271
individual data suggest they are valid in the main areas of interest back to
1900. We show additionally subsurface temperature measurements of the
North Atlantic which are described by Levitus et al. (1994).
3 The decadal variability in the North Pacific
3.1 Observations
In order to describe the decadal variability in the North Pacific, we performed a correlation analysis of low-pass filtered SST and 700 hPa height
anomalies observed during the period 1950 to 1990. The correlation analysis was performed as follows: First, the observations were detrended and
smoothed with a five year running mean filter. We then defined a SST
index by averaging the SST anomalies over the region 25° N - 40° Nand
170° E - 160 0 W (Fig. 1a). This particular region was chosen because it
is located in the region of maximum decadal variability. Finally, we computed locally the correlation coefficients of the SST index with the low-pass
filtered SST and 700 hPa anomalies.
The decadal variability is clearly seen in the North Pacific SST index
time series, with minima around the years 1960 and 1980 and maxima
around the years 1950, 1970, and 1990. Although the time period considered is definitely too short to estimate a reliable period for the decadal
variability, there is, however, some evidence of a 20-year time scale. The
results of the correlation analysis are shown in Figs. 1b, and 1c. The
dominant SST anomaly in the North Pacific is positive (by definition) and
surrounded by negative anomalies (Fig. 1b), a result that was found also
by Trenberth and Hurrell (1994). These features are statistically highly significant. The SST anomaly pattern shows a remarkable symmetry about
the equator, with equatorial Pacific SST anomalies of opposite sign and
SST anomalies of the same· sign in the Southern Hemisphere relative to
the main anomaly in the North Pacific.
The anomalous atmospheric circulation, as expressed by the 700 hPa
height anomaly field (Fig. 1c), is dominated by the reverse of the "PacificNorth-American" (PNA) pattern which is an eigenmode ofthe atmosphere
and also excited during extremes of the El Nino/Southern Oscillation
(ENSO) phenomenon (Horel and Wallace (1981)). Anomalous high pressure is found over the North Pacific, which was also noted by Trenberth
and Hurrell (1994). A negative height anomaly is centered over Canada
and a positive height anomaly over the southern part of the United States
individual data suggest they are valid in the main areas of interest back to
1900. We show additionally subsurface temperature measurements of the
North Atlantic which are described by Levitus et al. (1994).
3 The decadal variability in the North Pacific
3.1 Observations
In order to describe the decadal variability in the North Pacific, we performed a correlation analysis of low-pass filtered SST and 700 hPa height
anomalies observed during the period 1950 to 1990. The correlation analysis was performed as follows: First, the observations were detrended and
smoothed with a five year running mean filter. We then defined a SST
index by averaging the SST anomalies over the region 25° N - 40° Nand
170° E - 160 0 W (Fig. 1a). This particular region was chosen because it
is located in the region of maximum decadal variability. Finally, we computed locally the correlation coefficients of the SST index with the low-pass
filtered SST and 700 hPa anomalies.
The decadal variability is clearly seen in the North Pacific SST index
time series, with minima around the years 1960 and 1980 and maxima
around the years 1950, 1970, and 1990. Although the time period considered is definitely too short to estimate a reliable period for the decadal
variability, there is, however, some evidence of a 20-year time scale. The
results of the correlation analysis are shown in Figs. 1b, and 1c. The
dominant SST anomaly in the North Pacific is positive (by definition) and
surrounded by negative anomalies (Fig. 1b), a result that was found also
by Trenberth and Hurrell (1994). These features are statistically highly significant. The SST anomaly pattern shows a remarkable symmetry about
the equator, with equatorial Pacific SST anomalies of opposite sign and
SST anomalies of the same· sign in the Southern Hemisphere relative to
the main anomaly in the North Pacific.
The anomalous atmospheric circulation, as expressed by the 700 hPa
height anomaly field (Fig. 1c), is dominated by the reverse of the "PacificNorth-American" (PNA) pattern which is an eigenmode ofthe atmosphere
and also excited during extremes of the El Nino/Southern Oscillation
(ENSO) phenomenon (Horel and Wallace (1981)). Anomalous high pressure is found over the North Pacific, which was also noted by Trenberth
and Hurrell (1994). A negative height anomaly is centered over Canada
and a positive height anomaly over the southern part of the United States