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
Coriolis force causes wind deflection in what orientations in the northern and southern hemispheres?
- In the N hemishphere winds go to the RIGHT from HP to LP IN THE DIRECTION THAT THEY TRAVEL
- In the S hemisphere winds go to the LEFT from HP to LP IN THE DIRECTION THAT THEY TRAVEL
What happens to air in HP systems?
Dry air sinks in HP systems
Get dry and desert areas
In what P systems do we see the most ppt?
Low P systems
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
Surface ocean circulation is driven by what?
Wind
Explain the basic action of wind on surface waters
- When wind blows over the ocean, energy is transferred from the wind to the surface layers
- The transfer of energy is complex, but the greater the speed of the wind, the greater the frictional force acting on the sea surface and the stronger the surface current generated
- The wind stress (frictional force) also depends upon prevailing atmospheric conditions
- Surface current speed is typically about 3% of the wind speed
- The effect of the wind stress on the sea-surface is transmitted downwards as a result of internal friction within the upper ocean
- This creates turbulent surface ocean activity and the transfer of momentum (mass x velocity) by parcels of water down into the upper parts of the ocean. This is Eddy Viscosity.
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 geostrophic current and how is one formed?
Currents in which the horizontal pressure gradient is balanced by the Coriolis force
- The most ocean basins, the ocean is not indefinetely wide - there are land barriers which leads to the inevitable piling up of water
- Except in the Southern ocean where the currents circulate Antarctica
- But E.g. Sea surface elevtion is higher in the Eastern equatorial Pacific than in the Western equatorial Pacific by up to 0.5m
- The horizontal pressure gradient force acts to transport the water from where there is more to where there is less, so East to West
- Then Corilois force acts on a right angle to the flow, continually until the Coriolis force ends up acting in the opposite direction to that of the initial, internal horizntal pressure force i.e. Coriolis force acting W to E.
- This is now a geostrophic current.
- Geostrophic currents manifest in all of the big subtropical Gyres
*
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
In what scenarios do we see upwelling in the ocean?
- Coastal upwelling
- Wind blows parrallel to the coast line resulting in surface ocean flow in an offshore direction (R in NH, L in SH remember so orientation matters)
- Upwelling of colder water from deeper ocean at the coast = coastal upwelling areas
- Very prominent coastal upwelling areas in California, S America, Namibia
- The cold water from deep ocean is good for fishing because it is nutrient rich
- Equatorial upwelling
- trade winds meet at the ITCZ
- vacuum parrallel to ITCZ as warm water is pushed away from ITCZ
- Vacuum is countered by upwelling of cool deep ocean water from below
What is climate?
The average weather that persists for an extended period of time (years to decades, and above)
What is El Nino?
El Nino is one of the extreme events from the ENSO system
It is the oscillation of the ocean-atmosphere system in the tropical Pacific having important consequences for weather and climate around the globe
El Nino can cause for example major droughts and foresst fires in Aus while casuing massive flooding in California (1998)
What creates the ENSO system dyamics?
The Observed Southern Oscillation - the seesaw connection between S pacific High and Indonesian L P systems, and resulting dramatic sea surface T changes combine and lead to, not only the direct T and P effect but the creation of the Walker Cell
The Observed Southern Oscillation works in such a way that whenever one gets stronger the other gets weaker
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 do El Nino and La Nina affect weather in other parts of the world?
- Displacement of warm water in the equatorial pacific affects evap and condensation
- Ultimately it can alter the typical atmospheric jet stream patterns (particularly in the hemisphere experiencing winter)
- Effects transmitted to other parts of the world through atmospheric circulation
- It is the starting point for TELECONNECTIONS
- Teleconnections are large scale, long lasting shifts in atmospheric circulation that affect much of the globe
- Enso systems buildup over months and months and typically persist for a couple years and reoccur in like 2-7 years
- ENSO effects are most predictable for countries around the circum-Pacific. Effects are rarely felt in Europe.
- ENSO effects the hurricane seasons in N America - reletively benign in El Nino years
What are teleconnections?
large-scale, long-lasting shifts in atmospheric circulation that can affect much of the globe
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
What is the potential ppt impact of ENSO?
Most felt in the tropical area of the Pacific Ocean, results for December - Feb peak conditions
El Nino
- warm phase relative to SST in eastern equatorial pacific
- dry tendencies in AUS eg bush fires 1998
- lots of ppt in California
La Nina
- cold phase
- ppt over western eqatorial pacific
What is the potential temperature impact of ENSO?
Most felt in the tropical area of the Pacific Ocean, results for Dec - Feb = peak conditions
El Nino
- warm phase relative to SST in eastern equatorial pacific
- warming in S America
- cooler over mid US
La Nina
- S America cooling strongest response
What word of caution did the met office give to people trying to predict ENSO events from map reconstructions?
Each ENSO event is different, and occurs in conjunction with other climatic events. Not all impacts occur in all events, and impacts may not be confined to the regions indicated.
Thus these maps should not be regarded as forecasts for a current event, but rather as an indication of areas where impacts are likely, based on historical evidence.
An El Nino situation gives what T and P characteristics?
El Nino = Positive T deviation and small P difference
Low P = spreading of warm water
El Nino = higher average sea surface T
(La Nina = lower ave sea surface T)
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 is the NAO?
The North Atlantic Oscillation (NAO) is a large scale seesaw in atmospheric mass between the subtropical high and polar low.
Observations tell us that the mean position of the Gulf Stream is correlated with climatic conditions in north-western Europe (teleconnections)
Postive NAO = larger P difference
Negative NAO = weaker P difference
What happens in a postive NAO situation?
- Larger P difference between Icelandic Low and Azores High
- Increased westerlies
- More northern trajectory of strom tracks
- Mild/warm and wet winters in central Europe
- Dry and cold in N Africa
What happens in a negative NAO situation?
- Small P difference between the Icelandic Low and Azores high
- Suppresses westerlies
- Cold and dry winter and southernly strom tracks in central Europe
- Warmer, increased storm activity and rainfall to southern Europe and N Africa
What is a monsoon and why do they occur over India/the Indian ocean?
Winds that change seasonally
Near the Indian ocean, the massive land mass of asia occupies most of the northern hemisphere so only ocean in S hemisphere
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
What does the monsoon season do to surface ocean currents in the indian ocean?
really heavy rainfall in summer flips the direction of current to travel Wwards
Atmospheric carbon dioxide concentrations have changed significantly throughout earth history, with levels >1000ppm in the Greenhouse World and values below modern during cold times in the Icehouse World.
Name and discuss potential mechanisms that could contribute to the 100ppm changes in atmospheric CO2 observed on glacial-interglacial timescales of the past 800,000
- Antarctic ice cores indicate that eight glacial cycles occurred periodically over the past 800,000 years with interglacial and glacial periods corresponding to a max. 280ppm and ~190ppm CO2 respectively – that’s a 100,000 year cycle between ice ages.
- This corresponds to the Milankovitch cycle in which variations in the Earth’s orbital eccentricity occur on 100-400kyr timescales, changing the amount of solar radiation received by the Earth as it deviates to and from its circular orbit.
- Milankovitch cycles act as a natural pacemaker for glacial-interglacial variations, where we also see changes in the tilt of the rotation of the Earth’s axis (obliquity) on a 41kyr timescale which influences latitudinal distribution of solar radiation;
- and changes in the precession of the equinoxes (the wobble of the Earth’s axis) on 19-23kyr timescales.
- But the 100kyr orbital forcing is the primary external driver of glacial cycles in that it produces the temperature change.
- Reduction in Northern hemisphere summer insolation generates sufficient cooling to initiate ice sheet growth and so glacial cycles occur when there is low 65oN summer insolation.
- In colder clmiates, for example during the last glacial maximum (LGM) ocean circulation was more sluggish and salinity driven because average ocean T was colder
- Glacial North Atlantic Intermediate Water (GNAIW) underlain by GAABB up to 2000m
- = more stratified ocean in N and S
- = better for sequestration of CO2, reducing CO2 in atmosphere
Atmospheric carbon dioxide concentrations have changed significantly throughout earth history, with levels >1000ppm in the Greenhouse World and values below modern during cold times in the Icehouse World.
Name and discuss potential feedbacks that could contribute to the 100ppm changes in atmospheric CO2 observed on glacial-interglacial timescales of the past 800,000.
Atmospheric CO2 concentrations play an important role, initially as an internal feedback and then as an amplifier when it comes to glacial-interglacial climate. An amplifier is a feedback that involves the global carbon cycle and the deep ocean. Some simple feedback mechanisms include:
- Positive feedback: When ice caps are larger, a larger surface area of the earth is white, increased reflection of heat from the sun into space, cooling down earth further, leading to larger ice caps etc.
- Positive feedback: When ocean water cools, it can dissolve more CO2 (a gas) and thus takes up more CO2, leading to more cooling etc
- Negative feedback: When sea level drops during ice growth, we get more land, less ocean surface, vegetated land reflects less solar heat into space than the ocean, so lower sea levels may lead to warming
- Negative feedback: When ocean waters cool, less water evaporates, that means less precipitation world-wide, which means less snow on the ice caps, lower albedo and more warming.
What is radiative equilibrium?
- 30% of the shortwave solar radiation is refleced both by clouds (23%) in the atmosphere and from the surface (7%)
- 23% of that radiation is adsorbed by the atmosphere and 47% is adsorbed by the surface
- while the earth continually absorbs shortwave solar radiation, it will not simply continue to heat up
- in response to this heating, the earth emits longwave (infrared) radiation back into space
- radiative equilibrium means that all of the influencing factors are in balance and cancel each other out
- because of this, the temperature of the earth has been relatively constant over time
- we can measure the heat radiated back to space using satellites
why does our radiative equilibrium depend on greenhouse gases?
- 95% of emitted longwave radiation stays in the climiate system
- Measurements from satellites suggest that our planet is -18oC due to the radiation it emits. The actual surface T is +14oC (a 32oC difference)
- This is due to the Greenhouse Gas effect
How does the greenhouse gas effect work?
Some of the infrared radiation passes through the atmosphere but most is absorbed and re-emitted in all directions by GG molecules and clouds.
The effect of this is to warm the Earth’s surface and the lower atmosphere.
We have a habitable planet becasue of the GG effect.
How can GGs retain heat?
- Gas molecules can rotate at certain discrete frequencies - if frequency of incoming radiation is the same as the rotating molecule, radiation is adsorbed
- Gas molecules can also absorb radiation if their molecular vibration frequency matches the frequency of the infrared wave
What are the main GGs?
H2O (water vapour)
CO2 (carbon dioxide)
CH4 (methane)
N2O (nitrous oxide)
CFCs (chlorofluorocarbons)
Why doesn’t the atmosphere retain more heat from the GG effect?
Becasue the dominant atmospheric gases are not GG.
Main atmospheric gases are N2 (78%) and O2 (21%)
(and Ar (<1%))
Neither of them trap outgoing radiation as they are perfectly symmetrical molecules
They have no charges that interact, dont vibrate or rotate in a way that would interact w/ electromagnetic radiation
i.e. they are transparent to radiation
What is radiative forcing?
= net change in the energy balance of the Earth system due to some imposed perturbation
Or in other words: Radiative forcing is a concept used for quantitative comparisons of the strength of different human and natural agents in causing climate change
What is the radiative forcing for a doubling of
atmospheric CO2?
Radiative warming for doubling of CO2 is known as climate sensitivity and the best estimate of climate sensitivity today is 1.5-4.5oC.
The concept of radiative forcing describes the net change in the energy balance of the Earth system (expressed in W m–2) due to some imposed perturbation.
What are the main forcing agents contributing to the balance between 1750 and 2011?
Forcing agents consist of both natural and anthropogenic forcings. Tectonic processes, orbital changes and the strength of the sun are natural forcings.
Because forcing from tectonic processes and orbital changes only act on a kyr to Myr timescales, solar irradiance is the only significant natural process contributing between 1750 and 2011 and even still the radiative forcing was negligible at +0.05 Wm-2 (positive radiative forcing).
Another natural process to exert radiative forcing is volcanic eruptions, albeit on a short time-scale. Explosive volcanic eruptions can cool climate over intervals of a few years.
Total anthropogenic forcing amounts to +2.29Wm-2 and consists of:
- Well mixed greenhouse gases (e.g. CO2, CH4, N2O and Halocarbons)
- Ozone, and stratospheric water vapour
- Aerosols
Provide a brief description of solar irradience as a forcing agent and its approximate contribution.
The radiative forcing effects of solar irradiance varies with 11-year sunspot cycles and cause variations of 0.1% (less than 0.1oC). More sun-spots = more eruptions of energy = more solar irradiance.
radiative forcing was negligible at +0.05 Wm-2 (positive radiative forcing).
Provide a brief description of volcanism as a forcing agent and its approximate contribution.
- Erupted sulphur dioxide gas mixes with water vapour in the air to form droplets and particles called sulphate aerosols.
- If highly explosive eruptions can send aerosols up into the stratosphere (20-30km high) for long enough periods of time they can block incoming solar radiation and cooling can take place on short term timescales
- E.g. Pinatubo eruption in 1991 resulted in an observed cooling of 0.1-0.3oC for up to 3 years
Provide a brief description of well mixed GG as a forcing agent and its approximate contribution.
e.g. CO2 (+1.84 Wm-2),
CH4, N2O and Halocarbons (+1 Wm-2)
Halocarbons consist of CFCs (and were later replaced by HCFCs after the Montreal Protocol) that produce chlorine and bromine radicals that are harmful to ozone in the stratosphere, therefore leaving through more solar irradiance. Under the Montreal Protocol CFCs were phased out by the year 2000 and HCFCs are planned to be phased out by 2030.
Provide a brief description of ozone, and stratospheric water vapour as a forcing agent and its approximate contribution.
Stratospheric ozone (10-50km; 90%) absorbs harmful components of sunlight known as UV-B and is therefore regarded as ‘good ozone’ as it has a negative forcing effect (-0.05 Wm-2).
Tropospheric ozone (remaining 10%) is a non-primary, very toxic GS formed from photochemical reactions and is regarded as ‘bad ozone’. It has a positive forcing effect (+0.41 Wm-2).
Stratos. water vapour formed from the oxidation of CH4 has a minor positive forcing effect (+0.07 Wm-2) and acts more like a feedback or a response rather than an initial driver.
Provide a brief description of aerosols as a forcing agent and its approximate contribution.
-
Aerosol-Radiation Interaction (-0.45 Wm-2) or ‘direct aerosol effect’ (e.g. mineral dust, sulphate, nitrate, organic carbon)
- Aerosols scatter solar radiation. Less solar radiation reaches the surface which leads to localized cooling. The atmospheric circulation and mixing processes spread the cooling regionally and in the vertical.
-
Black carbon (from incomplete combustion) is the only aerosol to have a positive forcing effect as it lowers albedo (on snow) and absorbs heat
- As it absorbs solar radiation the aerosol layer is heated but the surface, which receives less solar radiation can cool locally. At the larger scale there is a net warming of the surface and atmosphere because the atmospheric circulation and mixing processes redistribute the thermal energy.
- Aerosol-Cloud Interaction (-0.45 Wm-2) where aerosols work as nucleation points for cloud formation, therefore increasing the amount of solar radiation being reflected.
- Aerosols have offset a substantial portion of GS forcing since 1750!
Briefly, what is the radiative forcing since 1750?
Total radiative forcing is positive and has led to an uptake of energy by the climate system. The largest contribution to total radiative forcing is caused by the increase in atmospheric concentration of CO2 since 1750.
What is an amplifier?
An amplifier is a feedback that involves the global carbon cycle and the deep ocean
