Climate Flashcards
Describe a La Niña condition.
- The atmospheric pressure difference between Tahiti (HIGH) and Darwin (LOW) is higher than usual
- Stronger trades
- More piling up of warm water
- Convective loop gets higher
- Thermocline deepens even further in the west,
and upwelling is more pronounced in the east
Describe a El Niño condition.
- The atmospheric pressure difference between Tahiti (HIGH) and Darwin (LOW) is lower than usual
- Weaker trades
- Less piling up of warm water
- Spreading of warm water
- Convective loop shifts eastwards
- Thermocline shallower in the west,
and deeper in the east (weaker/no upwelling)
What is thermohaline circulation?
Part of the ocean circulation that is driven by density differences (most of the ocean) dependent on temperature and salinity (mainly) and pressure.
- Warm seawater expands and is thus less dense than cold
- Saltier water is more dense than fresher water because dissolved salt fill interstices between water molecules
Overall a stable situation in the ocean is that less desnse water masses float over denser ones.
What is the main influence on ocean water temperature?
solar irradience
What does the T profile of the ocean look like?
- Surface T varies between -2-~36oC
- Upper ~200m: epipelagic/sunlight zone / mixed layer
- bio-pump action; tracers change in crazy dimensions t.f. surface oceans difficult to reconstruct
- Highest T gradient in upper water column
- 200-1000m = thermocline = area of steep T gradient
- Reletively stable below 1000m depth (~4oC in all ocean basins)
What are the main influences on the salinity of ocean waters?
- Evaporation = more saline
- Precipitation = fresher
- Freshwater input from rivers or melting ice (= fresher)
- Sea ice formation = more saline (sea ice has a salinity of almost zero)
What does salinity do to the freezing point of ocean water?
Lowers it from 0 to < -2oC
Which is the ocean has the fresher water?
The Pacific ocean is fresher than the Atlantic ocean and this influences how deep waters form
What is the equation of state?
- The equation of state expresses the relationship of T, S and P to density. And can be expressed as isopycnals on a T-S diagram.
- Isopycnal = line of equal density
- Rule of thumb: The colder or saltier a water mass, the denser it is.
- Water wasses which have the same density sit along the same isopycnal and so is helpful when infering ocean circulation.
- T and S combine to determine density
- Additional density increases with pressure (depth)
How do you form deep and bottom water?
- Dense surface water forms in winter at high latitudes in the Nordic and Labrador Seas in the N Atlantic
- When surface water becomes denser than the underlying water, the situation is unstable and the denser water sinks. It slides down an isopycnal into deep interior of the ocean forming NADW
- Sliding down density surfaces is easy - it requires no change of potential energy and takes no/little energy to achieve
- Deep water formation is called convection
One can trace the origin of deep water layers by matching them to similar density winter surface water
How is NADW formed?
As the Gulf stream travels northwards it gives out heat to the atmosphere, cools (+water is saline) and when surface water becomes denser than the underlying and surrounding waters we get deep water mass formation (convection) in the Nordic seas and intermediate water mass formation in Labrador sea. (Intermediate) Labrador sea water and overflows from the Nordic seas (bottom water) mix in the subpolar North Atlantic to form NADW - the major deep water mass of the global ocean.
What are the major water masses in the Atlantic ocean
Salinity outlines major water masses in the Atlantic ocean
- NADW
- AABW (Antarctic Bottom Water)
- AAIW (Antarctic Intermediate Water)
- Desnity not high enough to sink below ~1000m
- Fresh water and sinks when cold, particularly in winter
The Atlantic part of the global overturning circulation is called AMOC (Atlantic Meridional Overturning Circulation)
What are the major water masses in the Pacific ocean?
Salinity outlines the major water masses in the Pacific ocean
- NPIW (North Pacific Intermediate Water)
- PDW (Pacific Deep Water)
- AAIW (Antarctic Intermediate Water)
- AABW (Antarctic Bottom Water)
There is no deep water formation in the North Pacific, only NPIW is formed.
- The N Pacific is too fresh to form deep waters (ppt/evap balance)
PDW is the oldest water mass in the ocean (1200-1500 yrs old) because it forms as NADW in the Atlantic, travels through the southern ocean and then to the Pacific. PDW then upwells in a diffusive way to form surface currents that migrate towards the Indian ocean.
What part does the Southern Ocean have to play in ocean circulation?
The Southern Ocean connects/communicates bottom and surface waters from all 3 major ocean basins as there is no land barrier.
Explain how AABW is formed?
Antarctic bottom water is formed in the Weddell and Ross Seas from surface water cooling due cold surface wind blowing off the Antarctic continent and is also formed below the ice shelf. The winds are stronger during the winter months and thus AABW formation is more pronounced during the Antarctic winter season. Surface water is enriched in salt from sea ice formation. Due to its increased density, it flows down the Antarctic continental margin and continues north along the bottom.
It is the densest water in the free ocean, and underlies other bottom and intermediate waters throughout most of the southern hemisphere. AABW is very abundant everywhere but the North Atlantic. AABW drops to >4000m water depth.
What are the main mechanisms of transporting carbon in the Ocean?
- Carbon pumps
- Solubility pump
- Biological pump
- Organic carbon pump (soft tissue pump)
- CaCO3 counter pump
How does the solubulity pump work?
The solubility pump is resticted to surface waters and is to do with the exchange equilibria between the atmosphere and surface ocean.
Solubility of gases such as CO2 is higher in colder, fresher waters (although the salinity gradient is not that great across oceans) and under higher pressures deep in the ocean. So more CO2 is dissolved in cold waters at higher latitudes (compared to lower latitudes) and is transported by ocean circulation into the deep ocean where it can be sequestered.
In carbonate chemistry, what relates the amount of CO2 in the oceans and the atmosphere?
Henry’s Law
Under equilibrium condition, the concentration of CO2 in the surface ocean is related to the fugacity (or partial pressure) of CO2 in the atmosphere.
CO2 (g) = CO2 (aq)
Where do we see uptake of CO2 and degassing in the oceans?
The CO2 flux is determined by ocean circulation.
- Uptake where there is sinking cold water (NADW, AAIW, NPIM)
- Pressure relief and degassing and subsequent realease of CO2 in major upwelling areas (e.g. east Pacific, part of ENSO system)
Why can the ocean store so much more CO2 than the atmosphere?
Dissolved Inorganic Carbon (DIC) can be present as 4 different species in the oceans:
- H2CO3 (carbonic acid)
- HCO3- (bicarbonate ion)
- CO2 (aq)
- CO32- (carbonate ion)
And the carbonate ions are related thusly when dissolved:
CO2 (aq) + H2O = H2CO3 (put CO2 in ocean)
Carbonic acid is unstable so it dissociates soon after formation in two steps:
H2CO3 = HCO3- + H+ (K1 - 1st dissociation constant)
HCO3- = CO32- + H+ (K2 - 2nd dissociation constant)
K1 and K2 (and therefore the carbonate sp. formed) are dependent on T, salinity (S) and P.
Why are the oceans typically dominated by HCO3-?
Partitioning of DIC between CO2, HCO3- and CO32- in the ocean is a function of pH.
The modern surface sea water in the ocean has a pH of just above 8. This correlates to ~90% HCO3, ~9% CO32- and <1% CO2.
There is an anticorrelation between the carbonate ion (CO32-) and CO2 and HCO3- acts as the buffer in the reaction. As the pH is lowered, CO2 is favoured and thus waters get more acidic.
What is the Biological Pump?
The combined biological processes which transfer organic matter to depth.
It quickly removes carbon from surface ocean and atmosphere and puts it in the deep ocean.
Turning off the biological pump would lead to a 200ppm increase in atmospheric CO2 i.e. the biological pump locks away the equivalent of 200ppm of CO2 in organic carbon.
How does the organic carbon or soft tissue pump work?
- Small algae called phytoplankton that live in the euphotic zone of surface ocean waters comsume CO2 by photosynthesis and form organic matter as tissue (carbohydrate) and release oxygen. These tiny particles cannot sink and so cannot transport carbon deeper into the ocean.
- Phytoplankton is at the bottom of the food chain and so are grazed upon by zooplankton. The excretion produced by zooplankton contains millions of phytoplankton and has now accumulated into denser aggregate called marine snow. Marine snow is heavy/dense enough to start sinking.
- As the aggregate sinks deeper into the ocean it gets acted upon by bacteria. Under the uptake of 1 mole of oxygen, the organic C is decomposed, freeing 1 mole of dissolved CO2 which is then physically mixed and recycled.
- This carbon flux takes place in the ocean mixed layer (top 100m)
In modern oceans only 1% of Corg gets deposited on the sea floor before respiration takes place, but there were times in Earth history when black shales were getting deposited.
The amount of Corg getting deposited obviosly effects the climate balance and controls deep water ocean chemistry of O2, inorganic carbon, and nutrients (the elements marine organisms need for life = C, N, P, Si, Cd, Fe etc.)
What is the organic carbon or soft tissue pump?
The ‘organic carbon or soft tissue pump’ is the part of the biological pump that takes CO2 from the atmosphere, converts it to organic C, and carries it to the deeper ocean, where it gets remineralised again (i.e. decomposition).
What is the CaCO3 counter pump and how does it work?
The CaCO3 counter pump is the second type of biological pump that describes the process by which organisms build CaCO3 shells and skeletons (e.g. coccoliths, formenifera).
Ca2+ + 2HCO3- = CaCO3 + H2O + CO2
This process doesn’t just lock away carbon but releases CO2 too. In fact more C is released than is stored away, hence it’s named the counter pump.
What are the key observational changes of climate change so far?
- Rise in atmospheric GHG concentrations (CO2, CH4, N20)
- Rise in average global temperatures (ocean and land)
- Changing rainfall patterns
- More extreme regional temperature & rainfall events
- Decline in sea ice cover in the Arctic
- Retreat of mountain glaciers, degrading permafrost
- Ice loss in Greenland and Antarctica
- Rise in global sea level
- Ocean acidification (CO2)
Warming of the climate system is unequivocal, and since the 1950’s many of the observed changes are unprecedented over decades to millennia.
Name the key projections calculated for future climate change.
By the year 2100 it is projected that:
- Temperature change is likely to exceed 1.5oC relative to 1850 to 1900 (>2oC for RCP6.0 and RCP8.5)
- Precipitation: Changes will not be uniform across the globe. Contrast between regions/seasons will probably increase.
- The global Ocean will continue to warm, and heat will penetrate to the deep ocean and affect ocean circulation.
- Arctic sea ice will continue to shrink, global glacier volume and northern hemisphere snow cover will futher decrease.
- Sea level will continue to rise due to increased ocean warming and increased loss of mass from glaciers and ice sheets.
- Carbon cycle feedbacks will probably work to exacerbate climate change and ocean acidification will increase.
how can oxygen isotopes from ice cores tell us about past temperature
δ
- Light oxygen (O-16) is more easily extracted from ocean water during evap (less energy is needed to break the hydrogen bonds and accomplish the phase change)
- So in the climate system, light O isotopes prefer the vapour phase and heavy O isotopes (O-18) prefers the liquid phase (e.g. when forming rain from clouds)
- Temperature of the system controls this seperation
- At the tropics, there is preferential evaporation of O-16 and so the vapour phase formed is depleted in O18 and enriched in O16 i.e. it has a more negative δ18O
- As the water vapour in the atmosphere (tropical clouds) travel to higher latitudes, towards the poles due to atmospheric circulation to become temperate clouds, the condensation process takes over from the evaporation process
- i.e. as the air mass cools, its ability to hold water vapour decreases and results in ppt (rain)
- The rain preferentially removes O18 from the vapour, and is always heavier than the vapour from which it formed - resulting in the cloud becoming even lighter in its δ18O
- The rain that comes from the clouds is, however, lighter than the sea water still because it came from the lighter clouds relative to the sea water
- When the clouds move from temperate latitudes to the poles, this is exaggerated. More and more O18 is removed by ppt and the water vapour beomes lighter and lighter, i.e. more enriched in O16
- This is called the Rayleigh distillation/fractionation effect
- As you move from the tropics to the poles the atmosphere gets successively lighter in its δ18O.
- When at the poles, the snow that is ppt over ice sheets is rather depleted in O18 i.e. has a very negative δ18O
- To use δ18O as a palaeoT proxy, you must first understand that there is no way to create heavy oxygen in the atmosphere, and so there is a negative spectrum of δ18O from just below 0 at the tropics to -30 in Greenland and -50 in Antarctica
- I.e. δ18O gets more negative towards the poles and that can be correlated to T. It’s not a direct T effect, though, but the T effect is that AS AIR MASS COOLS ITS WATER VAPOUR CONTENT DROPS; AND SO DOES ITS O18 TO O16 RATIO
- And so when T is hotter, there is more evap and ppt, which means ice cores will be more depleted in O18 compared to O16 (have a more negative δ18O) than during colder times when there is less evap and ppt, and ice cores will have a more positive (but still negative) δ18O.
δ18O (%o) = ?
(18O/16O)sample - (18O/16O)standard / (18O/16O)standard
X 1000
how can oxygen isotopes from ocean sediment record tell us about past temperature
- Calcareous shells (benthic formenifera) in deep ocean sediments help derive the T signal further back in time
- There is a negative correlation when δ18O is plotted against T - tells that for every 4oC change there is a 1 %o change in δ18O
- But that does not mean that we can simply measure δ18O and put a T on in because the δ18O of sea water from which these shells form changes through time. And here’s how:
- Start off with a δ18O of 0 at the tropics
- Water vapour gets progressively more and more enriched in 16O from tropics to poles, and when its ppt at the poles it has very negative δ18O
- Because O16 that was evap from ocean is now locked up in the ice sheets on the continents, that O16 is lost from the mass balance and from the ocean
- This means that overall, the ocean has to become heavier - what stays behind in the ocean has a bit more O18 and hence the δ18O of the deep ocean actually becomes a bit positive
- In a greenhouse world, with all ice melted and O16 put back into the ocean shifts the δ18O back down to roughly 0
- In an ice-free world ~1%o lower δ18O in the ocean
- Therefore, what matters for δ18O of sea water is how much ice is tied up on the continents
- δw = isotopic comp of water = direct function of ice volume, and so also controls shell composition in the seas and T is a secondary effect.
- Luckily, δ18O in the seas decreases at the same time on the T and ice volume trend i.e. colder water T and more ice = more O18 in the water
What drives basic atmospheric circulation?
the basic atmospheric circulation pattern driven by (unequal) solar heating, pressure gradient force, and Coriolis Force
What is the ITCZ?
The Intertropical Convergence Zone
It is where the SE and NE trade winds meet
That is not neccessarily at the equator and often lies 10o N
How are winds named in atmospheric circulation?
Winds are named with the direction that they are coming from, not the direction they are blowing to
Explain how the global wind system is formed.
- Solar energy and the rotation of the Earth are the two main players in forming the global wind system. It’s driven by solar energy and modulated by the Coriolis F.
- Firstly, solar radiation hits surface and warms it unevenly.
- The tropics experience the most intense warming, with close to vertical incoming solar radiation
- Air close to the surface is warmed by conduction
- warm, moist air rises because it is less dense than the air around it, leaving behind a LP system at ground level (equatorial LP system)
- The warm, moist air condensates and builds a HP system up in the atmosphere above the equator
- Air moves towards the poles (due to the curvature of the Earth) and converges at ~30o N and S, sinks and builds the subtropical HP system at the surface
- The air at the surface flows along a pressure gradient from the subtropical HP systems towards the equatorial LP system, is acted upon by the Coriolis F and forms the trade winds
- At the poles cool dry air sinks, creating a HP system at the surface and a LP system in the atmosphere
- In the sub-polar regions (60o) there is a convergence of warm air masses coming from the subtropical HP systems (westerlies) and cold air masses coming from the polar HP systems (easterlies) which subsequently rises to create a HP system in the atmosphere, leaving behind the subpolar LP systems at the surface
Illustrate the basic atmospheric circulation pattern formed driven by (unequal) solar heating, pressure gradient force, and Coriolis Force.
Label your drawing.
- Warm moist air rises from the surface at ITCZ and from other LP surface systems, leaving behind a LP system
- Cool, dry air sinks at HP systems
- Label Hadley Cell, then Ferrel Cell, then Polar Cell
- Label names of winds
What is Ekman Transport and how does it work?
- Ekman transport describes the actual surface ocean flow direction relative to the prevailing wind direction and takes into account the effect of the Corlolis force
- Ekman transport takes place in the Ekman layer in the top 100m of the ocean
- The average motion of the Ekman layer is transport perpendicular to the wind direction (to the R in NH, to the L in SH)
- However, as you get deeper in the Ekman layer, subsurface ocean flow undergoes the continued effect of the coriolis force
- And so as the subsurface ocean flow becomes weaker the deeper into the ocean, it also becomes rotated futher and futher away from the prevailing wind direction at the surface until it flows in the opposite direction to the wind at the bottom of the Ekman layer
What is a Gyre and how is it formed?
- Gyre is a naturally occuring vortex of wind and currents
- Subtropical Gyres are formed by geostrophic flow
- Water piles up in the middle of a gyre creating a raised sea surface
- The rasied sea surface then becomes the starting point for a Horizontal Pressure Gradient Force
- And the geostrophic current keeps the Gyre moving around
Explain Ekman Pumping.
- Currents don’t just move around in a horizontal plane, but also have a vertical motion
IN N HEMISPHERE:
- In a cyclonic wind system (anticlockwise), the outward motion of water i.e. surface divergence creates a vacuum and subsequent upwelling of water in the middle of a gyre. This results in a dip in sea surface height in the center.
- In a anticyclonic wind system (clockwise), the inward motion of water i.e. surface convergence creates elevated surface waters in the center of the gyre and subsequent downwelling of water.
- During surface divergence the thermocline is brought nearer to the surface due to upwelling, and during surface convergence the thermocline is pushed deeper due to the downwelling of water
What is Walker Circulation?
Pacific-wide air circulation pattern that is caused by the difference in sea surface T between the western and eastern Pacific
In contrast to the Hadley cell which acts N-S to connect P systems, it acts E-W
What is the ‘normal’ state of ENSO?
- Warm water piling up in W Pacific, and surface flow from E to W due to trade wind direction (creating western equitorial Pacific warm pool)
- Causes big covectional loop over Eern equatorial pacific = Walker circulation (convective rising over Indonesia)
- Characteristic slope of thermocline = deep in the W due to expansion in warm waters, taking up more volume; thermocline crops up at surface in E, resulting in upwelling of cold water (thermocline almost non-existent in the E)
What is the thermocline?
The depth where oceanic T changes v. quickly / strong gradient of change
It occurs between the warm, well-mixed surface layer of the ocean (affected by solar radiation) and cold waters of the main ocean body where T is quite uniform
The strongest T gradient (and deepest thermocline) is seen at the tropics, where solar radiation is strongest, surface waters are >20oC and more warmth can percolate down into the water column.
The thermocline dissapears towards the poles where ocean T is more uniform, even in the surface waters.
How would a strong El Nino affect sea surface hight?
- Warm water that normally pools up in W equatorial pacific pools up in the central and eastern pacific ocean - sea level rises relative to mean sea surface height
- Sea surface height becomes lower than average in W
Do El Nino and La Nina influence global T records?
- They have a strong short term effect
- They create an internal variability in climate system (down to natural causes)
- Many distinct oscillating features in overall increaing T curve over last 45 years can be coupled to natural events like the ENSO system in a stunning co-variation
What happens during the summer monsoon season?
- Prevailing SW winds = southwest monsoon
- Low P over land / High P over sea
- More rapid heating of land surfaces compared to seawater
- Produces rising motion over the continents and draws moist air in from the ocean
- PPt over land
What happens during the winter monsoon season?
- Prevailing northeast winds = northeast monsoon
- HP over land / LP over the ocean
- More rapid cooling of the land surface compared to the ocean
- produces sinking motion over the continents and sends cold, dry air over the warmer ocean, shifting most winter ppt out to sea
