A synthetic view of rainfall intensification in the West African Sahel

The West African Sahel has been facing for more than 30 years an increase in extreme rainfalls 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 rainfalls 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 rainfalls 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 extreme rainfall is more attributable to an increase in the intensity of storms (80%) than to their occurrence (20%), reflecting a major disruption from the decadal variability of the rainfall regime documented in the region since 1950.

Another Record: Ocean Warming Continues through 2021 despite La Niña Conditions

The increased concentration of greenhouse gases in the atmosphere from human activities traps heat within the climate system and increases ocean heat content (OHC). Here, we provide the first analysis of recent OHC changes through 2021 from two international groups. The world ocean, in 2021, was the hottest ever recorded by humans, and the 2021 annual OHC value is even higher than last year’s record value by 14 ± 11 ZJ (1 zetta J = 1021 J) using the IAP/CAS dataset and by 16 ± 10 ZJ using NCEI/NOAA dataset. The long-term ocean warming is larger in the Atlantic and Southern Oceans than in other regions and is mainly attributed, via climate model simulations, to an increase in anthropogenic greenhouse gas concentrations. The year-to-year variation of OHC is primarily tied to the El Niño-Southern Oscillation (ENSO). In the seven maritime domains of the Indian, Tropical Atlantic, North Atlantic, Northwest Pacific, North Pacific, Southern oceans, and the Mediterranean Sea, robust warming is observed but with distinct inter-annual to decadal variability. Four out of seven domains showed record-high heat content in 2021. The anomalous global and regional ocean warming established in this study should be incorporated into climate risk assessments, adaptation, and mitigation.

The Variability of Air-sea O2 Flux in CMIP6: Implications for Estimating Terrestrial and Oceanic Carbon Sinks

The measurement of atmospheric O2 concentrations and related oxygen budget have been used to estimate terrestrial and oceanic carbon uptake. However, a discrepancy remains in assessments of O2 exchange between ocean and atmosphere (i.e. air-sea O2 flux), which is one of the major contributors to uncertainties in the O2-based estimations of the carbon uptake. Here, we explore the variability of air-sea O2 flux with the use of outputs from Coupled Model Intercomparison Project phase 6 (CMIP6). The simulated air-sea O2 flux exhibits an obvious warming-induced upward trend (∼1.49 Tmol yr−2) since the mid-1980s, accompanied by a strong decadal variability dominated by oceanic climate modes. We subsequently revise the O2-based carbon uptakes in response to this changing air-sea O2 flux. Our results show that, for the 1990–2000 period, the averaged net ocean and land sinks are 2.10±0.43 and 1.14±0.52 GtC yr−1 respectively, overall consistent with estimates derived by the Global Carbon Project (GCP). An enhanced carbon uptake is found in both land and ocean after year 2000, reflecting the modification of carbon cycle under human activities. Results derived from CMIP5 simulations also investigated in the study allow for comparisons from which we can see the vital importance of oxygen dataset on carbon uptake estimations.

Enhanced predictability of rapidly intensifying tropical cyclones over the western North Pacific associated with snow depth changes over the Tibetan Plateau

This study finds an enhanced relationship in recent years between January–March eastern Tibetan Plateau snow depth (TPSD) and the frequency of rapidly intensifying tropical cyclones (RITCs) over the western Northern Pacific (WNP) during the following peak TC season (July–November). The correlation between TPSD and RITCs is significant during 2000–2014 but was insignificant from 1979–1999. During 2000–2014, when TPSD increases, there is an enhanced low-level anomalous anticyclone over the subtropical eastern North Pacific mainly due to the combined effect of advection and dynamics of the climatological prevailing westerly jet. Northeasterly wind anomalies are observed on the flank of the anticyclonic circulation anomaly, favoring anomalously cool sea surface temperature (SST). These anomalies lead to an anomalous pattern similar to the Pacific meridional mode (PMM), via a wind-evaporation feedback and cold advection. A Gill-type Rossby response to the PMM-like negative phase results in an anticyclonic circulation anomaly over the WNP, suppressing RITCs during 2000–2014. A nearly opposite circulation anomaly occurred when TPSD was lower during 2000–2014. There is a weak relationship between TPSD and RITCs, due to the lack of a link between TPSD and the PMM-like pattern from 1979–1999. Decadal changes in the relationship between TPSD and RITCs are mainly due to the meridional displacement of the prevailing westerly jet which may be in response to decadal-to-multi-decadal variability of SST anomalies. These changes then result in changes in the relationship between January–March TPSD and the PMM-like pattern.

Local Ocean-Atmosphere Interaction in Indian Summer Monsoon Multi-Decadal Variability

The significant multi-decadal mode (MDM) of the Indian summer monsoon rainfall (ISMR) during the past two millennia provides a basis for decadal predictability of the ISMR and has a strong association with the North-Atlantic variability with the Atlantic Multi-decadal Oscillation (AMO) as a potential external driver. It is also known that the annual cycles and interannual variability of ISMR and sea surface temperatures (SST) over the tropical Indian Ocean (IO) are strongly coupled. However, the role of local air-sea interactions in maintaining or modifying the ISMR MDM remains unknown. A related puzzle we identify is that the IO SST has an increasing trend during two opposite phases of the ISMR MDM, namely during an increasing phase of ISMR (1901 to 1957) as well as a decreasing phase of ISMR (1958-2007). Here, using a twentieth-century reanalysis (20CR), we examine the role of air-sea interactions in maintaining two opposite phases of the ISMR MDM and unravel that the Bjerknes feedback is at the heart of maintaining the ISMR MDM but cannot explain the increasing trend of SST in the tropical IO during the opposite phases. Large-scale low-level vorticity influence on SST and net heat flux changes through circulation and cloudiness changes associated with the two phases of the ISMR MDM together contribute to the SST trends. The decreasing trend of low-level wind convergence during the period between 1958 and 2007 is a determining factor for the decreasing trend of ISMR in the backdrop of an increasing trend of atmospheric moisture content. Consistent with the lead of the AMO with respect to ISMR by about a decade, the AMO drives the transition from one phase of ISMR MDM to another by changing its phase first and setting up low-level equatorial zonal winds conducive for the transition.

Flow Indices Variability in Humid Subtropical of Upper Awash River Basin, Ethiopia

Investigating the hydrological extremes indices at high resolutions describing the whole stream spectrum is essential for the comprehensive assessment of watershed hydrology. The study focuses on a wide-ranging assessment of river discharge in annual mean, peak, and high and low percentiles flow at the Upper Awash River basin, Ethiopia. Statistical tests such as coefficient of variation, flood variability to characterize the flow regime and Tukey’s test to detect decadal variability. Modified Mann-Kendall test, Sen’s slope estimator, innovative trend analysis and Pettitt’s test were applied to see trends, and change points in time series, respectively. Results showed that the basin was characterized by moderate to high variability. Spatially, main tributaries showed a higher variability, almost in all-time step and characterized by higher flood variability. The large discharge receiving rivers resulted in a moderate to high and lower discharge variability. Test statistics resulted in a positive increasing trend dominating most time scales at a 5% significant level and higher magnitude of slope trend in peak flow. A negative trends were also exhibited. Hombole main outlet site experienced decreasing trend in high percentile flow. In comparison, complete trend direction agreements were observed (except in few series). Flow indices showed an upward shift and downward shift mainly in the year 2000s and the significant decadal variation resulted in comparable with change points. The study provides an understanding of water resources variability, which will be necessary to apply operational water resources strategies and management to restrain the potential impacts of variability nature of the streamflow.

Reconstructing 1200 years of Hydroclimate Variability in the Southern Margins of the Arabian Desert: Insights From a Paleo-Lake in Southern Yemen

The climate of the Arabian Desert is not well documented during the past two millennia due to the scarcity of continuous and well-dated terrestrial archives in the region. Reliable interpretation from the climatic records from this region are pivotal for identifying periodicities of inter-annual to multi-decadal variability and trends driven by shifts in position of the Intertropical Convergence Zone (ITCZ) and the strength of the monsoons. A high-resolution multiproxy approach is presented for a ∼3.3 m composite core, GBW, from a karst lake located in Ghayl ba Wazir, southern Yemen. Sedimentary proxies, including particle size distribution, coupled with magnetic susceptibility (MS) and geochemistry (XRF), provide a comprehensive picture of sediment depositional changes that may be linked to climate and environmental variability over the southern Arabian Desert. The chronology of the GBW core is provided by five radiocarbon (14C) dates from terrestrial macrofossils (wood and twigs) extracted from sediment samples and indicates the core extends to ∼900 CE. Our data indicates generally wetter conditions from 930 to 1400 CE corresponding to the “Medieval climate anomaly (MCA)” followed by arid phases during 1,410–1700 CE coinciding with the “Little Ice Age (LIA)”. Evidence for a drier LIA include high authigenic calcium precipitation [Ca/(Al + Fe + Ti)], decreased TOC/TIC values, and gypsum precipitation, whereas the wetter MCA is characterized by higher detrital element ratios (Ti/Al, K/Al, Rb/Sr), and increased TOC/TIC and deposition of finer sediments (EM1). Furthermore, end-member mixing analyses (EMMA) derived from the grain-size distribution (EM2 and EM3) corroborates the deposition of coarser silt sediment through wind erosion and production of carbonate sand during the LIA concurrently with low lake levels under generally dry conditions. Aridity during the LIA is consistent with evidence and theory for weakened boreal summer monsoons during intervals of northern hemisphere cooling.