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[1] Gridded precipitation from reanalysis can be a valid proxy for observations in many regions where local measurements are not available. Despite the continuous improvements in assimilation systems and the growing availability of in situ and remote observations, model errors can nevertheless still affect reanalysis quality. A temporal and spatial correction of ERA-Interim rainfall monthly means is proposed. It is shown that the strategy of postprocessing the data set is crucial for drought monitoring and forecasting.

[1] The Indian monsoon interannual variability is modulated by the El Nino Southern Oscillation (ENSO), with a drier than normal monsoon season usually preceding peak El Nino conditions, and vice versa for La Nina phase. Pacific sea surface temperature (SST) anomalies, however, are not the only player. Building upon our recent discovery that atmospheric teleconnections between the tropical Atlantic and the Indian basin contributed to the weakening of the ENSO-monsoon anticorrelation during the '80s and '90s, we investigate the role of south equatorial Atlantic SSTs in forcing the Indian monsoon rainfall (IMR). Using two observational data sets and two ensembles of simulations we show that the residual in the IMR time series for observed and modeled data, obtained by subtracting the ENSO-forced component of the IMR that is linearly related to the NINO34 index, is significantly correlated with south equatorial Atlantic SSTs. Our results have important implications for seasonal forecast efforts.

[1] Using ensemble integrations of an Atmospheric General Circulation Model (AGCM) forced globally with observed sea surface temperature (SST) anomalies it is shown that the observed decadal variability of Indian monsoon rainfall (IMR) can be reproduced, although amplitudes are somewhat overestimated. A second AGCM ensemble forced only in the Indian Ocean region suggests that the local Indian Ocean SST forcing is contributing significantly to the decadal IMR variability. Here, cold (warm) equatorial SSTs cause low-level divergence (convergence) that in turn modifies the local Hadley circulation and strengthens (weakens) the Indian monsoon circulation. This result is complementary to previous findings that IMR variability is mainly determined by an atmospheric teleconnection with ENSO.

[1] Ocean-atmosphere variability in the tropical Indian Ocean is investigated using observational data and ensemble experiments with a coupled general circulation model. In one ensemble (IO runs) the ocean-model domain is limited to the Indian Ocean and observed sea surface temperatures force the atmospheric model elsewhere. In a second ensemble (TPIO) the coupled domain includes the Tropical Pacific. The IO runs display a coupled mode of variability with several characteristics of the Indian Ocean Dipole (IOD), but independent on ENSO (El-Nino Southern Oscillation). Changes in the Walker circulation induced by ENSO are insufficient to trigger IOD events. In the TPIO runs ENSO variability is correlated with the IOD mode as observed. Oceanic processes are responsible for an essential component of ENSO forcing in the Indian Ocean. The ENSO phase conditions the thermocline depth in the Indonesian Throughflow region and in the southeastern IO. TPIO results are in agreement with SODA reanalysis.

[1] Using high-resolution global “time-slice” climate change simulations, we show the existence of a bimodal NAO distribution in future climate conditions under the IPCC A2 scenario forcing. The current climate exhibits a unimodal NAO distribution. The second “regime” in the future climate experiment is located in the positive region of the NAO phase space at about one standard deviation from the maximum, which is located near zero or at slightly negative NAO values. This additional regime grows at the expense of negative NAO occurrences. The change in the NAO distribution is physically related to negative PNA-like change occurring in the Pacific region. The interpretation of the occurrence of the second NAO maximum in the future climate simulation is that large positive NAO events will occur more frequently in future climate conditions, while the rest of the NAO frequency distribution shows only small changes. This implies that European winter climate is expected to show higher variability and increased occurrence of extreme climate conditions associated with the positive NAO phase.

[1] Recent analyses of historical data and output of numerical models have suggested a dipolar mode of decadal variability in the South Atlantic. In this work, the the atmospheric response to such anomalous SST dipole in the South Atlantic is investigated with a numerical model. The results show a deep baroclinic response over the equatorward pole and a shallow equivalent barotropic response over the poleward pole. The baroclinic response is strongest during the austral summer. It affects the meridional circulation over the tropical Atlantic with anomalous downward motion north of the ITCZ and anomalous ascending motion southward of it, resulting in a southward shift of the ITCZ. The ITCZ response is enhanced due to a positive feedback induced by anomalous low level convergence of moisture. Changes in cloud cover counteract the variations in latent heat flux north of the ITCZ, thereby prohibiting the generation of an SSTA of opposite sign as suggested by the wind-evaporation-SST (WES) feedback.