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Amplification of sea surface temperature (SST) seasonality in response to global warming is a robust feature in climate model projections but season‐dependent regional disparities in this amplification and the associated mechanisms are not well addressed. Here, by analyzing large ensemble simulations using Community Earth System Model version 2, we investigate detailed spatiotemporal characteristics of the amplification of SST seasonality focusing on the North Pacific and North Atlantic, where robust changes are projected to emerge around 2050 under SSP3‐7.0 scenario. Our results indicate that atmosphere‐ocean coupled processes shape regional changes in SST seasonality differently between warm (MAMJJAS) and cold seasons (ONDJF). During the warm season, the projected warming tendency is mainly due to increased net surface heat flux and weakening of vertical mixing. On the other hand, in the cold season, the projected cooling tendency is driven by strengthened vertical mixing over the North Pacific associated with the northward shift of storm tracks but weakened horizontal advection and mixing due to changes in ocean currents over the North Atlantic. Plain Language Summary: In addition to the increase in annual mean sea surface temperature (SST), the amplitude of SST seasonal cycle is projected to intensify in response to greenhouse warming over many parts of the global ocean due to more warming during local summer than winter. By analyzing large ensemble simulations, we show how different atmospheric and ocean processes influence spatiotemporal differences in SST seasonality changes in the North Pacific and North Atlantic. Our findings indicate that during local summer, the projected warming tendency intensifies with the increased heat input into the ocean and weakening of ocean mixing. Meanwhile, during local winter, the projected cooling tendency intensifies with the strengthening of vertical mixing associated with the enhanced storm track activities in the North Pacific, but the weakening of ocean horizontal advection and mixing attributable to the weakened ocean circulation over the North Atlantic. They together contribute to the amplification of SST seasonality, which could have implications for marine ecosystems, including the plankton phenology. Key Points: Using CESM2 large ensemble simulations, this study explores the robustness of future sea surface temperature (SST) seasonality changes and the associated atmospheric and surface ocean processesIn the warm season, the future SST warming is mainly driven by the reduced upper ocean mixing and the increased net surface heat influx into the oceanIn the cold season, the future SST cooling is attributable to strengthened vertical mixing over the North Pacific but weakened horizontal advection and mixing in the North Atlantic [ABSTRACT FROM AUTHOR]

Marine dissolved organic matter (DOM) cycles play a pivotal role in sustaining marine ecosystems and regulating the ocean's carbon sequestration from the atmosphere. However, the response of DOM cycles, including dissolved organic carbon (DOC) and dissolved organic phosphorus (DOP), to future climate change remains highly uncertain. Using the Community Earth System Model version 2 large ensemble simulations, we find that the C:P ratios in DOM are projected to increase by up to two‐fold in oligotrophic gyres by 2100. Increased upper ocean stratification reduces surface phosphate availability, thereby elevating phytoplankton C:P ratios and enhancing phytoplankton utilization of DOP, both acting to deprive DOM of P. Moreover, ocean stratification has a direct effect on exporting less DOC to the subsurface while accumulating more DOC at the sea surface. As a result of the strong sensitivity to ocean surface warming, the anthropogenically driven trends in upper ocean DOM concentration and its C:P ratios are estimated to emerge earlier from the simulated natural variability than upper ocean phosphate concentrations and net primary production—two key biogeochemical variables that are frequently monitored. This study suggests that changes in the C:P ratios of DOM could serve as a sensitive fingerprint of anthropogenic ocean warming, potentially exerting broad impacts on marine microbes. Our estimated 4% reduction in the globally integrated DOC export below 100 m is comparable to a 2% reduction in particulate organic carbon (POC) export by 2100, implying that global warming is likely to weaken the biological carbon pump through both DOC and POC. Plain Language Summary: Organic matter is produced by phytoplankton photosynthesis in the sunlit surface ocean. Particulate organic matter sinks to the deep ocean and decomposes back into inorganic forms over relatively short timescales. In contrast, dissolved organic matter (DOM), much smaller than particulate organic matter and formed through biogeochemical reprocessing, can be transported by ocean circulations and stored in the intermediate and deep ocean over timescales ranging from decades to millennia. Using an Earth system model, we show that DOM cycles respond sensitively to upper ocean warming through changes in biological production and uptake as well as changes in ocean stratification. Notably, the intensification of upper ocean stratification has a multiplicative effect on the carbon‐to‐phosphorus ratio of surface DOM due to its overall opposing effects on dissolved organic carbon (increasing) and dissolved organic phosphorus (decreasing). We also find that the future subsurface ocean may store less DOM, implying that more respired carbon could escape to the atmosphere through the shallow decomposition of organic carbon. Key Points: The C:P ratios in dissolved organic matter are projected to increase by up to two‐fold in low latitudes and decrease in high latitudes as of 2100Anthropogenically forced trends in C:P ratios of marine organic matter emerge earlier than those of marine net primary productionOcean stratification increases the accumulation of surface dissolved organic carbon while exporting less to the subsurface [ABSTRACT FROM AUTHOR]

Anatomically modern humans originated in Africa around 200 thousand years ago (ka).sup.1-4. Although some of the oldest skeletal remains suggest an eastern African origin.sup.2, southern Africa is home to contemporary populations that represent the earliest branch of human genetic phylogeny.sup.5,6. Here we generate, to our knowledge, the largest resource for the poorly represented and deepest-rooting maternal L0 mitochondrial DNA branch (198 new mitogenomes for a total of 1,217 mitogenomes) from contemporary southern Africans and show the geographical isolation of L0d1'2, L0k and L0g KhoeSan descendants south of the Zambezi river in Africa. By establishing mitogenomic timelines, frequencies and dispersals, we show that the L0 lineage emerged within the residual Makgadikgadi-Okavango palaeo-wetland of southern Africa.sup.7, approximately 200 ka (95% confidence interval, 240-165 ka). Genetic divergence points to a sustained 70,000-year-long existence of the L0 lineage before an out-of-homeland northeast-southwest dispersal between 130 and 110 ka. Palaeo-climate proxy and model data suggest that increased humidity opened green corridors, first to the northeast then to the southwest. Subsequent drying of the homeland corresponds to a sustained effective population size (L0k), whereas wet-dry cycles and probable adaptation to marine foraging allowed the southwestern migrants to achieve population growth (L0d1'2), as supported by extensive south-coastal archaeological evidence.sup.8-10. Taken together, we propose a southern African origin of anatomically modern humans with sustained homeland occupation before the first migrations of people that appear to have been driven by regional climate changes. Analyses of mitochondrial genomes from populations in southern Africa provide evidence of a southern African origin of anatomically modern humans and a sustained occupation of the homeland before the first migrations of people appear to be driven by regional climate shifts., Author(s): Eva K. F. Chan [sup.1] [sup.2] , Axel Timmermann [sup.3] [sup.4] , Benedetta F. Baldi [sup.1] , Andy E. Moore [sup.5] , Ruth J. Lyons [sup.1] , Sun-Seon Lee [...]

Extratropical cyclones are a dominant feature of the midlatitudes, and often occur as storm sequences. This phenomenon is known as cyclone clustering, which is common over regions like the eastern North Atlantic and western Europe. Here, intense clustered cyclones may lead to large cumulative socioeconomic impacts. There are several different approaches to quantify cyclone clustering, but a detailed evaluation on how clustering may change in a warmer climate is missing. We perform a cyclone clustering analysis for the Northern Hemisphere midlatitudes using the ERA5 reanalysis to characterize clustering during 1980–2020. Moreover, we use large ensemble simulations of the Community Earth System Model version 2 following the SSP3-7.0 scenario to compare clustering during 2060–2100 to 1980–2020. Our model simulations show significant enhancement in cyclone clustering over Europe for 3 and 4 cyclones within 7 days in the future decades, which is increasing by up to 25% on average during 2060–2100 compared to 1980–2020. In contrast, cyclone clustering decreases along the west coast of the United States and Canada by up to 24.3% and by 10.1% in the Gulf of Alaska for the same periods. In a warmer climate, clustered cyclones have lower minimum pressure and larger radii and depths compared to nonclustered events. Our findings suggest that change in future cyclone clustering depends on regions affected by global warming, with implications for the cumulative windstorm risk. Significance Statement: Storm sequences like the one of December 1999 (Anatol, Lothar, and Martin) have led to large socioeconomic impacts in Europe. It is still unclear how such events will change under global warming. We analyze storm sequences in a reanalysis and a large climate model ensemble for recent (1980–2020) and future climate conditions (2060–2100). Our results show a significant enhancement of storm sequences over Europe for 3 and 4 storms within 7 days, while a decrease is found along the west coast of the United States, western Canada, and in the Gulf of Alaska in future decades. Our findings suggest that the characteristics of cyclone clustering may change in a warmer world, and thus also the associated impacts. [ABSTRACT FROM AUTHOR]

Mountains play a vital role in shaping regional and global climate, altering atmospheric circulation and precipitation patterns. To this end, identifying projected changes in mountain precipitation is significantly challenging due to topographic complexity. This study explains how mountain precipitation could respond to rising greenhouse gases. Using a series of century‐long fully coupled high‐resolution simulations conducted with the Community Earth System Model, we aim to disentangle future changes in mountain precipitation in response to atmospheric carbon dioxide (CO2) perturbations. Our research findings indicate that the warming observed in global mountains is more pronounced when compared to the mean warming rates experienced globally and in the ocean under elevated CO2. We identify five low‐latitude mountain ranges with elevation‐dependent precipitation response, including New Guinea, East Africa, Eastern Himalayas, Central America, and Central Andes. Those mountains are expected to have a mixture of increasing and decreasing precipitation in response to CO2‐induced warming, especially over the summit and steep topography. To elucidate the mechanisms controlling future changes in mountain precipitation, we propose "Orographic moisture omega feedback" in which an increase in low‐level relative humidity enhances local precipitation by strengthening the upward motion through moist processes for the wetting response and vice versa for the drying response. The effects of Mountain precipitation changes can be extended to hydrology and could lead to significant consequences for human societies and ecosystems. Plain Language Summary: Recently, Mountain warming with increasing greenhouse warming has taken attention. However, predicting changes in mountain rainfall is hard and limitedly explored. There is a need to connect predominately idealized studies of change in orographic precipitation with warming to State‐of‐the‐art global climate model simulation (GCM). Therefore, we ran experiment simulations to study how mountain rainfall will change in the future due to the increase in atmospheric carbon dioxide. The impact of warming on mountain precipitation is complex and could result in a combination of results, such as increased and decreased precipitation. To our knowledge, this is the first kind of study that connects the change in mountain precipitation with future greenhouse warming using a 25 km high‐resolution GCM. This study found that the changes in rainfall are expected in high‐elevation mountains located in low‐latitude regions with steep slopes. This analysis concentrates on five major mountain ranges: New Guinea, East Africa, the Himalayas, Central America, and Central Andes. Furthermore, the feedback mechanism could explain how low‐level humidity affects Mountain rainfall and atmospheric convection. By studying mountain meteorology, we can gain insights into the complex interactions between mountains and the atmosphere, contributing to the overall understanding of our earth system. Key Points: This study uses a high‐resolution model experiment to explain how mountain precipitation could respond to rising atmospheric carbon dioxide (CO2) concentrationProjected precipitation changes are dominated over low‐latitude mountains, especially the summit and steep topographyWe propose a mechanism, "Orographic moisture omega feedback," that explains the unprecedented anomalous changes in the mountain climate [ABSTRACT FROM AUTHOR]