2020 Vol. 39, No. 7
Display Method:
2020, (7): 1-2.
Abstract:
2020, 39(7): 1-10.
doi: 10.1007/s13131-020-1574-4
Abstract:
In the past nearly two decades, the Argo Program has created an unprecedented global observing array with continuous in situ salinity observations, providing opportunities to extend our knowledge on the variability and effects of ocean salinity. In this study, we utilize the Argo data during 2004–2017, together with the satellite observations and a newly released version of ECCO ocean reanalysis, to explore the decadal salinity variability in the Southeast Indian Ocean (SEIO) and its impacts on the regional sea level changes. Both the observations and ECCO reanalysis show that during the Argo era, sea level in the SEIO and the tropical western Pacific experienced a rapid rise in 2005–2013 and a subsequent decline in 2013–2017. Such a decadal phase reversal in sea level could be explained, to a large extent, by the steric sea level variability in the upper 300 m. Argo data further show that, in the SEIO, both the temperature and salinity changes have significant positive contributions to the decadal sea level variations. This is different from much of the Indo-Pacific region, where the halosteric component often has minor or negative contributions to the regional sea level pattern on decadal timescale. The salinity budget analyses based on the ECCO reanalysis indicate that the decadal salinity change in the upper 300 m of SEIO is mainly caused by the horizontal ocean advection. More detailed decomposition reveals that in the SEIO, there exists a strong meridional salinity front between the tropical low-salinity and subtropical high salinity waters. The meridional component of decadal circulation changes will induce strong cross-front salinity exchange and thus the significant regional salinity variations.
In the past nearly two decades, the Argo Program has created an unprecedented global observing array with continuous in situ salinity observations, providing opportunities to extend our knowledge on the variability and effects of ocean salinity. In this study, we utilize the Argo data during 2004–2017, together with the satellite observations and a newly released version of ECCO ocean reanalysis, to explore the decadal salinity variability in the Southeast Indian Ocean (SEIO) and its impacts on the regional sea level changes. Both the observations and ECCO reanalysis show that during the Argo era, sea level in the SEIO and the tropical western Pacific experienced a rapid rise in 2005–2013 and a subsequent decline in 2013–2017. Such a decadal phase reversal in sea level could be explained, to a large extent, by the steric sea level variability in the upper 300 m. Argo data further show that, in the SEIO, both the temperature and salinity changes have significant positive contributions to the decadal sea level variations. This is different from much of the Indo-Pacific region, where the halosteric component often has minor or negative contributions to the regional sea level pattern on decadal timescale. The salinity budget analyses based on the ECCO reanalysis indicate that the decadal salinity change in the upper 300 m of SEIO is mainly caused by the horizontal ocean advection. More detailed decomposition reveals that in the SEIO, there exists a strong meridional salinity front between the tropical low-salinity and subtropical high salinity waters. The meridional component of decadal circulation changes will induce strong cross-front salinity exchange and thus the significant regional salinity variations.
2020, 39(7): 11-18.
doi: 10.1007/s13131-020-1576-2
Abstract:
A strong spring Wyrtki jet (WJ) presents in May 2013 in the eastern equatorial Indian Ocean. The entire buildup and retreat processes of the spring WJ were well captured by two adjacent Acoustic Doppler Current Profilers mounted on the mooring systems. The observed zonal jet behaved as one intraseasonal event with the significant features of abrupt emergence as well as slow disappearance. Further research illustrate that the pronounced surface westerly wind burst during late-April to mid-May, associated with the active phase of a robust eastward-propagating Madden–Julian oscillation in the tropical Indian Ocean, was the dominant reason for the rapid acceleration of surface WJ. In contrasting, the governing mechanism for the jet termination was equatorial wave dynamics rather than wind forcing. The decomposition analysis of equatorial waves and the corresponding changes in the ocean thermocline demonstrated that strong WJ was produced rapidly by the wind-generated oceanic downwelling equatorial Kelvin wave and was terminated subsequently by the westward-propagating equatorial Rossby wave reflecting from eastern boundaries of the Indian Ocean.
A strong spring Wyrtki jet (WJ) presents in May 2013 in the eastern equatorial Indian Ocean. The entire buildup and retreat processes of the spring WJ were well captured by two adjacent Acoustic Doppler Current Profilers mounted on the mooring systems. The observed zonal jet behaved as one intraseasonal event with the significant features of abrupt emergence as well as slow disappearance. Further research illustrate that the pronounced surface westerly wind burst during late-April to mid-May, associated with the active phase of a robust eastward-propagating Madden–Julian oscillation in the tropical Indian Ocean, was the dominant reason for the rapid acceleration of surface WJ. In contrasting, the governing mechanism for the jet termination was equatorial wave dynamics rather than wind forcing. The decomposition analysis of equatorial waves and the corresponding changes in the ocean thermocline demonstrated that strong WJ was produced rapidly by the wind-generated oceanic downwelling equatorial Kelvin wave and was terminated subsequently by the westward-propagating equatorial Rossby wave reflecting from eastern boundaries of the Indian Ocean.
2020, 39(7): 19-31.
doi: 10.1007/s13131-020-1575-3
Abstract:
Interannual variability (IAV) in the barrier layer thickness (BLT) and forcing mechanisms in the eastern equatorial Indian Ocean (EEIO) and Bay of Bengal (BoB) are examined using monthly Argo data sets during 2002–2017. The BLT during November–January (NDJ) in the EEIO shows strong IAV, which is associated with the Indian Ocean dipole mode (IOD), with the IOD leading the BLT by two months. During the negative IOD phase, the westerly wind anomalies driving the downwelling Kelvin waves increase the isothermal layer depth (ILD). Moreover, the variability in the mixed layer depth (MLD) is complex. Affected by the Wyrtki jet, the MLD presents negative anomalies west of 85°E and strong positive anomalies between 85°E and 93°E. Therefore, the BLT shows positive anomalies except between 86°E and 92°E in the EEIO. Additionally, the IAV in the BLT during December–February (DJF) in the BoB is also investigated. In the eastern and northeastern BoB, the IAV in the BLT is remotely forced by equatorial zonal wind stress anomalies associated with the El Niño-Southern Oscillation (ENSO). In the western BoB, the regional surface wind forcing-related ENSO modulates the BLT variations.
Interannual variability (IAV) in the barrier layer thickness (BLT) and forcing mechanisms in the eastern equatorial Indian Ocean (EEIO) and Bay of Bengal (BoB) are examined using monthly Argo data sets during 2002–2017. The BLT during November–January (NDJ) in the EEIO shows strong IAV, which is associated with the Indian Ocean dipole mode (IOD), with the IOD leading the BLT by two months. During the negative IOD phase, the westerly wind anomalies driving the downwelling Kelvin waves increase the isothermal layer depth (ILD). Moreover, the variability in the mixed layer depth (MLD) is complex. Affected by the Wyrtki jet, the MLD presents negative anomalies west of 85°E and strong positive anomalies between 85°E and 93°E. Therefore, the BLT shows positive anomalies except between 86°E and 92°E in the EEIO. Additionally, the IAV in the BLT during December–February (DJF) in the BoB is also investigated. In the eastern and northeastern BoB, the IAV in the BLT is remotely forced by equatorial zonal wind stress anomalies associated with the El Niño-Southern Oscillation (ENSO). In the western BoB, the regional surface wind forcing-related ENSO modulates the BLT variations.
2020, 39(7): 32-41.
doi: 10.1007/s13131-019-1627-8
Abstract:
The characteristics of the T/S structures, water mass exchange and deep circulation in the Andaman Sea are investigated based on the simulation from a high-resolution general circulation model (MITgcm). The results show that, below 1 000 m, the water mass is saltier, warmer and more homogeneous in the Andaman Sea than that in the Bay of Bengal, attributing to the strong vertical mixing at the depth of ~1 800 m. The water mass exchange between the Andaman Sea and the Bay of Bengal goes through three major channels, which manifests itself as follows: the northern channel (Preparis Channel) is the main passage of water mass transport from the Bay of Bengal to the Andaman Sea, whereas the Middle Channel (the south of Andaman Islands and the north of Nicobar Islands) has an opposite transport; the southern channel (Great Channel) features with a four-layer water exchange which results in the least net transport among the three channels; all the transports through the three channels have an intra-annual variation with a period of half a year. At 1 000-m depth, the entire Andaman Sea is occupied by a cyclonic circulation in January and July while by an anticyclonic one in April and October. The semiannual cycle found in both the deep circulation and water mass exchange is likely associated with the downwelling eastward-propagating Kelvin waves induced by the semiannual westerly component in the equatorial Indian Ocean during intermonsoon seasons.
The characteristics of the T/S structures, water mass exchange and deep circulation in the Andaman Sea are investigated based on the simulation from a high-resolution general circulation model (MITgcm). The results show that, below 1 000 m, the water mass is saltier, warmer and more homogeneous in the Andaman Sea than that in the Bay of Bengal, attributing to the strong vertical mixing at the depth of ~1 800 m. The water mass exchange between the Andaman Sea and the Bay of Bengal goes through three major channels, which manifests itself as follows: the northern channel (Preparis Channel) is the main passage of water mass transport from the Bay of Bengal to the Andaman Sea, whereas the Middle Channel (the south of Andaman Islands and the north of Nicobar Islands) has an opposite transport; the southern channel (Great Channel) features with a four-layer water exchange which results in the least net transport among the three channels; all the transports through the three channels have an intra-annual variation with a period of half a year. At 1 000-m depth, the entire Andaman Sea is occupied by a cyclonic circulation in January and July while by an anticyclonic one in April and October. The semiannual cycle found in both the deep circulation and water mass exchange is likely associated with the downwelling eastward-propagating Kelvin waves induced by the semiannual westerly component in the equatorial Indian Ocean during intermonsoon seasons.
2020, 39(7): 42-49.
doi: 10.1007/s13131-020-1613-1
Abstract:
Electron microprobe analysis was conducted on plagioclase from the plagioclase ultraphyric basalts (PUBs) erupted on the Southwest Indian Ridge (SWIR) (51°E) to investigate the geochemical changes in order to better understand the magmatic processes occurring under ultraslow spreading ridges and to provide insights into the thermal and dynamic regimes of the magmatic reservoirs and conduit systems. The phenocryst cores are generally calcic (An74–82) and are depleted in FeO and MgO. Whereas the phenocryst rims (An67–71) and the plagioclase in the groundmass (An58–63) are more sodic and have higher FeO and MgO contents than the phenocryst cores. The crystallization temperatures of the phenocryst cores and the calculation of the equilibrium between the phenocrysts and the matrix suggest that the plagioclase cores are unlikely to have crystallized from the host basaltic melt, but are likely to have crystallized from a more calcic melt. The enrichment in incompatible elements (FeO and MgO), as well as the higher FeO/MgO ratios of the outermost phenocryst rims and the groundmass, are the result of plagioclase-melt disequilibrium diffusion during the short residence time in which the plagioclase crystallized. Our results indicate that an evolved melt replenishing under the SWIR (51°E) drives the eruption over a short period of time.
Electron microprobe analysis was conducted on plagioclase from the plagioclase ultraphyric basalts (PUBs) erupted on the Southwest Indian Ridge (SWIR) (51°E) to investigate the geochemical changes in order to better understand the magmatic processes occurring under ultraslow spreading ridges and to provide insights into the thermal and dynamic regimes of the magmatic reservoirs and conduit systems. The phenocryst cores are generally calcic (An74–82) and are depleted in FeO and MgO. Whereas the phenocryst rims (An67–71) and the plagioclase in the groundmass (An58–63) are more sodic and have higher FeO and MgO contents than the phenocryst cores. The crystallization temperatures of the phenocryst cores and the calculation of the equilibrium between the phenocrysts and the matrix suggest that the plagioclase cores are unlikely to have crystallized from the host basaltic melt, but are likely to have crystallized from a more calcic melt. The enrichment in incompatible elements (FeO and MgO), as well as the higher FeO/MgO ratios of the outermost phenocryst rims and the groundmass, are the result of plagioclase-melt disequilibrium diffusion during the short residence time in which the plagioclase crystallized. Our results indicate that an evolved melt replenishing under the SWIR (51°E) drives the eruption over a short period of time.
2020, 39(7): 50-60.
doi: 10.1007/s13131-020-1623-z
Abstract:
The sea surface temperature (SST) seasonal cycle in the eastern equatorial Pacific (EEP) plays an important role in the El Niño–Southern Oscillation (ENSO) phenomenon. However, the reasonable simulation of SST seasonal cycle in the EEP is still a challenge for climate models. In this paper, we evaluated the performance of 17 CMIP6 climate models in simulating the seasonal cycle in the EEP and compared them with 43 CMIP5 climate models. In general, only CESM2 and SAM0-UNICON are able to successfully capture the annual mean SST characteristics, and the results showed that CMIP6 models have no fundamental improvement in the model annual mean bias. For the seasonal cycle, 14 out of 17 climate models are able to represent the major characteristics of the observed SST annual evolution. In spring, 12 models capture the 1–2 months leading the eastern equatorial Pacific region 1 (EP1; 5°S–5°N, 110°–85°W) against the eastern equatorial Pacific region 2 (EP2; 5°S–5°N, 140°–110°W). In autumn, only two models, GISS-E2-G and SAM0-UNICON, correctly show that the EP1 and EP2 SSTs vary in phase. For the CMIP6 MME SST simulation in EP1, both the cold bias along the equator in the warm phase and the warm bias in the cold phase lead to a weaker annual SST cycle in the CGCMs, which is similar to the CMIP5 results. However, both the seasonal cold bias and warm bias are considerably decreased for CMIP6, which leads the annual SST cycle to more closely reflect the observation. For the CMIP6 MME SST simulation in EP2, the amplitude is similar to the observed value due to the quasi-constant cold bias throughout the year, although the cold bias is clearly improved after August compared with CMIP5 models. Overall, although SAM0-UNICON successfully captured the seasonal cycle characteristics in the EEP and the improvement from CMIP5 to CMIP6 in simulating EEP SST is clear, the fundamental climate models simulated biases still exist.
The sea surface temperature (SST) seasonal cycle in the eastern equatorial Pacific (EEP) plays an important role in the El Niño–Southern Oscillation (ENSO) phenomenon. However, the reasonable simulation of SST seasonal cycle in the EEP is still a challenge for climate models. In this paper, we evaluated the performance of 17 CMIP6 climate models in simulating the seasonal cycle in the EEP and compared them with 43 CMIP5 climate models. In general, only CESM2 and SAM0-UNICON are able to successfully capture the annual mean SST characteristics, and the results showed that CMIP6 models have no fundamental improvement in the model annual mean bias. For the seasonal cycle, 14 out of 17 climate models are able to represent the major characteristics of the observed SST annual evolution. In spring, 12 models capture the 1–2 months leading the eastern equatorial Pacific region 1 (EP1; 5°S–5°N, 110°–85°W) against the eastern equatorial Pacific region 2 (EP2; 5°S–5°N, 140°–110°W). In autumn, only two models, GISS-E2-G and SAM0-UNICON, correctly show that the EP1 and EP2 SSTs vary in phase. For the CMIP6 MME SST simulation in EP1, both the cold bias along the equator in the warm phase and the warm bias in the cold phase lead to a weaker annual SST cycle in the CGCMs, which is similar to the CMIP5 results. However, both the seasonal cold bias and warm bias are considerably decreased for CMIP6, which leads the annual SST cycle to more closely reflect the observation. For the CMIP6 MME SST simulation in EP2, the amplitude is similar to the observed value due to the quasi-constant cold bias throughout the year, although the cold bias is clearly improved after August compared with CMIP5 models. Overall, although SAM0-UNICON successfully captured the seasonal cycle characteristics in the EEP and the improvement from CMIP5 to CMIP6 in simulating EEP SST is clear, the fundamental climate models simulated biases still exist.
2020, 39(7): 61-68.
doi: 10.1007/s13131-020-1626-9
Abstract:
We investigate the air-sea momentum flux in the marine atmospheric boundary layer using a tower-based direct measurement method. First, we compare the collected data with previous observations, and the results are roughly consistent. Next, in the low-to-moderate winds, the exchange coefficients (or drag coefficients) deviate between onshore and offshore winds, which exhibits the influence of surface wave on the momentum flux. Furthermore, we use a surface-wave-involved parameterization scheme to explain the dependence of momentum flux on surface wave. The results consolidate the influence of surface wave on momentum flux on the one hand, and validate the surface-wave-involved parameterization scheme on the other hand.
We investigate the air-sea momentum flux in the marine atmospheric boundary layer using a tower-based direct measurement method. First, we compare the collected data with previous observations, and the results are roughly consistent. Next, in the low-to-moderate winds, the exchange coefficients (or drag coefficients) deviate between onshore and offshore winds, which exhibits the influence of surface wave on the momentum flux. Furthermore, we use a surface-wave-involved parameterization scheme to explain the dependence of momentum flux on surface wave. The results consolidate the influence of surface wave on momentum flux on the one hand, and validate the surface-wave-involved parameterization scheme on the other hand.
2020, 39(7): 69-78.
doi: 10.1007/s13131-020-1573-5
Abstract:
Many typhoons pass through the East China Sea (ECS) and the oceanic responses to typhoons on the ECS shelf are very energetic. However, these responses are not well studied because of the complicated background oceanic environment. The sea surface temperature (SST) response to a severe Typhoon Rananim in August 2004 on the ECS shelf was observed by the merged cloud-penetrating microwave and infrared SST data. The observed SST response shows an extensive SST cooling with a maximum cooling of 3°C on the ECS shelf and the SST cooling lags the typhoon by about one day. A numerical model is designed to simulate the oceanic responses to Rananim. The numerical model reasonably simulates the observed SST response and thereby provides a more comprehensive investigation on the oceanic temperature and current responses. The simulation shows that Rananim deepens the ocean mix layer by more than 10 m on the ECS shelf and causes a cooling in the whole mixed layer. Both upwelling and entrainment are responsible for the cooling. Rananim significantly deforms the background Taiwan Warm Current on the ECS shelf and generates strong Ekman current at the surface. After the typhoon disappears, the surface current rotates clockwise and vertically, the current is featured by near inertial oscillation with upward propagating phase.
Many typhoons pass through the East China Sea (ECS) and the oceanic responses to typhoons on the ECS shelf are very energetic. However, these responses are not well studied because of the complicated background oceanic environment. The sea surface temperature (SST) response to a severe Typhoon Rananim in August 2004 on the ECS shelf was observed by the merged cloud-penetrating microwave and infrared SST data. The observed SST response shows an extensive SST cooling with a maximum cooling of 3°C on the ECS shelf and the SST cooling lags the typhoon by about one day. A numerical model is designed to simulate the oceanic responses to Rananim. The numerical model reasonably simulates the observed SST response and thereby provides a more comprehensive investigation on the oceanic temperature and current responses. The simulation shows that Rananim deepens the ocean mix layer by more than 10 m on the ECS shelf and causes a cooling in the whole mixed layer. Both upwelling and entrainment are responsible for the cooling. Rananim significantly deforms the background Taiwan Warm Current on the ECS shelf and generates strong Ekman current at the surface. After the typhoon disappears, the surface current rotates clockwise and vertically, the current is featured by near inertial oscillation with upward propagating phase.
2020, 39(7): 79-90.
doi: 10.1007/s13131-020-1596-y
Abstract:
The comprehensive three-dimensional structures of an anti-cyclonic mesoscale eddy (AE) in the subtropical northwestern Pacific Ocean were investigated by combining the Argo floats profiles with enhanced vertical and temporal sampling and satellite altimetry data. The AE originated near the Kuroshio Extension and then propagated westward with mean velocity of 8.9 cm/s. Significant changes and evolutions during the AE’s growing stage (T1) and further growing stage (T2) were revealed through composite analysis. In the composite eddy core, maximum temperature (T) and salinity (S) anomalies were of 1.7 (1.9)°C and 0.04 (0.07) psu in T1 (T2) period, respectively. The composite T anomalies showed positive in almost whole depth, but the S anomalies exhibited a sandwich-like pattern. The eddy’s intensification and its influence on the intermediate ocean became more significant during its growth. The trapping depth increased from 400×104 Pa to 580×104 Pa while it was growing up, which means more water volume, heat and salt content in deeper layers can be transported. The AE was strongly nonlinear in upper oceans and can yield a typical mean volume transport of 0.17×106 m3/s and a mean heat and salt transport anomaly of 3.6×1011 W and –2.1×103 kg/s during the observation period. The Energy analysis showed that eddy potential and kinetic energy increased notably as it propagated westward and the baroclinic instability is the major energy source of the eddy growth. The variation of the remained Argo float trapped within the eddy indicated significant water advection during the eddy’s propagation.
The comprehensive three-dimensional structures of an anti-cyclonic mesoscale eddy (AE) in the subtropical northwestern Pacific Ocean were investigated by combining the Argo floats profiles with enhanced vertical and temporal sampling and satellite altimetry data. The AE originated near the Kuroshio Extension and then propagated westward with mean velocity of 8.9 cm/s. Significant changes and evolutions during the AE’s growing stage (T1) and further growing stage (T2) were revealed through composite analysis. In the composite eddy core, maximum temperature (T) and salinity (S) anomalies were of 1.7 (1.9)°C and 0.04 (0.07) psu in T1 (T2) period, respectively. The composite T anomalies showed positive in almost whole depth, but the S anomalies exhibited a sandwich-like pattern. The eddy’s intensification and its influence on the intermediate ocean became more significant during its growth. The trapping depth increased from 400×104 Pa to 580×104 Pa while it was growing up, which means more water volume, heat and salt content in deeper layers can be transported. The AE was strongly nonlinear in upper oceans and can yield a typical mean volume transport of 0.17×106 m3/s and a mean heat and salt transport anomaly of 3.6×1011 W and –2.1×103 kg/s during the observation period. The Energy analysis showed that eddy potential and kinetic energy increased notably as it propagated westward and the baroclinic instability is the major energy source of the eddy growth. The variation of the remained Argo float trapped within the eddy indicated significant water advection during the eddy’s propagation.
2020, 39(7): 91-106.
doi: 10.1007/s13131-020-1603-3
Abstract:
Between June 2015 and June 2017, two pressure-recording inverted echo sounders (PIESs) and five current and pressure-recording inverted echo sounders (CPIESs) deployed along a section across the Kerama Gap acquired a dataset of ocean bottom pressure records in which there was significant 21-day variability (Pbot21). The Pbot21, which was particularly strong from July–December 2016, was coherent with wind stress curl (WSC) on the continental shelf of the East China Sea (ECS) with a squared coherence of 0.65 for a 3-day time lag. A barotropic ocean model demonstrated the generation, propagation, and dissipation of Pbot21. The modeled results show that the Pbot21 driven by coastal ocean WSC in the ECS propagated toward the Ryukyu Island Chain (RIC), while deep ocean WSC could not induce such variability. On the continental shelf, the Pbot21 was generated nearly synchronously with the WSC from the coastline to the southeast but dissipated within a few days due to the effect of bottom friction. The detection of Pbot21 by the moored array was dependent on the 21-day WSC patterns on the continental shelf. The Pbot21 driven southeast of the Changjiang Estuary by the WSC was detected while the Pbot21 generated northeast of the Changjiang Estuary was not.
Between June 2015 and June 2017, two pressure-recording inverted echo sounders (PIESs) and five current and pressure-recording inverted echo sounders (CPIESs) deployed along a section across the Kerama Gap acquired a dataset of ocean bottom pressure records in which there was significant 21-day variability (Pbot21). The Pbot21, which was particularly strong from July–December 2016, was coherent with wind stress curl (WSC) on the continental shelf of the East China Sea (ECS) with a squared coherence of 0.65 for a 3-day time lag. A barotropic ocean model demonstrated the generation, propagation, and dissipation of Pbot21. The modeled results show that the Pbot21 driven by coastal ocean WSC in the ECS propagated toward the Ryukyu Island Chain (RIC), while deep ocean WSC could not induce such variability. On the continental shelf, the Pbot21 was generated nearly synchronously with the WSC from the coastline to the southeast but dissipated within a few days due to the effect of bottom friction. The detection of Pbot21 by the moored array was dependent on the 21-day WSC patterns on the continental shelf. The Pbot21 driven southeast of the Changjiang Estuary by the WSC was detected while the Pbot21 generated northeast of the Changjiang Estuary was not.
2020, 39(7): 107-114.
doi: 10.1007/s13131-020-1625-x
Abstract:
This study presents an analysis of the spectral characteristics of remote sensing reflectance (Rrs) in northwestern South China Sea based on the in situ optical and water quality data for August 2018. Rrs was initially divided into four classes, classes A to D, using the max-classification algorithm, and the spectral properties of whole Rrs were characterized using the empirical orthogonal function (EOF) analysis. Subsequently, the dominant factors in each EOF mode were determined.The results indicated that more than 95% of the variances of Rrs are partly driven by the back-scattering characteristics of the suspended matter. The initial two EOF modes were well correlated with the total suspended matter and back–scattering coefficient. Furthermore, the first EOF modes of the four classes of Rrs (A–D Rrs–EOF1) significantly contributed to the total variances of each Rrs class. In addition, the correlation coefficients between the amplitude factors of class A–D Rrs–EOF1 and the variances of the relevant water quality and optical parameters were better than those of the unclassified ones. The spectral shape of class A Rrs–EOF1 was governed by the absorption characteristic of chlorophyll a and colored dissolved organic matter (CDOM). The spectral shape of class B Rrs–EOF1 was governed by the absorption characteristic of CDOM since it exhibited a high correlation with the absorption coefficient of CDOM (ag (λ)), whereas the spectral shape of class C Rrs–EOF1 was governed by the back-scattering characteristics but not affected by the suspended matter. The spectral shape of class D Rrs–EOF1 exhibited a relatively good correlation with all the water quality parameters, which played a significant role in deciding its spectral shape.
This study presents an analysis of the spectral characteristics of remote sensing reflectance (Rrs) in northwestern South China Sea based on the in situ optical and water quality data for August 2018. Rrs was initially divided into four classes, classes A to D, using the max-classification algorithm, and the spectral properties of whole Rrs were characterized using the empirical orthogonal function (EOF) analysis. Subsequently, the dominant factors in each EOF mode were determined.The results indicated that more than 95% of the variances of Rrs are partly driven by the back-scattering characteristics of the suspended matter. The initial two EOF modes were well correlated with the total suspended matter and back–scattering coefficient. Furthermore, the first EOF modes of the four classes of Rrs (A–D Rrs–EOF1) significantly contributed to the total variances of each Rrs class. In addition, the correlation coefficients between the amplitude factors of class A–D Rrs–EOF1 and the variances of the relevant water quality and optical parameters were better than those of the unclassified ones. The spectral shape of class A Rrs–EOF1 was governed by the absorption characteristic of chlorophyll a and colored dissolved organic matter (CDOM). The spectral shape of class B Rrs–EOF1 was governed by the absorption characteristic of CDOM since it exhibited a high correlation with the absorption coefficient of CDOM (ag (λ)), whereas the spectral shape of class C Rrs–EOF1 was governed by the back-scattering characteristics but not affected by the suspended matter. The spectral shape of class D Rrs–EOF1 exhibited a relatively good correlation with all the water quality parameters, which played a significant role in deciding its spectral shape.
2020, 39(7): 115-126.
doi: 10.1007/s13131-020-1622-0
Abstract:
This study examines wave reflection by a multi-chamber partially perforated caisson breakwater based on potential theory. A quadratic pressure drop boundary condition at perforated walls is adopted, which can well consider the effect of wave height on the wave dissipation by perforated walls. The matched eigenfunction expansions with iterative calculations are applied to develop an analytical solution for the present problem. The convergences of both the iterative calculations and the series solution itself are confirmed to be satisfactory. The calculation results of the present analytical solution are in excellent agreement with the numerical results of a multi-domain boundary element solution. Also, the predictions by the present solution are in reasonable agreement with experimental data in literature. Major factors that affect the reflection coefficient of the perforated caisson breakwater are examined by calculation examples. The analysis results show that the multi-chamber perforated caisson breakwater has a better wave energy dissipation function (lower reflection coefficient) than the single-chamber type over a broad range of wave frequency and may perform better if the perforated walls have larger porosities. When the porosities of the perforated walls decrease along the incident wave direction, the perforated caisson breakwater can achieve a lower reflection coefficient. The present analytical solution is simple and reliable, and it can be used as an efficient tool for analyzing the hydrodynamic performance of perforated breakwaters in preliminary engineering design.
This study examines wave reflection by a multi-chamber partially perforated caisson breakwater based on potential theory. A quadratic pressure drop boundary condition at perforated walls is adopted, which can well consider the effect of wave height on the wave dissipation by perforated walls. The matched eigenfunction expansions with iterative calculations are applied to develop an analytical solution for the present problem. The convergences of both the iterative calculations and the series solution itself are confirmed to be satisfactory. The calculation results of the present analytical solution are in excellent agreement with the numerical results of a multi-domain boundary element solution. Also, the predictions by the present solution are in reasonable agreement with experimental data in literature. Major factors that affect the reflection coefficient of the perforated caisson breakwater are examined by calculation examples. The analysis results show that the multi-chamber perforated caisson breakwater has a better wave energy dissipation function (lower reflection coefficient) than the single-chamber type over a broad range of wave frequency and may perform better if the perforated walls have larger porosities. When the porosities of the perforated walls decrease along the incident wave direction, the perforated caisson breakwater can achieve a lower reflection coefficient. The present analytical solution is simple and reliable, and it can be used as an efficient tool for analyzing the hydrodynamic performance of perforated breakwaters in preliminary engineering design.
2020, 39(7): 127-134.
doi: 10.1007/s13131-020-1610-4
Abstract:
HY-2A (Haiyang-2A), launched in 2011, is the first ocean dynamic environment satellite of China and is equipped with a radar altimeter as one of the primary payloads. HY-2A shifted the drift orbit in March 2016 and has been accumulating geodetic mission (GM) data for more than three years with 168-day cycle. In this paper, we present the preliminary gravity field inverted by the HY-2A/GM data from March 2016 to December 2017 near Taiwan (21°–26°N, 119°–123°E). The gravity anomaly is computed by Inverse Vening Meinesz (IVM) formula with a one-dimensional FFT method during remove-restore procedure with the EGM2008 gravity model as the reference field. For comparison, CryoSat-2 altimeter data are used to inverse the gravity field near Taiwan Island by the same method. Comparing with the gravity field derived from CryoSat-2, a good agreement between the two data sets is found. The global ocean gravity models and National Geophysical Data Center (NGDC) shipboard gravity data also are used to assess the performance of HY-2A/GM data. The evaluations show that HY-2A and CryoSat-2 are at the same level in terms of gravity field recovery and the HY-2A/GM altimeter-derived gravity field has an accuracy of 2.922 mGal. Therefore, we can believe that HY-2A will be a new reliable data source for marine gravity field inversion and has the potentiality to improve the accuracy and resolution of the global marine gravity field.
HY-2A (Haiyang-2A), launched in 2011, is the first ocean dynamic environment satellite of China and is equipped with a radar altimeter as one of the primary payloads. HY-2A shifted the drift orbit in March 2016 and has been accumulating geodetic mission (GM) data for more than three years with 168-day cycle. In this paper, we present the preliminary gravity field inverted by the HY-2A/GM data from March 2016 to December 2017 near Taiwan (21°–26°N, 119°–123°E). The gravity anomaly is computed by Inverse Vening Meinesz (IVM) formula with a one-dimensional FFT method during remove-restore procedure with the EGM2008 gravity model as the reference field. For comparison, CryoSat-2 altimeter data are used to inverse the gravity field near Taiwan Island by the same method. Comparing with the gravity field derived from CryoSat-2, a good agreement between the two data sets is found. The global ocean gravity models and National Geophysical Data Center (NGDC) shipboard gravity data also are used to assess the performance of HY-2A/GM data. The evaluations show that HY-2A and CryoSat-2 are at the same level in terms of gravity field recovery and the HY-2A/GM altimeter-derived gravity field has an accuracy of 2.922 mGal. Therefore, we can believe that HY-2A will be a new reliable data source for marine gravity field inversion and has the potentiality to improve the accuracy and resolution of the global marine gravity field.
2020, 39(7): 135-145.
doi: 10.1007/s13131-020-1612-4
Abstract:
In this study, oil spill experiments were performed in a water tank to determine changes in the surface scattering characteristics during the emulsification of oil spills. A C-band fully-polarimetric microwave scatterometer and a vector network analyzer were used to observe films of the following oils: crude oil with an asphalt content below 3% that is prone to emulsification (type A), fresh crude oil extracted from an oilfield (type B), and industrial crude oil that was dehydrated and purified (type C). The difference in the backscatter results between the emulsified oil film and the calm water surface under C-band microwaves and the influence of the emulsification of the oil film on the backscatter were analyzed in detail. The results demonstrate that under a low-wind and no-waves condition (the maximum wave height was below than 3 mm), the emulsification of crude oil could modulated the backscatter through changes in the surface roughness and the dielectric constant, where the surface roughness had the dominant effect. The surface backscatters of the type B oil were greater than that of the type C oil in both the emulsified and non-emulsified states. In the non-emulsified state, the average differences in the backscatter between the type B and C oils were 2.19 dB, 2.63 dB, and 2.21 dB for the polarization modes of VV, HH, and HV/VH, respectively. Smaller corresponding average differences of 0.98 dB, 1.49 dB, and 1.5 dB were found for the emulsified state with a 20% moisture constant for the oil film. The results demonstrated that the surface roughness of the different oil films could vary due to the differences in the oil compositions and the oil film properties, which in turn affect the backscatter of the oil film surface.
In this study, oil spill experiments were performed in a water tank to determine changes in the surface scattering characteristics during the emulsification of oil spills. A C-band fully-polarimetric microwave scatterometer and a vector network analyzer were used to observe films of the following oils: crude oil with an asphalt content below 3% that is prone to emulsification (type A), fresh crude oil extracted from an oilfield (type B), and industrial crude oil that was dehydrated and purified (type C). The difference in the backscatter results between the emulsified oil film and the calm water surface under C-band microwaves and the influence of the emulsification of the oil film on the backscatter were analyzed in detail. The results demonstrate that under a low-wind and no-waves condition (the maximum wave height was below than 3 mm), the emulsification of crude oil could modulated the backscatter through changes in the surface roughness and the dielectric constant, where the surface roughness had the dominant effect. The surface backscatters of the type B oil were greater than that of the type C oil in both the emulsified and non-emulsified states. In the non-emulsified state, the average differences in the backscatter between the type B and C oils were 2.19 dB, 2.63 dB, and 2.21 dB for the polarization modes of VV, HH, and HV/VH, respectively. Smaller corresponding average differences of 0.98 dB, 1.49 dB, and 1.5 dB were found for the emulsified state with a 20% moisture constant for the oil film. The results demonstrated that the surface roughness of the different oil films could vary due to the differences in the oil compositions and the oil film properties, which in turn affect the backscatter of the oil film surface.
2020, 39(7): 146-164.
doi: 10.1007/s13131-020-1619-8
Abstract:
In previous studies, Lagrangian analyses were used to assess large-scale ocean circulation, and the Lagrangian coherent structure could also reveal the evolution of the two-dimensional structure of the mesoscale eddies. However, few studies have demonstrated the three-dimensional structure of the mesoscale eddies via Lagrangian analysis. Compared with previous studies, which investigated the eddy structure via a Eulerian view, we used a Lagrangian view to provide a different perspective to study the eddy structure. An idealized cyclonic mesoscale eddy is built up over a seamount, and it presents downwelling inside the eddy and upwelling alongside the eddy formed within a closed circulation system. This structure is difficult to display via a Eulerian analysis. However, the trajectories of particles can well demonstrate the full cycle: the fluid sank and rotated inside the eddies, converged to the upwelling zone of the bottom layer and returned to the surface through upwelling. We also applied a Lagrangian analysis to a realistic simulation. As a significant phenomenon in the South China Sea, the dipole structure of the anticyclonic eddy (AE)/cyclonic eddy (CE) pair off of central Vietnam has been well studied but mainly at the sea surface. With a Lagrangian analysis, we illustrate the three-dimensional structure of the eddy pair: the fluid sank (rose) and rotated inside the AE (CE). More importantly, the trajectories of the particles suggested that there was no fluid exchange between the two eddies since the strong boundary jet separates them from each other. All the conclusions above have been verified and are supported by the computational error estimate. With a selected time step and integral period, the computational errors always present small values, although they increase with strong divergent and vertical diffusive flow.
In previous studies, Lagrangian analyses were used to assess large-scale ocean circulation, and the Lagrangian coherent structure could also reveal the evolution of the two-dimensional structure of the mesoscale eddies. However, few studies have demonstrated the three-dimensional structure of the mesoscale eddies via Lagrangian analysis. Compared with previous studies, which investigated the eddy structure via a Eulerian view, we used a Lagrangian view to provide a different perspective to study the eddy structure. An idealized cyclonic mesoscale eddy is built up over a seamount, and it presents downwelling inside the eddy and upwelling alongside the eddy formed within a closed circulation system. This structure is difficult to display via a Eulerian analysis. However, the trajectories of particles can well demonstrate the full cycle: the fluid sank and rotated inside the eddies, converged to the upwelling zone of the bottom layer and returned to the surface through upwelling. We also applied a Lagrangian analysis to a realistic simulation. As a significant phenomenon in the South China Sea, the dipole structure of the anticyclonic eddy (AE)/cyclonic eddy (CE) pair off of central Vietnam has been well studied but mainly at the sea surface. With a Lagrangian analysis, we illustrate the three-dimensional structure of the eddy pair: the fluid sank (rose) and rotated inside the AE (CE). More importantly, the trajectories of the particles suggested that there was no fluid exchange between the two eddies since the strong boundary jet separates them from each other. All the conclusions above have been verified and are supported by the computational error estimate. With a selected time step and integral period, the computational errors always present small values, although they increase with strong divergent and vertical diffusive flow.
2020, 39(7): 165-174.
doi: 10.1007/s13131-020-1624-y
Abstract:
Recent studies have revealed that the predominant tidal constituents have seasonal variations at some locations. However, how to accurately obtain these variations remains a problem for the traditional harmonic analysis (HA) due to the tradeoff between length of time window and resolution of constituents. Therefore, a method named as “two-step HA” is developed in this study, which consists of both long- and short-time-window HA. Through a series of ideal experiments, practical application at two tidal gauges and comparison with the traditional HA, the feasibility and accuracy of the two-step HA are verified: The two-step HA performs better than the traditional HA in estimating monthly amplitudes and phases for the predominant constituents, whether they have seasonal variability or not. In addition to capturing variations of the predominant constituents at tidal gauges, the two-step HA would be useful in investigation of the coherence and incoherence of internal tides.
Recent studies have revealed that the predominant tidal constituents have seasonal variations at some locations. However, how to accurately obtain these variations remains a problem for the traditional harmonic analysis (HA) due to the tradeoff between length of time window and resolution of constituents. Therefore, a method named as “two-step HA” is developed in this study, which consists of both long- and short-time-window HA. Through a series of ideal experiments, practical application at two tidal gauges and comparison with the traditional HA, the feasibility and accuracy of the two-step HA are verified: The two-step HA performs better than the traditional HA in estimating monthly amplitudes and phases for the predominant constituents, whether they have seasonal variability or not. In addition to capturing variations of the predominant constituents at tidal gauges, the two-step HA would be useful in investigation of the coherence and incoherence of internal tides.
