Rapid Climate Change: Heinrich/Bolling- Allerod Events and the Thermohaline Circulation By: Andy Lesage April 13, 2010 Atmos. 6030
Outline Background Heinrich Event I/Bolling-Allerod Transition (Liu et al.) Thermohaline Circulation Sensitivity (Lorenzo et al.) Conclusions
Background Three major climate events (Heinrich, Bolling-Allerod, Younger Dryas), occurred between 19k and 11k before present. (Liu et al. model covers 22k-14k) Heinrich - very cold phase at end of most recent glaciation period Bolling-Allerod - rapid temperature transition, warmer phase Younger Dryas - less extreme cold phase Timmerman and Menviel (2009) Proxy records used to determine multiple variables.
Sediment cores Ice cores (N, Ar) Reef corals (tropics) Deep-sea corals (O, C) Proxy Sources http://www.mun.ca/geog/images/content/102-1812845044.jpg http://www.arcticice.org/hotraxweb/arctic_basin/images/field_photos/ice_core2.jpg
Liu et al. Study First study using a coupled atmosphere-ocean general circulation model. (NCAR Atmospheric Research Community Climate System Model) Spans from Last Glacial Maximum (~20ka) to Bolling-Allerod warming (~14ka). Forcings added based on known observations (insolation, greenhouse gases, ice sheets, coastlines, meltwater flux over N. America)
Thermohaline Circulation Surface water flows northward. Cooler, denser (higher salinity) water sinks at poles. Upwelling at lower latitudes. Fresh water reduces overturning circulation. http://www.liv.ac.uk/physocean/schematics/thc.gif
Heinrich Event BA-Bolling-Allerod H1-Heinrich Event I insolation 22ka-19ka model has little temperature change (mostly insolation effects) sea level rise meltwater red-model data grayobservations 17.5ka enters Heinrich Event deep water production Meltwater flux increased in N. Atlantic and Gulf of Mexico matching with observations. Results: increase in sea level and a decrease in North Atlantic deep water production.
Meltwater Flux 19ka-17ka increase at rate consistent with sea-level rise. (peak flux of 20m/ky) BA-Bolling-Allerod H1-Heinrich Event I Leads to decrease in overturning circulation. meltwater reduction simulations Freshwater anomaly confined to upper Atlantic then gets transported in upper ocean, later into deep ocean by Bolling- Allred (15ky). Meltwater flux is then reduced under two scenarios: 1) linear decrease to 0 at 14.2ky 2) sudden shut-off 14.67ky effect on overturning Overturning circulation increases when meltwater flux is reduced, peaks at ~19 sverdrups (1 sverdrup = 10e6 cubic meters/second), ~6 sverdrups above glacial level Differs from previous studies which showed a two stable layer system where a gradual change in meltwater flux can induce sudden dramatic warming.
Temperature Evolution Heinrich Event - North Atlantic cools dramatically, Southern Hemisphere little change. Caused by decrease in northward heat transport of overturning circulation. Heinrich - Glacial Surface Temperature Change Bolling-Allerod - Heinrich Bolling-Allerod - warming globally, especially North Atlantic (up to 20C) ~15C over Greenland, 5C from overturning circulation recovery from glacial state, 10C from CO2 warming and overturning overshoot. Bolling-Allerod - Glacial
Temperature BA-Bolling-Allerod H1-Heinrich Event I Evolution Close relation between model results and ice core data for Greenland and Antarctica. Also, close relation between model results and Iberian Margin (off of portugal) and Cariaco Basin (off of Venezuela) Greenland ice core red/blue-model data gray-observations Antarctic ice core
Atlantic Meridional Overturning Circulation Graphs for the gradual drop in meltwater flux scenario Strength of overturning in Sverdrups Heinrich event corresponds with decrease in overturning. Bolling-Allerod corresponds with an increase in overturning. Glacial temperature Heinrich - glacial Bolling-Allerod - glacial
Thermohaline Circulation Sensitivity (Lorenzo et al.) Thermohaline circulation has had a large effect in past, may be useful for future projections. Lorenzo et al. studied decadal and multi-decadal variability. (Atmospheric- Ocean General Circulation Models) Resolution = few hundred km. Stochastic forcing (random noise) inserted. Other studies have shown, including the IPCC report, a slowing of the thermohaline circulation under global warming and increased likelihood of abrupt change.
Model Design 3-D coupled atmospheric-ocean-sea ice model. Atmospheric: 3-level quasi-geostrophic model, parameterized diabatic processes. Ocean: sophisticated vertical mixing parameterization Sea ice: sensible/latent heat storage in snow/ice, changes in snow/ice thickness. Random fluctuations added in freshwater flux (Greenland).
Results - Thermohaline Circulation Model run in steady state, near collapse of THC with a.8sv forcing in Greenland. (approximately that of deglaciation period). Overturning stream-function fell from 30 to ~2Sv, North Atlantic cooling. Multi-decadal freshwater forcings lead to dramatic changes in THC strength. no freshwater forcing decadal variability Thermohaline circulation with continuum discharge of.8sv in Greenland-Iceland-Norwegian Sea multi-decadal variability
Results - Surface Temperature Temperature changes most dramatic in North Atlantic. Cooling around Greenland except northeast (Greenland Sea). Warming is result of increased surface heat flux. decadal variability in surface temp. multi-decadal (70 yr) variability in surface temp.
Results - Other changes 500hPa stream function (polar jet) weakened due to thermohaline circulation weakening. Decrease in precipitation and evaporation over Northern Hemisphere, particularly ~60 degrees North (-2cm/yr) ITCZ shifts, leads to change in rainfall near equator. SST increases or near zero change globally except in North Atlantic. Salinity decreases significantly near western Greenland.
Conclusions Atlantic meridional overturning circulation can be connected to rapid climate change in the recent past (geologic scales). 2 stable state pattern may not be a feature of overturning circulation. Reconstructions of meltwater flux before Bolling-Allerod would improve results. Decadal or multidecadal signals (rapid melting of Greenland ice via global warming could qualify) can induce weakening of the thermohaline circulation and lead to changes in global climate. Model improvement (resolution of smaller scales) necessary for greater accuracy of future studies.
Sources Liu et al. (2009), Transient Simulation of Last Deglaciation with a New Mechanism for Bolling-Allerod Warming. Science, Vol. 325. no. 5938, pp. 310-314. DOI: 10.1126/science.1171041 Lorenzo et al. (2009). Sensitivity of thermohaline circulation to decadal and multidecadal variability. ICES Journal of Marine Science, Vol. 66: pp. 1439 1447. Severinghaus and Brook (1999), Abrupt Climate Change at the End of the Last Glacial Period Inferred from Trapped Air in Polar Ice. Science, Vol. 286. no. 5441, pp. 930-934. DOI: 10.1126/science.286.5441.930 Smith et al. (1997), Rapid climate change in the North Atlantic during the Younger Dryas recorded by deep-sea corals. Nature, 386, 818-820 (24 April 1997); doi:10.1038/386818a0 Timmermann and Menviel (2009), What Drives Climate Flip-Flops? Science, 325, 273 Science Vol. 325. no. 5938, pp. 273-274. DOI: 10.1126/science.1177159
Extra Figures - Stream-function
Extra Figures - Precipitation
Extra Figures - SST/Salinity