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One consequence of climate change is an increasing mismatch between timing of food requirements and food availability. Such a mismatch is primarily expected in avian long-distance migrants because of their complex annual cycle, and in habitats with a seasonal food peak. Here we show that insectivorous long-distance migrant species in The Netherlands declined strongly (1984–2004) in forests, a habitat characterized by a short spring food peak, but that they did not decline in less seasonal marshes. Also, within generalist long-distance migrant species, populations declined more strongly in forests than in marshes. Forest-inhabiting migrant species arriving latest in spring declined most sharply, probably because their mismatch with the peak in food supply is greatest. Residents and short-distance migrants had non-declining populations in both habitats, suggesting that habitat quality did not deteriorate. Habitat-related differences in trends were most probably caused by climate change because at a European scale, long-distance migrants in forests declined more severely in western Europe, where springs have become considerably warmer, when compared with northern Europe, where temperatures during spring arrival and breeding have increased less. Our results suggest that trophic mismatches may have become a major cause for population declines in long-distance migrants in highly seasonal habitats.

Recent findings indicate that rainfall variability over West Africa is characterized by more positive anomalies in the last four decades. The authors demonstrate that the recent interannual rainfall variability is linked to an air-sea phenomenon that occurs in the tropical Atlantic and eastern Pacific Ocean, and then propose the Trans-Atlantic-Pacific Ocean Dipole (TAPOD) index as a measure for this tropical ocean phenomenon, which is found to be closely correlated with the West African summer rainfall anomalies. Using observational and reanalysis datasets, composite analysis suggests that enhanced precipitation in West Africa is associated with the positive phase of the TAPOD, which is characterized by warm sea surface temperature anomalies (SSTAs) in the tropical Atlantic and cool SSTAs in the tropical eastern Pacific Ocean. During the positive phase of the TAPOD, there are significant westerly anomalies over the tropical Atlantic Ocean, which drives anomalous water vapor convergence over West Africa, leading to enhanced precipitation in the region.

The West African westerly jet (WAWJ) is a low-level jet centered near 10° N over the West African coast from June to September. This study quantifies the low-level moisture transport associated with the WAWJ on seasonal timescales, examines the relationship between the jet and West African precipitation using a moisture budget analysis, and further distinguishes the WAWJ from the West African monsoon (WAM) flow. Three reanalyses and five observational rainfall datasets are examined to build confidence. The WAWJ has a diurnal cycle with a minimum at 12 UTC and a maximum at 18 UTC, dominated by the ageostrophic wind component. According to the momentum budget analysis, the diurnal acceleration/deceleration of the WAWJ is controlled by variations in geopotential height gradient forces associated with the continental thermal low and its westward, offshore extension over the eastern Atlantic. The WAWJ wind speed and rainfall across a large region of the western Sahel (0°–10°W, 8°–18° N) are significantly and positively correlated. In this region, the moisture flux associated with the WAWJ is stronger than that associated with the southerly WAM flow from July 5 to August 20 (45 days). The moisture budget analysis reveals that the seasonal evolution of the rainfall in this analysis region is associated with zonal moisture convergence related to changes of the WAWJ. Enhanced (reduced) rainfall occurs in months with a strong (weak) WAWJ, accompanied by low-level moisture flux anomalies associated with the WAWJ instead of the southerly WAM.

Abstract The West African Sahel has been facing for more than 30 years an increase in extreme rainfall with strong socio-economic impacts. This situation challenges decision-makers to define adaptation strategies in a rapidly changing climate. The present study proposes (i) a quantitative characterization of the trends in extreme rainfall at the regional scale, (ii) the translation of the trends into metrics that can be used by hydrological risk managers, (iii) elements for understanding the link between the climatology of extreme and mean rainfall. Based on a regional non-stationary statistical model applied to in-situ daily rainfall data over the period 1983–2015, we show that the region-wide increasing trend in extreme rainfall is highly significant. The change in extreme value distribution reflects an increase in both the mean and variability, producing a 5%/decade increase in extreme rainfall intensity whatever the return period. The statistical framework provides operational elements for revising the design methods of hydraulic structures which most often assume a stationary climate. Finally, the study shows that the increase in annual maxima of daily rainfall is more attributable to stronger storm intensities (80%) than to more frequent storm occurrences (20%), reflecting a major rainfall regime shift in comparison to those observed in the region since 1950.

The strength of the simultaneous linear relationship between El Niño/Southern Oscillation (ENSO) and Indian summer monsoon (ISM) precipitation show strong variations on a decadal timescale. While some studies attribute this to shift in the state of the climate and consequent teleconnection pattern, some other argue this as natural variability between two random time series. In this study, we show that the relationship between West African Summer Monsoon (WASM) precipitation with ENSO also experiences decadal timescale oscillation. While the ENSO–ISM relationship weakened during the past seven decades, ENSO–WASM relationship strengthened to above the 95% significance level. We explain this multi-decadal see-saw of strong–weak impact of ENSO on ISM and WASM through a common mechanism. ENSO impacts ISM and WASM rainfall by modulating the upper tropospheric temperature of subtropical Africa and South Asia. While the impact of ENSO on this temperature anomaly was strong and concentrated over the northwest of Indian region before the 1980, the anomalies are spatially discontinuous and weak after 1980. Moreover, a westward shift of the center of this anomaly after 1980 help strengthen the ENSO–WASM relationship. We also show a dramatic change in the relationship between Atlantic Niño and ENSO before and after the 1980s. While before 1980 ENSO did not have much impact on Atlantic Nino index-3 (ATL3), after 1980 El Niño (La Niña) is coincidental with negative (positive) ATL3 index. Since a negative (positive) ATL3 reduce (enhance) WASM by increased south-westerly moisture flux, the ENSO–WASM relationship strengthens after 1980. Our study suggests that the decadal variations of ENSO–ISM and ENSO–WASM relationship is physically linked and possibly could not be due to pure noise in the time series.

This article presents an overview of the land ITCZ (Intertropical Convergence Zone) over West Africa, based on analysis of NCAR-NCEP Reanalysis data. The picture that emerges is much different than the classic one. The most important feature is that the ITCZ is effectively independent of the system that produces most of the rainfall. Rainfall linked directly to this zone of surface convergence generally affects only the southern Sahara and the northern-most Sahel, and only in abnormally wet years in the region. A second feature is that the rainbelt normally assumed to represent the ITCZ is instead produced by a large core of ascent lying between the African Easterly Jet and the Tropical Easterly Jet. This region corresponds to the southern track of African Easterly Waves, which distribute the rainfall. This finding underscores the need to distinguish between the ITCZ and the feature better termed the “tropical rainbelt”. The latter is conventionally but improperly used in remote sensing studies to denote the surface ITCZ over West Africa. The new picture also suggests that the moisture available for convection is strongly coupled to the strength of the uplift, which in turn is controlled by the characteristics of the African Easterly Jet and Tropical Easterly Jet, rather than by moisture convergence. This new picture also includes a circulation feature not generally considered in most analyses of the region. This feature, a low-level westerly jet termed the African Westerly Jet, plays a significant role in interannual and multidecadal variability in the Sahel region of West Africa. Included are discussions of the how this new view relates to other aspects of West Africa meteorology, such as moisture sources, rainfall production and forecasting, desertification, climate monitoring, hurricanes and interannual variability. The West African monsoon is also related to a new paradigm for examining the interannual variability of rainfall over West Africa, one that relates changes in annual rainfall to changes in either the intensity of the rainbelt or north-south displacements of this feature. The new view presented here is consistent with a plethora of research on the synoptic and dynamic aspects of the African Easterly Waves, the disturbances that are linked to rainfall over West Africa and spawn hurricanes over the Atlantic, and with our knowledge of the prevailing synoptic and dynamic features. This article demonstrate a new aspect of the West Africa monsoon, a bimodal state, with one mode linked to dry conditions in the Sahel and the other linked to wet conditions. The switch between modes appears to be linked to an inertial instability mechanism, with the cross-equatorial pressure gradient being a critical factor. The biomodal state has been shown for the month of August only, but this month contributes most of the interannual variability. This new picture of the monsoon and interannual variability shown here appears to be relevant not only to interannual variability, but also to the multidecadal variability evidenced in the region between the 1950s and 1980s.

The West African westerly jet is a low-level feature of the summer climatology that transports moisture from the eastern Atlantic onto the African continent at 8°–11°N. This study examines the relationship between the jet and Sahel precipitation variability in August, when both the jet and rainfall reach their seasonal maxima. Variations of the West African westerly jet are significantly positively correlated with precipitation variations over the Sahel on both interannual and decadal time scales. Three periods are identified (1958–71, 1972–87, and 1988–2009), corresponding to times with a wet Sahel–strong jet, dry Sahel–weak jet, and relatively wet Sahel–strong jet. In wet (dry) periods, enhanced (decreased) westerly moisture fluxes associated with a strong (weak) jet increase (decrease) the low-level moisture content over the Sahel, decreasing (enhancing) the stability of the atmosphere. This association between the jet and Sahel rainfall is also found in case studies of 1964, 1984, 1999, and 2007. The southerly moisture flux associated with the West African monsoon has less pronounced decadal variability than the westerly moisture flux of the West African westerly jet and weaker correlations with Sahel rainfall. When the monsoon flow is weak, for example, 1999 and 2007, the Sahel may still experience positive precipitation anomalies in association with strong westerly moisture transport by the jet. The West African westerly jet is also important for stabilizing the regional vorticity balance by introducing strong relative vorticity gradients. Northward flow advects low relative vorticity south of the jet to balance positive vorticity tendencies generated by midtropospheric condensation.

This research focuses on evaluating the High-Resolution Model Intercomparison Project (HighResMIP) simulations within the framework of the Coupled Model Intercomparison Project (CMIP) Phase 6 (CMIP6). We used seven of its consortiums to study how CMIP6 reproduced the West African precipitation features during the 1950–2014 historical simulation periods. The rainfall event was studied for two sub-regions of West Africa, the Sahel and the Guinea Coast. Precipitation datasets from the Climate Research Unit (CRU) TS v4.03, University of Delaware (UDEL) v5.01, and Global Precipitation Climatology Centre (GPCC) were used as observational references with the aim of accounting for uncertainty. The observed annual peak during August, which is greater than 200, 25, and 100 mm/month in the Guinea Coast, the Sahel, and West Africa as a whole, respectively, appears to be slightly underestimated by some of the models and the ensemble mean, although all the models captured the general rainfall pattern. Global climate models (GCMs) and the ensemble mean reproduced the spatial daily pattern of precipitation in the monsoon season (from June to September) over West Africa, with a high correlation coefficient exceeding 0.8 for the mean field and a relatively lower correlation coefficient for extreme events. Individual models, such as IPSL and ECMWF, tend to show high performance, but the ensemble mean appears to outperform all other models in reproducing West African precipitation features. The result from this study shows that merely improving the horizontal resolution may not remove biases from CMIP6.

This paper aims to model the occurrence of Yearly July–September (YJAS) extreme droughts in the West African Sahel (WAS) and to estimate return periods and return levels of these events through stationary peaks-over-threshold model. For this purpose, the historical gridded monthly rainfall data from Climatic Research Unit for the period 1901–2009 were used. The results show that return levels of YJAS dry extremes have increased since 1970, implying that YJAS extreme droughts are consistently more severe after 1970 than they were before. Approximately 62 % of the WAS area in the postchange period of 1971–2009 was dominated by dry spells not longer than 1 year. The dynamics of the YJAS extremes drying trend indicate that the changes at the tails of YJAS dry extreme distribution have contributed to the dry trend in mean YJAS rainfall in the WAS. The estimated 40-year return level of these events based on 1971–2009 period was less than the average of dry extremes of the same period, suggesting that droughts could intensify in the future even though with some amelioration. Such a finding could prove helpful in anticipation of climate risks in this region where adaptive capacities are very low.

The biannual seasonal rainfall regime over the southern part of West Africa is characterised by two wet seasons, separated by the ‘Little Dry Season’ in July–August. Lower rainfall totals during this intervening dry season may be detrimental for crop yields over a region with a dense population that depends on agricultural output. Coupled Model Intercomparison Project Phase 5 (CMIP5) models do not correctly capture this seasonal regime, and instead generate a single wet season, peaking at the observed timing of the Little Dry Season. Hence, the realism of future climate projections over this region is questionable. Here, the representation of the Little Dry Season in coupled model simulations is investigated, to elucidate factors leading to this misrepresentation. The Global Ocean Mixed Layer configuration of the Met Office Unified Model is particularly useful for exploring this misrepresentation, as it enables separating the effects of coupled model ocean biases in different ocean basins while maintaining air–sea coupling. Atlantic Ocean SST biases cause the incorrect seasonal regime over southern West Africa. Upper level descent in August reduces ascent along the coastline, which is associated with the observed reduction in rainfall during the Little Dry Season. When coupled model Atlantic Ocean biases are introduced, ascent over the coastline is deeper and rainfall totals are higher during July–August. Hence, this study indicates detrimental impacts introduced by Atlantic Ocean biases, and highlights an area of model development required for production of meaningful climate change projections over the West Africa region.