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Seasonal to Interannual Variability in the SOC Air-Sea Flux Dataset and Hadley Centre Atmospheric Model Version 3

Josey, S.A.. 1999 Seasonal to Interannual Variability in the SOC Air-Sea Flux Dataset and Hadley Centre Atmospheric Model Version 3. Southampton, UK, Southampton Oceanography Centre, 39pp. (Southampton Oceanography Centre Internal Document, 51)

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Abstract/Summary

Results from a comparison of surface heat flux forcing variability in the SOC flux dataset with an ensemble mean of runs of the Hadley Centre Atmospheric Model 3 (HadAM3) with prescribed sea surface temperature (SST) for 1980-1995 are presented. The ability of HadAM3 to represent the detailed time evolution of the major atmospheric pressure oscillations over this period has been tested. Time series of the HadAM3 model and SOC observational indices for the Southern Oscillation are in fairly good agreement (r2 = 0.52). In contrast, the North Atlantic Oscillation (NAO) indices are essentially uncorrelated (r2 = 0.02). The correlation between model and observed North Pacific indices (r2 = 0.16 ), is intermediate between these two cases. Despite the poor model representation of the NAO index time series, composite fields constructed from the HadAM3 and observed NAO indices reveal similar patterns in the dominant components of the heat exchange. However, the composite SST fields differ, suggesting that the anomalous heat exchange in the model is not strongly forced by SST anomalies. The major North Atlantic and North Pacific modes of variability in the model have been characterised by Empirical Orthogonal Function (EOF) analysis. The leading modes in the model surface fields tend to be similar, both in terms of spatial pattern and proportion of variance explained, to the observational modes characterised by Kent et al. (1999). Thus, HadAM3 appears to capture the spatial characteristics of the main modes of variability but not their time evolution. The difference in the relative ability of the model to represent the Southern Oscillation and NAO suggests that the latter process is not strongly coupled to the SST at interannual timescales. Causal relationships between the first four SST, pressure and net heat flux modes have been investigated for the North Atlantic using lag correlation analysis. Several coupled modes are found in the observational dataset in which the SST is lead by the pressure and net heat flux at intervals of 1-3 months. Little evidence is found for the SST leading atmospheric modes in either HadAM3 or the SOC dataset. The causal connection between SST, pressure and net heat flux anomalies associated with the NAO maximum state has been further investigated by a lead/lag composite analysis of the SOC dataset. Statistically significant SST anomalies with a persistent spatial pattern at lags of up to 5 months relative to the NAO index maximum are observed. These anomalies are caused by changes in surface heat exchange arising from the altered atmospheric circulation during the NAO maximum state. Prior to the NAO maximum significant SST patterns are not found. Thus, the NAO forces an ocean response via the SST field but is not driven by anomalies in this field at seasonal to interannual timescales.

Item Type: Publication - Report (Technical Report)
Additional Pages: & figs
Additional Keywords: North Atlantic; Discovery cruise 223; Acoustic Correlation Current Profiler, ACCP.
Date made live: 31 Jul 2013 15:54 +0 (UTC)
URI: http://nora.nerc.ac.uk/id/eprint/502834

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