Diagnosis of vertical velocities with the QG omega equation: an examination of the errors due to sampling strategy

Vertical motion at the mesoscale plays a key role in ocean circulation, ocean-atmosphere interaction, and hence climate. It is not yet possible to make direct Eulerian measurements of vertical velocities less than 1000 m day⁻¹. However, by assuming quasi-geostrophic (QG) balance, vertical velocities O (10 m day⁻¹) can be diagnosed from the geostrophic velocity field and suitable boundary conditions. Significant errors in the accuracy of this diagnosis arise from the necessary compromise between spatial resolution and synopticity of a hydrographic survey. This problem has been addressed by sampling the output of a numerical ocean model to simulate typical oceanographic surveys of mesoscale fronts. The balance between the number of observations and the synopticity of observations affects the apparent flow and in particular the diagnosed vertical motion. A combination of effects can typically lead to errors of 85% in the estimation of net vertical heat flux. An analytical two-layer model is used to understand components of this error and indicate the key parameters for the design of mesoscale sampling.     

The Autonomous Ocean Profiler: a current-driven oceanographicsensor platform

The development and initial field test results of the Autonomous Ocean Profiler (AOP) are described. The profiler uses a hydrodynamic lift device to fly the instrument package up and down the water column along a taut vertical cable. Because the local currents drive the platform's vertical motion, power requirements are low, and therefore long, unattached deployments are possible. By using ARGOS or GOES satellite retrieval networks, the system can supply near-real-time data. The system provides profile data at very high vertical resolution in contrast to conventional buoys, which gather data only at fixed sensor depths. Because only a single set of sensors is required to cover the vertical range desired, the system is low cost and, for many applications, expendable. The initial deployment configuration is as an Arctic drifting buoy     

The 1996 IOC contaminant baseline survey in the Atlantic Ocean from 33°S to 10°N: introduction,sampling protocols,and hydrographic data

The third in a series of cruises designed to establish the present-day concentrations of trace elements and synthetic organic compounds in major water masses of the ocean, the 1996 Intergovernmental Oceanographic Commission Contaminant Baseline Survey occupied six vertical profile stations in the subtropical and tropical Atlantic. Underway surface samples also were acquired in the transects between these stations. This paper uses the temperature, salinity, oxygen, nutrient, and chlorophyll results from the cruise to set the hydrographic background for the other papers in this special volume. Major features sampled during the surface transect include the Brazil Current, the South Equatorial Current, and the offshore Amazon Plume. Utilizing the above parameters to identify water masses, we observed Antarctic Bottom Water (AABW) that ranged from a relatively undiluted form at 33°S (Station 10) to a highly attenuated form at 8°N (Station 6). Similarly, North Atlantic Deep Water (NADW) was obtained in various mixing stages along its flow path, and samples of NADW and AABW exchanging through the Romanche Fracture Zone to the eastern Atlantic basins were also taken. In addition to these deep water masses, representative samples of Antarctic Intermediate Water and Circumpolar Deep Water were acquired. Besides standard hydrography, these data also were used to verify the sampling integrity of the trace metal-clean, Go Flo bottles deployed on a Kevlar hydrographic cable.     

Absolute velocity along the AR7W section in the Labrador Sea

Nearly every spring since 1990, hydrographic data have been collected along a section in the Labrador Sea known as AR7W. Since 1995, lowered acoustic doppler current profiler (LADCP) data have also been collected. In this work we use data from six of these sections, spanning the time period 1995 through 2008, to determine absolute velocity across AR7W and analyze the main features of the general circulation in the area. We find that absolute velocity fields are characterized by strong, nearly barotropic flows all along the section, meaning there is no “level of no motion” for geostrophic velocity calculations. There is strong variability from year to year, especially in the strength of the boundary currents at each end; nevertheless, combining data from.all 6 sections yields a well-organized velocity field resembling that presented by Pickart and Spall (2007), except that our velocities tend to be stronger: there is a cyclonic boundary current system with offshore recirculations at both ends of the line; the interior is filled with virtually uniform, top-to-bottom bands of velocity with alternating signs. At the southwestern end of the section, the LADCP data reveal a dual core of the Labrador Current at times when horizontal resolution is adequate. At the northeastern end, the location of the recirculation offshore of the boundary current is bimodal, and hence the apparent width of the boundary current is bimodal as well. In the middle of the section, we have found a bottom current carrying overflow waters along the Northwest Atlantic Mid-Ocean Channel, suggesting one of various possible fast routes for those waters to reach the central Labrador Sea. We have used the hydrographic data to compute geostrophic velocities, referenced to the LADCP profiles, as well as to compute ocean heat transport across AR7W for four of our sections. For all but one year, these fluxes are comparable to the mean air–sea heat flux that occurs between AR7W and Davis Strait from December to May (O(50–80 TW)), and much larger than the annual average values (O(10–20 TW)).     

海道测量业务支持系统设计与实现

介绍了海道测量资料在国防和国民经济建设中的重要应用价值,详细分析了海道测量资料的信息服务现状与迫切的保障需求之间的差距,设计了海道测量业务服务系统的总体结构框架,提出并解决了多维多源海道测量资料的分类分级管理、海量数据采集建库、入库数据质量控制、数据编目与元数据发布、地图服务等核心技术问题,在此基础上研制了新一代海道测量业务支持系统。     

The annual cycle of vertical mixing and restratification in the Northern Gulf of Eilat/Aqaba (Red Sea) based on high temporal and vertical resolution observations

The stratification in the Northern Gulf of Eilat/Aqaba follows a well-known annual cycle of well-mixed conditions in winter, surface warming in spring and summer, maximum vertical temperature gradient in late summer, and erosion of stratification in fall. The strength and structure of the stratification influences the diverse coral reef ecosystem and also affects the strength of the semi-diurnal tidal currents. Long-term (13 months) moored thermistor data, combined with high temporal and vertical resolution density profiles in deep water, show that transitions from summer to fall and winter to spring/summer occur in unpredictable, pulses and are not slow and gradual, as previously deduced from monthly hydrographic measurements and numerical simulations forced by monthly climatologies. The cooling and deepening of the surface layer in fall is marked by a transition to large amplitude, semi-diurnal isotherm displacements in the stratified intermediate layer. Stratification is rebuilt in spring and summer by intermittent pulses of warm, buoyant water that can increase the upper 100–150 m by 2 °C that force surface waters down 100–150 m over a matter of days. The stratification also varies in response to short-lived eddies and diurnal motions during winter. Thus, the variability in the stratification exhibits strong depth and seasonal dependence and occurs over range of timescales: from tidal to seasonal. We show that monthly or weekly single-cast hydrographic data under-samples the variability of the stratification in the Gulf and we estimate the error associated with single-cast assessments of the stratification.     

On the Uncertainty of Archive Hydrographic Data Sets

As the international hydrographic community continues to address the question of irreducible uncertainty in modern surveys, a similar question must be asked of archived vertical beam echosounder (VBES) and leadline data sets. The Office of Naval Research funded STRATAFORM project surveyed an area of the New Jersey shelf around 39$^circ$12' N 72$^circ$50' W using an EM1000 multibeam echosounder (MBES). This area is also covered by National Oceanic and Atmospheric Administration surveys from 1936 to 1938 (from early visual indicating fathometers) and 1975–1976 (VBES). The analysis shows that the earlier data are biased in deeper water, most probably because of “hydrographic rounding” or instrument limitations, and may be unrecoverable, but that the VBES data appear approximately unbiased. Estimates of uncertainty for a surface model generated from the archive data are constructed, taking into account measurement, interpolation, and hydrographic uncertainty (addressing the problems of unobserved areas and surface reconstruction stability). Comparison of predicted depths against the MBES data shows that the VBES-derived surface is consistent given the quoted uncertainty and that the uncertainty corresponds with appropriate hydrographic survey standards. However, spatial aliasing of the VBES surface is observed, which may be the limiting factor in the applicability of this data.     

Uncertainty Reduction of Estimated Geostrophic Volume Transports with Altimeter Observations East of Taiwan

The common geostrophic estimation of ocean current velocity uses only water temperature and conductivity profiles. The geostrophic volume transport of a western boundary current, like the Taiwan Current (Kuroshio east of Taiwan), between the coast and its eastern boundary can be easily estimated based on hydrographic survey data. But the eastern boundary of the Taiwan Current is very uncertain due to extremely variable hydrographic conditions. This uncertainty is strongly correlated with the propagating mesoscale eddies originating from the interior of the western North Pacific Ocean. The uncertainty of estimated transport can be greatly reduced if eddy distribution is considered when determining the integration boundaries with the assistance of satellite altimeter measurements. Eight hydrographic surveys east of Taiwan between November 1992 and June 1996 are demonstrated in this study. The average geostrophic transport of the Taiwan Current with a reference set to 1000 dbar at 22°N between the east coast of Taiwan and 124°E is 22.9 ±14.2 Sv and changes to 22.1 ± 8.3 Sv, the uncertainty of which is nearly halved after taking account of the eddy distribution. The estimation uncertainty is insensitive to vertical displacements of the reference level within the depth range between 800 and 2000 dbar. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modelling hydrographic changes in the Labrador sea over the past five decades

Inter-annual to inter-decadal changes of hydrographic structure and circulation in the subpolar North Atlantic are studied using a coarse resolution ocean circulation model. The study covers 1949 through 2001, inclusive. A “time-mean state nudging” method is applied to assimilate the observed hydrographic climatology into the model. The method significantly reduces model biases in the long-term mean distribution of temperature and salinity, which commonly exist in coarse-resolution ocean models. By reducing the time-mean biases we also significantly improve the model’s representation of inter-annual to inter-decadal variations. In the central Labrador Sea, the model broadly reproduces the heat and salt variations of the Labrador Sea Water (LSW) as revealed by hydrographic observations. Model sensitivity experiments confirm that the low-frequency hydrographic changes in the central Labrador Sea are closely related to changes in the intensity and depth of deep convection. Changes in surface heat flux associated with the winter North Atlantic Oscillation (NAO) index play a major role in driving the changes in T–S and sea surface height (SSH). Changes in wind stress play a secondary role in driving these changes but are important in driving the changes in the depth-integrated circulation. The total changes in both SSH and depth-integrated circulation are almost a linear combination of the separate influences of variable buoyancy and momentum fluxes.     

Vertical Land Motion in the Mediterranean Sea from Altimetry and Tide Gauge Stations

We have computed estimates of the rate of vertical land motion in the Mediterranean Sea from differences of sea level heights measured by the TOPEX/Poseidon radar altimeter and by a set of tide gauge stations. The comparison of data at 16 tide gauges, using both hourly data from local datasets and monthly data from the PSMSL dataset, shows a general agreement, significant differences are found at only one location. Differences of near-simultaneous, monthly and deseasoned monthly sea level height time-series have been considered in order to reduce the error in the estimated linear-term. In a subset of 23 tide gauge stations the mean accuracy of the estimated vertical rates is 2.3 ± 0.8 mm/yr. Results for various stations are in agreement with estimates of vertical land motion from geodetic methods. A comparison with vertical motion estimated by GPS at four locations shows a mean difference of ?0.04 ± 1.8 mm/yr, however the length of the GPS time-series and the number of locations are too small to draw general conclusions.     

Vertical Land Motion in the Mediterranean Sea from Altimetry and Tide Gauge Stations

We have computed estimates of the rate of vertical land motion in the Mediterranean Sea from differences of sea level heights measured by the TOPEX/Poseidon radar altimeter and by a set of tide gauge stations. The comparison of data at 16 tide gauges, using both hourly data from local datasets and monthly data from the PSMSL dataset, shows a general agreement, significant differences are found at only one location. Differences of near-simultaneous, monthly and deseasoned monthly sea level height time-series have been considered in order to reduce the error in the estimated linear-term. In a subset of 23 tide gauge stations the mean accuracy of the estimated vertical rates is 2.3 ± 0.8 mm/yr. Results for various stations are in agreement with estimates of vertical land motion from geodetic methods. A comparison with vertical motion estimated by GPS at four locations shows a mean difference of -0.04 ± 1.8 mm/yr, however the length of the GPS time-series and the number of locations are too small to draw general conclusions.     

Vertical exaggeration of reflection seismic data in geoscience publications 2006-2010

Reflection seismic data is widely-used in industry and academia. It is often interpreted and displayed with considerable vertical exaggeration. Vertical exaggeration is defined as the vertical to horizontal aspect ratio of a reflection seismic cross section in depth. Recent literature is reviewed here to quantify for the first time the extent and typical values of vertical exaggeration in published work. Results are collated from 1437 papers published in 68 journals in the period 2006-2010. This broad scope is intended to provide a global view but it does not claim to be exhaustive. One example from each paper was analysed. Depth conversion was necessary in 74% of cases (every case of time-domain seismic with vertical exaggeration not stated). The main findings are that only 12% of papers use aspect ratios with vertical scale set approximately equal to horizontal scale (vertical exaggeration in the range 0.8-1.2). 75% of papers use reflection seismic with vertical exaggeration greater than 2. Splits of the data in terms of shallow high resolution seismic and deep, crustal imaging seismic were also obtained. The cause of this very widespread vertical exaggeration of reflection seismic cross sections is generally some form of display optimisation, such as emphasis of stratigraphic architectures or displaying long sections that would otherwise not fit on screen, or effects related to seismic sampling, but a specific reason is rarely stated. Angular relationships in stratigraphic and structural architectures are distorted so commonly that true geometries are rarely seen, which becomes an issue if readers are unfamiliar with these geometries. There is a clear opportunity for authors who display and publish reflection seismic data to annotate an estimate of vertical exaggeration alongside other standard annotation on seismic displays.     

Bottom boundary layer in the Black Sea: Experimental data,turbulent diffusion,and fluxes

Measurements of 14 vertical profiles of currents and hydrological parameters in the near-bottom layer with a depth resolution of 0.1 m were carried out in several regions of the Black Sea shelf, at five points over the continental slope, and in three deep water regions. The upper boundary of the benthic boundary layer (BBL) was reliably determined at a point at a distance from 5–7 to 35–40 m from the bottom where the gradients of the density and the velocity of the currents changed. The experimental data obtained were used to determine the coefficient of the bottom friction, the friction velocity, the coefficients of the vertical diffusion of momentum and density, and the vertical fluxes of temperature and salinity in the BBL.     

GPS测高技术在无验潮水深测量中的应用

应用双频GPS动态后处理高精度定位技术，建立了一套完整的GPS无验潮海洋深度测量作业模式，通过海上试验与传统作业模式作了数值分析比较，结果表明，该作业模式不仅无需验潮，而且能够有效消除传统作业模式中船只动态吃水和涌浪等因素对测量成果的影响，显著提高水深测量成果的精度。     

The effects of vertical sampling resolution on estimates of plankton biomass and rate calculations in stratified water columns

The effects of vertical sampling resolution on estimates of plankton biomass and grazing calculations were examined using data collected in two different areas with vertically stratified water columns. Data were collected from one site in the upwelling region off Oregon and from four sites in the Northern Gulf of Mexico, three within the Mississippi River plume and one in adjacent oceanic waters. Plankton were found to be concentrated in discrete layers with sharp vertical gradients at all the stations. Phytoplankton distributions were correlated with gradients in temperature and salinity, but microzooplankton and mesozooplankton distributions were not. Layers of zooplankton were sometimes collocated with layers of phytoplankton, but this was not always the case. Simulated calculations demonstrate that when averages are taken over the water column, or coarser scale vertical sampling resolution is used, biomass and mesozooplankton grazing and filtration rates can be greatly underestimated. This has important implications for understanding the ecological significance of discrete layers of plankton and for assessing rates of grazing and production in stratified water columns.     

Spatial and Temporal Variability in a Vertical Section Across the Alaskan Stream and Subarctic Current

In the central North Pacific Subarctic Gyre, CTD hydrographic measurements were carried out yearly in late June from 1990 to 1998 at 9 stations along 180° meridian from 48°N to 51.2°N. Vertical sections of 9-year means, anomalies for each year and others of potential temperature, salinity, potential density and geostrophic velocity (referred to 3000 m) were calculated based on this data set. Empirical Orthogonal Function (EOF) analysis was adopted in the investigation of spatial characteristics and its temporal variation in vertical sections. The spatial distribution of the 1st mode EOF of velocity shows the westward Alaskan Stream and the eastward Subarctic Current. This mode explains 37.6% of the total variance. Two positive maxims appear in its amplitude in 1991 and 1997, which is similar to the variation in volume transport of the eastward Subarctic Current. These variations are closely related to the vertical movement of Ridge Domain deep water.     

南黄海春季水温分布特征的分析

利用美国海军的空间分辩率为10′×10′月平均的GDEM三维水温资料,研究了南黄海春季水温的分布特征及其演变过程。分析结果较清晰地显示了春季南黄海的水温分布如何从冬季的垂直均匀型过渡到夏季的层化结构。分析还表明:春季南黄海水温的水平和垂直结构皆比冬季更为复杂,并出现若干个较特殊的水文现象,例如,在34°40′~36°20′N的南黄海西侧出现了“青岛冷水团”,而在35°30′~37°20′N的南黄海东侧,初次发现存在着一个类似性质的冷水团,称其为“仁川外海冷水团”。此外,在冷水团的邻近海域还存在着中层冷水。     

Model initialization in a tidally energetic regime: A dynamically adjusted objective analysis

A simple improvement to objective analysis of hydrographic data is proposed to eliminate spatial aliasing effects in tidally energetic regions. The proposed method consists of the evaluation of anomalies from observations with respect to circulation model fields. The procedure is run iteratively to achieve convergence. The method is applied in the Bay of Fundy and compared with traditional objective analysis procedures and dynamically adjusted climatological fields. The hydrographic skill (difference between observed and model temperature and salinity) of the dynamically adjusted objective analysis is significantly improved by reducing bias and correcting the vertical structure. Representation of the observed velocities is also improved. The resulting flow is consistent with the known circulation in the Bay.     

Intercomparison of High-Resolution Bay of Bengal Circulation Models Forced with Different Winds

The high-resolution Bay of Bengal circulation modeling in the region [80E–95E; 5N–22N] is performed with a horizontal resolution of 10 km and the highest vertical resolution of 5 m near the surface. The intercomparison experiments, with ocean model forced with the near-surface (1) National Centers for Environmental Prediction (NCEP) reanalysis winds and (2) blended seawinds data (a combination of remotely sensed scatterometer and in situ observations) are carried out for a period of 17 years during 1998–2014. The seasonal variability of the realistically simulated surface hydrographic (temperature and salinity) and circulation (currents) variables from both the experiments is compared and contrasted with the observational data. The mixed layer depth seasonal variability of the region is also studied. The mesoscale features of currents at 50 and 100 m are also studied. The volume transport across different sections in the Bay of Bengal is computed and its relation with summer monsoon rainfall is investigated. The results suggest that there is no real advantage of using high-resolution blended seawinds over the much coarser NCEP reanalysis winds.     

Running GCM physics and dynamics on different grids: algorithm and tests

An 'alternate grid' (GridAlt) technique is presented, which allows the different components of a general circulation model's governing equations to be computed on distinct grids chosen for that component or process. In the implementation presented here, the tendencies of state variables from the physical parametrizations are computed on a vertical grid with very fine resolution near the surface, whereas the remaining terms in the equations of motion are computed using an Eta coordinate with coarser vertical resolution.
Results from a suite of aquaplanet experiments show that much of the benefit of increased vertical resolution in the whole model can be realized by enhancing the vertical resolution of the 'physics grid' using GridAlt. The benefit is realized in the fields which are computed directly in the physical parametrizations, and in the vertical structure of the relative humidity and mass streamfunction. Results from a suite of realistically configured simulations demonstrated an impact of GridAlt that was similar to its impact in the simplified simulations, as well as an improved response to El Niño Southern Oscillation forcing. It is concluded that the present implementation of GridAlt offers a practical way to allow GCMs to better capture the near-surface structure of the atmosphere.