The global XBT network Gustavo Goni (1), Molly Baringer (1) and Dean Roemmich (2)

1. The global XBT network Gustavo Goni (1), Molly Baringer (1) and Dean Roemmich (2) (1) NOAA/AOML, Miami, FL (2) SIO, La Jolla, CA NOAA Climate Program Office Climate Observation Division 8th Annual PI Meeting Annual System Review Meeting Washington, DC June 25-27, 2012

2. 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

3. The global XBT network OceanObs09 Recommended Transects High Density (HD) – mesoscale resolving, 4 times per year Frequently Repeated (FR) – 100-150 km spacing, 12-18 times per year Low Density – no longer recommended

4. The global XBT network International Collaboration with NOAA France: AX20, AX01, AX02 South Africa: AX25, AX08, AX18 Brazil: AX97 Argentina: AX18 Australia: IX01, IX22 Italy: 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)

5. The global XBT network Deployment/Observations 2001 2006 2011 Number of profiles on the GTS in units of 1,000 125K 20K XBTs Deployed (Red); Argo Deployed (Blue)

6. 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.

7. Boundary Current Array The High Resolution XBT (HRX) Network samples the world’s boundary currents - the subtropical WBCs and EBCs, the low latitude WBCs, and the ACC. 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) • … : Boundary currents sampled by the HRX Network • Global in scope (i.e. all 5 subtropical WBCs) • Enhanced BC sampling is highest priority, OO’09. • Argo provides complementary absolute and/or deep relative reference level velocities. • The HD (HRX) Network integrates the BCs and interior. AOML status map

8. Goal: The global network Of currents from XBTs Agulhas Current at 28S: IX21 (1994) Leeuwin Current 32S: IX15 (1987) Indonesian Throughflow: IX01 (1987) Upstream Kuroshio Current: Upstream: PX44 (since 1991) Downstream: PX05 (2009) East Australian Current: at 27S: PX30 (1991) at 33S: 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 Current South 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)

9. NEUC SECC NECC SEUC sSEC NEC nSEC cSEC Zonal currents in the Tropical Atlantic Goni and Baringer, 2002

10. Integrating the ocean observing system: HD (HRX), Argo, CalCOFI PX37S Line 90 Argo Steric Height 0/2000 Line 90 Transport Line 90 PX37S Argo PX37S PX37S The “real” boundary current is the northward California Undercurrent, not the southward California Current. Courtesy of D. Roemmich

11. Geostrophic volume transport in subtropical Pacific Integrated transport: Black: Argo RG high resolution Red: 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

12. Example: Northward Heat Transport in SA (AX18) Total = 0.51  0.15 PW Geos. = 0.40  0.16 PW Ekman = 0.11  0.16 PW • Geostrophic transport controls the total northward heat transport. • Geostrophic and Ekman transports experience comparable variability Both geostrophic and Ekman transports experience annual cycles, but they are out of phase. …now using altimetry Garzoli and Baringer (2007) Baringer and Garzoli (2007) …e.g. Brazil current

13. The global XBT network Scientific Publications On average 23 publications a year are published using XBTs as the primary data source.

14. NOAA Role in the global XBT network AOML SIO • 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.

15. The Future of XBTs: Fully implement and maintain the XBT network as recommended in OceanObs99 (phase out of LD, increases HD) 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) Support technological improvements (underway) Implement XBT calibrations based on CTDs (underway) Gouretski and Reseghetti, 2010 Continue XBT data analysis for scientific studies and increase its operational applications 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)

16. The global XBT network New technology Climate quality XBT with two pressure switches, which trigger signal at predetermined depth (By Sippican) < 1 m error First test with improved thermal sensor was carried out in 2012 on the Western Boundary Time Series cruise.

17. 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).

18. The global XBT network summary • XBT network provides 20,000 T(z) profiles each year globally. • Network has transitioned away from LD and FR towards HD. • Science emphasizes HD transects: • Monitoring currents • Monitoring heat budgets, transport, regional balances • In the future: • HD (HRX) network forms the backbone of a boundary current observing system • FRE studies refine corrections for historical XBT data • Probe improvements, T sensors, pressure switches produce climate quality measurements • First XBT Science Workshop (Australia, 2011) • Highlight scientific accomplishments • XBT Science Steering Team

21. In response to previous presentations: 1) International collaboration also for XBTs as there is for RAMA, Argo, TAO, Arctic,sea level 2) We do not provide XBTs to Australia, but we are still carrying studies together with CSIRO and BOM.  We just provided them with old XBTs that together with what they have will be used in a FRE experiment for old XBTs. 3) We started to complement the XBT-derived measurements of MHT in the south Atlantic with estimates performed with altimetry. 4) XBT and altimetry estimates confirmed (Goni et al, published last year) that the shift to the south of the Brazil Current from drifters (the Lumpkin-Garzoli manuscript mentioned by Legler), is actually part of a longer period motion.

23. HD Only • Only HD (17) • (5) ix10, ix15,ix21, ix28, ix31 • (7) px30, px34, px36, px38, px40, px50, px81 • (5) Ax03, ax18, ax22, ax25, ax97 • Total HD (HD only plus HD+FR lines) (29) • (6) Ix01, ix10, ix15,ix21, ix28, ix31 • (15) Px05, px06, px09, px10, px13, px30, px31, px34, • px36, px37, px38, px40, px44, px50, px81 • (9) ax02, ax03, ax07, ax08, ax10, ax18, ax22, ax25, ax97 • HD Active (22) • (3) Ix15, ix21, ix28 • (8) Px05, px06, px09, px10, px30, px31, px34, px37, px38, px44 • (9) Ax02, Ax07, ax08, ax10, ax18, ax20, ax22, ax25, ax97 • (2) Mx04, mx01 • HD Inactive (4) • (2) Ix10, ix31 • (2) Px50, px81 • FR • Only FR (23) • (7) Ix06, Ix07, ix08, ix09, ix12, ix14, ix22 • (9) Px02, Px04, px08, Px11, px17, px18, px21, px26, px83 • (7) ax01, ax02, ax11, ax15, ax20, ax29, ax34 • Total FR (Only FR and FR+HD) (35) • (8) Ix01, Ix06, Ix07, ix08, ix09, ix12, ix14, ix22 • (17) Px02, Px04, px05, px06, px08, px09, px10, Px11, • px13, px17, px18, px21, px26, px31, px37, • px44, px83 • (10) ax01, ax02, ax07, ax08, ax10, ax11, ax15, ax20, • ax29, ax34 • Active (18) • (5) Ix01, Ix09, ix12, ix14, ix22 • (8) px02, px08, px09, px11, px13, px31, px36, px40 • (5) ax01, ax02, ax03, Ax11, ax15 • Inactive (7) • (3) Ix06, Ix07, ix08, • (11) px04, px05, px06, px10, px17, px18, px21, • px26, px37, px44, px83 • (6) ax07, ax08, ax10, ax20, ax29, ax34 • HD and FR • Total (12) • (1) Ix01 • (8) Px05, px06, px09, px10, px13, px31, px37, px44 • (3) Ax07, ax08, ax10 • Active • Px09, px31 • ax02 • Inactive

24. Height difference, W-E (anomaly) Black: AVISO Blue: AVISO at HRX times (4-cruise running mean) Yellow: HRX (4-cruise running mean)

25. XBT Network Scientific Goals • Backbone observing system for monitoring boundary currents • Monitoring of Meridional Heat Transport, Regional Heat budgets bounded by HD lines • Complement Argo and other platforms to measure heat content (Largest historical heat content contribution) • Synergy with other observing platforms (TSG, pCO2, altimetry,…) • Provide high value research

26. Key Results: Pacific Ocean (PX06) • Min temp variance at both ends, • Eastward flow in distinct filaments • 4 year period variability, and decadal trend in transport Figure by Dean Roemmich; Goni et al, 2010

27. Key Results: The California Current and Undercurrent (PX37)

28. Key Results: Frontal regions in the ACC (AX25) • AX25 XBT obs + satellite altimetry, • Detection of fine scale features that form the fronts • Subantarctic front contributes to 50% of the total transport variance of the ACC, even when its transport is less than other fronts. Figure by Sebastiaan Swart; Goni et al, 2010

29. Key Results: Heat transport NA and AMO (AX07) Figure by M. Baringer; Goni et al, 2010

31. 1. XBT Recommendations: Fully implement and maintain the XBT network as recommended in OceanObs99 Expand transects to include interior and marginal seas, such as the Mediterranean Sea and the Gulf of Mexico Analyze and evaluate the correct temporal and spatial sampling rate for each deployment mode Evaluate the effectiveness of profiling floats to reproduce climatic signals that were previously captured by XBTs in LD mode Continue the support of real-time transmissions Support advisory panels such as GOSUD and SAMOS Support integration of XBT observations with those of other platforms Support technological improvements

32. 1. XBT Recommendations: Implement XBT calibrations based on CTDs to facilitate the comparison of XBT data every time research-quality CTD data are collected. Establish consistent data quality control procedures and data base management. To improve meta data To complete a high quality, historical and global XBT database. Continue XBT data analysis for scientific studies and increase its operational applications Gouretski and Reseghetti, 2009, submitted Support a strong presence of XBT science and operations results in scientific and operational panels and meetings. To recommend the creation of an international panel for upper ocean thermal observations to support and evaluate recommendations of the integration of the different platforms, including XBTs

33. The global XBT network

34. The AOML XBT network Meridional Heat Transport North Atlantic at ~ 27N South Atlantic at ~ 35S