Sea Level and Circulation Variability in the SCS and ENSO Implications

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Sea Level and Circulation Variability in the SCS
and ENSO Implications

• Guoqi Han
• Fisheries and Oceans Canada

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Background

• Semi-enclosed
• Seasonally reversing basin-scale gyre
• Regional/non-regional forcing
• Knowledge gaps in low-frequency variability.

(e.g. Wyrtki, 1961Chu et al., 1999 Qu, 2000
Wang et al., 2006)
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Issues

• Annual amphidromes (Zhang et al., 2006)
• Insufficient data length
• Interannual variability (Fang et al., 2006)
• Satellite data only

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Objectives

• To revisit annual sea level variations,
  especially amphidromes.
• To examine interannual and longer-term sea level
  changes.
• To explore potential impacts of ENSO.

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Merged Satellite Altimetry Data (AVISO)

• 1/3o sea level anomaly data from October 1992 to
  September 2006.
• Atmospheric corrections for ionospheric, and wet
  and dry tropospheric delays.
• Oceanographic corrections for sea state bias,
  inverse barometric response, and elastic ocean,
  solid earth, and pole tides.

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Modified Response Analysis Method

Annual Cycle Sub-tidal Residual
The sub-tidal residuals were averaged seasonally
to generate seasonal-mean anomalies by season and
year.
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Tide-gauge Data

• Monthly-mean sea level from PSMSL (variable
  duration).
• Least squares fit to extract the annual cycle
  (1993-2001).
• Seasonal-mean anomalies were calculated.
• Trend analysis for 1993-2001 and 1979-2001,
  respectively.

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T and S Climatology (Ishii et al., 2005)

• Monthly 1o T and S for the upper 700 m
  (1950-2003).
• Steric height relative to the 700 db.
• Least squares fit to extract the annual cycle
  (1993-2001).
• Seasonal-mean anomalies (by season and year) were
  calculated.
• Trend analysis for 1993-2001 and 1979-2001,
  respectively.

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Mean sea level and spectrum
ENSO
Annual
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Annual Amphidromes
Phase
Amplitude
cm
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1St mode represents the seasonally reversing
basin-scale gyre.
cm
cm
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2nd mode represents the Vietnam dipole in summer
and W. Luzon eddy in winter.
cm
cm
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Annual Cycle Altimetry vs. Tide Gauge (93-01)
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Averaged Altimetry Sea Level and ENSO
Correlation C-0.63 (-0.68) Rate of rise R
1.0 cm/yr!
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Averaged Altimetry Sea Level and ENSO
Correlation C-0.52 (-0.62) Rate of rise R
0.4 cm/yr Large sensitivity!
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Averaged Tide-gauge Sea Level and ENSO
C-0.67 (-0.61) R 0.7 cm/yr
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Steric Height and ENSO
Chigh negative R 0.92 cm/yr
TD
T/P
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Averaged Tide-gauge Sea Level and ENSO
C-0.53 (-0.50) R 0.22 cm/yr
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Steric Height and ENSO
Chigh negative R 0.12 cm/yr
TD
T/P
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Temperature and ENSO
1979-2001 C-0.57 R 0.0048 oC/yr
1993-2001 C-0.72 R 0.044 oC/yr
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Salinity and ENSO
1979-2001 Clow R-0.0008 psu/yr
1993-2001 Clow R -0.004 psu/yr
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Sea Level and ENSO

• High (Low) ENSO - Low (high) water temperature
  (700 m) - Low (high) sea level.

Sea Level Rise and T/S

• Increasing T and decreasing S can account for
• approximately the sea level rise in the 1990s
  (mostly representing the ENSO effects)
• only about half in the 1979-2001.

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Higher ENSO, weaker near-surface cyclonic gyre
cm
cm
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Higher ENSO, low intrusion?
cm
cm
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Conclusions

• Three nearly stationary annual amphidromes,
  located where the basin-scale gyre and meso-scale
  eddies have nil sea level variation.
• Sea level had strong interannual variations,
  correlated with ENSO negatively, mainly via
  temperature. Luzon Strait transport???
• The sea level rise rate (sensitive to data
  length) was about 1 cm/yr in the 1990s,
  influenced by the ENSO-frequency variability and
  dominated by steric height.
• The rates for 1979-2001 was much lower, closer to
  long-term global estimates. The steric height
  effect can account for about 50.

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Thank you for your attention.Questions/Comments?