Dynamics
Outline
1. Ocean Overview of Observed Physical Patterns
Winds, Ocean Temperature, Salinity, Surface + Deep Currents
2. Atmospheric Circulation
Coriolis Force, Atmospheric Convection, Hadley Circulation, Surface Wind Patterns
3. Density Structure of the Ocean
Salinity Variations, Ocean Heating and Cooling, Seawater Density, Buoyancy and Density Stratication
4. Wind-Driven Surface Circulation
Acting Forces, Ekman Transport in the Surface Ocean, Geostrophic Currents in the Subsurface Ocean, Subtropical Gyre Circulation, Equatorial and Coastal Upwelling
5. Thermohaline Circulation of the Deep Ocean
Water Mass Identication by T-S Signature, Rate of Movement with 14C, Location of Deepwater Formation, Global Conveyor Belt Circulation and Global Heat Transport
Density Structure of the Ocean
1. Ocean currents are forced by the direct action winds blowing over the ocean surface and by internal pressure forces created, in part, by the unequal distribution of density in the ocean.
2. The vertical distribution of density directly influences the vertical movement of water in the ocean and indirectly influences the horizontal movement.
3. Because salt concentration and temperature jointly determine seawater density, the concepts of ocean salinity and ocean heat content are presented in some detail.
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Surface Salinity
1. The total salt content of the ocean is essentially constant
a) b) c) Input: Weathering of continental rock, and subsequent transport by rivers, brings salt ions to the ocean Output: Mineral precipitation within the ocean removes salt ions from solution The input and output rates have been roughly equal for millions of year i.e. steady state conditions have been achieved
2. While the total amount of salt in the ocean does not vary, the unequal addition/removal of freshwater over the global oceans surface creates large regional differences in surface ocean salt concentration 3. Salinity is a measure of salt concentration and is often expressed as the number of grams of salt in a thousand grams of seawater, expressed as parts per thousand and denoted by the symbol . 35 grams of salt in 1000 grams of seawater has a salinity of 35
Note that the symbol is closely related the symbol for parts per hundred % also known as percentage.
Variation in Surface Salinity
1. A result of surface ocean evaporation and atmospheric precipitation (rain) 2. Evaporation at the ocean surface removes only freshwater and leaves behind salt - thus increasing surface ocean salinity 3. Atmospheric precipitation adds freshwater to the surface ocean - thus reducing surface ocean salinity 4. Overall, salinity is a direct function of evaporation minus precipitation
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Annual Average Precipitation Pattern
Millimeters Per Day
Hadley circulation produces upward convection and high precipitation along the equator and also at about 60 Latitude
Annual Average Evaporation Pattern
Millimeters Per Day
Hadley circulation at around 30 latitude is where cold dry air aloft descends and warms and spreads out north/south (and turned by Coriolis) over the earths surface. The warm and dry surface winds are conducive to strong evaporation in the subtropics regions centered at 30 latitude.
Ocean Surface Salinity
Parts per Thousand
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Comparison Between Salinity and Evaporation Minus Precipitation
Zonally Averaged Values (Along Lines of Constant Latitude)
Heating the Surface Ocean
Main Heat Flux Components In and Out of the Surface Ocean
1. 2. Ocean temperature is a measure of the heat energy contained in the ocean. When more energy enters than leaves the ocean surface, the ocean surface warms. Similarly, when more heat leaves than enters the ocean surface, the ocean surface layer cools. Sunlight energy always adds heat to the ocean (during daylight hours). The thermal/infrared radiation from the warm surface ocean causes heat loss. Heat is also lost from to ocean due to evaporative cooling. Just as a wind blowing over a wet skin cools the skin by evaporation, so too it cools the surface ocean. Heat can be lost or gain by the surface ocean through a process called heat conduction which can be thought of as heat diffusion. If the ocean is warmer than the atmosphere, heat will leave the ocean. If the atmosphere warmer is than the ocean, heat will enter the ocean
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Depth of Light Energy Penetration Into the Surface Ocean
1. In the clearest open-ocean waters: Light penetrates down to about 150-200 meters 2. In turbid coastal waters water: light may only penetrate a few meters to maybe 50 meters
Typical Vertical Profile of Temperature
1. Because sunlight is quickly absorbed in the ocean, all solar heating takes place very near the ocean surface. Vertical mixing of surface water can move some of the warm surface water deeper, but mixing only reaches at most to about 500 meters. Seawater below 500 meters is uniformly cold. The broad region centered at around 500 meters where seawater temperature changes from warm to uniformly cold is referred to as the permanent thermocline In some regions, there is a much shallower thermocline that only forms in summer and is then is erased in winter. This temporary thermocline is referred to as the seasonal thermocline
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Seawater Density and Vertical Density Stratification of the Water Column
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Graphical Depiction of Density (grams per liter) as a Function of Temperature and Salinity
Typical Vertical Density Profile
1. In a stably stratified ocean, the least dense water floats above the more dense water. The seasonal pycnocline (region of strong change in density with depth) comes and goes with net heat gains in spring and summer and net hear losses in fall and winter The permanent pycnocline remains in place and is the result of the long-term balance downward heating/mixing at the surface and upward mixing of cold water from below.
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The Seasonal Thermocline
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Seasonal Variation of Heating and Cooling of the Surface Ocean at Mid-Latitude Regions
Annual Range of Sea Surface Temperature
Seasonal Change in Surface Layer Temperature and Surface Layer Thickness
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Cartoon Depicting the Latitudinal Range of the Seasonal and Permanent Thermocline
Wind-Driven Circulation of the Surface Ocean
Acting Forces in the Wind-Drive Circulation
Surface Layer: 1. Fluid friction is high where there are strong changes in fluid velocity with change in depth. Pressure gradient force is present but dwarfed by the strong friction force so it is neglected. Coriolis Force is ever present.
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Deep Ocean: 1. The fluid in the thick deep layer moves essentially without friction! 2. 3. fluid friction is negligibly small because only small changes in fluid velocity with change.
Without friction the fluid easily moves in response to small pressure gradient forces. Coriolis Force is ever present.
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Ekman Transport of Water in the Surface Layer of the Ocean
Ekman Transport Due to Wind Force, Friction Force and Coriolis Force
1. Ekman Spiral: the spiraling of thin ocean currents within the Ekman Layer
top-most current moves at 45-degrees to the wind forcing. The bottom-most current moves directly opposite the wind direction.
Ekman Layer
2. Ekman Transport is the rate of total water transported in the Ekman Layer
Derived by summing all the individual thin currents over the entire Ekman Layer
Direction is exactly at 90 degrees of the wind direction
3. Oceanographers often treat the Ekman Layer as a slab of water that moves in unison at 90 degrees to the right (northern hemisphere) or left (southern hemisphere) of the wind direct
Geostrophic Motion in the Subsurface Layer of the Ocean
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Response of Fluid in the Deep Ocean Due to Pressure Gradient Force and Coriolis Force
1. Fluid first respond to pressure gradient force (blue arrow) and moves down the pressure gradient toward lower pressure Coriolis force (red arrow) always pushes the fluid to the right of it present direction of travel The fluid continues to be pushed by Coriolis until the Coriolis force is directed equal and opposite of the pressure gradient force - at which point the two forces cancel and the fluid move at steady speed (no acceleration) - remember too that there is not friction to bring this steady motion to a halt! The point at which currents move with steady speed with Coriolis and pressure gradient forces in prefect opposition is referred to as geostrophic balance and the resulting current is referred to as a geostrophic current.
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