5.2 Validation of the model NorESM
5.2.1 Comparison of long-term mean SST in NorESM with Observations
Figure 5.1: Long-term SST mean over 1982-2009 for a) OISST1, b) NorESM and c) the difference between NorESM and OISST1. Long-term mean over 1910-2009 for d) 10-ensemble mean HadISST2 e) NorESM and f) the difference between NorESM SST and 10-ensembles mean HadISST2.
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In this section, I compare the SST climatology of NorESM during the satellite era with OISST1 over the period 1982-2009 (Figure 5.1 upper panels) and then the SST climatology of NorESM is compared to HadISST2 during the entire period (1910-2009) (Figure 5.1 lower panels).
The NorESM model represents the large-scale features of the South-East Atlantic basin well, with relatively cool SST (SST <18°C) in the Benguela Upwelling system and the poleward sector.
Relatively warm SST (SST> 23°C) are realistically represented off Angola and in the equatorial Atlantic basin. However, the NorESM model does not represent the upwelling close to the Namibian coast and in the ABF well. Warm bias of up to 3.5°C is observed in the model along the Namibian coast and ABF when compared with OISST1 and up to 4°C when compare with HadISST2. The warmest bias is observed in the ABF. In Southern Benguela, the difference between NorESM and OISST1 or HadISST2 remains lower than 1°C. A comparison between NorESM and NEMO over the period 1958 -2009 also shows a warm bias up to 3°C in NorESM (not shown).
5.2.2 Comparison of SST timeseries in NorESM and 10-ensemble HadISST2 in different domains
Figure 5.2: 11-Year running mean of SST anomalies from 1910 to 2009 in the four coastal domains (from top to bottom and from left to right) ABF, NB, CB, and SB. The red line is NorESM and the blue lines are the 10-ensemble members of HadISST2.
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In this section, I compare the NorESM SST timeseries at lower frequencies. Figure 5.2 shows the 11-year running mean of SST anomalies from 1910 to 2009 in the same four domains (ABF, NB, CB, and SB) as in Chapter 3 and Figure 3.8, but with 4° longitudinal averages for NorESM (red line) and HadISST2 (blue line). The NorESM SST and HadISST2 show a marked low-frequency modulation in all the coastal regions. However, the low-frequency variability has more energy in the NorESM compared to HadISST2. In addition, NorESM seems to display a more consistent low- frequency SST timeseries across NB, CB, and SB, whereas the HadISST.2 data are able to capture more regional features. The ratio of the NorESM and HadiSST2 standard deviation (SD) is between 1.20 and 1.7 in all domains (Figure 5.2). In some domains, the period of lowest and highest values of SST are not always the same in the NorESM compared to HadISST2. In the ABF for instance (Figure 5.2 top left panel), the NorESM model displays positive SST anomalies of up to 0.4°C in 1931-1950, 1956-1964, and 1994-2004, while negative SST anomalies of up to -0.5°C occurred in 1964-1993. The HadISST2, contrariwise, displays positive SST anomalies in 1920- 1928, 1932-1940,1945-1952, 1961-1970, and 1993-2004 while negative SST anomalies occurred in 1953-1961 and 1975-1992. The correlation between NorESM SST and HadISST2 at lower frequencies, in the ABF domain, is 0.48. It is statistically significant at the 95% level (based on a Student t-test). I also note that the ABF SST in the NorESM model seems to undergo multi-decadal rather than decadal variability.
In NB and CB, the model displays positive SST anomalies up to 0.4°C in 1957-1967 and 1980-1990, while negative SST anomalies up to -0.35°C occurred in 1922-1933, 1943-1957, 1966- 1975, and 1989-1988. HadISST2, on the other hand, displays positive SST anomalies with less magnitude than the model (< 0.3°C) in 1935-1954 and 1960-1967 while negative SST anomalies up to -0.35°C are observed in 1925-1935, 1953-1962, and 1975-1983. I note that positive SST anomalies occur in 1985-1998 in CB using HadISST2, while a very weak SST anomaly is observed in NB. The correlations between the NorESM model SST and HadISST2 are 0.21 and 0.32, respectively in NB and CB. These correlations are not statistically significant at the 95% level. In SB (Figure 5.2 bottom right panel), the NorESM model SST low frequency variability matches the HadISST2 variability very well. There are positive SST anomalies in both the NorESM and 10- ensemble HadISST2 of up to 0.45°C in 1915-1923, 1959-1965, and 1982-1993, while negative SST
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anomalies up to 0.4°C occur in 1925-1935, 1966-1975, and 1992-2000. However, there are discrepancies between NorESM and HadISST2 during the period 1945-1953 when NorESM has negative SST anomalies and HadISST2 has positive SST anomalies. In SB, the correlation between NorESM SST and ensemble mean HadISST2 at lower frequencies is 0.61 and is statistically significant at the 95% level. In conclusion, the NorESM and HadISST2 present evidence of decadal variability across the whole region; however, the low-frequency variability in the NorESM model is not in good agreement with the low frequency of HadISST2 in most of the domains except in SB where the NorESM and 10-ensembles HadISST2 show the same pattern of low-frequency variability. This may be because the NorESM does not represent the quasi-permanent upwelling in NB and CB well, or because there were not enough observations to build the HadISST2 dataset, especially before the 1960s. Thus, I will focus my analysis on the Southern Benguela region in the following sections.