The global XBT network

Molly Baringer (1), Gustavo Goni (1), and Dean Roemmich (2)

(1) NOAA/AOML, Miami, FL(2) SIO, La Jolla, CA

NOAA Climate Program OfficeClimate Observation Division

8th Annual PI MeetingAnnual System Review Meeting

Washington, DCJune 25-27, 2012

XBT Network:

• Program goals: mesoscale resolving global array of repeated transects, real-time data delivery, providing synergy with other platforms

• U.S. roles in the XBT Program; Ocean Obs ‘09 recommendations, Strategy for meeting the program’s goals.

• Users and uses of XBT data : research, operational applications, ocean/climate assessment

• Challenges for the XBT network, technology improvement

The global XBT networkOceanObs09 Recommended Transects

High Density (HD) – mesoscale resolving, 4 times per year

Frequently Repeated (FR) – 100-150 km spacing, 12-18 times per yearLow Density – no longer recommended

The global XBT networkInternational Collaboration with NOAA

France: AX20, AX01, AX02South Africa: AX25, AX08, AX18Brazil: AX97Argentina: AX18Australia: IX01, IX22Italy: MX01, MX02, MX4

More than fourteen institutions collaborate on collection, quality control, and science from XBT data including:US (NOAA, SIO), France (IRD, UP), Australia (ABOM, CSIRO), South Africa (UCPT), Japan (JMA), Brazil (FURG, Navy), Italy (ENEA), India (NIO), Germany (BSH), Argentina (SHA)

The global XBT networkDeployment/Observations

2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

XBT 25 23 23 29 25 22 20 21 21 19 18

Argo 7 17 31 47 72 96 110 121 126 121 126

2001 2006 2011

125K

20K

XBTs Deployed (Red); Argo Deployed (Blue)

Number of profiles on the GTS in units of 1,000

Value of the HD (HRX) Network

• Ocean circulation: – The HD (HRX) Network samples the boundary currents and the ocean

interiors at high spatial resolution for transport estimation.– It provides a tool for integrating the observing system.– The combination of XBTs, Argo, and altimetry mitigates the limitations

of the individual datasets.

• Mass, heat, and freshwater budgets: – Estimates of reference velocities, heat storage, vertical advection, and

Ekman transport are all much improved in the past decade.– Time mean balances have good confidence.– Time varying balances remain a challenge, but progress is being made.

HRX transects are sampling:• Kuroshio (3 HRX tracks), Gulf Stream (3 HRX tracks)• Agulhas, Brazil Current, East Australian Current (2 HRX tracks)• Eastern boundary currents (California Current, Leeuwin Current, …)• Low latitude WBCs: Solomon Sea, Indonesian Throughflow• Antarctic Circumpolar Current (3 HRX tracks)• …

AOML status map

: Boundary currents sampled by the HRX Network

1. Global in scope (i.e. all 5 subtropical WBCs)

2. Enhanced BC sampling is highest priority, OO’09.

3. Argo provides complementary absolute and/or deep relative reference level velocities.

4. The HD (HRX) Network integrates the BCs and interior.

The High Resolution XBT (HRX) Network samples the world’s boundary currents - the subtropical WBCs and EBCs, the low latitude WBCs, and the ACC.

Boundary Current Array

Goal: The global networkOf currents from XBTs

Agulhas Current at 28S: IX21 (1994)Leeuwin Current 32S: IX15 (1987)Indonesian Throughflow: IX01 (1987)

Upstream Kuroshio Current: Upstream: PX44 (since 1991)Downstream: PX05 (2009)

East Australian Current: at 27S: PX30 (1991)at 33S: PX34 (1991)

East Auckland Current and Tasman outflow: PX06 (1986)

Solomon Sea current system: PX05 (2009)

California Current System: Undercurrent: PX37 (1991)California Current: PX37 (1991)

Alaska Current: PX38 (1993)

Antarctic Circumpolar CurrentSouth of Tasmania: IX28 (1993)Drake Passage: AX22 (1996)South of South Africa: AX25 (2004)

Gulf Stream: AX10 (1997), AX32 (1981) Florida Current: AX7 (2000)North Atlantic Drift Current: AX01 (1997)Labrador Current: AX02 (2010)Atlantic Ocean Equatorial Current System:

AX08 (2000), AX20 (2010)

Brazil Current: AX97 (2004)Brazil/Malvinas Confluence: AX18 (2002)Benguela Current and Agulhas Current

Rings: AX18 (2002) and AX08 (2000)

Zonal currents in the Tropical Atlantic

NECC

NEUCSEUC

SECC

NEC nSECcSEC

sSEC

Goni and Baringer, 2002

PX37S

Line 90

PX37S

Line 90

Line 90

PX37SArgo

Transport

PX37S

The “real” boundary current is the northward California Undercurrent, not the southward California Current.

Argo Steric Height 0/2000

Integrating the ocean observing system: HD (HRX), Argo, CalCOFI

Courtesy of D. Roemmich

Geostrophic volume transport in subtropical Pacific

Integrated transport:Black: Argo RG high resolutionRed: HRX during Argo era (29 cruises)Dark blue: HRX, all Hong Kong (44 cruises)

Argo era: 29 cruises; mean -12.9 Sv; σ = 3.66 Sv; Std error = 0.7 Sv

Differences between PX37 and Argo: • At high spatial resolution Argo has larger errors in the temporal mean• Argo misses the northward EBC

Courtesy D. Roemmich and J. Gilson

Example: Northward Heat Transport in SA (AX18)

Garzoli and Baringer (2007)Baringer and Garzoli (2007)

Both geostrophic and Ekman transports experience annual cycles, but they are out of phase.

Geostrophic transport controls the total northward heat transport.Geostrophic and Ekman transports experience comparable variability

Total = 0.51 0.15 PWGeos. = 0.40 0.16 PWEkman = 0.11 0.16 PW

…now using altimetry…e.g. Brazil current

On average 23 publications a year are published using XBTs as the primary data source.

The global XBT networkScientific Publications

SIOAOML

NOAA Role in the global XBT network

NOAA funds approximately 60% of XBTs, while international partners aid in the actual deployments

E.g. of the 11 HD transects done by AOML, international partners deploy XBTs on 9 lines.

The Future of XBTs:The Future of XBTs:

15. Create an international science panel for upper ocean thermal observations to support and evaluate recommendations of the integration of the different platforms, including XBTs (XBT Science Team created)

Gouretski and Reseghetti, 2010

1. Fully implement and maintain the XBT network as recommended in OceanObs99 (phase out of LD, increases HD)

2. Expand transects to include interior and marginal seas, such as the Mediterranean Sea and the Gulf of Mexico (Med Sea expansion, no Gulf of Mexico)

8.8. Support technological Support technological improvements (underway)improvements (underway)

9. Implement XBT calibrations based on CTDs (underway)

13.13. Continue XBT data analysis for scientific studies and increase its operational Continue XBT data analysis for scientific studies and increase its operational applications applications

The global XBT networkNew technology

Climate quality XBT with two pressure switches, which trigger signal at predetermined depth (By Sippican)

First test with improved thermal sensor was carried out in 2012 on the Western Boundary Time Series cruise.

< 1 m error

Challenges• Resources: Near level funded has forced a more rapid

transition to HD with a reduction in FR (and all LD).

• Spatial Coverage: Deployment opportunities are limited. For example transects in the Indian Ocean are extremely difficult, Ax18/Ax18* in the South Atlantic.

• Technical failures (e.g. variable fall rate): Quick detection and correction is essential. Important to have synergy of multiple platforms with different, enhancing goals.

• System Integration: Many other platforms require XBT program for logistical support (e.g. Argo float deployments, drifter deployments, pCO2/TSG calibrations and maintenance, weather service Met messages and US Coast Guard Amver alert system).

The global XBT networksummary

1. XBT network provides 20,000 T(z) profiles each year globally.2. Network has transitioned away from LD and FR towards HD.3. Science emphasizes HD transects:

Monitoring currentsMonitoring heat budgets, transport, regional balances

4. In the future:HD (HRX) network forms the backbone of a boundary

current observing systemFRE studies refine corrections for historical XBT dataProbe improvements, T sensors, pressure switches

produce climate quality measurements5. First XBT Science Workshop (Australia, 2011)

Highlight scientific accomplishmentsXBT Science Steering Team