Monitoring Heat Monitoring Heat Transport Changes using Transport Changes using

Expendable Expendable BathythermographsBathythermographs

Molly Baringerand

Silvia GarzoliNOAA, 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?

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:

0.0075 +/- 0.045 PW

• Levitus S(p) , T(p) below 850m:-0.0075 +/- 0.1109 PW

• Both:0.035 +/- 0.136 PW

Winds:NCEP mean monthly values

V at the bottom:Peterson (1989): Malvinas

Current-0.04 m/sec at 38°S

Currents on the shelf: results from Palma et al., 2004 (< 0.2 Sv)

Special Constraints used for Special Constraints used for South Atlantic Ax18South Atlantic Ax18

Examples Examples from from Ax18Ax18

Date HT (PW)

Jul-02 0.69 ±0.2 Nov-02 0.49±0.2 May-03 0.86±0.2 Nov-03 0.67±0.2 Mar-04 0.75±0.2 Jul-04 0.67±0.2 Sep-04 0.75±0.2 Dec-04 0.90±0.2 Feb-05 0.67±0.2

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.

SummarySummary