• Rebecca Cowley1 • Susan Wijffels1 • Lijing Cheng2 • Jiang Zhu2 • Tim Boyer3 • Shoichi Kizu4

FOR FURTHER INFORMATIONRebecca Cowley e rebecca.cowley@csiro.au t +61 3 6232 5446

CONTACT USt 1300 363 400 +61 3 9545 2176 e enquiries@csiro.au w www.csiro.au

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REFERENCESCowley, R., S. Wijffels, L. Cheng, T. Boyer, S. Kizu, 2013: Biases in Expendable

Bathythermograph Data: A New View Based on Historical Side-by-Side Comparisons. J. Atmos. Oceanic Technol., 30, 1195–1225.

Cheng, L., J. Zhu, R.Cowley, T. Boyer and S. Wijffels (submitted): Time, Probe Type and Temperature Variable Bias Corrections to Historical Expendable Bathythermograph Observations. J.Atmos.Ocean.Technol.

Hanawa, K., P. Rual, R. Bailey, A. Sy and M. Szabados, 1995: A New Depth Time Equation for Sippican Or TSK T-7, T-6 and T-4 Expendable Bathythermographs (XBT). Deep-Sea Res.Part I-Oceanogr.Res.Pap., 42, 1423-1451, doi:10.1016/0967-0637(95)97154-Z.

Hanawa, K., H. Yoritaka, 1986: Detection of systematic errors in XBT data and their correction. Journal of the Oceanographical Society of Japan, 43, 68-76, doi:10.1007/BF02110635.

Gouretski, V., K. P. Koltermann, 2007: How much is the ocean really warming? Geophys.Res.Lett., 34.

Levitus, S., J. I. Antonov, T. P. Boyer, R. A. Locarnini, H. E. Garcia and A. V. Mishonov, 2009: Global ocean heat content 1955-2008 in light of recently revealed instrumentation problems. Geophys.Res.Lett., 36, doi:10.1029/2008GL037155.

Improving estimates of Global Ocean Heat Content by removing biases in Expendable BathyThermograph (XBT) data

1 CSIRO Marine and Atmospheric Research, GPO Box 1538, Hobart, Tasmania 7001, Australia2 Chinese Academy of Sciences3 NODC, NOAA4 Tohoku University, Japan

Why so warm?XBT data comprises a large proportion (~30%) of available ocean temperature data. Previous calculations of global ocean heat content (GOHC) have been biased warm by errors in XBT data (Gouretski and Koltermann, 2007), caused by depth errors and a pure temperature bias (Cowley et al, 2013).

The pairs database We have collected 5,371 co-located (in space and time) XBT and CTD profile pairs. The pairs database is publicly available via the CSIRO Data Portal (https://data.csiro.au/dap/home).

Calculating the XBT depth error Using the method of Hanawa and Yoritaka (1986) where temperature gradients are matched between the CTD and XBT profiles, we are able to fit a linear model to the depth errors in the XBT data for each pair (dZ = a*z + b, where dZ is the depth error (m) and z is the XBT depth (m) corrected to the Hanawa (1995) fall rate).

(a) Location and decade of collection. (b) Distribution of XBT probe types over time.

0 50 100 150 200 250 300 350

−50

0

50

Longitude

Latit

ude

a

1960’s

1970’s

1980’s

1990’s

2000’s

>2010

1970 1980 1990 2000 20100

100

200

300

400

Cou

nt

Year

b

Sippican T4

Sippican T6

Sippican T7

Sippican DB

TSK T6

TSK T7

2 4 6 8 -0.1 0 0.1 -20 20010

0

100

200

300

400

500

600

700

800

900

Dep

th (

m)

Cor

rela

tion

(R)

CTD Sippican T7XBT Highest correlation Depth error linear model

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1a cb

Does the depth error vary with time? By looking at the a term from the linear model, we can see changes in the depth error over time. The grey shading indicates the number of pairs in each group and two times standard error is indicated by the size of each box. ‘S65’ indicates the original Sippican fall rate and ‘H95’ the Hanawa (1995) fall rate. ±0.02m/m is the manufacturer’s allowed error.

Using a global-scale pairs database to refine the corrections The XBT/CTD pairs database may not be representative of the operational XBT data collection. Most pairs were collected from research vessels and during testing of new equipment. A global-scale pairs database is used to better represent the historical database.

Correlations in the fall rate equation and thermal bias are used to correct the data (Cheng et al, submitted).

1970 1980 1990 2000 2010

−0.02

−0.01

0

0.01

0.02

0.03

0.04

H95

Year

S65

α (m

m−

1 )

Slower fall rate mid 1970’s

a

Sippican T4/T6

1970 1980 1990 2000 2010

−0.02

−0.01

0

0.01

0.02

0.03

0.04

H95

Year

S65

α (m

m−

1 )

Slowing fall rate to present

b

Sippican T7/DB

30

40

50

60

70

80

90

100

30

40

50

60

70

80

90

100

-50 0 50

0.25

-0.05

0.05

0

0.1

-0.1

0.15

0.2

Tem

pera

ture

diff

eren

ce (

°C)

600 m

Latitude (degree)

Original Temp-Cor All-Cor

1980 1990 2000 201019701960

5

0

-5

-25

-20

-10

-15

Oce

an h

eat c

onte

nt x

10

22 jo

ules

Year

No corrections

L09

TGCH

Application of XBT corrections to improve GOHC estimates After applying the corrections derived from the pairs database, we see a reduction in estimates of GOHC. GOHC estimates are shown from data with no XBT corrections (dotted line), with corrections using the method of Levitus et al (2009) (black line), with corrections derived from the pairs database (red line) and with corrections derived from the global-scale pairs database (green line).

XBT – CTD temperature differences at 600m by latitude. Uncorrected (red), after temperature-variable depth correction (blue), after all corrections are applied (yellow).

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