Guanghong Liao

Variation in the Kuroshio intrusion: Modeling and interpretation of observations collected around the Luzon Strait from July 2009 to March 2011

This study analyzes the observed subtidal currents, 1/12° global HYCOM model results, and the observed time series to interpret seasonal and interannual patterns in the behavior of the Kuroshio intrusion around the Luzon Strait (LS). The observations include current measurements conducted at mooring station N2 (20°40.441′N, 120°38.324′E) from 7 July 2009 to 31 March 2011, surface geostrophic currents derived from the merged absolute dynamic topography, and the trajectory of an Argo float during the winter of 2010–2011. Results from mooring station N2 confirmed the seasonal changes in the Kuroshio intrusion and the variation of the Kuroshio intrusion during El Nino event from July 2009 to April 2010 and La Nina even from June 2010 to March 2011. The strongest Kuroshio intrusion occurs in the winter, with successively weaker currents in spring, autumn, and summer. Comparison of relative differences (Δmax (z)) in the maximum absolute value of monthly average zonal velocity components |Umax (z)| showed that the Kuroshio intrusion was stronger during the 2009–2010 winter (El Nino) than the 2010–2011 winter (La Nina). Furthermore, the relative differences (Δmax (z)) in deeper layers exceed those of the surface layer. Circulation patterns in surface geostrophic currents and the Argo float trajectory confirmed the results of mooring station N2. The Kuroshio intrusion velocity variation modeled using the 1/12° global HYCOM model resembled the observation on both seasonal to interannual scales. Modeled variation in the zonal mean velocity anomaly was also consistent with Nino3, Nino4, and North Equatorial Current (NEC) bifurcation latitude indices, indicating concurrent impacts of the ENSO influence. Monsoon winds strongly affect the seasonal variation while the weak upstream Kuroshio transport induced by El Nino, strongly affects the interannual variation, such as 2009–2010 winter. In 2010–2011 winter, the impact of winter monsoon forcing still exists in the LS. However, the stronger upstream Kuroshio transport during this period did not allow the Kuroshio to penetrate into the LS deeply. This explains why the 2009–2010 winter Kuroshio intrusion (El Nino event) was stronger than that of the 2010–2011 winter (La Nina event).

Reciprocal sound transmission experiments for current measurement in a tidal river

Six repeat reciprocal sound transmission experiments for current measurement were successful carried out in the upstream region of the Qiantang River in Hangzhou city, about 90 km from the mouth of Hangzhou Bay during April to December 2009. Two Coastal Acoustic Tomography (CAT) Systems were set up at a distance of 3050 m for making a reciprocal transmission between the both banks of the Qiantang River. During the sound transmission experiments, thirty-two repeat shipboard Acoustic Doppler Profiler (ADP) surveys were also performed along the sound transmission line to get a comparison with the reciprocal sound transmission data. Range-averaged current velocities, determined from the travel time differences along the transmission line were in good agreement with those from the ADP, producing a root-mean-square difference of 0.03 m/s. The travel time differences data were well correlated with the river discharge estimated from the ADP data. The time series of river discharge during the experimental period was estimated through an empirical formula, relating to both the parameters. The variations of river discharge caused by tidal bores were well captured. The river discharge changed in the range of −7626 m3/s and 5096 m3/s, with a mean of 1246 m3/s. The above results suggest that the CAT is a powerful instrument for measuring continuously the river discharge in such tidal rivers with quite heavy shipping traffic.

The diel vertical migration of sound scatterers observed by an acoustic Doppler current profiler in the Luzon Strait from July 2009 to April 2011

Acoustic Doppler current profiler (ADCP) receives echoes from sound scatterers, then their speed is calculated by the Doppler effect. In the open ocean, most of these backscatterers are from the plankton. The sound scatterers descend down to depth at around dawn, their mean speed is 2.9 cm/s, then they ascend up to the surface layer at around dusk with a mean speed of 2.1 cm/s, in the Luzon Strait. The descending speed is faster, which suggests that this zooplankton population may accelerate its downward migration under the action of the gravity. The vertical distribution of a mean volume backscattering strength (MVBS) in the nighttime has two peaks, which locate near the upper and lower boundary layers of halocline, respectively. However, the backscatterers only aggregate near the surface layer in the daytime. The diel vertical migration (DVM) of sound scatterers has several characteristic patterns, it is stronger in summer, but weaker in winter, and the maximum peak occurs in September. The DVM occurrence is synchronous with the seawater temperature increasing at around dawn and dusk, it may affect the ocean mixing and water stratification.

Moored observation of abyssal flow and temperature near a hydrothermal vent on the Southwest Indian Ridge

Four moorings were deployed near “Dragon Flag,” an active hydrothermal vent in the valley of the Southwest Indian Ridge. The goal was to examine the variability of currents and temperature, which will guide the trajectory of spreading plumes. The mean current was cross-isobath, and the circulation was characterized by a submesoscale circulation. Observed currents also showed fluctuations with periods of 1–15 days. The inferred phase speed and wavelength for the wave with a period of 4.4 day are 10.4 km d−1 and 45.8km, respectively, which are consistent with the topographic Rossby wave theory. The persistent warming tendency with corresponding variation of salinity based on background θ-S properties may be caused by background circulation and divergence of the water column. The warming or cooling episodes were most likely as signatures of isopycnal surface depression or uplifting induced by the moving of mesoscale eddies. Well-resolved rotary spectra exhibited important nonlinear interactions between inertial and semidiurnal tide in the velocity and temperature records. Amplification of near-inertial currents in the near bottom is also exposed. These discoveries provided new evidence for the nonlinear interaction and trapped near-inertial waves by the ridge, which occurred in the deep ocean of the Southern Hemisphere. Such nonlinear interaction may represent a significant energy loss pathway for the internal waves, and part of the decay of such motion would likely result in increased mixing to maintain the abyssal stratification. Enhanced near-inertial motions can play a major role for the local advection of hydrothermal plumes.

Multiple-scale temporal variations and fluxes near a hydrothermal vent over the Southwest Indian Ridge

A deep-ocean mooring system was deployed 100 m away from an active hydrothermal vent over the Southwest Indian Ridge (SWIR), where the water depth is about 2,800 m. One year of data on ocean temperature 50 m away from the ocean floor and on velocities at four levels (44 m, 40 m, 36 m, and 32 m away from the ocean floor) were collected by the mooring system. Multiplescale variations were extracted from these data: seasonal, tidal, super-tidal, and eddy scales. The semidiurnal tide was the strongest tidal signal among all the tidal constituents in both currents and temperature. With the multiple-scale variation presented in the data, a new method was developed to decompose the data into five parts in terms of temporal scales: time-mean, seasonal, tidal, super-tidal, and eddy. It was shown that both eddy and tidal heat (momentum) fluxes were characterized by variation in the bottom topography: the tidal fluxes of heat and momentum in the along-isobath direction were much stronger than those in the cross-isobath direction. For the heat flux, eddy heat flux was stronger than tidal heat flux in the cross-isobath direction, while eddy heat flux was weaker in the along-isobath direction. For the momentum flux, the eddy momentum flux was weaker than tidal momentum flux in both directions. The eddy momentum fluxes at the four levels had a good relationship with the magnitude of mean currents: it increased with the mean current in an exponential relationship.

Variation of Indo-Pacific upper ocean heat content during 2001–2012 revealed by Argo

Understanding of the temporal variation of oceanic heat content (OHC) is of fundamental importance to the prediction of climate change and associated global meteorological phenomena. However, OHC characteristics in the Pacific and Indian oceans are not well understood. Based on in situ ocean temperature and salinity profiles mainly from the Argo program, we estimated the upper layer (0–750 m) OHC in the Indo-Pacific Ocean (40°S–40°N, 30°E–80°W). Spatial and temporal variability of OHC and its likely physical mechanisms are also analyzed. Climatic distributions of upper-layer OHC in the Indian and Pacific oceans have a similar saddle pattern in the subtropics, and the highest OHC value was in the northern Arabian Sea. However, OHC variabilities in the two oceans were different. OHC in the Pacific has an east-west see-saw pattern, which does not appear in the Indian Ocean. In the Indian Ocean, the largest change was around 10°S. The most interesting phenomenon is that, there was a long-term shift of OHC in the Indo-Pacific Ocean during 2001–2012. Such variation coincided with modulation of subsurface temperature/salinity. During 2001–2007, there was subsurface cooling (freshening) nearly the entire upper 400 m layer in the western Pacific and warming (salting) in the eastern Pacific. During 2008–2012, the thermocline deepened in the western Pacific but shoaled in the east. In the Indian Ocean, there was only cooling (upper 150 m only) and freshening (almost the entire upper 400 m) during 2001–2007. The thermocline deepened during 2008–2012 in the Indian Ocean. Such change appeared from the equator to off the equator and even to the subtropics (about 20°N/S) in the two oceans. This long-term change of subsurface temperature/salinity may have been caused by change of the wind field over the two oceans during 2001–2012, in turn modifying OHC.

The rotary multiple filter technique and its application in the analysis of ocean current data

The Multiple Filter Technique (MFT) is a filtering technique, which is used to investigate variations in the amplitude and phase of dispersive signals in the time (t), and frequency f) domain. The paper extends the MFT to process current vector time series. The rotary MFT is applied to investigate the evolution of nonlinear wave-wave interactions observed in the ocean. The purpose of the present paper is twofold, namely: I) to review the MFT method, and apply it to deal with rotary components of ocean vectors, and, II) to describe applications of the rotary MFT to examine the evolution of energies at different frequencies in the ocean current time series, especially the nonlinear interactions of internal waves in the ocean. The rotary MFT is demonstrated to be an effective tool for analyzing the current time series data.

Conversion of Pressure to Depth for Moored Instruments Using a Reference Bottom Mounted Pressure Sensor

ABSTRACT A new method is proposed to convert pressure measured by an instrument to water depth using an additional available bottom-moored pressure sensor. A perturbation analysis is used in this analysis, which leads to a simple formula for calculating water depth (defined as one from the mean sea surface to the instrument) from the pressure data. In field experiments, this method is easier to apply than existing methods. Based on the theoretical derivation, the error associated with the method comes from two sources when the instrument depth is known at the beginning of the measurement: temporal variation of deep water density at depths deeper than the instrument and variation in the gravitational acceleration with instrument displacement. These two sources contribute up to 4% of the error relative to the vertical displacement of the instrument, assuming the pressure sensor is accurate. With the vertical displacement of the instrument being on the order of 10 m, the absolute error is on the order of 0.4 m, which is expected and acceptable in oceanic measurements. The method is applied to data from a field experiment that took place along the Myanmar coast in December 2012.