6 - Ocean Circulation Flashcards
Describe the vertical structure of the ocean.
Surface mixed layer - 20-100m thick, interacts with atmosphere, exchanges kinetic energy & heat, typically well mixed
Pycnocline - upper ~1km, zone of transition between surface and deep water, rapid increase in density(usually result of temperature change but sometimes result of salinity change), steep density gradient stabilises layers
Describe the concept of thermohaline circulation.
Like air, denser water sinks and lighter water sinks - cold or more saline water sinks whilst warm or less saline water rises.
Describe the conveyor belt system of thermohaline circulation.
It distrbutes water and heat vertically throuhg the water column and horizontally across the globe - bottom water formation only in Atlantic whilst upwelling over large areas in Indian and Pacific Ocean
Cooling, evaporation and sea ice formation increase salinity and density
Describe the Gulf Stream.
Western boundary current in the Atlantic
Narrow, fast-moving from Cuba to Cape Hatteras
Decreases speed across North Atlantic
Flow broadens and slows, becoming North Atlantic Drift
Movement to the south along the Canary Current is very slow, shallow & broad
Driven by thermohaline circulation & winds(Bermuda High)
There would be a weak Gulf stream even without thermohaline circulation
What are the internal modes of climate variability?
Long term cycles(decadal/centennial) e.g. thermohaline variability
Short-term cycles(intra-annual, inter-annual, decadal) - tropics(e.g. El Nino Southern Oscillation(ENSO)) & extra-tropics(e.g. North Atlantic Oscillation(NAO))
Describe the cycle of El Nino.
Easterly winds pile up water in West Pacific
Easterly winds weaken with season around September
A wave carries warm water to East Pacific
This wave triggers warm event(El Nino)
Wave overshoots as a Kelvin wave and is transmitted back as a Rossby wave to the West Pacific
Return wave triggers normal to cold event(La Nina)
Easterly wind strengthens and a new cycle can begin
How long is the cycle of El Nino?
Every 3-7 years(post-1970 - every 2.8 years)
What are some global effects of El Nino?
Droughts in Australia & Indonesia
Warm, rainy weather in eastern Pacific/South America
What does the North Atlantic Oscillation explain climatically?
The variability of tropospheric circulation over North Atlantic during winter
What is the NAO index based on?
The sea-level pressure difference between Subtropical(Azores) High and Subpolar(Iceland) Low
Describe the positive phase of the NAO(the negative is the opposite) and how it affects pressure, temperature & precipitation.
Pressure Changes:
Below-normal pressure across high latitudes of North Atlantic
Above-normal pressure over central North Atlantic, eastern US & Western Europe
Temperature Changes:
Warmer in eastern US & northern Europe
Colder in Greenland and often across Southern Europe/Middle East
During prolonged periods of one particular phase, temperature patterns extend into central Russia and north-central Siberia
Precipitation Changes:
Wetter over Northern Europe & Scandinavia
Drier over Southern & Central Europe
