Preliminary results on

Formation and variability of North Atlantic sea surface

salinity maximum in a global GCM

Tangdong Qu

International Pacific Research Center, SOEST, University of Hawaii

(August 2, 2010)

In collaboration with

Shan Gao and Ichiro Fukumori

Aquarius will be launched in April 2011 to measure the sea surface salinity over the global ocean.

Of particular importance in the ocean’s salinity distribution is the SSS maxima (oceanic “deserts”) at subtropical latitudes. The highest SSS maximum (>37.2 psu) occurs in the North Atlantic, and a field experiment (SPURS) is scheduled

for 2012 to study this maximum.

SPURS

Questions to be addressed:

(1) What processes maintain this salinity maximum?

(2) What processes give rise to its temporal variability?

(3) What scales of temporal variability dominate?

(4) Where does the excess salt of E-P go?

(5) What is the transport of excess salt from the surface layer and can this be considered significant in driving MOC?

Their maxima don’t coincide, implying strong influence of ocean dynamics.

However, an observational assessment of ocean’s role in maintaining this SSS maximum is not possible because not all the necessary data are ordinarily available.

E-P (NCEP)

A careful examination of SSS and E-P distributions indicate:

Taking advantage of the rapid advance in ocean models, this study investigates the formation and variability of SSS maximum in the NA using results from the model of Consortium for Estimating the Circulation and Climate of the Ocean (ECCO). Our focus is to address:

(1) How the SSS maximum in the North Atlantic is maintained and changed, and what processes are governing its salinity budget;

(2) Where the SSS maximum water is subducted and where it goes (salt river).

Though no ocean models are perfect, analysis of models’ results may help understand the questions listed above and provide useful hints for the design and analysis of future observations (e.g., SPURSE).

The present study

ECCO model description

ECCO has a horizontal resolution of 1° globally, except within 20° of the equator, where its meridional grid spacing is gradually reduced to 0.3°. Following the spin up, the model is forced from 1980 to the present by wind stress, heat flux, and evaporation minus precipitation estimates of the NCEP Re-analysis. The equivalent of a fresh water flux is implemented by relaxing surface salinity to climatological values with a 60-day relaxation scale. The model results from 1993 to 2006 are used for the present study (see Fukumori et al., 2004).

The ECCO model reproduces the observed SSS reasonably well.

Mixed layer salinity budget

On the annual average, surface flux is balanced nearly equally by horizontal advection and vertical entrainment .

convergence

+ SSP

Small-scale processes

Vertical entrainment

Horizontal advectionSurface fluxSalinity

tendency

1.76×10-8 g/cm3/s(~100 cm/yr) -0.87×10-8 g/cm3/s

-0.75×10-8 g/cm3/s-0.15×10-8 g/cm3/s

In the verticalSalinity is maximum near the sea surface and vertical exchange takes salt away.

Seasonal cycle

(1) MLD matters to the salinity budget; (2) E-P directly impacts dS/dt (not SSS).

Gordon, 2009

+ SSP

Result from ECCO

Salinity budget analysis confirms that ocean dynamics is important in the SSS seasonal variability. Adding the contribution from ocean

dynamics better explains the SSS tendency, with the correlation increasing from ~0.7 to >0.9.

Seasonal salinity budget

SSS varies from year to year

The interannual amplitude is ~0.2 psu for the period of integration. The correlation of SSS with E-P is only about 0.3.

The correlation increases from 0.32 to 0.76 if the contribution of ocean dynamics is added, better explaining the SSS variability. Eddies and smalerl-scale processes?

Interannual budget

R=0.32

R=0.76

Subduction Subduction of SSS maximum water occurs mostly at the intersects of MLD fronts and outcropping lines near σθ=26.0 kg m-3

Properties near σθ=26.0 kg m-3

Salt River

Low PV

There are two subduction bands in the North Atlantic, and most of the SSS maximum water formation occurs in the southern band. Where the water goes after subducted?

ECCO

Qiu and Huang, 1995

ATL_vertical integral all.wmv

Passive tracer was first released in the winter mixed layer of the SSS maximum region, and then integrated forward for 28 years to see where the water goes with time.

Labrador

Sea

SSS maximum

20°-30°N, 24°-52°W

Denmark Strait

Vertically integrated tracer

Snapshots at Years 0, 2, 4, 6, 8, 10, 12, 14, 16, 20, 24, and 27, showing most of the SSS maximum water reaches the Labrador Sea.

Where the SSS maximum water goes?

Vertically integrated below the ML Vertically integrated within the ML

A large portion of the SSS maximum water subducted in the central North Atlantic, with some still remaining in the mixed layer. As the water moves northwestward, subduction continues to occur, and little remains in the ML at the end of the integration.

Zonally integrated tracer distribution

Meridional circulationThe salinity maximum water gets colder as it moves toward higher latitudes, and this produces a denser surface water that can reach as deep as up to 3000 m in the sub-polar North Atlantic (Labrador Sea?).

Interannual subduction volume

Subduction volume (North Atlantic)Subduction volume (SSS maximum)

The subduction volume varies from year to year and seems to have a downward trend during the period of integration. This needs further investigation.

The analysis of ECCO’s results has revealed the following information, which might be useful for the upcoming Ocean Salinity Field Campaign (SPURS).

(1) Horizontal advection and vertical entrainment are of equal importance in balancing the surface E-P flux and maintaining the SSS maximum.

(2) The SSS maximum varies on seasonal to interannual time scales, and both horizontal advection and vertical entrainment play an important role in this variability.

(3) The excess salt resulting from E-P is mostly subducted into the ocean’s thermocline, and from there it is conveyed poleward and directly contributes to the formation and circulation of NADW.

A remaining issue: Little subducted SSS maximum water spreads toward the equator, and its contribution to the shallow MOC seems to be minor. ???

Summary of the preliminary results