Lecture 9: Hydrologic Cycle (cont’d) and Ocean Circulation

Relative Humidity.
Relative actual water vapor content of the air
= ––––––––––––––––––––––––––––– x 100
Humidity max. water vapor capacity of the air

Bringing vapor pressure to saturation point:
o Evaporation increases as unsaturated air moves over lakes or the ocean
o And temperatures can decrease as air moves over cooler surfaces.
o Uplift - any process by which air is raised to a higher altitude. We can appreciate where precipitation is distributed around the g lobe, therefore, by understanding that most ppt occurs as air cools when it is forced to rise.

Uplift can result from 3 processes,
o mixing of air masses of different densities (as along the polar front
o convection
o Orographic movement of air up and over mountains

Deserts occur where there is
o inadequate supply of water (interior of large land masses),
o uplift is suppressed –
o Deserts also occur on the west coasts of large continents in areas that lie equator-ward of the midlatitude low pressure systems, because they often have cold offshore ocean currents. These are called Littoral deserts. E.g., desert of Baja California.

OCEAN CIRCULATION

Movement of the oceans is tied closely to the circulation of the atmosphere and both are ultimately driven by the distribution of available solar energy and their motions are linked by friction at the sea surface.

Coriolis force also influences ocean when it moves, which plays a role in circulation pattern.

The oceans are vertically stratified, with more dense water at the bottoms of the major ocean basins and less dense water near the surface.
Density is controlled by the temperature and the salt content (salinity) of the water.

Winds and Surface Currents:

o Do the oceans circulate for the same reason as the atmosphere
o 90% of solar radiation is absorbed in the first 100 m of ocean water.
o Temperature changes in the ocean occurs very slowly because of high heat capacity.
o Movement of Wind over the ocean surface: friction.
- surface – ocean wind-drift currents
- Coriolis effect - observed deflection ca. 20 – 25° from wind direction
- Trade winds produce westward flowing currents
- At higher latitudes Westerlies produce eastward flowing currents
- At ocean boundaries, water deflected
- Currents complete a large circular circulation pattern called a GYRE in the subtropical oceans. Clockwise in the north, Counter clockwise in the southern hemisphere
- mental model of how surface currents flow. But actual ocean surface circulation is more complex and has differences not seen in this simplified model.
o For example, our model would predict that water would pile up (converge) along the northeast and southwest corners of the gyre. In fact the convergence happens in the middle of the gyre.
o Complexities arise from coriolis effect, friction between wind and the water surface, friction between layers of the ocean and result in a net movement of water, 90° angle to the wind direction.
This net movement is referred to as Ekman Transport,
o An effect of Eckman Transport is to push water towards the center of the gyre, convergence
o We see areas of Divergence – where water is pulled away at right angle to the wind direction.

Lecture 10: Ocean Circulation

Upwellng and Downwelling
o Where surface water piles up, the sea surface rises and layer of water thickens.
o Accumulation of water causes it to sink Downwelling
o Conversely, where divergence occurs, sea surface drops and layer thins. Cooler water rises from below to replace it: Upwelling.
Boundary currents:
o Western boundary currents tend to be faster flowing with a narrower path
o Eastern boundary currents tend to be more diffuse and with a slower flow
o Examples:
o Western Boundary: Gulf Stream begins as a narrow flow (50-75 km wide) traveling with surface speeds of b/t 3 – 10 km/hour, and moving warm water (20° C or higher).
o eastern boundary current, the Canary current: Much broader – up to 1000 km across, slower flow
Summary: Surface Ocean Circulation
o Ocean currents aid in latitudinal redistribution of energy:
o e.g., No. Atlantic drift moves warm water up into northern Europe
o coast of Labrador where temperatures are much colder
o Cold currents flowing from the poles along CA coast, for example,(or the more developed HUMBOLDT current off So. America) resulting in littoral deserts

Circulation of the Deep Ocean
Driven by differences in water density
Density differences driven by differences in Salinity and Temperature
Salinity
o Salt content of water mass (g/kg) expressed as ppt (or ‰) parts per thousand
o Average salinity of oceans is ca. 35 ppt
o Primary constituents: Cl-, Na+ (other ions as well)
o Salts derived from weathering of crustal rocks
o Oceans remove salts from seawater at same rate overall as added

Thermohaline Circulation
o Horizontal changes in density are small; vertical changes can be large
o Vertical structure is stable
o Movement of water through deep ocean is relatively slow (1000 years)
Vertical Structure:
o Surface: 60 – 100 m (lower density); interacts with atmosphere. Mixed layer
o Transition zone: 1000 m thick, Pycnocline zone: steep gradient in density, salinity and temperature
o Deep Ocean water: little movement vertically; most is subhorizontal.


Salt content and the age of the earth

Thermohaline Ciruclation


The importance of oceans in the study of Earth’s past climates:
Ocean Sediments
Importance of oceans in Earth’s climates: Surface circulation and Deep water circulation