Pressure-recording Inverted Echo Sounders Research Articles

Eddy heat transport across the Luzon Strait derived from current- and pressure-recording inverted echo sounder observations

The heat transport across the Luzon Strait has crucial impact on the heat budget of the South China Sea. However, the quantum of heat transported is uncertain owing to the scarcity of observations. In this study, the zonal eddy heat transport (ZEHT) across the Luzon Strait was investigated using a mooring array of current- and pressure-recording inverted echo sounders. The ZEHT is generally westward and downgradient. The downgradient ZEHT is related to the positive baroclinic energy conversion. A new method was proposed to empirically determine the long-term (1993–2022) ZEHT from the sea surface height (ZEHTSSH). The long-term ZEHTSSH had a mean value of 3.8 TW (1 TW = 1012 W) directed westward. The seasonal mean ZEHTSSH was the largest in winter (4.5 TW) and smallest in summer (2.7 TW). The largest ZEHTSSH during winter may be caused by enhanced baroclinic instability facilitated by smaller stratification and higher vertical velocity shear. The long-term mean ZEHT is estimated as 2.6 TW if the gaps of the Luzon Strait not covered by the mooring section are considered. This study has proposed a new method to derive a long-term ZEHT time series based on observations, which will be meaningful for the study on the oceanic heat budget around the SCS and related climate study.

Regional Abyssal Vorticity Balance in the Northeast South China Sea: External and Internal Dynamics of Abyssal Circulation

Abstract Abyssal vorticity balance in the northeast South China Sea was assessed for over a year based on observations from 28 current- and pressure-recording inverted echo sounders distributed west of the Luzon Strait. The regional first-order balance was dominated by the planetary vorticity flux and bottom pressure torque, which reflect the external and internal dynamics of abyssal circulation. Vertical motion considerably contributed to the planetary vorticity flux, whereas the contribution of horizontal motion was negligible. Positive and negative planetary vorticity fluxes dominate the areas along the eastern and western boundaries, indicating upward and downward vertical transport, respectively. The opposite planetary vorticity fluxes in the different areas were accompanied by different current patterns; regional anticyclonic and cyclonic characteristics appeared near the western and eastern boundaries, respectively, owing to the deep topography as the abyssal current followed the boundary. The planetary vorticity flux near the eastern boundary was substantial in spring and autumn; in contrast, along the western boundary it was enhanced only in spring. Deep eddies played important roles in planetary vorticity flux and regional vorticity balance. The results of this study reveal the formation dynamics of abyssal circulation in the South China Sea as well as its spatiotemporal distributions, providing a more detailed description of abyssal circulation. Significance Statement The deep South China Sea (SCS) is a nearly enclosed basin characterized by cyclonic abyssal circulation. Based on the observations from 28 current- and pressure-recording inverted echo sounders distributed west of the Luzon Strait, the vorticity balance in the deep SCS was clarified. The planetary vorticity flux and bottom pressure torque maintain a first-order balance of vorticity, which acts as the external and internal dynamics of the abyssal circulation. The study describes the temporal variability and spatial distribution of vorticity terms in the deep ocean west of the Luzon Strait, which may contribute to a more detailed understanding of abyssal circulation formation and its evolution.

Near-Inertial Waves Reaching the Deep Basin in the South China Sea after Typhoon Mangkhut (2018)

Abstract Typhoon Mangkhut crossed the northeastern South China Sea (SCS) in September 2018 and induced energetic near-inertial waves (NIWs) that were captured by an array of 39 current- and pressure-recording inverted echo sounders and two tall moorings with acoustic Doppler current profilers and current meter sensors. The array extended from west of the Luzon Strait to the interior SCS, with the path of the typhoon cutting through the array. NIWs in the interior SCS had lower frequency than those near the Luzon Strait. After the typhoon crossed the SCS, Mangkhut-induced near-inertial currents in the upper ocean reached over 50 cm s−1. NIWs traveled southward for hundreds of kilometers, dominated by modes 2 and 3 in the upper and deep ocean. The horizontal phase speeds of mode 2 were ∼3.9 and ∼2.5 m s−1 north and south of the typhoon’s track, respectively, while those of mode 3 were ∼2.1 and ∼1.7 m s−1, respectively. Mode 5 was only identified in the north with a smaller phase speed. Owing to different vertical group velocities, the energy of mode-2 NIWs reached the deep ocean in 20 days, whereas the higher-mode NIWs required more time to transfer energy to the bottom. NIWs in the north were trapped and carried by a westward-propagating anticyclonic eddy, which enhanced the near-inertial kinetic energy at ∼300 m and lengthened the duration of energetic NIWs observed in the north. Significance Statement Near-inertial waves (NIWs), generally caused by wind (e.g., typhoons and monsoons) in the upper ocean, are one of the two types of energetic internal waves widely observed in the ocean. After their generation near the surface, energetic NIWs propagate downward and equatorward, thereby significantly contributing to turbulent mixing in the upper and deep ocean and acting as a mechanism of energy transfer from the surface to the deep ocean. The unprecedented NIW observations in the South China Sea describe the generation, propagation, and vertical normal modes of typhoon-induced NIWs in the upper and deep oceans, and contribute to knowledge regarding the dynamic responses of abyssal processes to typhoons.

Direct Evidence of Standing Internal Tide West of the Luzon Strait Observed by a Large-Scale Observation Array

Abstract Using a large-scale observation array of 27 simultaneous pressure-recording inverted echo sounders (PIESs), the standing wave features of the mode-1 M2 internal tide west of the Luzon Strait (LS) were identified. These features exhibited nonmonotonic spatial phase shifts and half-wavelength amplitude modulation, resulting in spatially varying amplitudes under PIES observations, which have not been previously observed in field observations west of the LS. Satellite altimeter measurements also identified standing-wave patterns consistent with the PIES observations. These patterns emanated from interference between the northwestward and southeastward beams from the LS and the slope of the southern Taiwan Strait, respectively. Near the LS, the two beams superimposed into partial standing waves, whereas the superimposed waves tended to become perfect standing waves near the slope of the southern Taiwan Strait. The nodes and antinodes of the wave shifted under the influence of an anticyclonic eddy. The eddy-induced background current modified the phase speed of the internal tides, and the superimposed standing-wave nodes and antinodes deflected clockwise. The node shifted during three anticyclonic eddy events, and two stations on two sides of the wave node showed opposite variations in amplitude. Significance Statement The internal tidal constituent (M2) propagating in opposite directions can result in standing waves, which have been frequently observed in global oceans but were absent west of the Luzon Strait (LS). Our observations (based on a large-scale array west of the LS) discovered a standing M2 internal tide, which stems from interference between the northwestward beams emanating from the LS and southeastward beams from the slope of the southern Taiwan Strait. Anticyclonic eddies play important roles in adjusting the amplitude of internal tides by deflecting the standing-wave nodes and antinodes clockwise. The study facilitates the understanding of the energy distribution and mixing processes west of the LS and provides a fresh perspective on the dynamic relationship between mesoscale perturbations and internal tides.

Validation and Evaluation of GRACE-FO Estimates with In Situ Bottom Pressure Array Measurements in the South China Sea

The Gravity Recovery and Climate Experiment (GRACE), and its follow-on mission (GRACE-FO), provides a novel measurement of the variations in ocean bottom pressure (OBP) at global and basin scales, including those in marginal seas. However, these measurements have not yet been validated rigorously for the South China Sea (SCS). In this study, the accuracy in the monthly GRACE-FO mascon solutions in the SCS from the Jet Propulsion Laboratory (JPL), Center for Space Research (CSR), and Goddard Space Flight Center (GSFC) was validated with the results of the comparison with the in situ OBP records from an array of 25 pressure-recording inverted echo sounders (PIESs) that are located west of the Luzon Strait (LS). The correlation coefficient (Cor) and root mean square difference (RMSD) between the 10-month period of GSFC and PIES, spanning from July 2018 to June 2019 (with missing satellite data for August and September 2018), were 0.77 (p-value = 0.005) and 0.41 mbar (1 mbar = 100 Pa), respectively. These values suggest that the accuracy of GSFC in the SCS in this period was substantially better than that of JPL (Cor = 0.35, p-value = 0.16; RMSD = 0.74 mbar) and CSR (Cor = 0.25, p-value = 0.24; RMSD = 0.89 mbar). Moreover, the volume transport anomaly of the SCS abyssal circulation was estimated and compared based on the OBP records from GSFC and PIES observations, indicating that the GRACE-FO OBP (GSFC) can be used to monitor seasonal or longer-period variations in the SCS abyssal volume transport. Additionally, the variations in OBP from GRACE-FO were significantly overestimated on the continental shelf of the SCS, which may be attributed to signal leakage. Our findings provide reliable evidence for the application of long-term, fully covered OBP records from GRACE-FO in the SCS, and also offer a valuable reference for the application of GRACE-FO in other regions.

Summer Anticyclonic Eddies Carrying Kuroshio Waters Observed by a Large CPIES Array West of the Luzon Strait

Abstract The Kuroshio intrusion into the South China Sea (SCS) in summer is weak and has rarely been reported by in situ observations. Here, we describe a new form of Kuroshio water intrusion that is strongest during the summer, the North Luzon Warm Eddy (NLWE), which is an anticyclonic eddy originating north of Luzon Island. From early July to mid-September 2018, two NLWEs moving northwestward were captured by a mooring array consisting of 27 current- and pressure-recording inverted echo sounders (CPIESs). The three-dimensional CPIES estimates reveal that the NLWEs carried large amounts of saline Kuroshio waters (S > 34.7 psu) in the subsurface, which was also evidenced by Argo float profiles. The Kuroshio intrusion was confined to waters shallower than the 14.8°C isotherm. Historical data for NLWEs suggest that they occur mostly during the summer but are absent between November and March, which is attributed to seasonal wind stress curl (WSC). However, because the seasonal signal of WSC during summer is small, intraseasonal WSC is directly responsible for the generation of NLWEs. Significance Statement This paper describes a new type of Kuroshio water intrusion into the South China Sea (SCS)—the North Luzon Warm Eddy (NLWE), which is an anticyclonic eddy generated north of Luzon Island. The eddy mostly occurs during summer when the Kuroshio intrusion is commonly considered the weakest. From observations of a large CPIES array, we provide a cradle-to-grave picture of the NLWE. NLWEs are estimated to contribute almost half of the westward Luzon Strait transport during the summer and, as such, play an important role in the seasonal stratification and circulation in the northeastern SCS.

Generation of high-frequency topographic Rossby waves in the Gulf of Mexico

The Loop Current Eddy (LCE) separation cycle energizes deep circulation in the eastern Gulf of Mexico, transferring energy from the surface intensified Loop Current (LC) to the typically quiescent lower layers. To document the generation and radiation of deep energy during this cycle, an array of 24 current and pressure recording inverted echo sounders (CPIES) is deployed in the region 89°W to 86°W, 25°N to 27.5°N with the intent to capture circulation near bathymetric features thought to be important for current-topographic interactions: Campeche Bank, Mississippi Fan, and West Florida Shelf. During the nearly two-year deployment, June 2019 to May 2021, three LCE separation events are observed, during which energy injected into the deep Gulf organizes into two distinct frequency bands (1/100 – 1/20 days–1 and 1/20 – 1/10 days–1). High-frequency variability dominates the array’s northwest corner in the vicinity of the Mississippi Fan. Wave properties are consistent with topographic Rossby Waves (TRWs) with wavelengths of 150 – 300 km. Their generation coincides with each LCE separation and is attributed to an upper-lower layer resonant coupling between surface meanders and the sloping topography of the Mississippi Fan. TRWs captured by the CPIES array will likely intensify as wavelengths shorten in steeper topography along propagation pathways towards the Sigsbee Escarpment, generating hazardous currents with the potential to disrupt oil and gas operations in the region.

Observation of Bottom-Trapped Topographic Rossby Waves to the West of the Luzon Strait, South China Sea

Abstract Topographic Rossby waves (TRWs) play an important role in deep-ocean dynamics and abyssal intraseasonal variations. Observational records from 15 current- and pressure-recording inverted echo sounders (CPIESs) and two moorings deployed in the northern Manila Trench (MT), South China Sea (SCS), for over 400 days were utilized to analyze the widely existing near-21-day bottom-trapped TRWs in the trench. The TRWs were generally generated in winter and summer, dominated by perturbations in the upper ocean. Kuroshio intrusion and its related variabilities contributed to the perturbations in winter, whereas the perturbations generated north of Luzon Island dominated in summer. Eddies north of Luzon propagated northwestward in the summer of 2018; however, these eddies caused the Kuroshio meanderings in the Luzon Strait (LS) in the summer of 2019. The variations in the Kuroshio path and the Kuroshio-related eddies induced TRWs in the deep ocean in regions with steep topography. However, the spatiotemporal distributions of TRWs were complex owing to the propagation of the waves. Some cases of TRWs showed no relation to the local upper-layer perturbations but propagated from adjacent regions. Some of these TRWs were induced by perturbations in the upper ocean in adjacent regions, and propagated anticlockwise in the MT with shallow water to their right, while others may be related to the intraseasonal variations in deep-water overflow in the LS and propagated northward. This study suggests that the Kuroshio and Kuroshio-related eddies significantly contribute to the dynamic processes associated with intraseasonal variations in the deep SCS through the generation of TRWs. Significance Statement Topographic Rossby waves (TRWs) are fluctuations generated when water columns travel across sloping topography under potential vorticity conservation. Based on observations of 15 current- and pressure-recording inverted echo sounders (CPIESs) and two moorings in the northern Manila Trench (MT) in the South China Sea (SCS), TRWs with periods of approximately 21 days were observed and analyzed. This study describes the generation, propagation, and spatiotemporal distribution of TRWs west of the LS and confirms that regional Kuroshio meanderings and upper eddies play important roles in the dynamic processes associated with intraseasonal variations in the deep SCS; the study may thus contribute to knowledge on the dynamic response of the abyssal current to mesoscale perturbations in the upper ocean.

Observation of Abyssal Circulation to the West of the Luzon Strait, South China Sea

Abstract South China Sea (SCS) abyssal circulation largely contributes to water renewal, energy budget, and sedimentary processes in the deep ocean. The three-dimensional abyssal circulation west of the Luzon Strait (LS) in the northern SCS was investigated using an array comprising 27 current- and pressure-recording inverted echo sounders. Over 400 days of measurements from June 2018 to July 2019 showed a narrow and strong (∼70 km, ∼2.3 cm s−1 at 2500 dbar) northward current near the steep eastern boundary, while a wide and weak (∼180 km, ∼1.5 cm s−1 at 2500 dbar) southwestward current lies along the subdued western boundary. The circulation showed conspicuous cyclonic patterns with a volume transport of ∼1.21 ± 0.93 Sv (1 Sv ≡ 106 m3 s−1) and ∼1.59 ± 0.95 Sv below 2500 dbar along the eastern and western boundaries, respectively. The current near the LS was strong in late autumn and early winter but weak in late winter and spring, following the seasonal variation of LS deep-water overflow. However, the southwestward current in the interior SCS was stronger in summer and early autumn but weaker in late winter and early spring. The different seasonal patterns identified near the LS and the interior SCS are attributed to the propagation of seasonal variation. The weak current along the western boundary in August 2018 and February 2019 was dominated by LS deep-water overflow with a time lag of ∼7.5 months. Although eddies in the upper ocean may also contribute to such variation through pressure work, the effect is minor. Significance Statement Cyclonic circulation in the deep South China Sea (SCS) largely contributes to water renewal, energy budget, and sedimentary processes and influences the transport of dissolved elements, minerals, and pollutants. As an important part of the SCS throughflow, an in-depth analysis of the SCS abyssal circulation may also contribute to understanding Indonesian Throughflow and global climate change. The three-dimensional abyssal circulation west of the Luzon Strait was investigated using large-scale data from June 2018 to July 2019, which provided unprecedented coverage of abyssal circulation in the northeast SCS. The study provides important observational evidence for the existence of SCS abyssal cyclonic circulation. Detailed spatiotemporal structure of abyssal circulation and its variations are presented, and related dynamic processes are discussed.

Brazil Current Volume Transport Variability During 2009–2015 From a Long‐Term Moored Array at 34.5°S

AbstractThe Brazil Current, the western limb of the subtropical gyre of the South Atlantic Ocean, is one of the major Western Boundary Currents of the global ocean. Here, we present the first multiyear continuous daily time series of Brazil Current absolute volume transport obtained using 6+ years of observations from a line of four pressure‐recording inverted echo sounders (PIES) deployed at 34.5°S. The array was augmented in December 2012 with two current meter‐equipped PIES and in December 2013 with a moored Acoustic Doppler Current Profiler on the upper continental slope. The Brazil Current is bounded by the sea surface and the neutral density interface separating South Atlantic Central Water and Antarctic Intermediate Water, which is on average at a reference pressure of 628 ± 46 dbar, and it is confined west of 49.5°W. The Brazil Current has a mean strength of −14.0 ± 2.8 Sv (1 Sv ≡ 106 m3 s−1; negative indicates southward flow) with a temporal standard deviation of 8.8 Sv and peak‐to‐peak range from −41.7 to +20 Sv. About 80% of the absolute transport variance is concentrated at periods shorter than 150 days with a prominent peak at 100 days. The baroclinic component accounts for 85% of the absolute transport variance, but the barotropic variance is not negligible. The baroclinic and barotropic transports are uncorrelated, demonstrating the need to measure both transport components independently. Given the energetic high frequency transport variations, statistically significant seasonal to interannual variability and trends have yet to be detected.

The Impact of Meanders, Deepening and Broadening, and Seasonality on Agulhas Current Temperature Variability

AbstractFor the first time, the temperature transport of the Agulhas Current is quantified in a time series. Over a 25-month mooring deployment at 34°S, seven tall moorings were instrumented to measure current velocity, temperature, and salinity. Current- and pressure-recording inverted echosounders were used to extend geostrophic velocity, temperature, and salinity records to 300 km offshore. In the mean, the current transports 3.8 PW of heat southward relative to 0°C: −76 Sv (1 Sv ≡ 106 m3 s−1) at a transport-weighted temperature of 12.3°C. A 0.9-PW standard deviation in temperature transport is due to variability in both volume transport and the temperature field. Meandering of the current core dominates variability in the temperature field by warming temperatures offshore and cooling temperatures near the coast. However, meandering has a limited impact on the temperature transport, which varies more closely with a deepening and broadening of the current associated with an inshore isotherm shoaling and an offshore isotherm deepening. Stronger southward temperature transports correspond to a deeper current transporting more volume, yet at a cooler transport-weighted temperature. Seasonality is not observed in the temperature transport time series, possibly because of the offsetting effects of cooler temperatures during times of seasonally stronger volume transports. Although volume transport and temperature transport are highly correlated, the large variability in transport-weighted temperature means that using volume transport alone to infer temperature transport results in an error that could be as large as 24% of the southern Indian Ocean heat transport.

Generation and propagation of 21-day bottom pressure variability driven by wind stress curl in the East China Sea

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.

Observed Storm Track Dynamics in Drake Passage

AbstractThe dynamics of an oceanic storm track—where energy and enstrophy transfer between the mean flow and eddies—are investigated using observations from an eddy-rich region of the Antarctic Circumpolar Current downstream of the Shackleton Fracture Zone (SFZ) in Drake Passage. Four years of measurements by an array of current- and pressure-recording inverted echo sounders deployed between November 2007 and November 2011 are used to diagnose eddy–mean flow interactions and provide insight into physical mechanisms for these transfers. Averaged within the upper to mid-water column (400–1000-m depth) and over the 4-yr-record mean field, eddy potential energy is highest in the western part of the storm track and maximum eddy kinetic energy occurs farther away from the SFZ, shifting the proportion of eddy energies from to about 1 along the storm track. There are enhanced mean 3D wave activity fluxes immediately downstream of SFZ with strong horizontal flux vectors emanating northeast from this region. Similar patterns across composites of Polar Front and Subantarctic Front meander intrusions suggest the dynamics are set more so by the presence of the SFZ than by the eddy’s sign. A case study showing the evolution of a single eddy event, from 15 to 23 July 2010, highlights the storm-track dynamics in a series of snapshots. Consistently, explaining the eddy energetics pattern requires both horizontal and vertical components of W, implying the importance of barotropic and baroclinic processes and instabilities in controlling storm-track dynamics in Drake Passage.

Internal tides in the northwestern South China Sea observed by pressure-recording inverted echo sounders

This study presents IT observations in the northwestern SCS from five pressure-recording inverted echo sounders (PIESs). The vertical round-trip acoustic travel times that PIESs measures are proportional to the vertical thermocline displacements; therefore, PIESs can be used to monitor the 1st mode internal tides. PIESs were deployed along a section between the continental shelf break in the north and Xisha Islands in the south. Measurements show that both diurnal and semidiurnal ITs were stronger on the northern and southern sides of the section than those in the middle. Their temporal variations were coherent with local surface tides, which implies locally generated ITs may be important. Complex empirical orthogonal function analysis reveals a small phase difference (the south leads the north by 70°) across the section for diurnal ITs, but a significant phase difference (the north leads the south by 360°) for semidiurnal ITs. These observations indicate that the dominant diurnal ITs propagate across the section (from east to west), while the semidiurnal ITs propagate along the section (from north to south). A ray-tracing model was utilized to demonstrate the impact of mesoscale circulation patterns on the propagation of diurnal ITs from the Luzon Strait to the PIES sites. The model explains the temporal IT variations, which were incoherent with local surface tides. The result indicates that diurnal ITs in the northwestern SCS can be affected by ITs propagating from the Luzon Strait, while semidiurnal ITs are primarily generated locally from the continental shelf break in the north.

Comparison of Measurements from Pressure-recording Inverted Echo Sounders and Satellite Altimetry in the North Equatorial Current Region of the Western Pacific

An array of 5 pressure-recording inverted echo sounders (PIESs) was deployed along the Jason-2 214 ground track in the North Equatorial Current (NEC) region of the western Pacific Ocean for about 2 years from June 2012. Round-trip acoustic travel time from the bottom to the sea surface and bottom pressure measurements from PIES were converted to sea level anomaly (SLA). AVISO along-track mono-mission SLA (Mono-SLA), reference mapped SLA (Ref-MSLA), and up-to-date mapped SLA (Upd-MSLA) products were used for comparison with PIES-derived SLA (ηtot). Comparisons of ηtot with Mono-SLA revealed that hump artifact errors significantly contaminate the Mono-SLA. Differences of ηtot from both Ref-MSLA and Upd-MSLA decreased as the hump errors were reduced in mapped SLA products. Comparisons of Mono-SLA measurements at crossover points of ground tracks near the observation sites revealed large differences though the time differences of their measurements were only 1.53 and 4.58 days. Comparisons between Mono-SLA and mapped SLA suggested that mapped SLA smooths out the hump artifact errors by taking values between the two discrepant Mono-SLA measurements at the crossover points. Consequently, mapped SLA showed better agreement with ηtot at our observation sites. AVISO mapped sea surface height (SSH) products are the preferable dataset for studying SSH variability in the NEC region of the western Pacific, though some portions of hump artifact errors seem to still remain in them.

Near 7-day response of ocean bottom pressure to atmospheric surface pressure and winds in the northern South China Sea

Ocean bottom pressures, observed by five pressure-recording inverted echo sounders (PIESs) from October 2012 to July 2014, exhibit strong near 7-day variability in the northern South China Sea (SCS) where long-term in situ bottom pressure observations are quite sparse. This variability was strongest in October 2013 during the near two years observation period. By joint analysis with European Center for Medium-Range Weather Forecasts (ECMWF) data, it is shown that the near 7-day ocean bottom pressure variability is closely related to the local atmospheric surface pressure and winds. Within a period band near 7 days, there are high coherences, exceeding 95% significance level, of observed ocean bottom pressure with local atmospheric surface pressure and with both zonal and meridional components of the wind. Ekman pumping/suction caused by the meridional component of the wind in particular, is suggested as one driving mechanism. A Kelvin wave response to the near 7-day oscillation would propagate down along the continental slope, observed at the Qui Nhon in the Vietnam. By multiple and partial coherence analyses, we find that local atmospheric surface pressure and Ekman pumping/suction show nearly equal influence on ocean bottom pressure variability at near 7-day periods. A schematic diagram representing an idealized model gives us a possible mechanism to explain the relationship between ocean bottom pressure and local atmospheric forcing at near 7-day periods in the northern SCS.

Near 5-Day Nonisostatic Response to Atmospheric Surface Pressure and Coastal-Trapped Waves Observed in the Northern South China Sea

AbstractOceanic, nonisostatic responses to near 5-day Rossby–Haurwitz atmospheric pressure waves have been observed in open oceans; however, such responses based on observations in marginal seas such as the South China Sea have not been reported, owing to the limited ocean bottom pressure Pbot records. The Pbot measurements from pressure recording inverted echo sounders (PIESs) at sites in the northern South China Sea revealed a nonisostatic-like response near 5 days, although the coastal-trapped waves (CTWs) appeared to obscure it because their broadband periods include the near 5-day band. Cross-spectral analysis revealed that the PIES Pbot records and the sea level (SL) records of Hong Kong all correlate strongly with the atmospheric pressure and winds over the East China Sea. This is indicative of remotely forced CTWs. The PIES Pbot records showed higher coherence near 5 days with the zonal low-pass wavelength filters applied to the atmospheric pressure, and the phase analysis results strongly suggest nonisostatic oceanic responses to the westward-propagating Rossby–Haurwitz waves. Effective separation of CTWs and the nonisostatic responses from the Pbot records at the near 5-day period was achieved. The oceanic responses to the Rossby–Haurwitz waves in the northern South China Sea were nonisostatic; a 1-mbar change in air pressure resulted in a 1.58-mbar change in Pbot with a phase lag of 14.8°. The mean phase speed of CTWs from Hong Kong to station P3 was estimated to be 9.9 m s−1.

Bottom Temperatures in Drake Passage

AbstractLong time series of bottom temperatures in the Southern Ocean are rare. The cDrake array with over 40 current- and pressure-recording inverted echo sounders, moored across Drake Passage to monitor the Antarctic Circumpolar Current (ACC) variability and transport, measured temperature at 1 and 50 m above the seafloor at depths ≥ 3500 m and at the southern continental margin. The 4-yr dataset provided an opportunity to examine the temporal and spatial scales of bottom temperature variability. High variability was observed; ranges were 0.5°–0.9°C in the northern passage and 0.3°–0.6°C in the southern passage. Standard deviations in the two regions were 0.1°–0.15°C and <0.05°C, respectively. Meandering of the ACC with its deep-reaching thermocline accounted for up to 50% of the observed bottom temperature variance. Northern passage temperatures, spaced less than 40 km apart, were correlated with each other, while those in the southern portion, separated by 60–70 km, were not. A gap in the West Scotia Ridge provided a deep passageway for cold water to reach the northern passage from the southern basin; an extreme event during February 2008 brought bottom waters with in situ temperatures below 0.38°C as far north as 57°S. Strong vertical temperature gradients between 1 and 50 m above the bottom occurred intermittently due to intrusions associated with deep eddy circulations arising beneath the meandering jet and to flow over steep topography, permitting the generation of internal waves. High variability in temperature on interannual time scales requires record lengths of 13–17 yr to estimate long-term trends reliably.

Estimates of Eddy Heat Flux Crossing the Antarctic Circumpolar Current from Observations in Drake Passage

AbstractThe 4-yr measurements by current- and pressure-recording inverted echo sounders in Drake Passage produced statistically stable eddy heat flux estimates. Horizontal currents in the Antarctic Circumpolar Current (ACC) turn with depth when a depth-independent geostrophic current crosses the upper baroclinic zone. The dynamically important divergent component of eddy heat flux is calculated. Whereas full eddy heat fluxes differ greatly in magnitude and direction at neighboring locations within the local dynamics array (LDA), the divergent eddy heat fluxes are poleward almost everywhere. Case studies illustrate baroclinic instability events that cause meanders to grow rapidly. In the southern passage, where eddy variability is weak, heat fluxes are weak and not statistically significant. Vertical profiles of heat flux are surface intensified with ~50% above 1000 m and uniformly distributed with depth below. Summing poleward transient eddy heat transport across the LDA of −0.010 ± 0.005 PW with the stationary meander contribution of −0.004 ± 0.001 PW yields −0.013 ± 0.005 PW. A comparison metric, −0.4 PW, represents the total oceanic heat loss to the atmosphere south of 60°S. Summed along the circumpolar ACC path, if the LDA heat flux occurred at six “hot spots” spanning similar or longer path segments, this could account for 20%–70% of the metric, that is, up to −0.28 PW. The balance of ocean poleward heat transport along the remaining ACC path should come from weak eddy heat fluxes plus mean cross-front temperature transports. Alternatively, the metric −0.4 PW, having large uncertainty, may be high.

The impact of monsoon winds and mesoscale eddies on thermohaline structures and circulation patterns in the northern South China Sea

We deployed 5 pressure-recording inverted echo sounders (PIES) along a section in the northern South China Sea (NSCS), and estimated well the distributions of temperature, salinity and velocity across the section. Applying the empirical orthogonal function (EOF) method, we found that variability of the estimates is dominated by two modes: one named the seasonal mode affecting strongly on the hydrographic distribution with explained variability of temperature/salinity by 62.9/72.2%; the other named the eddy mode, corresponding to the arrival of mesoscale eddies, affecting strongly on the circulation pattern with explained variability of velocity by 63.2%. Temporal variation of the seasonal mode is highly correlated with the monsoon winds southeast of Vietnam, suggesting a nonlocal forcing mechanism. Case studies looking at the structures and evolutions of three captured eddies, whose impacts were well quantified by the eddy mode. The monsoon (eddies) significantly affects temperature, salinity and velocity shallower than 635m (860m), 160m (150m) and 1055m (920m), respectively. The monsoon (eddies) can induce maximum temperature, salinity and velocity anomalies up to −1.6 to 2.1°C (−2.5 to 2.2°C), −0.11 to 0.14psu (−0.13 to 0.27psu) and −0.31 to 0.46m/s (−0.40 to 0.38m/s), respectively. Mean volume transport (VT) across the section is 1.0Sv (1Sv=1×106m3s−1, positive to the northeast). Seasonal VT (with eddy impacts removed) is −4.6Sv, 11.4Sv, −5.1Sv and −4.1Sv for spring, summer, autumn and winter, respectively.