Dynamic Response of the North Atlantic Circulation to

Dynamic Response of the North Atlantic Circulation to

Dynamic Response of the North
Atlantic Circulation to Ocean Heat
Content Changes between 1990
and 2014
Stuart A. Cunningham & Clare Johnson
Scottish Association for Marine Science (SAMS)
[email protected]
NACLIM ASM, Almada 30th Sept 2015
“The research leading to these results has received funding from the
European Union 7th Framework Programme (FP7 2007-2013) under grant
agreement n.308299 NACLIM Project”
OHC: 0 to 1868 m
Linear change in OHC, Geopotential
height and absolute dynamic sea-surface
height from 1991 to 2014
Geopotential anomaly [m] at 1615 m relative to 0 m
Met Office
Hadley
Centre
EN4: quality
controlled
subsurface ocean
temperature and
salinity profiles
and objective
analyses
AVISO+
Satellite
Altimetry
Data
MADT Change [m]
Good, S. A., M. J. Martin and N. A. Rayner, 2013. EN4:
quality controlled ocean temperature and salinity
profiles and monthly objective analyses with
uncertainty estimates, JGR: Oceans, 118, 6704-6716,
doi:10.1002/2013JC009067
Rio, M. H., S. et al. (2011). "New CNES-CLS09 global
mean dynamic topography computed from the
combination of GRACE data, altimetry, and in situ
measurements." JGR: Oceans 116(C7): C07018.
Calculate absolute velocity changes between 1991 and 2014
Resulting absolute
velocity field is a mass
balanced set of
velocity anomalies
that quantify the
linear least squares
change in velocity
between 1991 and
2014 across these two
sections.
Fluxes of Heat and Fresh-Water in the Labrador
Sea and from Greenland to Scotland due to the
linear change in absolute velocity from 1991 to
2014
Mean Potential Temperature
Mean Salinity
Mass balanced Absolute velocity anomalies v’
Results
Mean Circulation and linear change from 1993 to 2014
Mean Absolute Dynamic
Topography [m]
Linear least-squares change
(1991 to 2014)
Surface Speed [m/s]
Surface Speed [m/s]
Changes in Overturning and Horizontal Circulations
>27.0
>27.65 in
<27.65 out
>27.65
27.6-27.74
27.0-27.2
>27.0
<27.65
<27.74
27.2-27.6
South
North
Zonally Integrated
Volume transport
Anomaly [Sv]
Zonally Integrated Heat
Flux Anomaly [PW]
-0.02 PW
Context for Heat Flux Changes
Atlantic Ocean Heat Transport
Coupled models (CM2.1, CCSM4)
Radiation balance residual (NCEP,ECMWF,TF08)
Global hydro inverse (GW03 Ganachaud)
Air-sea flux climatology (LY09 Large)
RAPID & 41N: Direct
RAPID @ 26.5°N
0.2 PW decline 2004 to 2014
~20%
Bacon, S. (1997).
"Circulation and fluxes
in the North Atlantic
between Greenland and
Ireland." J. Phys.
Oceanogr. 27(7): 14201435.
Hobbs & Willis
@ 41°N
TF08 error bars
0.28±0.06 PW
Decline 1991
to 2014 -0.02
PW ~10%
Zonally Integrated Fresh-Water Fluxes [mSv]
FW Flux [mSv]
(negative = reduced southward flux of Fresh Water)
Recent Warming of the Labrador Sea
Igor Yashayaev and Allyn Clarke
Bedford Institute of Oceanography
[email protected]
-6 mSv
-26 mSv
Context for Fresh Water changes
Arctic Fresh Water Balance [mSv] (summer 2005)
Flux to Atlantic Total 260 mSv
• Davis Strait -119 ± 14 mSv.
• Fram Strait + Barents Sea 141 ± 28 mSv.
•
-26 mSv -22%
-6 mSv -4%
Tsubouchi, T., et al. (2012). "The Arctic Ocean in summer: A quasisynoptic inverse estimate of boundary fluxes and water mass
transformation.” JGR: Oceans 117(C1)
Context for Fresh Water Change
Arctic Fresh Water Storage
•
•
•
•
From mid-1990’s to 2010 Arctic stored 6000-10,000 km3 of fresh water.
Much of this in the Beaufort gyre – due to spin up.
Rate of storage ~19 to 32 mSv.
This study suggests that the export of Fresh Water in the Arctic Outflows and Labrador
Current has reduced by 26 mSv
Giles, K. A., S. W. Laxon, et al. (2012). "Western Arctic Ocean freshwater storage increased
by wind-driven spin-up of the Beaufort Gyre." Nature Geosci 5(3): 194-197.
Rabe, B., M. Karcher, et al. (2011). "An assessment of Arctic Ocean freshwater content
changes from the 1990s to the 2006–2008 period." Deep Sea Research Part I:
Oceanographic Research Papers 58(2): 173-185.
McPhee, M. G., A. Proshutinsky, et al. (2009). "Rapid change in freshwater content of the
Arctic Ocean." Geophysical Research Letters 36(10): n/a-n/a.
Summary of circulation and heat and fresh water flux
changes from 1991 to 2014
1. Atlantic OHC varies significantly both temporally and spatially.
2. Circulation has a dynamical response to OHC changes because of
large gradients between boundary currents and the gyre interior.
3. Vertical and horizontal anomalous circulations show a rich
structure.
4. Basin wide integrations are important for understanding net
changes.
5. Northward heat flux canonical value of ~0.2 PW has reduced by
0.02 PW.
6. Southward Fresh Water Flux has reduced – particularly in the
Labrador Basin – matching rate of Fresh Water storage in the
Arctic.
--- THE END -Linear OHC change from 1990 to 2014
• Thank you for listening
Results
Linear Change in Circulation, Heat and Fresh-Water Fluxes
in the Labrador Sea and from Greenland to Scotland
Calculate absolute velocity changes between 1991 and 2014
• Interpolate monthly EN4 temperatures and salinities to CTD station positions.
• Compute geopotential anomaly at each CTD station.
• Compute linear least-squares change of of geopotential anomaly in time (1991 to
2014).
• Compute geostrophic velocity anomalies from total linear change in geopotential.
• Compute geostrophic surface reference velocities from linear ADT change.
• For the west and east sections separately balance the anomalous mass flux to zero.
Resulting absolute velocity field is a mass balanced set of velocity anomalies
that quantify the linear least squares change in velocity between 1991 and
2014 across these two sections.
Atlantic Circulation
Relationship between OHC & Sea-Surface Height
Variability and Wind Stress Curl Forcing
• Hátún, H., et al. (2005). "Influence of the Atlantic subplar gyre on the thermohaline
circulation." Science 309: 1841-1844.
• Häkkinen, S., P. B. Rhines and D. L. Worthen (2013). "Northern North Atlantic sea
surface height and ocean heat content variability." JGR: Oceans 118(7): 3670-3678.
• Häkkinen, S., P. B. Rhines and D. L. Worthen (2011). "Atmospheric Blocking and Atlantic
Multidecadal Ocean Variability." Science 334(6056): 655-659.
Analysis and figure
courtesey of Dr Bee Berx,
Marine Science Scotland.
AVISO+ Satellite Altimetry Data
Rio, M. H., S. et al. (2011). "New CNES-CLS09 global mean dynamic
topography computed from the combination of GRACE data,
altimetry, and in situ measurements." JGR: Oceans 116(C7):
C07018.
1. ADT is the sum of sea level anomalies and a mean
dynamic topography, both referenced over a
twenty-year period (1993-2012).
2. Key improvements are the use of a new mean
dynamic topography calculated from GOCE satellite
data, and in-situ observations.
3. The geoid model has a horizontal resolution of
125 km.
4. Multivariate objective analysis (including wind and
in situ data) is used to improve the large-scale
solution so horizontal resolution of 0.25°.
5. In the subpolar North Atlantic the spatial resolution
is a ~15 x 28 km.
Met Office Hadley Centre
EN4: quality controlled subsurface ocean
temperature and salinity profiles and
objective analyses
• Observed subsurface ocean temperature
and salinity profiles with data quality
information.
• Objective analyses formed from the
profile data with uncertainty estimates.
• The objective analysis is on a regular 1°
grid at 42 depth levels from 5 to 5350 m.
Good, S. A., M. J. Martin and N. A. Rayner, 2013. EN4: quality controlled
ocean temperature and salinity profiles and monthly objective analyses
with uncertainty estimates, JGR: Oceans, 118, 6704-6716,
doi:10.1002/2013JC009067