Role of seaice in forcing the austral spring atmospheric circulation in the CPTEC AGCM

This work investigates the influence on the atmosphere's circulation of anomalous sea ice conditions during the austral spring season – defined here as ASO – through a sensitivity experiment performed with the CPTEC's AGCM. The synthetic sea ice boundary condition used was a mosaic of observed (1998) anomalous sea ice extension in the Ross Sea and climatology of sea ice and SST elsewhere, to exclude the effects due to ENSO episodes. The experiment was carried out with large ensemble size (60 members) to achieve a better recognition of signal. The results show that where the sea ice layer was in excess (deficiency) the surface fluxes of latent and sensible heat by the underneath ocean are attenuated (enhanced), leading to large negative (positive) anomalies of net upward surface fluxes. The magnitude of the anomalies was substantial, reaching values of the order of 100 W.m-2 in a seasonal average. In general, the sensible heat flux anomalies presented values twice larger than the latent heat flux anomalies. During ASO season, anomalous cooling fluxes in the central-western sector of the sea ice edge in the Ross Sea led to a decrease of the near surface temperature. The temperature falling was considerable (>-5.0 deg) near the surface but decreased rapidly with height (-0.7 deg at 850 hPa). There is an associated anomalous geopotential pattern at 925 hPa which is statistically significant (t-test) at 5% level. This pattern appears to be resultant from both the large scale circulation and the flux induced temperatures anomalies. There are indications that the configuration of the upper level zonal winds could be related to this anomalous geopotential pattern.