ocean heat content: estimates and uncertainties 1950-present

© Crown copyright 2004

Ocean heat content: Estimates and

uncertainties 1950-presentBruce Ingleby and Matt Palmer

[email protected]

C20C, Exeter, 14 March 2007


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


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


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


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


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

© Crown copyright 2004 Page 10

Ob-Bk mean and RMS statistics

Bathy ~ 0.1C warmerthan CTD

Buoy biases?

Differences in geographical coverage


EN3 (WOD05/GTSPP) data: Jan 1996- extra pre-ARGO profilers available cf EN2


EN3 (WOD05/GTSPP) data: Jan 2003- ARGO has since filled in S. Hem. gaps


XBTs: Fall rate and bias


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!


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


Ocean Heat Content


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


Heat content – Levitus and EN2


1970s hump doesn’t show inNoBT analyses

Five year runningmean (previous

slide) emphasises 1970s hump


Larger XBT bias in 1972-1981


1972-1981 All Data: warm in W Pacific


T300 anomaly for December 1996


Alarming local dip in T from PALACE data


Ob-Bk stats

Centre plot Tr IndMax bias at 100m

Suggests depth errorof PALACE data


Has the ocean cooled since 2003?

Lyman et al., [2006]


XBT biased almost 0.2 warm cf ARGO


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


Questions?

ocean heat content: estimates and uncertainties 1950-present

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