FEATURE

THE SLOCUMMISSION
Narrative and Illustration
By Henry Stommel"3I h e payoff in increaseof knowledge often is
greatest the moreunconventional theidea, especially when it
conflicts with collectivewisdom.

I T IS DIFFICULT to realize that twenty-five years
have passed since I first came to the Slocum Mission
Control Center on Nonamesset Island, one of theElizabeth Islands, in 1996. I was a post-doc in physical oceanography, and the Department of the Environment had just acquired the island from the descendants of a sea captain prominent in the China trade ofthe early nineteenth century. The government acquired Nonamesset to establish the World Ocean
Observing System [WOOS], a facility capable ofmonitoring the global ocean, using a fleet of small
neutrally-buoyant floats called Slocums that draw
their power from the temperature stratification of theocean. Nonamesset Island was chosen partly because
it is isolated from the mainland of Cape Cod, but
mostly because it is close to the Woods Hole Oceanographic Institution, the Marine Biological Laboratory. and a thriving scientific community.
Nestling low in the hills is the Mission ControlCenter itself, with its satellite antennas. Along thebeach, facing Buzzards Bay, there are a few houses
for a small permanent staff. Most of the scientificstaff commute from their homes in the Upper Capearea, reaching Nonamesset by a ferry. There is a large
dock at the cove, but the one that the ferry uses is at
Sheep-Pen Harbor on the Hole. There are no automobiles on the island. The buildings are connected by
footpaths and there is a narrow-gauge railroad formoving supplies from the docks to the facilities. Forthe most part the island has been left undisturbed. Itis in a pristine state of beauty, tranquil beyond theexperience of those who swarm about on the mainland.
The Slocum float is named after Joshua Slocum,
the Yankee skipper who first went around the world
singlehanded in a small sailing vessel. There wereSlocums on the Elizabeth Islands before, ever since
Peleg Slocum of Dartmouth purchased Cuttyhunk,Nashawena and Penikese in 1693. Whether Joshua
was related to them, I never have discovered. But myrelationship with Slocums has a different genealo g y - - a scientific and technical one. Perhaps I should
begin by saying what Slocums do.
They migrate vertically through the ocean bychanging ballast, and they can be steered horizontally by gliding on wings at about a 35 degree angle.
They generally broach the surface six times a day toHenry Stommel, Woods Hole Oceanographic institution, Woods
Hole. MA, 02543.

22contact Mission Control via satellite. During brief
moments at the surface, they transmit their accumulated data and receive instructions telling them howto steer through the ocean while submerged. Their
speed is generally about half a knot. There are military applications for them, but our work in WOOS isunclassified. We have a fairly large fleet of Slocums,about 1,000. Half are devoted to a program of routinehydrographic observation, much like the meteorologists' upper air network. The rest make soundings oftemperature, salinity, oxygen, nutrients, and thosegeochemically important tracers that the geochemists have been clever enough to find automatic measuring devices and sensors for. The other half of theSlocum fleet is devoted to purely scientific purposes:special research programs carried out under the instructions of academic scientists, by contract. Slocums
were originally designed with a 5-year lifetime, butmany have been in continuous service at sea for morethan 10 years. They are widely dispersed throughout
the world's ocean.
Our WOOS center and the Slocum Mission had
their start because of the growing concern with
monitoring the environment: Is the ocean heating
up? Where are the pollutants going? Can we construct theoretical models of the ocean circulation thatare useful in predicting the direction of climatechange? With a necessarily small fleet of researchships, how could numerous widely dispersed measurements throughout all depths of the ocean be obtained on a routine basis? In 1995, much of theoceanographic community had been involved for 15years in the World Ocean Circulation Experiment(WOCE), exploring the general circulation of theocean but still using the technology of the 80's. For
example, one of the keystones of WOCE was theWorld Hydrographic Program (WHP), which used asingle ship per year over a period of 12 years tosurvey 48 long hydrographic sections, with somerepeats. Only six stations each day could be occupied, giving a very low rate of data acquisition
compared to what the meteorologists were gettingfrom their upper air network.
A really new method was needed, one that would
provide subsurface data on a scale and at a frequency
that matched what remote sensing by satelIite provided for the sea surface. Multiplying the number ofships by a factor of 100 was economically out of thequestion. But a pioneering ocean engineer had adifferent vision of how to garner a harvest of data on
a deep-ocean global basis, and this led, after a few
vicissitudes, to the Department of the Environment's
determination to support the Slocum Mission and thepresent deployment of Slocums throughout the world.
It has been my career.
So here I am on a lovely October day sitting in thelibrary of Mission Control overlooking VineyardSound. On the grass bank outside the window there
is a flock of sheep grazing. I have the daily logbooksof those first deployments and experiments made in
the early days of Mission Control before me on theOCEANOGRAPHY-APRIL-1989

.

(7Each Slocum reports"-T.'_;~"~.- ---.

~l '~ll,7 ~ , - ~--~---= - ~ . . - _"'¢

Iinto Mission Control via
satellite about six times

The Slocum Mission Control Center on Nonamesset Island.

table. The assembled staff has asked me to describe
the excitement of those early days on a personallevel, so that is what I am going to try to do.
ASMOST of you know, the backbone of ourclimate monitoring capability is our permanent fleetof 480 Sentinel series of Slocums. These are able tooccupy, on a monthly basis, all the 48 hydrographic
sections that took the WHP 12 years to do once. We
acquire data from this widely dispersed network via
satellite telemetry. It is something of a wonder tofollow the intricate patrol pattern of the Slocums
assigned to the various sections. For example, there
are ten assigned to patrol a section along 24°N in theAtlantic. Each patrols a six degree segment of longitude. Pairs meet once a month to do an intercalibration. Sometimes we schedule an intricate cotillionlike dance of the Sentinel Slocums in which theyinterchange patrols, thus obtaining, over the courseof a year, a complete intercalibration of all Slocums
on the section. We also have special Calibration
Standard Slocums that wander randomly about thepatrol lines, like the inspectors on a subway system,
calibrating all Slocums that they encounter.
Each Slocum reports into Mission Control via
satellite about six times a day. At each report, conductivity, temperature and depth (CTD) data aretransmitted, along with auxiliary tracer data. The
satellite also reports the Slocum's position, which
informs us how much the float has been set by theocean currents it encounters and permits its automatic pilot to be reset according to a course-correction algorithm. During the day, the casts terminate at
OCEANOGRAPHY.APRIL-1989

a day.

different depths, and sometimes the floats dwell fora period at the bottom of the cast, so it is possible torecover information about the vertical modal structure of the currents.
Thus we have a basic set of Sentinels routinely
patrolling the ocean between 50°S and 50°N, equivalent to a fleet of 48 full-time hydrographic survey
ships, making no port stops, using no fuel, and byinternational law exempt from the restrictions onpassage through territorial waters exacted of manned
vessels. Each day we get about 3000 CTD profiles
from our Sentinel fleet.
The prototype Sentinel was launched in 1994,
before the Mission Control Center was built, and thelittle group of originators was still working in theattic of the Bigelow building at the Woods Hole
Oceanographic Institution. I can only imagine what
the sense of excitement must have been in the attic
control center when they started to control the navigation of Sentinel 1 after its launch off Bermuda.
According to the logbook, it was decided not toconfront the Gulf Stream at the start. The plan was tosteer the Slocum as nearly eastward along 32°N as
could be done, making a highly detailed hydrographic section along the way. With some delays due
to interrupted communication, this section took 198
days. It ended off lfni on the Northwest African
coast, where the RRS Discovery had ended her 1957
section long before. The ship managed to obtain 25
casts. Sentinel 1 obtained 3820 casts. On this leg theprototype successfully demonstrated that effectivehydrographic station work of a routine nature could
be done remotely by an unmanned, unfueled instru23

There is nothing likethe need to makedecisions to lay bareareas of ignorance thatare papered over in
textbooks.

24

Tent package, directed by controllers far away. The
atmosphere in the altic must have been e c s t a t i c - rather like that in the spacecraft control room when
Voyager 2 rendezvoused with the moon of Neptune
in August 1989. That first Slocum spawned thewhole fleet of moniloring Sentinels thai we have inthe ocean today.
But Sentinel 1 had further successes. On a returnwestward section the following year, it encountereda Mediterranean salt-lens eddy. According to thelogbook, the eddy was first detected in the velocity
structure. It was decided to go eddy chasing. Soon thehigh salt at 1200 m depth became evident. Sentinel 1became trapped by the eddy, but after three revolutions it was shaken loose, and the controllers were
now faced with an interesting p r o b l e m - - h o w torelocate the eddy and implant the instrument package
in it again. In those days there was very little information about the nature of the variability of the background current field, and it was difficult to find theeddy. Help was proffered by some numerical modelers, who had set up a dynamical model on the basisof the data obtained during the first three revolutions
before the eddy was lost. Their model suggested ageneral amplitude of pulsation and a direction ofdrift. As it turned out, the search was successful, andfor a further six years, the eddy was tracked, measured and studied as no eddy had ever been before. Onthe basis of this study, the Mediterranean Water
Experiment during lhe years 2003-2013 was designed and conducted with spectacular s u c c e s s - revealing an unexpected physical process of tracer
dispersal, and a bizarre mathematical way of parameterizing it in general circulation models.
One of the na'fve early experiments with S l o c u m s - betk)re the Department of the Environment assumed
responsibility for them and built the Nonamesset
facility--was the International Round-The-World
Race of three Slocums. At the time, the whole idea ofparticipating in a race sounded a little disreputable to
the rest of the scientific community, but a sense ofadventure reinforced by a lack of alternate funding
drove the Slocum advocates to accept the invitationof the New York Yacht Club, and of Australian andFrench equivalents, to build three Slocums for thepurpose. They were controlled from the attic office,but according to instructions received from the threerace committees. The U.S. committee had originallysuggested that the race be from Bermuda to Bermuda, but it was finally decided to begin and end atHawaii, since it was suspected to be more difficult tochoose good north-south courses in the Pacific thanin the Atlantic. Each committee was given the positions of all three Slocums. Because the Slocums did
not have to carry CTD and tracer sensors, they costless than $100,000 each, and funds of $200,000 ayear for the attic control center were guaranteed forfive years.
The race was a great popular success. Television
programs carried news of the progress of the race ona weekly basis. Lay people became familiar withequatorial undercurrents, western boundary currents,
and the Antarctic Circumpolar Current. Mesoscaleeddies became a part of everyday language. TheNational Geographic Society made two television
documentaries about the contest, Theoretical pundits
offered their advice to all sides, National computer
centers confidentially advised their national racecommittees. Cadets at Annapolis tried to organize alast minute entry of their own. The French won witha spectacular score of 708 days.
The race showed that one could develop skill inprogramming the Slocums. Having to decide what
heading to choose stimulated modelers and descriptive oceanographers to exercise their minds and theircomputers. As the results of decisions became obvious, knowledge grew, and as knowledge grew, performance improved. The challenge of the race inspired a marvellous exploration of geography, phenomenlogy, dynamical theory, computer programsand technical development. There is nothing like theneed to make decisions to lay bare areas of ignorance
that are papered over in textbooks. Suddenly one isfaced with the practical problem of finding a westernboundary current, riding internal Kelvin waves, ortraversing an unknown oceanic region, learning about
it as one goes.
I N LATER YEARS, the early U.S.-AustralianFrench race paid handsome dividends. It evolved
into our program of Explorer Slocums, today the predominant scientific program of the Mission Control
Center. More than 300 Slocums are dedicated toseveral dozen scientific studies at any one time.
There are presently 40 devoted to study of the recirculation regions of both the Gulf Stream and Kuroshio. To date these instruments have collected information for synoptic descriptions and mapping andhave gathered statistics about low frequency variability for over 15 years in each region. The result hasbeen a remarkable improvement in physical understanding and numerical modeling in both these regions. Other groups of Slocums study equatorialdynamics, map overflows and western boundary
currents in remote oceanic regions, and increase theprecision of local data bases, for example.
Some of these Explorer projects have gone on fora decade or more. One of the earliest was a study ofthe circulation in the equatorial Indian Ocean, suggested in 1988 by a few far-sighted oceanographers.
Several Slocums were launched in the western Pacific off Mindanao. They were steered into various
passages in the Indonesian Archipelago, such as theBanda Sea, and then held in position by gliding
against the prevailing currents in such a fashion as tomonitor the transport of water from the Pacific into
the Indian Ocean through the Archipelago. This partof the project lasted two years, until statistically goodtransport estimates had been obtained, and then theSlocums explored the water mass front near 10°S
latitude in the Indian Ocean, in particular that part ofthe circulation that passes toward the west, north ofO C E A N O G R A P I I Y - A P R I I . o I t~89

Cape Amber, Madagascar. It was found that most ofthe time this water flows northward across the equator to join the Somali Current. During later years, it
was discovered that intermediate waters escape fromthe Arabian Sea and manage to move southward
across the equator.
The projects that I have always liked best are theones conceived on the spur of the moment by an
inquisitive individual. We try to reserve twenty percent of our Slocums to pursue such sudden inspirations. They are generally the most exciting; they
evolve in unexpected ways and reveal new dimensions of the unknown about the ocean. We have usedsmall numbers to follow whale migrations, for example, and to decipher the language of whales. We
have conducted explorations in regions where political unrest would hamper the conduct of conventional
ship operations. And we always have delighted inbeing of service to scientists with unusual and new
ideas that demand examination, even at the expenseof the pressures to expand military obligations andmore routine civilian monitoring programs. We havefound, over the years, that the payoff in increase ofknowledge often is greatest the more unconventional
the idea, especially when it conflicts with collectivewisdom. This policy has not always been easy tojustify to our government sponsors, but they havebecome accustomed to allowing us to utilize a twentypercent fraction of the observational resources they
pay for in these imaginative risky speculative ways.
So we have our fun, and they have learned that it pays
off.
When I first came to Nonamesset the techniquesof graphic displays of data were just emerging fromthe flat screen of the cathode ray oscilloscope andliquid crystal devices of the late twentieth century.
Now, of course, the controller is literally animated. It
can be zoomed to any time or space scale, giving theobserver the impression of swimming through theocean, much like being aboard a fictional space shipfrom one of the old popular television programs. Ourcontrol room is a highly versatile submarine. We canobserve the data in a vast variety of ways. We canswitch from observations to predictions of the greatnumerical models running elsewhere; we can visualize the interplay and balances between various terms
in the dynamical equations as computed from data ormodels; we can scan the history of our subject; wecan project ourselves into the scenarios constructedby the paleo-oceanographers. In short, we can immerse ourselves in an infinite variety of ways oflooking at the real ocean and our mathematical abstractions of it. This ability is the result of twenty-fiveyears of accrued observation and skill and understanding. We can call upon the most informed scientific opinion available anywhere in the world. Andwe have become so accustomed to facile manipulation of this mass of information, we can scarcely
imagine the plodding ways of earlier days.
Y E S T E R D A Y I was shooting the breeze with acolleague who has a summer house on Cuttyhunk.
The view from his house is spectacular--nearly 270
degrees of horizon----extending from the Bourne
Bridge, all along the New Bedford shoreline to Point
Judith, and then offshore to Sow and Pigs. There isalso a wide view of Vineyard Sound over Canapitset
Channel. I guess his view beats what we see from
Nonamesset, although I do think ours is rather grandtoo. But then I walk into our control room, with its
panoply of views of the sea.
There are the updated global pictures from the remote sensors on satellites, there the evolving maps ofsubsurface variables, there the charts that show theposition and status of all our Slocum scientific platforms, and 1 am satisfied that we are looking at theocean more intensely and more deeply than anyone
anywhere else.

T h e projects thatI have always liked
best are the onesconceived on the spurof the moment by aninquisitive individual.

METEOROLOGICAL AND OCEANOGRAPHICGLOBAL OBSERVING NETWORKS" A PARALLEL
DURING THE THIRTY year interval 1945-1975, theworld meteorological communityestablished a considerable network of upper-air observing stations. By the1970's, wind observations reported by radiosondes("rawinds") were widely available for analysis of global
atmospheric circulation statistics. For example, inJanuary 1971, launches from about 600 rawind stations
were made twice daily in most cases. The stations wereunevenly distributed and concentrated near major landmasses, mostly in the Northern Hemisphere. Most ofthe wind data observations were from altitudes less than
500 mb. A few of the mid-ocean stations were, for awhile, weather ships, but many of the weather ships
eventually were discontinued because of the great
expense.
The World Ocean Observing System (WOOS) was
installed during the thirty years 1995-2025. The distribution of the Slocum fleet (includingthe routine Sentinel series and some of the special scientific Explorer
OCEANOGRAPHY-APRIL-1989

series) is not fixed geographically, because the Slocums
move about in patterns executing oceanographic sections. On a typical day, the approximately 550 Slocums
are distributed on a more or less regular rectangular 5°
x 5° grid that covers the major ocean basins from 60°N
to 60°S latitude.
Each Slocum can sample the full range of oceanographic depth six times a day. The data sets are reported
at each surface interval by means of satellite telemetrylinks. Synoptic oceanic features change more slowly
than their atmospheric counterparts, so it is possible tooptimize the collection of statistically independent databy moving the Slocums around, rather than keeping
them stationary.
In this way they can complete interconnected oceanographic sections, with an average individual length of
700 km every 35-50 days. They collect standard physical and chemical data, and they report current drifts
integrated over varying depth ranges. []25