2019 Vol. 38, No. 4
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2019, 38(4): .
Abstract:
2019, 38(4): 1-11.
doi: 10.1007/s13131-019-1410-x
Abstract:
Besides the Indonesian throughflow (ITF), the South China Sea throughflow (SCSTF) also contributes to the water transport from the Pacific to the Indian Ocean. However, this South China Sea (SCS) branch at the Karimata Strait is poorly observed until 2007, even though its importance has been suggested by numerical studies for decades. In this paper, we review the nearly 10-year field measurement in the Karimata Strait by the execution of the projects of "SCS-Indonesian Seas Transport/Exchange (SITE) and Impacts on Seasonal Fish Migration" and "The Transport, Internal Waves and Mixing in the Indonesian Throughflow regions (TIMIT) and Impacts on Marine Ecosystem", which extend the observations from the western Indonesian seas to the east to include the main channels of the ITF, is introduced. Some major achievements from these projects are summarized.
Besides the Indonesian throughflow (ITF), the South China Sea throughflow (SCSTF) also contributes to the water transport from the Pacific to the Indian Ocean. However, this South China Sea (SCS) branch at the Karimata Strait is poorly observed until 2007, even though its importance has been suggested by numerical studies for decades. In this paper, we review the nearly 10-year field measurement in the Karimata Strait by the execution of the projects of "SCS-Indonesian Seas Transport/Exchange (SITE) and Impacts on Seasonal Fish Migration" and "The Transport, Internal Waves and Mixing in the Indonesian Throughflow regions (TIMIT) and Impacts on Marine Ecosystem", which extend the observations from the western Indonesian seas to the east to include the main channels of the ITF, is introduced. Some major achievements from these projects are summarized.
2019, 38(4): 12-19.
doi: 10.1007/s13131-019-1411-9
Abstract:
This study examines a Kuroshio main path (KMP) cut-off event east of Taiwan Island occurred in fall-winter 2013-2014 and its impacts on the South China Sea (SCS) by analyzing satellite altimetry and mooring observations. Satellite altimeter sea level anomaly (SLA) images reveal a complete process that a huge cyclonic eddy (CE) from the Pacific collided with the Kuroshio and the western boundary from 15 October 2013 to 15 January 2014. Mooring observations evidenced that the Kuroshio upper ocean volume transport was cut off more than 82% from 17×106 m3/s in September to 3×106 m3/s in November 2013. The KMP cut-off event caused the Kuroshio branching and intruding into the SCS and strengthened the eddy kinetic energy in the northern SCS west of the Luzon Strait. Using the total momentum as a dynamic criterion to determine the role of eddy collision with the Kuroshio reasonably explains the KMP cut-off event.
This study examines a Kuroshio main path (KMP) cut-off event east of Taiwan Island occurred in fall-winter 2013-2014 and its impacts on the South China Sea (SCS) by analyzing satellite altimetry and mooring observations. Satellite altimeter sea level anomaly (SLA) images reveal a complete process that a huge cyclonic eddy (CE) from the Pacific collided with the Kuroshio and the western boundary from 15 October 2013 to 15 January 2014. Mooring observations evidenced that the Kuroshio upper ocean volume transport was cut off more than 82% from 17×106 m3/s in September to 3×106 m3/s in November 2013. The KMP cut-off event caused the Kuroshio branching and intruding into the SCS and strengthened the eddy kinetic energy in the northern SCS west of the Luzon Strait. Using the total momentum as a dynamic criterion to determine the role of eddy collision with the Kuroshio reasonably explains the KMP cut-off event.
2019, 38(4): 20-28.
doi: 10.1007/s13131-019-1409-3
Abstract:
Mesoscale eddy generation mechanisms in the eastern South China Sea (ESCS) are investigated using altimetry observations and solutions of a nonlinear, 1½-layer reduced-gravity model. We estimate the relative roles of the wind forcing in the interior South China Sea (SCS) and the remote forcing from the western tropical Pacific (WTP) in eddy generation in the ESCS. Model solutions show that the high-frequency wind in the interior SCS is the primary forcing for eddies, which explains about 54% of the mesoscale eddies generated in the ESCS. Signals from the WTP also play an important role. Wind-driven equatorial signals reach the west coast of Luzon Island through the Sibutu Passage and Mindoro Strait. The reflected Rossby waves from the west coast of Luzon Island propagate westward, become unstable, and turn into eddies. The signals driven by high-frequency wind from the WTP explain about 40% of the mesoscale eddies generated in the ESCS. The high-frequency wind forcing in both the SCS and the WTP is important for eddy generation in the ESCS.
Mesoscale eddy generation mechanisms in the eastern South China Sea (ESCS) are investigated using altimetry observations and solutions of a nonlinear, 1½-layer reduced-gravity model. We estimate the relative roles of the wind forcing in the interior South China Sea (SCS) and the remote forcing from the western tropical Pacific (WTP) in eddy generation in the ESCS. Model solutions show that the high-frequency wind in the interior SCS is the primary forcing for eddies, which explains about 54% of the mesoscale eddies generated in the ESCS. Signals from the WTP also play an important role. Wind-driven equatorial signals reach the west coast of Luzon Island through the Sibutu Passage and Mindoro Strait. The reflected Rossby waves from the west coast of Luzon Island propagate westward, become unstable, and turn into eddies. The signals driven by high-frequency wind from the WTP explain about 40% of the mesoscale eddies generated in the ESCS. The high-frequency wind forcing in both the SCS and the WTP is important for eddy generation in the ESCS.
2019, 38(4): 29-38.
doi: 10.1007/s13131-018-1222-4
Abstract:
The seasonal characteristics and formation mechanism of the thermohaline structure of mesoscale eddy in the South China Sea are investigated using the latest eddy dataset and ARMOR3D data. Eddy-centric composites reveal that the horizontal distribution of temperature anomaly associated with eddy in winter is more of a dipole pattern in upper 50 m and tends to be centrosymmetric below 50 m, while in summer the distribution pattern is centrosymmetric in the entire water column. The horizontal distribution of eddy-induced salinity anomaly exhibits similar seasonal characteristics, except that the asymmetry of the salinity anomaly is weaker. The vertical distribution of temperature anomaly associated with eddy shows a monolayer structure, while the salinity anomaly demonstrates a triple-layer structure. Further analysis indicates that the vertical distribution of the anomalies is related to the vertical structure of background temperature and salinity fields, and the asymmetry of the anomalies in upper 50 m is mainly caused by the horizontal advection of background temperature and salinity.
The seasonal characteristics and formation mechanism of the thermohaline structure of mesoscale eddy in the South China Sea are investigated using the latest eddy dataset and ARMOR3D data. Eddy-centric composites reveal that the horizontal distribution of temperature anomaly associated with eddy in winter is more of a dipole pattern in upper 50 m and tends to be centrosymmetric below 50 m, while in summer the distribution pattern is centrosymmetric in the entire water column. The horizontal distribution of eddy-induced salinity anomaly exhibits similar seasonal characteristics, except that the asymmetry of the salinity anomaly is weaker. The vertical distribution of temperature anomaly associated with eddy shows a monolayer structure, while the salinity anomaly demonstrates a triple-layer structure. Further analysis indicates that the vertical distribution of the anomalies is related to the vertical structure of background temperature and salinity fields, and the asymmetry of the anomalies in upper 50 m is mainly caused by the horizontal advection of background temperature and salinity.
2019, 38(4): 39-46.
doi: 10.1007/s13131-019-1412-8
Abstract:
In this study, subsurface eddies near the Vietnam coast of the South China Sea were observed with in situ observations, including Argo, CTD, XBT and some processed and quality controlled data. Based on temperature profiles from four Argo floats near the coast of Vietnam, a subsurface warm eddy was identified in spring and summer. The multi-year Argo and Global Temperature and Salinity Profile Programme (GTSPP) data were merged on a seasonal basis based on the data interpolating variational analysis (DIVA) method to reconstruct the three-dimensional temperature structure. There is a warm eddy in the central subsurface at 12.5°N, 111°E below 300 m depth in spring, which does not exist in autumn and is weak in winter and summer. From CSIRO Atlas of Regional Seas (CARS) and Generalized Digital Environment Model (GDEM) reanalysis data, this subsurface warm eddy is also verified in spring.
In this study, subsurface eddies near the Vietnam coast of the South China Sea were observed with in situ observations, including Argo, CTD, XBT and some processed and quality controlled data. Based on temperature profiles from four Argo floats near the coast of Vietnam, a subsurface warm eddy was identified in spring and summer. The multi-year Argo and Global Temperature and Salinity Profile Programme (GTSPP) data were merged on a seasonal basis based on the data interpolating variational analysis (DIVA) method to reconstruct the three-dimensional temperature structure. There is a warm eddy in the central subsurface at 12.5°N, 111°E below 300 m depth in spring, which does not exist in autumn and is weak in winter and summer. From CSIRO Atlas of Regional Seas (CARS) and Generalized Digital Environment Model (GDEM) reanalysis data, this subsurface warm eddy is also verified in spring.
2019, 38(4): 47-57.
doi: 10.1007/s13131-018-1224-2
Abstract:
Four trawl-resistant bottom mounts, with acoustic Doppler current profilers (ADCPs) embedded, were deployed in the Karimata Strait from November 2008 to June 2015 as part of the South China Sea-Indonesian Seas Transport/Exchange and Impact on Seasonal Fish Migration (SITE) Program, to estimate the volume and property transport between the South China Sea and Indonesian seas via the strait. The observed current data reveal that the volume transport through the Karimata Strait exhibits significant seasonal variation. The winter-averaged (from December to February) transport is -1.99 Sv (1 Sv=1×106 m3/s), while in the boreal summer (from June to August), the average transport is 0.69 Sv. Moreover, the average transport from January 2009 to December 2014 is -0.74 Sv (the positive/negative value indicates northward/southward transport). May and September are the transition period. In May, the currents in the Karimata Strait turn northward, consistent with the local monsoon. In September, the southeasterly trade wind is still present over the strait, driving surface water northward, whereas the bottom flow reverses direction, possibly because of the pressure gradient across the strait from north to south.
Four trawl-resistant bottom mounts, with acoustic Doppler current profilers (ADCPs) embedded, were deployed in the Karimata Strait from November 2008 to June 2015 as part of the South China Sea-Indonesian Seas Transport/Exchange and Impact on Seasonal Fish Migration (SITE) Program, to estimate the volume and property transport between the South China Sea and Indonesian seas via the strait. The observed current data reveal that the volume transport through the Karimata Strait exhibits significant seasonal variation. The winter-averaged (from December to February) transport is -1.99 Sv (1 Sv=1×106 m3/s), while in the boreal summer (from June to August), the average transport is 0.69 Sv. Moreover, the average transport from January 2009 to December 2014 is -0.74 Sv (the positive/negative value indicates northward/southward transport). May and September are the transition period. In May, the currents in the Karimata Strait turn northward, consistent with the local monsoon. In September, the southeasterly trade wind is still present over the strait, driving surface water northward, whereas the bottom flow reverses direction, possibly because of the pressure gradient across the strait from north to south.
2019, 38(4): 58-71.
doi: 10.1007/s13131-019-1413-7
Abstract:
So far, large uncertainties of the Indonesian throughflow (ITF) reside in the eastern Indonesian seas, such as the Maluku Sea and the Halmahera Sea. In this study, the water sources of the Maluku Sea and the Halmahera Sea are diagnosed at seasonal and interannual timescales and at different vertical layers, using the state-of-the-art simulations of the Ocean General Circulation Model (OGCM) for Earth Simulator (OFES). Asian monsoon leaves clear seasonal footprints on the eastern Indonesian seas. Consequently, the subsurface waters (around 24.5σθ and at~150 m) in both the Maluku Sea and the Halmahera Sea stem from the South Pacific (SP) during winter monsoon, but during summer monsoon the Maluku Sea is from the North Pacific (NP), and the Halmahera Sea is a mixture of waters originating from the NP and the SP. The monsoon impact decreases with depth, so that in the Maluku Sea, the intermediate water (around 26.8σθ and at~480 m) is always from the northern Banda Sea and the Halmahera Sea water is mainly from the SP in winter and the Banda Sea in summer. The deep waters (around 27.2σθ and at~1 040 m) in both seas are from the SP, with weak seasonal variability. At the interannual timescale, the subsurface water in the Maluku Sea originates from the NP/SP during El Niño/La Niña, while the subsurface water in the Halmahera Sea always originates from the SP. Similar to the seasonal variability, the intermediate water in Maluku Sea mainly comes from the Banda Sea and the Halmahera Sea always originates from the SP. The deep waters in both seas are from the SP. Our findings are helpful for drawing a comprehensive picture of the water properties in the Indonesian seas and will contribute to a better understanding of the ocean-atmosphere interaction over the maritime continent.
So far, large uncertainties of the Indonesian throughflow (ITF) reside in the eastern Indonesian seas, such as the Maluku Sea and the Halmahera Sea. In this study, the water sources of the Maluku Sea and the Halmahera Sea are diagnosed at seasonal and interannual timescales and at different vertical layers, using the state-of-the-art simulations of the Ocean General Circulation Model (OGCM) for Earth Simulator (OFES). Asian monsoon leaves clear seasonal footprints on the eastern Indonesian seas. Consequently, the subsurface waters (around 24.5σθ and at~150 m) in both the Maluku Sea and the Halmahera Sea stem from the South Pacific (SP) during winter monsoon, but during summer monsoon the Maluku Sea is from the North Pacific (NP), and the Halmahera Sea is a mixture of waters originating from the NP and the SP. The monsoon impact decreases with depth, so that in the Maluku Sea, the intermediate water (around 26.8σθ and at~480 m) is always from the northern Banda Sea and the Halmahera Sea water is mainly from the SP in winter and the Banda Sea in summer. The deep waters (around 27.2σθ and at~1 040 m) in both seas are from the SP, with weak seasonal variability. At the interannual timescale, the subsurface water in the Maluku Sea originates from the NP/SP during El Niño/La Niña, while the subsurface water in the Halmahera Sea always originates from the SP. Similar to the seasonal variability, the intermediate water in Maluku Sea mainly comes from the Banda Sea and the Halmahera Sea always originates from the SP. The deep waters in both seas are from the SP. Our findings are helpful for drawing a comprehensive picture of the water properties in the Indonesian seas and will contribute to a better understanding of the ocean-atmosphere interaction over the maritime continent.
2019, 38(4): 72-79.
doi: 10.1007/s13131-019-1414-6
Abstract:
Based on the high-resolution Eulerian fields of an ocean general circulation model simulation, the heat contribution of the Indonesian throughflow (ITF) to the Indian Ocean is estimated by Lagrangian tracing method. The heat transport of each particle of ITF waters is calculated by tracing temperature change along the trajectory until the particle exits the Indian Ocean. The simulation reveals that the ITF waters flow westward and branch near Madagascar, further showing the ITF waters are redistributed in both northern and southern Indian Ocean. Heat budget analysis indicates that the ITF waters gain 0.41 PW (Petawatts, 1015 W) in the northern Indian Ocean and lose 0.56 PW in the southern Indian Ocean, respectively. As a result, the ITF waters warm the whole Indian Ocean basin with only 0.15 PW, which shows an "insignificant" role of ITF on the Indian Ocean because of the heat exchange compensation between northern and southern Indian Ocean. Furthermore, the tracing pathways show that the ITF waters mainly flow out the Indian Ocean at both sides of the basin via Agulhas Current and Leeuwin Current. About 89% of the ITF waters leave along western boundary and the rest 11% along eastern boundary. Compared to seeding section, 0.10 PW and 0.05 PW are released to the Indian Ocean, respectively.
Based on the high-resolution Eulerian fields of an ocean general circulation model simulation, the heat contribution of the Indonesian throughflow (ITF) to the Indian Ocean is estimated by Lagrangian tracing method. The heat transport of each particle of ITF waters is calculated by tracing temperature change along the trajectory until the particle exits the Indian Ocean. The simulation reveals that the ITF waters flow westward and branch near Madagascar, further showing the ITF waters are redistributed in both northern and southern Indian Ocean. Heat budget analysis indicates that the ITF waters gain 0.41 PW (Petawatts, 1015 W) in the northern Indian Ocean and lose 0.56 PW in the southern Indian Ocean, respectively. As a result, the ITF waters warm the whole Indian Ocean basin with only 0.15 PW, which shows an "insignificant" role of ITF on the Indian Ocean because of the heat exchange compensation between northern and southern Indian Ocean. Furthermore, the tracing pathways show that the ITF waters mainly flow out the Indian Ocean at both sides of the basin via Agulhas Current and Leeuwin Current. About 89% of the ITF waters leave along western boundary and the rest 11% along eastern boundary. Compared to seeding section, 0.10 PW and 0.05 PW are released to the Indian Ocean, respectively.
2019, 38(4): 80-89.
doi: 10.1007/s13131-019-1415-5
Abstract:
Near-bottom currents play important roles in the formation and dynamics of deep-water sedimentary systems. This study examined the characteristics and temporal variations of near-bottom currents, especially the tidal components, based on two campaigns (2014 and 2016) of in situ observations conducted southeast of the Dongsha Island in the South China Sea. Results demonstrated near-bottom currents are dominated by tidal currents, the variance of which could account for~70% of the total current variance. Diurnal tidal currents were found stronger than semidiurnal currents for both barotropic and baroclinic components. The diurnal tidal currents were found polarized with predominantly clockwise-rotating constituents, whereas the clockwise and counterclockwise constituents were found comparable for semidiurnal tidal currents. It was established that diurnal tidal currents could induce strong current shear. Baroclinic tidal currents showed pronounced seasonal variation with large magnitude in winter and summer and weak magnitude in spring and autumn in 2014. The coherent components accounted for~65% and~50% of the diurnal and semidiurnal tidal current variances, respectively. The proportions of the coherent and incoherent components changed little in different seasons. In addition to tidal currents, it was determined that the passing of mesoscale eddies could induce strong near-bottom currents that have considerable influence on the deep circulation.
Near-bottom currents play important roles in the formation and dynamics of deep-water sedimentary systems. This study examined the characteristics and temporal variations of near-bottom currents, especially the tidal components, based on two campaigns (2014 and 2016) of in situ observations conducted southeast of the Dongsha Island in the South China Sea. Results demonstrated near-bottom currents are dominated by tidal currents, the variance of which could account for~70% of the total current variance. Diurnal tidal currents were found stronger than semidiurnal currents for both barotropic and baroclinic components. The diurnal tidal currents were found polarized with predominantly clockwise-rotating constituents, whereas the clockwise and counterclockwise constituents were found comparable for semidiurnal tidal currents. It was established that diurnal tidal currents could induce strong current shear. Baroclinic tidal currents showed pronounced seasonal variation with large magnitude in winter and summer and weak magnitude in spring and autumn in 2014. The coherent components accounted for~65% and~50% of the diurnal and semidiurnal tidal current variances, respectively. The proportions of the coherent and incoherent components changed little in different seasons. In addition to tidal currents, it was determined that the passing of mesoscale eddies could induce strong near-bottom currents that have considerable influence on the deep circulation.
2019, 38(4): 90-96.
doi: 10.1007/s13131-018-1251-z
Abstract:
A 1.5-layer quasi-geostrophic reduced gravity model is used to study the hysteresis of a periodic or leaking western boundary current (WBC) flowing by a gap. When the periods of the WBC variations are much longer than the Rossby adjustment time scales of the circulation in the vicinity of the gap, the Hopf bifurcations during the Re-increase and Re-decrease loops are delayed to produce a new domain of hysteresis of the Reynolds numbers, and the critical Reynolds numbers of the WBC regime transitions change significantly, with the domain of the hysteresis Reynolds number larger for shorter periodic forcing. When the periods of the WBC variations are comparable to those of the Rossby adjustment time scales of the circulation in the vicinity of the gap, the WBC path inside the gap becomes periodic without hysteresis. The intrusion of the WBC into the western basin generally gets smaller as the period decreases. In addition, the partial leakage of the WBC transport through the gap into the western basin is found to have significant impact on the hysteresis loop of the WBC path when the leaked transport is larger than 1/2 of the WBC. Both the intrusion extent and the critical Reynolds numbers of the WBC regime transition are changed, and the larger the throughflow transport, the larger the change.
A 1.5-layer quasi-geostrophic reduced gravity model is used to study the hysteresis of a periodic or leaking western boundary current (WBC) flowing by a gap. When the periods of the WBC variations are much longer than the Rossby adjustment time scales of the circulation in the vicinity of the gap, the Hopf bifurcations during the Re-increase and Re-decrease loops are delayed to produce a new domain of hysteresis of the Reynolds numbers, and the critical Reynolds numbers of the WBC regime transitions change significantly, with the domain of the hysteresis Reynolds number larger for shorter periodic forcing. When the periods of the WBC variations are comparable to those of the Rossby adjustment time scales of the circulation in the vicinity of the gap, the WBC path inside the gap becomes periodic without hysteresis. The intrusion of the WBC into the western basin generally gets smaller as the period decreases. In addition, the partial leakage of the WBC transport through the gap into the western basin is found to have significant impact on the hysteresis loop of the WBC path when the leaked transport is larger than 1/2 of the WBC. Both the intrusion extent and the critical Reynolds numbers of the WBC regime transition are changed, and the larger the throughflow transport, the larger the change.
2019, 38(4): 97-105.
doi: 10.1007/s13131-018-1337-7
Abstract:
Through analysis of the results of a verified high-fidelity numerical model, the intra-seasonal variations (ISVs) in the depth of the 22℃ isotherm (D22) in the South China Sea (SCS) basin are investigated. The results show that the ISVs in the D22 exhibit distinct seasonality in the SCS. The ISVs in the D22 are quite significant, especially within a band along the northwestern boundary of the basin and at the southern end of the basin during boreal winter. In these areas, the ratio of the standard deviations (STDs) of intra-seasonal band to the STDs of total data could exceed 0.6. Although the ISVs in the D22 are detectable in the area affected by the Vietnam Offshore Current during boreal summer and autumn, these variations are sometimes overwhelmed by oscillations with other frequencies. An analysis of the causes of the ISVs in the D22 in the SCS indicates that sea surface fluxes and wind stirring are not the dominant external driving mechanisms of the phenomena described above. The ISVs in the D22 are thought to be induced mainly by the thermodynamic adjustment of the ocean itself and the associated instabilities. The energy of the northern and southern bands that display strong ISVs in the D22 may be derived from eddy kinetic energy, rather than eddy available potential energy. The diversity of the propagation of the ISVs in the D22 is very conspicuous within these two bands.
Through analysis of the results of a verified high-fidelity numerical model, the intra-seasonal variations (ISVs) in the depth of the 22℃ isotherm (D22) in the South China Sea (SCS) basin are investigated. The results show that the ISVs in the D22 exhibit distinct seasonality in the SCS. The ISVs in the D22 are quite significant, especially within a band along the northwestern boundary of the basin and at the southern end of the basin during boreal winter. In these areas, the ratio of the standard deviations (STDs) of intra-seasonal band to the STDs of total data could exceed 0.6. Although the ISVs in the D22 are detectable in the area affected by the Vietnam Offshore Current during boreal summer and autumn, these variations are sometimes overwhelmed by oscillations with other frequencies. An analysis of the causes of the ISVs in the D22 in the SCS indicates that sea surface fluxes and wind stirring are not the dominant external driving mechanisms of the phenomena described above. The ISVs in the D22 are thought to be induced mainly by the thermodynamic adjustment of the ocean itself and the associated instabilities. The energy of the northern and southern bands that display strong ISVs in the D22 may be derived from eddy kinetic energy, rather than eddy available potential energy. The diversity of the propagation of the ISVs in the D22 is very conspicuous within these two bands.
2019, 38(4): 106-115.
doi: 10.1007/s13131-019-1416-4
Abstract:
The trends of the sea surface temperature (SST) and SST fronts in the South China Sea (SCS) are analyzed during 2003-2017 using high-resolution satellite data. The linear trend of the basin averaged SST is 0.31℃ per decade, with the strongest warming identified in southeastern Vietnam. Although the rate of warming is comparable in summer and winter for the entire basin, the corresponding spatial patterns of the linear trend are substantially different between them. The SST trend to the west of the Luzon Strait is characterized by rapid warming in summer, exceeding approximately 0.6℃ per decade, but the trend is insignificant in winter. The strongest warming trend occurs in the southeast of Vietnam in winter, with much less pronounced warming in summer. A positive trend of SST fronts is identified for the coast of China and is associated with increasing wind stress. The increasing trend of SST fronts is also found in the east of Vietnam. Large-scale circulation, such as El Niño, can influence the trends of the SST and SST fronts. A significant correlation is found between the SST anomaly and Niño3.4 index, and the ENSO signal leads by eight months. The basin averaged SST linear trends increase after the El Niño event (2009-2010), which is, at least, due to the rapid warming rate causing by the enhanced northeasterly wind. Peaks of positive anomalous SST and negatively anomalous SST fronts are found to co-occur with the strong El Niño events.
The trends of the sea surface temperature (SST) and SST fronts in the South China Sea (SCS) are analyzed during 2003-2017 using high-resolution satellite data. The linear trend of the basin averaged SST is 0.31℃ per decade, with the strongest warming identified in southeastern Vietnam. Although the rate of warming is comparable in summer and winter for the entire basin, the corresponding spatial patterns of the linear trend are substantially different between them. The SST trend to the west of the Luzon Strait is characterized by rapid warming in summer, exceeding approximately 0.6℃ per decade, but the trend is insignificant in winter. The strongest warming trend occurs in the southeast of Vietnam in winter, with much less pronounced warming in summer. A positive trend of SST fronts is identified for the coast of China and is associated with increasing wind stress. The increasing trend of SST fronts is also found in the east of Vietnam. Large-scale circulation, such as El Niño, can influence the trends of the SST and SST fronts. A significant correlation is found between the SST anomaly and Niño3.4 index, and the ENSO signal leads by eight months. The basin averaged SST linear trends increase after the El Niño event (2009-2010), which is, at least, due to the rapid warming rate causing by the enhanced northeasterly wind. Peaks of positive anomalous SST and negatively anomalous SST fronts are found to co-occur with the strong El Niño events.
2019, 38(4): 116-125.
doi: 10.1007/s13131-019-1417-3
Abstract:
Based on the daily sea surface height and absolute geostrophic velocity data from 1993 to 2015 provided by the AVISO Center of French Space Agency, the surface Kuroshio transport east of Taiwan and its adjacent eddy field (sea surface height anomaly) were analyzed. Four main periods of the surface Kuroshio transport and eddy field east of Taiwan were obtained, which were used to reveal their interactions. The main conclusions are as follows:(1) Based on the wavelet analysis, the surface Kuroshio transport east of Taiwan and its nearby eddy field showed significant seasonal, annual and interannual periods. In addition to the obvious periods of 182 days (0.5 year) and 365 days (1 year), there were also more obvious periods of about 860 days (2.35 years) and 2 472 days (6.8 years) for the surface Kuroshio transport. There were also four more obvious periods corresponding to the eddy field of 200 days (0.55 year), 374 days (1 year), 889 days (2.43 years) and 2 374 days (6.5 years), although there were latitudinal variations. (2) Based on both the correlation and causal analysis, the correlation between the surface Kuroshio transport and the nearby eddy field over the above four periods was analyzed, and different Kuroshio-eddy interactions, with period and latitudinal variability, were revealed.
Based on the daily sea surface height and absolute geostrophic velocity data from 1993 to 2015 provided by the AVISO Center of French Space Agency, the surface Kuroshio transport east of Taiwan and its adjacent eddy field (sea surface height anomaly) were analyzed. Four main periods of the surface Kuroshio transport and eddy field east of Taiwan were obtained, which were used to reveal their interactions. The main conclusions are as follows:(1) Based on the wavelet analysis, the surface Kuroshio transport east of Taiwan and its nearby eddy field showed significant seasonal, annual and interannual periods. In addition to the obvious periods of 182 days (0.5 year) and 365 days (1 year), there were also more obvious periods of about 860 days (2.35 years) and 2 472 days (6.8 years) for the surface Kuroshio transport. There were also four more obvious periods corresponding to the eddy field of 200 days (0.55 year), 374 days (1 year), 889 days (2.43 years) and 2 374 days (6.5 years), although there were latitudinal variations. (2) Based on both the correlation and causal analysis, the correlation between the surface Kuroshio transport and the nearby eddy field over the above four periods was analyzed, and different Kuroshio-eddy interactions, with period and latitudinal variability, were revealed.
2019, 38(4): 126-135.
doi: 10.1007/s13131-019-1418-2
Abstract:
Using the fuzzy cluster analysis and the temperature-salinity (T-S) similarity number analysis of cruise conductivity-temperature-depth (CTD) data in the upper layer (0-300 m) of the northern South China Sea (NSCS), we classify the upper layer water of the NSCS into six water masses:diluted water (D), surface water (SS), the SCS subsurface water mass (US), the Pacific Ocean subsurface water mass (UP), surface-subsurface mixed water (SU) and subsurface-intermediate mixed water (UI). A new stacked stereogram is used to illustrate the water mass distribution, and to examine the source and the distribution of UP, combining with the sea surface height data and geostrophic current field. The results show that water mass UP exists in all four seasons with the maximum range in spring and the minimum range in summer. In spring and winter, the UP intrudes into the Luzon Strait and the southwest of Taiwan Island via the northern Luzon Strait in the form of nonlinear Rossby eddies, and forms a high temperature and high salinity zone east of the Dongsha Islands. In summer, the UP is sporadically distributed in the study area. In autumn, the UP is located in the upper 200 m layer east of Hainan Island.
Using the fuzzy cluster analysis and the temperature-salinity (T-S) similarity number analysis of cruise conductivity-temperature-depth (CTD) data in the upper layer (0-300 m) of the northern South China Sea (NSCS), we classify the upper layer water of the NSCS into six water masses:diluted water (D), surface water (SS), the SCS subsurface water mass (US), the Pacific Ocean subsurface water mass (UP), surface-subsurface mixed water (SU) and subsurface-intermediate mixed water (UI). A new stacked stereogram is used to illustrate the water mass distribution, and to examine the source and the distribution of UP, combining with the sea surface height data and geostrophic current field. The results show that water mass UP exists in all four seasons with the maximum range in spring and the minimum range in summer. In spring and winter, the UP intrudes into the Luzon Strait and the southwest of Taiwan Island via the northern Luzon Strait in the form of nonlinear Rossby eddies, and forms a high temperature and high salinity zone east of the Dongsha Islands. In summer, the UP is sporadically distributed in the study area. In autumn, the UP is located in the upper 200 m layer east of Hainan Island.
2019, 38(4): 136-145.
doi: 10.1007/s13131-018-1338-6
Abstract:
After validated by the in-situ observation, the slab model is used to study the wind-generated near-inertial energy flux (NIEF) in the South China Sea (SCS) based on satellite-observed wind data, and its dependence on calculation methods and threshold criteria of the mixed layer depth (MLD) is investigated. Results illustrate that the total amount of NIEF in the SCS could be doubled if different threshold criteria of MLD are adopted. The NIEF calculated by the iteration and spectral solutions can lead to a discrepancy of 2.5 GW (1 GW=1×109 W). Results also indicate that the NIEF exhibits spatial and temporal variations, which are significant in the boreal autumn, and in the southern part of the SCS. Typhoons are an important generator of NIEF in the SCS, which could account for approximately 30% of the annual mean NIEF. In addition, deepening of the MLD due to strong winds could lead to a decrease of NIEF by approximately by 10%. We re-estimate the annual mean NIEF in the SCS, which is (10±4) GW and much larger than those reported in previous studies.
After validated by the in-situ observation, the slab model is used to study the wind-generated near-inertial energy flux (NIEF) in the South China Sea (SCS) based on satellite-observed wind data, and its dependence on calculation methods and threshold criteria of the mixed layer depth (MLD) is investigated. Results illustrate that the total amount of NIEF in the SCS could be doubled if different threshold criteria of MLD are adopted. The NIEF calculated by the iteration and spectral solutions can lead to a discrepancy of 2.5 GW (1 GW=1×109 W). Results also indicate that the NIEF exhibits spatial and temporal variations, which are significant in the boreal autumn, and in the southern part of the SCS. Typhoons are an important generator of NIEF in the SCS, which could account for approximately 30% of the annual mean NIEF. In addition, deepening of the MLD due to strong winds could lead to a decrease of NIEF by approximately by 10%. We re-estimate the annual mean NIEF in the SCS, which is (10±4) GW and much larger than those reported in previous studies.
2019, 38(4): 146-153.
doi: 10.1007/s13131-019-1394-6
Abstract:
The Island Rule, derived from the Sverdrup theory, is widely used to estimate and analyze water transport through a strait. Previous studies presented single-or multi-island rules with either lateral or bottom friction. In this paper, an analytical model of wind-driven circulation is assumed based on linear dynamics. Considering both lateral and bottom friction, the analytic solutions of the transport streamfunction around the islands are derived and the volume transport through the channel is presented. The results are similar to those of Wajsowicz, but the frictional constants represent different values. The analytic solution shows that the relationship between the lateral frictional and bottom frictional dissipation is complex in terms of the frictional constants. To understand the interaction between the two friction types, lateral and bottom friction values were randomly chosen on a barotropic beta plane. The result shows an approximately linear relationship between the lateral and bottom friction in consisting of the combined frictional constants. We studied the effect of the channel width on the transport through the channel. The results show that the friction enhances the flow under some widths, which is similar to the flow behavior when only the lateral friction is considered. We also compared the transport through the channel at different depths and founded that the deeper the water, the smaller the transport reduction ratio when the horizontal eddy viscosity coefficient and the bottom drag coefficient remained constants. To further present the combined role of lateral frictional and bottom frictional dissipation, we compared our model with the model of Wajsowicz for two islands, where only the lateral or bottom friction were considered, with different channel widths. The results showed that the effect of the lateral friction is greater than the bottom friction when the channel is narrow, especially in the Munk boundary layer thickness. When the channel is much wider than the Munk boundary layer thickness, the role of the bottom friction is greater than that of the lateral friction. The model was applied to the Indonesian throughflow and yielded a reduction of approximately 20% in the transport.
The Island Rule, derived from the Sverdrup theory, is widely used to estimate and analyze water transport through a strait. Previous studies presented single-or multi-island rules with either lateral or bottom friction. In this paper, an analytical model of wind-driven circulation is assumed based on linear dynamics. Considering both lateral and bottom friction, the analytic solutions of the transport streamfunction around the islands are derived and the volume transport through the channel is presented. The results are similar to those of Wajsowicz, but the frictional constants represent different values. The analytic solution shows that the relationship between the lateral frictional and bottom frictional dissipation is complex in terms of the frictional constants. To understand the interaction between the two friction types, lateral and bottom friction values were randomly chosen on a barotropic beta plane. The result shows an approximately linear relationship between the lateral and bottom friction in consisting of the combined frictional constants. We studied the effect of the channel width on the transport through the channel. The results show that the friction enhances the flow under some widths, which is similar to the flow behavior when only the lateral friction is considered. We also compared the transport through the channel at different depths and founded that the deeper the water, the smaller the transport reduction ratio when the horizontal eddy viscosity coefficient and the bottom drag coefficient remained constants. To further present the combined role of lateral frictional and bottom frictional dissipation, we compared our model with the model of Wajsowicz for two islands, where only the lateral or bottom friction were considered, with different channel widths. The results showed that the effect of the lateral friction is greater than the bottom friction when the channel is narrow, especially in the Munk boundary layer thickness. When the channel is much wider than the Munk boundary layer thickness, the role of the bottom friction is greater than that of the lateral friction. The model was applied to the Indonesian throughflow and yielded a reduction of approximately 20% in the transport.
2019, 38(4): 154-166.
doi: 10.1007/s13131-019-1419-1
Abstract:
A 72-h fine-resolution atmosphere-wave-ocean coupled forecasting system was developed for the South China Sea and its adjacent seas. The forecasting model domain covers from from 15°S to 45°N in latitude and 99°E to 135°E in longitude including the Bohai Sea, the Yellow Sea, the East China Sea, the South China Sea and the Indonesian seas. To get precise initial conditions for the coupled forecasting model, the forecasting system conducts a 24-h hindcast simulation with data assimilation before forecasting. The Ensemble Adjustment Kalman Filter (EAKF) data assimilation method was adopted for the wave model MASNUM with assimilating Jason-2 significant wave height (SWH) data. The EAKF data assimilation method was also introduced to the ROMS model with assimilating sea surface temperature (SST), mean absolute dynamic topography (MADT) and Argo profiles data. To improve simulation of the structure of temperature and salinity, the vertical mixing scheme of the ocean model was improved by considering the surface wave induced vertical mixing and internal wave induced vertical mixing. The wave and current models were integrated from January 2014 to October 2015 driven by the ECMWF reanalysis 6 hourly mean dataset with data assimilation. Then the coupled atmosphere-wave-ocean forecasting system was carried out 14 months operational running since November 2015. The forecasting outputs include atmospheric forecast products, wave forecast products and ocean forecast products. A series of observation data are used to evaluate the coupled forecasting results, including the wind, SHW, ocean temperature and velocity. The forecasting results are in good agreement with observation data. The prediction practice for more than one year indicates that the coupled forecasting system performs stably and predict relatively accurate, which can support the shipping safety, the fisheries and the oil exploitation.
A 72-h fine-resolution atmosphere-wave-ocean coupled forecasting system was developed for the South China Sea and its adjacent seas. The forecasting model domain covers from from 15°S to 45°N in latitude and 99°E to 135°E in longitude including the Bohai Sea, the Yellow Sea, the East China Sea, the South China Sea and the Indonesian seas. To get precise initial conditions for the coupled forecasting model, the forecasting system conducts a 24-h hindcast simulation with data assimilation before forecasting. The Ensemble Adjustment Kalman Filter (EAKF) data assimilation method was adopted for the wave model MASNUM with assimilating Jason-2 significant wave height (SWH) data. The EAKF data assimilation method was also introduced to the ROMS model with assimilating sea surface temperature (SST), mean absolute dynamic topography (MADT) and Argo profiles data. To improve simulation of the structure of temperature and salinity, the vertical mixing scheme of the ocean model was improved by considering the surface wave induced vertical mixing and internal wave induced vertical mixing. The wave and current models were integrated from January 2014 to October 2015 driven by the ECMWF reanalysis 6 hourly mean dataset with data assimilation. Then the coupled atmosphere-wave-ocean forecasting system was carried out 14 months operational running since November 2015. The forecasting outputs include atmospheric forecast products, wave forecast products and ocean forecast products. A series of observation data are used to evaluate the coupled forecasting results, including the wind, SHW, ocean temperature and velocity. The forecasting results are in good agreement with observation data. The prediction practice for more than one year indicates that the coupled forecasting system performs stably and predict relatively accurate, which can support the shipping safety, the fisheries and the oil exploitation.
2019, 38(4): 167-174.
doi: 10.1007/s13131-019-1420-8
Abstract:
This study investigates the temperature inversion phenomenon in the Zhujiang (Pearl) River Estuary (ZRE) using hydrological data collected in a summer cruise during July 6-17, 2015. The results suggest that temperature inversion occurred primarily near the salinity front, with an average temperature difference (ΔT) of 0.42℃ between the inversion layer and the underlying water. The inversion layer was approximately 4 m thick on average, with an upper boundary at a depth of 1-6 m and a lower boundary at a depth of 3-10 m. Different mechanisms and dynamic processes were responsible for temperature inversion in different parts of the study area. (1) At the salinity front in the west of the ZRE, the measurements collected by CTD (conductivity, temperature, and depth) showed that the low-salinity water mass on the inner side of the front was approximately 2℃ cooler than the high-salinity water mass on the outer side. Temperature inversion occurred when the cooler low-salinity water overlapped the warmer high-salinity water near the front due to the driving force of the background flow. (2) Inversion layers occurred at the mouth of the Taiping waterway as a result of varying horizontal flow between two different water masses under the effects of tides and runoff. (3) To the southwest of Hong Kong, temperature inversion occurred due to the interaction of upwelling and the salinity front.
This study investigates the temperature inversion phenomenon in the Zhujiang (Pearl) River Estuary (ZRE) using hydrological data collected in a summer cruise during July 6-17, 2015. The results suggest that temperature inversion occurred primarily near the salinity front, with an average temperature difference (ΔT) of 0.42℃ between the inversion layer and the underlying water. The inversion layer was approximately 4 m thick on average, with an upper boundary at a depth of 1-6 m and a lower boundary at a depth of 3-10 m. Different mechanisms and dynamic processes were responsible for temperature inversion in different parts of the study area. (1) At the salinity front in the west of the ZRE, the measurements collected by CTD (conductivity, temperature, and depth) showed that the low-salinity water mass on the inner side of the front was approximately 2℃ cooler than the high-salinity water mass on the outer side. Temperature inversion occurred when the cooler low-salinity water overlapped the warmer high-salinity water near the front due to the driving force of the background flow. (2) Inversion layers occurred at the mouth of the Taiping waterway as a result of varying horizontal flow between two different water masses under the effects of tides and runoff. (3) To the southwest of Hong Kong, temperature inversion occurred due to the interaction of upwelling and the salinity front.
2019, 38(4): 175-182.
doi: 10.1007/s13131-019-1421-7
Abstract:
In this study, the sectional characteristics of temperature, salinity and density off the central Zhejiang coast were analyzed using three sections of observational data in the spring of 2016. The results are as follows:(1) a cold water patch was observed in the middle layer of sections from 10 to 25 m, and a weak upwelling was observed at the upper layer near the central Zhejiang coast; (2) several thermoclines, inverted thermoclines, and haloclines were observed in the survey area; (3) the Taiwan Warm Current Water (TWCW) climbing from the slope towards the survey area affected the thermocline, making it thinner and intensified; however, the TWCW was not strong enough to break through the thermocline to reach the sea surface.
In this study, the sectional characteristics of temperature, salinity and density off the central Zhejiang coast were analyzed using three sections of observational data in the spring of 2016. The results are as follows:(1) a cold water patch was observed in the middle layer of sections from 10 to 25 m, and a weak upwelling was observed at the upper layer near the central Zhejiang coast; (2) several thermoclines, inverted thermoclines, and haloclines were observed in the survey area; (3) the Taiwan Warm Current Water (TWCW) climbing from the slope towards the survey area affected the thermocline, making it thinner and intensified; however, the TWCW was not strong enough to break through the thermocline to reach the sea surface.
2019, 38(4): 183-189.
doi: 10.1007/s13131-019-1422-6
Abstract:
For acoustic detection of internal waves, the core issue is to obtain the temporal and spatial distribution of the sound speed profile (SSP). In the inversion process, the SSP is usually expressed by a few parameters through expansion. However, information about internal waves may sometimes be hard to read directly from the inversion results. The aim of this paper is to characterize the internal waves directly though expansion coefficients. By deducing the dynamic equations of the internal waves, an orthogonal basis called the hydrodynamic normal modes (HNMs) can be extracted from a certain number of SSP samples. Unlike the existing widely used empirical orthogonal functions (EOFs), the HNMs have a more explicit physical meaning that is directly related to internal wave activity. The HNMs are then used to expand the SSP time series, and the expansion coefficients are derived. Eventually, information about internal waves can be read directly from the time derivative of the expansion coefficients of the first two modes. In this study, this method is applied to thermistor string profiles from the northern shelf of the South China Sea, where the SSP shows evident spatial and temporal variations due to internal waves. The results show that the SSP can be described approximately by the first two modes with adequate precision. The special oscillation structure of the time derivative of the expansion coefficients can be used to detect internal solitary waves. The expansion coefficients can also give information on internal solitary wave amplitude and width. According to theoretical and experimental analysis, it can be concluded that the internal waves monitoring method introduced in this paper is effective. The HNMs method is simple to apply and depends less on sample data than EOFs. It could be used as an efficient alternative to EOFs to expand the use of the SSP in highly variable areas, where internal waves are intensive.
For acoustic detection of internal waves, the core issue is to obtain the temporal and spatial distribution of the sound speed profile (SSP). In the inversion process, the SSP is usually expressed by a few parameters through expansion. However, information about internal waves may sometimes be hard to read directly from the inversion results. The aim of this paper is to characterize the internal waves directly though expansion coefficients. By deducing the dynamic equations of the internal waves, an orthogonal basis called the hydrodynamic normal modes (HNMs) can be extracted from a certain number of SSP samples. Unlike the existing widely used empirical orthogonal functions (EOFs), the HNMs have a more explicit physical meaning that is directly related to internal wave activity. The HNMs are then used to expand the SSP time series, and the expansion coefficients are derived. Eventually, information about internal waves can be read directly from the time derivative of the expansion coefficients of the first two modes. In this study, this method is applied to thermistor string profiles from the northern shelf of the South China Sea, where the SSP shows evident spatial and temporal variations due to internal waves. The results show that the SSP can be described approximately by the first two modes with adequate precision. The special oscillation structure of the time derivative of the expansion coefficients can be used to detect internal solitary waves. The expansion coefficients can also give information on internal solitary wave amplitude and width. According to theoretical and experimental analysis, it can be concluded that the internal waves monitoring method introduced in this paper is effective. The HNMs method is simple to apply and depends less on sample data than EOFs. It could be used as an efficient alternative to EOFs to expand the use of the SSP in highly variable areas, where internal waves are intensive.
