>In a prior post NINO3.4 Data Comparison – HADSST and ERSST.v2, I illustrated the difference between the HADSST and ERSST.v2 versions of NINO3.4 data. As a follow up to that post, the paper “Improvements to NOAA’s Historical Merged Land–Ocean Surface Temperature Analysis (1880–2006)”, Smith and Reynolds (2008), JOURNAL OF CLIMATE, VOLUME 21, provides an explanation for the difference between the ERSST.v3 and HADSST versions. Refer to page 2293, or pdf document page 11 of 14.
FIRST: THE DIFFERENCES BETWEEN ERSST.V2 AND ERSST.V3 DEPICTIONS OF NINO3.4
“Changes in the Niño-3.4 SST anomalies between ERSST.v2 and ERSST.v3 are very small after 1950. Earlier in the record the two are also highly correlated, but there are times when the ERSST.v2 anomaly is greatly damped from a lack of sampling (Fig. 8). These times include years before 1880 and around 1918, shown more clearly in the difference (Fig. 8, bottom panel).”
Their Figure 8, both panels, is shown below.
The two versions (ERSST.v2 and ERSST.v3) of the NINO3.4 SST anomalies are similar to one another over the period where I illustrated in the earlier post that HADSST and ERSST.v2 diverge. Therefore, there should also be a significant difference between the HADSST and ERSST.v3 depictions of NINO3.4 SST anomalies. A comparative graph of the HADSST and ERSST.v3 versions of NINO3.4 are illustrated in Figure 1. The difference (ERSST.v3 Minus HADSST) is shown in Figure 2.
SECOND: THE DIFFERENCES BETWEEN HADSST AND ERSST.V3 DEPICTIONS OF NINO3.4
The explanation in the linked paper for the difference is:
“Both analyses are clearly producing consistent interannual variations. But there are important differences in this region in periods when sampling is sparse. In Niño-3.4 prior to 1950, HadISST is biased about 0.3°C warmer than ERSST.v3. Much of the bias is due to the use of different historical bias adjustments in the two analyses prior to 1942. Another important difference depends on the method used to compute low frequency variations. In HadISST they are computed by fitting data to a global mode, while here simpler averaging and filtering is used, as discussed above.”
The HADSST version of NINO3.4 SST anomaly data is available through the Royal Netherland Meteorological Institute (KNMI) webpage:
ERSST.v3b data sets are available from the NCDC here:
Note that the NINO data page has an error. The data changes from SST to SST anomaly in 1985.
I used an earlier version in the preparation of this post.