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© Crown copyright Met Office Salinity: the accelerator, brake and warning lights of climate change? Richard Wood 1 With thanks to Ed Hawkins 2, Helene.

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Presentation on theme: "© Crown copyright Met Office Salinity: the accelerator, brake and warning lights of climate change? Richard Wood 1 With thanks to Ed Hawkins 2, Helene."— Presentation transcript:

1 © Crown copyright Met Office Salinity: the accelerator, brake and warning lights of climate change? Richard Wood 1 With thanks to Ed Hawkins 2, Helene Hewitt 1, Laura Jackson 1, José Rodríguez 1, Robin Smith 2, Michael Vellinga 1 and Peili Wu 1 1 Met Office Hadley Centre, Exeter, UK 2 NCAS-Climate, Reading University, UK

2 © Crown copyright Met Office Outline S2S2 S2S2 1.Can we use salinity to detect changes in the global water cycle? 2.Salinity as accelerator and brake of the climate system 3.Can we provide early warning of approaching AMOC thresholds? 4.Some modelling progress

3 © Crown copyright Met Office Outline S2S2 S2S2 1.Can we use salinity to detect changes in the global water cycle? 2.Salinity as accelerator and brake of the climate system 3.Can we provide early warning of approaching AMOC thresholds? 4.Some modelling progress

4 © Crown copyright Met Office Precipitation under climate change The rich get richer, the poor get poorer IPCC AR5, 2014 Low concentrationsHigh concentrations

5 © Crown copyright Met Office So how will this affect salinity? S2S2 Modelled salinity changes (1990s-1960s): fresh gets fresher, salty gets saltier (Banks & Bindoff 2004) Observed (global) changes ~1970 to ~2005 (Helm et al. 2010)

6 © Crown copyright Met Office Salinity minima and maxima expand S2S2 Projected salinity changes in Indian Ocean SAMW: salinity minimum freshens (Banks et al. 2000) Observed changes ~1970 to ~2005 (Helm et al. 2010)

7 © Crown copyright Met Office Do current models capture the observed rate of change? S2S2 Durack et al. 2012 Obs SSS change 1950-2000 Low-warming CMIP3 models Ocean model E-P + 5% High-warming CMIP3 models

8 © Crown copyright Met Office Terray et al. 2011 Formal detection? SSS detection scaling factors for modelled patterns of SSS change due to anthropogenic forcing vs. internal variability. Detection for global and Pacific, but not for Atlantic Global & TropicsPacificAtlantic

9 © Crown copyright Met Office Outline S2S2 S2S2 1.Can we use salinity to detect changes in the global water cycle? 2.Salinity as accelerator and brake of the climate system 3.Can we provide early warning of approaching AMOC thresholds? 4.Some modelling progress

10 © Crown copyright Met Office What drives the AMOC response to global warming? S2S2 Gregory et al. 2005 AMOC weakening in response to GHG increase...... is dominated by the effects of warming, but salinity is also slowing the AMOC Fraction of change due to radiative forcing only

11 Tropical fresh water response stabilises the AMOC Global warming leads to enhanced water transport from tropical Atlantic to tropical Pacific +ve Atlantic salinity anomalies transported northwards from subtropics to subpolar regions, accelerating (stabilising) the AMOC Components of Atlantic ocean salinity transport at 33°N (Thorpe at al., J. Climate 2001) Thorpe et al. 2001

12 © Crown copyright Met Office S2S2 Vellinga & Wu 2008 Global energy and water cycles, and AMOC are intimately linked Effect of artificial AMOC shutdown (hosing) Change in atmos energy transport Change in ocean heat transport

13 © Crown copyright Met Office ITCZ moves south in response to colder NH temperature Water input moves into S Atlantic N Atlantic gets saltier → feedback onto AMOC Change in Precipitation-Evaporation (mm/day) (Vellinga & Wood Climatic Change 2002) AMOC feeds back onto water cycle

14 © Crown copyright Met Office Vellinga & Wu 2008 Global energy and water cycles, and AMOC are intimately linked Net effect on global energy flows

15 © Crown copyright Met Office S2S2 S2S2 Salinity forcing in the right places could induce threshold crossing Fichefet et al. 2003 SRES B2 scenario: Change in AMOC in a low-resolution climate model with and without interactive Greenland ice sheet model Difference in surface air temperature around 2100 (with ice sheet – without ice sheet)

16 © Crown copyright Met Office Outline S2S2 S2S2 1.Can we use salinity to detect changes in the global water cycle? 2.Salinity as accelerator and brake of the climate system 3.Can we provide early warning of approaching AMOC thresholds? 4.Some modelling progress

17 © Crown copyright Met Office Do AMOC thresholds exist in coupled climate models? Hosing (Sv) (Hawkins et al. GRL 2011) MOC (Sv) F OV (Sv) FAMOUS YES! Hysteresis experiment using FAMOUS (low resolution version of HadCM3) Add fresh water to N Atlantic (hosing) Slowly increase hosing from 0 to 1 Sv, then back down to -0.5 Sv (5000 years) Initial state

18 © Crown copyright Met Office Dynamics of the threshold are very simple 5 boxes: represent contiguous water masses Similar assumptions to Stommel model All control parameters are emergent (observable) properties of the climate system, so can be diagnosed for any GCM (or the real world): NO TUNING! Atlantic Indo-Pacific 90S Latitude 90N Model Level 90S Latitude 90N Rodriguez et al. 2014

19 © Crown copyright Met Office Calibrate (not tune!) the box model to the FAMOUS GCM: unperturbed state only! Repeat hysteresis experiment with box model Thresholds and hysteresis quantitatively well described by box model Calibrate to FAMOUS at this point only Box Model FAMOUS Box ModelFAMOUS Dynamics of the threshold are very simple This suggests that by monitoring some large scale properties of the salinity & water transports we could provide early warning of the approach to an AMOC threshold Rodriguez et al. 2014

20 © Crown copyright Met Office Salinity accumulation stores up potential for AMOC overshoot Increase CO 2, then return to pre- industrial values after a delay (CO 2 removal technology) AMOC overshoots original strength, delaying the recovery of NH temperatures to pre- industrial values Wu et al. 2011

21 © Crown copyright Met Office Salinity accumulation stores up potential for AMOC overshoot Wu et al. 2011 DensityDensity due to SDensity due to T 30°S 90° N

22 © Crown copyright Met Office Outline S2S2 S2S2 1.Can we use salinity to detect changes in the global water cycle? 2.Salinity as accelerator and brake of the climate system 3.Can we provide early warning of approaching AMOC thresholds? 4.Some modelling progress

23 Salinity in top 500m (PSU) Meridional FW transport (Sv) Partitioning of fresh water transport between overturning and gyre impacts AMOC stability Generally wrong in many models (AMOC too stable?) Improved in some newer models Courtesy Laura Jackson Improved modelling of Atlantic fresh water transports Total Gyre Overturning

24 © Crown copyright Met Office S2S2 Improving modelling of large scale water cycle Demory et al Clim Dyn 2014 How much of the precip over land is recycled from evap, or due to dynamical moisture convergence? Atmos model resolution CMIP3/5 CMIP6

25 © Crown copyright Met Office S2S2 Summary 1.Observed large scale salinity changes consistent with expectations from climate change models. 2.Some progress on formal attribution. Do models under- estimate rate of water cycle change? 3.Salinity changes can act both to accelerate and slow the AMOC – complex interaction of processes/timescales 4.Possibility of monitoring distance from AMOC threshold 5.Salinity can store up commitment to long term change 6.Recent progress in modelling the large scale water cycle – new opportunities to bring together with observations

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