LOW FREQUENCY VARIATION OF SEA SURFACE SALINITY IN THE TROPICAL ATLANTIC

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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
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• 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?

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30 yr salinity difference between 198599 and
195569 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
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Linear trend of the zonally averaged pentadal
salinity 19551959 to 19941998 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.
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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.

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• 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  throughout the 1960s through 1980s has been
  replaced by a freshening trend in the recent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.

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Number of grid points in the 15S-15N, 50W-15E
Tropical Atlantic sector covered by observations
during a month.
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Salinity averaged 5oS to 5oN across the
equatorial Atlantic (WOD01 profile data)
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Salinity averaged 5oS to 5oN across the
equatorial Atlantic (WOD01 profile
datathermosalinograph)
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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)
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5-year running mean SSS anomaly in the tropical
Atlantic.
SSS change is 0.15psu/(20yr) or
0.08psu/decade
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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.
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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.
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• 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)

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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.

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

WeO(1) m/day gtgt E-PO(1) mm/day
Entrainment
Surface flux
EUC salinity
Examples 1. Salinity variation due to variation
of entrainment velocity. In the equatorial zone
wetxU2, so
0.2psu in response to 1m/s change in zonal
wind at mean zonal wind U5 m/s and S-h-S0.5
psu. 2. What change in precipitation induces 0.2
psu change in salinity?
3 mm/day at we0.5 m/day. 3. Mixed layer
salinity responds to change in precipitation at a
rate of S/we0.07 psu/(mm day-1).
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• Interannual variation of salinity, wind, and
  rainfall

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

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• 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.
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