Climate Dynamics最新文献

Significant changes have occurred during the last few decades across the North Atlantic climate system, including in the atmosphere, ocean, and cryosphere. These large-scale changes play a vital role in shaping regional climate and extreme weather events across the UK and Western Europe. This review synthesizes the characteristics of observed large-scale changes in North Atlantic atmospheric and oceanic circulations during past decades, identifies the drivers and physical processes responsible for these changes, outlines projected changes due to anthropogenic warming, and discusses the predictability of these circulations. On multi-decadal time scales, internal variability, anthropogenic forcings (especially greenhouse gases), and natural forcings (such as solar variability and volcanic eruptions) are identified as key contributors to large-scale variability in North Atlantic atmospheric and oceanic circulations. However, there remain many uncertainties regarding the detailed characteristics of these various influences, and in some cases their relative importance. We therefore conclude that a better understanding of these drivers, and more accurate quantification of their relative roles, are crucial for more reliable decadal predictions and projections of regional climate for the North Atlantic and Europe.

Supplementary information: The online version contains supplementary material available at 10.1007/s00382-025-07591-1.

"Single Model initial-condition Large Ensembles" (SMILEs) conducted with Earth system models have transformed our ability to quantify internal climate variability and forced climate change at local and regional scales. An important consideration in their experimental design is the choice of initialization procedure as this influences the duration of initial-condition memory, with implications for interpreting the temporal evolution of both the ensemble-mean and ensemble-spread. Here we leverage the strategic design of the 100-member Community Earth System Model version 2 (CESM2) SMILE to investigate the dependence of ensemble spread on the method of initialization (micro- vs. macro- perturbations) and the effects of ocean initial-condition memory. We find that the evolution of ensemble spread in 10-year low-pass filtered data is relatively insensitive to the method of initialization beyond the second decade, with the notable exception of the tropical Indo-Pacific in the 4th decade, when macro-initialization significantly enhances ensemble spread, possibly as a result of a state-dependent response to major volcanic activity. Initial-condition memory associated with the chosen Atlantic Meridional Overturning Circulation (AMOC) states unfolds in two stages: First, in the North Atlantic lasting 4-5 decades, and subsequently, in the Indo-Pacific sector of the Southern Ocean appearing 35-years after initialization and lasting 3-4 decades. Known AMOC dynamics explain the first stage, but the role of AMOC and the mechanisms responsible for the delayed appearance of initial-condition memory in the Southern Ocean remain to be fully elucidated. Implications and recommendations for the design of future SMILEs are provided.

Supplementary information: The online version contains supplementary material available at 10.1007/s00382-024-07553-z.

This study investigates the impact of model resolution on simulating South Asian monsoon rainfall, focusing on the Ganges-Brahmaputra-Meghna (GBM) basin. By comparing high- and low-resolution versions of four CMIP6 HighResMIP model families against reference datasets (MSWEP and ERA5), we emphasize the advantages of high-resolution models in accurately simulating key monsoon characteristics, including annual rainfall, timing, intensity, and duration. Our results show that the high-resolution models align more closely with observed data, outperforming their low-resolution counterparts. Between 1979 and 2014, the high-resolution model ensemble (HR-models) captures key shifts in monsoon timing, such as delayed onset and withdrawal, leading to a slight increase in monsoon duration. In contrast, the low-resolution ensemble (LR-models) showed more pronounced delays in onset. The observational datasets, MSWEP and ERA5, indicate earlier (7 ± 3 days) and later (3 ± 1.2 days) onsets, respectively, with both showing delays in withdrawal, indicating extended monsoon duration. Notably, the increase in monsoon duration is more pronounced in MSWEP observations than in the model simulations, particularly for LR-models. Regarding rainfall trends, the HR-models more accurately reflect observed changes in both total rainfall and extreme rainfall from 1979-2014 compared to LR-models. Future projections (2015-2050) indicate further delays in monsoon onset, with HR-models projecting larger increases in total rainfall and extreme events (up to 4.5%/decade for the 95th percentile of rainfall) compared to LR-models, which show smaller increases and higher variability in total and extreme rainfall. These findings highlight the critical role of model resolution in improving the accuracy of monsoon simulations, with HR models offering more reliable simulations of historical monsoon behaviour and therefore likely more robust projections of future monsoon behavior. These are essential for informed water management and agricultural decision-making over the complex topography of the GBM basin.

Supplementary information: The online version contains supplementary material available at 10.1007/s00382-025-07716-6.

The last glacial period, between around 115 and 12 thousand years before present, exhibited strong millennial-scale climate variability. This includes abrupt transitions between cold and warm climates, known as Dansgaard-Oeschger (D-O) cycles. D-O cycles have been linked to switches in dynamical regimes of the Atlantic Overturning Meridional Circulation (AMOC), but the exact mechanisms behind abrupt climate changes and AMOC regime shifts remain poorly understood. This paper introduces the convection-advection oscillator mechanism to explain the millennial-scale oscillations observed in a set of HadCM3 general circulation model simulations forced with snapshots of deglacial meltwater history. The oscillator can be separated into two components acting on different time scales. The fast convection component responds to changes in vertical stratification in the North Atlantic by activating or deactivating deep water formation sites. The slow advection component regulates the accumulation and depletion of salinity in the North Atlantic. This oscillator mechanism is triggered under specific background conditions and freshwater release patterns. The freshwater perturbation causes an instability that triggers a global salt reorganisation, modifying the North Atlantic stratification. For a given forcing pattern, the system oscillates if the salt transport can lead to an alternating reactivation and deactivation of the AMOC. Otherwise, the climate settles in a warm or cold steady state. This mechanism expands existing theories of millennial-scale variability and provides a general framework for understanding abrupt climate change in general circulation models.

Supplementary information: The online version contains supplementary material available at 10.1007/s00382-025-07630-x.

Heatwaves and summer droughts across Europe are likely to intensify under anthropogenic global warming thereby affecting ecological and societal systems. To place modern trends and extremes in the context of past natural variability, annually resolved and absolutely dated climate reconstructions are needed. Here, we present a network of 153 yew (Taxus baccata L.) tree-ring width (TRW) series from 22 sites in southern England that cover the past 310 years. Significant positive correlations were found between TRW chronologies and both April-July precipitation totals (r > 0.7) and July drought indices (r > 0.59) back to 1901 CE (p < 0.05). We used a suite of residual and standard TRW chronologies to reconstruct interannual to multi-decadal spring-summer precipitation and mid-summer drought variability over western Europe, respectively. Our yew hydroclimate reconstructions capture the majority of reported summer droughts and pluvials back to 1710 CE. Clusters of severe drought spells occurred in the second half of the 18th and mid-twentieth century. Our study suggests that the frequency and intensity of recent hydroclimate extremes over western Europe are likely still within the range of past natural variability.

Supplementary information: The online version contains supplementary material available at 10.1007/s00382-025-07601-2.

Accurate seasonal prediction of the Indian Ocean Dipole (IOD) is crucial given its socioeconomic impacts on countries surrounding the Indian Ocean. Using hindcasts from the Met Office Global Seasonal Forecasting System (GloSea6), coupled mean-state biases in the western and eastern equatorial Indian Ocean (WEIO and EEIO) and their impacts on IOD prediction are examined. Results show that GloSea6 exhibits a pronounced cold bias in the EEIO that rapidly develops after the monsoon onset in boreal summer (JJA, July-August) and persists into autumn (SON, September-November). This cold bias is linked to erroneous easterlies and a shallow thermocline, likely associated with the monsoon circulation. The seasonal evolution and relative timing of the precipitation biases, such that they develop through JJA in the EEIO but follow in the WEIO in SON, suggests that the EEIO plays the leading role in the development of coupled feedbacks that lead to the large dipole pattern of coupled biases. Analysis of skill metrics for the IOD shows that GloSea6 achieves a high anomaly correlation coefficient at short lead times, though it tends to overestimate IOD amplitude. This overestimation is larger in the eastern IOD pole than in the western pole and is likely linked to the poor representation of the evolution of the sea surface temperature anomalies in the EEIO during IOD events in SON. This study highlights the crucial role of regional biases, particularly in the EEIO, in shaping IOD variability and demonstrates that addressing such biases in GloSea6 could improve IOD prediction.

The Barents-Kara Sea ice concentration (BKS) has undergone dramatic declines in recent decades, consistent with the overall reduction in sea ice across the Arctic region. There has been a long-standing scientific question whether this BKS loss significantly influences winter temperature extremes over mid-to-high latitudes. While there is ongoing debate on this point, it is generally acknowledged that BKS loss affects the stratospheric polar vortex (SPV) through the enhancement of upward propagating waves, which itself can subsequently influence surface weather and climate conditions. However, due to the large internal variability within the climate system and the limited observational data, the strength of the BKS-SPV linkage and its dependence on different background states remain unclear. In this work, we investigate the causal effect of BKS change on SPV using a climate model with large ensemble simulations. Consistent with previous literature, the results indicate that BKS loss significantly weakens the SPV, with the magnitude of the response varying with El Niño-Southern Oscillation (ENSO) and Quasi-Biennial Oscillation (QBO) phases, indicating a state-dependent causal effect. In particular, El Niño is found to suppress the causal effect of BKS change on the SPV, whereas La Niña and neutral ENSO strengthen it, which is consistent with what is found from observations. In contrast, the effect of QBO alone is relatively weak but becomes more pronounced when combined with ENSO. Dynamical analyses reveal that both tropospheric wave forcing and modulation of stratospheric wave propagation contribute to the state-dependent causal effects. By leveraging large ensemble simulations and combining statistical and physical analyses, this study provides an additional perspective on understanding the factors influencing the SPV response to BKS loss, which could ultimately impact surface climate.