Deep Ocean Circulation & implications for Earth s climate

Transcription

1 Deep Ocean Circulation & implications for Earth s climate

2 I. Ocean Layers and circulation types

3 1) Ocean Layers Ocean is strongly Stratified Consists of distinct LAYERS controlled by density takes huge amounts of energy to mix up the stable layers!

4 Temperature slice in the central Atlantic North South Thermocline a sharp transition in temperature and hence DENSITY (marked by many contour intervals) The main thermocline divides WARM SURFACE from COLD DEEP water.

5 Ocean Layers SURFACE OCEAN THERMOCLINE DEEP OCEAN

6 What forces can move layers of water? Water above the thermocline can be moved by wind (via friction with the atmosphere) Water below thermocline- much larger barrier, due to density layers.

7 Circulation types two very separate types of ocean circulation 1) surface -driven by wind 2) deep driven by density

8 Recall: Surface currents Affect surface water above the main thermocline (<10% of ocean water!) Driven by major wind belts Geostrophic circulation : balance of CORIOLIS driven transport vs. gravity

9 BUT: what about below geostrophic flow? Surface Deep

10 The Other kind of Ocean currents Deep currents Affect deep water below pycnocline (90% of ocean water!) Driven by density differences Larger and slower (way.. Way slower) than surface currents Critical to overall long-term circulation of ocean Balance of HEAT TRANSPORT..(climate..)

11 Deep currents Density-driven Factors affecting density of water: Temperature (most important factor) Salinity Thus, THERMOHALINE Circulation Two Processes Deep water formation (Vertical) Surface water becomes dense -> sinks Deep water movement (Horizontal) Slow, along constant density strata

12 Deep ocean circulation: Also called thermo-haline circulation Temperature (T) Salinity (S) Seawater Density depends on both T and S

13 Annual Mean Precipitation Rate (Kg/m2/day) Indo- Pacific - Excess Rain Atlantic - Excess Evaporation

14 Annual Sea Surface Temperature Annual Sea Surface Salinity

15 So where does ocean deep water actually form? How would you predict?

16 KEY: Gulf Stream Current Carries Salty and Warm water to high latitude North Atlantic Gulf stream

17 LABRADOR SEA ICELAND SEA * * Northern cold water deeps: North Atlantic Deep Water (NADW) formation Is the main source of deep water to entire world ocean

18 One other place..

19 * WEDDELL SEA Antarctic Bottom Water Formation

20 ANTARCTIC BOTTOM WATER FORMATION WEDDELL SEA 0 m Sea ice forms in winter 250 m Shelf water becomes salty and dense Freshwater used to form sea ice Continental Shelf Open ocean water is fresher and less dense Antarctica Slope

21 N Atlantic Ocean Cross Section S

22 Deep water masses

23 Summary Two main sources of Deep Water North Atlantic Deep Water Salty surface water cools as it reaches high latitudes Antarctic Bottom Water Cold water becomes salty as sea ice forms on continental shelves NO SOURCES in North Pacific Too fresh

24 Main Paradigm: the Global Conveyer Belt

25 Global Ocean Conveyor Belt represents net ocean circulation

26 Global Circulation NADW forms, and flows south joins the Antarctic Circumpolar Current flows north into Indian Oc. then Pacific Oc. returns to surface in Upwelling regions (spots all over the globe)

27 Surface water flows: from Pacific through Indonesia through Indian Ocean around tip of Africa and up to tropical Atlantic Gulf Stream carries water to N Atlantic to start journey again

28 Global Conveyor Belt total turnover time of ocean ~ 1000 years Surface Ocean residence time = 100 years Upwelling Deep Water Formation Deep Cold Ocean residence time = 1000 years Residence time is the average amount of time a substance (in this case water) spends in a reservoir

29 How we know this?

30 Slight Seque

31 How old is deep water?

32 EXTREME deep sea water!

33 Natural Energy Laboratory near Kona Hawaii Pipelines 10 to 600 and 1000 meters, in central N Pacific ocean gyre-

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35 What does old water even mean?

36 But how would you know How old is water? Deep-water formed at surface NADW and AABW In equilibrium w/atm at surface Background fact: CO2 atm >> ocean CO2 most soluble gas in water Once deepwater sinks, isolated CO2 is isolated too!

37 How old is deep water?

38 How s it done? Isolate CO2 from seawater Some chemistry: Inorganic carbon in the ocean H 2 O + CO 2 > H 2 CO 3 <> HCO 3- + H + <> CO 3= + H + Acidify

39 How s it done? Carbon (awesome atom) 12 C major isotope 13 C stable isotope ~1% of all C 14 C radioisotope % (part per trillion) Undergoes constant radioactive decay Half-life of 5730 yrs

40 How s it done? Measure radiocarbon at start Measure radiocarbon at end Difference due to decay (gives time) Calculate how long it took

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46 Into the lab Isolate CO 2 (and anything else of interest) Bring to your neighborhood Linear Accelerator Mass Spectrometer

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48 14 C- (Radiocarbon): Compound- Specific measurements- Lawrence Livermore, CAMS

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50 Sound like fun?

51 Start (Youngest) RESULT End (Oldest)

52 What about all those magic micronutrients? (that come from Greenland glaciers?)

53 RECALL: Nutrient Profiles! below main thermocline = oldest water in ocean! Deep water ~ 1000 yrs old

54 Time for a BREAK?

55 END A CLIMATE LINK?

56 The Holocene Period= post last ice age The height of the last glacial period was ~ 20,000 yrs ago. The Earth began to warm rapidly ~ 15,000 yrs ago, entered The Holocene Period. During this time, global sea level rose, glaciers melted, and erosion dramatically reshaped landscapes.

57 The Younger Dryas Event BUT : warming came to an abrupt, (but fortunately temporary!), end ~13,000 yrs ago A reversal of climate occurred plunged parts of Europe back into almost another iceage. Commonly called The Younger Dryas Event (named after an Alpine flower common at the time.)

58 Greenland ice core data: Recall low precip means low temp. The Younger Dryas Event and Abrupt Climate Change Abrupt change Abrupt change Rapid termination of last glacial Last ice age.. Climate record punctuated by very rapid changes

59 An Explanation? One of best explanations for this sudden change is related to the formation of North Atlantic Deep Water. Recall NADW formation region and the processes involved

60 NADW Formation Region The Global Ocean Conveyor Belt

61 Currents effects..

62 A Temporary Shutdown of NADW Formation? BUT HOW? End of last glacial period, Arctic polar ice melt = large source of freshwater. What influence would this freshwater have on NADW formation? What influence would you imagine NADW formation might have on the Gulf Stream? And what role does the Gulf Stream play in controlling climate of Europe?

63 The Westerlies and Storm Track In wintertime, cold air from N. America cools the Gulf Stream EUROPE Net transport of heat over Europe

64 Is there any clear modern precedent for this idea?

65 1960 s: The Great Salinity Anomaly (GSA): A mass of ocean water in the N. Atlantic with anomalously low salinity, caused by Arctic sea-ice melt, that moved around the ocean until the 1970 s!

66 The GSA made the surface ocean fresher and less dense, reducing overturning and deep water formation rates.

67 The Great Salinity Anomaly

68 The Gulf Stream and Global Warming? The Gulf Stream is in part drawn northward by the sinking NADW global warming will be largest in the polar regions (snow-ice albedo feedback) This will lead to an increase in sea-ice melt, = freshening of the N Atlantic =reduction in the rate of NADW formation? As a result, the Gulf Stream will not be drawn as far north, meaning that northern Europe may become colder.

69 Projections: down ~50% by 2050? Sea ice down ~20% since 1955

70 Global Ocean Conveyor Belt Will Slow Down

71 The day after tomorrow? Abrupt climate change a reality? Probably way more rapid than we ever thought (decades)

72 What about Feedbacks? If northern Europe is colder, this could mean longer winters, accumulation of snow and ice, and further cooling via the snow-ice albedo effect. What will be the end result? We can only speculate.. (Too much untestable complexity..) But an abrupt change in climate like that which occurred during the Younger Dryas may be possible.

73 Overall: could a Younger Dryas-like event in future? Or will global warming dominate? New York City in the future??

74 END