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Andrew M. Chiodi and Don E. Harrison

Abstract

This study shows that, since 1979 when outgoing longwave radiation (OLR) observations became reliably available, most of the useful U.S. seasonal weather impact of El Niño events is associated with the few events identified by the behavior of outgoing longwave radiation (OLR) over the eastern equatorial Pacific (“OLR–El Niño events”). These events produce composite seasonal regional weather anomalies that are 95% statistically significant and robust (associated with almost all events). Results also show that there are very few statistically significant seasonal weather anomalies, even at the 80% level, associated with the non-OLR–El Niño events. A major enhancement of statistical seasonal forecasting skill over the contiguous United States appears possible by incorporating these results. It is essential to respect that not all events commonly labeled as El Niño events lead to statistically useful U.S. seasonal forecast skill.

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Andrew M. Chiodi and Don E. Harrison

Abstract

The North Atlantic hurricane seasons of 2005 and 2006 were dramatically different for the Gulf Coast and eastern seaboard of the United States. The 2005 hurricane season was one of the most destructive seasons in history, whereas there was limited impact in 2006. Hurricane activity had been forecast to be above normal in 2006, but it was not. One of the conspicuous differences in environmental conditions between these two years was sea surface temperature anomaly (SSTA) over a region of the western Atlantic and Caribbean (15°–30°N, 70°–40°W), which is important for hurricane formation and intensification. SSTA was more than 1.5 standard deviations warmer during the 2005 hurricane season, but it was much less in 2006 through most of its hurricane season. The intent of this study is to determine the mechanisms responsible for this SSTA difference. It is shown that the difference can be reproduced using a simple one-dimensional (1D) ocean mixed layer model forced with surface fluxes from the NCEP–NCAR reanalysis project. It is found that there are two causes of SSTA difference over this region during July through September: the first is latent heat flux variability caused by wind speed effects, and the second is nonlinear ocean warming caused by submonthly atmospheric variability. The observed SSTA difference is reproduced by our model even though solar forcing damps the observed difference, contrary to previous hypotheses.

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