Lecture 6: Ocean circulation and the carbon cycle 2 Flashcards
MERIDIONAL OVERTURNING CIRCULATION
What drives ocean circulation?
• Differences in water density
• Temperature • Salinity
• Cool, salty water most dense
THERMOHALINE CIRCULATION – THE BASICS
- Warm water has lower density than cold water
- Water with higher salt contents is heavier than fresher water
- Cold salty water is the heaviest water
- Evaporation increases salinity of the water in warm regions
- During its journey to cold regions the salty water cools and sinks towards the ocean floor
- By contrast, diffuse upwelling occurs in warm regions
- Therefore both temperature and salinity gradients are the driver of an overturning circulation
THE GULF STREAM & NORTH ATLANTIC DRIFT
- The Gulf Stream near the U.S. East Coast
- Moves away from the coast at Cape Hatteras
- Has a speed of about 10 km/h ( 6 mph)
- Carries many times the world’s energy demand
- Warm, clear, blue water contrasts with the cooler, darker, more productive water to the north and west
- Clouds form over the warm current as water vapor evaporates from the ocean surface
MERIDIONAL OVERTURNING CIRCULATION: ATLANTIC
- A simplified view of the Meridional Overturning Circulation (MOC) in the Atlantic
- Surface water becomes dense and sinks in the north and south polar regions
- Being denser, Antarctic Bottom Water slips beneath North Atlantic Deep Water
- The water then gradually rises across a very large area in the tropical and temperate zones, then flows poleward to repeat the cycle
SURFACE CURRENTS & DEEP CURRENTS
Seawater solution consists mainly of: dissolved salts (ions) mainly from breakdown of crustal rocks (e.g. Na+, Ca2+)
dissolvedgases(e.g.O2)
dissolved organic material
large range of particles in suspension
Major constituents (> 1 ppm by mass) account for 99.9% of Salinity S which is on overage 35‰
Salinity is affected by:
precipitation, runoff, melting of ice and snow (decrease) evaporation(increase) input of dissolved ions is balanced by removal to sediments
global oceans are in steady state
Regional variations in salinity affect water density
THE THERMOHALINE CIRCULATION
Meridional Overturning Circulation (MOC) is part of the global Thermohaline Circulation (THC), sometimes called the “ocean conveyor belt”
Red colours represent surface currents, blue colours represent deep currents
Density (temperature and salinity) gradients are the driver for this circulation
It contributes to the global distribution of energy in form of heat
THE IMPORTANCE OF THE PACIFIC
- The solar energy that reaches the Pacific Ocean provides a large proportion of the heat needed to drive the global atmospheric circulation.
- The tropical Pacific ocean and atmosphere represent a giant heat engine that converts solar energy into kinetic energy of the winds.
- Small changes to the atmosphere-ocean interaction in the Pacific will have important consequences across the globe.
WALKER CIRCULATION
- In normal conditions trade winds blow towards the west across the tropical Pacific, resulting in a pile up of warm surface water in the west Pacific.
- Cold nutrient-rich water wells up at the South American coast, supporting diverse marine ecosystems (fishery), dry climate conditions
- Higher water temperatures results in extensive convection near Indonesia
- This forms a convective loop, the Walker Circulation, which forms an important part of the Hadley Cell and the general atmospheric circulation, reinforcing the trade winds
- This results in the formation of a zonal gradient in water temperatures
EL NIÑO CONDITIONS
- During an El-Nino Event the trade winds relax in the central and western Pacific
- Consequentially the thermocline in the Eastern Pacific gets depressed, in the Western Pacific it is elevated
- Warm surface water moves eastward followed by rainfall
- Results in flooding in South America (Peru) and drought in Indonesia and Australia
- With the eastward displacement of the heat source changes in atmospheric circulation occur affecting weather patterns in regions even at great distance from the Pacific
LA NIÑA CONDITIONS
- A La-Niña Event is characterised by unusually cold sea surface temperatures
- Trade winds are stronger than usual
- La Niña sometimes (but not always) follows an El Niño event
- El Niño and La Niña are opposite phases of the El Niño-Southern Oscillation cycle, with La Niña sometimes referred to as the cold phase of ENSO and El Niño as the warm phase of ENSO
- The impact on global circulation and weather patterns tend to be opposite those of El Niño
NATURAL SOURCES OF ATMOSPHERIC CO2
Natural Sources:
• Volcanoes – annual volcanic emissions are a tiny fraction (1/150th) compared to anthropogenic emissions!
- Combustion processes (e.g. wildfires)
- Respiration of aerobic organisms (animals, humans)
MAP OF GLOBAL OCEANIC CARBON UPTAKE
Estimate of the sea-to-air flux of carbon dioxide, based on 940,000 measurements of surface water pCO2 collected since 1956.
Generally areas with colder water temperature represent regions of net CO2 uptake, whereas a positive sea-to-air flux is found in warm water regions.
OCEAN ACIDIFICATION CAUSED BY CO2 UPTAKE
Uptake of increased CO2 reduces surface ocean pH values
- increase of acidity of ocean water
This increase in acidification is expected to affect
- coral reefs (declining calcification)
- cold water corals
- ecosystems where aragonite is essential (shell building)
- temperature increase results in coral bleachin
