U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications

Variability in the Eastern Equatorial Pacific Ocean During 1986-1988

Michael J. McPhaden and Stanley P. Hayes

NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

Journal of Geophysical Research, 95(C8), 13,195-13,208 (1990)
Not subject to U.S. copyright. Published in 1990 by the American Geophysical Union.

Appendix B: Mean Seasonal Cycles

Mean seasonal cycles have been estimated from monthly averaged equatorial mooring time series measurements at 110W and 140W. Monthly averages were determined by smoothing the time series with a 31-day running mean filter and subsampling on the fifteenth of each month. Three years of data were used at 140W (January 1984 to December 1986), and 5 years of data at 110W (April 1980 to March 1982; January 1984 to December 1986). The first several months of 1986-1987 ENSO variability are included in these estimates, but the large interannual variations in 1987 and 1988 are not. Similarly, variability from the 1982-1983 ENSO episode is not included in the 110W mean seasonal calculation.

One can compute a standard deviation for each month in the mean seasonal cycle at 110W where 5 years of data were used. Characteristically, these deviations are about 1C (SST), 10 m (20C isotherm depth), 4 dyn. cm (dynamic height), 20 cm s (zonal currents), and 1 m s (winds). These numbers typify non-ENSO interannual variations on a monthly basis. Thus, when discussing anomalies from the mean seasonal cycle, emphasis should be given to those variations persisting longer than 1 month in excess of the values quoted above.

Note that we expect differences between these climatologies and others that may exist in the eastern equatorial Pacific. Figure B1, for example, shows a comparison of estimates based on moored measurements with the Reynolds [1988] SST climatology and the Wyrtki and Meyers [1975] wind climatology. The moored data show SSTs that are consistently colder by l-2C and zonal winds that are consistently stronger by 1-3 m s. There are several possible explanations for these differences. First, they could represent real interdecadal variations since the Reynolds and the Wyrtki and Meyers climatologies are based on pre-1980 data. Second, Reynolds and Wyrtki and Meyers include ENSO years in their climatologies, whereas we have excluded data from the 1982-1983 ENSO and from most of the 1986-1987 ENSO. This could lead to slightly warmer temperatures and, at 140W, slightly weaker easterlies compared to the mooring climatologies. (At 110W easterlies tend to be stronger than usual during ENSO, with the notable exception of the 1982-1983 event, so that ENSO biases cannot explain the wind differences at 110W.) Third, the previously published climatologies are spatially smoothed over several degrees of latitude and longitude. In the case of the Reynolds climatology this would lead to warmer equatorial SSTs because of the strong meridional SST gradients flanking the equatorial minimum. Fourth, the Reynolds and the Wyrtki and Meyers climatologies are based on different measurement techniques than the mooring climatologies; i.e., merchant ship wind observations were often reported on the Beaufort scale, and SST observations were based on bucket or engine room intake temperatures. These climatologies may be biased because of uneven spatial and temporal sampling and, in the case of SST, because of engine room heating from the intake method [Saur, 1963]. We have therefore favored the mooring-based mean seasonal cycles in this study in spite of the relative shortness of the records, because they allow for a consistent discussion of interannual anomalies in all the moored measurements (including subsurface velocity, for which no other climatology exists).

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