Climate change and impact on ocean variability

Transcription

1 Climate change and impact on ocean variability The ocean circulation - a global system of surface and deep currents - is powered by two different 'engines'. Movement in the top few hundred to a thousand metres is driven mainly by the prevailing winds. 1

2 1000m Depth The Shallow, Swift Wind-driven Circulation 4000m Temperature along a section in the mid-pacific (152W) The ocean circulation - a global system of surface and deep currents - is powered by two different 'engines'. Movement in the top few hundred to a thousand metres is driven mainly by the prevailing winds. Vertical circulation is driven by cold, salty water sinking at high latitudes, returning towards the equator at depth and being replaced by warm water moving towards the poles at the surface. 2

3 1000m Depth The Slow, Deep Thermohaline Circulation 4000m Temperature along a section in the mid-pacific (152W) The ocean circulation - a global system of surface and deep currents - is powered by two different 'engines'. Movement in the top few hundred to a thousand metres is driven mainly by the prevailing winds. Vertical circulation is driven by cold, salty water sinking at high latitudes, returning towards the equator at depth and being replaced by warm water moving towards the poles at the surface. This is known as the thermohaline circulation from the This is known as the thermohaline circulation from the combination of temperature (~thermo) and saltiness (~haline) that controls high-latitude sinking. 3

4 Nomenclature Meridional Overturning Circulation (MOC): Total northward/southward flow, over latitude and depth Thermohaline Circulation (THC): Part of MOC driven by heat & water exchange with atmosphere MOC is observable quantity; THC an interpretation Often used synonymously, but wind-driven MOC part should be considered separately 4

5 Why does it matter? 5

6 Why do we care if the oceans change? Why do we care if the oceans change? Potential for positive feedbacks influencing global climate Changing cryosphere Carbon uptake Thermohaline circulation Sea-level rise Impact of acidification on ecosystems Impact of climate change on ecosystems (warming, freshening, mixed layer, light, circulation, sea ice, winds) 6

7 Why do we care if the oceans change? Potential for positive feedbacks influencing global climate Changing cryosphere Carbon uptake Thermohaline circulation Sea-level rise Impact of acidification on ecosystems Impact of climate change on ecosystems (warming, freshening, mixed layer, light, circulation, sea ice, winds) 7

8 General pattern of the surface currents a balance of heat 8

9 The ocean s heat capacity is about 1,000 times larger than that of the atmosphere. Heat flux related to ocean regions. The oceans net heat uptake since 1960 is around 20 times greater than that of the atmosphere (Levitus et al., 2005). Heat Flux across the Ocean/Atmosphere (Watts/m 2 ) Da Silva 9

10 v Western boundary currents = negative heat flux into the atmosphere v Eastern boundary currents = postive heat flux from the atmosphere 10

11 Meridional heat transport T Anomaly (N-S sections) Atlantic Pacific Indian The heat anomaly is due to upwelling of anomalously warm deep layers in the Southern Ocean...role of eddies? Results have shown that the majority of transport between ocean basins occurs within the Southern Ocean =? 11

12 However. the meridional heat flux required to completely balance this transport must include another form of mechanism other than the mean flow. Such as areas of high variability that result in the generation of eddies Morrow et al., 2004 The meridional heat flux required to balance the PW (1PW=10 15 W) of heat lost by the ocean to the atmosphere at high southern latitudes must come from eddy transport Eddy kinetic energy (Ke) from 5 years Topex/Poseidon (Dec 1992 to Dec 1997). Source: Stammer 12

13 Projecting the behaviour of the Southern Ocean, including the carbon sink, in greenhouse scenarios will thus require models that capture realistically the effect of the ACC eddy variability. Boning et al., Nature 2008 So its important!! Zonally averaged NCEP SST data suggest increasing temperatures.. 13

14 Energy content changes (blue) and (burgundy). 1000m Depth The Slow, Deep Thermohaline Circulation 4000m Temperature along a section in the mid-pacific (152W) 14

15 So.. Why does this happen? What are the mechanics driving the GTH? What is a stable water column? Salinity increases with depth, temperature decreases with depth A stable water column is layered or stratified, like a three layered cake Now let s look at an unstable water column Salinity uniform with depth, temperature uniform with depth An unstable water column is not stratified, it is well mixed Dense water continually sinking 15

16 Where is coldest surface water? ~ at the poles A component of salt through brine rejection.. At the poles, the water column is unstable and is well mixed because of sinking cold and salty water 16

17 The instability of the higher latitudes gives rise to deep water formation 1. South Pole off coast of Antarctica Antarctic Bottom Water (AABW) ~ 1 C, 34.7 ppt ~ densest water in the ocean NADW and AABW form NADW and AABW form at surface, sink and then spread out in horizontal direction at the bottom of the ocean 17

18 Causes of fresher shelf water Increased glacial ice melt? More precipitation? Less sea ice formation? Change in winds and ocean circulation? Davis et al., Vaughan; Science,

19 The instability of the higher latitudes gives rise to deep water formation 2. North Pole off coast of Greenland North Atlantic Deep Water (NADW) ~ 3 C, 34.9 ppt ~ very dense but not as dense as AABW NADW and AABW form at surface, sink and then spread out in horizontal direction at the bottom of the ocean 19

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21 But what is the Importance of the Thermohaline circulation? Lets say the heat pump can be turned ON or OFF.what will happen? On today! There is vigorous mixing at the poles ~ dense surface water sinks at the poles ~ thermohaline circulation is initiated ~ this is the switch that turns the conveyor belt on As the global conveyor belt returns water to the poles through surface currents, the oceans give off the heat picked up at the lower latitudes to the land masses at the higher latitudes (i.e. northern Europe) ~ oceans acting as a heat pump to warm the land masses ~ Hence UK has a mild climate, Norway ice free ports.. 21

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23 BUT WHAT IF WE TURN IT OFF? There is no vigorous mixing at the poles water column becomes stable ~ there is no dense water sinking at the poles (surface waters warmed, polar ice caps melt) ~ thermohaline circulation slows down ~ the global conveyor belt switch is turned off There is no heat pump to warm the land masses ~ much colder in northern Europe How could the THC slow down? Increased rainfall, melting of the cryosphere are all possible consequences of higher temperatures, and could reduce North Atlantic surface salinity sufficiently to slow down the formation of deep water. If this happens, the THC may shut down. Once stopped, the heat conveyor may take time to recover, and the consequences would be a cooling of northwest Europe. 23

24 Effect on Europe if the THC slows down HADCM3 simulation where large amounts of fresh water was added to the North Atlantic at year London 1683 little ice age 24

25 The Importance of the Global Conveyer Belt (cont d) What do we think happened to cause the Younger Dryas? As earth warmed during warming 2000 year warming period ~ the surface waters also warmed ~ polar ice caps melted (surface waters less salty) ~ northern Atlantic surface water less dense ~ no vigorous mixing, interruption in thermohaline circulation ~ global conveyor belt turned off ~ no heat transfer to northern Europe ~ ice sheets, no forests grow Another example of when the global conveyor belt is turned off: We also have records of a prolonged period of cold in northern Europe from ~ known as the Little Ice Age ~ could have been caused by an interruption or slow-down in thermohaline circulation, conveyor belt slowed down (sluggish) 25

26 Salinity a key indicator! Ocean salinity changes are a sensitive indicator for detecting changes in precipitation, evaporation, river runoff and ice melt. Estimates of changes in the freshwater Estimates of changes in the freshwater content of the global ocean suggest that the global ocean is freshening. 26

27 Salinity increase due to an increase in evaporation Salinity decrease due to an increase in precipitation Ice melt, changes in the GTH/MOC 27

28 Is there evidence for salinity change? Yes in the Southern Ocean formation site of the AABW 28

29 Freshening of AABW 115E, 61S to 63.3S THETA psu SALINITY Rintoul

30 Freshening of the Mode Waters: global warming? (Wang et al. 1999).temperature shows a similar trend 30

31 Warming in the Southern Ocean has been attributed to a southward shift and increased intensity of the Southern Hemisphere westerlies, which would shift the ACC slightly southward and intensify the subtropical gyres (e.g., Cai, 2006). 31

32 Growing evidence of a slow down? Growing evidence of a slow down? 32

33 Growing evidence of a slow down? 33

34 Transport data from moorings deployed across the tropical Atlantic suggest that since 1957 the volume transport has slowed by 30% 34

35 Climate models show that the Earth s climate system responds to changes in the MOC and suggest that this overturning might gradually decrease in transport in the 21st century as a consequence of anthropogenic warming and additional freshening in the North Atlantic. However, observations of changes in the MOC strength and variability are fragmentary. Observed changes in MOC transport, water properties and water mass formation are inconclusive. This is partially due to decadal variability and partially due to inadequate long-term observations. From repeated hydrographic sections in the subtropics, Bryden et al. (2005) concluded that the MOC transport at 25 N had decreased by 30% between 1957 and 2004, but the presence of significant unsampled variability in time and the lack of supporting direct current measurements may reduce confidence in this estimate. What Causes Sea Level to Change? 35

36 The Bathtub Sea Level Model Precipitation over Oceans Runoff from Continents + - Evaporation from Oceans 36

37 Global mean surface temperatures have increased 37

38 Regional variability from historical tide gauges New York Brest Honolulu Buenos-Aires 1900 Time 2000 Sea Level Budget (IPCC, mm/year) Thermal Expansion 1.6 ± ± Mountain Glaciers 0.8 ± ± Greenland Ice Melt 0.2 ± ± Antarctic Ice Melt 0.2 ± ± 0.4 Land Water Storage?? = Total of Observed Contributions Observed Sea Level Change 2.8 ± ± ± ±

39 What does the IPCC say? The oceans are warming. Over the period 1961 to 2003, global ocean temperature has risen by 0.10 C from the surf ace to a depth of 700 m. Global ocean heat content (0 3,000 m) has increased at a rate of 0.21 ± 0.04 W m 2 globally. Global ocean heat content observations show considerable interannual and inter-decadal variability. Measurements of Sea Level Change GRAVITY 39

40 Tide Gauges with Greater Than 10 Years of Measurements > 50 Years of Measurements Tide Gauges with Greater Than 10 Years of Measurements > 50 Years of Measurements Of course still poorly sampled 40

41 150 Tide Gauge Observations 3.2 mm/year 100 MSL (mm) mm/year 2.0 mm/year -50 Average Rate ~ 1.8 mm/year Year [Church and White, 2006] TOPEX/Poseidon Satellite Altimeters Jason ? OSTM/Jason

42 Geographical distribution of sea level trends ( ) Global Mean Sea Level from Satellite Altimetry Average Rate = 3.5 mm/year ( ) El Nino [Mitchum and Nerem,

43 20 Thermal Expansion: Contribution to Sea Level Rate = 0.4 mm/year ( ) 15 Rate = mm/year ( ) 10 MSL (mm) [Levitus Year et al., 2005; Antonov et al., 2005] Greenland Melt Extent 43

44 Melting of the Greenland Icesheet 44

45 Alaska Glacier Mass Changes from GRACE Sea Level Contribution of 0.3 mm/year over [Tamisiea et al., 2005] Arctic Sea-ice melting ~10% decrease in sea-ice per decade 45

46 Antarctic Ice Mass Flux from -2 kminsar 3-4 /yr 3 /yr -2 km 3 /yr SLR 0.4 to 0.6 mm/yr +5 km 3 /yr -37±20 km 3 /yr -49±20 km 3 /yr -2 km 3 /yr -3 km 3 /yr -38 km 3 /yr +48 km 3 /yr -4 km 3 /yr -114 km 3 /yr -22 km 3 /yr -56 km 3 /yr +33 km 3 /yr +21 km 3 /yr -33 km 3 /yr +5 km 3 /yr [Rignot, 2005] 46

47 Is Sea Level Rise Accelerating? Short answer: probably The satellite sea level record is too short (~14 years) to rule out that the recent rise is due to natural decadal variability. This is only likely to be resolved by having a longer satellite data record (~30 years). The decline in satellite programs in recent years has put this in jeopardy. NEAR FUTURE PERSPECTIVES ALTIMETRY for measuring sea level Long sea level time series ARGO Thermal expansion + salinity GRACE Land waters (climate + human activities) Ice sheets mass balance Ocean mass change + thermal expansion Swath altimetry Surface Waters monitoring 47

48 Summary Observations of sea level change are consistent with how we expect sea level to respond in a warming climate. Sea level rose faster in the last decade than the 20th century average. Whether the current rate of rise is accelerating can only be resolved with longer satellite time series. Presently, ocean warming, melting of mountain glaciers, and melting of the polar ice caps are contributing in roughly equal amounts to the observed rise. The largest uncertainty in future sea level rise projections is the contribution of Greenland and Antarctica. Many of the remaining questions about sea level rise can only be answered with continued satellite measurements, which are in serious jeopardy. Effects of Sea Level Rise 1 meter 2 meters 4 8 GFD 48

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50 IPCC year rates of global sea level change from tide gauge (black) satellite altimetry (green) and contributions from thermal expansion (red) The ocean varies over a broad range of time scales, from seasonal to decadal (e.g., circulation in the main subtropical gyres) to centennial and longer (associated with the MOC). The main modes of climate variability are the El Niño-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), the Northern Annular Mode (NAM), which is related to the North Atlantic Oscillation (NAO), and the Southern Annular Mode (SAM). Forcing of the oceans is often related to these modes, which cause changes in ocean circulation through changed patterns of winds and changes in surface ocean density. 50

51 March El Niño October La Niña 51

52 Changing Southern Hemisphere climate: the Southern Annular Mode Sen Gupta & England

53 Northern Annular Mode 53

54 Oceans and CO 2 Passive ocean CO 2 uptake Reduction in surface ocean ph ocean acidification Passive uptake cannot keep up with increased anthropogenic CO 2 active uptake ( CO 2 sequestration ) ocean acidification 54

55 Anthropogenic C0 2 Invasion Impacts of invading CO 2 on ocean chemistry and biology Original methodological challenges related to measuring the anthropogenic part of DIC Models of future DIC concentrations are key: by by 2300 (Caldeira & Wickett, 2005) Impacts on Ocean Biology Corals (ocean acidification induces a decrease in C++; cumulative impacts: >T, >ph) Reduced growth, calcification and survival of many other shallow benthic species Affects fish physiology Invertebrates physiology also affected (also cumulative impacts: >T, <dissolved O 2 ) 55

56 Impacts of Ocean Acidification Generally, ocean acidification affects ocean biology and how the ocean functions as a system Passive uptake of CO 2 will gradually invade the deep ocean Tropical (above) and subtropical pteropods; stony cold water corals (right) CO 2 Sequestration in the Deep? Naturally, most deep organisms tend to avoid natural ph variations (e.g. vent plumes) Liquid CO 2 lake scenario on the deep ocean floor: many factors affecting stability Judicious choices should be made (delivery schemes, injection rates, droplet size, bottom bathymetry, water column injection, etc.) Iron fertilization as a mitigation strategy? Side effects include <dissolved 0 2, >atmospheric N 2 O, <nutrients downstream from a fertilization side) 56

57 Are We in Trouble? Geo-engineering schemes are not well understood. Planet-sized geo-engineering means planet-sized risks. Caldeira, K. What Can We Do? IOC-SCOR Ocean Acidification Symposium Series Policy side: Royal Society Policy Report Recommendations There is a clear risk of significant adverse effects of ocean acidification. This risk should be taken into account by policymakers and other relevant national and international bodies. Any targets set for CO2 emission reductions should take account of the impact on ocean chemistry and acidification as well as climate change. Ocean acidification and its impacts on the oceans needs to be taken into account by the Intergovernmental Panel on Climate Change and kept under review by international scientific bodies. 57

58 Royal Society Policy Report Recommendations (cont.) The increased fragility and sensitivity of marine ecosystems due to ocean acidification, climate change, deteriorating water quality, coastal deforestation, fisheries and pollution needs to be taken into consideration during the development of any policies that relate to their conservation, sustainable use and exploitation, or effects on the communities that depend on them. Tackling ocean acidification cannot be done by any country alone. A major internationally-coordinated research effort (including monitoring) into ocean chemical changes should be launched, with additional investments. International research collaboration should be enhanced, from laboratory, mesocosm and field studies to global monitoring. Action needs to be taken now to reduce global emissions of CO2 to the atmosphere to avoid the risk of large and irreversible damage to the oceans. We recommend that all possible approaches be considered to prevent CO2 reaching the atmosphere. No option that can make a significant contribution should be dismissed. 58