Representation of the Indian Ocean Walker circulation in climate models and links to Kenyan rainfall

Abstract

Reliable climate change projections over East Africa are vital because of regional vulnerability to precipitation changes. However, global climate models from Coupled Model Intercomparison Project Phase 5 (CMIP5) display significant biases in their representation of key East African rainfall seasons, which call into question the reliability of projected climate change. We investigate the links between models' representation of rainfall over Kenya during the long and short rains and the proximate Walker circulation. There is a strong correlation in the short rains between model biases in Kenyan rainfall and in the mid-to-upper tropospheric vertical velocity associated with this circulation. The overturning Indian Ocean Walker cell at the equator is absent in 5/25 models during the short rainsthese models exhibit wet biases. In the long rains, dry biased models overestimate the strength of the descending limb of the circulation over East Africa. Omega biases over the Congo Basin are linked to broader Walker circulation biases. During the long rains, models overestimate equatorial descent more generally across the Western Hemisphere Tropics (0 E-200 E). A significant correlation is obtained across the model ensemble between model rainfall over Kenya and Western Hemisphere equatorial ascent during November. Atmosphere-only models display some improvements over coupled models, but biases of a similar magnitude remain. We therefore propose Indian Ocean Walker circulation errors as a key source of bias in CMIP5 East African rainfall. The results add to recent work on CMIP5 biases in this region, demonstrating that the Indian Ocean Walker circulation should be a focus for future model improvement and a consideration when assessing the reliability of climate projections over East Africa. Further work is needed on the causes of Walker circulation biases (in particular the role of SST), and on understanding the impact of Walker circulation biases on modelled tropical rainfall elsewhere in the world.

Future Changes in the Indian Ocean Walker Circulation and Links to Kenyan Rainfall

“…The previous section has demonstrated that CMIP5 model projections, which are generally toward a wetter East Africa, occur concomitantly with changes to the IOWC including reduced positive omega and enhanced easterly winds. This supports other recent studies arguing that the IOWC is an important control on East African rainfall in both observations and models; it has been suggested that the dynamics control rainfall rather than the other way around (e.g., Chadwick et al, 2013;King et al, 2020;Mutai & Ward, 2000;Nicholson, 2017;Williams & Funk, 2011;Zhao & Cook, 2021), especially given the important role of SST in IOWC variability (Black, 2005;Hoell et al, 2017;Hoell & Funk, 2014;Liebmann et al, 2014). The important task that follows from this analysis is to assess the plausibility of these future projections in the context of the models' known biases in representation of the historical IOWC and associated rainfall conditions.…”

“…Therefore, the ensemble mean rainfall averages for both historical and future simulations of the long rains are thus dominated by a few very wet or very dry models. The wet biased models in MAM could be considered more realistic, as the ensemble mean has a dry bias during the season, but King et al (2020) have shown that these highly biased models have unrealistic dynamics over East Africa and the tropical Indian Ocean in the historical coupled simulations.…”

“…This is reflected in the less strong descent in the composites relative to ascent over East Africa in Figure 7. This can be linked to the differing model behavior in the 1975-2005 baseline conditions; King et al (2020) find that the equatorial overturning associated with the IOWC is not represented during this period in November in INMCM4, for example.…”

“…The IOD links East Africa to wider tropical interannual variability via non-stationary teleconnections to the El Niño Southern Oscillation (ENSO;MacLeod et al, 2020;Nicholson, 2015). The climatological overturning cell features ascent over the Maritime Continent and descent over East Africa from September to March (Zhao & Cook, 2021), which is a contributor to the relative aridity of Kenya despite its location on the equator (King et al, 2020;Nicholson, 2017). Statistical and dynamic relationships between Indian Ocean ascent and descent and rainfall over East Africa are well established during the East African rainy seasons (Behera et al, 2005;Black, 2005;King et al, 2020;Liebmann et al, 2014Liebmann et al, , 2017Vigaud et al, 2017).…”

“…Since these zonal winds form part of the solenoidal structure of the IOWC (Nicholson, 2017), this indicates an important limitation in the models' simulation of East African rainfall. King et al (2020) demonstrated that CMIP5 models' rainfall biases over Kenya can be dynamically linked to their representation of the Walker Circulation, with dry (wet) biases linked to descending (ascending) biases over East Africa. Jones (2019), andMunday (2019) found a weakening of the zonal overturning circulation over the Congo Basin in CMIP5 projections, which is projected to become a more important influence on East African rainfall in future (Giannini et al, 2018).…”