Abstract. The South Vietnam upwelling (SVU) develops off the Vietnamese coast (South China Sea, SCS) during the southwest summer monsoon over four main areas: the northern coastal upwelling (NCU), the southern coastal upwelling (SCU), the offshore upwelling (OFU) and the shelf off the Mekong River mouth (MKU). An ensemble of 10 simulations with perturbed initial conditions were run with the fine-resolution SYMPHONIE model (1 km inshore) to investigate the daily to intraseasonal variability of the SVU and the influence of the ocean intrinsic variability (OIV) during the strong SVU of summer 2018. The intraseasonal variability is similar for the SCU, MKU and OFU, driven to the first order by the wind variability. The MKU and SCU are induced by stable ocean dynamics (the northeastward then eastward boundary current) and have very little chaotic variability. The OIV has a stronger influence on OFU. In July, OFU mainly develops along the northern flank of the eastward jet. The influence of the OIV is strongest and related to the chaotic variability of the meridional position of the jet. In August, this position is stable and OFU develops mainly in the area of positive wind curl and cyclonic eddies north of the jet. The influence of the OIV, weaker than in July, is related to the organization of this mesoscale circulation. The NCU shows a behavior different from that observed in the other areas. In the heart of summer, a large-scale circulation formed by the eastward jet and eddy dipole is well established with an alongshore current preventing the NCU development. In early and late summer, this circulation is weaker, allowing a mesoscale circulation of strongly chaotic nature to develop in the NCU area. During those periods, the OIV influence on the NCU is very strong and related to the organization of this mesoscale circulation: the NCU is favored (annihilated) by offshore-oriented (alongshore) structures.
{"title":"Intraseasonal variability of the South Vietnam upwelling, South China Sea: influence of atmospheric forcing and ocean intrinsic variability","authors":"M. Herrmann, Thai To Duy, C. Estournel","doi":"10.5194/os-19-453-2023","DOIUrl":"https://doi.org/10.5194/os-19-453-2023","url":null,"abstract":"Abstract. The South Vietnam upwelling (SVU) develops off the Vietnamese coast (South\u0000China Sea, SCS) during the southwest summer monsoon over four main areas: the\u0000northern coastal upwelling (NCU), the southern coastal upwelling (SCU), the\u0000offshore upwelling (OFU) and the shelf off the Mekong River mouth (MKU). An\u0000ensemble of 10 simulations with perturbed initial conditions were run with\u0000the fine-resolution SYMPHONIE model (1 km inshore) to investigate the daily\u0000to intraseasonal variability of the SVU and the influence of the ocean\u0000intrinsic variability (OIV) during the strong SVU of summer 2018. The intraseasonal variability is similar for the SCU, MKU and OFU, driven to the\u0000first order by the wind variability. The MKU and SCU are induced by stable ocean\u0000dynamics (the northeastward then eastward boundary current) and have very\u0000little chaotic variability. The OIV has a stronger influence on OFU. In\u0000July, OFU mainly develops along the northern flank of the eastward jet. The\u0000influence of the OIV is strongest and related to the chaotic variability of the\u0000meridional position of the jet. In August, this position is stable and OFU\u0000develops mainly in the area of positive wind curl and cyclonic eddies north\u0000of the jet. The influence of the OIV, weaker than in July, is related to the\u0000organization of this mesoscale circulation. The NCU shows a behavior different\u0000from that observed in the other areas. In the heart of summer, a large-scale\u0000circulation formed by the eastward jet and eddy dipole is well established\u0000with an alongshore current preventing the NCU development. In early and late\u0000summer, this circulation is weaker, allowing a mesoscale circulation of\u0000strongly chaotic nature to develop in the NCU area. During those periods,\u0000the OIV influence on the NCU is very strong and related to the organization of\u0000this mesoscale circulation: the NCU is favored (annihilated) by\u0000offshore-oriented (alongshore) structures.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"38 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-04-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76822481","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. We use output from a freely running NEMO model simulation for the equatorial Pacific to investigate the utility of linearly removing the local influence of vertical displacements of the thermocline from variations in sea surface height. We show that the resulting time series of residual sea surface height, denoted ηnlti, measures variations in near-surface heat content that are independent of the local vertical displacement of the thermocline and can arise from horizontal advection, surface heat flux, and diapycnal mixing processes. We find that the variance of ηnlti and its correlation with sea surface temperature are focused on the Niño4 region. Furthermore, ηnlti averaged over the Niño4 region is highly correlated with indices of central Pacific El Niño–Southern Oscillation (CP ENSO), and its variance in 21-year running windows shows a strong upward trend over the past 50 years, corresponding to the emergence of CP ENSO following the 1976/77 climate shift. We show that ηnlti can be estimated from observations, using satellite altimeter data and a linear multi-mode model. The time series of ηnlti, especially when estimated using the linear model, show pronounced westward propagation in the western equatorial Pacific, arguing for an important role for zonal advective feedback in the dynamics of CP ENSO, in particular for cold events. We also present evidence that the role of the thermocline displacement in influencing sea surface height increased strongly after 2000 in the eastern part of the Niño4 region, at a time when CP ENSO was particularly active. Finally, the diagnostic is easy to compute and can be easily applied to mooring data or coupled climate models.
{"title":"A simple diagnostic based on sea surface height with an application to central Pacific ENSO","authors":"Jufen Lai, R. Greatbatch, M. Claus","doi":"10.5194/os-19-421-2023","DOIUrl":"https://doi.org/10.5194/os-19-421-2023","url":null,"abstract":"Abstract. We use output from a freely running NEMO model simulation for the equatorial Pacific to investigate the utility of linearly removing the local influence of vertical displacements of the thermocline from variations in sea surface height. We show that the resulting time series of residual sea surface height, denoted ηnlti, measures variations in near-surface heat content that are independent of the local vertical displacement of the thermocline and can arise from horizontal advection, surface heat flux, and diapycnal mixing processes. We find that the variance of ηnlti and its correlation with sea surface temperature are focused on the Niño4 region. Furthermore, ηnlti averaged over the Niño4 region is highly correlated with indices of central Pacific El Niño–Southern Oscillation (CP ENSO), and its variance in 21-year running windows shows a strong upward trend over the past 50 years, corresponding to the emergence of CP ENSO following the 1976/77 climate shift. We show that ηnlti can be estimated from observations, using satellite altimeter data and a linear multi-mode model. The time series of ηnlti, especially when estimated using the linear model, show pronounced westward propagation in the western equatorial Pacific, arguing for an important role for zonal advective feedback in the dynamics of CP ENSO, in particular for cold events. We also present evidence that the role of the thermocline displacement in influencing sea surface height increased strongly after 2000 in the eastern part of the Niño4 region, at a time when CP ENSO was particularly active. Finally, the diagnostic is easy to compute and can be easily applied to mooring data or coupled climate models.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"64 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-04-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80836088","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}
E. Møller, A. Christensen, J. Larsen, K. Mankoff, M. Ribergaard, Mikael K. Sejr, P. Wallhead, M. Maar
Abstract. The Greenland ice sheet is melting, and the rate of ice loss has increased 6-fold since the 1980s. At the same time, the Arctic sea ice extent is decreasing. Meltwater runoff and sea ice reduction both influence light and nutrient availability in the coastal ocean, with implications for the timing, distribution, and magnitude of phytoplankton production. However, the integrated effect of both glacial and sea ice melt is highly variable in time and space, making it challenging to quantify. In this study, we evaluate the relative importance of these processes for the primary productivity of Disko Bay, west Greenland, one of the most important areas for biodiversity and fisheries around Greenland. We use a high-resolution 3D coupled hydrodynamic–biogeochemical model for 2004–2018 validated against in situ observations and remote sensing products. The model-estimated net primary production (NPP) varied between 90–147 gC m−2 yr−1 during 2004–2018, a period with variable freshwater discharges and sea ice cover. NPP correlated negatively with sea ice cover and positively with freshwater discharge. Freshwater discharge had a strong local effect within ∼ 25 km of the source-sustaining productive hot spots during summer. When considering the annual NPP at bay scale, sea ice cover was the most important controlling factor. In scenarios with no sea ice in spring, the model predicted a ∼ 30 % increase in annual production compared to a situation with high sea ice cover. Our study indicates that decreasing ice cover and more freshwater discharge can work synergistically and will likely increase primary productivity of the coastal ocean around Greenland.
{"title":"The sensitivity of primary productivity in Disko Bay, a coastal Arctic ecosystem, to changes in freshwater discharge and sea ice cover","authors":"E. Møller, A. Christensen, J. Larsen, K. Mankoff, M. Ribergaard, Mikael K. Sejr, P. Wallhead, M. Maar","doi":"10.5194/os-19-403-2023","DOIUrl":"https://doi.org/10.5194/os-19-403-2023","url":null,"abstract":"Abstract. The Greenland ice sheet is melting, and the rate of ice\u0000loss has increased 6-fold since the 1980s. At the same time, the Arctic sea\u0000ice extent is decreasing. Meltwater runoff and sea ice reduction both\u0000influence light and nutrient availability in the coastal ocean, with\u0000implications for the timing, distribution, and magnitude of phytoplankton\u0000production. However, the integrated effect of both glacial and sea ice melt\u0000is highly variable in time and space, making it challenging to quantify. In\u0000this study, we evaluate the relative importance of these processes for the\u0000primary productivity of Disko Bay, west Greenland, one of the most important\u0000areas for biodiversity and fisheries around Greenland. We use a\u0000high-resolution 3D coupled hydrodynamic–biogeochemical model for 2004–2018 validated against in situ observations and remote sensing products. The model-estimated net primary production (NPP) varied between 90–147 gC m−2 yr−1 during 2004–2018, a period with variable freshwater discharges\u0000and sea ice cover. NPP correlated negatively with sea ice cover and\u0000positively with freshwater discharge. Freshwater discharge had a strong\u0000local effect within ∼ 25 km of the source-sustaining productive hot\u0000spots during summer. When considering the annual NPP at bay scale, sea ice\u0000cover was the most important controlling factor. In scenarios with no sea\u0000ice in spring, the model predicted a ∼ 30 % increase in annual\u0000production compared to a situation with high sea ice cover. Our study\u0000indicates that decreasing ice cover and more freshwater discharge can work\u0000synergistically and will likely increase primary productivity of the coastal\u0000ocean around Greenland.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"37 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-04-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77742928","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}
Shanice Bailey, S. Jones, R. Abernathey, A. Gordon, X. Yuan
Abstract. This study investigates the variability of water mass transformation (WMT) within the Weddell Gyre (WG). The WG serves as a pivotal site for the Meridional Overturning Circulation (MOC) and ocean ventilation because it is the primary origin of the largest volume of water mass in the global ocean: Antarctic Bottom Water (AABW). Recent mooring data suggest substantial seasonal and interannual variability of AABW properties exiting the WG, and studies have linked the variability to the large-scale climate forcings affecting wind stress in the WG region. However, the specific thermodynamic mechanisms that link variability in surface forcings to variability in water mass transformations and AABW export remain unclear. This study explores how current state-of-the-art data-assimilating ocean reanalyses can help fill the gaps in our understanding of the thermodynamic drivers of AABW variability in the WG via WMT volume budgets derived from Walin's classic WMT framework. The three ocean reanalyses used are the following: Estimating the Circulation and Climate of the Ocean state estimate (ECCOv4), Southern Ocean State Estimate (SOSE) and Simple Ocean Data Assimilation (SODA). From the model outputs, we diagnose a closed form of the water mass budget for AABW that explicitly accounts for transport across the WG boundary, surface forcing, interior mixing and numerical mixing. We examine the annual mean climatology of the WMT budget terms, the seasonal climatology and finally the interannual variability. Our finding suggests that the relatively coarse resolution of these models did not realistically capture AABW formation, export and variability. In ECCO and SOSE, we see strong interannual variability in AABW volume budget. In SOSE, we find an accelerating loss of AABW during 2005–2010, driven largely by interior mixing and changes in surface salt fluxes. ECCO shows a similar trend during a 4-year time period starting in late 2007 but also reveals such trends to be part of interannual variability over a much longer time period. Overall, ECCO provides the most useful time series for understanding the processes and mechanisms that drive WMT and export variability in the WG. SODA, in contrast, displays unphysically large variability in AABW volume, which we attribute to its data assimilation scheme. We also examine correlations between the WMT budgets and large-scale climate indices, including El Niño–Southern Oscillation (ENSO) and Southern Annular Mode (SAM), and find no strong relationships.
{"title":"Water mass transformation variability in the Weddell Sea in ocean reanalyses","authors":"Shanice Bailey, S. Jones, R. Abernathey, A. Gordon, X. Yuan","doi":"10.5194/os-19-381-2023","DOIUrl":"https://doi.org/10.5194/os-19-381-2023","url":null,"abstract":"Abstract. This study investigates the variability of water mass transformation (WMT) within the Weddell Gyre (WG).\u0000The WG serves as a pivotal site for the Meridional Overturning Circulation (MOC) and ocean ventilation because it is the primary origin of the largest volume of water mass in the global ocean: Antarctic Bottom Water (AABW).\u0000Recent mooring data suggest substantial seasonal and interannual variability of AABW properties exiting the WG, and studies have linked the variability to the large-scale climate forcings affecting wind stress in the WG region.\u0000However, the specific thermodynamic mechanisms that link variability in surface forcings to variability in water mass transformations and AABW export remain unclear.\u0000This study explores how current state-of-the-art data-assimilating ocean reanalyses can help fill the gaps in our understanding of the thermodynamic drivers of AABW variability in the WG via WMT volume budgets derived from Walin's classic WMT framework. The three ocean reanalyses used are the following: Estimating the Circulation and Climate of the Ocean state estimate (ECCOv4), Southern Ocean State Estimate (SOSE) and Simple Ocean Data Assimilation (SODA).\u0000From the model outputs, we diagnose a closed form of the water mass budget for AABW that explicitly accounts for transport across the WG boundary, surface forcing, interior mixing and numerical mixing.\u0000We examine the annual mean climatology of the WMT budget terms, the seasonal climatology and finally the interannual variability. Our finding suggests that the relatively coarse resolution of these models did not realistically capture AABW formation, export and variability.\u0000In ECCO and SOSE, we see strong interannual variability in AABW volume budget.\u0000In SOSE, we find an accelerating loss of AABW during 2005–2010, driven largely by interior mixing and changes in surface salt fluxes.\u0000ECCO shows a similar trend during a 4-year time period starting in late 2007 but also reveals such trends to be part of interannual variability over a much longer time period.\u0000Overall, ECCO provides the most useful time series for understanding the processes and mechanisms that drive WMT and export variability in the WG.\u0000SODA, in contrast, displays unphysically large variability in AABW volume, which we attribute to its data assimilation scheme.\u0000We also examine correlations between the WMT budgets and large-scale climate indices, including El Niño–Southern Oscillation (ENSO) and Southern Annular Mode (SAM), and find no strong relationships.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"64 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-04-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86881468","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}
O. Vergara, R. Morrow, M. Pujol, G. Dibarboure, C. Ubelmann
Abstract. The ocean's sea surface height (SSH) field is a complex mix of motions in geostrophic balance and unbalanced motions including high-frequency tides, internal tides, and internal gravity waves. Barotropic tides are well estimated for altimetric SSH in the open ocean, but the SSH signals of internal tides remain. The transition scale, Lt, at which these unbalanced ageostrophic motions dominate balanced geostrophic motions is estimated for the first time using satellite altimetry. Lt is critical to define the spatial scales above which surface geostrophic currents can be inferred from SSH gradients. We use a statistical approach based on the analysis of 1 Hz altimetric SSH wavenumber spectra to obtain four geophysical parameters that vary regionally and seasonally: the background error, the spectral slope in the mesoscale range, a second spectral slope at smaller scales, and Lt. The mesoscale slope and error levels are similar to previous studies based on satellite altimetry. The break in the wavenumber spectra to a flatter spectral slope can only be estimated in midlatitude regions where the signal exceeds the altimetric noise level. Small values of Lt are observed in regions of energetic mesoscale activity, while larger values are observed towards low latitudes and regions of lower mesoscale activity. These results are consistent with recent analyses of in situ observations and high-resolution models. Limitations of our results and implications for reprocessed nadir and future swath altimetric missions are discussed.
{"title":"Global submesoscale diagnosis using along-track satellite altimetry","authors":"O. Vergara, R. Morrow, M. Pujol, G. Dibarboure, C. Ubelmann","doi":"10.5194/os-19-363-2023","DOIUrl":"https://doi.org/10.5194/os-19-363-2023","url":null,"abstract":"Abstract. The ocean's sea surface height (SSH) field is a complex mix of motions in\u0000geostrophic balance and unbalanced motions including high-frequency tides,\u0000internal tides, and internal gravity waves. Barotropic tides are well\u0000estimated for altimetric SSH in the open ocean, but the SSH signals of\u0000internal tides remain. The transition scale,\u0000Lt, at which these unbalanced\u0000ageostrophic motions dominate balanced geostrophic motions is estimated\u0000for\u0000the first time using satellite altimetry. Lt is critical to\u0000define the\u0000spatial scales above which surface geostrophic currents can be inferred\u0000from\u0000SSH gradients. We use a statistical approach based on the analysis of 1 Hz\u0000altimetric SSH wavenumber spectra to obtain four geophysical parameters\u0000that\u0000vary regionally and seasonally: the background error, the spectral slope in\u0000the mesoscale range, a second spectral slope at smaller scales, and\u0000Lt. The\u0000mesoscale slope and error levels are similar to previous studies based on\u0000satellite altimetry. The break in the wavenumber spectra to a flatter\u0000spectral slope can only be estimated in midlatitude regions where the\u0000signal exceeds the altimetric noise level. Small values of Lt\u0000are observed\u0000in regions of energetic mesoscale activity, while larger values are\u0000observed\u0000towards low latitudes and regions of lower mesoscale activity. These\u0000results\u0000are consistent with recent analyses of in situ observations and\u0000high-resolution models. Limitations of our results and implications for\u0000reprocessed nadir and future swath altimetric missions are discussed.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"73 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-03-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83731831","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 current study aims to analyse the vertical structure of the ocean during upwelling events using in situ and modelled data. Additionally, the influence of climate patterns, namely the North Atlantic Oscillation (NAO) and the East Atlantic (EA) pattern, on the vertical structure and their impact on the upwelling activity are assessed for a period of 25 years (1993–2017). The study focuses on the central part of the Canary Current (25–35∘ N) with persistent upwelling throughout the year, with an annual cycle and the strongest events from June to September. Upwelling is determined using two different approaches: one index is calculated based on temperature differences between the coastal and the offshore area, and the other is calculated based on wind data and the resulting Ekman transport. Different datasets were chosen according to the indices. Stable coastal upwelling can be observed in the study area for the analysed time span, with differences throughout the latitudes. A deepening of the isothermal layer depth and a cooling of temperatures are observed in the vertical structure of coastal waters, representing a deeper mixing of the ocean and the rise of cooler, denser water towards the surface. During years of a positive NAO, corresponding to a strengthening of the Azores High and the Icelandic Low, stronger winds lead to an intensification of the upwelling activity, an enhanced mixing of the upper ocean, and a deeper (shallower) isothermal layer along the coast (offshore). The opposite is observed in years of negative NAO. Both effects are enhanced in years with a coupled, opposite phase of the EA pattern and are mainly visible during winter months, where the effect of both indices is the greatest. The study therefore suggests that upwelling activities are stronger in winters of positive North Atlantic Oscillation coupled with a negative East Atlantic pattern and emphasizes the importance of interactions between the climate patterns and upwelling.
{"title":"The signature of NAO and EA climate patterns on the vertical structure of the Canary Current upwelling system","authors":"Tina Georg, M. C. Neves, P. Relvas","doi":"10.5194/os-19-351-2023","DOIUrl":"https://doi.org/10.5194/os-19-351-2023","url":null,"abstract":"Abstract. The current study aims to analyse the vertical structure\u0000of the ocean during upwelling events using in situ and modelled data.\u0000Additionally, the influence of climate patterns, namely the North Atlantic\u0000Oscillation (NAO) and the East Atlantic (EA) pattern, on the vertical\u0000structure and their impact on the upwelling activity are assessed for a\u0000period of 25 years (1993–2017). The study focuses on the central part of\u0000the Canary Current (25–35∘ N) with persistent upwelling\u0000throughout the year, with an annual cycle and the strongest events from June to\u0000September. Upwelling is determined using two different approaches: one index is\u0000calculated based on temperature differences between the coastal and the\u0000offshore area, and the other is calculated based on wind data and the resulting Ekman\u0000transport. Different datasets were chosen according to the indices. Stable coastal upwelling can be observed in the study area for the analysed\u0000time span, with differences throughout the latitudes. A deepening of the\u0000isothermal layer depth and a cooling of temperatures are observed in the\u0000vertical structure of coastal waters, representing a deeper mixing of the\u0000ocean and the rise of cooler, denser water towards the surface. During years of a positive NAO, corresponding to a strengthening of the\u0000Azores High and the Icelandic Low, stronger winds lead to an intensification\u0000of the upwelling activity, an enhanced mixing of the upper ocean, and a\u0000deeper (shallower) isothermal layer along the coast (offshore). The opposite\u0000is observed in years of negative NAO. Both effects are enhanced in years\u0000with a coupled, opposite phase of the EA pattern and are mainly visible\u0000during winter months, where the effect of both indices is the greatest. The study therefore suggests that upwelling activities are stronger in winters of\u0000positive North Atlantic Oscillation coupled with a negative East Atlantic\u0000pattern and emphasizes the importance of interactions between the climate\u0000patterns and upwelling.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"34 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-03-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88013914","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 Gulf of Thailand (GoT), a shallow semi-enclosed basin located in the western equatorial Pacific, undergoes much wind variabilities on both seasonal and interannual timescales that produce complex surface circulation. The local Ekman pumping modifies sea level in the northern GoT, while remote wind forcing influences sea level variability at the GoT western boundary, potentially through the coastal trapped Kelvin waves. The importance of the Ekman current on ageostrophic current is also important; the stronger influence of the Ekman current is found toward the southern part of the GoT. The GoT circulation reverses its direction seasonally following the monsoon wind reversal which is well-captured by the most dominant complex empirical orthogonal function explaining 28 % of the total circulation variance. During the monsoon transition, a strong meridional current along the western boundary that connects to the flow at the GoT southeastern entrance is observed. This implies high exchange between the GoT and the South China Sea and thus modification of the GoT water. On the interannual timescale, the GoT circulation is directly impacted by both the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Interestingly, the two climate modes have different spatial influences on the GoT circulation. The IOD dominates the interannual current along the GoT western boundary and the southern boundary of the observing domain (8∘ N), while the ENSO correlates with that in the interior. The results highlight the complex circulation pattern as being contributed by different dynamics over each region of the GoT.
{"title":"Surface circulation in the Gulf of Thailand from remotely sensed observations: seasonal and interannual timescales","authors":"A. Anutaliya","doi":"10.5194/os-19-335-2023","DOIUrl":"https://doi.org/10.5194/os-19-335-2023","url":null,"abstract":"Abstract. The Gulf of Thailand (GoT), a shallow semi-enclosed basin located in the western equatorial Pacific, undergoes much wind variabilities on both seasonal and interannual timescales that produce complex surface circulation. The local Ekman pumping modifies sea level in the northern GoT, while remote wind forcing influences sea level variability at the GoT western boundary, potentially through the coastal trapped Kelvin waves. The importance of the Ekman current on ageostrophic current is also important; the stronger influence of the Ekman current is found toward the southern part of the GoT. The GoT circulation reverses its direction seasonally following the monsoon wind reversal which is well-captured by the most dominant complex empirical orthogonal function explaining 28 % of the total circulation variance. During the monsoon transition, a strong meridional current along the western boundary that connects to the flow at the GoT southeastern entrance is observed. This implies high exchange between the GoT and the South China Sea and thus modification of the GoT water. On the interannual timescale, the GoT circulation is directly impacted by both the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Interestingly, the two climate modes have different spatial influences on the GoT circulation. The IOD dominates the interannual current along the GoT western boundary and the southern boundary of the observing domain (8∘ N), while the ENSO correlates with that in the interior. The results highlight the complex circulation pattern as being contributed by different dynamics over each region of the GoT.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"46 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-03-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"79673234","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}
A. Barnoud, J. Pfeffer, A. Cazenave, Robin Fraudeau, V. Rousseau, M. Ablain
Abstract. We investigate the performances of Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) satellite gravimetry missions in assessing the ocean mass budget at the global scale over 2005–2020. For that purpose, we focus on the last years of the record (2015–2020) when GRACE and GRACE Follow-On faced instrumental problems. We compare the global mean ocean mass estimates from GRACE and GRACE Follow-On to the sum of its contributions from Greenland, Antarctica, land glaciers, terrestrial water storage and atmospheric water content estimated with independent observations. Significant residuals are observed in the global mean ocean mass budget at interannual timescales. Our analyses suggest that the terrestrial water storage variations based on global hydrological models likely contribute in large part to the misclosure of the global mean ocean mass budget at interannual timescales. We also compare the GRACE-based global mean ocean mass with the altimetry-based global mean sea level corrected for the Argo-based thermosteric contribution (an equivalent of global mean ocean mass). After correcting for the wet troposphere drift of the radiometer on board the Jason-3 altimeter satellite, we find that mass budget misclosure is reduced but still significant. However, replacing the Argo-based thermosteric component by the Ocean Reanalysis System 5 (ORAS5) or from the Clouds and the Earth's Radiant Energy System (CERES) top of the atmosphere observations significantly reduces the residuals of the mass budget over the 2015–2020 time span. We conclude that the two most likely sources of error in the global mean ocean mass budget are the thermosteric component based on Argo and the terrestrial water storage contribution based on global hydrological models. The GRACE and GRACE Follow-On data are unlikely to be responsible on their own for the non-closure of the global mean ocean mass budget.
{"title":"Revisiting the global mean ocean mass budget over 2005–2020","authors":"A. Barnoud, J. Pfeffer, A. Cazenave, Robin Fraudeau, V. Rousseau, M. Ablain","doi":"10.5194/os-19-321-2023","DOIUrl":"https://doi.org/10.5194/os-19-321-2023","url":null,"abstract":"Abstract. We investigate the performances of Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) satellite gravimetry missions in assessing the ocean mass budget at the global scale over 2005–2020. For that purpose, we focus on the last years of the record (2015–2020) when GRACE and GRACE Follow-On faced instrumental problems. We compare the global mean ocean mass estimates from GRACE and GRACE Follow-On to the sum of its contributions from Greenland, Antarctica, land glaciers, terrestrial water storage and atmospheric water content estimated with independent observations. Significant residuals are observed in the global mean ocean mass budget at interannual timescales. Our analyses suggest that the terrestrial water storage variations based on global hydrological models likely contribute in large part to the misclosure of the global mean ocean mass budget at interannual timescales. We also compare the GRACE-based global mean ocean mass with the altimetry-based global mean sea level corrected for the Argo-based thermosteric contribution (an equivalent of global mean ocean mass). After correcting for the wet troposphere drift of the radiometer on board the Jason-3 altimeter satellite, we find that mass budget misclosure is reduced but still significant. However, replacing the Argo-based thermosteric component by the Ocean Reanalysis System 5 (ORAS5) or from the Clouds and the Earth's Radiant Energy System (CERES) top of the atmosphere observations significantly reduces the residuals of the mass budget over the 2015–2020 time span. We conclude that the two most likely sources of error in the global mean ocean mass budget are the thermosteric component based on Argo and the terrestrial water storage contribution based on global hydrological models. The GRACE and GRACE Follow-On data are unlikely to be responsible on their own for the non-closure of the global mean ocean mass budget.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"45 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-03-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"84140028","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}
G. Lyu, Armin Koehl, Xinrong Wu, Meng Zhou, D. Stammer
Abstract. The adjoint assimilation method has been applied to coupled ocean and sea ice models for sensitivity studies and Arctic state estimations. However, the accuracy of the adjoint model is degraded by simplifications of the adjoint of the sea ice model, especially the adjoint sea ice rheologies. As part of ongoing developments in coupled ocean and sea ice estimation systems, we incorporate and approximate the adjoint of viscous-plastic sea ice dynamics (adjoint-VP) and compare it with the adjoint of free-drift sea ice dynamics (adjoint-FD) through assimilation experiments. Using the adjoint-VP results in a further cost reduction of 7.9 % in comparison to adjoint-FD, with noticeable improvements in the ocean temperature over the open water and the intermediate layers of the Arctic Ocean. Adjoint-VP adjusts the model input more efficiently than adjoint-FD does by involving different sea ice retreat processes. For instance, adjoint-FD melts the sea ice up to 1.0 m in the marginal seas from May to June by overadjusting air temperature (>8 ∘C); adjoint-VP reproduces the sea ice retreat with smaller adjustments to the atmospheric state within their prior uncertainty range. These developments of the adjoint model here lay the foundation for further improving Arctic Ocean and sea ice estimations by comprehensively adjusting the initial conditions, atmospheric forcings, and parameters of the model.
{"title":"Effects of including the adjoint sea ice rheology on estimating Arctic Ocean–sea ice state","authors":"G. Lyu, Armin Koehl, Xinrong Wu, Meng Zhou, D. Stammer","doi":"10.5194/os-19-305-2023","DOIUrl":"https://doi.org/10.5194/os-19-305-2023","url":null,"abstract":"Abstract. The adjoint assimilation method has been applied to coupled ocean and sea ice models for sensitivity studies and Arctic state estimations. However, the accuracy of the adjoint model is degraded by simplifications of the adjoint of the sea ice model, especially the adjoint sea ice rheologies. As part of ongoing developments in coupled ocean and sea ice estimation systems, we incorporate and approximate the adjoint of viscous-plastic sea ice dynamics (adjoint-VP) and compare it with the adjoint of free-drift sea ice dynamics (adjoint-FD) through assimilation experiments. Using the adjoint-VP results in a further cost reduction of 7.9 % in comparison to adjoint-FD, with noticeable improvements in the ocean temperature over the open water and the intermediate layers of the Arctic Ocean. Adjoint-VP adjusts the model input more efficiently than adjoint-FD does by involving different sea ice retreat processes. For instance, adjoint-FD melts the sea ice up to 1.0 m in the marginal seas from May to June by overadjusting air temperature (>8 ∘C); adjoint-VP reproduces the sea ice retreat with smaller adjustments to the atmospheric state within their prior uncertainty range. These developments of the adjoint model here lay the foundation for further improving Arctic Ocean and sea ice estimations by comprehensively adjusting the initial conditions, atmospheric forcings, and parameters of the model.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"33 4 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-03-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88350976","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 Chukchi Slope Current is a westward-flowing current along the Chukchi slope, which carries Pacific-origin water from the Chukchi shelf into the Canada Basin and helps set the regional hydrographic structure and ecosystem. Using a set of experiments with an idealized primitive equation numerical model, we investigate the energetics of the slope current during the ice-covered period. Numerical calculations show that the growth of surface eddies is suppressed by the ice friction, while perturbations at mid-depths can grow into eddies, consistent with linear instability analysis. However, because the ice stress is spatially variable, it is able to drive Ekman pumping to decrease the available potential energy (APE) and kinetic energy of both the mean flow and mesoscale eddies over a vertical scale of 100 m, well outside the frictional Ekman layer. The rate at which the APE changes is determined by the vertical density flux, which is negative as the ice-induced Ekman pumping advects lighter (denser) water upward (downward). A scaling analysis shows that Ekman pumping will dominate the release of APE for large-scale flows, but the effect of baroclinic instability is also important when the horizontal scale of the mean flow is the baroclinic deformation radius and the eddy velocity is comparable to the mean flow velocity. Our numerical results highlight the importance of ice friction in the energetics of the slope current and eddies, and this may be relevant to other ice-covered regions.
{"title":"A numerical investigation on the energetics of a current along an ice-covered continental slope","authors":"Hengling Leng, Hailun He, M. Spall","doi":"10.5194/os-19-289-2023","DOIUrl":"https://doi.org/10.5194/os-19-289-2023","url":null,"abstract":"Abstract. The Chukchi Slope Current is a westward-flowing current\u0000along the Chukchi slope, which carries Pacific-origin water from the Chukchi\u0000shelf into the Canada Basin and helps set the regional hydrographic\u0000structure and ecosystem. Using a set of experiments with an idealized\u0000primitive equation numerical model, we investigate the energetics of the\u0000slope current during the ice-covered period. Numerical calculations show\u0000that the growth of surface eddies is suppressed by the ice friction, while\u0000perturbations at mid-depths can grow into eddies, consistent with linear\u0000instability analysis. However, because the ice stress is spatially variable,\u0000it is able to drive Ekman pumping to decrease the available potential energy\u0000(APE) and kinetic energy of both the mean flow and mesoscale eddies over a\u0000vertical scale of 100 m, well outside the frictional Ekman layer. The rate\u0000at which the APE changes is determined by the vertical density flux, which\u0000is negative as the ice-induced Ekman pumping advects lighter (denser) water\u0000upward (downward). A scaling analysis shows that Ekman pumping will dominate\u0000the release of APE for large-scale flows, but the effect of baroclinic\u0000instability is also important when the horizontal scale of the mean flow is\u0000the baroclinic deformation radius and the eddy velocity is comparable to the\u0000mean flow velocity. Our numerical results highlight the importance of ice\u0000friction in the energetics of the slope current and eddies, and this may be\u0000relevant to other ice-covered regions.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"52 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-03-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82901230","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}