Download Lecture 4:the observed mean circulation

Lecture 4:the observed mean
circulation
Atmosphere, Ocean, Climate
Dynamics
EESS 146B/246B
The observed mean circulation
• Lateral structure of the surface circulation
• Vertical structure of the circulation
• Abyssal circulation
Satellite Altimetry
Figure courtesy NASA/JPL
•The TOPEX/POSEIDON satellite altimeter measures the distance between itself
and the surface of the ocean based on the travel time of a radio wave sent from the
satellite and bounced off the surface.
The Geoid
GEOID
Lab
Earth
•Similar to a rotating tank of water in the lab, the ocean surface is distorted even in the
absence of currents. Rather than being spherical it takes on an ellipsoidal shape,
known as the geoid.
•Currents will lead to modulations of the surface relative to the geoid.
The dynamic topography
Figure/animation courtesy NASA/JPL
•The satellite altimeter measures sea surface height anomalies relative to the geoid,
known as the dynamic topography
•Using the geostrophic balance, gradients in the dynamics topography can be used to
estimate the surface circulation.
10 year mean dynamic topography
•Highs in the subtropics (20º<|latitude|<40º), lows in the subpolar regions.
Geostrophic surface currents and the
dynamic topography
Subtropical
gyres
•Highs in the dynamic topography drive the subtropical gyres with their
anticyclonic circulation.
Geostrophic surface currents
Subpolar
gyres
Antarctic
Circumpolar
Current
•Lows in the dynamic topography drive the cyclonic circulations in the subpolar
gyres and the Antarctic Circumpolar Current.
Western boundary currents
•Currents are much stronger on the western than eastern side of ocean basins this
is a phenomenon known as western intensification.
•In these western boundary currents, the speed can exceed 100 cm/s
•Flows in the center of the gyres is < 10 cm/s.
Important wind-driven surface currents
•The strong currents on the western side of ocean basins transport warm water from low to high
latitude in the subtropics and cool water from high to low latitudes in the subpolar regions.
Important wind-driven surface currents
•In the Southern Ocean the Antarctic Circumpolar Current is unabated by continents and transports
vast amounts of water from west to east.
•On the eastern side of ocean basins, weaker cold currents move water from high to low latitudes.
Vertical structure of the circulation
temperature (ºC)
January 2006 CLIMODE cruise
z
latitude
(m)
Down-stream velocity (m/s) and
Density (kg/m3)
2
1
4
3
www.climode.org
Cross-stream distance (km)
longitude
•The circulation tends to be strongest near the surface and decays with depth.
•The vertical shear is associated with horizontal density gradients consistent with
the thermal wind balance.
Compensation of the pressure gradient force
in the deep ocean by the SSH
•Density surfaces dip down in the subtropical gyres and rise up in the subpolar
gyres.
Compensation of the pressure gradient force
in the deep ocean by the SSH
•Density surfaces dip down in the subtropical gyres and rise up in the subpolar
gyres.
Compensation of the pressure gradient force
in the deep ocean by the SSH
•SSH is high (low) in the subtropics (subpolar) and thus generates a PGF that
counteracts the PGF in the deep associated with the horizontal density gradient.
Bottom intensified currents
(cm/s)
North-South velocity
isopycnals
•In the deep abyss, on the western side of
ocean basins, bottom intensified flows known
as Deep Western Boundary Currents are
observed.
DWBC
flowing to
the south
•They are associated with tilted isopycnals
that lean on the topography.
Figures from Joyce et al DSR 2005
High latitude origin of DWBCs
Pot. Temperature
•DWBCs transport cooler and
fresher waters from the high
latitudes and thus are a critical
component of the thermohaline
circulation.
psu
ºC
Salinity
DWBCs in the Pacific
Higher O2
concentration
•In the south Pacific, isopycnals lean on the topography in the western part of the basin.
•Isopycnal tilt would be associated with a northward flowing bottom intensified flow.
•Enhanced oxygen concentration in these deep waters suggests that they were recently
in contact with the surface and formed in the seas around Antarctica.