The ocean and the global hydrologic cycle

Jim Carton (University of Maryland)Paulo Nobre (INPE)

São Paulo Summer School on Global Climate Modeling October, 2011

Guiding Question: Does the oceanic component of the hydrologic

cycle vary, and if so what are the consequences for climate?

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Outline

• Overview: – mean global salt and freshwater budgets

• Measuring the oceanic hydrologic cycle• Observed and modeled trends• Connection between salinity trends and

climate• Introduction to ocean modeling

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The global hydrologic cycle

Dai and Trenberth

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Some Numbers

• Net evaporation from the ocean: 1.2 m/yr = 3.8x10-8 m/s• Area of the ocean: 3.6x1014 m2

• Surface volume flux due to evap: 13.7x106 m3/s or 13.7Sv (similar to the rate of formation of North Atlantic Deep Water!)

• Amazon River discharge: 0.2x106 m3/s (so think of evaporation as: 70 Amazons!

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How does this freshwater budget relate to the salinity budget? First a little math

𝐷𝜌𝐷𝑡 +∇ ∙ (�� 𝜌 )=(𝑃 −𝐸)𝛿(𝑧)

Mass is conserved, so storage and advection must balance net surface flux. The mass budget is:

Salinity must be conserved, but for salinity there are no sources or sinks! (except a very weak river source)

𝐷𝑆𝜌𝐷𝑡 +∇ ∙ (��𝑆 𝜌 )=0 (2

(2) Can be rewritten using (1) to give us the salt budget:

𝑆 [𝐷𝜌𝐷𝑡 +∇ ∙ (��𝜌 )]+𝜌 [𝐷𝑆𝐷𝑡 +𝛻 ∙ (��𝑆 )]=0

[𝐷𝑆𝐷𝑡 +𝛻 ∙ (�� 𝑆 )]=𝑆

𝜌 (𝐸−𝑃 )𝛿(𝑧 ) (4

(1

Storage + advection = effective net surface flux

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Climatological sea surface salinity

In warm water: ST 00078.000015.0So, change in density due to 1psu is equivalent to 5C

Stephens et al., NOAA, 2002

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Observed E-P

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Annual Range of Salinity

Observing systems

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Aquarius 7dy

Argo

Present and future salinity sampling

PIRATA

Current Argo distribution

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Salinity from the PIRATA mooring at 21N, 23W: evidence of eddies!

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Time series at the 21N, 23W PIRATA mooring

OBS S(20m) vs EVAP at 21N, 23W

36.2

36.4

36.6

36.8

37

37.2

M-07 N-07 M-08 N-08 M-09 N-09 M-10 N-10

Salin

ity

0.0E+00

2.0E-08

4.0E-08

6.0E-08

8.0E-08

1.0E-07

1.2E-07

EVAP

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Sea surface salinity

Lagerloef et al., 2007: The Aquarius/SAC-D Mission: Designed to Meet the Salinity Remote-Sensing Challenge, Oceanography Magazine.

Microwave brightness temperature varies with salinity. Panel to the right shows the variations of radiation expected from a flat surface (no waves). Note that the dependence is highest at higher temperatures.

Aquarius exploits this dependence to obtain an SSS measurement with an expected ~0.2PSU accuracy at monthly timescales.

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WOW!

Trends

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(psu/50yr)

surface salinity linear trend

Durack and Wijffels (2010)

Observed 50yr drying trend over Africa

Held et. al., 2005

psu/50yr

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Vertical structure of the salinity change in the Atlantic (zonally averaged)

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Freshening trends in the deep North Atlantic

Dickson, et. al., Nature, 2002

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Surface salinity change in an atmosphere-ocean coupled GCM (CM2.1) in response to elevated CO2

Stouffer et al. (2006)

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Salinity and climate

• Heat and freshwater cycles are linked directly through the relationship between latent heat loss and evaporation:– 0.6 Sv Freshwater loss corresponds to

1.5 petawatts of latent heat flux• They are linked indirectly through the

impact of salinity changes on density.

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How the equation of state depends on salinity

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The meridional overturning circulation

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Heat transport by the oceans

Houghton et al., (1996: 212)

Some Numbers

• Assume 15Sv = 1.5x106 m3/s northwards at the surface and southwards below 2000m depth

• Assume a 15oC temperature difference between the two flows

• The net heat transport is: 1.5x107 x 15 x 4x106 = 0.9x1015 W! or nearly the total amount of heat being transported northwards in the North Atlantic.

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Freshwater transport

Jourdan et al., J. Phys. Oceanogr., 27, 457-467, 1997

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Will the Atlantic MOC change in response to increasing greenhouse gasses?

IPCC 4th Assessment

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Ocean general circulation modeling

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Basic Equations

2

222

z

UUpUDt

UD

o

windszz

U

)0(

0

zwU

heatingz

TTDt

DT

2

22

fluxsaltz

SSDtDS

2

22

G e o s t r o p h y

Numerics• Arakawa-B grid in

horizontal (2nd order)• Upstream advection• Leap frog time

differencing• Rigid lid (w(z=0)=0)• Separate internal and

extenral modes• SHMEM, MPI, shared

memory, multi-threaded version

u,v

T,S

Arakawa-B

u,v

u,v

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Our model• General circulation ocean model

using POP2 numerics• Global grid• 0.1ox0.1ox42Levels (3x108)• ‘Normal year’ forcing. 64yr spinup• Compute/save full salt budgets

(1yr so far)• 1 year requires 12K PE hrs on an

IBM Power6

10% of actual resolutionCarton

Observations Model Simulation

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What terms balance E-P in the salt budget of the mixed layer?

( ) ( , )

( )

H

z z z z h

h

d SE P S u v S subsurface ML mixing

dthsubsurface S w S St

S S S

??

Do eddies contribute?

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Mean salt balance 0-100m average

Cool color means exporting salt

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What we’ve learned• Oceanic hydrologic cycle overview

– The oceans play a central role in the global hydrologic cycle– Patterns of surface salinity mainly reflect patterns of E-P

• Observing systems: rapidly improving– ARGO born 2001– Aquarius born 2011

• Salinity trends: – In the past 50 years salty places are getting saltier, fresh places

are getting fresher– In particular, the subpolar North Atlantic has been getting fresher– These results seem to be consistent with CO2 effects based on

coupled models– The implications for the meridional overturning circulation

(AMOC) are still not clear• Ocean General Circulation Modeling

– We’ll use this tool to look at the salt budget of the upper 100m.

• High salinity ‘ocean deserts’ (source waters for the tropical thermocline) are maintained by: 1) surface evaporation, 2) poleward wind-driven transport of freshwater, and 3) horizontal eddy exchanges. How will they change?

• The ‘dry’ parts of the ocean have been getting saltier possibly reflecting an intensification of the hydrologic cycle. What will this mean for Saudi Arabia?– What changes have occurred historically?– How can we improve our guesses about future conditions?– What are the impacts of these physical changes on marine

biogeochemical processes?

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Does the oceanic component of the hydrologic cycle vary, and if so what are the consequences for climate?

The answer to the first part is clearly yes. But what about the second?