Variability of TC Model ENSO

Before addressing the variation of model TC frequency, we present the features of model ENSO. Figure 5a shows the time series of the simulated SST anomalies in the Nino3 region (5°S-5°N, 150°W-90°W) averaged during the months from August to October. Though observed ENSO typically occurs every 2 to 7 years (e.g., McPhaden et al. 2007), the CGCM simulates the quasi-periodic ENSO with a significant narrow spectrum peak at about 5 years (Fig. 5b). The standard deviation

30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Year

Fig. 5 (a) Time series of model Nino3 (5°S-5°N, 150°W-90°W) SST anomaly averaged during the months from August to October (ASO), denoted by solid line. Dashed lines are standard deviation. (b) Power spectra of model ASO Nino3 SST anomaly (solid line) and dashed curve shows the power spectra of red noise

60N 40N 20N EQ 20S 40S a

60N 40N 20N EQ 20S 40S

EI Nino(OBS)

La Nina(OBS)

EI Nino(Model)

Fig. 5 (a) Time series of model Nino3 (5°S-5°N, 150°W-90°W) SST anomaly averaged during the months from August to October (ASO), denoted by solid line. Dashed lines are standard deviation. (b) Power spectra of model ASO Nino3 SST anomaly (solid line) and dashed curve shows the power spectra of red noise

EI Nino(OBS)

EI Nino(Model)

La Nina(Model)

La Nina(OBS)

La Nina(Model)

60E 120E 180 120W 60W 0

60E 120E 180 120W 60W 0

60E 120E 180 120W 60W 0

60E 120E 180 120W 60W 0

Fig. 6 Composite maps of observed SST anomalies averaged during the months from August to October of El Nino (a) and La Nina years (b), respectively. Contour interval is 0.5°C. (c)-(d) Same as in (a)-(b) but for model of model Nino3 SST anomaly is 2.12°C that is twice larger than the observed one (0.91°C). However, this deficiency implies that the model may provide useful information for understanding the impact of a strong ENSO on TC frequency.

Figure 6 presents the composite maps of model SST anomalies on Nino3 SST anomalies, together with the observed counterparts. We here define El Nino (La Nina) years when Nino3 SST anomaly averaged during the months from August to October is more (less) than the standard deviation. Comparing with the observations, there are discrepancies in the SST anomalies in the equatorial Atlantic and the eastern equatorial Indian Ocean. This is because air-sea coupled modes in the tropical Atlantic and Indian Oceans concur with ENSO in this model, while those modes do not always occur with ENSO in the observations (Zebiak 1993; Saji et al. 1999). The deficiencies may be related to the larger amplitude of ENSO in this model, resulted from the difference in convection scheme (Iizuka et al. 2003). However, the spatial structures of SST anomalies in the model during model El Nino and La Nina years show generally good agreement with the observed ones.

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