How do coupled models represent the African Easterly Jets and their associated dynamics over Central Africa during the September–November rainy season?

Abstract

Climate models are vital to the assessment of the impacts of climate change in the Central African regions. Establishing how well models reproduce key processes is important to the confidence we attach to these tools. This study examines model representation of the September to November characteristics, such as location and intensity, of the African Easterly Jet (AEJ) north and south in a sample of 16 commonly evaluated CMIP5 and CMIP6 models and in two reanalyses (ERA5 and MERRA2). The analysis evolves to assess key drivers of the AEJ from energetic interactions, the characteristics of mid-level highs and thermal lows and the nature of surface thermal heating. Over West Africa, several models miss the southeast-northwest orientation of the AEJ-N core, leading to a gap of around 60 in the location of the jet while most CMIP5 models also fail to locate AEJ-S over southern Central Africa. In general, the spread of simulated AEJ locations around reanalyses is larger for the CMIP5 sample compared to CMIP6 equivalent models, indicating improvement from CMIP5 to CMIP6 in this 16 model subset. However, this improvement in some CMIP6 models (e.g. GISS-E2-1-G and MIROC6) is not related to a maximum surface meridional gradient in temperature. Most CMIP5 and CMIP6 models underestimate the surface temperature gradient over AEJ-N region. As a first order diagnostic of the jet's acceleration, most coupled models better simulate the atmospheric energetic interactions over AEJ-N region that leads to its strong contribution to AEJ-N maintenance compared to AEJ-S. This study strengthens our understanding of the mid-level circulation over Central Africa by detecting gaps in the mechanisms maintaining the AEJ in coupled models and highlights processes that should be improved in future ensembles.

Mechanisms of Added Value of a Coupled Global Ocean‐Regional Atmosphere Climate Model Over Central Equatorial Africa

“…This assumption seems likely given that MPI‐ESM‐LR simulates, for instance, more biased SSTs than ERA‐Interim (Figure 4). In case this assumption is attested, the dynamical downscaling by means of a coupled ocean‐atmosphere RCM would therefore result beneficial especially in regions where the climate system is greatly influenced by the land‐sea interaction and feedback (such as CEA), and where ESMs can show large SST biases (e.g., Creese & Washington, 2018; Kuete et al., 2022; Taguela et al., 2022). To this, it is critically important to assess the plausibility of the AV resulting from the coupling.…”

Contribution of Lakes in Sustaining Greening of the Sahara during the Mid-Holocene

“…While the monsoon westerly flow was at ~10° N in PIref, it moved to ~15° N in MHref. Given that the model bias and uncertainty in reproducing the AEJ still require improvement in reanalysis datasets (Kuete et al, 2022), our climate model efficiently captured the WAM patterns for investigating the sensitivity of WAM to lake expansions in the Sahara.…”

“…For example, compared with the enhanced localized water cycling forced by lake evaporation (Broström et al, 1998;Carrington et al, 2001), Krinner et al (2012) considered that the cooling effect that stabilizes convection is only locally applicable to deep lakes but increases the predictable water in summer and delays cooling in autumn, thereby extending monsoon. Using the latest information on the NAf monsoon system, the contribution of the Sahara Heat Low (SHL) and Sahara Highs in western Sahara, the African Easterly Jet (AEJ) in the middle atmosphere (600 hPa), and Tropical Easterly Jet (TEJ) in the upper atmosphere (200 hPa) on the near-surface westerly flow northward and rainfall has been further discussed (Biasutti & Sobel, 2009;Claussen et al, 2017;Kuete et al, 2022). Chandan and Peltier (2020) further stressed that such a cooling effect could weaken the SHL and local convection, reducing the precipitation.…”

Contribution of lakes in sustaining the Sahara greening during the mid-Holocene

“…Because the model bias and uncertainty in reproducing the AEJ still require improvement in reanalysis datasets (Kuete et al, 2022), our climate model efficiently captured the WAM patterns for investigating its sensitivity to lake expansions in the Sahara. The corresponding high-pressure system, low-pressure system, African Easterly Jet, and Tropical Easterly Jet are marked as "H", "L", "AEJ", and "TEJ", respectively.…”

“…For example, compared with the enhanced localized water cycling forced by lake evaporation (Broström et al, 1998;Carrington et al, 2001), Krinner et al (2012) considered that the cooling effect that stabilizes convection is only locally applicable to deep lakes but increases the precipitation rates in summer and delays cooling in autumn, thereby extending the monsoon. Recent studies have explored the mechanisms of how various components of the NAf monsoon system, including the Sahara Heat Low (SHL) and Sahara highs in the western Sahara, African Easterly Jet (AEJ) in the middle atmosphere (600 hPa), and Tropical Easterly Jet (TEJ) in the upper atmosphere (200 hPa) influence the near-surface westerly flow northward and rainfall (Biasutti and Sobel, 2009;Claussen et al, 2017;Kuete et al, 2022). However, discrepancies exist regarding the effects of these components.…”