Atmosphere, Ocean, Climate Dynamics: the Ocean Circulation EESS 146B/246B

Transcription

1 Atmosphere, Ocean, Climate Dynamics: the Ocean Circulation EESS 146B/246B Instructor: Leif Thomas TA: Gonçalo Zo Zo Gil

2 Course Objectives Identify and characterize the components of the ocean circulation that play a key role in regulating the Earth s climate and explain the basic physics that governs their dynamics. Animation of Sea Surface Temperature (SST) courtesy of NASA

3 In what ways does the ocean regulate the Earth s climate?

4 Role of the ocean in the sequestration of CO 2 Since the start of the industrial revolution, the ocean has been the only net sink of anthopogenic CO2 on Earth (Sabine et al. 2004). Without the oceanic carbon uptake the concentration of CO2 in the atmosphere would be ~55 ppm higher. Column inventory of CO 2 The distribution and storage of the anthropogenic CO 2 in the ocean is shaped by the circulation.

5 Subduction and the sequestration of anthropogenic CO 2 at ocean fronts Anthropogenic CO 2 (µmol kg -1 ) 60 S Equator 40 N Figure from Sabine et al, Science 2004 In the Southern Ocean anthropogenic CO 2 is subducted along density surfaces that outcrop at the strong ocean fronts. The circulation plays an important role in pumping carbon into the deep ocean where it can be stored there for decades to centuries.

6 Heat transport in the atmosphereocean climate system out in out The movement of air in the atmosphere and water in the ocean both act to regulate the Earth s climate, keeping the temperatures on the Earth from being extreme.

7 Atmosphere-ocean climate system: an analogy out in Poles Tropics insulating wall

8 Atmosphere-ocean climate system: an analogy out in insulating wall

9 Atmosphere-ocean climate system: an analogy out in Remove wall to allow airflow.

10 Atmosphere-ocean climate system: an analogy out in air flow

11 Atmosphere-ocean climate system: an analogy out in Pump water through tubes in the floor.

12 Atmosphere-ocean climate system: an analogy out in in out Ocean circulation Pump water through tubes in the floor.

13 Atmosphere-ocean climate system: an analogy out in Atmospheric circulation 2/3 of the transport in out Ocean circulation 1/3 of the transport

14 The Gulf Stream Why are the strongest currents found in the western side of the Atlantic? What processes set the transport of mass and in the Gulf Stream? How might a change in climate affect the Gulf Stream and vice versa?

15 Eddies in the Gulf Stream SST of the Gulf Stream Courtesy of the Ocean Color Group URI The circulation is turbulent. How do these turbulent motions affect the large scale flow and mix, salt, and dissolved gases?

16 29-March Introduction and motivation 31-March Physical characteristics of the ocean (9.1) 2-April LAB: Constraints imposed by rotation and stratification; Course Schedule 5-April Coriolis force, geostrophy, and the thermal wind balance (6.6, 7.1, 7.3) 7-April Observed large-scale circulation. (9.1, 9.2) 9-April LAB: Taylor-Proudman effect; geostrophic balance 12-April Wind-stress and Ekman layers (10.1) 14-April Ekman pumping and suction and the Taylor-Proudman effect on a sphere (10.1, 10.2) 16-April LAB: Ekman pumping/suction, vortex stretching; 19-April Wind-driven gyres and the Sverdrup balance (10.3) 21-April Western boundary currents, vorticity balance, and Rossby waves. (10.3,11.3.2) 23-April LAB: Wind-driven ocean gyres 26-April Taylor-Proudman in a layered ocean and recirculation gyres (10.4) 28-April Air-sea fluxes, the ocean mixed layer, and water mass formation (11.1) 30-April LAB: Thermal-wind balance; 3-May Three-dimensional structure of the gyres, water masses, and subduction 5-May Antarctic Circumpolar Current and Antarctic Intermediate Water (10.5, 8.3) 7-May LAB: Eddies in the ocean: baroclinic instability 10-May Deep water formation and the global density budget (11.1, 11.2) 12-May Dynamical models of the abyssal, meridional overturning circulation (11.3) 14-May Observations of the abyssal circulation and its water masses (11.4); 17-May Energetics of the ocean circulation 19-May Ocean, freshwater, and carbon transport and budget (11.5,11.6) 21-May LAB: Abyssal circulation 24-May Equatorial circulation (12.2) 26-May ENSO/El Niño (12.2) OVERVIEW AND REVIEW OF BASIC FLUID MECHANICS WIND-DRIVEN CIRCULATION ABYSSAL CIRCULATION ATM-OCEAN COUPLING

17 Prerequisites PHYSICS: course in mechanics and some exposure to the equations of fluid motion. MATH: multivariable/vector calculus (should know what partial derivatives, the gradient, divergence, and curl are).

18 ROTATION Laboratory Demonstrations The Earth rotates about its axis at an angular velocity Experiments with a rotating table will be used to illustrate the constraints that the Earth s rotation imposes on the ocean circulation.

19 Layered nature of the ocean North-South section of density in the Pacific, 170 W Depth (m) 50 S Equator 50 N The ocean is stratified, i.e. layered in density. Because it takes energy to move water across density surfaces, the 3D shape of these isopycnals influences the path of the movement of water. Changes in density are associated with variations in the water s temperature and salinity.

20 Exploring the distribution of temperature and salinity in the ocean using Ocean Data View Identify and gain a familiarity with the geographical distribution of major ocean water masses (i.e. waters with a characteristic temperature and salinity combination).

21 Textbook Chapters 6-7 review of the basic fluid mechanics governing the circulation of the ocean and atmosphere. Chapter 9: description of the largescale ocean circulation Chapter 10: the wind-driven circulation Chapter 11: the abyssal/thermohaline circulation

22 Expectations Grades will be based on four homeworks (80%) and a lab notebook (20%). You will be required to participate in the laboratory demonstrations held nearly every Friday and document what they observe in a lab notebook to be turned in on the Monday after the lab at the beginning of the class period. Problem sets should be turned in at the beginning of the class period. Late assignments will receive a point deduction of 50% of the total.