Expendable Bathythermograph Research Articles

A three-dimensional variational ocean data assimilation system: Scheme and preliminary results

A new 3DVAR-based Ocean Variational Analysis System (OVALS) is developed. OVALS is capable of assimilating in situ sea water temperature and salinity observations and satellite altimetry data. As a component of OVALS, a new variational scheme is proposed to assimilate the sea surface height data. This scheme considers both the vertical correlation of background errors and the nonlinear temperature-salinity relationship which is derived from the generalization of the linear balance constraints to the nonlinear in the 3DVAR. By this scheme, the model temperature and salinity fields are directly adjusted from the altimetry data. Additionally, OVALS can assimilate the temperature and salinity profiles from the ARGO floats which have been implemented in recent years and some temperature and salinity data such as from expendable bathythermograph, moored ocean buoys, etc. A 21-year assimilation experiment is carried out by using OVALS and the Tropical Pacific circulation model. The results show that the assimilation system may effectively improve the estimations of temperature and salinity by assimilating all kinds of observations. Moreover, the root mean square errors of temperature and salinity in the upper depth less than 420 m reach 0.63℃ and 0.34 psu.

Semidiurnal M2 internal tides in the Indo‐Australian Basin

Clear signals of internal tide propagation are found at the semidiurnal M2 frequency in the eastern South Indian Ocean over the abyssal plain of the Indonesian Australian Basin. We find a spatially and temporally coherent internal tide with phase propagation estimated from satellite altimetry suggesting generation along the Indonesian Archipelago and the Australian North West Shelf east of 115°E. A sinusoidal fit is used to estimate internal tide phase and amplitude along vertical cross sections from expendable bathythermograph (XBT) data along volunteer merchant shipping lines. It is likely tidal energy in the middle of the Indonesian Australian Basin is from both northern and southern source regions, and may sustain a second region of enhanced mixing for the Indonesian Throughflow.

Why do Intrathermocline Eddies Form in the Japan/East Sea? A Modeling Perspective

Abstract : Intrathermocline eddies (ITEs) are characterized by a subsurface lens of relatively homogeneous water. By definition, they are situated within the thermocline and therefore split the stratified water column, taking the form of a dome in the upper part of the thermocline and a bowl in the lower part. Observations of ITEs in diverse regions of the world ocean indicate typical spatial scales of 10-100 km horizontally and 100 m vertically. In the Japan/East Sea (JES) there are at least three mechanisms for the formation of ITEs from pre-existing non-ITE eddies based on results from the HYbrid Coordinate Ocean Model (HYCOM). Those mechanisms include advection of the stratified seasonal variations of temperature and salinity through the Tsushima Strait, restratification of the upper water column due to seasonal heating and cooling of the upper ocean, and subduction of ITE water originating from the Tsushima Strait beneath the wintertime Subpolar Front. The formation mechanisms are not mutually exclusive. Indeed, all three are shown to be interactively affecting the formation of an ITE in at least one case. Gordon et al. (2002) reported the existence of ITEs in the JES based on observations from SeaSoar instrumentation, conductivity-temperature depth (CTD) sensors, and airborne expendable bathythermographs (AXBTs). Their paper contains extensive analysis of ITEs in the JES and observational evidence of formation mechanisms based on cruise data collected during 1999-2000 as part of the Office of Naval Research (ONR) JES Department Research Initiative as well as results from earlier studies. The Gordon et al. work inspired a numerical modeling study to examine whether or not similar features could be simulated. If they could be simulated, could the ocean model be used as a tool to elucidate the formation mechanisms of the ITEs? This study uses HYCOM to simulate JES ITEs that have domed stratification at the top, forming a lens-shaped interior of nearly unstratified water.

Wind‐driven and steric fluctuations of sea surface height in the southwest Pacific

Large fluctuations in sea surface height (SSH) occurred in the southwest Pacific between New Zealand and Fiji in the late 1990s. A model of SSH including steric heating and wind‐driven Rossby waves explains more than 40% of the observed SSH variance in the region over the 12 years of satellite measurements. The modelled SSH also agrees with dynamic height calculated from subsurface temperatures measured along an expendable bathythermograph (XBT) line between New Zealand and Fiji. The model simulations show a large high in SSH was created by anomalous downward Ekman pumping during 1998 when the seasonal change to upwelling failed to occur. The 50‐year wind record shows other downwelling events have occurred in the region predominantly during El Niño conditions.

Ocean mixed layer depth: A subsurface proxy of ocean‐atmosphere variability

A new criterion, based on the shallowest extreme curvature of near surface layer density or temperature profiles, is established for demarking the mixed layer depth, hmix. Using historical global hydrographic profile data, including conductivity‐temperature‐depth and expendable bathythermograph data obtained during World Ocean Circulation Experiment, its seasonal variability and monthly to interannual anomalies are computed. Unlike the more commonly used Δ criterion, the new criterion is able to deal with both different vertical resolutions of the data set and a large variety of observed stratification profiles. For about two thirds of the profiles our algorithm produces an hmix/c that is more reliable than the one of the Δ criterion. The uncertainty for hmix/c is ±5 m for high‐ (<5 m) and ±8 m for low‐ (<20 m) resolution profiles. A quality index, QImix, which compares the variance of a profile above hmix to the variance to a depth of 1.5 × hmix, shows that for the 70% of the profile data for which a clearly recognizable well‐mixed zone exists near the surface, our criterion identifies the depth of the well‐mixed zone in all cases. The standard deviation of anomalous monthly hmix/c is typically 20–70% of the long‐term mean hmix/c. In the tropical Pacific the monthly mean anomalies of hmix/c are not well correlated with anomalies of sea surface temperature, which indicate that a variety of turbulent processes, other than surface heat fluxes, are important in the upper ocean there. Comparisons between observed hmix/c and Massachusetts Institute of Techonology/ocean general circulation model/Estimating the Circulation and Climate of the Ocean model simulated mixed layer depth indicate that the KPP algorithm captures in general a 30% smaller mixed layer depth than observed.

Validation of the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR‐E) sea surface temperature in the Southern Ocean

Satellite sea surface temperature (SST) measurements from Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR‐E) are compared with in situ temperature observations from high‐resolution expendable bathythermograph and hull‐mounted thermosalinograph data along two sections (south of Australia and Drake Passage) in the Southern Ocean. To eliminate the effects of diurnal warming and low wind speed, we use only AMSR‐E data collected within 5 hours of the in situ observations, with wind speeds exceeding 6 m s−1. The AMSR‐E measurements are warmer than in situ observations during summer and are colder than in situ observations during winter. Factors that may cause the temperature difference are examined, including wind speed, columnar water vapor, columnar cloud water, geographic location, local temperature, and time of observation. Of these, wind speed and columnar water vapor are found to be the major factors contributing to the temperature difference between AMSR‐E SST and in situ SST observations. The temperature difference decreases with increasing wind speed and water vapor. AMSR‐E and in situ SST observations are also compared with simultaneous Moderate Resolution Imaging Spectroradiometer (MODIS) SST and weekly Reynolds Optimum Interpolated (OI) SST. Results suggest that the OI SSTs have a warm bias for both summer and winter; MODIS SSTs indicate a cold bias. In contrast, AMSR‐E SSTs show little bias relative to expendable bathythermographs.

Interannual variability in northeast Pacific circulation

Interannual variability of the circulation in the northeast Pacific Ocean is explored through a joint analysis of expendable bathythermograph (XBT) and expendable conductivity‐temperature‐depth (XCTD) data, satellite altimetry, and output from a model that was constrained by ocean data. XBT temperature profiles with high spatial resolution are available in the eastern North Pacific along two repeated transects. These ship tracks, along with the coast of North America, define a closed “box” which is used to study the time‐mean circulation and its variability on interannual timescales. Geostrophic velocities from XBT data are compared with geostrophic velocities from model output as well as the full model velocity fields. Correlations in variability on interannual timescales between transport in the subpolar gyre and in the subtropical gyre are present in both model output and data. The nature of the variability, and its relation to the changes of the strength of the North Pacific Current (NPC), which supplies the water for both gyres, are explored. Interannual variability in gyre transport is found to be related to both the bifurcation of the NPC, resulting in an anticorrelation in transport between the two gyres, and to variations in NPC strength, resulting in simultaneous changes in the two gyres. The dominant signal is found to be a long‐term increase in the NPC, which results in a strengthening of the subtropical gyre. Possible connections with local‐scale wind stress changes and with the El Niño/Southern Oscillation phenomenon are also explored.

Decadal temperature changes in the Tasman Sea

Ocean temperature changes between 1991 and 2005 in the eastern Tasman Sea were analysed. This area was chosen because of a combination of data availability, low mesoscale variability and because of its importance in determining the climate of the downwind New Zealand landmass. A large warming extending to the full depth of the water column (c. 800 m) was found to have occurred between 1996 and 2002. This warming was seen in measurements by expendable bathythermographs and also in satellite sea surface temperature and sea surface height products, and has a clear impact on New Zealand's terrestrial temperature. The nature of the warming is discussed, together with likely forcing mechanisms. No local forcing mechanisms are consistent with the observed warming, leading to the conclusion that the signal seen in the Tasman Sea is part of a larger South Pacific‐wide phenomenon.

Temperature contrasts in the water column inferred from amplitude‐versus‐offset analysis of acoustic reflections

We show that seismic amplitude‐versus‐offset (AVO) analysis can be used to remotely quantify the temperature contrasts responsible for acoustic reflections in the ocean. We inferred the sound speed and density contrasts associated with two reflections in the Norwegian Sea by comparing their AVO response to that predicted by the Zoeppritz equations. Estimates of temperature contrasts were calculated from sound speed contrasts using Wilson's equation and compared with in situ temperature measurements from coincident expendable bathythermographs (XBTs). A strong reflection from Line 9 is best explained by a −6 m/s step in sound speed, corresponding to a temperature decrease of ∼1.5°C. A weaker reflection from Line 11 yielded a sound speed contrast of −1.2 m/s, corresponding to a temperature decrease of ∼0.3°C. The inferred temperature contrasts match those found in XBT data remarkably well. L1‐norm misfits between predicted and calculated values indicate that the acoustic impedance contrasts are principally controlled by sound speed contrasts, rather than density contrasts. AVO analysis offers an accurate and robust technique for estimating sound speed (and therefore temperature) contrasts in the water column.

The baroclinic transport of the Antarctic Circumpolar Current south of Africa

Five hydrographic transects at nominal longitudes 0°E and 30°E, and fourteen expendable bathythermograph (XBT) sections near the former longitude are used to study the baroclinic transport of the Antarctic Circumpolar Current (ACC) between Africa and Antarctica. The bottom‐referenced geostrophic transport between the Subtropical Front and the ACC Southern Boundary is 147 ± 10 Sv. Estimating the transport from the XBTs using a technique previously employed south of Australia proves delicate because of an irregular bathymetry and water mass variations. It nevertheless confirms ACC transports around 150 Sv. Gathering these and other estimates from the Atlantic sector suggests that, while North Atlantic Deep Water is injected in the current west of 35°W, a partially compensating loss of Deep Circumpolar Water occurs east of this longitude. Another transport increase from 0°E to 30°E might reflect southward transfer across the Subtropical Front south of the Agulhas retroflection.

Initialization of an ENSO Forecast System Using a Parallelized Ensemble Filter

Abstract As a first step toward coupled ocean–atmosphere data assimilation, a parallelized ensemble filter is implemented in a new stochastic hybrid coupled model. The model consists of a global version of the GFDL Modular Ocean Model Version 4 (MOM4), coupled to a statistical atmosphere based on a regression of National Centers for Environmental Prediction (NCEP) reanalysis surface wind stress, heat, and water flux anomalies onto analyzed tropical Pacific SST anomalies from 1979 to 2002. The residual part of the NCEP fluxes not captured by the regression is then treated as stochastic forcing, with different ensemble members feeling the residual fluxes from different years. The model provides a convenient test bed for coupled data assimilation, as well as a prototype for representing uncertainties in the surface forcing. A parallel ensemble adjustment Kalman filter (EAKF) has been designed and implemented in the hybrid model, using a local least squares framework. Comparison experiments demonstrate that the massively parallel processing EAKF (MPPEAKF) produces assimilation results with essentially the same quality as a global sequential analysis. Observed subsurface temperature profiles from expendable bathythermographs (XBTs), Tropical Atmosphere Ocean (TAO) buoys, and Argo floats, along with analyzed SSTs from NCEP, are assimilated into the hybrid model over 1980–2002 using the MPPEAKF. The filtered ensemble of SSTs, ocean heat contents, and thermal structures converge well to the observations, in spite of the imposed stochastic forcings. Several facets of the EAKF algorithm used here have been designed to facilitate comparison to a traditional three-dimensional variational data assimilation (3DVAR) algorithm, for instance, the use of a univariate filter in which observations of temperature only directly impact temperature state variables. Despite these choices that may limit the power of the EAKF, the MPPEAKF solution appears to improve upon an earlier 3DVAR solution, producing a smoother, more physically reasonable analysis that better fits the observational data and produces, to some degree, a self-consistent estimate of analysis uncertainties. Hybrid model ENSO forecasts initialized from the MPPEAKF ensemble mean also appear to outperform those initialized from the 3DVAR analysis. This improvement stems from the EAKF’s utilization of anisotropic background error covariances that may vary in time.

Inter-manufacturer Difference and Temperature Dependency of the Fall-Rate of T-5 Expendable Bathythermograph

The fall-rate of the T-5 expendable bathythermograph (XBT) produced by Tsurumi Seiki (TSK) Co., Ltd and that by Sippican Inc., are intercompared by a series of contemporaneous and colocated measurements with conductivity-temperature-depth (CTD) profilers. It is confirmed that the fall-rates of the two manufacturers' T-5 differ by about 5 percent, despite the fact that they had been believed to be identical for many years. The cause of the difference is discussed on the basis of a detailed cross-examination of the two T-5 models. It is found for the first time that the two models are different in several respects. The manufacturer's fall-rate equation is only applicable to the Sippican T-5, for which Boyd and Linzell's (1993) equation seems to be slightly more accurate. Kizu et al.'s (2005) equation gives a clearly less biased depth than the manufacturers' equation for the TSK T-5. It is also found that the fall-rates of both T-5 models are dependent on water temperature, perhaps because of viscosity. The temperature-dependency of the fall-rate of the TSK T-5 is larger than that of the Sippican T-5.

Impact of Combining Temperature Profiles from Different Instruments on an Analysis of Mixed Layer Properties

Abstract For a joint analysis of temperature profiles from floats, expendable bathythermographs (XBTs), and other instruments (e.g., thermistor chains) the consistency of the profiles from the different sources needs to be ensured to avoid results with systematic errors. In this study profiles from the different instrument types are compared after they passed through a series of quality control tests. The different methods for quality control are presented. After ensuring that only high-quality profiles remain in the dataset, a statistical analysis of the temperature differences between adjacent profiles (in space and time) is performed. Potential regional differences as well as possible differences between the various float types are addressed. Finally, the impact of combining the profiles from floats with those from other instruments on gridded fields of the mixed layer temperature, thickness, and heat budget is discussed. It is found that the joint analysis yields more reliable results for the gridded fields and the heat storage rate. A large part of this improvement is a result of the reduced seasonal bias.

Operational multivariate ocean data assimilation

AbstractA fully three‐dimensional, multivariate, optimum‐interpolation ocean data assimilation system has been developed that produces simultaneous analyses of temperature, salinity, geopotential and vector velocity. The system is run in real‐time, and can be executed as a stand‐alone analysis or cycled with an ocean forecast model in a sequential incremental update cycle. Additional capabilities have been built into the system, including flow‐dependent background‐error correlations and background‐error variances that vary in space and evolve from one analysis cycle to the next. The ocean data types assimilated include: remotely sensed sea surface temperature, sea surface height, and sea‐ice concentration; plus in situ surface and sub‐surface observations of temperature, salinity, and currents from a variety of sources, such as ships, buoys, expendable bathythermographs, conductivity–temperature–depth sensors, and profiling floats. An ocean data quality‐control system is fully integrated with the multivariate analysis, and includes feedback of forecast fields and prediction errors in the quality control of new observations. The system is operational at the US Navy oceanographic production centres both in global and in regional applications. It is being implemented as the data assimilation component of the Hybrid Coordinate Ocean Model as part of the US contribution to the Global Ocean Data Assimilation Experiment, and in a limited‐area ensemble‐based forecasting system that will be used in an adaptive sampling, targeted observation application. Copyright © 2005 Royal Meteorological Society

Effect of Salinity on Estimating Geostrophic Transport of the Indonesian Throughflow along the IX1 XBT Section

Geostrophic transport of the Indonesian Throughflow (ITF) is estimated from optimally-interpolated temperature data along a frequently repeated expendable bathythermograph (XBT) section between Fremantle, Australia and Sunda Strait, Indonesia and from two historical temperature-salinity (T/S) relationship products, CSIRO Atlas for Regional Seas (CARS) and Levitus (1982). The annual mean ITF geostrophic transport relative to 400 in during 1984-2001 is estimated to be 4.6 Sv using the CARS T/S relationship, which is about 20% higher than that found using the Levitus T/S relationship. This transport increment is due to the fact that the CARS T/S relationship, which incorporates more recent hydrographic data, better resolves the low-salinity signature of the ITF water. Isothermal averaging in the CARS T/S relationship may also improve representations of the water mass signatures in deep layers.

Determination of dynamic heights in the Bay of Bengal from XBT profiles and climatological salinities

Dynamic height (DH) computations are necessary for the estimation of geostrophic currents, heat transport studies, identification of eddies and to validate the altimeter sea surface height (SSH) observations. To compute DHs we need the temperature and salinity profiles to a reasonable deeper level. But, temperature profiles alone, such as from XBT (expendable bathy thermograph), are more numerous than combined temperature and salinity measurements from CTD (conductivity, temperature and depth). To use these voluminous temperature measurements from XBT observations in the computation of DHs, temperature/salinity relations were developed in many oceanic regions. Since such an exercise cannot be carried out in the Bay of Bengal due to the absence of adequate CTD measurements, we have proposed a method of utilizing XBT measurements and climatological (Levitus and Boyer, 1994) salinities to compute the DHs. The approach is demonstrated by replacing the CTD salinities with the climatological salinities. The relative errors compared to the DH signals are 3.8%, 2.7% and 2.6% for 200, 700 and 1000 dbar levels, respectively. The DHs relative to 700 dbar computed using the XBT temperature and climatological salinity profiles are compared with the SSH observations from TOPEX altimeter. The standard deviation of the difference between the DHs computed after converting to geometrical units and SSH is 7.3 cm.

Closing the Time-Varying Mass and Heat Budgets for Large Ocean Areas: The Tasman Box

Abstract The role of oceanic advection in seasonal-to-interannual balances of mass and heat is studied using a 12-yr time series of quarterly eddy-resolving expendable bathythermograph (XBT) surveys around the perimeter of a region the authors call the Tasman Box in the southwestern Pacific. The region contains the South Pacific’s subtropical western boundary current system and associated strong mesoscale variability. Mean geostrophic transport in the warm upper ocean (temperature greater than 12°C) is about 3.8 Sv (1 Sv ≡ 106 m3 s−1) southward into the box across the Brisbane, Australia–Fiji northern edge. Net outflows are 3.3 Sv eastward across the Auckland, New Zealand–Fiji edge, and 2.7 Sv southward across Sydney, Australia–Wellington, New Zealand. Mean Ekman convergence of 2.2 Sv closes the mass budget. Net water mass conversions in the upper ocean consist of inflow of waters averaging about 26°C and 35.4 psu balanced by outflow at about 18°C and 35.7 psu, and reflect the net evaporation and heat loss in the formation of South Pacific Subtropical Mode Water. The mean heat balance shows good agreement between ocean heat flux convergence (42.3 W m−2), heat loss to the atmosphere from the NCEP–NCAR reanalysis (39.2 W m−2), and heat storage calculated from data in the box interior (1.3 W m−2). On interannual time scales, volume transport through the box ranges from about 1 to 9 Sv, with heat flux convergence ranging from about 20 to 60 W m−2. An interannual balance in the heat budget of the warm layer is achieved to within about 10 W m−2 (or 6 W m−2 for the upper 100 m alone). Maxima in the advective heat flux convergence occurred in 1993, 1997, and 1999–2000, and corresponded to maxima in air–sea heat loss. The evolution of surface-layer temperature in the region is the residual of nearly equal and opposing effects of ocean heat flux convergence and air–sea exchange. Hence, ocean circulation is a key element in the interannual heat budget of the air–sea climate system in the western boundary current region.

Synoptic temperature structure of the East China and southeastern Japan/East Seas

Abstract Air-deployed expendable bathythermograph (AXBT) surveys are used to investigate the synoptic temperature structure of the East China Sea (ECS), southern Yellow Sea, and southeastern Japan/East Sea (JES) from 1992–1995, in the depth range 0–400 m. The main results of this study are as follows. The regional winter mixed layer is shallow along the paths of the Kuroshio and Polar Front due to strong advection of warm water, which suppresses vertical mixing. The region where mixing is to the bottom is generally northwest of the 100-m isobath in the ECS and in the Tsushima Strait. Two September surveys reveal two different Kuroshio Front positions: in 1992 this front was far on-shelf of its mean position, preventing the formation of eddies at Mien Hwa Canyon; in 1993, the Kuroshio Front had migrated off-shelf, and eddies along the front had formed. The Kuroshio Current Branch West of Kyushu is not seen in summer at 60 m depth, only winter and spring; however, there is evidence of this branch current in all seasons at 100 m depth. Along the entire length of the ECS in all surveys except winter (where wind mixing had homogenized the water column), cross sections show that cold water (

Comparison of Indonesian Throughflow transport observations, Makassar Strait to eastern Indian Ocean

The Pacific to Indian Ocean transport within the passageways of the Indonesian seas (ITF) varies on interannual and longer time scales associated with ENSO, the Asian monsoons and interannual climate variability of the Indian Ocean. Although direct current measurements of the ITF are of limited duration, none long enough to properly describe greater than annual variability of the ITF, observations indicate that the bulk of the ITF passes through Makassar Strait. The repeat expendable bathythermograph (XBT) IX1 section begun in 1983 provides the longest time series of the full ITF introduced into the Indian Ocean between northwest Australia and Java. We find that the surface to 600 dbar Makassar Strait transport, as measured by current meter moorings from December 1996 to July 1998, correlates at r = 0.77 ± 0.14 with the geostrophic transport constructed from IX1 XBT data for that time interval, with a 98 day lag.

A New Fall-Rate Equation for T-5 Expendable Bathythermograph (XBT) by TSK

The accuracy of the manufacturer’s fall-rate equation for the T-5 Model of expendable bathythermograph (XBT) has been investigated based on about 300 collocated pairs of XBT-CTD (Conductivity-Temperature-Depth profiler) measurements in various climatological regions. We found that the equation systematically overestimates depth by about 5% for the T-5 produced by Tsurumi Seiki, Co. Ltd. (TSK), but almost no bias is associated with the T-5 produced by Sippican, Inc., in USA. The cause of this difference is not clear, because the two manufacturers’ T-5 probes are reported to have identical shape and weight in water. We propose a new fall-rate equation for the TSK T-5: z(t) = 6.54071t - 0.0018691t 2, where z(t) is depth in meters at time, t, in seconds.