2022 Vol. 41, No. 5
Display Method:
2022, 41(5): 1-2.
Abstract:
2022, 41(5): 1-11.
doi: 10.1007/s13131-021-1879-y
Abstract:
Based on a two-level nested model from the global ocean to the western Pacific and then to the South China Sea (SCS), the high-resolution SCS deep circulation is numerically investigated. The SCS deep circulation shows a basin-scale cyclonic structure with a strong southward western boundary current in summer (July), a northeast-southwest through-flow pattern across the deep basin without a western boundary current in winter (January), and a transitional pattern in spring and autumn. The sensitivity model experiments illustrate that the Luzon Strait deep overflow is the main factor controlling the seasonal variation in the SCS deep circulation. The SCS surface wind can significantly influence the SCS deep circulation in winter. The Luzon Strait deep overflow transport from the Pacific into the SCS ranges from 0.68×106 m3/s to 1.83×106 m3/s, reaching its maximum in summer (July, up to 1.83×106 m3/s), less in autumn and winter, and the minimum in spring (May, 0.68×106 m3/s). In summer, the strong Luzon Strait deep overflow dominates the SCS deep circulation when the role of the SCS surface wind is small. In winter, the weaker Luzon Strait deep overflow and SCS surface wind jointly drive the SCS deep circulation into a northeast-southwest through-flow pattern. The potential vorticity (PV) dissipation in the SCS deep basin reaches its maximum (−0.122 m2/s2) in May and its minimum (−0.380 m2/s2) in July.
Based on a two-level nested model from the global ocean to the western Pacific and then to the South China Sea (SCS), the high-resolution SCS deep circulation is numerically investigated. The SCS deep circulation shows a basin-scale cyclonic structure with a strong southward western boundary current in summer (July), a northeast-southwest through-flow pattern across the deep basin without a western boundary current in winter (January), and a transitional pattern in spring and autumn. The sensitivity model experiments illustrate that the Luzon Strait deep overflow is the main factor controlling the seasonal variation in the SCS deep circulation. The SCS surface wind can significantly influence the SCS deep circulation in winter. The Luzon Strait deep overflow transport from the Pacific into the SCS ranges from 0.68×106 m3/s to 1.83×106 m3/s, reaching its maximum in summer (July, up to 1.83×106 m3/s), less in autumn and winter, and the minimum in spring (May, 0.68×106 m3/s). In summer, the strong Luzon Strait deep overflow dominates the SCS deep circulation when the role of the SCS surface wind is small. In winter, the weaker Luzon Strait deep overflow and SCS surface wind jointly drive the SCS deep circulation into a northeast-southwest through-flow pattern. The potential vorticity (PV) dissipation in the SCS deep basin reaches its maximum (−0.122 m2/s2) in May and its minimum (−0.380 m2/s2) in July.
2022, 41(5): 12-26.
doi: 10.1007/s13131-021-1935-7
Abstract:
Using observations and numerical simulations, this study examines the intraseasonal variability of the surface zonal current (u ISV) over the equatorial Indian Ocean, highlighting the seasonal and spatial differences, and the causes of the differences. Large-amplitude u ISV occurs in the eastern basin at around 80°–90°E and near the western boundary at 45°–55°E. In the eastern basin, the u ISV is mainly caused by the atmospheric intraseasonal oscillations (ISOs), which explains 91% of the standard deviation of the total u ISV. Further analysis suggests that it takes less than ten days for the intraseasonal zonal wind stress to generate the u ISV through the directly forced Kelvin and Rossby waves. Driven by the stronger zonal wind stress associated with the Indian summer monsoon ISO (MISO), the eastern u ISV in boreal summer (May to October) is about 1.5 times larger than that in boreal winter (November to April). In the western basin, both the atmospheric ISOs and the oceanic internal instabilities contribute substantially to the u ISV, and induce stronger u ISV in boreal summer. Energy budget analysis suggests that the mean flow converts energy to the intraseasonal current mainly through barotropic instabilities.
Using observations and numerical simulations, this study examines the intraseasonal variability of the surface zonal current (u ISV) over the equatorial Indian Ocean, highlighting the seasonal and spatial differences, and the causes of the differences. Large-amplitude u ISV occurs in the eastern basin at around 80°–90°E and near the western boundary at 45°–55°E. In the eastern basin, the u ISV is mainly caused by the atmospheric intraseasonal oscillations (ISOs), which explains 91% of the standard deviation of the total u ISV. Further analysis suggests that it takes less than ten days for the intraseasonal zonal wind stress to generate the u ISV through the directly forced Kelvin and Rossby waves. Driven by the stronger zonal wind stress associated with the Indian summer monsoon ISO (MISO), the eastern u ISV in boreal summer (May to October) is about 1.5 times larger than that in boreal winter (November to April). In the western basin, both the atmospheric ISOs and the oceanic internal instabilities contribute substantially to the u ISV, and induce stronger u ISV in boreal summer. Energy budget analysis suggests that the mean flow converts energy to the intraseasonal current mainly through barotropic instabilities.
2022, 41(5): 27-40.
doi: 10.1007/s13131-021-1969-x
Abstract:
The statistical characteristics and mechanisms of mesoscale eddies in the North Indian Ocean are investigated by adopting multi-sensor satellite data from 1993 to 2019. In the Arabian Sea (AS), seasonal variation of eddy characteristics is remarkable, while the intraseasonal variability caused by planetary waves is crucial in the Bay of Bengal (BOB). Seasonal variation of the eddy kinetic energy (EKE) is distinct along the west boundary of AS, especially in the Somali Current region. In the BOB, larger EKE occurs at the northwest basin from March to May, to the east of Sri Lanka from June to September, and along the east coast of India from November to December. The wind stress work (WW) is further studied to figure out the direct influence of wind forcing on EKE. The WW exerts positive effects on EKE along the west boundary of AS and in the south of India/Sri Lanka during the two monsoon seasons. Besides, the WW also has impact on EKE along the east coast of India in November and December. Eventually, we investigate the characteristics and the driving mechanisms of long lifespan eddies. In the AS, long lifespan anti-cyclonic eddies (AEs) mainly generate in the Socotra, the West Indian Coastal Current and the East Arabian Current regions, while cyclonic eddies (CEs) are concentrated in the northwest region. In the BOB, long lifespan AEs mostly form near the west of Myanmar, while CEs are accumulated at the north and northwest basin. The instabilities caused by Rossby waves, coastal Kelvin waves, seasonal currents, together with wind stress forcing exert enormous efforts on the generation and evolution of these eddies.
The statistical characteristics and mechanisms of mesoscale eddies in the North Indian Ocean are investigated by adopting multi-sensor satellite data from 1993 to 2019. In the Arabian Sea (AS), seasonal variation of eddy characteristics is remarkable, while the intraseasonal variability caused by planetary waves is crucial in the Bay of Bengal (BOB). Seasonal variation of the eddy kinetic energy (EKE) is distinct along the west boundary of AS, especially in the Somali Current region. In the BOB, larger EKE occurs at the northwest basin from March to May, to the east of Sri Lanka from June to September, and along the east coast of India from November to December. The wind stress work (WW) is further studied to figure out the direct influence of wind forcing on EKE. The WW exerts positive effects on EKE along the west boundary of AS and in the south of India/Sri Lanka during the two monsoon seasons. Besides, the WW also has impact on EKE along the east coast of India in November and December. Eventually, we investigate the characteristics and the driving mechanisms of long lifespan eddies. In the AS, long lifespan anti-cyclonic eddies (AEs) mainly generate in the Socotra, the West Indian Coastal Current and the East Arabian Current regions, while cyclonic eddies (CEs) are concentrated in the northwest region. In the BOB, long lifespan AEs mostly form near the west of Myanmar, while CEs are accumulated at the north and northwest basin. The instabilities caused by Rossby waves, coastal Kelvin waves, seasonal currents, together with wind stress forcing exert enormous efforts on the generation and evolution of these eddies.
2022, 41(5): 41-50.
doi: 10.1007/s13131-021-1947-3
Abstract:
This paper presents an efficient algorithm for generating a spherical multiple-cell (SMC) grid. The algorithm adopts a recursive loop structure and provides two refinement methods: (1) an arbitrary area refinement method and (2) a nearshore refinement method. Numerical experiments are carried out, and the results show that compared with the existing grid generation algorithm, this algorithm is more flexible and operable.
This paper presents an efficient algorithm for generating a spherical multiple-cell (SMC) grid. The algorithm adopts a recursive loop structure and provides two refinement methods: (1) an arbitrary area refinement method and (2) a nearshore refinement method. Numerical experiments are carried out, and the results show that compared with the existing grid generation algorithm, this algorithm is more flexible and operable.
2022, 41(5): 51-63.
doi: 10.1007/s13131-021-1874-3
Abstract:
Knowledge of sediment variation processes is essential to understand the evolution mechanism of beach morphology changes. Thus, a field measurement was conducted at the Heisha Beach, located on the west coast of the Zhujiang River (Pearl River) Estuary, to investigate the short-term variation in suspended sediment concentrations (SSCs) and the relationship between the SSC and turbulent kinetic energy, bottom shear stress (BSS), and relative wave height. Based on extreme event analysis results, extreme events have a greater influence on turbulent kinetic energy than SSC. Although a portion of the turbulent kinetic energy dissipates directly into the water column, it plays an important role in suspended sediment motion. Most of the time, the wave-current interaction is strong enough to drive sediment incipience and resuspension. When combined, the wave-current interaction and wave-induced BSSs have a greater influence on suspended sediment transport and SSC variation than current-induced BSS alone. The relative wave height also has a strong correlation with SSC, indicating that the combined effect of water depth and wave height significantly impacts SSC variation. Water depth is mainly controlled by the tide on the beaches; thus, the effects of tides and waves should be conjunctively considered when analyzing the factors influencing SSC.
Knowledge of sediment variation processes is essential to understand the evolution mechanism of beach morphology changes. Thus, a field measurement was conducted at the Heisha Beach, located on the west coast of the Zhujiang River (Pearl River) Estuary, to investigate the short-term variation in suspended sediment concentrations (SSCs) and the relationship between the SSC and turbulent kinetic energy, bottom shear stress (BSS), and relative wave height. Based on extreme event analysis results, extreme events have a greater influence on turbulent kinetic energy than SSC. Although a portion of the turbulent kinetic energy dissipates directly into the water column, it plays an important role in suspended sediment motion. Most of the time, the wave-current interaction is strong enough to drive sediment incipience and resuspension. When combined, the wave-current interaction and wave-induced BSSs have a greater influence on suspended sediment transport and SSC variation than current-induced BSS alone. The relative wave height also has a strong correlation with SSC, indicating that the combined effect of water depth and wave height significantly impacts SSC variation. Water depth is mainly controlled by the tide on the beaches; thus, the effects of tides and waves should be conjunctively considered when analyzing the factors influencing SSC.
2022, 41(5): 64-77.
doi: 10.1007/s13131-021-1860-9
Abstract:
Threatening millions of people and causing billions of dollars in losses, tropical cyclones (TCs) are among the most severe natural hazards in the world, especially over the western North Pacific. However, the response of TCs to a warming or changing climate has been the subject of considerable research, often with conflicting results. In this study, the abilities of Coupled Model Intercomparison Project (CMIP) Phase 6 (CMIP6) models to simulate TC genesis are assessed through historical simulations. The results indicate that a systematic humidity bias persists in most CMIP6 models from corresponding CMIP Phase 5 models, which leads to an overestimation of climatological TC genesis. However, the annual cycle of TC genesis is well captured by CMIP6 models. The abilities of 25 models to simulate the geographical patterns of TC genesis vary significantly. In addition, seven models are identified as well simulated models, but seven models are identified as poorly simulated ones. A comparison of the environmental variables for TC genesis in the well-simulated group and the poorly simulated group identifies moisture in the mid-troposphere as a key factor in the realistic simulation of El Niño-Southern Oscillation (ENSO) impacts on TC genesis. In contrast with the observations, the poorly simulated group does not reproduce the suppressing effect of negative moisture anomalies on TC genesis in the northwestern region (20°–30°N, 120°–145°E) during El Niño years. Given the interaction between TC and ENSO, these results provide a guidance for future TC projections under climate change by CMIP6 models.
Threatening millions of people and causing billions of dollars in losses, tropical cyclones (TCs) are among the most severe natural hazards in the world, especially over the western North Pacific. However, the response of TCs to a warming or changing climate has been the subject of considerable research, often with conflicting results. In this study, the abilities of Coupled Model Intercomparison Project (CMIP) Phase 6 (CMIP6) models to simulate TC genesis are assessed through historical simulations. The results indicate that a systematic humidity bias persists in most CMIP6 models from corresponding CMIP Phase 5 models, which leads to an overestimation of climatological TC genesis. However, the annual cycle of TC genesis is well captured by CMIP6 models. The abilities of 25 models to simulate the geographical patterns of TC genesis vary significantly. In addition, seven models are identified as well simulated models, but seven models are identified as poorly simulated ones. A comparison of the environmental variables for TC genesis in the well-simulated group and the poorly simulated group identifies moisture in the mid-troposphere as a key factor in the realistic simulation of El Niño-Southern Oscillation (ENSO) impacts on TC genesis. In contrast with the observations, the poorly simulated group does not reproduce the suppressing effect of negative moisture anomalies on TC genesis in the northwestern region (20°–30°N, 120°–145°E) during El Niño years. Given the interaction between TC and ENSO, these results provide a guidance for future TC projections under climate change by CMIP6 models.
2022, 41(5): 78-89.
doi: 10.1007/s13131-021-1878-z
Abstract:
Seasonal and interannual variability of ocean bottom pressure (OBP) in the Southern Ocean was investigated using Gravity Recovery and Climate Experiment (GRACE) data and a Pressure Coordinate Ocean Model (PCOM) based on mass conservation. By comparing OBP, steric sea level, and sea level, it is found that at high latitudes the OBP variability dominates the sea level variability at seasonal-to-decadal time scales. The diagnostic OBP based on barotropic vorticity equation has a good correlation with the observations, indicating that wind forcing plays an important role in the variability of the OBP in the Southern Ocean. The unique interannual patterns of OBP in the Southern Ocean are closely associated with El Niño-Southern Oscillation (ENSO) and Southern Annular Mode (SAM). Regression analysis indicates that ENSO and SAM influence the OBP through altering the Ekman transport driven by surface wind. The leading pattern of OBP from PCOM are very similar to observations. Sensitive experiments of PCOM show that surface wind forcing explains the observed OBP variability quite well, confirming the importance of wind forcing and related oceanic processes. In the eastern South Pacific, the averaged OBP shows a decrease (increase) trend before (after) 2011, reflecting the reverse trend in westerly wind. In the South Indo-Atlantic Ocean, the averaged OBP has a weak increase trend during 2003–2016.
Seasonal and interannual variability of ocean bottom pressure (OBP) in the Southern Ocean was investigated using Gravity Recovery and Climate Experiment (GRACE) data and a Pressure Coordinate Ocean Model (PCOM) based on mass conservation. By comparing OBP, steric sea level, and sea level, it is found that at high latitudes the OBP variability dominates the sea level variability at seasonal-to-decadal time scales. The diagnostic OBP based on barotropic vorticity equation has a good correlation with the observations, indicating that wind forcing plays an important role in the variability of the OBP in the Southern Ocean. The unique interannual patterns of OBP in the Southern Ocean are closely associated with El Niño-Southern Oscillation (ENSO) and Southern Annular Mode (SAM). Regression analysis indicates that ENSO and SAM influence the OBP through altering the Ekman transport driven by surface wind. The leading pattern of OBP from PCOM are very similar to observations. Sensitive experiments of PCOM show that surface wind forcing explains the observed OBP variability quite well, confirming the importance of wind forcing and related oceanic processes. In the eastern South Pacific, the averaged OBP shows a decrease (increase) trend before (after) 2011, reflecting the reverse trend in westerly wind. In the South Indo-Atlantic Ocean, the averaged OBP has a weak increase trend during 2003–2016.
2022, 41(5): 90-98.
doi: 10.1007/s13131-021-1953-5
Abstract:
Contrasting decrease and increase trends of sea surface temperature (SST) have been documented in the western Subarctic (WSA) and the rest of the Northwest Pacific (NWP) from 1958 to 2017, respectively. Consequently, more (less) total carbon dioxide (TCO2) due to ocean cooling (warming) is transported to the surface, which leads to increase (decrease) of oceanic surface partial pressure of carbon dioxide (pCO2). With the combined influence of the rising atmospheric carbon dioxide (CO2) level and changing ocean conditions, a prominent increase in oceanic surface pCO2 occurred with different rates of increase in summer and winter in the NWP. The oceanic surface pCO2 is mainly controlled by the variation of TCO2 at the interdecadal timescale and by SST at the seasonal timescale. Our results also indicate that increasing SST tends to strengthen the capability of ocean in absorbing anthropogenic CO2 in the NWP, while ocean’s uptaking ability is weakened in the cooling area of the WSA.
Contrasting decrease and increase trends of sea surface temperature (SST) have been documented in the western Subarctic (WSA) and the rest of the Northwest Pacific (NWP) from 1958 to 2017, respectively. Consequently, more (less) total carbon dioxide (TCO2) due to ocean cooling (warming) is transported to the surface, which leads to increase (decrease) of oceanic surface partial pressure of carbon dioxide (pCO2). With the combined influence of the rising atmospheric carbon dioxide (CO2) level and changing ocean conditions, a prominent increase in oceanic surface pCO2 occurred with different rates of increase in summer and winter in the NWP. The oceanic surface pCO2 is mainly controlled by the variation of TCO2 at the interdecadal timescale and by SST at the seasonal timescale. Our results also indicate that increasing SST tends to strengthen the capability of ocean in absorbing anthropogenic CO2 in the NWP, while ocean’s uptaking ability is weakened in the cooling area of the WSA.
2022, 41(5): 99-109.
doi: 10.1007/s13131-021-1951-7
Abstract:
The El Niño-Southern Oscillation (ENSO) ensemble prediction skills of the Beijing Climate Center (BCC) climate prediction system version 2 (BCC-CPS2) are examined for the period from 1991 to 2018. The upper-limit ENSO predictability of this system is quantified by measuring its “potential” predictability using information-based metrics, whereas the actual prediction skill is evaluated using deterministic and probabilistic skill measures. Results show that: (1) In general, the current operational BCC model achieves an effective 10-month lead predictability for ENSO. Moreover, prediction skills are up to 10–11 months for the warm and cold ENSO phases, while the normal phase has a prediction skill of just 6 months. (2) Similar to previous results of the intermediate coupled models, the relative entropy (RE) with a dominating ENSO signal component can more effectively quantify correlation-based prediction skills compared to the predictive information (PI) and the predictive power (PP). (3) An evaluation of the signal-dependent feature of the prediction skill scores suggests the relationship between the “Spring predictability barrier (SPB)” of ENSO prediction and the weak ENSO signal phase during boreal spring and early summer.
The El Niño-Southern Oscillation (ENSO) ensemble prediction skills of the Beijing Climate Center (BCC) climate prediction system version 2 (BCC-CPS2) are examined for the period from 1991 to 2018. The upper-limit ENSO predictability of this system is quantified by measuring its “potential” predictability using information-based metrics, whereas the actual prediction skill is evaluated using deterministic and probabilistic skill measures. Results show that: (1) In general, the current operational BCC model achieves an effective 10-month lead predictability for ENSO. Moreover, prediction skills are up to 10–11 months for the warm and cold ENSO phases, while the normal phase has a prediction skill of just 6 months. (2) Similar to previous results of the intermediate coupled models, the relative entropy (RE) with a dominating ENSO signal component can more effectively quantify correlation-based prediction skills compared to the predictive information (PI) and the predictive power (PP). (3) An evaluation of the signal-dependent feature of the prediction skill scores suggests the relationship between the “Spring predictability barrier (SPB)” of ENSO prediction and the weak ENSO signal phase during boreal spring and early summer.
2022, 41(5): 110-123.
doi: 10.1007/s13131-021-1949-1
Abstract:
In the northwestern North Pacific, annual net air-sea CO2 flux is greatest in the Kuroshio Extension (KE) zone, owing to its low annual mean partial pressure of CO2 (pCO2), and it decreases southward across the basin. To quantify the influences of factors controlling the latitudinal gradient in CO2 uptake, sea surface pCO2 and related parameters were investigated in late spring of 2018 in a study spanning the KE, Kuroshio Recirculation (KR), and subtropical zones. We found that the sea-to-air pCO2 difference (ΔpCO2) was negative and at its lowest in the KE zone. ΔpCO2 gradually increased southward across the KR zone, and the sea surface was nearly in air-equilibrium with atmospheric CO2 in the subtropical zone. We found that northward cooling and vertical mixing were the two major processes governing the latitudinal gradient in surface pCO2 and ΔpCO2, while biological influences were relatively minor. In the KE zone affected by upwelling, the vertical-mixing-induced increase in surface pCO2 likely canceled out approximately 61% of the decrease in surface pCO2 caused by cooling and biological activities. Moreover, the prolonged air-sea equilibration for CO2 and relatively short hydraulic retention time jointly led to the low surface pCO2 in the KE zone in spring. Ultimately, the cooling KE current flows out of the region before it can be re-equilibrated with atmospheric CO2.
In the northwestern North Pacific, annual net air-sea CO2 flux is greatest in the Kuroshio Extension (KE) zone, owing to its low annual mean partial pressure of CO2 (pCO2), and it decreases southward across the basin. To quantify the influences of factors controlling the latitudinal gradient in CO2 uptake, sea surface pCO2 and related parameters were investigated in late spring of 2018 in a study spanning the KE, Kuroshio Recirculation (KR), and subtropical zones. We found that the sea-to-air pCO2 difference (ΔpCO2) was negative and at its lowest in the KE zone. ΔpCO2 gradually increased southward across the KR zone, and the sea surface was nearly in air-equilibrium with atmospheric CO2 in the subtropical zone. We found that northward cooling and vertical mixing were the two major processes governing the latitudinal gradient in surface pCO2 and ΔpCO2, while biological influences were relatively minor. In the KE zone affected by upwelling, the vertical-mixing-induced increase in surface pCO2 likely canceled out approximately 61% of the decrease in surface pCO2 caused by cooling and biological activities. Moreover, the prolonged air-sea equilibration for CO2 and relatively short hydraulic retention time jointly led to the low surface pCO2 in the KE zone in spring. Ultimately, the cooling KE current flows out of the region before it can be re-equilibrated with atmospheric CO2.
2022, 41(5): 124-135.
doi: 10.1007/s13131-021-1936-6
Abstract:
Seismicity in ocean ridge-transform systems reveals fundamental processes of mid-ocean ridges, while comparisons of seismicity in different oceans remain rare due to a lack of detection of small events. From 1996 to 2003, the Pacific Marine Environmental Laboratory of the National Oceanic and Atmospheric Administration (NOAA/PMEL) deployed several hydrophones in the eastern Pacific Ocean and the northern Atlantic Ocean. These hydrophones recorded earthquakes with small magnitudes, providing us with opportunities to study the seismic characteristics of ridge-transform systems at different spreading rates and make further comparisons of their differences. This study comparatively analyzed hydroacoustic and teleseismic data recorded on the fast-spreading East Pacific Rise (EPR, 10°S to 12°N), intermediate-spreading Galapagos Ridge (103° W to 80° W), and slow-spreading Mid-Atlantic Ridge (MAR, 15°N to 37°N). We present a systematic study of the spatial and temporal distribution of events, aftershock seismicity, and possible triggering mechanisms of aftershock sequences. Our analysis yields the following conclusions. (1) From the hydroacoustic data, the EPR transform faults had the highest average seismicity rate among the three regions. (2) Along-ridge event distributions show that a high number of earthquakes were concentrated on the EPR, while they became dispersed on the GR and fewer and more scattered on the MAR, reflecting that the different tectonic origins were closely correlated with the spreading rate. (3) Analysis from mainshock-aftershock sequences shows no significant differences in the aftershock decay rate among the three regions. (4) Multiple types of aftershock triggering models were inferred from Coulomb stress changes: strike-slip mainshocks triggered strike-slip aftershocks and normal faulting aftershocks, and normal faulting mainshocks triggered normal faulting aftershocks. Although these results are case studies, they may be applicable to other ocean ridge-transform systems in future investigations. Our results provide important new insights into the seismicity of global ocean ridge-transform systems.
Seismicity in ocean ridge-transform systems reveals fundamental processes of mid-ocean ridges, while comparisons of seismicity in different oceans remain rare due to a lack of detection of small events. From 1996 to 2003, the Pacific Marine Environmental Laboratory of the National Oceanic and Atmospheric Administration (NOAA/PMEL) deployed several hydrophones in the eastern Pacific Ocean and the northern Atlantic Ocean. These hydrophones recorded earthquakes with small magnitudes, providing us with opportunities to study the seismic characteristics of ridge-transform systems at different spreading rates and make further comparisons of their differences. This study comparatively analyzed hydroacoustic and teleseismic data recorded on the fast-spreading East Pacific Rise (EPR, 10°S to 12°N), intermediate-spreading Galapagos Ridge (103° W to 80° W), and slow-spreading Mid-Atlantic Ridge (MAR, 15°N to 37°N). We present a systematic study of the spatial and temporal distribution of events, aftershock seismicity, and possible triggering mechanisms of aftershock sequences. Our analysis yields the following conclusions. (1) From the hydroacoustic data, the EPR transform faults had the highest average seismicity rate among the three regions. (2) Along-ridge event distributions show that a high number of earthquakes were concentrated on the EPR, while they became dispersed on the GR and fewer and more scattered on the MAR, reflecting that the different tectonic origins were closely correlated with the spreading rate. (3) Analysis from mainshock-aftershock sequences shows no significant differences in the aftershock decay rate among the three regions. (4) Multiple types of aftershock triggering models were inferred from Coulomb stress changes: strike-slip mainshocks triggered strike-slip aftershocks and normal faulting aftershocks, and normal faulting mainshocks triggered normal faulting aftershocks. Although these results are case studies, they may be applicable to other ocean ridge-transform systems in future investigations. Our results provide important new insights into the seismicity of global ocean ridge-transform systems.
2022, 41(5): 136-146.
doi: 10.1007/s13131-021-1946-4
Abstract:
The Huanghe River (Yellow River) Delta has a wide distribution of fine-grained soils. Fluvial alluviation, erosion, and wave loads affect the shoal area, resulting complex physical and mechanical properties to sensitive fine-grained soil located at the river-sea boundary. The cone penetration test (CPT) is a convenient and effective in situ testing method which can accurately identify various soil parameters. Studies on undrained shear strength only roughly determine the fine content (FC) without making the FC effect clear. We studied four stations formed in different the Huanghe River Delta periods. We conducted in situ CPT and corresponding laboratory tests, examined the fine content influence on undrained shear strength (Su), and determined the cone coefficient (Nk). The conclusions are as follows. (1) The fine content in the area exceeded 90%, and the silt content was high, accounting for more than 70% of all fine particle compositions. (2) The undrained shear strength gradually increased with depth with a maximum of approximately 250 kPa. When the silt content was lower than 60%–70%, the undrained shear strength decreased. (3) The silt and clay content influenced undrained shear strength, and the fitted fsh/qt function model was established, which could be applied to strata with a high fine content. The cone coefficients were between 20 and 25, and the overconsolidated soil layer had a greater cone coefficient.
The Huanghe River (Yellow River) Delta has a wide distribution of fine-grained soils. Fluvial alluviation, erosion, and wave loads affect the shoal area, resulting complex physical and mechanical properties to sensitive fine-grained soil located at the river-sea boundary. The cone penetration test (CPT) is a convenient and effective in situ testing method which can accurately identify various soil parameters. Studies on undrained shear strength only roughly determine the fine content (FC) without making the FC effect clear. We studied four stations formed in different the Huanghe River Delta periods. We conducted in situ CPT and corresponding laboratory tests, examined the fine content influence on undrained shear strength (Su), and determined the cone coefficient (Nk). The conclusions are as follows. (1) The fine content in the area exceeded 90%, and the silt content was high, accounting for more than 70% of all fine particle compositions. (2) The undrained shear strength gradually increased with depth with a maximum of approximately 250 kPa. When the silt content was lower than 60%–70%, the undrained shear strength decreased. (3) The silt and clay content influenced undrained shear strength, and the fitted fsh/qt function model was established, which could be applied to strata with a high fine content. The cone coefficients were between 20 and 25, and the overconsolidated soil layer had a greater cone coefficient.
2022, 41(5): 147-162.
doi: 10.1007/s13131-021-1948-2
Abstract:
On average, five to six storms occur in the Qiongzhou Strait every year, causing significant damage to coastal geomorphology and several property losses. Tropical Storm Bebinca is the most unusual and complex storm event that has occurred in this region over the last 10 years. To detect the high-frequency beachface responses to the storm, a pressure sensor was deployed in the surf zone to record the free sea surface height, and the heights of grid pile points on the beachface were measured manually to determine beach elevation changes during this storm. Empirical Mode Decomposition and related analysis techniques were used to analyze the high-frequency topography and wave data. The results showed that: (1) the beachface response process occurred in three stages. The first stage was the rapid response stage, wherein the spring tide berm began to erode significantly, and the front edge of the beach berm reacted closely. The two beach sections resisted the harmful energy of the main storm. In the second stage, the beach slope increased after a large sediment loss on the beach berm and its front edge. To adapt to the storm energy, the beach at the low tide line began to erode, and the beach slope decreased. In the third stage, after the storm turned, the wave energy was significantly attenuated, and the beach berm eroded to resist the residual wave energy. The beachface began to oscillate and recover. (2) The main wave surface was the superimposed product of a few internal mode functions. Similar results were observed in beachface changes. High-frequency driving factors determine the local characteristics of beach evolution, and low-frequency driving factors determine the beach evolution trend. (3) The response of sediment to the storm was not a single sea-transportation, but a single- or two-way conversion driven by factors such as wave energy, swash flow, and secondary wave breaking. (4) The Ω-RTR model is not completely applicable to beach states that undergo rapid changes during storms. Therefore, it is necessary to carry out further research on beach state identification during storms.
On average, five to six storms occur in the Qiongzhou Strait every year, causing significant damage to coastal geomorphology and several property losses. Tropical Storm Bebinca is the most unusual and complex storm event that has occurred in this region over the last 10 years. To detect the high-frequency beachface responses to the storm, a pressure sensor was deployed in the surf zone to record the free sea surface height, and the heights of grid pile points on the beachface were measured manually to determine beach elevation changes during this storm. Empirical Mode Decomposition and related analysis techniques were used to analyze the high-frequency topography and wave data. The results showed that: (1) the beachface response process occurred in three stages. The first stage was the rapid response stage, wherein the spring tide berm began to erode significantly, and the front edge of the beach berm reacted closely. The two beach sections resisted the harmful energy of the main storm. In the second stage, the beach slope increased after a large sediment loss on the beach berm and its front edge. To adapt to the storm energy, the beach at the low tide line began to erode, and the beach slope decreased. In the third stage, after the storm turned, the wave energy was significantly attenuated, and the beach berm eroded to resist the residual wave energy. The beachface began to oscillate and recover. (2) The main wave surface was the superimposed product of a few internal mode functions. Similar results were observed in beachface changes. High-frequency driving factors determine the local characteristics of beach evolution, and low-frequency driving factors determine the beach evolution trend. (3) The response of sediment to the storm was not a single sea-transportation, but a single- or two-way conversion driven by factors such as wave energy, swash flow, and secondary wave breaking. (4) The Ω-RTR model is not completely applicable to beach states that undergo rapid changes during storms. Therefore, it is necessary to carry out further research on beach state identification during storms.
2022, 41(5): 163-172.
doi: 10.1007/s13131-021-1859-2
Abstract:
A 10-year (2003–2012) hindcast was conducted to study the wave field in the Zhe-Min coastal area (Key Area OE-W2) located off Zhejiang and Fujian provinces of China. Forced by the wind field from a weather research and forecasting model (WRF), high-resolution wave modelling using the SWAN was carried out in the study area. The simulated wave fields show a good agreement with observations. Using the simulation results, we conducted statistical analysis of wave power density in terms of spatial distribution and temporal variation. The effective duration of wave energy in the sea area was discussed, and the stability of wave energy was evaluated using the coefficient of variation of wave power density. Results indicate that the wave energy resource in the study area was about 4.11×106 kW. The distribution of wave energy tends to increase from the north (off Zhejiang coast) to the south (off Fujian coast), and from near-shore area to the open sea. The sea areas with wave power density greater than 2 kW/m are mostly distributed seaward of the 10-m isobath, and the contours of the wave power density are almost parallel to the shoreline. The sea areas around the islands that are far from the mainland are rich in wave energy, usually more than 6 kW/m, and therefore are of obvious advantages in planning wave energy development and utilization. The effective duration of wave energy in the offshore area shows an increasing trend from north (off Zhejiang coast) to south (off Fujian coast), with values of ~3 500 h in the north and ~4 450 h in the south. The coefficient of variation of wave energy in this region is mostly in the range of 1.5–3.0, and gradually decreases from the north to the south, suggesting that the wave energy in the south is more stable than that in the north.
A 10-year (2003–2012) hindcast was conducted to study the wave field in the Zhe-Min coastal area (Key Area OE-W2) located off Zhejiang and Fujian provinces of China. Forced by the wind field from a weather research and forecasting model (WRF), high-resolution wave modelling using the SWAN was carried out in the study area. The simulated wave fields show a good agreement with observations. Using the simulation results, we conducted statistical analysis of wave power density in terms of spatial distribution and temporal variation. The effective duration of wave energy in the sea area was discussed, and the stability of wave energy was evaluated using the coefficient of variation of wave power density. Results indicate that the wave energy resource in the study area was about 4.11×106 kW. The distribution of wave energy tends to increase from the north (off Zhejiang coast) to the south (off Fujian coast), and from near-shore area to the open sea. The sea areas with wave power density greater than 2 kW/m are mostly distributed seaward of the 10-m isobath, and the contours of the wave power density are almost parallel to the shoreline. The sea areas around the islands that are far from the mainland are rich in wave energy, usually more than 6 kW/m, and therefore are of obvious advantages in planning wave energy development and utilization. The effective duration of wave energy in the offshore area shows an increasing trend from north (off Zhejiang coast) to south (off Fujian coast), with values of ~3 500 h in the north and ~4 450 h in the south. The coefficient of variation of wave energy in this region is mostly in the range of 1.5–3.0, and gradually decreases from the north to the south, suggesting that the wave energy in the south is more stable than that in the north.
2022, 41(5): 173-182.
doi: 10.1007/s13131-021-1968-y
Abstract:
This study focuses on the development of a farm of tidal turbines in the Khuran Channel. The important factors include the location of turbines and their hydrodynamic effects on the environment. A three-dimensional circulation model for the Persian Gulf is employed for the comprehensive evaluation of the tidal energy potential throughout the study area. The model is validated by using in situ observations of water level and current data. The appropriate potential points for extracting the tidal energy were identified in the Persian Gulf using the model results. The Khuran Channel, located in the north of Qeshm Island, was found to be the best place to extract tidal energy inside the Persian Gulf. By adding the term of momentum losses to the governing equations, the feedback of extracting energy on the hydrodynamic around Qeshm Island was studied. The simulation results show that the average daily power production of a tidal farm with 99 turbines for one month is approximately 1.3 MW. This tidal farm also has a significant impact on the water level inside the Khuran Channel, especially near the tidal farm where these fluctuations exceed 4 cm. The change in the current speed caused by wake reaches 0.4 m/s. Wake effects were active up to 7 km downstream of the turbines. The current velocity was also estimated to be 1.6 m/s and 2.1 m/s during the spring and ebb tides within the channel, respectively.
This study focuses on the development of a farm of tidal turbines in the Khuran Channel. The important factors include the location of turbines and their hydrodynamic effects on the environment. A three-dimensional circulation model for the Persian Gulf is employed for the comprehensive evaluation of the tidal energy potential throughout the study area. The model is validated by using in situ observations of water level and current data. The appropriate potential points for extracting the tidal energy were identified in the Persian Gulf using the model results. The Khuran Channel, located in the north of Qeshm Island, was found to be the best place to extract tidal energy inside the Persian Gulf. By adding the term of momentum losses to the governing equations, the feedback of extracting energy on the hydrodynamic around Qeshm Island was studied. The simulation results show that the average daily power production of a tidal farm with 99 turbines for one month is approximately 1.3 MW. This tidal farm also has a significant impact on the water level inside the Khuran Channel, especially near the tidal farm where these fluctuations exceed 4 cm. The change in the current speed caused by wake reaches 0.4 m/s. Wake effects were active up to 7 km downstream of the turbines. The current velocity was also estimated to be 1.6 m/s and 2.1 m/s during the spring and ebb tides within the channel, respectively.
2022, 41(5): 183-194.
doi: 10.1007/s13131-021-1970-4
Abstract:
With the development of satellite altimetry technology, the resolution of sea-level anomaly (SLA) datasets is constantly improving. Current spatial resolution levels can reach a grid size of (1/4)° × (1/4)°, with daily measurements that span from 1993 to 2018, allowing for the precise identification and tracking of individual eddies. In the current study, in addition to the internal circulation and migration of eddies, a new aspect in eddy kinematics is revealed and investigated for the first time: shape-based overall eddy rotation (SOER), based on the intrinsic elliptical shape of eddies identified from a high-resolution SLA dataset. We found that eddies can maintain an elliptical shape and a slow and stable SOER during their migration process. The SOER speed was observed to be negatively correlated to eddy lifetime, and exhibited a dependence on latitude, decreasing from low- and high- to mid-latitudes. The SOER direction tended to be consistent with the direction of internal circulation, particularly for long-lived eddies. In addition, we identified a negative relationship between internal circulation speed and SOER speed while the migration speed was positively related to SOER speed. These findings further expand and improve eddy kinematics, which is of great significance for the future study of eddy dynamics.
With the development of satellite altimetry technology, the resolution of sea-level anomaly (SLA) datasets is constantly improving. Current spatial resolution levels can reach a grid size of (1/4)° × (1/4)°, with daily measurements that span from 1993 to 2018, allowing for the precise identification and tracking of individual eddies. In the current study, in addition to the internal circulation and migration of eddies, a new aspect in eddy kinematics is revealed and investigated for the first time: shape-based overall eddy rotation (SOER), based on the intrinsic elliptical shape of eddies identified from a high-resolution SLA dataset. We found that eddies can maintain an elliptical shape and a slow and stable SOER during their migration process. The SOER speed was observed to be negatively correlated to eddy lifetime, and exhibited a dependence on latitude, decreasing from low- and high- to mid-latitudes. The SOER direction tended to be consistent with the direction of internal circulation, particularly for long-lived eddies. In addition, we identified a negative relationship between internal circulation speed and SOER speed while the migration speed was positively related to SOER speed. These findings further expand and improve eddy kinematics, which is of great significance for the future study of eddy dynamics.
