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Andrew Hoell
,
Andrea E. Gaughan
,
Shraddhanand Shukla
, and
Tamuka Magadzire

Abstract

Southern Africa precipitation during December–March (DJFM), the height of the rainy season, is closely related with two modes of climate variability, El Niño–Southern Oscillation (ENSO) and the subtropical Indian Ocean dipole (SIOD). Recent research has found that the combined effects of ENSO and SIOD phasing are linked with changes to the regional southern Africa atmospheric circulation beyond the individual effects of either ENSO or SIOD alone. Here, the authors extend the recent research and examine the southern Africa land surface hydrology associated with the synchronous effects of ENSO and SIOD events using a macroscale hydrologic model, with particular emphasis on the evolution of the hydrologic conditions over three critical Transfrontier Conservation Areas: the Kavango–Zambezi Conservation Area, the Greater Limpopo Transfrontier Park, and the Kgalagadi Transfrontier Park. A better understanding of the climatic effects of ENSO and SIOD phase combinations is important for regional-scale transboundary conservation planning, especially for southern Africa, where both humans and wildlife are dependent on the timing and amount of precipitation. Opposing ENSO and SIOD phase combinations (e.g., El Niño and a negative SIOD or La Niña and a positive SIOD) result in strong southern Africa climate impacts during DJFM. The strong instantaneous regional precipitation and near-surface air temperature anomalies during opposing ENSO and SIOD phase combinations lead to significant soil moisture and evapotranspiration anomalies in the year following the ENSO event. By contrast, when ENSO and SIOD are in the same phase (e.g., El Niño and a positive SIOD or La Niña and a negative SIOD), the southern Africa climate impacts during DJFM are minimal.

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Andrew Hoell
,
Trent W. Ford
,
Molly Woloszyn
,
Jason A. Otkin
, and
Jon Eischeid

Abstract

Characteristics and predictability of drought in the midwestern United States, spanning the from the Great Plains to the Ohio Valley, at local and regional scales are examined during 1916–2015. Given vast differences in hydroclimatic variability across the Midwest, drought is evaluated in four regions identified using a hierarchical clustering algorithm applied to an integrated drought index based on soil moisture, snow water equivalent, and 3-month runoff from land surface models forced by observed analyses. Highlighting the regions containing the Ohio Valley (OV) and Northern Great Plains (NGP), the OV demonstrates a preference for subannual droughts, the timing of which can lead to prevalent dry epochs, while the NGP demonstrates a preference for annual-to-multiannual droughts. Regional drought variations are closely related to precipitation, resulting in a higher likelihood of drought onset or demise during wet seasons: March–November in the NGP and all year in the OV, with a preference for March–May and September–November. Due to the distinct dry season in the NGP, there is a higher likelihood of longer drought persistence, as the NGP is 4 times more likely to experience drought lasting at least one year compared to the OV. While drought variability in all regions and seasons is related to atmospheric wave trains spanning the Pacific–North American sector, longer-lead predictability is limited to the OV in December–February because it is the only region/season related to slow-varying sea surface temperatures consistent with El Niño–Southern Oscillation. The wave trains in all other regions appear to be generated in the atmosphere, highlighting the importance of internal atmospheric variability in shaping Midwest drought.

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