Editorial Type:
Article
Article Type:
Research Article

Tropical Pacific Sea Level Variations (1948–98)

Thomas M. Smith Climate Prediction Center, NCEP/NWS/NOAA, Camp Springs, Maryland

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Print Publication:
01 Aug 2000
DOI:
https://doi.org/10.1175/1520-0442(2000)013<2757:TPSLV>2.0.CO;2
Page(s):
2757–2769
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Abstract

Monthly sea level in the Pacific Ocean (30°S–30°N) is analyzed for the 1948–98 period. It is shown that most sea level variations in this region have large scales, and so the available network of tide gauge stations is sufficient for analysis over this period. Analysis is done using statistically optimal interpolation and the full covariance structure defined by a more recent well-sampled period. The analysis reflects major variations in the station data, including warm and cool episodes in the tropical Pacific and an increase in variance in the second half of the analysis period. At the tide gauge stations, the analysis correlation with observations generally exceeds 0.9. Cross-validation tests show that errors in the tropical Pacific sea level analysis are typically less than 4.5 cm throughout the analysis period and less than 4 cm in the best-sampled most recent period. This result compares well with the TOPEX/Poseidon (Ocean Topography Experiment) satellite error estimate of 2 cm.

Rotated EOF analysis of the 1948–98 sea level anomalies shows the dominance of ENSO variations. Taken together, the first three modes account for most of the variance and represent different phases of the ENSO cycle, with two buildup modes and a mature phase mode. One of the buildup modes is associated with a western Pacific warm pool centered north of the equator and is negatively correlated with variations in the east-equatorial Pacific. The other buildup mode is associated with a western Pacific warm pool centered south of the equator, which gives a strong indication of east-equatorial Pacific variations a year in advance. An increasing trend in the sea level over the record period may be related to increasing variance in the sea level, which is reflected by stronger and more frequent ENSO variations in the second half of the record.

Corresponding author address: Dr. Thomas M. Smith, National Centers for Environmental Prediction, Analysis Branch—Climate Prediction Center, Camp Springs, MD 20746.

Abstract

Monthly sea level in the Pacific Ocean (30°S–30°N) is analyzed for the 1948–98 period. It is shown that most sea level variations in this region have large scales, and so the available network of tide gauge stations is sufficient for analysis over this period. Analysis is done using statistically optimal interpolation and the full covariance structure defined by a more recent well-sampled period. The analysis reflects major variations in the station data, including warm and cool episodes in the tropical Pacific and an increase in variance in the second half of the analysis period. At the tide gauge stations, the analysis correlation with observations generally exceeds 0.9. Cross-validation tests show that errors in the tropical Pacific sea level analysis are typically less than 4.5 cm throughout the analysis period and less than 4 cm in the best-sampled most recent period. This result compares well with the TOPEX/Poseidon (Ocean Topography Experiment) satellite error estimate of 2 cm.

Rotated EOF analysis of the 1948–98 sea level anomalies shows the dominance of ENSO variations. Taken together, the first three modes account for most of the variance and represent different phases of the ENSO cycle, with two buildup modes and a mature phase mode. One of the buildup modes is associated with a western Pacific warm pool centered north of the equator and is negatively correlated with variations in the east-equatorial Pacific. The other buildup mode is associated with a western Pacific warm pool centered south of the equator, which gives a strong indication of east-equatorial Pacific variations a year in advance. An increasing trend in the sea level over the record period may be related to increasing variance in the sea level, which is reflected by stronger and more frequent ENSO variations in the second half of the record.

Corresponding author address: Dr. Thomas M. Smith, National Centers for Environmental Prediction, Analysis Branch—Climate Prediction Center, Camp Springs, MD 20746.

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