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- Author or Editor: Chun Zhou x
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Abstract
The Luzon Strait, with its deepest sills at the Bashi Channel and Luzon Trough, is the only deep connection between the Pacific Ocean and the South China Sea (SCS). To investigate the deep-water overflow through the Luzon Strait, 3.5 yr of continuous mooring observations have been conducted in the deep Bashi Channel and Luzon Trough. For the first time these observations enable us to assess the detailed variability of the deep-water overflow from the Pacific to the SCS. On average, the along-stream velocity of the overflow is at its maximum at about 120 m above the ocean bottom, reaching 19.9 ± 6.5 and 23.0 ± 11.8 cm s−1 at the central Bashi Channel and Luzon Trough, respectively. The velocity measurements can be translated to a mean volume transport for the deep-water overflow of 0.83 ± 0.46 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) at the Bashi Channel and 0.88 ± 0.77 Sv at the Luzon Trough. Significant intraseasonal and seasonal variations are identified, with their dominant time scales ranging between 20 and 60 days and around 100 days. The intraseasonal variation is season dependent, with its maximum strength taking place in March–May. Deep-water eddies are believed to play a role in this intraseasonal variation. On the seasonal time scale, the deep-water overflow intensifies in late fall (October–December) and weakens in spring (March–May), corresponding well with the seasonal variation of the density difference between the Pacific and SCS, for which enhanced mixing in the deep SCS is possibly responsible.
Abstract
The Luzon Strait, with its deepest sills at the Bashi Channel and Luzon Trough, is the only deep connection between the Pacific Ocean and the South China Sea (SCS). To investigate the deep-water overflow through the Luzon Strait, 3.5 yr of continuous mooring observations have been conducted in the deep Bashi Channel and Luzon Trough. For the first time these observations enable us to assess the detailed variability of the deep-water overflow from the Pacific to the SCS. On average, the along-stream velocity of the overflow is at its maximum at about 120 m above the ocean bottom, reaching 19.9 ± 6.5 and 23.0 ± 11.8 cm s−1 at the central Bashi Channel and Luzon Trough, respectively. The velocity measurements can be translated to a mean volume transport for the deep-water overflow of 0.83 ± 0.46 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) at the Bashi Channel and 0.88 ± 0.77 Sv at the Luzon Trough. Significant intraseasonal and seasonal variations are identified, with their dominant time scales ranging between 20 and 60 days and around 100 days. The intraseasonal variation is season dependent, with its maximum strength taking place in March–May. Deep-water eddies are believed to play a role in this intraseasonal variation. On the seasonal time scale, the deep-water overflow intensifies in late fall (October–December) and weakens in spring (March–May), corresponding well with the seasonal variation of the density difference between the Pacific and SCS, for which enhanced mixing in the deep SCS is possibly responsible.
Abstract
The role of mesoscale eddies in modulating the semidiurnal internal tide (SIT) in the northern South China Sea (SCS) is examined using the data from a cross-shaped mooring array. From November 2013 to January 2014, an anticyclonic eddy (AE) and cyclonic eddy (CE) pair crossed the westward SIT beam originating in Luzon Strait. Observations showed that, because of the current and stratification modulations by the eddy pair, the propagation speed of the mode-1 SIT sped up (slowed down) by up to 0.7 m s−1 (0.4 m s−1) within the AE’s (CE’s) southern portion. As a result of the spatially varying phase speed, the mode-1 SIT wave crest was clockwise rotated (counterclockwise rotated) within the AE (CE) core, while it exhibited convex and concave (concave and convex) patterns on the southern and northern peripheries of the AE (CE), respectively. In mid-to-late November, most of the mode-1 SIT energy was refracted by the AE away from Dongsha Island toward the north part of the northern SCS, which resulted in enhanced internal solitary waves (ISWs) there. Corresponding to the energy refraction, responses of the depth-integrated mode-1 SIT energy to the eddies were generally in phase at the along-beam-direction moorings but out of phase in the south and north parts of the northern SCS at the cross-beam-direction moorings. From late December to early January, intensified mode-2 SIT was observed, whose energy was likely transferred from the mode-1 SIT through eddy–wave interactions. The observation results reported here are helpful to improve the capability to predict internal tides and ISWs in the northern SCS.
Abstract
The role of mesoscale eddies in modulating the semidiurnal internal tide (SIT) in the northern South China Sea (SCS) is examined using the data from a cross-shaped mooring array. From November 2013 to January 2014, an anticyclonic eddy (AE) and cyclonic eddy (CE) pair crossed the westward SIT beam originating in Luzon Strait. Observations showed that, because of the current and stratification modulations by the eddy pair, the propagation speed of the mode-1 SIT sped up (slowed down) by up to 0.7 m s−1 (0.4 m s−1) within the AE’s (CE’s) southern portion. As a result of the spatially varying phase speed, the mode-1 SIT wave crest was clockwise rotated (counterclockwise rotated) within the AE (CE) core, while it exhibited convex and concave (concave and convex) patterns on the southern and northern peripheries of the AE (CE), respectively. In mid-to-late November, most of the mode-1 SIT energy was refracted by the AE away from Dongsha Island toward the north part of the northern SCS, which resulted in enhanced internal solitary waves (ISWs) there. Corresponding to the energy refraction, responses of the depth-integrated mode-1 SIT energy to the eddies were generally in phase at the along-beam-direction moorings but out of phase in the south and north parts of the northern SCS at the cross-beam-direction moorings. From late December to early January, intensified mode-2 SIT was observed, whose energy was likely transferred from the mode-1 SIT through eddy–wave interactions. The observation results reported here are helpful to improve the capability to predict internal tides and ISWs in the northern SCS.
Abstract
Spatiotemporal variations in internal solitary wave (ISW) polarity over the continental shelf of the northern South China Sea (SCS) were examined based on mooring-array observations from October 2013 to June 2014. Depression ISWs were observed at the easternmost mooring, where the water depth is 323 m. Then, they evolved into elevation ISWs at the westernmost mooring, with a depth of 149 m. At the central mooring, with a depth of 250 m, the ISWs generally appeared as depression waves in autumn and spring but were elevation waves in winter. Seasonal variations in stratification caused this seasonality in polarity. On the intraseasonal time scales, anticyclonic eddies can modulate ISW polarity at the central mooring by deepening the thermocline depth for periods of approximately 8 days. During some days in autumn and spring, depression ISWs and ISWs in the process of changing polarity from depression to elevation appeared at time intervals of 10–12 h because of the thermocline deepening caused by internal tides. Isotherm anomalies associated with eddies and internal tides have a more significant contribution to determining the polarity of ISWs than do the background currents. The observational results reported here highlight the impact of multiscale processes on the evolution of ISWs.
Abstract
Spatiotemporal variations in internal solitary wave (ISW) polarity over the continental shelf of the northern South China Sea (SCS) were examined based on mooring-array observations from October 2013 to June 2014. Depression ISWs were observed at the easternmost mooring, where the water depth is 323 m. Then, they evolved into elevation ISWs at the westernmost mooring, with a depth of 149 m. At the central mooring, with a depth of 250 m, the ISWs generally appeared as depression waves in autumn and spring but were elevation waves in winter. Seasonal variations in stratification caused this seasonality in polarity. On the intraseasonal time scales, anticyclonic eddies can modulate ISW polarity at the central mooring by deepening the thermocline depth for periods of approximately 8 days. During some days in autumn and spring, depression ISWs and ISWs in the process of changing polarity from depression to elevation appeared at time intervals of 10–12 h because of the thermocline deepening caused by internal tides. Isotherm anomalies associated with eddies and internal tides have a more significant contribution to determining the polarity of ISWs than do the background currents. The observational results reported here highlight the impact of multiscale processes on the evolution of ISWs.
Abstract
Although observational efforts have been made to detect submesoscale currents (submesoscales) in regions with deep mixed layers and/or strong mesoscale kinetic energy (KE), there have been no long-term submesoscale observations in subtropical gyres, which are characterized by moderate values of both mixed layer depths and mesoscale KE. To explore submesoscale dynamics in this oceanic regime, two nested mesoscale- and submesoscale-resolving mooring arrays were deployed in the northwestern Pacific subtropical countercurrent region during 2017–19. Based on the 2 years of data, submesoscales featuring order one Rossby numbers, large vertical velocities (with magnitude of 10–50 m day−1) and vertical heat flux, and strong ageostrophic KE are revealed in the upper 150 m. Although most of the submesoscales are surface intensified, they are found to penetrate far beneath the mixed layer. They are most energetic during strong mesoscale strain periods in the winter–spring season but are generally weak in the summer–autumn season. Energetics analysis suggests that the submesoscales receive KE from potential energy release but lose a portion of it through inverse cascade. Because this KE sink is smaller than the source term, a forward cascade must occur to balance the submesoscale KE budget, for which symmetric instability may be a candidate mechanism. By synthesizing observations and theories, we argue that the submesoscales are generated through a combination of baroclinic instability in the upper mixed and transitional layers and mesoscale strain-induced frontogenesis, among which the former should play a more dominant role in their final generation stage.
Abstract
Although observational efforts have been made to detect submesoscale currents (submesoscales) in regions with deep mixed layers and/or strong mesoscale kinetic energy (KE), there have been no long-term submesoscale observations in subtropical gyres, which are characterized by moderate values of both mixed layer depths and mesoscale KE. To explore submesoscale dynamics in this oceanic regime, two nested mesoscale- and submesoscale-resolving mooring arrays were deployed in the northwestern Pacific subtropical countercurrent region during 2017–19. Based on the 2 years of data, submesoscales featuring order one Rossby numbers, large vertical velocities (with magnitude of 10–50 m day−1) and vertical heat flux, and strong ageostrophic KE are revealed in the upper 150 m. Although most of the submesoscales are surface intensified, they are found to penetrate far beneath the mixed layer. They are most energetic during strong mesoscale strain periods in the winter–spring season but are generally weak in the summer–autumn season. Energetics analysis suggests that the submesoscales receive KE from potential energy release but lose a portion of it through inverse cascade. Because this KE sink is smaller than the source term, a forward cascade must occur to balance the submesoscale KE budget, for which symmetric instability may be a candidate mechanism. By synthesizing observations and theories, we argue that the submesoscales are generated through a combination of baroclinic instability in the upper mixed and transitional layers and mesoscale strain-induced frontogenesis, among which the former should play a more dominant role in their final generation stage.
Abstract
The complex behaviors of internal solitary waves (ISWs) in the Andaman Sea were revealed using data collected over a nearly 22-month-long observation period completed by two moorings. Emanating from the submarine ridges northwest of Sumatra Island and south of Car Nicobar, two types of ISWs, referred to as S- and C-ISWs, respectively, were identified in the measurements, and S-ISWs were generally found to be stronger than C-ISWs. The observed S- and C-ISWs frequently appeared as multiwave packets, accounting for 87% and 43% of their observed episodes, respectively. The simultaneous measurements collected by the two moorings featured evident variability along the S-ISW crests, with the average wave amplitude in the northern portion being 36% larger than that in the southern portion. The analyses of the arrival times revealed that the S-ISWs in the northern portion occurred more frequently and arrived more irregularly than those in the southern portion. Moreover, the temporal variability of ISWs drastically differed on monthly and seasonal time scales, characterized by relatively stronger S-ISWs in spring and autumn. Over the interannual time scale, the temporal variations in ISWs were generally subtle. The monthly-to-annual variations of ISWs could be mostly explained by the variability in stratification, which could be modulated by the monsoons, the winds in equatorial Indian Ocean, and the mesoscale eddies in the Andaman Sea. From careful analyses preformed based on the long-term measurements, we argued that the observed ISWs were likely generated via internal tide release mechanism and their generation processes were obviously modulated by background circulations.
Abstract
The complex behaviors of internal solitary waves (ISWs) in the Andaman Sea were revealed using data collected over a nearly 22-month-long observation period completed by two moorings. Emanating from the submarine ridges northwest of Sumatra Island and south of Car Nicobar, two types of ISWs, referred to as S- and C-ISWs, respectively, were identified in the measurements, and S-ISWs were generally found to be stronger than C-ISWs. The observed S- and C-ISWs frequently appeared as multiwave packets, accounting for 87% and 43% of their observed episodes, respectively. The simultaneous measurements collected by the two moorings featured evident variability along the S-ISW crests, with the average wave amplitude in the northern portion being 36% larger than that in the southern portion. The analyses of the arrival times revealed that the S-ISWs in the northern portion occurred more frequently and arrived more irregularly than those in the southern portion. Moreover, the temporal variability of ISWs drastically differed on monthly and seasonal time scales, characterized by relatively stronger S-ISWs in spring and autumn. Over the interannual time scale, the temporal variations in ISWs were generally subtle. The monthly-to-annual variations of ISWs could be mostly explained by the variability in stratification, which could be modulated by the monsoons, the winds in equatorial Indian Ocean, and the mesoscale eddies in the Andaman Sea. From careful analyses preformed based on the long-term measurements, we argued that the observed ISWs were likely generated via internal tide release mechanism and their generation processes were obviously modulated by background circulations.
Abstract
Near-inertial waves (NIWs) trapped in a propagating anticyclonic eddy (AE) are investigated along the eddy path at three areas spanning 660 km by using two mooring arrays and a cruise transect. In the upstream area, the reconstructed three-dimensional structure reveals that NIWs are concentrated within the eddy core with wave current amplitudes exceeding 0.2 m s−1; vertically, due to the critical layer effect caused by eddy baroclinicity, NIWs are trapped at depths around 200 and 315 m with frequencies estimated to be ω 1 ≈ 0.918f and ω 2 ≈ 0.985f, respectively. After the AE propagates southwestward for hundreds of kilometers, the NIWs of frequency ω 1 are still detectable inside the AE, while NIWs of frequency ω 2 are absent because of the equatorward migration of the AE on a beta plane. Meanwhile, the wave kinetic energy downstream is trapped closer to the eddy center in radial direction, with the wave amplitude decaying roughly in a Gaussian form along the eddy radius, and becomes more homogeneous in the azimuthal direction, showing a more regular trapping form in the three-dimensional view. Investigation on wind shows that trapped NIWs are likely to be generated by a typhoon but less affected by the wind during the eddy passage time. By an energy analysis, we find that enhanced wave dissipation near the critical layer is roughly balanced by the energy transfer from mean flows, and therefore the trapped wave kinetic energy is largely conserved during the long-distance migration.
Abstract
Near-inertial waves (NIWs) trapped in a propagating anticyclonic eddy (AE) are investigated along the eddy path at three areas spanning 660 km by using two mooring arrays and a cruise transect. In the upstream area, the reconstructed three-dimensional structure reveals that NIWs are concentrated within the eddy core with wave current amplitudes exceeding 0.2 m s−1; vertically, due to the critical layer effect caused by eddy baroclinicity, NIWs are trapped at depths around 200 and 315 m with frequencies estimated to be ω 1 ≈ 0.918f and ω 2 ≈ 0.985f, respectively. After the AE propagates southwestward for hundreds of kilometers, the NIWs of frequency ω 1 are still detectable inside the AE, while NIWs of frequency ω 2 are absent because of the equatorward migration of the AE on a beta plane. Meanwhile, the wave kinetic energy downstream is trapped closer to the eddy center in radial direction, with the wave amplitude decaying roughly in a Gaussian form along the eddy radius, and becomes more homogeneous in the azimuthal direction, showing a more regular trapping form in the three-dimensional view. Investigation on wind shows that trapped NIWs are likely to be generated by a typhoon but less affected by the wind during the eddy passage time. By an energy analysis, we find that enhanced wave dissipation near the critical layer is roughly balanced by the energy transfer from mean flows, and therefore the trapped wave kinetic energy is largely conserved during the long-distance migration.
Abstract
Based on long-term mooring-array and satellite observations, three-dimensional structure and interannual variability of the Kuroshio Loop Current (KLC) in the northeastern South China Sea (SCS) were investigated. The 3-yr moored data between 2014 and 2017 revealed that the KLC mainly occurred in winter and it exhibited significant interannual variability with moderate, weak, and strong strengths in the winters of 2014/15, 2015/16, and 2016/17, respectively. Spatially, the KLC structure was initially confined to the upper 500 m near the Luzon Strait, but it became more barotropic, with kinetic energy transferring from the baroclinic mode to the barotropic mode when it extended into the SCS interior. Through analyzing the historical altimeter data between 1993 and 2019, it is found that the KLC event in 2016/17 winter is the strongest one since 1993. Moored-data-based energetics analysis suggested that the growth of this KLC event was primarily fed by the strong wind work associated with the strengthened northeast monsoon in that La Niña–year winter. By examining all of the historical KLC events, it is found that the strength of KLC is significantly modulated by El Niño–Southern Oscillation, being stronger in La Niña and weaker in El Niño years. This interannual modulation could be explained by the strengthened (weakened) northeast monsoon associated with the anomalous atmospheric cyclone (anticyclone) in the western North Pacific during La Niña (El Niño) years, which inputs more (less) energy and negative vorticity southwest of Taiwan that is favorable (unfavorable) for the development of KLC.
Abstract
Based on long-term mooring-array and satellite observations, three-dimensional structure and interannual variability of the Kuroshio Loop Current (KLC) in the northeastern South China Sea (SCS) were investigated. The 3-yr moored data between 2014 and 2017 revealed that the KLC mainly occurred in winter and it exhibited significant interannual variability with moderate, weak, and strong strengths in the winters of 2014/15, 2015/16, and 2016/17, respectively. Spatially, the KLC structure was initially confined to the upper 500 m near the Luzon Strait, but it became more barotropic, with kinetic energy transferring from the baroclinic mode to the barotropic mode when it extended into the SCS interior. Through analyzing the historical altimeter data between 1993 and 2019, it is found that the KLC event in 2016/17 winter is the strongest one since 1993. Moored-data-based energetics analysis suggested that the growth of this KLC event was primarily fed by the strong wind work associated with the strengthened northeast monsoon in that La Niña–year winter. By examining all of the historical KLC events, it is found that the strength of KLC is significantly modulated by El Niño–Southern Oscillation, being stronger in La Niña and weaker in El Niño years. This interannual modulation could be explained by the strengthened (weakened) northeast monsoon associated with the anomalous atmospheric cyclone (anticyclone) in the western North Pacific during La Niña (El Niño) years, which inputs more (less) energy and negative vorticity southwest of Taiwan that is favorable (unfavorable) for the development of KLC.
Abstract
At 26.5°N in the North Atlantic, a continuous transbasin observational array has been established since 2004 to detect the strength of the Atlantic meridional overturning circulation. The observational record shows that the subtropical Atlantic meridional overturning circulation has weakened by 2.5 ± 1.5 Sv (as mean ± 95% interval; 1 Sv ≡ 106 m3 s−1) since 2008 compared to the initial 4-yr average. Strengthening of the upper southward geostrophic transport (with a 2.6 ± 1.6 Sv southward increase) derived from thermal wind dominates this Atlantic meridional overturning circulation decline. We decompose the geostrophic transport into its temperature and salinity components to compare their contributions to the transport variability. The contributions of temperature and salinity components to the southward geostrophic transport strengthening are 1.0 ± 2.5 and 1.6 ± 1.3 Sv, respectively. The variation of salinity component is significant at the 95% confidence level, while the temperature component’s variation is not. This result highlights the vital role that salinity plays in the subtropical Atlantic meridional overturning circulation variability, which has been overlooked in previous studies. We further analyze the geostrophic transport variations and their temperature and salinity components arising from different water masses, which shows that a warming signal in Labrador Sea Water and a freshening signal in Nordic Sea Water are two prominent sources of the geostrophic transport increase. Comparison of the temperature and salinity records of the 26.5°N array with the upstream records from repeated hydrographic sections across the Labrador Sea suggests that these thermohaline signals may be exported from the subpolar Atlantic via the deep western boundary current.
Abstract
At 26.5°N in the North Atlantic, a continuous transbasin observational array has been established since 2004 to detect the strength of the Atlantic meridional overturning circulation. The observational record shows that the subtropical Atlantic meridional overturning circulation has weakened by 2.5 ± 1.5 Sv (as mean ± 95% interval; 1 Sv ≡ 106 m3 s−1) since 2008 compared to the initial 4-yr average. Strengthening of the upper southward geostrophic transport (with a 2.6 ± 1.6 Sv southward increase) derived from thermal wind dominates this Atlantic meridional overturning circulation decline. We decompose the geostrophic transport into its temperature and salinity components to compare their contributions to the transport variability. The contributions of temperature and salinity components to the southward geostrophic transport strengthening are 1.0 ± 2.5 and 1.6 ± 1.3 Sv, respectively. The variation of salinity component is significant at the 95% confidence level, while the temperature component’s variation is not. This result highlights the vital role that salinity plays in the subtropical Atlantic meridional overturning circulation variability, which has been overlooked in previous studies. We further analyze the geostrophic transport variations and their temperature and salinity components arising from different water masses, which shows that a warming signal in Labrador Sea Water and a freshening signal in Nordic Sea Water are two prominent sources of the geostrophic transport increase. Comparison of the temperature and salinity records of the 26.5°N array with the upstream records from repeated hydrographic sections across the Labrador Sea suggests that these thermohaline signals may be exported from the subpolar Atlantic via the deep western boundary current.
Abstract
The deep water overflow at three gaps in the Heng-Chun Ridge of the Luzon Strait is investigated based on long-term continuous mooring observations. For the first time, these observations enable us to assess the detailed structure and variability in the deep water overflow directly spilling into the South China Sea (SCS). The strong bottom-intensified flows at moorings WG2 and WG3 intrude into the deep SCS with maximum along-stream velocities of 19.2 ± 9.9 and 15.2 ± 6.8 cm s−1, respectively, at approximately 50 m above the bottom. At mooring WG1, the bottom current revealed spillage into the Luzon Trough from the SCS. The volume transport estimates are 0.73 ± 0.08 Sv at WG2 and 0.45 ± 0.02 Sv at WG3, suggesting that WG2 is the main entrance for the deep water overflow crossing the Heng-Chun Ridge into the SCS. By including the long-term observational results from previous studies, the pathway of the deep water overflow through the Luzon Strait is also presented. In addition, significant intraseasonal variations with dominant time scales of approximately 26 days at WG2 and WG3 have been revealed, which tend to be enhanced in spring and may reverse the deep water overflow.
Abstract
The deep water overflow at three gaps in the Heng-Chun Ridge of the Luzon Strait is investigated based on long-term continuous mooring observations. For the first time, these observations enable us to assess the detailed structure and variability in the deep water overflow directly spilling into the South China Sea (SCS). The strong bottom-intensified flows at moorings WG2 and WG3 intrude into the deep SCS with maximum along-stream velocities of 19.2 ± 9.9 and 15.2 ± 6.8 cm s−1, respectively, at approximately 50 m above the bottom. At mooring WG1, the bottom current revealed spillage into the Luzon Trough from the SCS. The volume transport estimates are 0.73 ± 0.08 Sv at WG2 and 0.45 ± 0.02 Sv at WG3, suggesting that WG2 is the main entrance for the deep water overflow crossing the Heng-Chun Ridge into the SCS. By including the long-term observational results from previous studies, the pathway of the deep water overflow through the Luzon Strait is also presented. In addition, significant intraseasonal variations with dominant time scales of approximately 26 days at WG2 and WG3 have been revealed, which tend to be enhanced in spring and may reverse the deep water overflow.
Abstract
The deep western boundary current (DWBC) was studied based on a full-depth mooring east of Luzon Island in the Northern Philippine Sea deep basin during the period from January 2018 to May 2020. On average, the DWBC in the Philippine Sea flows southward with a velocity of approximately 1.18 cm s−1 at a depth of 3050 m. Significant intraseasonal and seasonal variations of the DWBC are identified. The intraseasonal variations have multiple spectral peaks in the range of 30–200 days, with the most obvious peak at approximately 120 days. On the seasonal time scale, the DWBC intensifies in summer/autumn and weakens in winter/spring, corresponding well with the seasonal variation of the ocean bottom pressure (OBP) from the Gravity Recovery and Climate Experiment. Both intraseasonal and seasonal variations have no significant correlation with the temporal variations in the upper and middle layers but have a certain correlation with transport through the Yap–Mariana Junction (YMJ). A set of experiments based on an inverted-reduced-gravity model and the OBP data reveal that the temporal variations originating from the YMJ could propagate counterclockwise along the boundary of the deep basin to the western boundary of the deep Philippine Sea, dominating the temporal variations of DWBC.
Abstract
The deep western boundary current (DWBC) was studied based on a full-depth mooring east of Luzon Island in the Northern Philippine Sea deep basin during the period from January 2018 to May 2020. On average, the DWBC in the Philippine Sea flows southward with a velocity of approximately 1.18 cm s−1 at a depth of 3050 m. Significant intraseasonal and seasonal variations of the DWBC are identified. The intraseasonal variations have multiple spectral peaks in the range of 30–200 days, with the most obvious peak at approximately 120 days. On the seasonal time scale, the DWBC intensifies in summer/autumn and weakens in winter/spring, corresponding well with the seasonal variation of the ocean bottom pressure (OBP) from the Gravity Recovery and Climate Experiment. Both intraseasonal and seasonal variations have no significant correlation with the temporal variations in the upper and middle layers but have a certain correlation with transport through the Yap–Mariana Junction (YMJ). A set of experiments based on an inverted-reduced-gravity model and the OBP data reveal that the temporal variations originating from the YMJ could propagate counterclockwise along the boundary of the deep basin to the western boundary of the deep Philippine Sea, dominating the temporal variations of DWBC.
