Lecture 4:the observed mean circulation. Atmosphere, Ocean, Climate Dynamics EESS 146B/246B

Transcription

1 Lecture 4:the observed mean circulation Atmosphere, Ocean, Climate Dynamics EESS 146B/246B

2 The observed mean circulation Lateral structure of the surface circulation Vertical structure of the circulation Abyssal circulation

3 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.

4 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.

5 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.

6 10 year mean dynamic topography Highs in the subtropics (20º< latitude <40º), lows in the subpolar regions.

7 Geostrophic surface currents and the dynamic topography Subtropical gyres Highs in the dynamic topography drive the subtropical gyres with their anticyclonic circulation.

8 Subpolar gyres Geostrophic surface currents Antarctic Circumpolar Current Lows in the dynamic topography drive the cyclonic circulations in the subpolar gyres and the Antarctic Circumpolar Current.

9 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.

10 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.

11 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.

12 Vertical structure of the circulation Down-stream velocity (m/s) and Density (kg/m 3 ) January 2006 CLIMODE cruise z (m) latitude temperature (ºC) 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.

13 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.

14 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.

15 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.

16 Bottom intensified currents North-South velocity (cm/s) 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 isopycnals They are associated with tilted isopycnals that lean on the topography. Figures from Joyce et al DSR 2005

17 High latitude origin of DWBCs Salinity Pot. Temperature psu ºC DWBCs transport cooler and fresher waters from the high latitudes and thus are a critical component of the thermohaline circulation.

18 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.