U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
Mean seasonal cycles have been estimated from monthly averaged equatorial mooring time series measurements at 110°W and 140°W. 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 140°W (January 1984 to December 1986), and 5 years of data at 110°W (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 110°W mean seasonal calculation.
One can compute a standard deviation for each month in the mean seasonal cycle
at 110°W where 5 years of data were used. Characteristically, these deviations
are about 1°C (SST), 10 m (20°C 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°-2°C 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 140°W, slightly
weaker easterlies compared to the mooring climatologies. (At 110°W 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
110°W.) 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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