Monitoring Heat Monitoring Heat Transport Changes Transport Changes using Expendable using Expendable Bathythermographs Bathythermographs Molly Baringer and Silvia Garzoli NOAA, AOML •What are time/space scales of climate signals? •How can XBTs be used to measure heat transport? •How can these measurements/estimates be improved?
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Monitoring Heat Transport Changes using Expendable Bathythermographs Molly Baringer and Silvia Garzoli NOAA, AOML What are time/space scales of climate.
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Monitoring Heat Monitoring Heat Transport Changes using Transport Changes using
•What are time/space scales of climate signals?•How can XBTs be used to measure heat transport?•How can these measurements/estimates be improved?
Map of net surface heat fluxMap of net surface heat flux
Integrated Net surface Heat FluxIntegrated Net surface Heat Flux
Time series of modeled meridional heat transport in the Atlantic.
Häkkinen et al (1999)
Time scales of heat transport Time scales of heat transport variabilityvariability
Short time scales: 0.9 PW over months
Long time scales: 0.6 PW over decades
Meridional Heat Transport
25N
45N
High Resolution XBT High Resolution XBT Lines:Lines:
Ax7 North AtlanticAx7 North Atlantic
High Resolution XBT High Resolution XBT Lines:Lines:
Ax18 South AtlanticAx18 South Atlantic
Direct estimates of lateral volume (V), mass (M) and heat Direct estimates of lateral volume (V), mass (M) and heat (H) transport across a vertical section require T, S and (H) transport across a vertical section require T, S and velocity observations:velocity observations:
V v dx dz Sv 106 m3 /s
M v dx dz Kg /s
H c p v dx dz PW 1015Watts Uncertainty in v.
v t vg vag vb
Reference levelWind products ??
MethodologyMethodology
• XBT data is collected using Sippican T-7 probes typically to depths of about 800 meters
• Data is extended to the ocean bottom using Levitus 2001 data set.
• Salinity in the upper ocean is estimated for each XBT by using Salinity and Temperature relationships at different locations and depths using
Salinity
Tem
pera
ture
•ARGO and CTD•Levitus 2001 climatology
•Transports were computed in layers and summed for the entire water column. The level of no motion velocity was adjusted so that the net mass transport across the section was zero using a single velocity correction for each section. Typically, values of this velocity ranged from 10-4 to 10-6 m s-1.
•Additional corrections/constraints to the net transports were made to account for barotropic currents:
•Florida Current (Ax7)•Brazil/Malvinas Currents (Ax18)and•Ax18 XBT sections that terminated before fully crossing the entire ocean basin.
•Ekman transports: were determined using climatological wind fields interpolated to the location of the XBT lines.
M y x
fx
Special Constraints applied to Special Constraints applied to subtropical North Atlantic subtropical North Atlantic
Section: Ax7Section: Ax7Winds:Hellerman and Rosenstein climatological annual mean
Initial Reference Velocity:no motion along = 27.6 kg/m3 (~1000m)
Boundary Currents:+32.1 Sv in the Florida Straits (long-term
cable transport average)
Example Example from from Ax7Ax7
Interannual variability in heat Interannual variability in heat transport along Ax7transport along Ax7
Annual Cycle along Ax7Annual Cycle along Ax7
North Atlantic Results North Atlantic Results Summary: Ax7Summary: Ax7
•Mean Heat transport from 1995 to present:
–1 PW +/- 0.2 PW
•Interannual variability range:– 0.4 +/- 0.2 PW or 35% of total
•Annual cycle range:– 0.1 +/- 0.2 PW or 10% of total
Methodology Error Bars? Methodology Error Bars? Tested Against 24N Full Water Tested Against 24N Full Water
Column CTD SectionsColumn CTD Sections• Levitus Salinity for upper 850m:
South Atlantic Heat Transport: Ax18South Atlantic Heat Transport: Ax18
0
0.2
0.4
0.6
0.8
1
1.2
2 4 6 8 10 12 14
Hea
t T
rans
port
(P
W)
month
Annual Cycle: Ax18
0.8 PW
0.9 PW
0.7 PW)
0.7 PW
0.7 PW
0.5 PW)0.7 PW
0.9 PW
Values are ±0.2
0.8 PW
Febr 05
Annual Cycle: Ax18Annual Cycle: Ax18
Ekman Heat Transport (PW): NCEP Monthly Climatology
Methodology Error Bars?Methodology Error Bars?Method tested against A10 WOCE Method tested against A10 WOCE
CTD sectionCTD section
Unlike the North Atlantic, Levitus T(p), S(p) below 850m produces a mean bias:
+0.2 PW Bias
Still left with wind errors and other unknown variability
+/- 0.2 PW
MERIDIONAL HEAT TRANSPORT= 0.7 ± 0.2 PW
BIAS due to using Levitus below 800m of 0.2 PW
0.5 ± 0.2 PW
Summary for South Atlantic: Summary for South Atlantic: Ax18Ax18
What we need to improve What we need to improve these estimates:these estimates:
Additional constraints to improve initial estimate of total velocity field and its error covariances. • Deeper observations (fast-deep T5 to 2000+ meters?)• Specific transport estimates (e.g. Malvinas Current): XCPs?• Improved Salinity estimation (ARGO/XCTDs?)• Improved wind products and ship based observations• Improved subsurface climatologies (ARGO?)• Formalized procedure including error estimates (Inversion?)
• XBTs show mean heat transports similar to historical estimates
• In the North Atlantic, there has been a pronounced decrease in heat transport over the past 10 years.
•More sections, improved data are necessary to determine meaningful error estimates.
•Until an improved system is established, data from the high density lines can be used to obtained an estimate of the heat transport variability.