ARGO—Fit for Purpose?

Reblogged from Watts Up With That:

By Rud Istvan

This is the second of two guest posts on whether ‘big’ climate science missions are fit for purpose, inspired by ctm as seaside lunch speculations.

The first post dealt with whether satellite altimetry, specifically NASA’s newest Jason3 ‘bird’, was fit for sea level rise (SLR) ‘acceleration’ purpose. It found using NASA’s own Jason3 specs that Jason3 (and so also its predecessors) likely was NOT fit–and never could have been–despite its SLR data being reported by NASA to 0.1mm/yr. We already knew that annual SLR is low single digit millimeters. The reasons satellite altimetry cannot provide that level of precision are very basic, and were known to NASA beforehand—Earth’s requisite reference ellipsoid is lumpy, oceans have varying waves, atmosphere has varying humidity—so NASA never really had a chance of achieving what they aspired to: satalt missions to measure sea level rise to fractions of a millimeter per year equivalent to tide gauges. NASA claims they can, but their specifications say they cannot. The post proved lack of fitness via overlap discrepancies between Jason2 and Jason3, plus failure of NASA SLR estimates to close.

This second related guest post asks the same question of ARGO.

Unlike Jason3, ARGO had no good pre-existing tide gauge equivalent mission comparable. Its novel oceanographic purposes (below) tried to measure several things ‘rigorously’ for the very first time. “Rigorously’ did NOT mean precisely. One, ocean heat content (OHC), was previously very inadequately estimated. OHC is much more than just sea surface temperatures (SSTs). SSTs (roughly but not really surface) were formerly measured by trade route dependent buckets/thermometers, or by trade route and ship laden dependent engine intake cooling water temperatures. Deeper ocean was not measured at all until inherently depth inaccurate XBT sensors were developed for the Navy.

Whether ARGO is fit for purpose involves a complex unraveling of design intent plus many related facts. The short ARGO answer is probably yes, although OHC error bars are provably understated in ARGO based scientific literature.

For those WUWT readers wishing a deeper examination of this guest post’s summary conclusions, a treasure trove of ARGO history, implementation, and results is available at www.ARGO.uscd.edu. Most of this post is either directly derived therefrom, or from references found therein, or leads to Willis Eschenbach’s previous WUWT ARGO posts (many searchable using ARGO), with the four most relevant directly linked below.

This guest post is divided into three parts:

1. What was the ARGO design intent? Unlike simple Jason3 SLR, ARGO has a complex set of overlapping oceanographic missions.

2. What were/are the ARGO design specs relative to its missions?

3. What do facts say about ARGO multiple mission fitness?

Part 1 ARGO Intent

ARGO was intended to explore a much more complicated set of oceanography questions than Jason’s simple SLR acceleration. The ideas were developed by oceanographers at Scripps circa 1998-1999 based on a decade of previous regional ocean research, and were formulated into two intent/design documents agreed by the implementing international ARGO consortium circa 2000. There were several ARGO intended objectives. The three most explicitly relevant to this summary post were:

1. Global ocean heat climatology (OHC with intended accuracy explicitly defined as follows)

2. Ocean ‘fresh water storage’ (upper ocean rainfall salinity dilution)

3. Map of non-surface currents

All providing intended “global coverage of the upper ocean on broad spatial scales and time frames of several months or longer.”

Unlike Jason3, no simple yes/no ‘fit for purpose’ for ARGO’s multiple missions is possible. It depends on which mission over what time frame.

Part 2 ARGO Design

The international design has evolved. Initially, the design was ~3000 floats providing a random roughly 3 degree lat/lon ocean spacing, explicitly deemed sufficient spatial resolution for all ARGO intended oceanographic purposes.

There is an extensive discussion of the array’s accuracy/cost tradeoffs in the original intent/design documentation. The ARGO design “is an ongoing exercise in balancing the array’s requirements against the practical limitations imposed by technology and resources”. Varying perspectives still provided (1998-99) “consistent estimates of what is needed.” Based on previous profiling float experiments, “in proximate terms an array with spacing of a few hundred kilometers is sufficient to determine surface layer heat storage (OHC) with an accuracy of about 10W/m2 over areas (‘pixels’) about 1000km on a side.” Note the abouts.

The actual working float number is now about 3800. Each float was to last 4-5 years battery life; the actual is ~4.1 years. Each float was to survive at least 150 profiling cycles; this has been achieved (150 cycles*10 days per cycle/365 days per year equals 4.1 years). Each profile cycle was to be 10 days, drifting randomly at ~1000 meters ‘parking depth’ at neutral buoyancy for 9, then descending to 2000 meters to begin measuring temperature and salinity, followed by a ~6 hour rise to the surface with up to 200 additional measurement sets of pressure (giving depth), temperature, and salinity. This was originally followed by 6-12 hours on the surface transmitting data (now <2 hours using the Iridium satellite system) before sinking back to parking depth.

The basic ARGO float design remains:

clip_image002

And the basic ARGO profiling pattern remains:

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‘Fit for purpose’ concerning OHC (via the 2000 meter temperature profile) presents two relevant questions. (1) Is 2000 meters deep enough? (2) Are the sensors accurate enough to estimate the 10W/m2 per 1000km/side ‘pixel’?

With respect to depth, there are two differently sourced yet similar ‘yes’ answers for all mission intents.

For salinity, the ARGO profile suffices. Previous oceanographic studies showed (per the ARGO source docs) that salinity is remarkably unvarying below about 750 meters depth in all oceans. This fortunately provides a natural salinity ‘calibration’ for those empirically problematic sensors.

It also means seawater density is roughly constant over about 2/3 of the profile, so pressure is a sufficient proxy for depth (and pressure can also be calibrated by measured salinity above 750 meters translated to density).

For temperature, as the following figure (in °F not °C) typical thermocline profiles show, ARGO ΔT depth profile does not depend very much on latitude since 2000 meters equaling ~6500 feet reaches the approximately constant deep ocean temperature equilibrium at all latitudes, providing another natural ARGO ‘calibration’. The 2000 meters ARGO profile was a wise intent/design choice.

clip_image005

Part 3 Is ARGO fit for purpose?

Some further basics are needed as background to the ARGO objectives.

When an ARGO float surfaces to transmit its data, its position is ascertained via GPS to within about 100 meters. Given the vastness of the oceans, that is an overly precise position measurement for ‘broad spatial scales’ of deep current drift and 1000000km2 OHC/salinity ‘pixels’.

Thanks to salinity stability below 750 meters, ARGO ‘salinity corrected’ instruments are accurate (after float specific corrections) to ±0.01psu, giving reasonable estimates of ‘fresh water storage’. A comparison of 350 retrieved ‘dead battery’ ARGO floats showed that 9% were still out of ‘corrected’ salinity calibration at end of life, unavoidably increasing salinity error a little.

The remaining big ‘sufficient accuracy’ question is OHC, and issues like Trenberth’s infamous “Missing Heat” covered in the eponious essay in ebook Blowing Smoke. OHC is a very tricky sensor question, since the vast heat capacity of ocean water means a very large change in ocean heat storage translates into a very small change in absolute seawater temperature.

How good are the ARGO temperature sensors? On the surface, it might seem to depend, since as an international consortium, ARGO does not have one float design. There are presently five: Provor, Apex, Solo, S2A, and Navis.

However, those 5 only ever embodied two temperature sensors, FS1 and SBE. Turns out—even better for accuracy—FS1 was retired late in 2006 when JPL’s Willis published the first ARGO OHC analysis after full (3000 float) deployment, finding (over too short a time frame, IMO) OHC was decreasing (!). Oops! Further climate science analysis purportedly showed FS1 temperature profiles in a few hundred of the early ARGO floats were probably erroneous. Those floats were taken out of service, leaving just SBE sensors. All five ARGO float designs use current model SBE38 from 2015.

SeaBirdScientific builds that sensor, and its specs can be found at www.seabird.com. The SeaBird E38 sensor spec is the following (sorry, but it doesn’t copy well from their website where all docs are in a funky form of pdf probably intended to prevent partial duplication like for this post).

Measurement Range

-5 to +35 °C

Initial Accuracy 1

± 0.001 °C (1 mK)

Typical Stability

0.001 °C (1 mK) in six months, certified

Resolution

Response Time 2

500 msec

Self-heating Error

< 200 μK

1 NIST-traceable calibration applying over the entire range.
2 Time to reach 63% of nal value following a step change in temperature

That is a surprisingly good seawater temperature sensor. Accurate to a NIST calibrated 0.001°C, with a certified temperature precision drift per 6 months (1/8 of a float lifetime) of ±0.001°C. USCD says in its ARGO FAQs that the ARGO temperature data it provides is accurate to ±0.002°C. This suffices to estimate the about 10W/m2 OHC intent per 1000000 km2 ARGO ‘pixel’.

BUT, there is still a major ‘fit for purpose’ problem despite all the ARGO strong positives. Climate papers based on ARGO habitually understate the actual resulting OHC uncertainty—about 10W/m2. (Judith Curry has called this one form of her ‘uncertainty monster’). Willis Eschenbach has posted extensively here at WUWT (over a dozen guest posts already) on ARGO and its findings. His four most relevant for the ‘fit for purpose’ scientific paper uncertainty question are from 2012-2015, links that WE kindly provided via email needing no explanation:


Decimals of Precision

An Ocean of Overconfidence

More Ocean-Sized Errors In Levitus Et Al.

Can We Tell If The Oceans Are Warming

And so we can conclude concerning the ARGO ‘fit for purpose’ question, yes it probably is—but only if ARGO based science papers also correctly provide the associated ARGO intent uncertainty (error bars) for ‘rigorous albeit broad spatial resolution’.

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