Hurricanes as an enabler of Amazon fires

Abstract

<p>We investigated a teleconnection between North Atlantic tropical storms and Amazon fires as a possible case of compound remote extreme events. South America, together with Equatorial Asia, are the two major regions contributing to the global net fire carbon emission. High-frequency fires have also been shown to impact the unique ecological structure and biodiversity in the Amazon. The drivers of fires in the Amazon region have gained much interest. It is well known that Amazonian fires are ignited by humans and are thus not purely meteorological events. In this study however, the meteorological conditions that enable fires and are thus a cause of interannual variability are the principal concern.</p><p>Chen et al. (2015) identified a relationship between June to November Amazon fires, and earlier (March to June) North Atlantic SST anomalies and the number of annual North Atlantic tropical cyclones. Intriguingly, they showed that the correlation between fires and tropical storms is actually greater than that between SST and either fires or tropical storms. They concluded that the correlation between fires and tropical storms cannot be fully explained by assuming only a common linkage of the two with the SST. However, they did not identify other possible causes or explanations for this. Here we hypothesise that tropical cyclones rainfall could play an active role in promoting Amazon fire conditions and the role of SST is actually minor. We identify this link as through anomalous precipitation in the ITCZ and in particular, for the first time, the important role of Caribbean storms.</p><p>We found that the seasonal cycles of North Atlantic tropical storms and South Amazon fires are in phase with a maximum around September and have significant interannual correlation driven exclusively by Caribbean tropical storms. Years of high fire activity are associated with atmospheric conditions over the Caribbean which favour tropical cyclones, enhanced precipitation, local ascent and remote anomalous Amazon subsidence. We hypothesise a direct physical link of tropical storms in the Caribbean to fires in the South Amazon through a thermal direct circulation, Hadley-type response driven by off-equatorial heating partly caused by the storms. Anomalous precipitation over the Caribbean regardless of cause releases latent heat there and contributes to the thermal direct circulation. However, Caribbean tropical storms play an important role based on our analysis as they account for much of the precipitation anomalies there. The anomalous Amazon subsidence we observed promotes favourable fire conditions there. By focusing on the active fire season, our study leads to a deeper understanding of Amazon fire variability than previous studies which considered the pre-season. Our study helps build a more complete understanding of the drivers of Amazon fires and provides evidence of a case of remote physically linked compound extreme events.</p><p>Figure: Thermal direct cell between the Caribbean (green box) and the Amazon (red box).</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.3b564075b0e164646112461/sdaolpUECMynit/22UGE&app=m&a=0&c=96b10b7fbd8427fb251b94cd639771b1&ct=x&pn=gnp.elif&d=1" alt=""></p>