LOW FREQUENCY VARIATION OF SEA SURFACE SALINITY IN THE TROPICAL ATLANTIC

1. LOW FREQUENCY VARIATION OF SEA SURFACE SALINITY IN THETROPICAL 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

2. 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 change? AQUARIUS/SAC-D Workshop, WHOI , May 10-12, 2006

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

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

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

6. OUTLINE • We analyze the mixed layer salinity using historical data from the WOD01 and thermosalinograph data. • We find noticeable decadal variability of the tropical Atlantic SSS in response to changes in surface forcing. • Major near surface salinification which persisted throughout the 1960s through 1980s has been replaced by a freshening trend in the recent decades. AQUARIUS/SAC-D Workshop, WHOI , May 10-12, 2006

7. Number of grid points in the 15S-15N, 50W-15E Tropical Atlantic sector covered by observations during a month. AQUARIUS/SAC-D Workshop, WHOI , May 10-12, 2006

8. Salinity averaged 5oS to 5oN across the equatorial Atlantic (WOD01 profile data) AQUARIUS/SAC-D Workshop, WHOI , May 10-12, 2006

9. Salinity averaged 5oS to 5oN across the equatorial Atlantic (WOD01 profile data+thermosalinograph) AQUARIUS/SAC-D Workshop, WHOI , May 10-12, 2006

10. Spatial pattern of the Atlantic sea surface salinity (SSS) from the WOD01 salinity profiles and thermosalinograph data. SSS difference between 1980-1999 and 1960-1979 averages Time mean SSS (psu); AQUARIUS/SAC-D Workshop, WHOI , May 10-12, 2006

11. 5-year running mean SSS anomaly in the tropical Atlantic. SSS change is ~ 0.15psu/(20yr) or ~ 0.08psu/decade AQUARIUS/SAC-D Workshop, WHOI , May 10-12, 2006

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

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

14. Stronger equatorial easterly can affect the mixed layer salinity by • Enhancing entrainment of salt sub-mixed layer (EUC) water • Enhancing evaporation • Reducing rainfall (stronger eatsterly  northward shift of the ITCZ, warm NTA phase of the interhemispheric mode) AQUARIUS/SAC-D Workshop, WHOI , May 10-12, 2006

15. E P EQ Lat SEC S S-h-S~0.5 psu z=-h We S-h We~0.5 m/day EUC P-E~mm/day z 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. AQUARIUS/SAC-D Workshop, WHOI , May 10-12, 2006

16. Variation of the mixed layer salinity averaged spatially over the equatorial box is due to We=O(1) m/day >> E-P=O(1) mm/day Entrainment Surface flux EUC salinity Examples: 1. Salinity variation due to variation of entrainment velocity. In the equatorial zone we~tx~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). AQUARIUS/SAC-D Workshop, WHOI , May 10-12, 2006

17. At interannual periods entrainment, evaporation, and precipitation work in accord. The freshening occurs in phase with lower winds and higher rainfalls. Interannual variation of salinity, wind, and rainfall AQUARIUS/SAC-D Workshop, WHOI , May 10-12, 2006

18. Summary • Gradual salinification of the tropical Atlantic by 0.2 psu occurred during the three decades of the 1960s through the 1980s, which increased the density of the mixed layer by an amount equivalent to a 0.8oC drop in SST. • During the past decades the oceanic mixed layer has freshened to values observed in mid 1960s. • Decadal change in SSS develops in accord with the decadal change in the equatorial zonal wind that impacts entrainment of salt water into the mixed layer. • At interannual time scales 0.1psu variations in SSS are present with fluctuations at a rate of 0.05 psu/(mm/day). Interannual freshenning occurs in accord with weakening winds and higher rainfalls. AQUARIUS/SAC-D Workshop, WHOI , May 10-12, 2006

19. Acknowledgements. • We gratefully acknowledge support from the NASA Oceans Program. • Agencies collecting and maintaining data archives. • Dr. Alain Dessier kindly provided an update of the Atlantic sea surface salinity archive. • CORIOLIS salinity data were collected and made freely available by the CORIOLIS project and programs that contribute to it. My Coauthors and me appreciate giving us an opportunity to present this research. AQUARIUS/SAC-D Workshop, WHOI , May 10-12, 2006