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Ocean heat content: Estimates and uncertainties 1950-present. Bruce Ingleby and Matt Palmer [email protected] C20C, Exeter, 14 March 2007. Talk overview. Ocean data EN3 quality control and model-free analyses Data types and data coverage XBTs: Fall-rate and bias - PowerPoint PPT Presentation
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© Crown copyright 2004 Page 1
Ocean heat content: Estimates and
uncertainties 1950-presentBruce Ingleby and Matt Palmer
C20C, Exeter, 14 March 2007
© Crown copyright 2004 Page 2
Talk overview
Ocean dataEN3 quality control and model-free analysesData types and data coverageXBTs: Fall-rate and bias
Global heat contentPublished estimates and controversiesThe hump in the 1970sDifferences in estimates for the 1990sHeat content since 2003
Summary
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EN3 Data sources and processing
WOD05 – Levitus / NODC All subsurface data types:
Ocean station/CTD, XBT, Buoys, Profilers GTSPP from 1990-2006
In 1990s filling a few gaps in WOD05 Main source in last few years
ARGO data from US GODAE GDAC (1999-) Mainly to pick up salinity corrections (~ 30%)
New climatology: EN2 1971-2000 More effort (cf EN2) on removing biased or corrupt
cruises via intervention Ingleby and Huddleston (JMS, 2007) – EN2
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EN3 products/analyses
Quality controlled observations (1950-2005)Comprehensive set of automated checksEN2 used by various other groupsAvailable on www.hadobs.org
Objective analyses (1950-2005)Temperature and salinityPerformed as part of monthly QC cycle, using OI40 Z levels, 1.25° grid (0.3° N-S near equator)No model involved: damped persistence
backgroundPreliminary results for 2006 also available
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Ocean data types at a glance!
Type Instruments Max Depth
Quality
Research(TESAC)
BottleCTD
Some to 5000 m
High
Bathy MBTXBT (c 1965-)
300 m450, 750m
Low
Buoy TAO (c 1990-)PIRATA
750 m High
Profilingfloat
PALACEARGO
1000 m2000 m
High – mostly
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Coverage T 0-300m:
1 – observations complete
0 – climatology only
(depends on “spreading” in analysis scheme)
BATHYs very important in tropics, less so in extratropics
Deeper layers: coverage worse
BATHYs less important
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Ob-Bk mean and RMS statistics
Bathy ~ 0.1C warmerthan CTD
Buoy biases?
Differences in geographical coverage
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EN3 (WOD05/GTSPP) data: Jan 1996- extra pre-ARGO profilers available cf EN2
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EN3 (WOD05/GTSPP) data: Jan 2003- ARGO has since filled in S. Hem. gaps
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XBTs: Fall rate and bias
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XBT fall-rate correction
Most T-4/6/7 XBTs fall faster than original equation
Hanawa et al (1995), Zcor=Zx1.0336 Should vary with viscosity (Thadathil el al, 2001)
In 1995/96 many ships changed to revised equation – metadata not always available
EN2/3: Check instrument type – apply correction if necessary Ships without instrument type (and <840m) –
correction applied up to and including 1996, not afterwards
If correction not applied T appears too cold If correction applied twice then T too warm!
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XBT biases?
Heinmiller et al (1983) found XBTs 0.19 (T-4) and 0.13 (T-7) warm relative to CTDs Compared T at 25m or at depths with small T gradient to
minimise impact of depth errorsTwo studies (tank calibration) found smaller or zero
biasAll the authors found significant probe to probe, or
cruise to cruise, variability in the biasReseghetti et al (2006), also NRL internal reportsGouretski and Koltermann (2007) found warm but
variable XBT bias – our results similarUndetected gross errors may be biased?
XBTs were not designed as climate-quality instruments
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Ocean Heat Content
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Recent publications on Ocean Heat Content
Levitus et al (2000, Science): “Warming of the world ocean” – L2000
Gregory et al (2004, GRL): L2000 has too much decadal variability in OHC (or model has too little) – sampling issues
Levitus et al (2005, GRL): Oh no we don’tAchutarao et al (2006,JGR): You might have; “Subsampling model data with actual observational coverage has a large impact on the inferred temperature variability”
Gouretski and Koltermann (2007): XBT bias
Lyman et al (2006, GRL): “Recent cooling of the upper ocean” 2003-2005
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Heat content – Levitus and EN2
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1970s hump doesn’t show inNoBT analyses
Five year runningmean (previous
slide) emphasises 1970s hump
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Larger XBT bias in 1972-1981
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1972-1981 All Data: warm in W Pacific
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T300 anomaly for December 1996
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Alarming local dip in T from PALACE data
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Ob-Bk stats
Centre plot Tr IndMax bias at 100m
Suggests depth errorof PALACE data
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Has the ocean cooled since 2003?
Lyman et al., [2006]
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XBT biased almost 0.2 warm cf ARGO
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Summary
QC and bias issues – insufficient attentionPartly addressed here – more work needed
XBTs slightly warm – bias varies1970s hump: not in noBT analyses
XBT bias larger in that period1995-2000 Indian Ocean: PALACE data too cold – depth error
2003-2006: decrease in OHC (all data run)Partly due to XBT-ARGO transition + XBT bias
1990s: increase in OHC, all analyses agree!
Bias correct XBTs by type/country/ship/year?Look at Argo data more closely
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Questions?