Ocean eddy structure by satellite radar altimetry required for iceberg towing

Cold Regions Science and Technology, 1 (1980) 211-221 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

211

OCEAN EDDY STRUCTURE BY SATELLITE RADAR ALTIMETRY REQUIRED FOR ICEBERG TOWING

W.J. CAMPBELL U.S. Geological Survey, Tacoma, WA 98416

NASA/Goddard

R.E. CHENEY J.G. MARSH Space Flight Center, Greenbelt,

University

MD 20771

N.M. MOGNARD of Washington, Seattle, WA 98015

Abstract

Several satellite programs presently being

Models for the towing of large tabular

planned call for flying radar altimeters

in

icebergs give towing speeds of 0.5 knots to

polar or near-polar orbits in the mid-1980

1.0 knots relative to the ambient near sur-

time frame.

face current.

icebergs will probably be attempted,

indicates

Recent oceanographic

research

that the world oceans are not

principally

possible synoptic observations

composed of large steady-state

current systems,

Thus, by the time tows of large

like the Gulf Stream, but

it is

of ocean

rings and eddies which can be used to ascertain their location,

size, intensity,

and

that most of the ocean momentum is probably

translation velocity will be a reality.

involved in intense rings,

I.

formed by mean-

ders of the large streams, and in mid-ocean eddies.

These rings and eddies have typi-

cal dimensions

on the order of 200 km with

Introduction When the problem of towing icebergs as

a fresh water source was first approached a quantitative manner approximately

in

a decade

dynamic height anomalies across them of

ago, knowledge of the mesoscale structure of

tens-of-centimeters

the surface currents of the world oceans was

to a meter.

They

migrate at speeds on the order of a few

scant.

cm/sec.

believed that most of the momentum of the

Current velocities

knots have been observed

as great as 3

in rings, and

currents of i knot are common. successful

currents was contained

Thus, the

towing of icebergs is dependent

on the ability to locate, measure, ocean rings and eddies. systematically

and track

To accomplish

At that time it was generally

this

on synoptic scales appears

in the flow of large

systems like the Gulf Stream and the Kuroshio Current.

Consequently,

the early

iceberg towing models of Weeks and Campbell (1973) and Hult and Ostrander the ocean as a steady-state

(1973) treated

system and used

to be possible only by using satellite-

seasonal mean charts of dynamic height

borne radar altimeters.

anomalies

eddy structures

Ocean current and

as observed by the radar

to construct

transit trajectories.

These crude first approximation

studies

altimeters

on the GEOS-3 and Seasat-i

concluded with the exciting result that it

satellites

are presented and compared.

did indeed appear possible to tow icebergs

212

to certain areas.

However,

the assumption

equal to or greater than the above ambient

of a steady-state current system was seen by

current velocities at which icebergs will

Weeks and Campbell as a key shortcoming in

be towed.

Successful iceberg towing will

their "analysis of a geophysically difficult

be dependent upon one's ability to locate,

problem.

measure, and track ocean eddies and rings.

In particular,

it can be argued

that the use of seasonally averaged maps of

It will be imperative to steer icebergs into

Antarctic winds and ocean surface currents,

the sector of an eddy where its velocity is

produced as they were from an admitted

in the direction one wishes to go, since to

scarcity of data, will give results that

steer it into the wrong sector would mean

would not hold for the actual case of ice-

that the iceberg would go nowhere or back-

berg drift under the influence of short

wards.

time scale events."

be one where the berg is pulled/pushed from

Recent oceanographic studies such as the Mid-Ocean Dynamics Experiment

Thus, a successful iceberg tow will

eddy-to-eddy within the moving eddy field

(MODE

in such a way that the maximum cumulative

Group, 1978) have revealed that the surface

ocean current velocity vector in the pre-

of the oceans is not made u p p r i n c i p a l l y

ferred direction is obtained.

of

This tech-

large steady-state current systems but that

nique can be called "preferential eddy

most of the ocean momentum is probably

jumping."

involved in intense rings, formed by cur-

seen to be absolutely dependent on one's

rent meanders,

ability to use ocean eddies and rings.

and in mid-ocean eddies.

These rings and eddies have typical dimen-

In short, iceberg towing is now

To accurately locate, measure, and

sions on the order of 200 km with dynamic

track eddies and rings on a synoptic scale

height anomalies across them on the order

at sufficiently frequent time intervals

of tens-of-centimeters to a meter.

throughout the world oceans seems to be

They

migrate at speeds on the order of a few

cm/sec.

radar and/or laser altimeters.

This being

possible only by using satellite-borne

discovery

the

dominant

circulation iceberg

mode

is a very

towing.

Campbell

of mesoscale

(1973)

of Job

(1978)

towing

velocities

velocities

The

early

model

and

suggest

range

of ocean important

the

that

above between

surface

flown on the GEOS-3 satellite launched in

for

and

recent

optimum

ambient 0.5

major radar altimeter equipment in space was

one

Weeks

April 1975 and one of the most exciting discoveries with this system was that it

one iceberg

current

knots

cm/s) and 1.4 knots (70 cm/s).

The first

eddies

(25

The current

was possible to directly observe ocean currents and rings.

An improved radar altim-

eter was flown on the Seasat-i satellite, which unfortunately operated for only the three summer months of 1978 but succeeded

velocities observed directly within eddies

in obtaining excellent data.

and inferred by satellite radar altimeter

we present radar altimeter results from both

measurements of their dynamic height

of these satellites.

anomalies are normally in the order of 1

II.

knot (50 cm/s) and are as great as 3 knots (150 cm/s).

Therefore,

the current velo-

In this paper

TechniRue Figure 1 is a schematic diagram showing

the various factors which must be considered

cities within the eddies, which appear to

in the determination of sea surface topo-

exist in all the world oceans, will be

graphy from satellite radar altimetry.

The

213 observed altitude of the spacecraft above

the sea surface with respect to a reference

the ocean surface at any instant must be

ellipsoid with the origin at the center-of-

corrected for a variety of phenomenon such

mass of the earth.

as tropospheric refraction,

and the equation shown in Figure I show all

tides and the

The schematic diagram

offset of the antenna from the center-of-

the factors which must be considered in this

mass of the spacecraft.

computation.

Orbit computations

A model of the geoid or mean sea sur-

based upon precise laser ranging data and electronic Doppler data are used to correct

face for the Western North Atlantic Ocean

for satellite perturbations and thus pro-

off the East Coast of the U.S. has been

vide a means for calculating the height of

recently computed based upon a combination

SEASAT CENTER OF GRAVITY ANTENNA ELECTRICAL CENTER

r

D G : GEOMETRIC DISTANCE

I"

H: TRUE RADAR RANGE

h:

HEIGHT ABOVE ELLIPSOID

I

pJ

°~

ELECTRICAL SEA SURFACE GEOMETRIC SEA SURFACE

j ~

~

CORRECTIONS DD : DRYTROPOSPHERE Dw : WET TROPOSPHERE D i :IONOSPHERE D E :INSTRUMENT DELAY

D A : ATTITUDE

) DSwH'SlGNIFICANTWAVE

_...~.___....._.~_!DT.:TIDEs HEIGHT

) D B " BAROTROPIC PRESSURE MEAN SEA LEVEL

~_~.~) .

REFERENCE ELLIPSOID

GH=h-

Ds :STERICANOMALY

GH: APPROXIMATION TO J ~ GEOID HEIGHT

D G -- H + D D + D w + D ~ + D E

--D A --Dsw H --D T-D

B-D s

Figure i - Factors involved in satellite radar altimetry measurements.

214

of satellite derived and surface-observed

22 m relative to the northern coast of

gravity data (Marsh and Chang, 1978).

Puerto Rico is noted.

this computation,

In

By combining the altimeter measured

the satellite-derived

gravity data were used to describe the long

height of the satellite above the reference

Wavelength geoid features (> i000 km) and

ellipsoid obtained from precision orbit

surface data averaged over 5' x 5' areal

computations, one obtains the height of the

blocks were used to provide information on

sea surface with respect to the reference

the short wavelength features.

ellipsoid.

(i)

map of this surface,

A contour

(i.e., the height of

This surface departs from the

equipotential surface by i to 2 m due to

the geoid above the reference ellipsoid),

the effects of dynamic ocean processes,

is presented in Figure 2.

Several features

e.g., currents, eddies, and tides. Thus by

are apparent in this map.

The Hatteras

comparing the altimeter-derived geoid

abyssal plain appears as a relatively flat

heights with gravimetrically-determined

area in the central portion of the map.

geoid heights, signatures due to these

In the vicinity of Bermuda a feature with

phenomena can be calculated.

a height of about 5 m covering an area of about i° x i ° is noted.

Detailed surface gravity data are not

Over the Puerto

available on a global basis, however with

Rico Trench a geoid trough with a depth of

the comprehensive coverage provided by the

3~ 3| 3~ 34 32 3~ 2J 26 24 22 20 10 16 276

280

282

284

286

288

290

292

294

296

298

300

Figure 2 - NASA/Goddard detailed gravlmetrle geoid based on surface gravity data and the GEM-8 Earth Model. Contours are meters relative to the reference ellipsoid, (from Marsh and Chang, 1979). (i):

(') refers to minutes of longitude/latitude

215

Seasat and GEOS-3 altimeter systems it is

height difference between the passes is

now possible to compute mean sea surfaces

obtained directly.

based upon data collected over a period of

data set of this nature was collected during

several years which will average out the

the last month of the Seasat Mission.

short period transient effects.

ing this time period, the orbit was maneu-

An example

A particularly important

Dur-

of such a mean sea surface for the Northwest

vered so that a repeat of the groundtrack

Atlantic is presented in Figure 3 (Marsh,

was obtained every three days.

et al., 1979).

of this data is presented later in this

This surface is based upon

An example

over 400 individual tracks of GEOS-3 altim-

paper.

eter data collected over a period of about

III. Altimetric Observations of Ocean Fea-

two years.

The precision of this surface

is in the sub-meter range.

The deviations

tures in the Gulf Stream System Potential applications of satellite

of individual tracks or seasonal mean sur-

altimetry can be best demonstrated in the

faces with respect to the overall mean sur-

Western Sargasso Sea and Gulf Stream region.

faces will be due to the presence of time-

Not only is the geoid known within sub-meter

dependent effects which have been averaged

accuracy here, but the circulation is well

out in the computation of the mean surfaces.

documented and has strong surface topography

Another technique which can be employed

expression.

Difference in dynamic height

to detect eddies and other time-dependent

across the Gulf Stream, as inferred from the

phenomena is the inspection of collinear

sub-surface density structure,

passes.

order of 1 m.

By subtracting such passes one

from another,

the time-invariant geoid

signal is removed and the dynamic ocean

is on the

One of the most interesting aspects of the Stream is its horizontal wave motion in

<

Fisure 3-- Contour map of the ocean surface derived from GEOS-3 altimeter crossover data (after Marsh, Martin, McCarthy, and Chovitz, 1979).

216

the open ocean.

Several times a year elon-

the warm rings can be readily determined

gated meanders split off to form detached

from satellite infrared imagery.

current rings.

rings, however, usually lose their surface

Rings occur on both sides of

The cold

the Stream and sometimes last two years.

temperature expression a few months after

Those that form to the south are cyclonic,

formation.

cold-core rings, and are associated with

of particular interest for the detection of

sea surface depressions of the same magnitude

these features, whose distribution and move-

as the difference across the Gulf Stream.

ment are not well known.

Similarly, warm rings north of the Gulf Stream represent sea surface highs.

Satellite altimetry is therefore

As described in the previous section,

As

the most direct method of obtaining the

many as 12 rings have been observed in a

ocean dynamic signal from altimetry data is

4-month period (Richardson, Cheney, and

to subtract out the best available gravi-

Worhington,

metric geoid.

1978).

During cloud-free 9on-

ditions, locations of the Gulf Stream and

Figure 4 shows a Seasat pass

through the western North Atlantic and the

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Fisure 4 - Altimeter data from Seasat on August 6, 1978. When the data are differenced with the geoid surface along the same track, the result is dynamic topography due to ocean currents (low profile). Comparison with independent observations of the Gulf Stream, rings, and no anomaly regions indicates good agreement.

217

corresponding profile from the 5 ~ detailed

few cm.

geoid (Figure 2).

the geoid model and the altimetry can be

The difference between

the two is shown below.

Overall tilt and

For this limited region, at least,

combined to yield reasonably accurate

bias of the residual are due primarily to

results.

inaccuracy of Seasat's orbit.

should be possible to construct maps of sea

Shorter wave~

Given sufficient

data density,

length features are shown to correspond well

surface topography representing

with independent

ditions over periods of a few weeks.

satellite infrared and ship~.

board observations

indicated along the track.

it

average con-

A second technique is to approximate

The Gulf Stream appears as a 115 cm step with

the geoid with an altimetry-derived mean sea

a width of 95 km

surface.

and its north edge agrees

This surface contains both oceanic

well with that indicated by surface tempera~

and gravitational

ture.

aged over a long enough period, transient

An average slope of 1.2 x 10 -5 across

the Stream corresponds

to a geostrophic

cur-

components,

but when aver-

features such as current rings tend to be

rent speed of 133 cm/s, and a maximum speed

smoothed out. Altimetry observations

of 230 cm/s is implied by a steeper slope

shorter intervals can then be differenced

across the northern half of the Stream. Just

with the long-term average surface to search

south of the Gulf Stream is a 30 cm depres-

for anomalies.

sion which may be the edge of a cold ring

and Parra

known to be in the vicinity.

from 6 months of GEOS-3

Further south

In a study by Huang, Leitao,

(1978) a mean surface was derived altimetry (July to

at 33.5°N another cold ring stands out

December 1975).

clearly as a 65 cm depression.

also constructed for each month.

The region

over

Individual surfaces were The six

between 30°-28 ° appears relatively flat in

sets of monthly differences were examined

accordance with oceanographic

for characteristic

observations,

cold ring depressions.

indicated in Figure 5, a series of altimetry-

which indicate no anomalies greater than a

T

-

-

~

m

A

te~

CAPE Jt~LY

/

N)V N"m

/ I

7B

Ft //A eM I

I

74

MOVEMENT OF RING D 1975-1976 I

As

I

72,

I

I

70

I

I

68

I

66

Figure 5 - Movement of a cold Gulf Stream ring as observed with standard oceanographic measurements (dashed line) and satellite radar altimetry (shaded areas). Dots indicate position of the ring at the beginning of each month, whereas shaded areas represent average monthly positions (after Huang, Leitao, and Parra, 1979).

218

derived cold ring observations corresponds

A third technique for arriving at the

quite well with the previously documented

ocean dynamic signal is to use collinear

movement of a cold ring (Richardson, Cheney,

passes, which contain identical gravita-

and Worthington,

tional components.

1978).

These results are

By subtracting one pass

remarkable considering the 30-50 cm accuracy

from another the dynamic height difference

limitation of the GEOS-3 altimeter and the

between the two passes is obtained.

relatively few number of passes available

method can be used to study time-varying

(163) for this six month period.

phenomenon such as current meandering, trans-

currents,

Permanent

such as the Gulf Stream, are not

port variations, and transient rings and

as well suited for study by the mean sea

eddies.

surface technique.

Figure 6.

Meandering causes the

Gulf Stream to appear blurred, with perhaps twice its normal width,

This

An example from Seasat is shown in On September 17 Seasat flew di-

rectly over a cold ring (within 18 km of its

in any averaging

center)

technique.

located at 33.5°N, 68.8°W.

Three

weeks later on October 8 when Seasatrepeated --44 COLLINEAR SEASAT PASSED

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1 3, o°m 67.2°W

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LATITUDE

Fisure 6 - Two collinear Seasat altimetry passes, three weeks apart. The first passed over a cold ring at 33.5°N which was not present during the second pass. When subtracted to remove the gravitational component, the ring is clearly evident as a 40 cm depression, 275 km wide.

219

its track, the ring had moved westward and

Vladimirov (1978) who surveyed a cyclonic

was centered 126 km away from the satellite

ring south of Australia.

path.

dence of ring/meander activity along the

One would expect the characteristic

Historical evi-

sea surface depression associated with cold

entire length of the ACC has been presented

rings to be in the first pass but not the

by Lutjeharms and Baker (1979).

second.

When the two are subtracted a dis-

Based on these observations it is

tinct dip is found at 33.5°N, clearly due to

possible to construct a general description

the cold ring.

of cyclonic rings that originate from

The ring has a 40 cm dynamic

height signature and a width of approximately

meanders of the Polar Front.

275 km.

typical diameters of 100-200 km and surface -i current speeds of approximately 40 cm sec

The slope of its sides indicates

average surface current speeds of 34 cm -i sec IV.

They have

There is little vertical shear of the cur-

Potential Application in the Southern

rent in the upper layers, but between 500-

Ocean

i000 m current speed begins to decrease and

The Antarctic Circumpolar Current (ACC),

reaches zero at about 2000 m depth.

When

which encircles the Antarctic Continent, is

first formed, cyclonic rings in the Southern

the ocean's most powerful current.

Ocean possess sea surface temperature gradi-

Its net

eastward transport is on the order of 200 x

ents of a few °C but, like cyclonic Gulf

106 m 3 sec -I (Callahan, 1971), several times

Stream rings, these apparently disappear

greater than that of the Gulf Stream.

with time.

This

Their sub-surface structures

large volume flux is due not to the current's

are dramatic.

speed, which is in the range of only 25-50 cm -i sec , but to its size. The ACC consists of

approximately 500 m at the ring center and,

multiple currents spread over several de-

colder than outside.

grees of latitude and extending to 2000-

the rings migrate a few km/day northeastward

3000 m depth.

toward the Subantarctic Front, where they

Southwest of Australia along

II5°E, for example,

the current system oc-

cupies the region between 47°-52°S 1977).

(Emery,

The flow coincides with a band of

Deep isotherms are uplifted

at the core, 200 m temperature is 2°-4°C It is believed that

may eventually coalesce. Monitoring rings in the Southern Ocean appears to be a difficult task using stand-

temperature and salinity gradients generally

ard oceanographic techniques.

known as the Antarctic Polar Frontal Zone.

temperatures gathered from satellites can

The strongest current jets are found at the

undoubtedly provide useful information, but

northern and southern boundaries of this

detection of rings will be limited by their

zone and are called the Subantarctic and

relatively small surface thermal gradients,

Polar Fronts, respectively.

not to mention the relatively persistent

It is likely that the Antarctic Polar

Sea surface

cloud cover in the region around the con-

Frontal Zone is populated with rings or

vergence zone.

eddies formed from current meanders

ships are too scarce to be of value.

Gordon, and Molinelli,

1978).

(Taylor,

Direct evi-

Reports from transiting Satel-

lite tracked drifters or moored buoy sta-

dence of these rings has recently been ob-

tions could be used, but a large number of

tained by Joyce and Patterson (1977), who

instruments would be needed.

cbserved a cyclonic ring forming in the Drake Passage, and by Savchenko, Emery, and

Because Antarctic rings have dynamic height signatures of approximately 30 cm,

220 however,

satellite altimetry provides a

sensing techniques for observing these phe-

means of periodically mapping the eddy field

nomena under all weather conditions and

on synoptic scales.

during night or day are exciting.

Little is known about

the decay of these rings, but if they are

Mognard and Lago (1979) and Fedor,

at all similar to those formed by the Gulf

et al. (1979) have shown how the instan-

Stream, one might expect them to maintain

taneous return signal from the sea surface

relatively strong dynamic height gradients

to the satellite radar altimeter can be

throughout their lifetimes.

analyzed to give mean wave height and sur-

The problem is

made more difficult by the fact that bottom

face wind speed.

These studies used data

topography appears to be an important factor

from the GEOS-3 satellite, which did not

in ring generation (Lutjeharms and Baker,

carry a scatterometer as did Seasat-l.

1979); rings are therefore most common in

Pierson, et al. (1978) have used scatter-

regions having complex geoid surfaces.

ometer data from Skylab to measure sea

Success of this technique will probably de-

surface wind speed and direction.

pend upon the accuracy with which mean sea

coupled use of satellite scatterometer and

height models can be constructed in this

radar altimeter observations can give ac-

area using altimetry data.

There is little

The

curate information on sea surface mean wave

hope of obtaining a detailed gravimetric

height and wind velocity, as the analyses

geoid in the near future, but because rings

of the Seasat-i data will soon show.

are transients,

the mean surface method

The unfortunate early demise of Seasat-i

demonstrated by Huang, Leitao, and ParrN

and the anticipated demise of the long-lived

(1978) may provide a solution.

radar altimeter on GEOS-3, which greatly

Seasat data

are presently being studied with this appli-

exceeded its design life-time, puts us in

cation in mind.

the sad position of not having a radar al-

V.

Conclusions

timeter in orbit for several years, a period

Satellite altimetric observations of

in which test tows of icebergs might occur.

the sea surface can potentially provide ac-

However, several satellite programs pres-

curate measurements of ocean rings and

ently in the planning stage call for flying

eddies, their locations, sizes, intensities,

radar altimeters in polar or near-polar

and translational velocities, scales.

on synoptic

For the successful towing of ice-

bergs as a water resource such information is essential.

Yet for this important task

orbits in the mid-1980 time frame. by t h e t i m e

Thus,

iceberg towing starts in earnest,

synoptic satellite remote sensing of ocean eddies, rings, and currents and the wind and

satellite altimetry can provide equally es-

wave fields associated with them will hope-

sential information on ocean phenomena other

fully be a reality and will greatly enhance

thansea

surface topography, namely the

waves and wind.

Indeed, wave and wind

forces on icebergs along w i t h w a t e r

drag and

ocean current forces are the dominant forces controlling iceberg dynamics.

Since icebergs

exist in environments where accurate synoptic scale data on surface wind and'waves is either scarce or non-existent,

the new remote

the possibility for successful tows. References Callahan, J.E. (1971), Velocity structure and flux of the Antarctic Circumpolar Current south of Australia, J. Geophys. Res., 76: 5859-5864. Emery, W.J. (1977), Antarctic Polar Frontal Zone from Australia to the Drake Passage, J. Phys. Oceanosr.,~, (6):811-822.

221

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