DOI: 10.25349/d9t034
title: Dataset - Frequent but Predictable Droughts in East Africa Driven By A Walker Circulation Intensification

description: The decline of the eastern East African (EA) March-April-May (MAM) rains
 poses a life-threatening ‘enigma’, an enigma linked to sequential droughts
 in the most food insecure region in the world. The MAM 2022 drought was
 the driest on record, preceded by three poor rainy seasons, and followed
 by widespread starvation. Connecting these droughts is an interaction
 between La Niña and climate change, an interaction that provides exciting
 opportunities for long lead prediction and proactive disaster risk
 management. Using observations, reanalyses, and climate change
 simulations, we show here, for the first time, that post-1997 OND La Niña
 events are robust precursors of: (1) strong MAM ‘Western V Gradients’ in
 the Pacific, which help produce (2) large increases in moisture
 convergence and atmospheric heating near Indonesia, which appear
 associated with (3) regional shifts in moisture transports and vertical
 velocities, which (4) help explain more frequent dry EA rainy seasons.
 Understanding this causal chain will help make long-lead forecasts more
 actionable. Increased Warm Pool atmospheric heating and moisture
 convergence sets the stage for dangerous sequential droughts in EA. At
 20-yr time scales, we show that these Warm Pool heating increases are
 attributable to observed Western V warming, which is in turn largely
 attributable to climate change. As energy builds up in the oceans and
 atmosphere, we see stronger convergence patterns, which offer
 opportunities for prediction. Hence, linking EA drying to a stronger
 Walker Circulation can help explain the ‘enigma’ while underscoring the
 predictable risks associated with recent La Niña events.
description: This dataset was created to support the paper 'Frequent but
 Predictable Droughts in East Africa Driven By A  Walker Circulation
 Intensification'. This analysis draws together data from six
 categories:     1. Observed gridded rainfall
 values<br>    2. Observed sea surface temperatures (SST)<br> 
   3. Climate change simulations of  SST time series<br>    4.
 Seasonal predictions of SST time series<br>    5. ERA5 atmospheric
 reanalysis fields<br>    6. MERRA2 atmospheric reanalysis
 fields Rainfall comes from
 the
 Climate Hazard Center Infrared Precipitation with Stations archive
 (CHIRPS, https://www.nature.com/articles/sdata201566) and Centennial
 Trends (https://www.nature.com/articles/sdata201550) archives. The
 observed SST data are from the NOAA Extended Reconstruction sea
 surface temperature data set (version 5). The seasonal SST forecasts from
 the North American Multi-Model Ensemble (NMME). The projected SST and
 precipitation simulation time-series are from Phase 6 of the
 Climate Model Intercomparison Project (CMIP6). The reanalyses evaluated
 were the ERA5 and MERRA2. While all of these data are publicly
 available, we pull together here salient time series supporting the basic
 results of our paper. Our key points are:
 -- Human-induced warming in the
 western V area of the Pacific combined with La Niña, has produced
 frequent, predictable March-April-May droughts.
 --
 Thermodynamic analyses link these
 droughts to a stronger Walker Ciruclation, driven by predictable warming
 in the Western V region. --
 CMIP6 simulations indicate that
 western V warming is largely human-induced, this warming has enhanced and
 will enhance the Walker Circulation.
   The NMME seasonal climate forecasts
 are based on coupled ocean-atmosphere models, intialized monthly with
 observed conditions. The coupled ocean-atmosphere models in
 the CMIP6 archive, on the other hand, are initialized in the early 19th
 century, and then run into the future, constrained by changes in aerosols
 and greenhouse gasses. The NMME provide operational forecasts. The CMIP6
 provides climate change simulations. For
 the rainfall and SST data, the only major processing has been seasonal and
 spatial averaging and the calculation of anomalies. For
 the ERA5 and MERRA2 reanalyses, seasonal 'diabatic heating'
 terms were calculated using existing precipitaiton, radiation and sensible
 heat flux fields. This was combined with vertically integrated heat
 convergence to obtain a total estimate of 'atmospheric heating'
 -- which we use to examine changes in the strength of the Walker
 Circulation.    
    
description: By design, no special software or programming expertise is
 required to access the spreadsheet containing our results.