The Role of Inter-ocean Exchanges on Long-term Variability of the Northward Heat

Transport in the South Atlantic

Shenfu DongShenfu DongCIMAS/UM and NOAA/AOML

S. L. Garzoli, M.BaringerNOAA/AOML, Miami, Florida USA

June 9, 2010

R. Lumpkin, NOAA/AOML

Objective:

To investigate the role of the inter-ocean exchanges in the variability of the northward heat transport in the South Atlantic.

Specific Question: Where does the variability in the northward heat transport come from?

• Inter-ocean exchanges with the Pacific Ocean.• Inter-ocean exchanges with the Indian Ocean.• Air-sea heat exchanges.• heat storage in the basin.

SAfricaDrake HTHTHTQthc

35

AfricaDrakeS HTHTQthc

HT 35

OGCM for the Earth Simulator (OFES)

The model code is based on MOM3. OFES covers 75S to 75N. 0.1 horizontal resolution. 54 vertical levels. Spun up for 50 years

followed by a 57-yr hindcast run (1950–2006). Forcing from the NCEP–NCAR reanalysis.

Time-Mean Volume and Temperature Transport

Results based on XBT measurements at nominal 35S: Total volume transport: 0.5 Sv Total heat transport: 0.54 PW AMOC strength: 17.9 Sv

AMOC from OFES: 15 Sv

Relationship between AMOC and Northward Heat Transport at 35S

HT(PW) 0.05 * AMOC (Sv)

XBT observationsOFES Model

The Strength of the AMOC at 35S

Linear increasing trend in AMOC: 0.9 Sv per decade

Northward Temperature Transport across 35S - I

0.03 PW per decade

Northward Temperature Transport across 35S - III

),()(),( zxvzvvzxv ),()(),( zxzzx

Baroclinic: , Horizontal: )()( zzvcp ),(),( zxzxvcp

Volume Transports across Drake Passage and South of Africa

Temperature Transports across Drake Passage and South of Africa

0.03 PW/decade

-0.04 PW/decade

AfricaDrakeS HTHTQthc

HT 35

0.03 0.03 -0.04Linear Trend (PW/decade)

0.07 PW per decade

Anomalous Air-Sea Heat Flux

Decreasing trend: -0.03 PW/decade

AfricaDrakeS HTHTQthc

HT 35

0.03 -0.3 0.07Linear Trend (PW/decade)

Temperature Transports--- South of Africa

Total-0.04 PW per decade

ACC-dominant eastward transport

0.03 PW per decade

Agulhas leakage-0.07 PW per decade

AgulhasACCAfricaDrakeS HTHTHTQthc

HT ,35

0.03 -0.3 0.03 0.3 0.7Trend

PW/decade

Conclusions Both XBT measurements and the OFES model suggest that 1 sverdrup increase in the AMOC strength would cause an increase in the northward heat transport of 0.05 PW at approximately 35S.

The OFES model suggests an increase in the northward heat transport across 35S from 1980 to 2006.

The increasing trend in the northward heat transport is likely due to the increase in Agulhas leakage into the South Atlantic.

About half of the increase in Agulhas leakage is lost to the atmosphere through air-sea heat exchange.

Temperature Transports across Drake Passage and South of Africa

Northward Temperature Transport across 35S - II