Southern Ocean Floats on Isopycnals Experiment (SOFIE):Observing Mixing

Project Summary

The Southern Ocean Floats on Isopycnals Experiment (SOFIE) will use autonomous floats tomeasure mixing along isopycnals.

Intellectual Merit: SOFIE focuses on isopycnal mixing because oceanic mixing of heat and saltoccurs primarily along isopycnals. The experiment is planned for the Southern Ocean, where isopy-cnals tilt steeply so that the distinction between isopycnal and isobaric processes is pronounced.Isopycnal mixing rates obtained in SOFIE will be used to advance both the physical understand-ing of processes governing meridional overturning of the ocean and the numerical simulation ofthese processes. Numerical modeling studies indicate that the Southern Ocean overturning circula-tion is more realistic when mixing is parameterized to occur along isopycnals, but Southern Oceanisopycnal mixing rates have not previously been measured.

An important part of SOFIE are two proposed field surveys in the Southeast Pacific Basin.These cruises are essential to allow staggered float deployment, as there is no merchant shippingin the Southern Ocean. The high-quality, full-depth CTD/LADCP data collected during these sur-veys will provide the necessary measurements for float calibration in this historically data-voidregion. Float deployment in the relatively low-eddy environment of the southeast Pacific ensuresthat isopycnal mixing measurements will be obtained both from quiescent regions and the highlyturbulent region associated with Drake Pasage and the Brazil-Malvinas Confluence area.

Analysis of the float data will focus on estimating eddy heat, salt, and layer thickness fluxesalong isopycnals, as well as determining velocity variance on the isopycnal surfaces. Observedstatistical quantities will be compared with fluxes derived from repeat measurements in DrakePassage and with results from isopycnal and level models. Numerical model data will allow us toestimate the high-frequency component of eddy fluxes and the vertical variations in fluxes, whichcannot be determined from floats alone.

The SOFIE float and CTD/LADCP data will not only provide estimates of isopycnal mixing, butwill also augment the Argo database for the Southern Ocean. Recent comparisons of float data andhistoric hydrographic data indicate that between 700 and 1100 m depth, the Antarctic CircumpolarCurrent region has warmed at a rate of nearly 0.1˚C per decade over the last 50 years, a ratecomparable to the observed rate of atmospheric warming on islands located within the SouthernOcean. SOFIE’s database will provide an important record to continue monitoring temperaturechange in the Southern Ocean. Moreover, SOFIE’s isopycnal mixing estimates will quantify oneof the mechanisms that may be responsible for warming the ACC.

Broader Impact: SOFIE will develop and deploy new isopycnal-following autonomous floats andcarry out pathbreaking research using the floats. The SOFIE floats will augment the Argo databasefor the Southern Ocean, and hydrographic cruise data will also return important observations tomonitor ocean climate change in this rapidly varying part of the world. The project will involveone graduate student and will offer research opportunities to approximately one undergraduate peryear.

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Results from Prior NSF SupportCAREER: Linking Southern Ocean Dynamics to Global Climate OCE-9985203/OCE-0049066(S. Gille); 4/1/00 - 3/31/05, $472,046.

Research carried out in the first 20 months of this project has focused on analysis of SouthernOcean autonomous float and drifter data, with comparisons to historic hydrography and wind data.Findings have been presented at 4 conferences and have led to 1 newsletter article and 3 submittedjournal manuscripts. Results show that float temperatures at depths between 700 and 1100 m aresystematically warmer than older hydrographic temperatures, indicating that the Southern Oceanhas warmed approximately 0.01˚C/year since the 1950s (Gille, 2001c, 2002). Bottom velocitiesbelow the core of the Antarctic Circumpolar Current (ACC) are eastward, while outside the current,they appear to be westward or indistinguishable from zero (Gille, 2001a). Eddy heat fluxes implypoleward heat transport along the length of the ACC with elevated poleward heat transport nearDrake Passage and the Agulhas Retroflection (Gille, 2001b). Additional work soon to be submittedhas examined the behavior of floats during their time at the ocean surface (Romero and Gille, 2002).

The Drake Passage High Density XBT/XCTD Program OCE9632983/OPP003618 (J. Sprint-all); 7/1/01-6/30/04, $155,000.Shipboard acoustic Doppler current profiling on R/V Nathaniel B. Palmer and R/V LaurenceM. Gould OPP9816226 (T. K. Chereskin and E. Firing); 1/1/99-12/31/03, $328,437.

These awards enabled the PIs to implement a time series of upper ocean observations from theAntarctic supply vessels that cross Drake Passage, with a goal of understanding the co-variability ofupper ocean currents and temperatures on seasonal to interannual time scales. Sprintall began theXBT/XCTD program in September of 1996 (with original funding from NOAA and OCE9632983)and makes 6-8 high-resolution sections annually. Chereskin began the ADCP program in Septem-ber of 1999 and makes 2 to 4 sections/month.

Both of these projects have required technical developments specific to the extreme environmentof the Antarctic. Initially the XBTs were hand launched every half hour by a roster of volunteers,but in the past year use of an autolauncher has proven viable even in near freezing conditions,and the high density section can be made by a single operator. Technical developments for theADCP included the installation of a sound velocity probe in the well where the ADCP transduceris housed in an antifreeze solution behind an acoustic window. Automated processing routinesand e-mail transfer of data from the ship allow the ADCP data quality to be monitored in nearreal-time. The shipboard setup, routines, and daily emails from the ship are documented on a web-site (http://tryfan.ucsd.edu) and technical details have been presented at the International MarineTechnicians Workshop (Chereskin, 2000).

Although these are ongoing grants, and most of the publications to date are conference proceed-ings, they are the most relevant to the present proposal. References for preliminary results on theseasonal (Sprintall et al., 1997, 2000; Sprintall, 2001b), and mesoscale variability (Sprintall, 2001a)of upper ocean temperature and geostrophic flow in the major ACC fronts across Drake Passage;the structure of the narrow jets of the ACC (Chereskin et al., 2000); and the seasonal variability ofa marine ecosystem subject to seasonal ice cover located at Deception Island (Lenn et al., 2002)are listed in proposal section D.

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Figure 1: Meridional section of potential temperature (color) and neutral-density layer interfaces,which are roughly equivalent to isopycnals, (dashed lines) at 88˚W. Contour intervals are 1˚C and1 kg m−3, respectively. Note the significant change in depth of the neutral-density surfaces withlatitude. (From Gille (1999).)

PROJECT DESCRIPTION

1 Introduction

The Southern Ocean Floats on Isopycnals Experiment (SOFIE) will use autonomous floats to mea-sure mixing along isopycnals in the Southern Ocean.

Measuring mixing along isopycnals is important, because most of the transport of heat and saltin the ocean occurs along isopycnal surfaces. Theoretical and numerical models of the ocean (e.g.Bleck et al., 1992; Gent and McWilliams, 1996; Karsten et al., 2002) are built on the central ideathat oceanic processes are controlled by isopycnal mixing. However, little observational informa-tion exists at present about the processes that govern isopycnal mixing. What are typical mixingrates? How do mixing rates vary in time or space? Does eddy mixing along isopycnals correlatewith easily measured quantites such as surface eddy kinetic energy? Can time variable mixingprocesses be thought of as conserving properties on time-averaged isopycnals?

We have chosen to carry out this experiment in the Southern Ocean, because isopycnals tiltstrongly across the Antarctic Circumpolar Current (ACC), rising 1000 m over 1000 km, as shownin Figure 1. As a result, the distinction between isopycnal and isobaric mixing is greater in theSouthern Ocean than it is in other parts of the global ocean. Numerical modeling studies havefound that the introduction of isopycnal mixing parameterizations has greater impact on the merid-ional overturning of the Southern Ocean than it does on circulation in the Northern Hemisphereor midlatitude regions (e.g. Danabasoglu et al., 1994). We expect that by working in the SouthernOcean, we will have less difficulty distinguishing isopycnal processes from isobaric processes thanwe might in other parts of the world, and that measurements of isopycnal mixing rates will havemore immediate benefits, both for modeling and for our theoretical understanding of the ocean’smeridional overturning.

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SOFIE will make use of autonomous Sounding Oceanographic Lagrangian Observer (SOLO)floats (Davis et al., 2001). The floats will closely resemble isobaric floats that have been developedfor the Argo program (Roemmich et al., 2001), but will introduce new technological advances toallow them to follow isopycnal surfaces. Autonomous floats are needed for this study, because noother measurement device could obtain the observations that we require to estimate isopycnal mix-ing. For example, although current meters or isobaric floats can measure horizontal mixing (Gille,2001a), they provide no information about the location or slope of isopycnal surfaces, so cannotbe used alone to infer isopycnal mixing. Hydrographic inverse models are able to estimate fluxesalong isopycnal surfaces (Gille, 1999; Sloyan and Rintoul, 2001), but are not able to capture theeffects of temporal variability. Acoustically-tracked isopycnal floats have been used to investigatedispersion in the Gulf Stream (Bower and Rossby, 1989) and in the North Atlantic Current (Zhanget al., 2001), but because of the rapid advection rates in the ACC, acoustic tracking would be nearlyimpossible in the Southern Ocean. Dye release experiments have also been used to estimate isopy-cnal diffusivities in the deep ocean, but they are difficult to carry out in regions of strong advection(where survey costs are large) or on steeply tilted density surfaces, such as those found in the ACC,that may outcrop at the surface (thus losing tracer to the atmosphere).

Eddy energy varies along the length of the ACC. To ensure that we obtain isopycnal mixingobservations from quiescent regions as well as from eddy-rich regions, we will deploy our floatsin the relatively low-eddy region west of Drake Passage and allow them to drift eastwards into thehighly turbulent region associated with Drake Passage and the Brazil-Malvinas Confluence area.By deploying floats in the southeast Pacific, we will obtain excellent float sampling within DrakePassage, thus facilitating statistical comparisons with observations from the regular XBT/ADCPrepeat lines across Drake Passage. Few historic observations exist from the region upstream ofDrake Passage, so we will conduct a full CTD and LADCP survey during our deployment cruise,which will allow us to better quality control the temperature and salinity data from our SOFIEfloats and to obtain an independent estimate of the three-dimensional velocity field in our studyregion. These data will augment the historic database used to quality control all floats deployed inthe Southern Ocean.

Because isopycnal mixing plays a critical role in ocean general circulation models (OGCMs),the observational component of this project will be closely guided by numerical models. We haveestablished ties with the groups developing the new 1/10˚ Parallel Ocean Program (POP) and theglobal quarter degree Hybrid Coordinate Ocean Model (HYCOM). The models have sufficientresolution to capture many of the eddy features that are responsible for mixing in the ocean. (Seeletters of support by Bleck, Maltrud, and McClean.) Model output will allow us to estimate thefull vertical structure of isopycnal mixing and to evaluate which aspects of isopycnal mixing arelikely to be best sampled by the floats. We will also be able to provide direct feedback to the modeldevelopers to aid in the ongoing improvement of eddy-resolving OGCMs and of parameterizationsfor coarser resolution climate OGCMs.

In addition to providing observations of isopycnal mixing, as an ancillary benefit SOFIE willaugment the profiling float observations that are currently being collected through the Argo pro-gram. Monitoring upper ocean temperature and salinity is important in the Southern Ocean, be-cause the region has been shown to be extremely sensitive to climate change in numerical climatemodels (Banks and Wood, 2002), and observations suggest that it is warming at a rate of nearly1˚C/century in the core of the ACC, which is significantly faster than the global ocean as a whole(Gille, 2002). Scales of variability are as short as 10 to 30 km in the Southern Ocean (Houry et al.,

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Figure 2: Upper ocean temperature time series collected by a SOLO-carbon float deployed nearOcean Weather Station Papa. The yellow line shows the depth at which potential density differsfrom the surface value by 0.05. Triangles show dusk sample locations and circles indicate mid-daysamples. SOLO-carbon floats sampled at higher frequency than SOFIE floats will sample, and wereable to collect as many as 400 temperature profiles over 8 months. Their data were successfullyrelayed to land via ORBCOMM (Bishop and Davis, 2001).

1987), which would justify high-resolution float sampling. However, because of other pressures onthe program, Argo is unlikely to achieve even its target 3˚ spatial resolution in the Southern Oceanwithout supplemental observations through other programs, such as this one.

2 Experiment Design

2.1 Float Design: Following Isopycnals through Drake Passage

SOFIE will deploy a total of 40 profiling floats that will obtain isopycnal-following velocity tra-jectories and to estimate temperature and salinity changes on isopycnal surfaces. SIO’s InstrumentDevelopment Group (IDG) will build 20 floats per year, which will be based on the instrumentdesign developed for SOLO floats (Davis et al., 2001). IDG has considerable experience design-ing and building profiling floats and has already solved the major technical challenges required tobuild isopycnal following floats. SOLO float technology is well proven; Figure 2 shows an upperocean temperature time series collected near Ocean Weather Station Papa. The SOFIE float designrepresents an incremental but important step forward. While the original profiling AutonomousLagrangian Circulation Explorer (ALACE) floats deployed during WOCE were ballasted to a pre-determined depth, SOLO floats use active pressure sensors to find their target depth. Each SOLO

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Figure 3: (a) Depth of the isopycnal surface σ1 = 32. (b) Depth of the isopycnal surface σ1 = 32.25.Both of these surfaces outcrop within or south of the ACC and are comparatively deep within theACC. SOFIE floats will be programmed to select from several target isopycnal depths so that theyremain in the depth range between 1500 and 500 meters.

float will be equipped with a CTD that will allow it to determine density at its drift depth. ThusSOLO floats can easily be fitted with alternate software to make them follow isopycnals. The soft-ware to instruct a float how to follow an isopycnal will be similar to software already in existencethat is used to make the SIO Seasoar follow isotherms and isopycnals (Ferrari and Rudnick, 2001).

The floats will track σ1 isopycnal surfaces between 31.75 and 32.35 kg m−3 (Figure 3) and willbe programmed to adjust their target isopycnal depths in order to remain in the depth range between500 and 1500 m. On the north side of the ACC, these σ1 surfaces vary between about 1700 and800 m depth, and they outcrop within and south of the Circumpolar Current. (Since outcroppingisopycnals cannot be followed within the mixed layer, SOFIE floats will attempt to stay out of themixed layer.) The target depths are easily accessible with existing SOLO float pump technology.Some Southern Ocean analyses have used neutral density (Jackett and McDougall, 1997); however,neutral density and potential density surfaces do not differ substantially within the comparativelynarrow depth range targeted in this study, and we will rely on σ1.

Like Argo floats, SOFIE floats will measure vertical profiles of temperature and salinity asthey travel between mid-depth and the ocean surface. In order to provide reasonable samples ofmesoscale ocean features, SOFIE floats will rise to the surface every 10 days to transmit their obser-vations to shore. Similar sampling time periods have been used by ALACE floats deployed duringthe World Ocean Circulation Experiment (WOCE) and by Argo floats currently being deployedas part of the CLIVAR program. The floats have battery power sufficient to allow 200 profiles to1000 m depth, with expected total lifetimes exceeding 5 years.

The SOFIE floats are planned to transmit their data via the ORBCOMM satellite system. ORB-

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COMM offers several advantages over the Service Argos system used in many earlier float deploy-ments. First, ORBCOMM permits two-way satellite communications, which will allow us to sendnew instructions to the floats, to reprogram their missions when necessary. Second, ORBCOMM isable to transmit larger messages than does Service Argos, which means that more detailed data maybe sent back to land, and in many cases messages can be relayed more quickly than they would withService Argos. ORBCOMM also provides continuous coverage equatorward of 55˚, and at least4 usable satellite passes a day poleward of 55˚; by timing float arrivals at the surface to coincidewith satellite overpasses, we will be able to reduce the amount of time that floats spend at the sur-face, which should decrease their susceptibility to wave impact and biofouling. Finally, althoughall satellite communications companies have faced turbulent economic situations, ORBCOMM iscurrently financially solvent. In the event that ORBCOMM does not prove to be a stable partner,we have several alternate options. Service Argos will establish two-way communications capabil-ities starting in November 2002. We will be able to build our floats to use Service Argos two-waytransceivers for the same budget. If necessary, we also have the option of eliminating the two-way communication feature in our floats and using the standard Service Argos data transmissioncapabilities.

Past profiling floats have been plagued by salinity drift problems. However, Riser and Swift(2002) report that SeaBird sensors now have an accuracy of 0.01 PSU over 3 years. On the basis ofa Monte Carlo calculation, we estimate that in the ACC, this would result in a depth error of 10 to 25meters. For comparison, in this region the isopycnal surfaces rise 100 to 300 m per degree latitude.Thus a slow drift in the salinity sensors is unlikely to substantially bias our results. Moreover,in comparison with earlier floats, SOFIE salinity sensors may experience improved performance,because of their shortened exposure to the biological blooms at the ocean surface. Continuedtechnical efforts from the float development groups will also help minimize salinity problems.Nonetheless, we cannot assume that SOFIE floats will be immune to salinity sensor drift. In ourquality control procedure (discussed below) we will closely track the salinities recorded by ourfloats in comparison with historic observations. Using two-way communication capabilities, ifnecessary we will be able to reprogram the floats in real time, providing new salinity algorithms tofloats for which we understand how to correct the drift problems. In cases where salinity drift is noteasily corrected, either because little historical data is available or because of the inherent variabilityin the Southern Ocean, we will be able to instruct the float to follow isothermal or isobaric surfacesrather than isopycnals. As Figure 1 illustrates, within the Subantarctic Front region (∼ 59˚S),isotherms and isopycnals are closely aligned, so isotherms may be substituted for isopycnals withlittle degradation of the data. The Polar Front (∼ 62˚S) has stronger salinity variability, and thereisotherms may not be a suitable substitute for isopycnals.

2.2 Field Program: The Southeast Pacific Basin

The SOFIE floats will be launched during two cruises, planned for austral summers 2003-4 and2004-5 (Figure 4). We will run our cruises from east to west, taking advantage of the large-scaleeastward flow in the region to spread the floats out in time and space. From their deploymentin the deep-water low-eddy region upstream of Drake Passage, the floats will be advected into theshallower topography and high eddy energy region associated with Drake Passage and the MalvinasCurrent. This will allow us to obtain statistically independent samples of eddy flux statistics in botheddy-rich and quiescent segments of the ACC.

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Figure 4: Proposed cruise track, CTD/LADCP station locations, and float deployments. The cruiseplan is designed to encompass the Subantarctic and Polar Fronts (shown in red and blue, respec-tively, based on Orsi et al. (1995)) and to keep the floats north of the Southern ACC Front and theice edge. Drake Passage ADCP/XBT transects are also shown.

WOCE ALACE floats between 700 and 1100 m depth in the Southern Ocean (south of 45˚S)travelled at a mean speed of 9 cm s−1, with the fastest 20% averaging 20 cm s−1. Since SOFIEfloats will be deployed in the ACC, many of them will travel at the speed of the ACC core. Overthe course of their lifetimes, the fastest moving floats are likely to travel the entire length of theACC and return to their deployment region, providing one profile every 2 to 3 degrees longitudealong the way. Overall this coverage will exceed the spatial coverage obtained during WOCE andwill allow us to estimate fluxes, as discussed below.

The SOFIE deployment region in the southeast Pacific has fewer historical observations thanany other sector of the Southern Ocean (e.g. Olbers et al., 1992). With the exception of the recentWOCE section P19/P17E along 88˚W in January 1993 (Rubin et al., 1998), no high-quality hy-drographic observations exist in this region. Thus the temperature-salinity relationship is poorlyknown, and further, the WOCE sections show that the ambient salinity gradients are very weak(Gille, 1999). Monitoring temperature, salinity and velocity in the region is only possible fromdedicated ship surveys, due to the lack of merchant shipping. Obviously the profiling floats withtemperature and salinity sensors provide the most cost-effective means of continuously monitoringthe Southern Ocean, although their deployment also relies on a devoted ship transect. For this rea-son, during the float deployment cruises, we will carry out a complete survey of the region withshipboard and lowered ADCP velocity measurements as well as top-to-bottom CTD casts. CTDcasts at each of the float deployment locations will not only provide calibration for the float CTDsbut will also augment the historical data base on which we rely to estimate fluxes from float ob-servations. Lowered ADCP measurements collected during the CTD casts will provide referenceinstantaneous velocities for comparison with the time-averaged velocities obtained by the floats.

The cruise track crosses the Subantarctic Front and Polar Front at four longitudes separated by

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7 degrees from 74˚W to 95˚W (Figure 4). These major frontal features that characterize the ACCare on the order of 50-100 km wide (Nowlin and Clifford, 1982; Whitworth and Nowlin, 1987),and are associated with current cores that carry the bulk of the eastward transport in the ACC. The20 SOLO floats will be deployed at uniformly spaced intervals along the cruise track to ensureindependent sampling (Figure 4). Because the cruise track lies within the main ACC fronts, thefloats should remain within the high velocity cores of the ACC in their circumpolar journey, andnorth of the Southern ACC front and ice edge. The southern limit of the cruise track at 64˚S ishistorically ice-free during the austral summer months (http://geochange.er.usgs.gov/pub/sea ice/).The cruise track and associated CTD/LADCP survey will take approximately 19 days to complete,with an additional 7 days transit time required from a Chilean port such as Punta Arenas.

Along each section, CTD casts will be separated by ∼30 nautical miles (0.5 degrees latitude)and by 40 nautical miles (0.66 degrees latitude) to the south of 61˚S. This CTD station spacing isthe same as that achieved by WOCE sections in the region. The section at 88˚W will repeat theCTD casts from the ACC crossing carried out during WOCE P19/P17E, providing a measure ofACC temporal change. The UCSD/SIO Oceanographic Data Facility (ODF) will carry out CTDdata acquisition and processing to WOCE standards (e.g. Joyce and Corry, 1994a,b) or better. Thishigh quality is essential not only to distinguish different water masses, to estimate geostrophicshears, and to enable comparisons on isopycnal surfaces within a data set, but also to allow com-parison with other data sets and calibration of the float data. The scarcity of high quality data fromthe Southern Ocean makes the SOFIE measurements particularly important as a standard referencefor the region. ODF will supply experienced seagoing technicians to provide pressure and temper-ature measurements calibrated to laboratory measurements and salinities corrected against bottlesamples.

Under current NSF guidelines, ODF data aquisition is supported through NSF’s OceanographicTechnical Services Program and will not be charged to our science budget. Although ODF willdeliver scientifically usable data at sea, the final data processing and documentation will be com-pleted on shore, after completion of the cruises. Following current NSF policy, these shore dataprocessing and documentation services appear in our science budget (as a self-contained item under“Other”).

Shipboard and lowered ADCP measurements will provide complementary information on thecurrent structure. Donohue et al. (2001) recently showed that shipboard and lowered ADCP mea-surements in the Southern Ocean are able to detect nonzero bottom velocities below the core jets ofthe ACC. The shipboard ADCP has the best horizontal resolution, and this is particularly importantdue to the small Rossby radius. The barotropic nature of the current, excellent horizontal resolu-tion of the SADCP, and the multiple longitudes sampled by our cruise track will more stronglyconstrain the geostrophic reference from the shipboard ADCP (e.g. Chereskin and Trunnell, 1996)than was possible with the sparse sectional WOCE data available to Donohue et al. (2001). Thelowered ADCP will provide a full depth profile of current at the resolution of the CTD stations,yielding a detailed view of the current and current shear at the locations of the float deployments.We expect barotropic tidal velocities in this region of order 0-3 cm s−1, small compared to the largetidal currents of Antarctic marginal seas, (e.g. Robertson et al., 1998). The tide will be estimatedand removed from the measured currents using the OSU TPXO model (Egbert et al., 1994; Firing,1998b,a). Additionally, the CTD/LADCP measurements used together with parameterizations ofturbulent dissipation (Polzin et al., 1995; Polzin and Firing, 1997) will be useful in characterizingmixing in the region of float deployments, upstream of Drake Passage.

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In summary, by running two dedicated cruises, we will not only be able to stagger the launchof our floats in optimal positions upstream of Drake Passage, but we will also produce the firsthigh-resolution, high-quality data set of concurrent temperature, salinity and velocity full-depthobservations for examining property gradients and flow of the ACC within the Southeast PacificBasin.

2.3 Quality Control of the Profiling Float Data

Because SOLO floats are autonomous and nonrecoverable, their CTDs cannot be returned to the labfor recalibration. Therefore data quality control will be an important component of this experiment.Data from SOFIE will be relayed to us at Scripps following standard Argo procedures, and qualitycontrol will follow techniques under development for the Argo program.

Tropical Argo floats are calibrated by comparing the measured T-S profiles to historic datafrom the region. Argo has so far deployed few (if any) floats in the Southern Ocean, so Argoprocedures have not been tested in the region. We assume that the Southern Ocean data may offerchallenges, particularly because of the limited supply of historical data, and we have budgeted staffand scientist time to develop quality control methods appropriate for the Southern Ocean. Gillehas previous experience analyzing Southern Ocean ALACE data, and Sprintall is responsible forquality control of the temperature data collected in the ongoing Drake Passage XBT program (seePrior Results for a description of both programs). Our approach will compare SOFIE T-S profileswith historic data (Olbers et al., 1992; Levitus et al., 1998), as well as measurements from ourown deployment cruises, and repeat XBT/XCTD profiles from Drake Passage in order to developreliable sensor drift statistics for each of the floats. In the ACC frontal region, we expect to find fine-scale T-S structure, and we will take this into account in developing our quality control methods toavoid discarding real variability. Float positions and velocities will be determined using standardmethods developed through the SIO ALACE and Argo programs.

One priority in our analysis will be real-time evaluation of the sensor calibrations. Some floatsare likely to have salinity sensors that appear to have drifted in identifiable ways, and we will sendsoftware updates to these floats to allow more appropriate sampling missions.

3 Interpreting the Results

3.1 Converting Floats into Fluxes

Once SOFIE floats begin returning data, we will infer isopycnal mixing rates from their measur-ments of isopycnal velocity, isopycnal depth, temperature, and salinity. To assess isopycnal mixing,we will calculate both eddy fluxes and velocity variance (which is linked to dispersion), using thebackground mean field as a reference. Our goals are to answer the following questions:

• What are realistic isopycnal diffusivities in regions of strongly tilting isopycnals such as theACC? Within error bars, do diffusivities vary spatially or with depth? What controls the sizeof the diffusivities?

• To what extent do floats indicate that eddy mixing is exclusively along isopycnal surfaces?Do eddy-induced processes tend to mix along time-averaged or spatially-averaged iso-surfaces,as many numerical model parameterizations assume?

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Figure 5: (a) Along-ACC and (b) cross-ACC heat flux at 900 m, estimated from isobaric ALACEfloats and averaged along dynamic height contours. Dashed error bars use mean fields determinedfrom gridded atlas data (Gouretski and Jancke, 1998) and solid error bars are based on means fromALACE data. In panel b, heat fluxes are negative (poleward) across most of the ACC.

3.1.1 Lagrangian estimates of eddy fluxes

Eddy fluxes measure the correlation of velocity with a scalar value (such as temperature). In thescalar conservation equations, the divergence of the eddy fluxes determines the total mixing inducedby eddies. For SOFIE, we will compute eddy fluxes using a method developed by Gille (2001a)to derive eddy heat fluxes from isobaric ALACE floats deployed in the Southern Ocean. In thistechnique, each 10-day float cycle provides an estimate of vector velocity u and of a scalar quan-tity, such as potential temperature θ. From these instantaneous observations, we subtract separatemean estimates 〈u〉 and 〈θ〉, to obtain u´ and θ´. Mean fields can be derived from independent hy-drographic data (e.g. Gouretski and Jancke, 1998; Levitus et al., 1998) or from objectively mappedfloat observations (e.g. Bretherton et al., 1976; Davis, 1998; Gille, 2001a). For our analysis, wewill map velocity as a function of depth to account for the slope in isopycnal depth and will rotatevelocities into a coordinate system aligned with the mean flow, so that u´ represents the anomalousalong-stream velocity and v´ the cross-stream velocity. Finally, temporal and geographic averagesof 〈v´h´〉 will yield cross-stream eddy flux estimates. (Angular brackets here represent temporaland spatial averages.)

Although heat fluxes are highly inhomogeneous, at 900 m depth, Gille (2001b)’s cross-streameddy heat fluxes are statistically different from zero, as shown in Figure 5. (Along-stream eddy heatfluxes are not distinguishable from zero, but eddies are not expected to transport heat in the directionof mean flow.) When integrated vertically and circumpolarly, the cross-stream fluxes imply a netpoleward eddy heat flux across the ACC of 0.2 to 0.6×1015 W, consistent with past estimates of eddyheat flux in the Southern Ocean (deSzoeke and Levine, 1981; Gordon and Owens, 1987; Keffer andHolloway, 1988). In Figure 6, spatial variations of eddy heat fluxes indicate strong poleward eddyheat flux associated with Drake Passage and the Agulhas Retroflection in the western Indian Oceanas well as moderate poleward eddy heat fluxes along the length of the ACC.

Because the floats return measurements that represent averages over 10 day time intervals, the

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high-frequency component of the diffusivity will be lost. Estimates by Gille (2001b) from currentmeter data suggest that with 10-day sampling, floats are able to capture roughly 80% of the totaleddy variability. Current meters do not provide good means of measuring along-isopycnal variabil-ity, but comparisons with numerical model output will allow us to estimate the size of the missinghigh-frequency fluctuations for our analysis. Numerical models will also allow us to evaluate thevertical structure of eddy fluxes, which is inaccessible from a single float experiment. We haveestablished links with the POP and HYCOM modeling groups (see letters by Bleck, Maltrud, andMcClean) and will examine the behavior of numerical floats and the Eulerian eddy fluxes computedby the models.

With SOFIE data, we will be able to compute a suite of different eddy fluxes. Eddy fluxesof isopycnal depth 〈v´h´〉 are easy to compute but difficult to interpret physically. Heat fluxeson isopycnal surfaces are 〈v´θ´〉, while the heat flux associated with isopycnal displacement is〈v´h´∂θ/∂z〉. Salinity fluxes are analogous to heat fluxes. Eddy heat and salt fluxes will tell us ratesof along-isopycnal mixing and implied poleward fluxes associated with isopycnal displacements.Neither of these quantities can be obtained from isobaric floats.

Eddy thickness fluxes are the most intriguing quantity that SOFIE will allow us to measure.Eddy thickness fluxes 〈v´δh´〉 are commonly used in numerical models to represent along-isopycnalmixing (Gent and McWilliams, 1990). However, they have never been measured in the ocean, andrealistic thickness flux values are not known. From the SOFIE isopycnal float observations, thick-ness fluxes can be estimated as 〈v´h´∂δh/∂z〉 or obtained directly using the profile data in combi-nation with the isopycnal velocities as 〈v´δh´〉. These estimates will provide important constraintsfor numerical model parameterizations of eddy mixing.

The Gent and McWilliams (1990) subgrid-scale parameterization requires that thickness fluxesbe directed along mean isopycnals, but detailed evaluation of these processes in an idealized nu-merical model has indicated that this is not necessarily true (Gille and Davis, 1999). While eddydensity fluxes are adiabatic, they may appear diabatic relative to temporally and spatially averagedisopycnals. Using SOFIE observations, we will evaluate the extent to which thickness fluxes (v´δh´)are actually aligned along mean isopycnals (1/〈h〉) versus instantaneous isopycnals (1/(〈h〉+δh´)).

3.1.2 Velocity variance and dispersion statistics

The velocity variance terms, 〈u´u´〉 and 〈v´v´〉, can be calculated in the same way as eddy fluxes, bysubtracting estimated mean velocities from the observed float velocities. If eddy motions resemblea random walk, then in analogy with molecular diffusivity, the effective diffusivity κ (for the fixedtime lag ∆) is related to the velocity variance:

κ =12

d〈X2〉

dt=

12

∆〈v´2〉, (1)

where X represents particle displacements, ∆ is the time step between independent observations,and v´ represents the velocities relative to the mean background flow (Taylor, 1921; Freeland et al.,1975), and directional components can be identified by examining the along-stream and cross-stream velocity variances separately. Dispersion statististics are sometimes estimated by lookingat the relative separation of floats deployed from the same location, but this method is inappropri-ate for SOLO floats, because the floats are rapidly advected by the wind during their time at the

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Figure 6: Cross-ACC heat flux, averaged (a) in 30˚ longitude by 5˚ latitude bins or (b) in 30˚longitude by 0.2 dynamic meter bins. Poleward heat fluxes (negative) are prominent in the IndianOcean and in the eddy-active region in Drake Passage and just downstream of Drake Passage, wherethe SOFIE experiment will sample extensively.

ocean surface (Romero and Gille, 2002), and the resulting statistics will not be representative ofsubsurface isopycnal processes.

Because consecutive float trajectories should not be concatenated, SOFIE floats will providea limited estimate of particle dispersion representing only the net effect averaged over 10 days.This omits the low frequency components of velocity that control dispersion over long time pe-riods (Davis, 1991). The 10-day float sampling also filters out high-frequency variance, and as aresult, floats will always underestimate total variance. Because of these sampling issues, velocityvariances and dispersion statistics derived from SOFIE may need to be interpreted carefully. Aswith eddy flux estimates, we will assess the impact of 10-day velocity averaging by calculatingequivalent statistics from POP and HYCOM numerical model output.

3.2 Shipboard surveys

3.2.1 Upstream: Velocity and potential vorticity fields

In addition to providing a platform for float deployment and in-situ float calibration, our highresolution surveys will allow us to map the velocity and potential vorticity structure of the ACC.Potential vorticity (PV) is an important quantity in ocean dynamics, derived from statements ofconservation of mass and angular momentum. If dissipation and diabatic processes are negligible,PV is conserved and can be used as a tracer. In the current cores of the ACC where shears are large,the PV structure can indicate where dissipation may be important. The strength of simultaneousfull-depth CTD/LADCP measurements is that all three components of PV (planetary vorticity,

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relative vorticity from horizontal shear and from vertical shear) can be calculated directly andwithout interpolation (Beal and Bryden, 1999).

3.2.2 Downstream: Eulerian estimates of eddy fluxes

For comparison with Lagrangian estimates from floats, we will compute Eulerian estimates of upperocean eddy fluxes from Chereskin’s and Sprintall’s XBT/XCTD/ADCP time series data in DrakePassage. By 2004 we expect to have about 35 sections with concurrent temperature and velocitymeasurements and about 100 sections of velocity alone. From these synoptic sections, we can sub-tract mean fields derived from independent hydrographic data combined with objectively mappedfloats or from the time mean of the XBT/ADCP observations themselves. We will use a coordinatesystem aligned with the mean flow since our three principal crossings (Figure 4) intersect the ACCin different orientations. We will calculate Eulerian estimates of velocity variance, 〈u´v´〉, 〈v´θ´〉,and 〈v´h´〉, where these quantities are defined as for the float data. In the mixed layer the eddyfluxes are expected to be diabatic. The time series data will complement the Lagrangian estimatesthat we obtain from the floats, yielding good statistics for the surface layer in the energetic eddyregime of Drake Passage.

4 Time Line and Management Plan

The investigators involved in this project have previous experience working in the Southern Oceanand each bring separate areas of expertise to this program. Gille, who has extensive experiencemapping and analyzing Southern Ocean float data, will establish a final deployment strategy andwill oversee mapping and analysis of the isopycnal-following floats. The Instrument DevelopmentGroup (IDG) at Scripps, under the direction of Russ Davis, will take responsibility for developmentand construction of the isopycnal-following SOLO floats. IDG builds approximately 100 floats peryear for the Argo program and has considerable experience with the technology. Sprintall andChereskin will share responsibilities for the two SOFIE cruises: Sprintall will be Chief Scientistin 2003-4, and Chereskin in 2004-5. Sprintall will work with the programmer to establish a dataquality control system for the Southern Ocean float and CTD data. Chereskin will quality controlthe ADCP/LADCP data collected on the cruises, and prepare velocity data from Drake Passage forcomparison with float observations. A graduate student will participate in this project. Becauseof the broad range of activities associated with SOFIE, we anticipate offering undergraduate re-search opportunities each year, allowing selected students the opportunity to earn academic creditby participating in the project.

The proposed work will span four years.Year 1: Construction of first 20 floats and cruise preparation. Analysis of historic data and numer-ical model output to prepare final cruise deployment strategy.Year 2: Cruise number 1 deploys 20 floats. Analysis of cruise CTD and ADCP data. Preliminaryfloat data quality control. Construction of 20 additional floats.Year 3: Cruise number 2 deploys 20 floats. Analysis of cruise CTD and ADCP data. Ongoing floatdata quality control. Preliminary analysis of full data records.Year 4: Ongoing data quality control. Analysis and publication of preliminary results. Transitionof observation system to Argo for continued operation.

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5 Links to Other Projects

SOFIE is a self-contained process study, but the observations collected in this program will com-plement several proposed and ongoing Southern Ocean programs.

5.1 Drake Passage ADCP and XBT lines

As discussed in the prior NSF support section, Chereskin and Sprintall currently run regular ADCPand XBT/XCTD lines across Drake Passage. These measurements will provide velocity, temper-ature, and salinity anomaly data that can be directly compared with the float observations to helpinterpret isobaric and isopycnal fluxes through Drake Passage.

5.2 Southern Ocean CLIVAR

The CLIVAR Program documents support the establishment of a float array in the Southern Ocean.The region is poorly sampled by ships of opportunity or regular research cruises, and it is spatiallyinhomogeneous, so that dynamical regimes observed in one location may not extend to the fullSouthern Ocean. The CLIVAR Program has noted that floats have the unique capability to observethe entire ACC.

Joint field programs proposed by Lynne Talley (of SIO) and Bernadette Sloyan (of WHOI) inthe context of Southern Ocean CLIVAR will observe the formation and evolution of SubAntarcticMode Water (SAMW) using CTDs, moorings, RAFOS floats, and profiling floats. The experimentsare planned for the southeast Pacific, just north of the SOFIE float deployment region. While floatsand moorings used in the SAMW experiments are intended to stay in the study region, our SOFIEfloats will rapidly advect away from their launch sites. SOFIE will complement the SubAntarcticMode Water programs by tracking the fate of SAMW as it passes through Drake Passage andbeyond. The programs will work together to coordinate research cruises and float deploymentplans in order to maximize the value of the combined data set. (See attached letters by Talley andSloyan).

5.3 Why SOFIE will complement Argo without duplicating Argo

The goals of SOFIE are distinctly different from the goals of Argo. First, the Argo program intendsto deploy profiling isobaric floats at roughly 3˚ resolution over the global ocean in order to obtainupper ocean temperature and salinity profiles to monitor climate variability. In contrast, this projectintroduces new technology that is not part of Argo instrument capabilities in order to analyze mo-tions along isopycnals within the Southern Ocean. Second, although Argo intends to collect dataglobally, the Southern Ocean may not be sampled at full resolution. Moreover, the 3˚ resolutionplanned by Argo will not resolve the small-scale structures that typify the Southern Ocean, wherethe first baroclinic Rossby radius is substantially less than 50 km.

Although this program is not part of Argo, the Argo program may take advantage of the SOFIEprogram to deploy floats, and Argo has agreed to manage the float data stream after this proposal’sfunding ends. (See attached letter by Koblinsky). The SOFIE project will enrich the Argo data setby adding isopycnal-following floats to its suite of archived measurements. Because few researchships visit the Southern Ocean and ships of opportunity provide samples in only a limited portion

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of the Southern Ocean, floats are an essential tool for obtaining data about Southern Ocean climateand variability. The deployment opportunities offered by SOFIE and the temperature and salinityprofiles obtained in SOFIE will be an important augmentation to the Argo database.

6 Summary: Significance of proposed work

6.1 Intellectual Merit

SOFIE focuses on measuring along-isopycnal eddy fluxes in the Southern Ocean using floats. In-terpretation of the float observations will depend on hydrographic data collected during float de-ployment and historic hydrographic data, which together will help identify the background meantemperature and salinity fields.

• Isopycnal-following floats will be deployed in the Southern Ocean in order to assess thealong-isopycnal eddy fluxes across the ACC. Understanding these fluxes is essential if weare to evaluate Southern Ocean overturning, quantify the ocean component of meridionalheat transport, and determine the origins of the mid-depth warming of the Southern Ocean.

• As part of the float deployment, CTD and LADCP data will also be collected. Analysisof these data will allow us to better quality control the float data. Since the deployment isplanned for one of the most poorly sampled portions of the world’s ocean, the data collectedduring deployment will serve as an important addition to hydrographic data archives. Thefull SOFIE hydrographic data set will be made available to the community 2 years after thecruises.

• Fluxes obtained through SOFIE will provide the first observation-based estimates of heat andthickness fluxes along the steeply tilting isopycnals of the ACC. The results will provide avaluable picture of the dynamical processes that carry water upwards along isopycnals in theSouthern Ocean. They will also provide quantitative estimates of along-isopycnal fluxes tohelp evaluate and constrain ocean general circulation models.

6.2 Broader Impacts

• SOFIE will pioneer the use of autonomous isopycnal-following floats to study oceanic mix-ing processes and will make innovate use of two-way communication to ensure that the floatdata quality is maintained.

• Float data collected through this program will also add to the archive of temperature profilesand mid-depth velocities collected through the Argo program and available for assimilationand climate studies.

• SOFIE will involve a graduate student as well as approximately one undergraduate researcherper year.

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BUDGET JUSTIFICATION

Salary: Partial salary support is requested for the PIs Sarah Gille, Janet Sprintall and TeresaChereskin. Gille will be responsible for establishing a final float deployment strategy (year 1),mapping and analyzing the float data (years 2-4), and supervision of the graduate and undergraduatestudents (years 1-4). Sprintall will be responsible for overseeing float development and construction(year 1); cruise preparation and participation (years 2: Chief Scientist, and year 3); establishingdata quality control for float data, cruise and scientific data analysis (years 2-4). Chereskin will beresponsible for cruise preparation and participation (year 2 and 3: Chief Scientist); quality controlof the ADCP/LADCP cruise data and scientific analysis (years 2-4).

Funds are requested for a Programmer/Analyst (John Gilson) in years 2-4 for quality control ofthe SOFIE float data. Gilson is experienced in quality controling data from profiling Argo floats.We also request support for a second Programmer/Analyst to participate in the cruises, qualitycontrol LADCP/ADCP data, and provide support for the scientific analysis carried out as part ofthis project.

Funds are requested for a graduate student (years 1-4) with a thesis topic related to the SOFIEfloat project, to be supervised by Gille.

Salaries for the Research Project Assistant are for tasks that will specifically benefit this project,will be assigned by the PIs and charged on a time reported basis. These tasks should normallyinclude technical typing and editing, copying project literature, making travel arrangements, andco-ordination of efforts between project participants.

Computer Costs: Funds are requested for computer support at $XX per month, which coverscost of software maintenance, network support and facilities, storage devices, internet connectionsetc. Funds are also requested for a desktop computer for the graduate student (year 1) for analysisrelated to the SOFIE float data, and for a computer for data processing and quality control (year 2).

Equipment: Funds are requested for the purchase of 20 modified SOLO floats from SIO Instru-ment Development Group (IDG) in years 1 and 2. The 40 SOLO floats will be modified to allowthem to follow isopycnal velocity trajectories, and to estimate temperature and salinity changes onisopycnal surfaces.

Supplies and Materials: Supply and expense items categorized as project specific are forexpenses that specifically benefit this project, are reasonable and necessary for the performance ofthis project, and can be readily allocable to this project. These costs are slightly increased in years2 and 3, to support supplies needed for the SOFIE cruises.

Travel: Travel and per diem is requested for 4 participants (Sprintall, Chereskin, a programmer,and the graduate student) in the SOFIE cruises during years 2 and 3, that we anticipate to bePunta Arenas, Chile. Funds are also requested for travel and per diem to Punta Arenas for an IDGdevelopment engineer to conduct a pre-cruise check of the SOFIE floats. The IDG engineer has thenecessary expertise for ensuring that all floats are in good working order after their shipment fromSan Diego, and before deployment on the SOFIE cruises.

Travel and per diem are also requested for two PIs to present results from the SOFIE cruises andfloat deployments at the AMS Southern Oceanography and Meteorological Conference in Welling-

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ton, NZ in year 3. Funds are also requested in year 4, for one PI and the student to attend andpresent SOFIE results at the AGU Ocean Sciences meeting in Hawaii, February 2006, and for 2 PIsto attend and present results at the AGU Fall Meeting in San Francisco, December 2006.

Other: Funds are requested for publication costs associated with a scientific journal in years 3and 4.

Funds are also requested for project specific costs such as communication charges, xeroxing,mailing etc. that specifically benefit and are necessary for the performance of this project. Addi-tional communication costs are requested in the SOFIE cruise years 2 and 3.

Funds are requested to support the SIO Ocean Data Facility for data documentation and dataprocessing of the CTD casts in the SOFIE cruises in years 2 and 3.

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Date: Tue, 05 Feb 2002 17:28:19 -0500To: sgille@ucsd.eduFrom: Chet Koblinsky <koblinsky@gsfc.nasa.gov>Subject: Proposed float program in the Southern OceanCC: “Steve Piotrowicz” <Steve.Piotrowicz@noaa.gov>, roem@kakapo.ucsd.edu

Dear Sarah,As a co-chair of the US Argo Advisory Panel, I am pleased to hear that you intend propose a

complementary float field investigation in the Southern Ocean to the National Science Foundation.I am writing to clarify how the U.S. Argo project can interact with your proposed program. Thegeneral issue of related projects was discussed at the first meeting of the U.S. Argo AdvisoryPanel in December 2001, and the following is the consensus of that group. The Advisory Panel iscomposed of representatives of the NOPP consortium and of Argo user groups, such as GODAE,CLIVAR and others.

In general, the Argo Project encourages work that makes profiling float data available throughthe Argo Data System, leading to increased efficiency and improved global coverage for the ArgoProject. There are a couple of ways that US Argo can interact with such other programs, for mutualbenefit:

1. U.S. Argo float deployments.U.S. Argo is participating in the deployment of a global broad-scale profiling float array. Float

deployment opportunities from research vessels in remote ocean locations are of high value toArgo. Consequently, Argo should provide floats for R/V cruises to regions that are identified ashigh priority by the U.S. Argo Advisory Panel. The Southern Ocean is a high priority region forU.S. Argo.

The Argo floats would be deployed in accordance with the international protocols (spatial distri-bution, profiling depth, 10-day operational cycle, ‘real-time’ data delivery) for the program. If theArgo floats are deployed from an R/V conducting a regional process study the Argo floats might,for example, be targeted for regions outside of the focus area of the study. Such deployments shouldbe of clear benefit to both experiments - Argo gaining a deployment opportunity and the regionalstudy gaining an improved broad-scale context provided by Argo data.

2. Float communications.To the extent that non-Argo floats provide Argo-equivalent data, then Argo should assume

costs of data transmission following the expiration of the other project’s grant. So, for example, ifa cluster of floats deployed in a process study dispersed to become a broad-scale array, and the datafulfilled Argo requirements, then Argo clearly benefits by assuming transmission costs that cannotbe supported by the original program’s grant. However, floats deployed in shallow coastal areas ormarginal seas, or remaining in clusters are of lower priority to Argo.

Real-time processing and distribution of the data would be implemented by the U.S. ArgoData Center. The Principal Investigator would need to provide the Data Center with the meta-datanecessary to process and quality control the real-time data. The operational delayed-mode dataprocessing, and maintenance of the Argo data on the GODAE server, could be handled to somedegree by the program. The final quality control check in the U.S. Argo program that results in the‘best’ profile resident on the GODAE server involves the participation of the principal investigator

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responsible for the float of interest. The U.S. program does not have a mechanism or funding tosupport a non-U.S. Argo Principal Investigator for delayed-mode processing of data.

I hope this addresses your questions. If you need any further clarification, please send me anemail or call (301-614-5696). Please let me know if you need a signed paper copy of this letter foryour proposal.

We wish you the best success with your proposed program. Thank you for your interest in theUS Argo Project.

Sincerely,

Chet KoblinskyCo-Chairman U.S. Argo Advisory Panel

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Date: Fri, 8 Feb 2002 08:59:28 -0500 (EST)From: Rainer Bleck <bleck@asterix.rsmas.miami.edu>Reply-To: rbleck@rsmas.miami.eduTo: Sarah Gille <sgille@ucsd.edu>CC: rbleck@rsmas.miami.edu, Mathew Maltrud <maltrud@lanl.gov>Subject: Re: isopycnal mixing from floats

Dear Sarah,Your proposal to deploy floats to study mixing on isopycnal surfaces in the Southern Ocean is of

considerable interest to the isopycnic modeling community, and to me in particular. At Los Alamoswe are presently testing MICOM as well as HYCOM, its hybrid coordinate cousin, in a near-globaldomain at 0.225 deg x cos(latitude) resolution. We have been experimenting with various densitycoodinate choices (sigma 0, sigma 2 with and without thermobaric correction) and intend to start arun based on sigma 1 in the near future. This latter run would allow a very direct comparison withyour float observations. For this purpose, we will outfit the model with an online float simulator.

Many of our assumptions concerning the intensity of lateral stirring are based on numericalnecessities and have not been systematically tested by direct comparison to observations. I amlooking forward to the opportunity to tackle this issue with your help. Let’s both hope that fundingfor this project will materialize.

Best regards, -r.

– (home: 505-662-2368)Rainer Bleck (bleck@lanl.gov) voice: 505-665-9150 fax: -667-5921Los Alamos National Laboratory Mail Stop B296 Los Alamos, NM 87545

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