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Common cause for severe droughts in South America and marine heatwaves in the South Atlantic

Abstract

In 2013/14 eastern South America experienced one of its worst droughts. At the same time an unprecedented marine heatwave developed in the western South Atlantic. The drought was linked to suppression of the South Atlantic convergence zone and its associated rainfall, which led to water shortages in Brazil and impacted food supplies globally. Here we show from observations that such droughts and adjacent marine heatwaves have a common remote cause. Atmospheric blocking triggered by tropical convection in the Indian and Pacific oceans can cause persistent anticyclonic circulation that not only leads to severe drought but also generates marine heatwaves in the adjacent ocean. We show that increased shortwave radiation due to reduced cloud cover and reduced ocean heat loss from weaker winds are the main contributors to the establishment of marine heatwaves in the region. The proposed mechanism, which involves droughts, extreme air temperature over land and atmospheric blocking explains approximately 60% of the marine heatwave events in the western South Atlantic. We also identified an increase in frequency, duration, intensity and extension of marine heatwave events over the satellite period 1982–2016. Moreover, surface primary production was reduced during these events with implications for regional fisheries.

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Data availability

The SST and atmospheric data used in this work are freely available from www.esrl.noaa.gov/psd/data/gridded/data.noaa.oisst.v2.highres.html and http://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=sfc/, respectively. The precipitation and OLR data are provided freely at www.esrl.noaa.gov/psd/data/gridded/. The ocean colour data are also freely available from https://oceancolor.gsfc.nasa.gov. OSCAR currents are available from https://podaac.jpl.nasa.gov/dataset/OSCAR_ L4_OC_third-deg. Surface turbulent heat fluxes from OAFlux were obtained from http://oaflux.whoi.edu/data.html. The SARAH-2 SWR data are available from https://wui.cmsaf.eu/safira/action/viewDoiDetails?acronym=SARAH_V002_01. Ocean data from ORAS4 are available from www.ecmwf.int/en/research/climate-reanalysis/ocean-reanalysis.

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Acknowledgements

R.R.R. is supported by CNPq (no. 401873/2016-1) and CAPES (no. 88881.145866/2017-1). A.S. and A.S.T. (FT160100495) are supported by the Australian Research Council. This work is part of the research conducted by the Programmes INCT-MCII (CNPq no. 465501/2014-1 and CAPES/FAPS no. 16/2014) and Rede CLIMA (FINEP no. 01.13.0353-00). G.R.F. was supported by base funds to NOAA/AOML.

Author information

The main idea was developed by R.R.R. in collaboration with A.S.T. and A.S. Most of the text was written by R.R.R., who also did the data preparations and most of the data analyses and made the figures. G.R.F. provided the temperature budget calculations. The schematic was prepared by A.S. All the authors contributed with ideas, discussed the results and implications and contributed to the text.

Correspondence to Regina R. Rodrigues.

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Fig. 1: The 2013/14 western South Atlantic MHW event.
Fig. 2: Western South Atlantic MHW during austral summer for the period 1982–2016.
Fig. 3: Precursor of atmospheric blocking and MHWs.
Fig. 4: Mechanism of onset and decay of the 2013/2014 MHW event.
Fig. 5: Schematic representation of the forcing mechanisms of the western South Atlantic MHWs.