Causes of Tropical Circulation Variability
Peter Brandt, Marcus Dengler, Rebecca Hummels, Josefine Herrford
Large Scale Thermohaline Circulation
Kuhlbrodt et al. 2007 2
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
AMOC variability
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
AMOC Transport 36°N
53°N
Collapse of the Atlantic Thermohaline Circulation
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
precipitation evaporation
[m/yr]
Vellinga and Wood 2002
°C
Temperature
anomaly 20-30 yr after the collapse
Chang et al., 2008
Change of Tropical Climate
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.
Model Study
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
Change of Tropical Climate
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.
Chang et al., 2008
From J.-Y. Park
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
Role of Agulhas leakage
High correlation between MOC and NBC transport
Biastoch et al. 2009
Biastoch et al. 2008
MOC NBC
Increased Agulhas leakage:
salinity increase within the NBC
Lübbecke et al. 2015
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
Two pathways of the upper limp of the MOC:
1) Supply of EUC and NECC/NEUC mainly from the southern hemisphere
2) NBC rings
South of the equator, the DWBC eddies are a
major component of the cold water branch of the Meridional Overturning Circulation
Recife
Western Boundary Circulation
Dengler et al. 2004
Hummels et al. 2015
& update
Western Boundary Mooring Array at 11°S
Mooring array installed from 2000 to 2004 and from 2013 ongoing
Hummels et al. 2015
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
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.
Hummels et al. 2015
Water Mass Changes at 11°S
Decadal salinity trend with increasing salinities in the central water range and salinity decrease in NADW
Increase in oxgen albeit higher variability
Large Scale Advection of North Atlantic Deep Water
Rhein et al. 2013
Herrford et al. 2016, submitted
AABW Temperature Trends
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
