EXPANDING THE CAPABILITIES OF ARGO-TYPE FLOATS

Stephen C. RiserUniversity of WashingtonSeattle, Washington USA

riser@ocean.washington.edu

Longevity of US Argo floats as a function of deployment year; the US longevity is consistently 5-10% higher than for non-US float programs

Presently: 2 types of US Argo floats

APEX(UW; PMEL)

SOLO(SIO; WHOI)

Argos antenna

Cowling(air and oil bladders inside)

SeaBirdCTD unit

Configuration:T, S, p sensors

ARGOS70 samples, 0-2000 m

Up to 275 profiles10-day sampling

Air bladder outside, oil bladder inside

Float technology, communications, and new sensors…

Low latitude profiling

A new type of float: SOLO-II Communications: Iridium

Other sensors: O2 , nitrate, wind speed, rainfall, SST, SSS

Profiling under sea ice

Displacement volume (cm3) necessary to profile to 2000 m for a float with the volume and mass of APEX 260 ( 240 cm3 usable).

Note the lack of necessary displacement volume for profiles to 2000 m at low latitudes. There are now remedies for this problem.

[ Note: problems within 13 of the Equator ]

Main Piston

Oil

Piston

Oil

N2 Gas

Oil Bladder

[ 200 dbar ]

Result: 330 cm3 displacement

For N2 floats, the additional oil and piston connected to the gas canister provide a means to store energy as the gas is compressed during the float descent and recover this energy and use it to increase the float’s buoyancy during the ascent phase.

An APEX N2 float…

N2 APEX float showing the main piston and pump assembly and N2 canister

[ marginal cost $400]

N2 canister

Main piston

Cowling(bladder inside)

[ see Argonautics No. 10 for more details ]

2000 m profiles….

t = 20.76

[data from 71 levels]

Solo-II Profiling Float

• Compact, lightweight, efficient.• Reduced labor for assembly. • 2000 m profiles anywhere.• Long life (~6 years).• No air bladder.• Reciprocating pump (same as

Spray glider)• Scalable (in length, batteries,

sensors).• Pumping system adaptable for

deep-ocean profiling.

Prototype SOLO-2 floats are now undergoing lab-testing; deployments soon.

SOLO-I and SOLO-II

SOLO-I SOLO-II

# of dive cycles ~180 ~200

Energy (kJ)/divew/SBE-41cp 22.5 10.3

Max depth (dBar) 2300 2300

Ocean coverage @ Max depth ~45% 100%

Telemetry ARGOS Iridium

Surface time (hr) 14 0.25

Mass (kg) 30.4 18.6

Main pressure-case length (in) 41 26

Seek capability Bidirectional Bidirectional

SOLO Version Comparison

SOLO-II internal view, SOLO-I in back

Dissolved oxygen measurements from profiling floats

UW float 894, with SBE O2 sensor, operated for more than 3 years in the N. Pacific and showed evidence of blooms and net O2 production (Riser and Johnson, Nature, 2008; Martz et al., L-O, in press).

2 types of O2 sensors:SBE and Aanderaa Optode

Float 8948/02 – 7/0599 profilesThe first SBE O2 sensor

worked well for nearly3 years, showing onlysmall instrument drift over that time.

[ now more than 150 floats with O2

deployed in Argo; 2 types of sensors;see O2 White Paper for details ]

Profiling float end capwith PAL hydrophone addedthat can be used to make acoustic measurements of wind speed and rainfall, ason UW float 0006

Year-long trajectory of float 0006 in the Bay of Bengal

h

2h

Wind speed and rainfall (PAL)…

[ h 650 m ]

The wind speed from the float and from QwikScat agree well at speeds below about 10 m/sec.

[similar good comparisons between float/PAL and TRMM rainfall]

Surface salinity in the Bay of Bengal can be very low due to rain events and large river input

Surface salinity in the Bay of Bengal is correlated to both position of the float in the basin and rain input events (Riser et al., L-O, in press)

TRMM

2004 2005

5 m SALINITYWIND SPEED

RAIN RATE

rivers/near coast

rainfall effect

SW

mo

nso

on

(w

et)

SW

mo

nso

on

(w

et)

NE

mo

nso

on

(d

ry)

Faster communications: IRIDIUM

2-way communication with floats (mission changeable after deployment; many downloadable commands: profile depth,interval between profiles, sampling interval, etc.)

Fast communication compared to ARGOS ( 180 byte/seccompared to < 1 byte/sec)

Short times on the surface ( 6 minutes for a 55 KB, high resolution deep profile, compared to 9 hours for a low resolution 0.8 KB profile with ARGOS)

Use of new sensors and high-resolution experiments become possible

Cost per profile greater than ARGOS; cost per byte << ARGOS (in the US only)

Iridium floats (approximately 150 deployed):

An antenna suitable for both Iridium and GPS communications is required for Iridium floats, as well as numerous changes to internal electronics.

CTD data from UW Iridium float 5037, showing high resolution T and S data (p = 2 dbar) over a 2000 m profile (1000 T/S/p samples); surface time 6 minutes

✭

1000 m parking depth

[data from 1000 levels]

CTD and O2

data from float5209 in the Bayof Bengal

Uses of Iridium….

Drift-phase sampling

More complex sensors that require shore-based processing

Applications requiring 2-way communication

Operation under sea-ice

Aug 1 2007

Antarctic sea ice cover: recent seasonal extremes

There are now over 75 UW Argo floats in the Antarctic ice zone, with more to be deployed late in 2008. The data from these will yield new clues to the nature of ocean/atmosphere/ice interaction and climate change in the Antarctic.

Polarstern 2/25/08

Argo float positions in and around the seasonal ice zone in the Antarctic.

For the first time, large amounts of data can be collected under the Antarctic ice in winter.

These floats store profile data internally while under ice and transmit the saved profiles during ice-free periods.

The floats are programmed with an ice-avoidance algorithm.

Water properties in Oct. 2007

Water properties in Feb. 2008

Research questions: heat budget;fresh water budget; seasonal variabilityin the ice zone; oxygen uptake/production; nutrient (NO3) variability; long-term changes?

Nitrate Sensors on Profiling Floats:A collaboration between UW and MBARI

Iridiumantenna

CTD unit

Optode

Carbon fiber hull

ISUS

Spectro- photometer

UV light source

ISUSelectronics

ISUS sensor

ISUS electronics

ISUS sensor

Sensor guard

The NO3 sensor (ISUS) consists of a spectrophotometer and a light source. With3 Li battery packs, this float should be capable of about 275 profiles. If a good pH sensor was added, a nearly complete carbon budget could be constructed from a single float.

Profile 21, 3/10/08[near Hawaii]

T/S

T/O2

T/NO3

(WMO 5901468)

To examine the relationship between the near surface (5 m)salinity and the true sea surface salinity, we have recentlydeployed an Argo-type float with a 2nd CTD sensor thatcan continue to collect CTD data all the way to the sea surface,known as the STS (surface temperature/salinity) unit.

The data stream from theSTS float is designed to allowthe STS sensor to be comparedand recalibrated to the mainSBE41CP CTD unit on eachprofile, thus insuring thespecified accuracy.

This is an example of typical temperature and salinity profilesfrom the STS sensors. Note that the measurments continue to be collected until the float breaks the sea surface during its ascent.

SUMMARY:

There are a number of new technical developments taking place with profiling floats, including new float designs, new sensors, and faster communications. Some of these new features are already commercially available.

When planning for the observing system of the future, the utility of these developments should be taken into consideration, both globally and regionally.