low frequency variation of sea surface salinity in the tropical atlantic

LOW FREQUENCY VARIATION OF SEA SURFACE SALINITY IN THE

TROPICAL ATLANTIC

Semyon A. Grodsky1, James A. Carton1, and Frederick M. Bingham2 1 Department of Atmospheric and Oceanic Science, University of

Maryland at College Park, 2 Center for Marine Science, University of North Carolina at Wilmington

AQUARIUS/SAC-D Workshop, WHOI , May 10-12, 2006

BACKGROUND Curry et al. [2003] have found the salinification of

subtropical and tropical between 1985−99 and 1955−69 at a rate of 0.015 psu/decade.

Boyer et al. [2005] have found Atlantic salinity trends exceeding 0.03 psu/decade at nearsurface levels throughout the subtropics and tropics.

Does the salinification persist in the tropical Atlantic?

Is linear trend appropriate to characterize the slow salinity change?

What mechanisms are behind the slow salinity What mechanisms are behind the slow salinity change?change?


30 yr salinity difference between 1985–99 and 1955–69 averages versus depth in the western Atlantic. Adapted

from Curry et al., Nature, 2003.


Salinification tendency of up to 0.05psu/(3decades)=0.015psu/decade in the western tropical and subtropical Atlantic

Linear trend of the zonally averaged pentadal salinity 1955–1959 to 1994–1998 for the Atlantic Ocean.

Adapted from Boyer et al. [2005], Geoph. Res. Lett.


Salinification intensifies approaching the surface and the equator up to 0.05 psu/decade.

Time variation of salinity averaged 5oS to 5oN across the equatorial Atlantic. Data courtesy of Boyer et al. [2005],

Geoph. Res. Lett.


•Salinity variation intensifies nearsurface.

•There are substantial deviations from linear trend at nearsurface levels.

OUTLINEOUTLINE We analyze the mixed layer salinity using We analyze the mixed layer salinity using

historical data from the WOD01 and historical data from the WOD01 and thermosalinograph data.thermosalinograph data.

We find noticeable decadal variability of the We find noticeable decadal variability of the tropical Atlantic SSS in response to changes tropical Atlantic SSS in response to changes in surface forcing. in surface forcing.

Major near surface salinification which Major near surface salinification which persisted throughout the 1960s through persisted throughout the 1960s through 1980s has been replaced by a freshening 1980s has been replaced by a freshening trend in the recent decades. trend in the recent decades.



Number of grid points in the 15S-15N, 50W-15E Tropical Atlantic sector covered by observations during a month.

Salinity averaged 5oS to 5oN across the equatorial Atlantic (WOD01 profile data)


Salinity averaged 5oS to 5oN across the equatorial Atlantic (WOD01 profile data+thermosalinograph)


Spatial pattern of the Atlantic sea surface salinity (SSS) from the WOD01 salinity profiles and thermosalinograph data.


Time mean SSS (psu); SSS difference between 1980-1999 and 1960-1979 averages

5-year running mean SSS anomaly in the tropical Atlantic.


SSS change is ~ 0.15psu/(20yr) or ~ 0.08psu/decade

5-year running mean SSS and reanalysis rainfall anomaly in the tropical Atlantic.


Freshening in the recent decades occurs in accord with local rainfall. But over the whole record the decadal changes of SSS and rainfall are poorly correlated.

Regression of the equatorial Atlantic SSS time series on winds.


Stronger easterly corresponds to increased salinity.

Equatorial zonal wind anomaly and the Hadley Center MSL pressure difference between the eastern and western boxes.

Stronger equatorial easterly can affect the Stronger equatorial easterly can affect the mixed layer salinity bymixed layer salinity by

Enhancing entrainment of salt sub-mixed Enhancing entrainment of salt sub-mixed layer (EUC) waterlayer (EUC) water

Enhancing evaporationEnhancing evaporation Reducing rainfall (stronger eatsterly Reducing rainfall (stronger eatsterly

northward shift of the ITCZ, warm NTA northward shift of the ITCZ, warm NTA phase of the interhemispheric mode)phase of the interhemispheric mode)



E P

We

z

Lat

SEC

EUC

z=-hS

S-h

S-h-S~0.5 psu

We~0.5 m/day

P-E~mm/day

EQ

Simplified 1-D balance of the mixed layer salinity

•Decadal salinity anomalies are surface intensified focus on mixed layer mechanisms.

•Consider the vertically averaged mixed layer salinity that is also averaged zonally across the basin and across the equatorial belt where mean entrainment is positive.

•Assuming that salinity is nearly uniform horizontally gives an approximate balance between the time variations of entrainment, and net surface freshwater flux.

])[(][ SPESSw he

Variation of the mixed layer salinity averaged Variation of the mixed layer salinity averaged spatially over the equatorial box is due tospatially over the equatorial box is due to


he

he

e SSw

PESS

w

wS

)()(

Entrainment Surface flux EUC salinity

We=O(1) m/day >> E-P=O(1) mm/day

Examples:

1. Salinity variation due to variation of entrainment velocity.

In the equatorial zone we~x~U2, so =0.2psu in response to 1m/s change in zonal wind at mean zonal wind U=5 m/s and S-h-S=0.5 psu.

2. What change in precipitation induces 0.2 psu change in salinity?

=3 mm/day at we=0.5 m/day.

3. Mixed layer salinity responds to change in precipitation at a rate of S/we=0.07 psu/(mm day-1).

)()/(2 SSUUS h

ewSSP )/(

Interannual variation of salinity, wind, and Interannual variation of salinity, wind, and rainfallrainfall


At interannual periods entrainment, evaporation, and precipitation work in accord. The freshening occurs in phase with lower winds and higher rainfalls.

SummarySummary Gradual salinification of the tropical Atlantic by Gradual salinification of the tropical Atlantic by 0.2 0.2

psupsu occurred during the three decades of the occurred during the three decades of the 1960s through the 1980s, which increased the 1960s through the 1980s, which increased the density of the mixed layer by an amount density of the mixed layer by an amount equivalent to a 0.8equivalent to a 0.8ooC drop in SST. C drop in SST.

During the past decades the oceanic mixed layer During the past decades the oceanic mixed layer has freshened to values observed in mid 1960s. has freshened to values observed in mid 1960s.

Decadal change in SSS develops in accord with Decadal change in SSS develops in accord with the decadal change in the equatorial zonal wind the decadal change in the equatorial zonal wind that impacts entrainment of salt water into the that impacts entrainment of salt water into the mixed layer. mixed layer.

At interannual time scales At interannual time scales 0.1psu0.1psu variations in variations in SSS are present with fluctuations at a rate of SSS are present with fluctuations at a rate of 0.05 0.05 psu/(mm/day)psu/(mm/day). Interannual freshenning occurs in . Interannual freshenning occurs in accord accord with weakening winds and higher rainfalls.


Acknowledgements.Acknowledgements. We gratefully acknowledge support from the NASA We gratefully acknowledge support from the NASA

Oceans Program.Oceans Program. Agencies collecting and maintaining data archives. Agencies collecting and maintaining data archives. Dr. Alain Dessier kindly provided an update of the Dr. Alain Dessier kindly provided an update of the

Atlantic sea surface salinity archive. Atlantic sea surface salinity archive. CORIOLIS salinity data were collected and made CORIOLIS salinity data were collected and made

freely available by the CORIOLIS project and freely available by the CORIOLIS project and programs that contribute to it.programs that contribute to it.


My Coauthors and me appreciate giving us an opportunity to present this research.

low frequency variation of sea surface salinity in the tropical atlantic

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