The Role of Inter-ocean Exchanges on Long-term Variability of the Northward Heat Transport in the South Atlantic Shenfu Dong CIMAS/UM and NOAA/AOML S. L. Garzoli, M.Baringer NOAA/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.

OGCM for the Earth Simulator (OFES) The model code is based on MOM3. OFES covers 75S to 75N. 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 35S: 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 35S XBT observations OFES Model HT(PW)  0.05 * AMOC (Sv)

The Strength of the AMOC at 35S Linear increasing trend in AMOC: 0.9 Sv per decade

Northward Temperature Transport across 35S - I 0.03 PW per decade

Northward Temperature Transport across 35S - III Baroclinic: , Horizontal:

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

Linear Trend (PW/decade) 0.03 0.03 -0.04 Linear Trend (PW/decade) 0.07 PW per decade

Anomalous Air-Sea Heat Flux Decreasing trend: -0.03 PW/decade

Linear Trend (PW/decade) 0.03 -0.3 0.07

Temperature Transports--- South of Africa ACC-dominant eastward transport 0.03 PW per decade Total -0.04 PW per decade Agulhas leakage -0.07 PW per decade

Trend PW/decade 0.03 -0.3 0.03 0.3 0.7

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 35S. The OFES model suggests an increase in the northward heat transport across 35S 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 35S - II