Impact of freshwater release in the North Atlantic

Impact of freshwater release in the North Atlantic under different climate conditions in an OAGCM Didier Swingedouw, Masa Kageyama, Juliette Mignot, Pascale Braconnot, Eloi Mosquet, Charline Marzin, Ramdane Alkama, Olivier Marti LSCE-IPSL, France

Abrupt climate changes during glacial period Cooling and iceberg surge Monsoon changes (Wang et al. 2001) ITCZ migration(Peter à Origin? son et al. 2000) à Mechanisms for the teleconnections ? Courstesy of M. Kageyama

What about warm climates? Ü Reduction of the Greenland ice sheet during the Eemian Ü Potential collapse in the future What are the freshwater inputs AMOC No Greenl. CTRL under different climate conditions? Are the climatic responses and mechanisms similar under different climate conditions? With Greenl. Swingedouw et al. Clim. Dyn. 2007

Experimental design 10 simulations using the IPSL-CM 4 AOGCM of 250 years, with and without (computed) freshwater input Insolation at the top of the atmosphere PRE, FUT HOL-PRE LGM-PRE LIG-PRE Ü Last Interglacial (126 kyr BP, LIG) Ü Last Glacial Maximum (21 kyr BP LGM) Ü Holocene (6 kyr BP, HOL) Ü Preindustriel (PRE) : 0. 1 Sv hosing following Stouffer et al. 2006) Ü Future (2 x. CO 2, FUT) -126 kyr LIG -21 kyr -6 kyr 1860 2250 LGM HOL PRE FUT

Freshwater input and AMOC response Freshwater is computed according to the heat flux budget over land-ice area in the model (except for PRE and LGM) Atlantic Meridional Overturning Circulation (year 220 -250) LIGc LIGh LGMc LGMh HOLc HOLh PREc PREh FUTc FUTh Greenland melting after 250 years 16 14 12 10 8 6 4 2 0 Calving LIG LGM PRE FUT HOL

Climatic impact of « hosing » Temperature (year 220 -250, hosing-CTRL) LIG HOL FUT Precipitation LGM LIG LGM PRE HOL PRE FUT

SLP (year 220 -250, hosing-CTRL) Sea level pressure response Ü Change in the mean state LIG LGM HOL PRE Ü Resembling the Aleutian Low-Icelandic Low seesaw (Honda et al. 2001) Ü Adjustment of the storm FUT track to different temperature gradients En TW

Explanation for the « slight warming » pattern Sea-ice cover and wind stress changes (year 220 -250, hosing-CTRL) LIG HOL LGM PRE Ü Changes in surface winds associated with large-scale SLP changes Ü Interaction with sea ice cover FUT Ü Albedo feedback leads to the observed « slight warming » En TW

A complex energetic adjustment SW Net LW TOA AHT Atmosphere Sea ice OHT Land Ocean 25°N 90°N

A complex energetic adjustment Common mechanisms: • Oceanic heat transport decreases due to AMOC weakening • Partial compensation by the atmosphere SW= -172 Net=+ 13 LW=+ 185 TOA AHT= +99 Atmosphere Atm=-2 Glace=-91 Differences: • AMOC changes • Radiative adjustment and changes in reservoirs (sea ice) OHT= -232 Oce=-17 25°N Hosing preind. (en TW) 90°N

ITCZ migration amplitude Zonal precipitation changes over the Atlantic Ü The amplitude of the ITCZ shift (year 220 -250, hosing-CTRL) depends on the experiment Ü What drives these changes? LIG LGM FUT HOL PRE

ITCZ migration mechanism SST and wind stress changes (year 220 -250, hosing-CTRL) LIG HOL Ü Fast signal Ü C-shape wind change in the tropics Ü Atmospheric bridge and oceanic tunnel Ü Wind-Evaporation-SST feedback (Chiang et al. 2008) FUT LGM PRE

Linearity Ü Phase diagram of the response of different climate indicators (after 250 years) Ü Temperature changes in the NH linear with clear sky radiative changes (sea ice) Ü ITCZ changes linear with cooling (atmospheric bridge)

Conclusions Ü The Greenland freshwater input amount computed during the Eemian is larger than a 2 x. CO 2 (using a simple heat flux budget) after 250 years Ü In the IPSL-CM 4, the climatic impact of a freshwater input shares similarities∀ theclimate Ü Cooling of the Northern Hemisphere Ü Southward ITCZ migration, mainly in the Atlantic Ü Slight warming on the west coast of Greenland Ü Complex energetic adjustment: important role of the sea ice

Thank you Swingedouw et al. Impact of freshwater release in the North Atlantic under different climate conditions in an OAGCM. Journal of Climate 22, 6377 -6403, 2009