global warming and climate changes impact on overturning circulation and global conveyor belt.

1. Presentation of the Review
Article:
Weakening of the Atlantic Meridional
Overturning Circulation (AMOC)
1. Caesar, L., Rahmstorf, S.,
Robinson, A., & Feulner,
G. (2018). Article
Observed fingerprint of a
weakening Atlantic
Ocean overturning
circulation.
2. Rahmstorf, S., Box, J. E.,
Feulner, G., Mann, M. E.,
Robinson, A., Rutherford,
S., & Scha, E. J. (2015).
Exceptional twentieth-
century slowdown in
Atlantic Ocean overturning
circulation. 5(March).
https://doi.org/10.1038/N
CLIMATE2554.

2. INTRODUCTION
PAPER ONE
AMOC is one of
Earth’s major ocean
circulation systems,
redistributing heat on
our planet and
thereby affecting its
climate
DEFINNITION
OF TERMS
The twentieth-
century AMOC
weakening
DIFFERENT
INDICATORS FOR
ATLANTIC OCEAN
CIRCULATION
CAUSES OF THE
WEAKENING AND
IMPLICATIONS FOR
THE FUTURE
EVIDENCE OF
AMOC
WEAKENING

3. Definition of the AMOC index.
the AMOC index IAMOC as the difference
between the mean SST of the geographic
region that is most sensitive to a reduction
in the AMOC (the subpolar gyre region, sg)
and that of the whole globe:
Definition of the subpolar gyre region
SST differences in the subpolar North Atlantic relative
to the global mean SST are dominated by variations in
the AMOC
AMOC index for a certain year
The mean SST in the subpolar gyre region for the following
November−May season, minus the global mean SST for that
season.
ACC: Antarctica circumpolar current
Definition of the Gulf Stream region
the Gulf Stream region is defined as the region that
covers the above-average long- term warming east of
the US coast that results from an AMOC slowdown in
both observations and model
the terms “Gulf Stream region” and “subpolar gyre” region do
not refer directly to ocean circulation features, but rather to
SST features

4. Black circles : grid points of the region under consideration
White : model-average 2 ◦C cooling contour from a climate model
intercomparison1in which the models were subject to a strong AMOC
reduction induced by adding a freshwater anomaly to the northern Atlantic.
analysis for 1901–2000.
Global equal
area map
(Hammer
projection) for
1901–2013;
white indicates
insufficient/inho
mogeneous/poor
data coverage
EVIDENCE OF AMOC WEAKENING

5. Time series of the
maximum overturning
stream function (red) and
the AMOC index (blue).
• Thin lines show annual
values, thick lines
smoothed curves over
11 years.
Connection between the AMOC stream function and the temperature-
based AMOC index in a global warming scenario (RCP8.5)
The twentieth-century AMOC weakening
Spectral analysis of the proxy-
based AMOC index
subpolar gyre reaches nearly its lowest temperatures of the
past millennium in the late twentieth century
Reconstructing the AMOC index over the past millennium • Reconstructing AMOC index based on
• the Northern Hemisphere mean
temperature and SST of the subpolar
gyre.
• the land-and-ocean reconstruction with
the Errors in variables(EIV) method using
all the available proxies based on
Reduction of Error and Coefficient of
Efficiency.
• this series provides a skilful
reconstruction back to AD 900 and
beyond (95% significance compared to
a red-noise null).
AMOC index

6. DIFFERENT INDICATORS FOR ATLANTIC OCEAN CIRCULATION
temperature-based AMOC index
index based on NASA GISS temperature data48 (scale on left).
curve with uncertainty range shows
coral proxy data25 (scale on right).
Analyses of data from hydrographic sections
across the Atlantic at 25◦ N, where a 1 K change
in the AMOC index corresponds to a 2.3 Sv
change in AMOC transport based on the model
simulation
Annually resolved δ15 N data
represent a tracer for water mass
changes in the region.
• high values are characteristic of the
presence of Labrador Slope Water.
• The time evolution of the 15N
tracer agrees well with that of the
reconstructed AMOC index
Other estimates from oceanographic data similarly suggest relatively strong AMOC in the 1950s and 1960s, weak AMOC in
the 1970s and 1980s and stronger again in the 1990s
oceanic nitrogen- 15 proxy data from
corals off the US north-east coast

7. CAUSES OF THE WEAKENING AND IMPLICATIONS FOR THE FUTURE
Mass balance terms of the Greenland Ice Sheet.
• Cumulative anomaly relative to the mean over 1840–1900,
• pre-industrial period during which the Greenland Ice Sheet
was approximately in balance.
Dilution of the surface ocean could have
weakened deep-water formation, slowing the
AMOC
Indirect evidence from multiple sources
provides a consistent picture of causes of the
weakening:
 The time evolution of surface temperature
 ocean circulation
 Greenland ice mass balance
Further freshening of the subpolar Atlantic
in the next few decades might lead to
further weakening of the
AMOC within a decade or two,
 and possibly even more permanent
shutdown of Labrador Sea convection as
a result of global warming

8. COMPARING CLIMATE
MODEL AND SST
OBSERVATIONS
PAPER TWO
AMOC
SEASONAL
VARIATION IN SSTS IN
THE SUBPOLAR GYRE
REGION
AMOC Time
Evolution
Impacts and
Conclusion
PERFORMANCE
OF THE AMOC
INDEX IN
MODELS

9. COMPARING CLIMATE MODEL AND SST OBSERVATIONS
linear trend in SST over the ‘CO2-doubling’ experiment and the
corresponding control run, compared with the observed trend from
1870 to 2016 for November–May season.
Scales are of differing amounts of CO2 forcing
Both patterns comprise an area of below-average warming (normalized
trend < 1) and cooling (normalized trend < 0) in the subpolar gyre region
Comparison of
time series of SST
anomalies and the
strength of the
overturning
circulation in the
CM2.6 model.
anomaly of the actual AMOC overturning rate relative
to the control run

10. SEASONAL VARIATION IN SSTS IN THE SUBPOLAR GYRE REGION
annual-mean, global-mean
warming The SST trends in the subpolar gyre
region are well below the global-
mean warming year-round
(differences are given in numbers
along the x axis for the CM2.6 model
(light grey) and the HadISST data
(dark grey) and highlighted by
arrows), yet are smallest during the
cold part of the year for both
observations and model

11. PERFORMANCE OF THE AMOC INDEX IN MODELS
The graph shows the linear trend in the simulated AMOC decline versus the SST-
based AMOC index in the CMIP5 ensemble regression analysis
 The region defining the
subpolar cold patch is
chosen to be large
enough to encompass the
cooling found across all
models, because its exact
location differs in each
model shows the linear
1870–2016 trend in the
AMOC index, as well as in
the actual AMOC, in these
models

12. AMOC Time Evolution
Time evolution of the
AMOC, reconstructed
from
observational SST data
(blue curve)from the
period 1870–2016
using the calibration
factor 3.8 Sv K −1
found from the CMIP5
models
The calibration factor is
obtained through
regression analysis,

13. Impacts and Conclusion
Over Europe, Summer precipitation
decreases (increases) in northern
(southern) Europe has been associated
with a negative summer North Atlantic
Oscillation signal
 E.g 2015 European heat wave in
addition to Stationary Rossby wave
event
 Increase in storms in Europe.
 Sea Level rise and extreme warming
observed at the US’s east coast and
increase in draught in Sahel.
 Change in hydrological cycle, sea ice
IMPACTS
 Amoc weakening by 15% (3 ± 1 sv)
 Cooling in subpolar gyer region due to
reduction in heat transport and warming of
gulf stream region and northward shift of
gulf stream
 High resolution cliamate models with
increased carbon dioxide concentration
 Reduced northward heat transfer
 Emerging northward shift of the gulf stream
 Direct measurements from RAPID_AMOC
project and other studies show low AMOC
values in recent years.
CONCLUSION