The global XBT network

1
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 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
  programs 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
Deployment Mode Transects
High Density only 17 (22/29)
Frequently Repeated only 23 (18/35)
HD and FR 12
Low Density 0
TOTAL 52 (40/64)
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
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
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
worlds boundary currents - the subtropical WBCs
and EBCs, the low latitude WBCs, and the ACC.
Boundary currents sampled by the HRX Network

1. Global in scope (i.e. all 5 subtropical WBCs)
2. Enhanced BC sampling is highest priority, OO09.
3. Argo provides complementary absolute and/or deep
   relative reference level velocities.
4. The HD (HRX) Network integrates the BCs and
   interior.

AOML status map

• 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)

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
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
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 s 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)
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
SIO
AOML
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

1. 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)
lt 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