Marine heatwaves in the dynamics of the Pantanal´s historical drought and unprecedented fires

<p>Fire is a natural disturbance in the Neotropical savannas, and rather frequent in the relatively<br>dry and well-drained seasonal savannas of the Brazilian Cerrado. The neighboring Pantanal, on<br>the other hand, is a seasonally flooded savanna and the largest wetland in the world (≈138,000<br>km 2 ). Due to its wetter condition, fires in the Pantanal are much less frequent and spatially<br>restricted. But, given an ongoing extreme drought, the 2020’s fires in the Pantanal have been<br>unprecedented in extent and duration: About one third (≈45,000 km 2 ) of the area of this<br>important wetland has gone up in flames since last January. Regarding this historical drought,<br>climate change has been identified as one of the most important threats to the Pantanal.<br>Reductions in precipitation may cause significant disturbances in its ecological functioning,<br>affecting hydrological, floodplain inundation dynamics, as well as fire regime. Climate change<br>models from a recent study (Thielen et al. 2020, doi:10.1371/journal.pone.0227437) indicate<br>that, for the Pantanal, extended severe droughts are to be expected from the warming of Sea<br>Surface Temperatures (SST) at Northern Hemisphere oceans.<br>The present study analyses the spatial and temporal dynamics of precipitations during the<br>series 1981-2020 in the Upper Paraguay River Basin (UPRB), which comprises the Pantanal and<br>the neighboring Highlands, along with a co-evaluation of the SST trends at three oceanic<br>regions from Northern Hemisphere. Precipitation anomalies were analyzed by mean of the<br>Standardized Precipitation Drought Index (SPDI) based on the 1981-2010 climate normals.<br>Results show that for the UPRB, negative precipitation anomalies occur in pulses lasting<br>several years. A drought starting in 2019 has been the strongest and most extended on record,<br>persistently reaching the Extremely Dry condition (SPDI≤-2.0) during 2020. As early as Mar,<br>over 64% of the Pantanal is affected by such drought, and around 83% by Dec. For the UPRB,<br>four distinctive groups of subregions were identified according to their temporal dynamics of<br>mean SPDI values, mainly during Sep2019/Feb2020 and Mar2020/Dec2020. Here, precipitation<br>anomalies from southernmost subregions of the Pantanal were less intense and even not<br>affected by the drought.<br>As for SST, the Northeast Pacific region (PAC-NE) showed the most important dynamics. In this<br>region, SSTs have been anomalously warm since Jun 2019, with 64% of the time SSTA<br>surpassing the 90 th percentile: reaching the Heatwave condition. With a lead of one to two<br>months, PAC-NE showed the strongest (and negative) correlation with precipitation at UPRB<br>(r=-0.87) during Jan2019 to Dec2020. There is a significant trend for an increase in SST at the</p><p>Northeast Pacific, a trend that will certainly generate a rather continuous Heatwave in PAC-NE.<br>As a result, one expects an extension of the current extreme drought in the Pantanal area, at<br>least during 2021, and the intensification of fires with unprecedented duration and intensity,<br>extending now to areas historically flooded or perhumid. Concomitantly, we predict a most<br>definite impact on non-fire-resistant vegetation cover, as well as ecosystem functioning and<br>biodiversity.</p>