Etudes et évaluation de processus océaniques par des hiérarchies ...
Etudes et évaluation de processus océaniques par des hiérarchies ... Etudes et évaluation de processus océaniques par des hiérarchies ...
52 CHAPITRE 4. ETUDES DE PROCESSUS OCÉANOGRAPHIQUES A. Wirth et al. / Deep-Sea Research II 49 (2002) 1279–1295 1289 tel-00545911, version 1 - 13 Dec 2010 Fig. 6. Interannual variability of the GW on August 1. Left panels show values for 1995 minus 1993, right panels 1996 minus 1993. Top panels gives SSH from Topex/Poseidon (in mm), middle and bottom panels show depth integrated transport stream function from exp. PE-lo (middle) and PE-hi (bottom), both in Sv. the externally forced interannual variability is only moderately reduced (maximum near 20 m layer thickness). In the RG-hi models, the GW therefore shows no substantial internal variability, a result that is consistent with Luther and O’Brien (1989). Fig. 10 shows the GW meridional location vs. intensity as measured by the mean maximum layer thickness for the integrations with the RG model. In the high viscosity experiments, the wind forcing leaves a clear imprint in the GW intensity and to a lesser extent also in the GW location, with 1993
4.1. VARIABILITY OF THE GREAT WHIRL FROM OBSERVATIONS AND MODELS53 1290 A. Wirth et al. / Deep-Sea Research II 49 (2002) 1279–1295 Fig. 7. Layer thickness anomalies (in m) relative to 10-year mean of all months at 3 consecutive August 1 in the low-friction experiment RG-lo95. tel-00545911, version 1 - 13 Dec 2010 being the strongest and 1996 the weakest, and very little internal variations among the different ensemble members. In the low-friction experiments, the wind influence remains clearly visible but is superposed by a considerable scatter among the different ensemble members, with on average stronger GW realizations located more southward. Since the different members of each ensemble are subject to identical forcing, this scatter, which amounts to differences in meridional location by more than 100 km and in maximum layer thickness by more than 30 m; can only have internal causes. The differences in GW location between the three ensembles also follow from Table 2 where the meridional GW positions for the 3 years; and the respective standard deviations are given. Furthermore, from the observed SSH anomalies it follows that the differences in location of the altimetric anomalies are larger than the differences in GW location between any of the models, suggesting that in the ocean the large-scale variability is even higher than in the models considered here. Indeed, experiments with the RG model with even lower friction values (not shown) indicate that the variability of large-scale features increases even further. Within each of the three sets of Exps. RG-lo93, RG-lo95 and RG-lo96, we also have calculated the correlation of GW location and strength between July 1 and August 1 and between July 1 and September 1. The results are similar for the GW location and GW strength, showing significant correlations between July 1 and August 1 ranging from 0.45 to 0.72. The correlations between July 1 and September 1 are however insignificant. This suggests that the knowledge of the GW position and strength on July 1 does not improve a prediction of the same variables on September 1. This alludes to a correlation time for the GW location of roughly one month. 7. Discussion The western Arabian Sea, like the western parts of all the ocean basins, is dominated by strong boundary currents that lead to instabilities, especially in low latitudes. In these areas, nonlinearity is higher than in the interior and eastern parts of the basins, leading to a higher ratio of internal vs. external variability. At the beginning of this work, it was intended to describe and classify the behavior of the GW during its life cycle in comparison with observations. We found, however, that this was not possible in eddypermitting models. After a more or less annually recurrent birth period of the GW in July and
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52 CHAPITRE 4. ETUDES DE PROCESSUS OCÉANOGRAPHIQUES<br />
A. Wirth <strong>et</strong> al. / Deep-Sea Research II 49 (2002) 1279–1295 1289<br />
tel-00545911, version 1 - 13 Dec 2010<br />
Fig. 6. Interannual variability of the GW on August 1. Left panels show values for 1995 minus 1993, right panels 1996 minus 1993.<br />
Top panels gives SSH from Topex/Poseidon (in mm), middle and bottom panels show <strong>de</strong>pth integrated transport stream function from<br />
exp. PE-lo (middle) and PE-hi (bottom), both in Sv.<br />
the externally forced interannual variability is only<br />
mo<strong>de</strong>rately reduced (maximum near 20 m layer<br />
thickness). In the RG-hi mo<strong>de</strong>ls, the GW therefore<br />
shows no substantial internal variability, a<br />
result that is consistent with Luther and O’Brien<br />
(1989).<br />
Fig. 10 shows the GW meridional location vs.<br />
intensity as measured by the mean maximum layer<br />
thickness for the integrations with the RG mo<strong>de</strong>l.<br />
In the high viscosity experiments, the wind forcing<br />
leaves a clear imprint in the GW intensity and to a<br />
lesser extent also in the GW location, with 1993
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