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

© Crown copyright Met Office

Salinity: the accelerator, brake and warning lights of climate

change?

Richard Wood1

With thanks to Ed Hawkins2, Helene Hewitt1, Laura Jackson1, José Rodríguez1, Robin Smith2, Michael Vellinga1 and Peili Wu1

1Met Office Hadley Centre, Exeter, UK2NCAS-Climate, Reading University, UK


Outline

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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


Outline

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Precipitation under climate change

The rich get richer, the poor get poorer

IPCC AR5, 2014

Low concentrations High concentrations


So how will this affect salinity?S2

Observed (global) changes ~1970 to ~2005

(Helm et al. 2010)


Salinity minima and maxima expand

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Projected salinity changes in Indian Ocean SAMW: salinity minimum freshens

(Banks et al. 2000)

Observed changes ~1970 to ~2005

(Helm et al. 2010)


Do current models capture the observed rate of change?

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Durack et al. 2012

Obs SSS change 1950-2000

Low-warming CMIP3 models

Ocean model E-P + 5%

High-warming CMIP3 models

© 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 & Tropics Pacific Atlantic


Outline

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What drives the AMOC response to global warming?

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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

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


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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


• 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

© Crown copyright Met OfficeVellinga & Wu 2008

Global energy and water cycles, and AMOC are intimately linked

Net effect on global energy flows


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S2Salinity 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)


Outline

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Do AMOC thresholds exist in coupled climate models?

Hosing (Sv)(Hawkins et al. GRL 2011)

MO

C (

Sv)

FO

V (

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


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 90NM

odel

Lev

el

M

ode

l Lev

el

90S Latitude 90N

Rodriguez et al. 2014


•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


Salinity accumulation stores up potential for AMOC overshoot

Increase CO2, then return to pre-industrial values after a delay (CO2 removal technology)

AMOC overshoots original strength, delaying the recovery of NH temperatures to pre-industrial values

Wu et al. 2011


Salinity accumulation stores up potential for AMOC overshoot

Wu et al. 2011

Density Density due to SDensity due to T


Outline

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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


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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

CMIP3/5CMIP6


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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

Documents

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