Changes in the global water cycle inferred using the water mass transformation

framework.Nikolaos Skliris1, Jan D. Zika1, George Nurser2, Robert Marsh1, Simon Josey2,

Jeremy Grist2, Bablu Sinha2, , Adam Blaker2 and Frédéric Laliberté3. 1University of Southampton, 2National Oceanography Centre Southampton, 3University of Toronto

Volu

me

Salinity

Net evap.Net precip. Volu

me

Salinity

Mixing

Before After

This work is part of the Natural Environment Research Council funded project CLimate scale analysis of Air and Water masses (CLAW)

Wet get wetter & dry get drier?• Air holds ~7%

more moisture per 1K of warming (Clausius-Clapeyron).

• The water cycle increases by ~2-3%/K in climate models as atmospheric circulation weakens.

Held and Sodon (2006)

More Drought?

More Floods?

• Uncertainty in directly estimating changes in the water cycle remains a long-standing problem.

• Lack of robust observational estimates of precipitation and evaporation over the ocean

• Major problems when assessing the long-term E and P changes from atmospheric re-analysis products (often violate basic physical constraints, often inconsistent with observational estimates, generally showing too-intense water cycling)

• Oceanic observations of salinity with higher spatiotemporal coverage offer a unique opportunity in terms of measuring the integrated effect of changes in the water cycle

Measuring water cycle changes

Salinity and the water cycle

• Distinct high and low salinity regions in the ocean.

• These are coincident with evaporation minus precipitation (e-p) patterns.

• Do changes in the e-p pattern cause an amplification in the salinity pattern?

Is the ocean a rain gauge?

Salinity trend 1950-2010 (pss/60yrs): Global mean meridional sectionEn3

CSIRO

Durack and Wijffels 2010

Skliris et al. 2014

Skliris et al. 2014: Salinity trends estimated using Met Office/Hadley Centre En3 dataset objectively analysed monthly fields (1x1 deg. grid)

CSIRO (Durack and Wijffels, 2010): The salinity trends are fitted concurrently with the mean climatology and the ENSO signal through a multiple linear regression A variable search radius is also used, dependent on spatial and temporal sampling, which leads to highly-variable “spatial footprints” of mapped trends to emphasize broad-scale trends.

Salinity and the water cycle

19502010

Salinity mean contours

The Ocean as a rain gaugeΔSSS

Slope ΔSSS vs SSS Slop

e Δ(

e-p)

vs

(e-p

)

SSSDurack et al. 2012: Relate zonally averaged sea surface salinity (SSS) to its change(ΔSSS) observed over1950-2000:

-Get slope of ΔSSS vs SSS mean anomaly -Pattern Amplification ~8%

- Assume ΔSSS slope linearly related to Δ(e-p) diagnosed from CMIP3 models:ΔSSS slope increases at twice the rate of Δ(e-p)-Relate Δ(e-p) to global surface warming ΔT.

Suggests 4% increase of the water cycleor 8% increase/°K, but uncertainty is large.

Slop

e Δ(

e-p)

vs

(e-p

)

Global surface warming

• Skliris et al. 2014The water cycle amplification rate over specific regions is inferred from the observed 3-D salinity change field using the salt conservation equation in variable isopycnal volumes allowing for outcropping density surface migrations Assumptions: No change in diapycnal mixing/advection. Estimate of global E-P intensification ~2.1% over 1950-2010,~4% over 1979-2010

The Ocean as a rain gauge

Strong assumptions about advection processes and large uncertainty. Is there a more robust method?

Inferring Water Cycle Change from the observed 3-D salinity change

•Helm et al. 2010 E P change over 1970-2005 is inferred by integrating the salinity ‐change and volume along neutral density surfaces from the outcrop region to the equator in each hemisphere.Assumptions: No change in diapycnal mixing, no migration of isopycnal surfaces. Estimate of a 3 ± 2% decrease in P E over the mid and low ‐latitudes, a 7 ± 4% increase in the Northern Hemisphere high latitudes, and a 16 ± 6% increase in the Southern Ocean

Volu

me

Salinity

Dep

th

Latitude

Consider a histogram of waters of different salinities. The total Volume of water less than S is called V(S) (This is something we can measure!).

The water mass transformation framework

Volu

me

Salinity

Dep

th

Latitude

V(S)

V(S)S

S

Consider a histogram of waters of different salinities. The total Volume of water less than S is called V(S) (This is something we can measure!).

The water mass transformation framework

The water mass transformation framework

Movement of an individual water parcel in salinity coordinates is given by

Volu

me

Salinity

Dep

th

Latitude

Surface FWF + Diffusive salt flux

A fluid parcel changes its salinity, S, through evaporation (e), precipitation (p; including all water fluxes) and mixing.

Consider a histogram of waters of different salinities. The total Volume of water less than S is called V(S) (This is something we can measure!).

Water cycle change is estimated from the observed 3-D salinity change field using a novel thermodynamic framework based on the water mass transformation theory (Walin, 1982).

(Walin, 1977, 1982)

So the total movement of water across S is the integral of both terms

Volu

me

Salinity

Dep

th

Latitude

Changes in the volume of water (V) bound by an S=S* iso-surface are set by p-e and mixing. (δ is a dirac delta function, S0 is the mean ocean salinity and K is a positive definite diffusion tensor)

The water mass transformation framework

Movement of an individual water parcel in salinity coordinates is given by

Consider a histogram of waters of different salinities. The total Volume of water less than S is called V(S) (This is something we can measure!).

(Walin, 1977, 1982)

The water mass transformation framework

Lets integrate it (even easier to measure!)

Volu

me

Salinity

Dep

th

Diffusive salt fluxTotal Displacement of fresh water

Net Precipitation

Latitude

The water mass transformation framework

Lets integrate it (even easier to measure!)

In steady state

Mixing across a salinity surface is always positive

Net precipitation over the least saline waters and Net evaporation over the most saline waters

Diffusive salt fluxTotal Displacement of fresh water

Net Precipitation

The water mass transformation framework

Volu

me

Salinity

Intuitively mixing will always take salt from the saltiest water and mix it with the fresh water – collapsing the distribution

The water mass transformation framework

Volu

me

Salinity

Intuitively mixing will always take salt from the saltiest water and mix it with the fresh water – collapsing the distribution

Volu

me

So in steady state e-p must do the opposite freshening the fresh and evaporating the salty regions

Salinity

Application of the water mass framework

The real distribution has two peaks. The second being due to the North Atlantic

Volumetric Distribution of the Ocean in Salinity Coordinates

Application of the water mass framework

Accumulated (e-p-r) - least saline (p+r-e>0) most saline (p+r-e>0)

Water Cycle Amplitude in salinity space (~2.6 Sv)E-P: CORE2.0River runoff + melting: Dai et al. 2009, Dai and Trenberth, 2002

Application of the water mass framework

Skliris e al. 2014

Increase in precipitation?

Increase in evaporation?

Application of the water mass frameworkWhat is the total displacement of fresh water?

‘Displacement’ is fairly consistent between data sets although total fresh water content is not . Both datasets show a large net global freshening

0.037 Sv

0.042 Sv

Can we attribute this to water cycle changes?

-0.021 Sv

-0.041 Sv

Net freshening

Relating water mass change to the water cycleThe salinity distribution is getting wider. This could be due increased e-p alone. For the two data sets the implied change would be 1.5-2% of the total water cycle.

Mean water cycle ≈ 2.6 Sv

Additional p-e ≈ 0.04 Sv?

Relating water mass change to the water cycleThe salinity distribution is getting wider. This could be due to increased e-p alone. For the two data sets the implied change would be 1.5-2% of the total water cycle.

But how much mixing is going on?

Mean water cycle ≈ 2.6 Sv

Additional p-e ≈ 0.04 Sv?

Relating water mass change to the water cycleIn a model we have a complete time series of p-e, and the 3-D salinity distribution

Water cycle amplitude in CCSM4 (Sv)

CCSM4 (U. Of Toronto) Historical+RCP4.5 run (1950-2100)Although inter-annual variability is large, p-e trends are positive with the RCP4.5 scenario.

However the fresh water displacement diverges from the p-e change. Mixing dampens the signal

Accum. (p-e)’Accum. FW volume displacement (S)

Diffusive salt fluxTotal FW Displacement Net Precipitation

0 0.5 1 1.5 2 2.5 3 3.5 40.00

5.00

10.00

15.00

20.00

25.00

f(x) = 6.05296986814686 x + 0.188615824270567R² = 0.866144091580873

SST change (° C)

Wa

ter

Cy

cle

ch

an

ge

(%

)10 CMIP5 models (IPSL, NOAA-GFDL, HadGEM2, CNRM, MRI, MPI, Nor-NCC, CMCC, EC-EARTH,ACCESS1-3)Historical (1850-2005), RCP4.5 and RCP8.5 (2006-2100) runs

Historical RCP4.5

RCP8.5

CMIP5 ensemble mean: Water Cycle Amplitude sensitivity to warming ~6%/°K (Clausius Clapeyron rate ~7 %/°K)

Application of the water mass framework to CMIP5

CMIP5 models: Freshwater displacement (Sv) over 1950-2005

Application of the water mass framework to CMIP5

CMIP5 1950-2100 runs (Historical+RCP4.5)

Application of the water mass framework to CMIP5

Diffusive salt fluxNet PrecipitationTotal Displacement of fresh water

The mean deviation of salinity (W) is our state variable:

Fw , the p-e integrated up to the mean salinity, is our metric of the water cycle:

Mixing is modelled with an e-folding time scale τ.

A simple model for the salinity distribution

Fw =

=

where V0 is the total volume and So the mean salinity of the ocean

W increases with the accumulated p-e (e-p) over the freshest (saltiest) side of the distribution (Fw) and decreases with mixing.

So

Becomes

A simple model for the salinity distribution

MRI

ACCESS1-3

CMCC L

R

IPSL

LR

NCC-Nor

NOAA-GFDL

MPI

LR

EC-EARTH

CNRM

HadGEM

2

CMIP

5 M

EAN

CESM1

En30

0.050.1

0.150.2

0.250.3

0.35Wmean

En3: Wmean~ 0.195 g/kg

CMIP5 mean: Wmean~ 0.21 ±0.03 g/kg

MRI

ACCESS1-3

CMCC L

R

IPSL

LR

NCC-Nor

NOAA-GFDL

MPI

LR

EC-EARTH

CNRM

HadGEM

2

CMIP

5 M

EANEn3

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

Series1

Series2

Series3

HHistoric (1950-2005)

HRCP4.5 (2006-2100)

HRCP8.5 (2006-2100)

W

Historical periodEn3: ΔW~ 0.035 g/kgCMIP5 mean: ΔW ~ 0.034 ±0.020g/kg

A simple model for the salinity distribution

Use change in W to predict change in Fw

Estimate from pre-industrial mean (steady state): dW/dt=0 mean= Vo Wmean /(2SoFwmean )

mean (yrs)

MRI

ACCESS1-3

CMCC L

R

IPSL

LR

NCC-Nor

NOAA-GFDL

MPI

LR

EC-EARTH

CNRM

HadGEM

2

CESM1

CMIP

5 M

EAN

En3/C

ORE2.0

0

20

40

60

En3: mean ~ 48 yrs

CMIP5 mean: mean ~ 42 ±7 yrs

Quantifying water cycle change

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

0.5

1

1.5

2

f(x) = 0.95615773068775 xR² = 0.935124514360408

Historical (1950-2005)

RCP4.5 (2006-2100)

RCP8.5 (2006-2100)

1015m3

1015m3

Predicted ∫Fw’

∫Fw’

Quantifying water cycle change

CMIP5 runs: Historical (1950-2005), RCP4.5, RCP8.5 (2006-2100)

∫Fw’= Accumulated volume of freshwater added

(extracted) from the fresh (salty regions) since 1950 (historical) and 2006 (RCP4.5 and RCP8.5).

How is the water cycle changing?The observed mean salinity distribution (EN3, CSIRO) and water cycle (CORE2.) suggest a mixing timescale of 50 years. A fresh water displacement of ~0.037 Sv (0.042Sv) in En3 (CSIRO) implies the water cycle has increased by approximately 0.065 (0.080) Sv (assuming e-p increases linearly) thus ~2.5-3% or ~5-6%/°K over 1950-2010

Mean water cycle ≈ 2.6 Sv

Water Cycle change ≈ 0.065-0.080 Sv

Diffusive flux ≈ 0.028 – 0.038 Sv

Displacement of freshwater ≈ 0.037-0.042 Sv

mean ~ 48 yrs

Conclusions

• The salinity distribution is maintained by the stretching effect of a ~2.6 Sv water cycle and the collapsing effect of mixing with a timescale of order 50 years

• CMIP5 models show a robust relationship between change (increase)in the width of salinity distribution and change (increase) in the water cycle amplitude over the historical period and RCP projections

• , The increase in the distribution width in observations indicates an

increase in the water cycle of 0.065-0.080 Sv, i.e. ~ 2.5-3% (~5-6%/°K)amplification over 1950-2010.

References-Dai, A., T. Qian, K. E. Trenberth, and J. D Milliman, 2009: Changes in continental freshwater discharge from 1948-2004. J. Climate, 22, 2773-2791 -Dai, A., and K. E. Trenberth, 2002: Estimates of freshwater discharge from continents: Latitudinal and seasonal variations. J. Hydrometeorol., 3, 660-687.-Held IM, Soden BJ (2006) Robust responses of the hydrological cycle to global warming. J Clim 19:5686-5699. doi:10.1175/JCLI3990.1-Durack PJ, Wijffels SE (2010) Fifty-year trends in global ocean salinities and their relationship to broad-scale warming. J Clim 23:4342-4362. doi:10.1175/2010JCLI3377.1-Durack PJ, Wijffels SE, Matear RJ (2012) Ocean salinities reveal strong global water cycle intensification during 1950 to 2000. Science 336:455-458. doi:10.1126/science.1212222-Helm KP, Bindoff NL, Church JA (2010) Changes in the global hydrological-cycle inferred from ocean salinity. Geophys Res Lett 37:L18701. doi:10.1029/2010GL044222-Held IM, Soden BJ (2006) Robust responses of the hydrological cycle to global warming. J Clim 19:5686-5699. doi:10.1175/JCLI3990.1-Hosoda S, Suga T, Shikama N, Mizuno K (2009) Global surface layer salinity change detected by Argo and its implication for hydrological cycle intensification. J Oceanogr 65:579-586. doi:10.1007/s10872-009-0049-1-Skliris N, Marsh R, Josey SA, Good SA, Liu C, Allan RP (2014) Salinity changes in the World Ocean since 1950 in relation to changing surface freshwater fluxes. Clim Dyn. doi: 10.1007/s00382-014-2131-7-Walin, G., (1982) On the relation between sea-surface heat flow and thermal circulation in the ocean. Tellus, 34, 187-195

Ocean circulation in temperature-salinity coordinates.

Atmospheric circulation in temperature-moisture coordinates.

Water masses by region Air masses by region

So

Becomes

In CCSM4 the pre-industrial mean ≈ 55 yrs. And based on the transient run ≈ 45 yrsHence the mean may be a god indicator of the transient .

Relating water mass change to the water cycle

W

From salinity observations (En3) and assuming steady state, τ ≈ 50 years