léger, M. Araújo
Abstract. Geostrophic currents derived from altimetry are used to investigate the surface circulation in the western tropical Atlantic over the 1993–2017 period. Using six horizontal sections defined to capture the current branches of the study area, we investigate their respective variations at both seasonal and interannual timescales, as well as the spatial distribution of these variations, in order to highlight the characteristics of the currents on their route. Our results show that the central branch of the South Equatorial Current and its northern branch near the Brazilian coast, the North Brazil Current component located south of the Equator, and the Guyana Current have similar annual cycles, with maxima (minima) during late boreal winter (boreal fall) when the Intertropical Convergence Zone is at its southernmost (northernmost) location. In contrast, the seasonal cycles of the North Brazil Current branch located between the Equator and 7–8∘ N, its retroflected branch, the northern branch of the South Equatorial Current to the west of 35∘ W, and the North Equatorial Countercurrent show maxima (minima) during late boreal summer (boreal spring), following the remote wind stress curl strength variation. West of 32∘ W, an eastward current (the Equatorial Surface Current, ESC) is observed between 2–2∘ N, identified as the equatorial extension of the retroflected branch of the North Brazil Current. It is part of a large cyclonic circulation observed between 0–6∘ N and 35–45∘ W during boreal spring. We also observed a secondary North Brazil Current retroflection flow during the second half of the year, which leads to the two-core structure of the North Equatorial Countercurrent and might be related to the wind stress curl seasonal changes. To the east, the North Equatorial Countercurrent weakens and its two-core structure is underdeveloped due to the weakening of the wind stress. At interannual scales, depending on the side of the Equator examined, the North Brazil Current exhibits two opposite scenarios related to the phases of the tropical Atlantic Meridional Mode. At 32∘ W, the interannual variability of the North Equatorial Countercurrent and of the northern branch of the South Equatorial Current (in terms of both strength and/or latitudinal shift) are associated with the Atlantic Meridional Mode, whereas the variability of the Equatorial Surface Current intensity is associated with both the Atlantic Meridional Mode and Atlantic Zonal Mode phases.
{"title":"Revisiting the tropical Atlantic western boundary circulation from a 25-year time series of satellite altimetry data","authors":"D. M. Dimoune, F. Birol, F. Hernandez, F. Léger, M. Araújo","doi":"10.5194/os-19-251-2023","DOIUrl":"https://doi.org/10.5194/os-19-251-2023","url":null,"abstract":"Abstract. Geostrophic currents derived from altimetry are used to\u0000investigate the surface circulation in the western tropical Atlantic over\u0000the 1993–2017 period. Using six horizontal sections defined to capture the\u0000current branches of the study area, we investigate their respective\u0000variations at both seasonal and interannual timescales, as well as the\u0000spatial distribution of these variations, in order to highlight the\u0000characteristics of the currents on their route. Our results show that the\u0000central branch of the South Equatorial Current and its northern branch near the\u0000Brazilian coast, the North Brazil Current component located south of the\u0000Equator, and the Guyana Current have similar annual cycles, with\u0000maxima (minima) during late boreal winter (boreal fall) when the Intertropical\u0000Convergence Zone is at its southernmost (northernmost) location. In contrast, the\u0000seasonal cycles of the North Brazil Current branch located between the\u0000Equator and 7–8∘ N, its retroflected branch, the northern branch\u0000of the South Equatorial Current to the west of 35∘ W, and the North\u0000Equatorial Countercurrent show maxima (minima) during late boreal\u0000summer (boreal spring), following the remote wind stress curl strength\u0000variation. West of 32∘ W, an eastward current (the Equatorial Surface Current, ESC) is observed between 2–2∘ N, identified\u0000as the equatorial extension of the retroflected branch of the North Brazil\u0000Current. It is part of a large cyclonic circulation observed between\u00000–6∘ N and 35–45∘ W during\u0000boreal spring. We also observed a secondary North Brazil Current\u0000retroflection flow during the second half of the year, which leads to the\u0000two-core structure of the North Equatorial Countercurrent and might be\u0000related to the wind stress curl seasonal changes. To the east, the North\u0000Equatorial Countercurrent weakens and its two-core structure is\u0000underdeveloped due to the weakening of the wind stress. At interannual\u0000scales, depending on the side of the Equator examined, the North Brazil Current\u0000exhibits two opposite scenarios related to the phases of the tropical\u0000Atlantic Meridional Mode. At 32∘ W, the interannual variability of\u0000the North Equatorial Countercurrent and of the northern branch of the South\u0000Equatorial Current (in terms of both strength and/or latitudinal shift) are\u0000associated with the Atlantic Meridional Mode, whereas the variability of the Equatorial Surface\u0000Current intensity is associated with both the Atlantic Meridional Mode and Atlantic Zonal\u0000Mode phases.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"80 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-03-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"73614642","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
P. Chapman, S. DiMarco, A. Knap, Antonietta Quigg, N. Walker
Abstract. Hurricane Harvey deposited over 90×109 m3 of rainwater over central Texas, USA, during late August/early September 2017. During four cruises (June, August, September and November 2017) we observed changes in hydrography and nutrient and oxygen concentrations in Texas coastal waters. Despite intense terrestrial runoff, nutrient supply to the coastal ocean was transient, with little phytoplankton growth observed and no hypoxia. Observations suggest this was probably related to the retention of nutrients in the coastal bays and rapid uptake by phytoplankton of nutrients washed out of the bays, as well as dilution by the sheer volume of rainwater and the lack of significant carbon reserves in the sediments, despite the imposition of a strong pycnocline. By the November cruise conditions had apparently returned to normal, and no long-term effects were observed.
{"title":"The effects of Hurricane Harvey on Texas coastal-zone chemistry","authors":"P. Chapman, S. DiMarco, A. Knap, Antonietta Quigg, N. Walker","doi":"10.5194/os-19-209-2023","DOIUrl":"https://doi.org/10.5194/os-19-209-2023","url":null,"abstract":"Abstract. Hurricane Harvey deposited over 90×109 m3 of rainwater over\u0000central Texas, USA, during late August/early September 2017. During four\u0000cruises (June, August, September and November 2017) we observed changes in\u0000hydrography and nutrient and oxygen concentrations in Texas coastal waters.\u0000Despite intense terrestrial runoff, nutrient supply to the coastal ocean was\u0000transient, with little phytoplankton growth observed and no hypoxia.\u0000Observations suggest this was probably related to the retention of nutrients\u0000in the coastal bays and rapid uptake by phytoplankton of nutrients washed out\u0000of the bays, as well as dilution by the sheer volume of rainwater and the\u0000lack of significant carbon reserves in the sediments, despite the imposition\u0000of a strong pycnocline. By the November cruise conditions had apparently\u0000returned to normal, and no long-term effects were observed.","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"13 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82072310","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract. Autonomous and expendable profiling-float arrays such as those deployed in the Argo Program require the transmission of reliable data from remote sites. However, existing satellite data transfer rates preclude complete transmission of rapidly sampled turbulence measurements. It is therefore necessary to reduce turbulence data on board. Here we propose a scheme for onboard data reduction and test it with existing turbulence data obtained with a modified SOLO-II profiling float. First, voltage spectra are derived from shear probe and fast-thermistor signals. Then, we focus on a fixed-frequency band that we know to be unaffected by vibrations and that approximately corresponds to a wavenumber band of 5–25 cpm. Over the fixed-frequency band, we make simple power law fits that – after calibration and correction in post-processing – yield values for the turbulent kinetic energy dissipation rate ϵ and thermal-variance dissipation rate χ. With roughly 1 m vertical segments, this scheme reduces the necessary data transfer volume 300-fold to approximately 2.5 kB for every 100 m of a profile (when profiling at 0.2 m s−1). As a test, we apply our scheme to a dataset comprising 650 profiles and compare its output to that from our standard turbulence-processing algorithm. For ϵ, values from the two approaches agree within a factor of 2 87 % of the time; for χ, they agree 78 % of the time. These levels of agreement are greater than or comparable to that between the ϵ and χ values derived from two shear probes and two fast thermistors, respectively, on the same profiler.
{"title":"A turbulence data reduction scheme for autonomous and expendable profiling floats","authors":"K. Hughes, J. Moum, D. Rudnick","doi":"10.5194/os-19-193-2023","DOIUrl":"https://doi.org/10.5194/os-19-193-2023","url":null,"abstract":"Abstract. Autonomous and expendable profiling-float arrays such as those deployed in the Argo Program require the transmission of reliable data from remote sites. However, existing satellite data transfer rates preclude complete transmission of rapidly sampled turbulence measurements. It is therefore necessary to reduce turbulence data on board. Here we propose a scheme for onboard data reduction and test it with existing turbulence data obtained with a modified SOLO-II profiling float. First, voltage spectra are derived from shear probe and fast-thermistor signals. Then, we focus on a fixed-frequency band that we know to be unaffected by vibrations and that approximately corresponds to a wavenumber band of 5–25 cpm. Over the fixed-frequency band, we make simple power law fits that – after calibration and correction in post-processing – yield values for the turbulent kinetic energy dissipation rate ϵ and thermal-variance dissipation rate χ. With roughly 1 m vertical segments, this scheme reduces the necessary data transfer volume 300-fold to approximately 2.5 kB for every 100 m of a profile (when profiling at 0.2 m s−1). As a test, we apply our scheme to a dataset comprising 650 profiles and compare its output to that from our standard turbulence-processing algorithm. For ϵ, values from the two approaches agree within a factor of 2 87 % of the time; for χ, they agree 78 % of the time. These levels of agreement are greater than or comparable to that between the ϵ and χ values derived from two shear probes and two fast thermistors, respectively, on the same profiler.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"23 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-02-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90550932","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
S. Jones, N. Fraser, S. Cunningham, A. Fox, M. Inall
Abstract. The Atlantic Meridional Overturning Circulation (AMOC) transports heat and salt between the tropical Atlantic and Arctic oceans. The interior of the North Atlantic subpolar gyre (SPG) is responsible for the much of the water mass transformation in the AMOC, and the export of this water to intensified boundary currents is crucial for projecting air–sea interaction onto the strength of the AMOC. However, the magnitude and location of exchange between the SPG and the boundary remains unclear. We present a novel climatology of the SPG boundary using quality-controlled CTD (conductivity–temperature–depth) and Argo hydrography, defining the SPG interior as the oceanic region bounded by 47∘ N and the 1000 m isobath. From this hydrography we find geostrophic flow out of the SPG around much of the boundary with minimal seasonality. The horizontal density gradient is reversed around western Greenland, where the geostrophic flow is into the SPG. Surface Ekman forcing drives net flow out of the SPG in all seasons with pronounced seasonality, varying between 2.45 ± 0.73 Sv in the summer and 7.70 ± 2.90 Sv in the winter. We estimate heat advected into the SPG to be between 0.14 ± 0.05 PW in the winter and 0.23 ± 0.05 PW in the spring, and freshwater advected out of the SPG to be between 0.07 ± 0.02 Sv in the summer and 0.15 ± 0.02 Sv in the autumn. These estimates approximately balance the surface heat and freshwater fluxes over the SPG domain. Overturning in the SPG varies seasonally, with a minimum of 6.20 ± 1.40 Sv in the autumn and a maximum of 10.17 ± 1.91 Sv in the spring, with surface Ekman the most likely mediator of this variability. The density of maximum overturning is at 27.30 kg m−3, with a second, smaller maximum at 27.54 kg m−3. Upper waters (σ0<27.30 kg m−3) are transformed in the interior then exported as either intermediate water (27.30–27.54 kg m−3) in the North Atlantic Current (NAC) or as dense water (σ0>27.54 kg m−3) exiting to the south. Our results support the present consensus that the formation and pre-conditioning of Subpolar Mode Water in the north-eastern Atlantic is a key determinant of AMOC strength.
摘要大西洋经向翻转环流(AMOC)在热带大西洋和北冰洋之间输送热量和盐。北大西洋次极环流(SPG)的内部负责AMOC的大部分水团转换，而这些水向强化的边界流的输出对于将海气相互作用投射到AMOC的强度上至关重要。然而，火炮和边界之间的交换幅度和位置仍然不清楚。我们利用质量控制的CTD(电导率-温度-深度)和Argohydrography提出了SPG边界的新气候学，将SPG内部定义为以47°N和1000米等深线为界的海洋区域。从这个水文图中，我们发现从SPG流出的地转流在大部分边界周围具有最小的季节性。水平密度梯度在西格陵兰岛周围反转，在那里地转流进入SPG。地表Ekman强迫在所有季节驱动SPG净流量，具有明显的季节性，夏季为2.45±0.73 Sv，冬季为7.70±2.90 Sv。冬季平流进入SPG的热量为0.14±0.05 PW，春季为0.23±0.05 PW;夏季平流出SPG的淡水为0.07±0.02 Sv，秋季为0.15±0.02 Sv。这些估计值大致平衡了SPG域上的地表热量和淡水通量。SPG的翻转有季节变化，秋季最小为6.20±1.40 Sv，春季最大为10.17±1.91 Sv，地表Ekman最有可能是这种变化的中介。最大倾覆密度为27.30 kg m - 3，第二个较小的最大值为27.54 kg m - 3。上游水(σ027.54 kg m−3)向南流出。我们的结果支持目前的共识，即大西洋东北部亚极模态水的形成和预处理是AMOC强度的关键决定因素。
{"title":"Observation-based estimates of volume, heat, and freshwater exchanges between the subpolar North Atlantic interior, its boundary currents, and the atmosphere","authors":"S. Jones, N. Fraser, S. Cunningham, A. Fox, M. Inall","doi":"10.5194/os-19-169-2023","DOIUrl":"https://doi.org/10.5194/os-19-169-2023","url":null,"abstract":"Abstract. The Atlantic Meridional Overturning Circulation (AMOC) transports heat and\u0000salt between the tropical Atlantic and Arctic oceans. The interior of the\u0000North Atlantic subpolar gyre (SPG) is responsible for the much of the water\u0000mass transformation in the AMOC, and the export of this water to intensified\u0000boundary currents is crucial for projecting air–sea interaction onto the\u0000strength of the AMOC. However, the magnitude and location of exchange\u0000between the SPG and the boundary remains unclear. We present a novel\u0000climatology of the SPG boundary using quality-controlled CTD (conductivity–temperature–depth) and Argo\u0000hydrography, defining the SPG interior as the oceanic region bounded by\u000047∘ N and the 1000 m isobath. From this hydrography we find\u0000geostrophic flow out of the SPG around much of the boundary with minimal\u0000seasonality. The horizontal density gradient is reversed around western\u0000Greenland, where the geostrophic flow is into the SPG. Surface Ekman forcing\u0000drives net flow out of the SPG in all seasons with pronounced seasonality,\u0000varying between 2.45 ± 0.73 Sv in the summer and 7.70 ± 2.90 Sv\u0000in the winter. We estimate heat advected into the SPG to be between 0.14 ± 0.05 PW in the winter and 0.23 ± 0.05 PW in the spring, and\u0000freshwater advected out of the SPG to be between 0.07 ± 0.02 Sv in the\u0000summer and 0.15 ± 0.02 Sv in the autumn. These estimates approximately\u0000balance the surface heat and freshwater fluxes over the SPG domain.\u0000Overturning in the SPG varies seasonally, with a minimum of 6.20 ± 1.40 Sv in the autumn and a maximum of 10.17 ± 1.91 Sv in the spring,\u0000with surface Ekman the most likely mediator of this variability. The density\u0000of maximum overturning is at 27.30 kg m−3, with a second, smaller\u0000maximum at 27.54 kg m−3. Upper waters (σ0<27.30 kg m−3) are transformed in the interior then exported as either\u0000intermediate water (27.30–27.54 kg m−3) in the North Atlantic Current\u0000(NAC) or as dense water (σ0>27.54 kg m−3)\u0000exiting to the south. Our results support the present consensus that the\u0000formation and pre-conditioning of Subpolar Mode Water in the north-eastern\u0000Atlantic is a key determinant of AMOC strength.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"35 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-02-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"78969318","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract. Increasing Greenland Ice Sheet melting is anticipated to impact water mass transformation in the subpolar North Atlantic and ultimately the meridional overturning circulation. Complex ocean and climate models are widely applied to estimate magnitude and timing of related impacts under global warming. We discuss the role of the ocean mean state, subpolar water mass transformation, mesoscale eddies, and atmospheric coupling in shaping the response of the subpolar North Atlantic Ocean to enhanced Greenland runoff. In a suite of eight dedicated 60- to 100-year-long model experiments with and without atmospheric coupling, with eddy processes parameterized and explicitly simulated and with regular and significantly enlarged Greenland runoff, we find (1) a major impact by the interactive atmosphere in enabling a compensating temperature feedback, (2) a non-negligible influence by the ocean mean state biased towards greater stability in the coupled simulations, both of which make the Atlantic meridional overturning circulation less susceptible to the freshwater perturbation applied, and (3) a more even spreading and deeper mixing of the runoff tracer in the subpolar North Atlantic and enhanced inter-gyre exchange with the subtropics in the strongly eddying simulations. Overall, our experiments demonstrate the important role of mesoscale ocean dynamics and atmosphere feedback in projections of the climate system response to enhanced Greenland Ice Sheet melting and hence underline the necessity to advance scale-aware eddy parameterizations for next-generation climate models.
{"title":"On the ocean's response to enhanced Greenland runoff in model experiments: relevance of mesoscale dynamics and atmospheric coupling","authors":"T. Martin, A. Biastoch","doi":"10.5194/os-19-141-2023","DOIUrl":"https://doi.org/10.5194/os-19-141-2023","url":null,"abstract":"Abstract. Increasing Greenland Ice Sheet melting is anticipated to impact water mass transformation in the subpolar North Atlantic and ultimately the meridional overturning circulation. Complex ocean and climate models are widely applied to estimate magnitude and timing of related impacts under global warming.\u0000We discuss the role of the ocean mean state, subpolar water mass transformation, mesoscale eddies, and atmospheric coupling in shaping the response of the subpolar North Atlantic Ocean to enhanced Greenland runoff.\u0000In a suite of eight dedicated 60- to 100-year-long model experiments with and without atmospheric coupling, with eddy processes parameterized and explicitly simulated and with regular and significantly enlarged Greenland runoff, we find\u0000(1) a major impact by the interactive atmosphere in enabling a compensating temperature feedback,\u0000(2) a non-negligible influence by the ocean mean state biased towards greater stability in the coupled simulations,\u0000both of which make the Atlantic meridional overturning circulation less susceptible to the freshwater perturbation applied, and\u0000(3) a more even spreading and deeper mixing of the runoff tracer in the subpolar North Atlantic and enhanced inter-gyre exchange with the subtropics in the strongly eddying simulations.\u0000Overall, our experiments demonstrate the important role of mesoscale ocean dynamics and atmosphere feedback in projections of the climate system response to enhanced Greenland Ice Sheet melting and hence underline the necessity to advance scale-aware eddy parameterizations for next-generation climate models.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"36 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-02-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"87267915","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract. The Angolan shelf system represents a highly productive ecosystem. Throughout the year the sea surface is cooler near the coast than further offshore. The lowest sea surface temperature (SST), strongest cross-shore temperature gradient, and maximum productivity occur in austral winter when seasonally prevailing upwelling-favourable winds are weakest. Here, we investigate the seasonal mixed layer heat budget to identify atmospheric and oceanic causes for heat content variability. By using different satellite and in situ data, we derive monthly estimates of surface heat fluxes, mean horizontal advection, and local heat content change. We calculate the heat budgets for the near-coastal and offshore regions separately to explore processes that lead to the observed SST differences. The results show that the net surface heat flux warms the coastal ocean stronger than further offshore, thus acting to damp spatial SST differences. Mean horizontal heat advection is dominated by meridional advection of warm water along the Angolan coast. However, its contribution to the heat budget is small. Ocean turbulence data suggest that the heat flux, due to turbulent mixing across the base of the mixed layer, is an important cooling term. This turbulent cooling, being strongest in shallow shelf regions, is capable of explaining the observed negative cross-shore temperature gradient. The residuum of the mixed layer heat budget and uncertainties of budget terms are discussed.
{"title":"Seasonal cycle of sea surface temperature in the tropical Angolan Upwelling System","authors":"Mareike Körner, P Brandt, M. Dengler","doi":"10.5194/os-19-121-2023","DOIUrl":"https://doi.org/10.5194/os-19-121-2023","url":null,"abstract":"Abstract. The Angolan shelf system represents a highly productive\u0000ecosystem. Throughout the year the sea surface is cooler near the coast than\u0000further offshore. The lowest sea surface temperature (SST), strongest\u0000cross-shore temperature gradient, and maximum productivity occur in austral\u0000winter when seasonally prevailing upwelling-favourable winds are weakest.\u0000Here, we investigate the seasonal mixed layer heat budget to identify\u0000atmospheric and oceanic causes for heat content variability. By using\u0000different satellite and in situ data, we derive monthly estimates of surface\u0000heat fluxes, mean horizontal advection, and local heat content change. We\u0000calculate the heat budgets for the near-coastal and offshore regions\u0000separately to explore processes that lead to the observed SST differences.\u0000The results show that the net surface heat flux warms the coastal ocean\u0000stronger than further offshore, thus acting to damp spatial SST differences.\u0000Mean horizontal heat advection is dominated by meridional advection of warm\u0000water along the Angolan coast. However, its contribution to the heat budget\u0000is small. Ocean turbulence data suggest that the heat flux, due to turbulent\u0000mixing across the base of the mixed layer, is an important cooling term.\u0000This turbulent cooling, being strongest in shallow shelf regions, is capable\u0000of explaining the observed negative cross-shore temperature gradient. The\u0000residuum of the mixed layer heat budget and uncertainties of budget terms\u0000are discussed.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"84 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-01-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89320677","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
S. Widdicombe, K. Isensee, Y. Artioli, J. Gaitán‐Espitia, C. Hauri, J. Newton, M. Wells, S. Dupont
Abstract. Approximately one-quarter of the CO2 emitted to the atmosphere annually from human activities is absorbed by the ocean, resulting in a reduction of seawater pH and shifts in seawater carbonate chemistry. This multi-decadal process, termed “anthropogenic ocean acidification” (OA), has been shown to have detrimental impacts on marine ecosystems. Recent years have seen a globally coordinated effort to measure the changes in seawater chemistry caused by OA, with best practices now available for these measurements. In contrast to these substantial advances in observing physicochemical changes due to OA, quantifying their biological consequences remains challenging, especially from in situ observations under real-world conditions. Results from 2 decades of controlled laboratory experiments on OA have given insight into the likely processes and mechanisms by which elevated CO2 levels affect biological process, but the manifestation of these process across a plethora of natural situations has yet to be fully explored. This challenge requires us to identify a set of fundamental biological and ecological indicators that are (i) relevant across all marine ecosystems, (ii) have a strongly demonstrated link to OA, and (iii) have implications for ocean health and the provision of ecosystem services with impacts on local marine management strategies and economies. This paper draws on the understanding of biological impacts provided by the wealth of previous experiments, as well as the findings of recent meta-analyses, to propose five broad classes of biological indicators that, when coupled with environmental observations including carbonate chemistry, would allow the rate and severity of biological change in response to OA to be observed and compared. These broad indicators are applicable to different ecological systems, and the methods for data analysis suggested here would allow researchers to combine biological response data across regional and global scales by correlating rates of biological change with the rate of change in carbonate chemistry parameters. Moreover, a method using laboratory observation to design an optimal observing strategy (frequency and duration) and observe meaningful biological rates of change highlights the factors that need to be considered when applying our proposed observation strategy. This innovative observing methodology allows inclusion of a wide diversity of marine ecosystems in regional and global assessments and has the potential to increase the contribution of OA observations from countries with developing OA science capacity.
{"title":"Unifying biological field observations to detect and compare ocean acidification impacts across marine species and ecosystems: what to monitor and why","authors":"S. Widdicombe, K. Isensee, Y. Artioli, J. Gaitán‐Espitia, C. Hauri, J. Newton, M. Wells, S. Dupont","doi":"10.5194/os-19-101-2023","DOIUrl":"https://doi.org/10.5194/os-19-101-2023","url":null,"abstract":"Abstract. Approximately one-quarter of the CO2 emitted to the\u0000atmosphere annually from human activities is absorbed by the ocean,\u0000resulting in a reduction of seawater pH and shifts in seawater carbonate\u0000chemistry. This multi-decadal process, termed “anthropogenic ocean\u0000acidification” (OA), has been shown to have detrimental impacts on marine\u0000ecosystems. Recent years have seen a globally coordinated effort to measure\u0000the changes in seawater chemistry caused by OA, with best practices now\u0000available for these measurements. In contrast to these substantial advances\u0000in observing physicochemical changes due to OA, quantifying their\u0000biological consequences remains challenging, especially from in situ\u0000observations under real-world conditions. Results from 2 decades of\u0000controlled laboratory experiments on OA have given insight into the likely\u0000processes and mechanisms by which elevated CO2 levels affect biological\u0000process, but the manifestation of these process across a plethora of natural\u0000situations has yet to be fully explored. This challenge requires us to\u0000identify a set of fundamental biological and ecological indicators that are\u0000(i) relevant across all marine ecosystems, (ii) have a strongly demonstrated\u0000link to OA, and (iii) have implications for ocean health and the provision of\u0000ecosystem services with impacts on local marine management strategies and\u0000economies. This paper draws on the understanding of biological impacts\u0000provided by the wealth of previous experiments, as well as the findings of\u0000recent meta-analyses, to propose five broad classes of biological indicators\u0000that, when coupled with environmental observations including carbonate\u0000chemistry, would allow the rate and severity of biological change in\u0000response to OA to be observed and compared. These broad indicators are\u0000applicable to different ecological systems, and the methods for data\u0000analysis suggested here would allow researchers to combine biological\u0000response data across regional and global scales by correlating rates of\u0000biological change with the rate of change in carbonate chemistry parameters.\u0000Moreover, a method using laboratory observation to design an optimal\u0000observing strategy (frequency and duration) and observe meaningful\u0000biological rates of change highlights the factors that need to be considered\u0000when applying our proposed observation strategy. This innovative observing\u0000methodology allows inclusion of a wide diversity of marine ecosystems in\u0000regional and global assessments and has the potential to increase the\u0000contribution of OA observations from countries with developing OA science\u0000capacity.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"28 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-01-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89668641","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract. The seminal Ekman (1905) f-plane theory of wind-driven transport at the ocean surface is extended to the β plane by substituting the pseudo-angular momentum for the zonal velocity in the Lagrangian equation. When the β term is added, the equations become nonlinear, which greatly complicates the analysis. Though rotation relates the momentum equations in the zonal and the meridional directions, the transformation to pseudo-angular momentum greatly simplifies the longitudinal dynamics, which yields a clear description of the meridional dynamics in terms of a slow drift compounded by fast oscillations; this can then be applied to describe the motion in the zonal direction. Both analytical expressions and numerical calculations highlight the critical role of the Equator in determining the trajectories of water columns forced by eastward-directed (in the Northern Hemisphere) wind stress even when the water columns are initiated far from the Equator. Our results demonstrate that the averaged motion in the zonal direction depends on the amplitude of the meridional oscillations and is independent of the direction of the wind stress. The zonal drift is determined by a balance between the initial conditions and the magnitude of the wind stress, so it can be as large as the mean meridional motion; i.e., the averaged flow direction is not necessarily perpendicular to the wind direction.
{"title":"Extension of Ekman (1905) wind-driven transport theory to the β plane","authors":"N. Paldor, L. Friedland","doi":"10.5194/os-19-93-2023","DOIUrl":"https://doi.org/10.5194/os-19-93-2023","url":null,"abstract":"Abstract. The seminal Ekman (1905) f-plane theory of wind-driven\u0000transport at the ocean surface is extended to the β plane by\u0000substituting the pseudo-angular momentum for the zonal velocity in\u0000the Lagrangian equation. When the β term is added, the\u0000equations become nonlinear, which greatly complicates the analysis.\u0000Though rotation relates the momentum equations in the zonal and the\u0000meridional directions, the transformation to pseudo-angular momentum\u0000greatly simplifies the longitudinal dynamics, which yields a clear\u0000description of the meridional dynamics in terms of a slow drift\u0000compounded by fast oscillations; this can then be applied to\u0000describe the motion in the zonal direction. Both analytical\u0000expressions and numerical calculations highlight the critical role\u0000of the Equator in determining the trajectories of water columns\u0000forced by eastward-directed (in the Northern Hemisphere) wind stress\u0000even when the water columns are initiated far from the Equator. Our\u0000results demonstrate that the averaged motion in the zonal direction\u0000depends on the amplitude of the meridional oscillations and is\u0000independent of the direction of the wind stress. The zonal drift is\u0000determined by a balance between the initial conditions and the\u0000magnitude of the wind stress, so it can be as large as the mean\u0000meridional motion; i.e., the averaged flow direction is not\u0000necessarily perpendicular to the wind direction.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"4 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-01-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88591892","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"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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