Bottom (Vertical) Currents – AKA Thermohaline circulation
•
I. Introduction
•
Major Influence is Density
D=mass (gm)/unit volume (ml or cm3)
•
Density of ocean water influenced by several properties:
Temperature
Salinity
Pressure
•
II. Temperature
Increased heat causes increased molecular motion.
Warm water is less dense and tends to rise or float
Cold water more dense tends to sink
Temperature measurements easily made
most commonly measured oceanographic variable
•
Surface of ocean is Heated by:
Radiation from sun
Conduction of heat from atmosphere
Condensation of water vapor
Surface of ocean is Cooled by:
Reflection of radiation (solar) back to atmosphere
Conduction of heat back to atmosphere
Evaporation
•
Amount of solar radiation depends on latitude and time of year
•
Ocean currents transfer heat from one area to another by convection (contact)
Convection is heat transfer
•
III. Salinity
Defined as the total amount of dissolved salt (NaCl) in parts per thousand 0/00
NaCl is very soluble in water; dissolved in rain runoff from continents and
concentrated in oceans by evaporation
Salinity is measured by electrical conductivity - its ability to transmit an electric
current and dependent upon concentration of ions (charged ions)
Hydrometer or refractometer
•
Factors influencing Salinity
Evaporation- as freshwater evaporates, salt is concentrated (at surface).
Precipitation- salt is diluted at surface
Freezing of Ice- ice is mostly freshwater, thus salt is concentrated
Thawing of Ice- salt is diluted
•
IV. Pressure
Increased depth----Increased pressure
In Arctic and Antarctic Regions
Low temperatures causes ice formation and increased salinity
–
High salinity coupled with very low temperature produces very dense water,
which plunges down. These bottom currents move slowly across ocean floor
toward the equator
•
V. Density Problems
•
A liquid has a mass of 5 gms and has a volume of 5ml (cm3). What is the density?
•
An object has a volume of 15 liters and a mass of 7.5 kg. What is the density?
•
A liquid has a mass of 2.5 gm and a density of 2gm/ml. What is the volume?
•
Movement of Bottom Currents
•
Driven by Density Differences; causes stratification (or layering) of water
–
More dense water tends to stay on the bottom (colder, saltier, high pressure)
–
Less dense water tends to stay on top (warmer, less salty, less pressure)
Layers create invisible boundaries
•
Thermocline zone where temperature changes rapidly
•
Halocline zone - where salinity changes rapidly
•
Pycnocline zone –where both temperature and salinity change rapidly
•
Importance of Layering
•
Plants (algae) are the surface and produce oxygen. This oxygenated water tends to
stay at the surface (due to warmth).
•
No Plants (algae) are at the bottom to produce oxygen. Bottom waters receive oxygen
through slow diffusion or atmospheric mixing at the poles (where they begin). Generally there is
less oxygen at greater depths and fewer animals with depth.
•
Nutrients (minerals, organic material) are at the bottom. Plants (algae) that need these
are at the top.
•
Away from shores (in middle of ocean), the water is mostly devoid (barren)to life, due to
a lack of nutrients. The water is very clear. This is due to layering!
•
Layering is an important limiting factor to the amount of life present.
•
Wherever nutrient-rich bottom waters are brought to the surface, there is much life.
Helps set up and sustain the FOOD CHAIN!
•
Along coasts (Shallow Marine Environment) and continental shelves, the water is mixed:
due to wind and wave action. There is very little layering–many organisms will be present.
•
Special Kinds of Deep Currents
•
Upwellings – winds blow surface waters out to sea allowing nutrient-rich water to rise.
ex: West coasts of South America and Africa
•
Deep Counter Currents - water flows in opposite direction to surface current above.