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Interannual Variability of Synthesized FSU and NCEP-NCAR Reanalysis Pseudostress Products over the Pacific Ocean

William M. PutmanCenter for Ocean–Atmospheric Prediction Studies, The Florida State University, Tallahassee, Florida

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David M. LeglerCenter for Ocean–Atmospheric Prediction Studies, The Florida State University, Tallahassee, Florida

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James J. O’BrienCenter for Ocean–Atmospheric Prediction Studies, The Florida State University, Tallahassee, Florida

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Print Publication:
15 Aug 2000
DOI:
https://doi.org/10.1175/1520-0442(2000)013<3003:IVOSFA>2.0.CO;2
Page(s):
3003–3016
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Abstract

A technique is applied to seamlessly blend height-adjusted Florida State University (FSU) surface wind pseudostress with National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis-based pseudostress over the Pacific Ocean. The FSU pseudostress is shown to be of higher quality in the equatorial Pacific and thus dominates the analysis in that region, while the NCEP–NCAR reanalysis-based pseudostress is used outside the equatorial region. The blending technique is based on a direct minimization approach. The functional to minimize consists of five constraints; each constraint is given a weight that determines its influence on the solution. The first two constraints are misfits for the FSU and NCEP–NCAR reanalysis datasets. A spatially dependent weighting that highlights the regional strengths of each dataset is designed for these misfit constraints. Climatological structure information is used as a weak smoothing constraint on the solution through Laplacian and kinematic (divergence and curl) constraints. The weights for the smoothing constraints are selected using a sensitivity analysis and evaluation of solution fields. The resulting 37 yr of monthly pseudostress fields are suitable for use in a variety of modeling and climate variability studies.

The monthly mean analyses are produced for 1961 through 1997, over the domain 40°S–40°N, 125°E–70°W. NCEP–NCAR reanalysis data, from 40° to 60°N, are added to the minimization solution fields, and the monthly mean climatologies, based on the solution fields, are removed from the combined fields. The resulting pseudostress anomalies are filtered with an 18-month low-pass filter to focus on interannual and ENSO timescales, and a complex empirical orthogonal function (CEOF) analysis is performed on the filtered anomalies. The CEOF analysis reveals tropical and extratropical linkages, for example, the presence of a strengthening of the Aleutian low in the North Pacific, coincident with the anomalous westerlies along the equator associated with El Niño events. The analysis also reveals a weakening of the Aleutian low during the winter–spring preceding the El Niño events of 1973 and 1983, and during the peak period of El Viejo, the cold phase of ENSO. A change in the nature of the tropical and extratropical linkages is observed from the warm events of the 1960s to those of the 1980s. These linkages are not found using NCEP–NCAR reanalysis data alone.

* Current affiliation: Computational Climate Dynamics Group, Oak Ridge National Laboratory, Oak Ridge, Tennessee.

Corresponding author address: Dr. David M. Legler, USCLIVAR Office, 400 Virginia Ave. SW, Suite 750, Washington, DC 20024.

Email: legler@usclivar.org

Abstract

A technique is applied to seamlessly blend height-adjusted Florida State University (FSU) surface wind pseudostress with National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis-based pseudostress over the Pacific Ocean. The FSU pseudostress is shown to be of higher quality in the equatorial Pacific and thus dominates the analysis in that region, while the NCEP–NCAR reanalysis-based pseudostress is used outside the equatorial region. The blending technique is based on a direct minimization approach. The functional to minimize consists of five constraints; each constraint is given a weight that determines its influence on the solution. The first two constraints are misfits for the FSU and NCEP–NCAR reanalysis datasets. A spatially dependent weighting that highlights the regional strengths of each dataset is designed for these misfit constraints. Climatological structure information is used as a weak smoothing constraint on the solution through Laplacian and kinematic (divergence and curl) constraints. The weights for the smoothing constraints are selected using a sensitivity analysis and evaluation of solution fields. The resulting 37 yr of monthly pseudostress fields are suitable for use in a variety of modeling and climate variability studies.

The monthly mean analyses are produced for 1961 through 1997, over the domain 40°S–40°N, 125°E–70°W. NCEP–NCAR reanalysis data, from 40° to 60°N, are added to the minimization solution fields, and the monthly mean climatologies, based on the solution fields, are removed from the combined fields. The resulting pseudostress anomalies are filtered with an 18-month low-pass filter to focus on interannual and ENSO timescales, and a complex empirical orthogonal function (CEOF) analysis is performed on the filtered anomalies. The CEOF analysis reveals tropical and extratropical linkages, for example, the presence of a strengthening of the Aleutian low in the North Pacific, coincident with the anomalous westerlies along the equator associated with El Niño events. The analysis also reveals a weakening of the Aleutian low during the winter–spring preceding the El Niño events of 1973 and 1983, and during the peak period of El Viejo, the cold phase of ENSO. A change in the nature of the tropical and extratropical linkages is observed from the warm events of the 1960s to those of the 1980s. These linkages are not found using NCEP–NCAR reanalysis data alone.

* Current affiliation: Computational Climate Dynamics Group, Oak Ridge National Laboratory, Oak Ridge, Tennessee.

Corresponding author address: Dr. David M. Legler, USCLIVAR Office, 400 Virginia Ave. SW, Suite 750, Washington, DC 20024.

Email: legler@usclivar.org

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