Role in Climate Science
Within the next several years, a monitoring system using Argo floats will be completed, and approximately 3000 Argo floats will provide temperature-salinity profiles from the sea surface to the depth of 2000m in the worlds oceans every 10 days. We are to obtain numerous hydrographic data. The Argo project will provide one hundred thousand profiles a year, and the impact of the project on the academic field and the resultant societal field will be tremendous.
It goes without saying that the Argo project is a most
important tool in observing the ocean.
In my presentation, the
expectations of the Argo project in the CLIVAR sciences will be
discussed, based on our recent studies.
Argo and GODAE
The Global Ocean Data Assimilation Experiment (GODAE) is conducting its main demonstration phase from 2003 to 2005. From 2003 to 2005, operational and research institutions from Europe, Australia, Japan and the United States will be performing global ocean data assimilation and ocean forecast in order to provide regular and comprehensive descriptions of ocean fields such as temperature, salinity and currents at high temporal and spatial resolution.
GODAE will provide an integrated description of the ocean that combines remote sensing data, in-situ data and models through data assimilation. One of the objectives of GODAE is thus to maximize the benefits from the data (in particular altimetry and Argo). To achieve these goals, it is necessary to develop effective assimilation techniques of Argo data. A central issue is to effectively merge Argo data with remote sensing data for the description of the global ocean at high space and time resolution.
A review of GODAE progress will be first given. We will then focus the presentation on the use of Argo data by GODAE and the contribution of Argo data to the experiment. Important issues and perspectives for the coming years will be finally given.
Argo Float Technology:
Achievements and Challenges
The predecesors of the profiling floats employed in Argo were developed for use in the World Ocean Circulation Experiment (WOCE) in the early 1990s. The floats in WOCE were used mainly for making direct estimates of the deep velocity field of the world ocean. The Argo floats of today are equipped with CTD sensors that provide data of a quality comparable to that of shipboard observations, and these CTD profiles are probably the most important part of the data provided by the floats; clearly profiling float technology has evolved a great deal since WOCE in the 1990s. This ability to collect high quality CTD data from unattended platforms over the course of several years is a major technological achievement that is required by Argo and in a sense makes the program viable. As originally planned, the Argo program consists of 3000 floats that collect CTD profiles at regular intervals over the course of 4-5 years. Present float technology is adequate for realizing these goals: floats can be deployed nearly anywhere in the world ocean by a variety of methods, and float reliability has improved greatly in recent years. At the present time, new communication systems and biological, acoustical, and chemical sensors are being tested that will likely become operational in the next few years. These developments are likely to revolutionize our view of the world ocean and contribute directly to improved ocean models and forecasts.
The Argo data system
The Argo Data System is built around the individual processing systems of Argo Project contributors and on a global data server, mirrored at two sites, from which the master versions of the data are available. In order for this highly distributed system to work, it is necessary to standardize many aspects. In particular standard quality control procedures have been developed for the real-time data, and are being finalized for data processed in delayed mode. A standard exchange format allows data to be uploaded to the global servers and for users to visit and download data of interest. The global servers permit both ftp and www access and have tools to allow subsetting and some data display capabilities.
Other components of the data system include the Argo Information Centre, a global data repository for long term archiving, and regional centres. Not all of these are in place as yet. This presentation will explain the data system with a focus on how to get data, how to use them, and how to know what has been done to them.
Argo delayed-mode salinity data
One of the main ocean parameters measured by Argo
is salinity. However, salinity measurements from profiling floats
often experience artificial drifts and jumps that are
Design Requirements for an ARGO Float Array in the Indian Ocean Inferred from Observing System Simulation Experiments
Andreas Schiller, Susan E. Wijffels and Gary A. Meyers
CSIRO Marine Research
GPO Box 1538
Model experiments with an OGCM have been performed to assess sampling strategies for ARGO floats in the Indian Ocean on intraseasonal-to-seasonal time scales. The results suggest that a minimum requirement for resolving intraseasonal oscillations in the upper ocean is predominantly determined by their spatial scales, i.e. about 500km in the zonal and about 100km in the (equatorial) meridional direction.
Frequent temporal sampling becomes particularly important in dynamically active areas such as the western boundary current regime and the equatorial waveguide. High frequency sampling is required in these areas to maintain an acceptable signal-to-noise ratio, suggesting a minimum sampling interval of 6 days for capturing intraseasonal oscillations in the upper Indian Ocean.
Sampling of seasonal-and-longer-term variability down to 2000m depth is less critical within the range of sampling options of Argo floats, as signal-to-noise ratios for sampling intervals up to about 20 days are almost always larger than one. However, these results are based on a single OGCM and are subject to model characteristics and errors. Based on a coordinated effort, results from various models could provide more robust estimates by minimizing the impact of individual model errors on sampling strategies.
Experience of delayed mode quality control for float salinity data in the Atlantic
Yves Desaubies, Carole Grit, Y. Izenic
U. Send, L. Boehme
As part of the GyroScope project, some 84 floats were deployed in the Northern and North East Atlantic, from summer 2001 to autumn 2002. With a view to compare instrumental performances, two types of floats were used, in approximately equal numbers : APEX with Seabird sensors, and Provor with FSI sensors. The deployment area include such diverse ocean provinces as the Irminger and Labrador Seas in the subpolar gyre; the inter-gyre region of the NE Atlantic; the North Atlantic and Azores currents; and the Canaries basin in the Subtropical gyre. Correspondingly, a variety of water masses and fronts are encountered.
In an effort to perform Delayed Mode QC (DM-QC), the algorithm of Wong et al. (2003), in its latest release, has been implemented and tested on the float data. On average, the floats have about 50 profiles. Calibration of the algorithm on CTD indicated that it performs well, with error bars of the order of 0.01 psu.
The WOD 2001 has been augmented with recent CTD data, leading to reduced variance in the estimates.
Because of high space and time variability in the subpolar gyre, the parameters in the method have been adapted to account for fronts and anisotropic co-variances, with the result of reduced error bars in the correction estimates.
Float to float comparisons have been introduced, as a complementary indication of whether departures from climatology are real or due to instrumental error.
Preliminary results indicate that it will be difficult to implement automatic DM-QC procedures; in general the floats are reasonably stable (to within 0.04 psu), with no evidence of linear drift. There are several instances where the climatological mapping errors underestimate the variability, and therefore over-constrain the method. Complementary methods (see poster by Gaillard et al), which systematically use a wider and more recent data set, will be useful in building up progressively updated climatologies, and therefore reducing uncertainty in the corrections.
Partially funded under the 5th Framework Programme, "Global change, climate and biodiversity " ; contract EVK2-CT-2000-00087
What observation scheme should we use for profiling floats
to achieve the Argo goal on the accuracy of salinity measurement?
To attain the goal of the salinity measurement accuracy of the Argo Programme (±0.01 psu) the preferable measurement scheme for profiling float observations is examined from the view of the delayed-mode quality control. In the subtropical and subarctic regions of the North Pacific the goal will be generally achieved by measuring profiles constantly to the depth of 1500 dbar or more. In the tropical Pacific and the eastern Indian Ocean, the 1000 dbar profiles give sufficient quality of salinity data. In Mixed Water Region, however, it is difficult to reduce the correction errors within ±0.01 psu even if the profiling floats measure the profiles down to 2000 dbar constantly.
About a new observation scheme developed by Prof. Riser at University of Washington, we can find almost no reasons to positively adopt it for the profiling floats, at least from the view of the delayed-mode quality control of the Argo Programme.
Float to float drift intercomparison in an intermediate current regime
An improved series of underwater drift vectors has been estimated from a 15 days-cycling APEX float in the presence of an intermediate current regime near the Mid-Atlantic Ridge. In a statistical approach these estimates are compared with the velocities inferred from a park ensemble of four eddy-resolving RAFOS floats. They were deployed at the same location and ballasted for the same mission depth. We develop an extrapolation scheme to correct the surface drift information of the cycling float by data transmission parameters intrinsic to the satellite tracking ARGOS system.
A tendency towards a slightly (3%) higher in situ APEX speed is notable. The initial float separation between the cycling float and a simultaneously deployed RAFOS float has also been studied in terms of spatial and time scales of the velocity field. Additional displacement errors induced by geostrophic shear and wind forced currents are found to express only a second order source of inaccuracy. Estimated underwater velocities from the cycling float are in good agreement with the synoptic velocity field deduced from the RAFOS ensemble. Though the application of these results in comparable cases without a mean current field may be limited, such oceanic observations support the exploitation of the Lagrangian nature of the cycling floats with an improved statistical confidence.
A comparison of profiling float and XBT representations of upper layer temperature structure in the Northwestern subtropical North Atlantic
Robert L. Molinari
National Oceanic and Atmospheric Administration
Atlantic Oceanographic and Meteorological Laboratory
4301 Rickenbacker Causeway,
Profiling floats were deployed in the subtropical and subpolar North Atlantic as a component of the Atlantic Climate and Circulation Experiment (ACCE), the last field phase of the World Ocean Circulation Experiment. The floats were primarily instrumented with temperature sensors and ballasted to drift at 1000m for 10 days. ACCE floats were active from 1998 to 2001.
Previous studies have identified a shallow recirculation gyre located above the total water column inertial recirculation gyre observed south of the Gulf Stream. The shallow gyre has a seasonal cycle that has been characterized using satellite and expendable bathythermograph data. Maximum gyre size is observed in the fall and minimum size in the spring. Mapping the float data with bi-monthly resolution reproduces this annual cycle. Similarly, a decadal signal appears in the size of the gyre. The floats reproduce the changes in gyre size associated with this signal over the period of concurrent data.
The impact of Argo data on ocean and climate forecasting
Exeter, EX1 3PB
A number of operational ocean forecasting systems,
including both seasonal coupled forecast systems and higher
resolution short-range ocean forecast systems, have been assimilating
Argo data since it became routinely available. Studies to assess the
direct impact of different data types have been carried out. These
have withheld Argo temperature and salinity data and/or altimeter
data from the assimilation. The results of these studies will be
presented and discussed.
Ocean state estimation by 4D-VAR data assimilation using Argo Data
Frontier Research System for Global Change
3173-25 Showa-cho, Kanazawa-ku
Yokohama, 236-0001, Japan
Data assimilation approaches have recently focused on obtaining an optimal synthesis of observational data and model results for better descriptions of the ocean state. By using an ocean general circulation model and the 4D-VAR adjoint method, we have constructed a global ocean data assimilation system, which provides a comprehensive 4-dimensional reanalysis dataset based on available observational data.
Firstly, a data assimilation experiment is performed to define a mean seasonal state of the global ocean. The assimilated elements in this experiment are the climatological monthly temperature and salinity fields from WOD98 and sea surface height anomaly data derived from TOPEX/Poseidon altimetry. The obtained reanalysis dataset shows good consistency with previous knowledge of the ocean state and surface conditions. For example, the North Pacific Intermediate Water, which is characterized by a subsurface salinity minimum, is successfully reproduced. However, some important features in subsurface layers are missed due to the absence of the corresponding structures in WOD98 (e.g., dichothermal and mesothermal structures in the eastern subarctic region). This result shows that higher quality subsurface data is required for further improvement of the ocean state estimation by the state-of-the-art data assimilation system.
The ARGO data is most promising in this aspect because it can provide higher quality subsurface information covering the entire ocean. Using this advantage of the ARGO data, our system is currently applied to a long-term experiment for the last decade with an aim to investigate the impact of the ARGO data on the ocean state estimation.
A regional model assimilating ARGO, XBT, and altimeter data in the central North Pacific
Bruce Cornuelle, Josh Willis, Dean Roemmich
Scripps Institution of Oceanography,
University of California San Diego
9500 Gilman Drive
In the central ocean basins, far from the boundaries and from steep topography, quasi-geostrophic (QG) dynamics are thought to hold. Chelton and other authors have examined the westward propagation of sea surface height anomalies and have seen a good match with QG theory. In this work, we attempt to fit an eddy-resolving, regional, nonlinear quasi-geostrophic model to altimeter and in-situ observations, including ARGO profiles.
The quasi-geostrophic relations reduce the unknowns to a single field, the streamfunction, from which all other variables are derived by assuming geostrophic balance and adiabatic motion. This is more efficient than estimating temperature, salinity, density, and velocity separately and applying diagnostic constraints (adiabaticity, geostrophy and non-divergence) explicitly. The streamfunction expansion in vertical eigenmodes works best in a region with weak horizontal variation in the background ocean and in the topography. For this application, the model used the barotropic and the first two baroclinic modes, with the horizontal structure of each mode specified by a Fourier expansion periodic in each direction.
The model produces a dynamical synthesis of the multiple data streams that has greater skill in tracking the eddies than statistically based mapping schemes, and we hope to demonstrate model skill in forecasting and in cross-validation. It is anticipated that Argo data will be critical in providing the information on the baroclinic structure of the eddies that will allow the model to converge to the true field. This will be illustrated using twin experiments and fits to observations. While the QG model is not appropriate for global assimilation, it is a powerful tool for regional studies and for understanding the impact of data in assimilations.New elements of the Japan Sea oceanography found with the use of profiling floats.
Assimilating S(T) from Argo
Assimilating salinity data, along with temperature, into ocean models should allow for the better analysis of water properties and volumes over whole regions of the ocean. This would allow for better assessment of climatically important changes in water characteristics, that are normally only possible from repeat section hydrography.
The paper will present preliminary results from assimilating Argo data into the ocean component of the ECMWF seasonal forecast model. A two-stage process is used. (1) Temperature (from Argo and other data sources, XBT, TAO ...) is assimilated and the salinity is incremented to retain the S(T) relationship present in the model. This has previously been shown to greatly improve the salinity reproduction in the ECMWF model compared with leaving the salinity field unchanged. (2) Then the S(T) (from Argo) is assimilated directly, using a covariance function which is dependent on both horizontal separation and separation in temperature space. The method allows for larger spatial scales to be used, thus allowing a wider influence of the salinity data as a measurement of S(T). Some illustrations of this scale difference from using a conventional covariance function for S will be shown.
Ongoing plans are to produce a 40 year reanalysis of ocean properties and circulation, from which changes in S(T) properties and volumes of water masses can be diagnosed, where and when sufficient data are available.
Argo impact experiment with a JMA ENSO forecast model,
Climate Prediction Division,
Japan Meteorological Agency
1-3-4, Otecmachi, Chiyodaku,
The operational global ocean data assimilation system and ENSO forecast model at Japan Meteorological Agency (JMA) was revised in 2003. In a new ocean data assimilation system, salinity observations and altimeter data were assimilated in addition to temperature. A 3D variational analysis scheme and an incremental update scheme were introduced. A coupled general circulation model was replaced to an advanced model with higher resolution. These changes, particularly inclusion of salinity, may be favorable for effective use of Argo data for ENSO forecast.
Several forecast experiments initialized with and without Argo data were made in the period of 2001-2002. The results are compared and the practical impact of Argo data on ENSO forecast is discussed.
Argos role in closing the oceanic heat and freshwater budgets
Dean Roemmich, Josh Willis, and John Gilson
Scripps Institution of Oceanography
University of California San Diego
La Jolla CA92093-0230
A primary objective of global ocean observations is to close the mass, heat, and freshwater budgets of the oceans - regionally and globally on seasonal to decadal time-scales. Three illustrative calculations are presented to demonstrate the central role of Argo in this endeavor, as well as Argos integrative value with other observing system elements.
First it is shown that on seasonal time-scales, where a close balance is known to exist between ocean heat storage and air-sea flux, there is already sufficient Argo data to provide a quantitative global comparison of ocean heat storage with operational air-sea fluxes. The agreement between zonally averaged storage, calculated from successive cycles of Argo floats, and air-sea flux, is surprisingly good. Second, a new technique for combining subsurface data with satellite altimetry and SST (Willis et al, JGR 2003, In press) is applied to estimate global ocean heat content and storage for 1993 - 2002. Heat gain by the upper ocean, 0-800 m, averaged 1.0 +/- 0.3 W per meter-squared of ocean surface, accounting for about 65% of sea level rise during the period, through thermal expansion.
The 1997 El Nino produced the strongest feature in global heat gain, but the maximum in heat content occurred in late 1999. In a zonally integrated sense the greatest heat gain occurred at 40-degrees S (5 W/m**2), but there was also strong warming in the northern North Atlantic and the western tropical Pacific. The calculation illustrates both the power of combining Argo and altimetry and the further advantage that will be gained when the Argo array is fully deployed.
Finally, a closed regional heat budget is described using a decade of data in the southwestern Pacific, where broadscale temperature profiling has been comparable in coverage to that planned for Argo. This region includes the East Australian Current and is bounded by High Resolution XBT/XCTD transects that are used to calculate the time-varying circulation across the faces of the region and the advective heat flux convergence. Over the 12-year period of the study, mean advective heat flux convergence averaged over the region was 43 W/m**2, of which 3 W/m**2 was stored and 39 W/m**2 lost to air-sea exchange. The interannual heat budget was closed with residuals of 10 W/m**2 and interannual variability of about 30 W/m**2 in all three components (storage, air-sea flux, and advection). Argos contribution is in making similar heat budget calculations possible on a global basis, enabling the analogous freshwater budgets, and substantially decreasing the errors in ocean heat flux convergence and storage (to 0.1 W/m**2 for the global decadal storage calculation).
World Ocean heat content, 1955-1999
Sydney Levitus, John Antonov, Tim
We update our earlier estimates (Levitus et al., 2000) of ocean heat content for the world ocean with the additional 1.7 million temperature profiles that have become available as part of "World Ocean Database 2001". The results are quantitatively similar through the early 1990s but the newer estimates are lower for the 1990s due to an error in processing the XBT data for this period in our earlier results. This difference does not affect any previous conclusions regarding the dominance of ocean heat content in the earth's heat balance for this period. The results of multivariate analyses of ocean heat content are presented.
Upper ocean response before and after the cyclone from Argo floats
Muthalagu Ravichandran, G. Anita and Uday Bhaskar
Indian National Center for Ocean Information Services (INCOIS)
No.3, Nandagiri Hills, Jubilee Hills, Hyderabad 500 033
Significant progress has been made over the
past to improve our understanding of the basic oceanic and
atmospheric processes that occur during the passage of tropical
cyclone. Tropical cyclone passage over an ocean represents one of the
more extremeupper ocean response cases as the ocean loses heat to the
atmosphere (Shay 1994). Precipitation effects on the ocean mixed
layer during tropical cyclone passage has generally remained
unresolved due to lack of upper ocean temperature and salinity
profile measurements during and subsequent to storm passage. In order
to understand upper ocean response before and after cyclone, Argo
float is an additional observational capability, that will provide
temperature and salinity profile upto 2000 m in a near real-time. In
this paper, we have studied the behaviour of upper ocean responses
before and after the cyclone using the Argo floats in the Indian
ocean region. We have selected the floats/cyclones such a way that
Argo float data available on both side of the track. We have analyzed
how the difference profiles (pre-post) in the upper 200m, upper ocean
heat content, difference in the depth of the 26°C and 20°C
isotherm (cyclone heat potential), mixed layer depth varies in this
region before and after the passage of storm on both side of the
cyclone track. Also, a float in the Arabian Sea has provided
temperature and salinity profile everyday and has given the
opportunity to see how the upper ocean responded everyday before and
after the cyclone. It has been observed that the themocline just
below the mixed layer shows a well defined inertial oscillation of
about 12 m amplitude after the storm.
The decreasing of sea surface salinity by typhoon in the summer of 2002 and 2003 in the northwestern Pacific
Key Laboratory of Ocean Dynamic Processes and Satellite Oceanography
Second Institute of Oceanography, SOA,
9# Xi Xihexia Road
The tropical storms in Atlantic can induce increased SSS and decreased sea surface temperature (SST), which is evident from the 100 ARGO floats results (Kwon and Riser, 2002, in press). The decreasing of SST is because the losing of heat from ocean to the atmosphere and the intensified mixing with the cold water below the thermocline for the strong wind. The increased mixing by the fierce wind of storm also intenses the mixing in the mixed-layer and deepen the mixed-layer, which results in increased SSS for the mixing of SSS with the Subsurface Maximum Salinity Layer.
But the heavy rain during the Typhoon passage can decrease the SSS. We use the data of Argo in northwestern Pacific to study the effect of Typhoon Chataan (July 01～10, 2002), Halong (July 07～15, 2002) passed over Northwest Pacific. The salinity profile of Argo floats of ID: 5900173, 5900175 and 2900184 all show decreased SSS. Only float ID 5900126 shows increased SSS. In 2003, floats 5900222, 5900223 and 5900220 all show the decreased SSS phenomena. The decreasing of SSS and the salinity in the mixed-layer (about 100m thick in this area) after the pass of Typhoons is very prominent in most cases. The role of rain of typhoon and the mixing processes of mixed-layer with the thermocline on the change of SSS need further study. The reason for the different response of SSS of Atlantic and northwestern Pacific need further study.
Lagrangian observations of the mode water formation in the North Pacific
Lagrangian observations of the formation and modification processes of North Pacific Central Mode Water (NPCMW) have been pursued by means of profiling floats programmed to drift at a specific isopycnal surface rather than an isobaric surface since early summer in 2002. One of these floats captured the mixed layer around 37°N, 165°E, which was as deep as 100 m in November 2002 and deepened successively through March 2003, ending up as a perfectly uniform layer of 26.02sigma-theta thicker than 330m. Its temperature, 12.82-12.84°C, and salinity, 34.48psu correspond to those of the lightest variety of NPCMW identified in the isopycnally averaged climatology, North Pacific HydroBase.
The formation of such thick NPCMW had not been directly observed before. The float had been drifting at the 26.5sigma-theta surface lying at 500-550dbar within the maximum deviation of +/-0.1sigma-theta. It had remained at 36.7-38°N, 164.2-165.8°E and the total distance of its excursion was about 200km. The heat loss from the top 300m of the water column was 700-800W/m2 on average during the 4 months from November 2002, which implies the net surface heat release of 500-700W/m2, assuming the heat advection by the Ekman flow 100-200W/m2 and that by the geostrophic flow negligible. This value is much larger than the climatological monthly mean net surface heat flux, ranging from 200 to 300W/m2 in this region for the winter months, estimated based on the surface marine data.
The detection of subsurface theta-S changes
The quality of salinity data from Argo floats has been even better than anticipated when the program was conceived. Careful consideration of available reference profiles, for example CTDs taken on deployment cruises, indicate that many floats are providing real-time salinity data with absolute accuracy of O(0.01), which was originally thought of as a target accuracy for carefully adjusted delayed mode data. This initial accuracy, combined with good stability for selected floats, has made it possible to observe subsurface salinity changes as small as 0.04, with a high degree of confidence, and without needing long time series.
The abstracts for this conference report observations of property changes at various depths and in all the oceans. In March 2002 there was a UK research cruise from Durban to Freemantle along 32°S in the Indian Ocean. CTD data from the cruise revealed changes in properties since 1987, between the surface and the Intermediate Water: generally there was an increase in salinity in the upper thermocline and a freshening in the lower thermocline. A set of 25 APEX floats deployed on the cruise confirm that the Mode Water properties are stable (at the 0.01 level) over large horizontal scales (meridional and zonal) and over a 12-month timescale. Therefore the inferred changes are not due to aliased sampling of spatial gradients, nor are they due to seasonal or interannual variability.
We can therefore be very optimistic that with careful selection and calibration of profiles in regional analyses, data from the Argo array can be used to interpolate between research cruises and so provide unprecedented temporal and spatial descriptions of subsurface property variability and trends.
Observations of N. Atlantic subtropical mode water using profiling floats
Young-Oh Kwon and Stephen C. Riser
Basic characteristics of the N. Atlantic Subtropical Mode Water, also called Eighteen Degree Water (EDW), were investigated using temperature observations from profiling floats during January, 1998 - December, 2001. The data allowed a detailed analysis with monthly time resolution and full spatial coverage of the EDW region throughout the study period.
The EDW showed a maximum volume in winter (4.68 ± 0.53 x 10**14 m**3), followed by a gradual destruction throughout the rest of the year with an annual destruction rate of 2.24 ± 0.61 x 10**14 m**3. Experiments using a simple one dimensional diffusion model suggest an enhanced vertical mixing corresponding to a vertical diffusivity of 10**-4 m**2/s is needed to explain the observed annual destruction rate. Salt fingering at the bottom of the EDW seems to be the primary factor responsible for the elevated mixing.
Production of the EDW was observed during the experimental period, as patches of the deep winter mixed layer that were concentrated along the southern flank of the Gulf Stream between 45 and 65°W. The production was most active in March, and the temperature of newly produced EDW showed a zonal gradient with the warmest water on the western end. Interannual variability of the production was well-correlated with the total volume of the EDW, which also showed good correlation with the North Atlantic Oscillation (NAO) index.
Formation region of North Pacific Subtropical Mode Water in the late winter of 2003
The structure and properties of the mixed layer in the formation region of the North Pacific Subtropical Mode Water (STMW) east of Japan are presented for the late winter of 2003, using the temperature and salinity data mainly from Argo profiling floats. The formation region extends south of the Kuroshio Extension between 30°N and 35°N as far east as 175°E, characterized by the mixed layer deeper than 200db, the meridionally uniform temperature between 16.5°C and 18.2°C, and the associated mixed layer front at the southern boundary. The mixed layer in the formation region becomes colder, fresher, denser, and shallower toward the east with the significant mesoscale variations, leading to the formation of warmer, saltier, lighter, and thicker STMW in the farther western part of the region. The spatial variation of the mixed layer depth in the formation region depends largely on that of depth of the underlying permanent thermocline.
Winter conditions in the Irminger Sea revealed by Argo floats
Luca Centurioni and W John Gould
Scripps Institution of Oceanography
9500 Gilman Drive
La Jolla, CA 92093-0213
Argo float profiles have been used to reconstruct the evolution of the wintertime circulation, stratification and water mass properties from 1996 - 2003 in the Irninger Sea, SE of Greenland. Ship board CTDs were used to calibrate the year-round Argo float salinity data, resulting in a typical accuracy of about 0.02 .
The data (900 float profiles) reveal the small area in which stratification (in February and March) becomes weakest and a secondary elongated (parallel to the bottom topography) region in which stratification also becomes weak.
There has been much speculation about whether the Irninger Sea could be a site for deep winter convection. While there have been many CTD surveys of the area in recent years, virtually all have been in the spring, summer and autumn months. In the region of densest mixed layers no profiles show mixed conditions from the surface to depths greater than 500m. The heat content of the water column show that deep convection, for the period examined, is not likely to have occured.
The large seasonal and interannual variability at all depths in this region requires the continuing collection of ship based CTDs. Even if these data are collected in summer they can be used to calibrate floats so as to explore variability throughout each year and between years.
Structure and Variability of the Indian Ocean as revealed from ARGO observations
email@example.com / firstname.lastname@example.org
Lack of continuous subsurface measurements in the past, has been a major limitation in documenting the structure and variability of the Indian Ocean. The recently deployed ARGO float network in the Indian Ocean has provided a unique and unprecedented opportunity to study the large-scale dynamical processes contributing to the upper ocean variability in this region.
By synthesizing ARGO measurements along with XBT observations (from cruises) for the period (Jan 2002 - Sep 2003) and the WOA2001 dataset, we have carried out a detailed analysis of the spatio-temporal evolution of subsurface temperature and Sea Surface Temperature (SST) in the Indian Ocean. In addition, other datasets such as Sea Surface Height anomalies from TOPEX/POSEIDON; surface winds from QUICKSAT; SST from TMI; Rainfall from CMAP dataset and Outgoing Longwave Radiation from NOAA satellite - are used to understand the interactions between the upper ocean and the overlying atmosphere. The seasonal cycle of SST and subsurface temperatures clearly shows the semiannual component of the Indian Ocean response to seasonal reversal of the monsoon winds.
In addition, prominent intra-seasonal signals are noted in the oceanic temperature variations. Furthermore, the observations clearly reveal that significant changes both in the SST and subsurface temperature are associated with the inter-annual variability of the Southwest Monsoon circulation. The findings from the present observational analysis will be presented in detail.
A preliminary study of mesoscale eddy effects on the formation of the North Pacific Subtropical Mode Water
Pan Aijun and Liu Qinyu
We provide an elegant view on the formation process of the North Pacific Subtropical Mode Water (NPSTMW) in 2001 using newly observed temperature and salinity data from Argo profiling floats in the Kuroshio recirculation region.
Vertical homogeneous water mass characterized by Potential Vorticity (PV) lower than 1.5-10m-1s-1 is formed south of the Kuroshio with temperature about 16-18°C and potential density about 25.1-25.4 to depth from about 150m to 380m. The role of mesoscale eddies on the formation of NPSTMW are examined. It is shown that by strengthening the vertical mixing process and thus the ventilation process of isotherms of the permanent thermocline, anticyclonic (warm) eddies help to the formation of thicker NPSTMW than cyclonic (cold) ones.
Potential of Argo data for detection and attribution of oceanic climate change
Richard A. Wood,
Helene T. Banks, Michael Vellinga and Sheila Stark
Argo was conceived as a long-term, sustained observing system for the ocean. One of the benefits of such a system will be the provision of long, quasi-homogeneous timeseries of hydrographic data, with near-global coverage, which will fill the huge gaps in our current knowledge of low frequncy variability and trends in the ocean. But how will those timeseries be used to advance our understanding of the climate system, and feed into decision-making on global change?
We present results from three studies aimed at
identifying what types of statement may - and may not - be possible
in future based on 10 to 50 years of Argo data. The studies focus
Our approach is to use simulations using the HadCM3 climate model to generate the type of synthetic 'observations' that are likely to be gathered by Argo over the coming years. While the model almost certainly does not capture all the behaviour of the real world, and surprises are inevitable, our results demonstrate the potential of the developing Argo dataset to answer some fundamental questions in climate science.
Salinity change in the southeastern Indian Ocean
Helen Phillips, Susan Wijffels and Ming Feng
Nearly 4 years of Argo float data now exists in the Indonesian Throughflow Region of the southeast Indian Ocean (90°E-130°E, 25°S-5°S). These data are compared with the high resolution Australian-region hydrographic climatology CARS (CSIRO Atlas of Regional Seas).
Salinity during the period 1999-2003 is markedly different than the climatology. Waters above 200 dbar, where potential temperature is warmer than about 18°C, are consistently fresher over the entire region by approximately 0.2 psu. The freshening is greatest south of 15°S, south of the South Equatorial Current (SEC); there is little zonal variation in the extent of the freshening. Between 200 and 400 dbar ( theta~ 10-18°C), float salinities are close to climatology except between 14°S and 18°S where they are saltier than climatology by up to 0.1 psu, possibly due to variation in the location of the SEC. Below 400 dbar, ( theta< 10°C) there is weak freshening of approximately 0.03 psu. The surface freshening covers the entire period of the float record. During early 1998 to early 2001, the year preceding and two years during the float record, the Southern Oscillation Index was persistently positive, and rainfall in the region was persistently higher than average, suggesting one possible reason for the surface freshening.
Argo as an Aid to Environmental Monitoring and Assessment - the example of the Gulf of Alaska.
Howard Freeland and Steve MacDonald
Department of Fisheries and Oceans
Institute of Ocean Sciences
The spring of 2002 saw an unusual climate anomaly
develop within the waters of the Gulf of Alaska. This anomaly was
characterised by rapid warming at the surface and an injection of a
large T/S anomaly between 80m to 150m depth. Much has already been
written about this anomaly, its development and origin, and some
descriptions of its impacts have been prepared. In the northern Gulf
of Alaska the anomalies resulted in a massive stabilisation of the
water column through the spring, summer and fall of 2002. By
February 2003 it was apparent that very little mixing had taken place
in the upper ocean and that this would be restricting the supply of
nutrients which potentially could be damaging to the productivity of
the marine ecosystem during the spring and summer of 2003. These
observations, largely based on results derived directly from the Argo
array formed a large part of ecosystem assessments conducted in early
The Bering Slope Current System Revisited
Gregory C. Johnson and
Phyllis J. Stabeno
Stephen C. Riser
Mean circulation and water properties within the Aleutian Basin of the Bering Sea are investigated using hydrographic and subsurface park pressure displacement data from a regional array of 14 profiling CTD floats. After 10 days drifting at 1000 dbar each float measures temperature and salinity profiles as it rises to the surface, and then transmits these data via satellites which also make several fixes of the float surface positions before it sinks again. Every fourth cycle the floats sink from 1000 dbar to a 2000-dbar target just prior to ascent to measure deeper profiles.
The 1000-dbar displacements estimated from the float surface position fixes reveal a coherent few cm s-1 northwestward flow along the northeasternboundary, the deep signature of the Bering Slope Current. Mid-depth water property distributions are consistent with cyclonic advection of warm water from the south around the basin, eastward in the Aleutian North Slope Current, and then northwestward in the Bering Slope Current. Geostrophic transport estimates relative to 1000 dbar also show cyclonic motion, although with significant noise, likely owing to the influence of mesoscale eddies.The mean along-slope geostrophic transport of the Bering Slope Current is determined between 0 and 1900 dbar relative to 1000 dbar and then combined with mean along-slope velocities at 1000 dbar. The result is an absolute geostrophic transport estimate with 95% confidence intervals for the along-slope current offshore of the 1000-m isobath and between 0 to 1900 dbar, 5.8 (±1.7) x 10**6 m3 s-1.
Eddy-induced temperature anomalies in the Kuroshio extension recirculation gyre from Argo, TMI and altimetry measurements
Qiu, Shuiming Chen and Peter Hacker
The past decade has seen an unprecedented advancement in satellite measurement of sea surface height (SSH; by TOPEX/Poseidon, Jason-1, and ERS-1/2) and sea surface temperature (SST; e.g., by TRMM Microwave Imager). While these measurements have helped revealing many new features of the ocean circulation, their dynamic interpretations are often hampered by lack of observational data beneath the sea surface. The contemporaneous Argo measurements provide us now with a unique opportunity to unravel the sea surface signatures and their subsurface connections.
The recirculation gyre region south of the Kuroshio Extension jet is abundant with energetic mesoscale eddies. High-resolution satellite measurements reveal that warm (cold) SST anomalies associated with the mesoscale eddies tend to located ~150km to the west of high (low) SSH anomalies in this region. An examination of the available Argo data indicates that this phase misalignment is confined to the seasonal mixed layer (0-200 m depth). Below the seasonal mixed layer, the SST anomalies are in-phase with the SSH anomalies. The phase difference between the SST and SSH anomalies is found to be caused by meridional temperature advection associated with the mesoscale eddies and it contributes to the northward eddy heat transport in the Kuroshio Extension recirculation gyre region.
Distribution and variation of the mixed layer in the western North Pacific observed by Argo floats
Ohno1, Taiyo Kobayashi2, Naoto Iwasaka1,2, Toshio Suga2
2 Frontier Observational Research System for Global Change
In this study we describe the distributions and time variations of the mixed layer measured by Argo floats which have been deployed in the western North Pacific. Then its observed characteristics are compared with those inferred from World Ocean Atlas 1998 (WOA98). Profile data obtained by Argo floats in the North Pacific from April 2000 to July 2003 are used. The mixed layer depth (MLD) is defined as the depth at which potential density is larger than that at the sea surface by 0.125kg/m3. The mixed layers are developed most in February and March; its average depths are about 130m in the analyzed region. The mixed layers deeper than 250m are concentrated in the region south of the Kuroshio Extension. During the boreal summer of June to August the mixed layers become as shallow as 50m or less. The temporal and spatial variability of the MLDs is very large in winter. The observed mixed layer tends to be deeper than that from WOA98 in the subtropical region and to be shallower in the subarctic region. Especially the observed MLDs in March are deeper than those from WOA98 by up to 174m around 35°N, 145-155°E and are shallower by up to 233m around 40°N, 155-165°E. These results suggest that climatological MLDs calculated from gridded data like WOA98 may be far from realistic in some regions.
Estimating ocean velocity fields from Argo and other data
W Brech Owens
Intermediate level circulation in the southwestern part of the Japan/East Sea
Young-Gyu Park, Kyung-Hee Oh, Moon-Sik Suk and Kyung-Il Chang
Global Environment Research Laboratory,
Korea Ocean Research and Development Institute
P.O. Box 29,
Using trajectory data from the subsurface floats (PALACE and APEX) deployed into the Japan/East Sea for the past few years, we were able to obtain a detailed intermediate level circulation pattern over the south-western part of JES for the first time. The flow in the region is largely controlled by the bottom topography.
features we have found are as follows:
Our results, which are consistent with previous ones from current meter mooring (Chang et al. 2002) and pressure gauges (Teague et al. 2003), have been obtained with less effort and expenses but cover wider area than those previous studies.
The absolute geostrophic velocity field and wintertime convection in the Japan/East Sea estimated from an array of profiling floats
The subpolar gyre in the Japan/East Sea is a region of strong air-sea interaction, especially in winter, resulting in intermediate and deep convection and ventilation. The relatively high dissolved oxygen values in the subsurface waters of the Sea are signatures of these winter processes. In order to study the circulation and water mass properties of the subpolar gyre in the Japan/East Sea in detail, an array of 37 profiling floats were deployed by the US, with strong collaboration with Japan, Korea, and Russia, during the summer of 1999.
These floats sampled the subpolar gyre of the Japan/East Sea for over 3 years (some continue to operatate late in 2003). The profiles and deep velocities collected from these floats have been used to construct seasonal maps of the absolute geostrophic circulation of the Japan/East Sea at levels above 800 m. The profiles reveal sites in the western Japan/East Sea where wintertime convection occurs. A number of features revealed by the float trajectories and profiles appear to be consistent with the predictions of state-of-the-art numerical models of the Japan/East Sea.
New elements of the Japan Sea oceanography found with the use of profiling floats
4-years (1999-2003) work of 32 (after 190 cycles only 4 are still working ) deep floats-profilers (APEX and ) revealed deep currents, features of water structure and their changes.
All floats drifted at 800 dB level (measurements were made every 7 days in upper 800 dB layer).
Decoding and primary processing of data were made in UW. Constant for water column salinity difference was input on the base of salinity difference (climatic and measured) at lowest level. It was unusually high (up to 0.5 psu) for two floats only.
New elements of regional oceanography were found after analysis of floats data:
- large-scale gyre in
deep basin and new current in its northern part;
The use of floats measurements to addition of ship-based data allow to trace the Japan Sea Deep Water formation in winter of 2001.
Heat content of 250-750 dB layer in winter of 2001 year was minimal and just after it (in winter of 2002)- maximal.
Features of temporal variations of water characteristics gave interesting conclusions too.
Global statistics of inertial motions from Argo floats
241-318, Seoul National University,
San 56-1, Shilim-Dong,
Jong Jin Park
Brian A. King
Stephen C. Riser
Statistics of inertial motion have been compiled in the North Pacific, North Atlantic, the Indian Ocean, and the East (Japan) Sea respectively using the surface trajectories of profiling floats. The phase and amplitude of inertial motions and mean drift velocity can be derived from a weighted function-fit to a set of discrete Argos position fixes for each surface cycle. The statistical distributions of the inertial speeds in the North Atlantic and the East Sea are narrower than that in the Indian Ocean. The ranges of inertial speeds which occupy 99% of all cycles are 0~41 cm/s in the North Atlantic, 0~48cm/s in the East (Japan) Sea, and 0~81cm/s in the Indian Ocean. The accuracy of the inertial speed can be below 5cm/s if the drifting time at the surface is longer than 60% of local inertial period and mean position error is less than 600m.
Response of the Arabian Sea to the summer monsoons of 2002 and 2003
The Indian summer monsoon rainfall was 19% below average during 2002 and 5% above average (so far) this year. The relative strength of the monsoon winds showed considerable month to month variability. The southwesterlies were stronger in June 2002 than June 2003 and weaker in July 2002 than July 2003. However, the SST was warmer and the sea level higher during both June and July of 2003. Our knowledge about the response of the Arabian Sea to contrasting monsoons is limited primarily to the surface features due to the unavailability of sufficient subsurface observations. Argo floats provide an opportunity to examine the surface and subsurface changes to contrasting monsoons of 2002 and 2003.
Space time variations of the Arabian Sea thermohaline structure in the upper ocean has been examined using temperature and salinity profiles from Argo floats. The float density increased from 2002 to 2003 but there are still large gaps in the western and eastern parts of the Arabian Sea which has restricted the present analysis to the central Arabian Sea. In the northern Arabian Sea, the mixed layer deepened from the presmonsoon conditions by about 10m during 2002 compared to about 10m during 2003. In the south, both 2002 and 2003 showed a deepening of about 20m. The thermocline response was markedly different between 2002 and 2003. In 2002, the thermocline deepened south of about 10N and shallowed further north whereas in 2003 it deepened almost everywhere. The results are being verified using complementary data sets and ocean model outputs and the underlying mechanisms are being explored.
Acknowledgments : This study was funded by the Indian National Centre for Ocean Information Services (INCOIS), Department of Ocean Development, Government of India.
Observations of the Okhotsk Sea circulation and water masses using profiling floats
and Masaaki Wakatsuchi
Since mid-2000, 18 profiling floats have been deployed in the Sea of Okhotsk in a program of collaboration between Hokkaido University and Hokkaido Wakkanai Fisheries Experimental Station (Japan), the University of Washington (USA), and the Far East Regional Hydro-meteorolgical Research Institute (Russia). This work was sponsored through CREST in Japan. The floats were targeted for depths of 500 m, 750 m, and 1650 m. Trajectories of the floats have revealed the extent of the cyclonic gyre in the central Okhotsk basin and active mesoscale motions in the Kuril Basin. Some floats have survived throughout the winter, under the sea ice, and continued to function normally in spring after the ice melted; the data from these floats is very useful for examining the time evolution of the winter mixed layer under the ice. The seasonal heat content in the upper portion of the Okhotsk Sea as measured by the floats compares well with the surface heat flux. The heat budget estimates provided by the floats suggest that the anomalously early appearance of sea ice in the winter of 2000/2001 was caused by an anomalously large loss of heat loss in the fall of 2000. The freshwater budget associated with the ice formation/melting cycle has also been examined from the profiling float data.
Argo and other observing systems - issues and challenges
Applications of Argo observations - an exciting prospect