[en] Climate response to low warming scenarios that meet 2015 Paris Agreement is distinct from that to the medium/high-emissions scenarios analyzed in previous studies. The present study investigates changes in the North Pacific subtropical gyre and the underlying mechanisms under 1.5 °C low warming scenario by 11-member ensemble simulations from the Community Earth System Model. Specifically, atmospheric CO₂ concentration peaks in 2036 and then decreases, with global mean surface temperature (GMST) first increasing and then stabilizing after 2045. The changes of the lower thermocline depth are consistently weaker in the subtropics than in the tropics or high latitudes through 2100. During GMST increasing stage, the subtropical circulation strengthens in the upper 1000 m north of 30° N as a result of the enhanced wind stress. When GMST stabilizes, the subtropical circulation strengthens in the surface layer (0–150 m) and the main thermocline (150–600 m) but displays insignificant change in the intermediate layer (600–1000 m). Wind and stratification changes are both important in driving the North Pacific subtropical gyre changes, because sea surface warming is much weaker in the low warming scenario than that in the medium/high-emissions scenarios. In addition, as the surface wind change displays substantial natural variability and is strongly model-dependent, it dominates the differences in projected subtropical circulation changes across models. This study highlights the importance of wind changes in projections of the subtropical circulation changes under low warming scenarios.

[en] Remote forcing from El Niño-Southern oscillation (ENSO) and local ocean–atmosphere feedback are important for climate variability over the North Tropical Atlantic. These two factors are extracted by the ensemble mean and inter-member difference of a ten-member Pacific Ocean-Global Atmosphere (POGA) experiment, in which sea surface temperatures (SSTs) are restored to the observed anomalies over the tropical Pacific but fully coupled to the atmosphere elsewhere. POGA reasonably captures main features of the observed North Tropical Atlantic variability. Both ENSO forced and local North Tropical Atlantic Modes (NTAMs) develop with wind-evaporation-SST feedback. Notable biases exist. The seasonality of the simulated NTAM is delayed by 1 month, due to the late development of the North Atlantic Oscillation (NAO) in the model. This suggests the importance of NAO in setting the seasonality of NTAM and of the extratropical-tropical teleconnection. The simulated NTAM is closely related to the Atlantic Niño in the subsequent summer, a relationship not so obvious in observations. Local variability, represented by the preseason NAO and SST persistence, contributes considerably to NTAM variability. Including these two indicators, together with ENSO, improves the predictability of NTAM. The South Tropical Atlantic Mode can be forced by ENSO, and a cross-equatorial dipole is triggered by ENSO instead of local air-sea coupling within the tropical Atlantic.

[en] The Working Group I contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) provides a comprehensive assessment of the physical science basis of climate change. It builds upon the Working Group I contribution to the IPCC's Fourth Assessment Report in 2007 and incorporates subsequent new findings from the Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation, as well as from research published in the extensive scientific and technical literature. The assessment considers new evidence of past, present and projected future climate change based on many independent scientific analyses from observations of the climate system, paleo-climate archives, theoretical studies of climate processes and simulations using climate models. During the process of scoping and approving the outline of its Fifth Assessment Report, the IPCC focussed on those aspects of the current understanding of the science of climate change that were judged to be most relevant to policy-makers. In this report, Working Group I has extended coverage of future climate change compared to earlier reports by assessing near-term projections and predictability as well as long-term projections and irreversibility in two separate chapters. Following the decisions made by the Panel during the scoping and outline approval, a set of new scenarios, the Representative Concentration Pathways, are used across all three Working Groups for projections of climate change over the 21. century. The coverage of regional information in the Working Group I report is expanded by specifically assessing climate phenomena such as monsoon systems and their relevance to future climate change in the regions. The Working Group I Report is an assessment, not a review or a text book of climate science, and is based on the published scientific and technical literature available up to 15 March 2013. Underlying all aspects of the report is a strong commitment to assessing the science comprehensively, without bias and in a way that is relevant to policy but not policy prescriptive. This report consists of a short Summary in French for Policy-makers followed by the full version of the report in English comprising a longer Technical Summary and fourteen thematic chapters plus annexes. An innovation in this Working Group I assessment is the Atlas of Global and Regional Climate Projections (Annex I) containing time series and maps of temperature and precipitation projections for 35 regions of the world, which enhances accessibility for stakeholders and users. The Summary for Policy-makers and Technical Summary of this report follow a parallel structure and each includes cross-references to the chapter and section where the material being summarised can be found in the underlying report. In this way, these summary components of the report provide a road-map to the contents of the entire report and a traceable account of every major finding