1. Causes of Tropical Circulation Variability
February 1, 2013
Tropical Ocean Dynamics - Lecture 12
Causes of Tropical Circulation Variability
Peter Brandt, Marcus Dengler, Rebecca Hummels, Josefine Herrford
Peter Brandt - GEOMAR

2. Large Scale Thermohaline Circulation
November 12, 2010
Tropical Ocean Dynamics - Lecture 3
The western boundary regime of the tropical South Atlantic Ocean is a crossroads of important meridional transfers of warm and cold water masses. As part of the meridional overturning circulation (MOC) of the Atlantic, approximately 17–20 Sv (Sv =10^6 m^3 s^-1) of North Atlantic Deep Water (NADW) pass southward through the equatorial zone, compensated by a net northward transport of warm and intermediate waters and of Antarctic Bottom Water (e.g. Schott et al., 2005). Furthermore, the upper-layer western boundary flow, carried by the North Brazil Undercurrent (NBUC), is a crucial link within the Atlantic subtropical cell (STC) connecting the subduction regions of the subtropical South Atlantic and the eastward equatorial and off-equatorial undercurrents that supply the equatorial and eastern boundary upwelling regimes (Malanotte-Rizzoli et al. 2000; Zhang et al. 2003; Snowden and Molinari 2003; Schott et al. 2004). In addition to the shallow and deep overturning cells, the transport pattern of the tropical South Atlantic is complicated by the southward cross-equatorial Sverdrup transport, of the order of 10 Sv (depending on the wind stress climatology used, Mayer et al. 1998; Townsend et al. 2000), that also requires northward compensating return flow along the western boundary.
Kuhlbrodt et al. 2007
Peter Brandt / Marcus Dengler

3. RACE Contribution to AMOC Observing System
Denmark Strait Overflow
Deep Water Export from the Labrador Sea
Exchange between Subtropical and Subpolar Gyres
West-East Exchange, North Atlantic Current
AMOC variability in the tropics and impact on tropical circulation and climate

4. Tropical Ocean Dynamics - Lecture 12
AMOC variability
February 1, 2013
Tropical Ocean Dynamics - Lecture 12
Heat flux and wind stress associated with the North Atlantic Oscillation (NAO) responsible for AMOC changes
AMOC changes in the subpolar gyre reverberate in the subtropical North Atlantic.
Böning et al. 2006
(a) Variability of the dense portion (Labrador Sea Water and deeper) of Western Boundary Current transport at 53°N in the 1/3°- (dashed black) and 1/12°-models (blue), in relation to the convection intensity as given by the anomalies in the mean depth of the mixed layer in winter (in green; the anomalies represent the deviations from a time-mean depth of 1030 m, taken over the area of 56.5°–58.5°N, 55°–53°W; the green shading highlights strong convection episodes); all time series 2-year filtered. (b) Anomalies in the MOC transport (in Sv) as a function of latitude and time in 1/3°-HEAT, showing the rapid southward spreading of the dynamic response to the variation in LSW formation. (MOC anomalies are defined as the deviations from the time-mean MOC at each latitude; the maximum mean MOC transport is 18 Sv at about 40°N.) (c) Variability of the MOC transport at 36°N in the 1/3°- and 1/12°-models (thin purple and blue curves, respectively), and in 1/3°-HEAT (red), in relation to the deep WBC at 53°N in the 1/3°-model.
AMOC Transport 36°N
53°N
Peter Brandt - GEOMAR

5. Collapse of the Atlantic Thermohaline Circulation
February 4, 2016
Thermohaline Circulation - Lecture 12
Collapse was forced in the model by applying a strong initial freshening to the top layers of the North Atlantic
Immediate reduction of the northward heat transport and cooling of the northern hemisphere
Slower warming of the southern hemisphere
Temperature anomaly yr after the collapse °C
Vellinga and Wood 2002
(right) Change in surface air temperature during years 20–30 after the collapse of the THC. Areas where the anomaly is not significant have been masked.
(bottom) Significant changes during years 20–30 of the experiment of annual mean (a) precipitation and (b) evaporation into the atmospheric boundary layer. Units are m year−1, and blue (red) colours indicate anomalously wet (dry) conditions. The precipitation pattern indicate a southward shift of the ITCZ.
precipitation
evaporation
[m/yr]
Peter Brandt - GEOMAR

6. Change of Tropical Climate
February 1, 2013
Tropical Ocean Dynamics - Lecture 12
Southern hemisphere warming was found to be the results of modified STC pathways due to a substantially weakened AMOC (Chang et al. 2008)
When the AMOC is substantially weakened, the northern STC becomes a closed cell and transports warmer NACW toward the equator.
Chang et al., 2008
Peter Brandt - GEOMAR

7. Tropical Ocean Dynamics - Lecture 12
Model Study
February 1, 2013
Tropical Ocean Dynamics - Lecture 12
OAME: AMOC collapse (no northward mass transport) and heat flux from climate model water hosing runs
OME: only AMOC collapse
AME: only heat flux
Wen et al., 2011
The results of the experiments reveal the relative importance of oceanic processes and atmospheric processes in AMOC-induced tropical Atlantic variability/change. It is found that the oceanic processes are a primary factor contributing to the warming at and south of the equator and the precipitation increase over the Gulf of Guinea, while atmospheric processes are responsible for the surface cooling of the tropical north Atlantic and southward displacement of ITCZ.
Peter Brandt - GEOMAR

8. Change of Tropical Climate
February 1, 2013
Tropical Ocean Dynamics - Lecture 12
Warming of the tropical thermocline and weakened stratification south of the equator.
Result is a weaker seasonal cycle and weaker interannual variability in the cold tongue region.
c, Upper ocean temperature averaged over the area between 30°W and 0°E in longitude and 5°S and 0°N in latitude indicated by the rectangle in a as a function of depth and time. The dotted line indicates the abrupt changes in temperature. d, Time evolution of the SST averaged over the area indicated by the rectangle in a as a function of time. Rapid equatorial warming is seen about 20–25 years into the simulation in b–d. Ensemble averaging has been applied to all of the fields.
Chang et al., 2008
Peter Brandt - GEOMAR

9. Future Sahel Draughts
Large uncertainty in the projections of Sahel rainfall
Park et al. (2015) shows the importance of the differential warming of extratropics and tropics
From J.-Y. Park

10. Role of Agulhas leakage
High correlation between MOC and NBC transport
MOC
NBC
Biastoch et al. 2008
Increased Agulhas leakage: salinity increase within the NBC
Biastoch et al. 2009

11. Heat Content change due to Agulhas Leakage Dynamics
Increased Agulhas leakage contributes to tropical Atlantic (20°S-10°N) warming trend during recent decades
Hindcast: 1.27Sv/decade
Observations
Idealized simulations with different
Agulhas leakage
REF: 16.8 Sv
LOCAL 19.2 Sv
Lübbecke et al. 2015

12. Western Boundary Circulation
February 1, 2013
Tropical Ocean Dynamics - Lecture 12
Two pathways of the upper limp of the MOC:
1) Supply of EUC and NECC/NEUC mainly from the southern hemisphere
2) NBC rings
Recife
South of the equator, the DWBC eddies are a major component of the cold water branch of the Meridional Overturning Circulation
We call the northward flow at the western boundary “North Brazil Undercurrent” south of about 5°S, where the surface flow is dominantly southward due to westward wind stress. We call it “North Brazil Current” north of about 5°S, where the surface flow is dominantly northward, fed by the central branch of the South Equatorial Current.
Dengler et al. 2004
Peter Brandt - GEOMAR

13. Western Boundary Mooring Array at 11°S
Mooring array installed from 2000 to 2004 and from 2013 ongoing
Hummels et al. 2015
& update

14. NBUC Transport Anomaly
Geostrophic calculation (Zhang et al. 2011) and 1/10° model simulation show multidecadal variability
Mooring data in general agreement with model results
Hummels et al. 2015
Using additional eastern boundary transport measurements, bottom pressure measurements and interior hydrographic and satellite data, an AMOC time series will be estimated at 11°S.

15. Water Mass Changes at 11°S
Hummels et al. 2015
Decadal salinity trend with increasing salinities in the central water range and salinity decrease in NADW
Increase in oxgen albeit higher variability

16. Large Scale Advection of North Atlantic Deep Water
Rhein et al. 2013

17. AABW Temperature Trends
Herrford et al. 2016, submitted

18. Summary
RACE provides an important contribution to the Atlantic Ocean observing system
Decadal variability impact tropical Atlantic circulation and climate: long-term effect on rainfall, seasonal cycle and interannual variability
Observed decadal NBUC water mass variability associated with Agulhas leakage
Decadal deep and bottom water changes
Continuous observing systems are required to address long-term climate changes