South Atlantic oceanographic and meteorological

characteristics

Claudia Schmid (NOAA/AOML, USA)

Outline

• Hydrography & Water masses• Circulation:

– surface– intermediate depth– THC & inter-ocean exchange

• Atmosphere ocean interaction & Meteorology– upwelling off Africa– forcing the circulation– correlations– South Atlantic tropical storms

Water masses

Peterson & Withworth (1989)

Meridional sections30W 1E

27.35

27.18

27.00

Zonal section at about 30oS

27.35

27.18

27.00

Mean zonal currents at 35oW

Schott et al (2003)

Velocities in 700-900m

EIC was defined as the westward flow under the EUC.EDJ make it hard to derive mean currents below the EUC due to their poorly understood variability on seasonal to interannual time scales.

Intermediate depth circulation

Nunez-Riboni et al. (2005)

Intermediate depth Circulation

Mean currents (20d integrated trajectories)

Surface circulation

Conveyor belt

After Broecker (1991)

The compensating meridional flows

After Stramma & England (1999)

NADW (>2000m)

AAIW (500-1200m)

SACW (100-500m)

THC in the South Atlantic

Transports (from Gordon et al., 1992)

Donners and Drijfhout (2004)

T>9oC:~63% from IO

T<9oC, p>1500dbar: ~56% from IO

Transports from OCCAM

Donners and Drijfhout (2004)

T>9oC:~73% from IO

T<9oC, p>1500dbar: ~69% from IO

Intermediate depth transports (Schmid et al., 2000)

AAIW: <=45% from IO?

Brazil-Malvinas Confluence

Eddies in the Agulhas Region

Boebel et al. (2003)

SSH movie

Ring A

Schmid et al. (2003)

Eddy kinetic energy (cm2/s2)

Cape Cauldron

Wind and SLP anomaly off Africa

Risien et al.(2003)

January–March 2000 falls within the protracted 1998–2001 La Nina -> high pressure anomaly in mid-latitude South Atlantic and South Indian Oceans and low pressure anomaly over southern Africa, stronger easterlies over Benguela region.

Upwelling regions off Africa & wind variability

Risien et al.(2003)Lutjeharms and Meeuwis (1987)

pulses of stronger winds during the April–October 2000 period

Benguela

34oS

26oS

22oS

12oS

31oS

17oS

Variability at periods:

4 – 16 days in 10-23.5oS

4-12 and 25-50 days in 24-35oS

Angola-Benguela Front

John et al. (2004)

quiescent situation after strong upwelling

Angola-Benguela Front

John et al. (2004)

SST and faunistic advection due to the anticyclonic circulation off Angola.

Correlation of SSTA and Wind

Enfield and Mestas-Nunez (2000)

SSTA and

SLPA

Sterl and Hazeleger (2003)

Based on 52 years of data (1949–2000) from the NCEP/NCARReanalysis (Kalnay et al. 1996).

maxima of SST coincide with regions in which the pressure has a large gradient

Lagged regression SSTA (colors) and SLPA (10 hPa)

Sterl and Hazeleger (2003)

Build-up and destruction of the anomalies takes 6 months. Relationship strong during build-up, weak during destruction phase.

Lagged regression SSTA (colors) and

Sterl and Hazeleger (2003)

Heat flux acts to dampen (positive heat flux -> cooling) or enhance (negative heat flux) the SSTA anomalies.

total heat flux (3e+7 Km/s) wind stirring (3e+7 Km/s)

the regions of maximum SSTA and maximum wind stirring are displaced relative to one another

Lagged regression SSTA (colors) and

Sterl and Hazeleger (2003)

Ekman pumping (1e+7 Km/s) merid. Ekman Tr. (1e+7 Km/s)

Impacts of Ekman heat transport and Ekman pumping are at least a factor of three smaller than heat flux.

South Atlantic SSTA+ dipole

Haarsma et al (2003)

Atmospheric pressure and wind anomalies in response to the SSTA+ dipole

Haarsma et al (2003)

Most prominent is the response centered at 30W 15S during the austral summer (Jan.–March), mainly due to the equatorward SSTA pole (anomalous low MSLP and cyclonic circulation, anomalous anticyclonic circulation at 200 hPa).

MSLP anomaly 200 hPa streamfunction anomaly

Anomalies of precipitation and near-surface wind in response to the SSTA+ dipole

Haarsma et al (2003)

precipitation (mm/day) near surface wind (m/s)

Anomalous low (high) precipitation associated with anomalous advection of moisture (dry -> N, moist -> S) causing anomalous down- (up-) welling -> ITCZ is displaced southward towards warm SSTA.

Anomalies of net upward surface heat flux and cloud cover in response to the SSTA+ dipole

Haarsma et al (2003)

Anomaly of NSHF Anomaly of total cloud cover

Shift of ITCZ -> shift of cloud cover, but no significant change of NSHF since SSTA is too far south. Over the SSTA anomalies significant NSHF change mainly due to latent heat flux.

South Atlantic tropical cyclones• Angola tropical depression, April 10-15, 1991 (cyclone on

13.?) – for now the only one in the eastern South Atlantic

• Tropical depression, January 18-21 (cyclone on 19.?) southeast of Salvador, Brazil

• Hurricane Catarina March 22-28, 2004

• 2006: two short-lived tropical/subtropical systems were observed over 27oC water in a region of strong wind shear (Jeff Masters, www.wunderground.com):– February 23 near 29S, 36W (600 miles SE of Rio de Janeiro),

survived about 4 hours

– March 15 off South Brazil

Angola cyclone

Hurricane Catarina

Mean SST for March 21-28, 2004.Circle indicates Storm location on March 26.

animation