Explore the vast range of titles available.

Marine heatwaves (MHWs) and cold‐spells (MCSs) are extreme sea surface temperature events with significant impacts on marine ecosystems. However, the connection between these events and mixed layer depth (MLD) variations, as well as how their intensity relates to MLD changes, remains unclear. Integrating OISST V2.1 data with Argo profiles, this analysis finds that during MHWs, MLD decreases by 8.10% globally, while during MCSs, it increases by 8.13%. In 5° × 5° bins, 80.46% of ocean regions show MLD shallowing during MHWs, while 67.69% show deepening during MCSs. A significant global correlation between the intensity of MHWs/MCSs and MLD changes, with coefficients of −0.85 and −0.86, respectively. MHWs are more common in mesoscale anticyclonic eddies (AEs) (19.45%) than in cyclonic eddies (CEs) (10.11%). For MCSs, the pattern reverses, with 8.57% in AEs and 20.82% in CEs. Restratification and mesoscale eddies are two important factors driving MLD changes during these events. Plain Language Summary Marine heatwaves (MHWs) involve prolonged periods of sea surface temperatures (SSTs) above the 90th percentile of the climatological threshold, while marine cold‐spells (MCSs) involve SSTs below the 10th percentile. MHWs and MCSs both significantly impact marine ecosystems, particularly fragile coral reef ecosystems. A substantial amount of literature currently examines the characteristics of MHWs/MCSs, such as their frequency, duration, and cumulative days. However, the relationship between MHWs/MCSs and internal oceanic factors like mixed layer depth (MLD) variation is not fully understood. By combining OISST V2.1 data with Argo profiles, this study finds that MHWs are linked to significant MLD shallowing compared to background values. There is also a strong, statistically significant correlation between MHW intensity and the degree of MLD shallowing, at the 99% confidence level. In contrast, during MCSs, the MLD typically deepens relative to the climatological background. However, the degree of this deepening varies regionally with MCS intensity. Interestingly, MLD shallowing is observed during MCSs when the intensity exceeds −2.4°C. Further analysis indicates that mesoscale eddies and restratification are two mechanisms driving the variation in MLD during MHWs and MCSs. Key Points During marine heatwaves (MHWs), the mixed layer depth (MLD) shallows by 8.10% on average globally, while during MCSs, it deepens by 8.13% A significant correlation between the intensity of MHWs/MCSs and the relative change ratio of MLD Mesoscale eddies occurring alongside MHWs/MCSs can modify the usual trend of MLD shallowing during MHWs and deepening during MCSs

Reanalysis data reveals that the South China Sea (SCS) experiences intensified subsurface marine heatwaves (MHWs) and marine cold spells (MCSs) near the thermocline. On the interannual scale, surface and subsurface events demonstrate an opposite correlation with ENSO, primarily due to distinct drivers at different depths. During the developing phase of El Niño, the Luzon Strait transport, indicative of the SCS throughflow (SCSTF), increases and necessitates stronger upwelling for mass balance. This upwelling, acting on large subsurface vertical temperature gradients, induces significant subsurface cooling and the occurrence of MCSs. As El Niño matures, excessive atmospheric heat flux warms the surface layer, triggering surface MHWs. This heat is then transported downward mainly by vertical turbulent mixing, diminishing subsurface cooling, and terminating subsurface MCSs. The scenario reverses during La Niña events. The SCSTF serves as a vital oceanic pathway, transmitting ENSO signals into the SCS and profoundly modulating subsurface MHWs and MCSs.

Despite 2023 being the warmest year in the past 45 years, northern Europe experienced an extreme cold spell in mid-November, followed by a record-breaking cold spell in East Asia in early December. This study utilized various reanalysis datasets and a numerical model to investigate the mechanisms behind these consecutive extreme cold spells over Eurasia and whether these mechanisms can explain similar cases from 1979 to 2022. Our analysis reveals that these extreme cold spells are associated with increased snow cover over Northern Europe. Increased snow cover in early winter affects the energy balance, leading to a local cyclonic anomaly that promotes cold spells in Northern Europe. This anomaly then moves eastward, causing cold air masses to gather and strengthen in Siberia, which contributes to extreme cold spells in East Asia. Additionally, intensified upward tropospheric planetary waves due to increased snow cover weaken the stratospheric polar vortex, reducing its constraint on cold air and facilitating subsequent extreme cold spells in East Asia. Statistical analysis indicates that approximately 61.5% of the consecutive extreme cold spells over the past 44 years are influenced by these mechanisms. Furthermore, simulations with increased snow cover over Northern Europe show a higher probability of consecutive cold spells over Eurasia, supporting these findings.

This study investigates changes in linkages between atmospheric blocking and winter (December–February) cold spells over the Pacific-North America region in two large-ensembles of Canadian Earth System Models (CanESM2 and CanESM5 under high-emission scenarios). The two ensembles show decreases in winter blocking frequency over the North Pacific from 1981–2010 baseline to 2071–2100, with larger decreases in CanESM5 (− 3.08%/decade) than CanESM2 (− 1.73%/decade). Using a time-invariant (stationary) threshold estimated from the baseline to define cold days, the two ensembles project a decline in cold spell events as future air temperature increases; the occasional occurrence of cold spell events is still projected to occur at the end of the century. Using a time-dependent (nonstationary) climatological threshold, CanESM2 and CanESM5 ensembles project modest decreases in cold spell days over North America (− 2.0 and − 2.3%/decade). With the nonstationary threshold, the two ensembles project decreases in winter cold spell frequency during blocking, with larger decreases in CanESM5 (13%) than CanESM2 (3%) for 2071–2100 period compared to the baseline. The two ensembles display similar blocking-cold spell linkages between the baseline and future periods; however, the linkage is weaker and exhibits larger uncertainty in the future. Moreover, temperature advection and net heat flux anomalies during blocking are generally weaker for the future period, resulting in weaker impacts on North American cold spells with larger uncertainty associated with increases in internal-variability.

Marine cold-spells (MCSs), the cold ocean temperature extremes as the counterpart to marine heatwaves (MHWs), may have brutal impacts on marine ecosystems and fisheries. However, recent MCS-related researches mainly focused on the ocean surface, while the character of bottom MCSs (BMCSs) remains unclear. Here, we provide an analogous assessment of BMCSs and surface MCSs (SMCSs) over the continental shelf of China based on a 1/12° ocean reanalysis dataset during period of 1993–2023. We found that the duration and cumulative intensity of BMCSs can be longer and more intense than SMCSs, up to 20 d and ‒20 °C days, respectively. The mean and cumulative intensities of BMCSs and SMCSs increased in most regions, reaching 0.2 °C–0.4 °C/10a and 20 °C–40 °C days/10a, respectively. In contrast, there are no clear trends in the area-averaged time series of BMCS/SMCS metrics over the period 1993–2023. In terms of category and seasonal variations, BMCS generally has longer days than SMCS in the Moderate and Strong categories, persists longer than SMCS in summer and autumn (ranging from 2–6 d), and is more intense than SMCS in autumn and winter (ranging from 1 °C–2 °C). There are clear differences in category and seasonal changes between BMCS and SMCS. Furthermore, annual BMCS days are positively correlated with bottom depth, while mean intensity is negatively correlated. In addition, BMCS and SMCS often co-occur over shallow areas where the mixed layer depth can extend to the bottom.

Due to the significant negative consequences of winter cold extremes, there is need to better understand and simulate the mechanisms driving their occurrence. The impact of atmospheric blocking on winter cold spells over North America is investigated using ERA-Interim and NCEP-DOE-R2 reanalyses for 1981–2010. Initial-condition large-ensembles of two generations of Canadian Earth System Models (CanESM5 and its predecessor, CanESM2) are evaluated in terms of their ability to represent the blocking-cold spell linkage and the associated internal-variability. The reanalysis datasets show that 72 and 58% of cold spells in southern and northern North America coincide with blocking occurring in the high-latitude Pacific-North America. Compared to the two reanalyses, CanESM2 and CanESM5 ensembles underestimate by 19.9 and 14.3% cold spell events coincident with blocking, due to significant under-representation of blocking frequency over the North Pacific (− 47.1 and − 29.0%), whereas biases in cold spell frequency are relatively small (6.6 and − 4.7%). In the reanalyses, regions with statistically significant above-normal cold spell frequency relative to climatology lie on the east and/or south flanks of blocking events, whereas those with below-normal frequency lie along the core or surrounding the blocking. The two ensembles reproduce the observed blocking-cold spell linkage over North America, despite underestimating the magnitude of blocking frequency. The two ensembles also reproduce the physical drivers that underpin the blocking-cold spell linkage. Spatial agreement with the reanalyses is found in the simulated patterns of temperature advection and surface heat flux forcing anomalies during blocking events. While CanESM5 shows an improved representation of the blocking climatology relative to CanESM2, both yield similar results in terms of the blocking-cold spell linkage and associated internal-variability.

Heatwave and cold spell have been linked to cardiovascular disease (CVD) mortality. However, due to the varying definitions of heatwave and cold spell, their impacts on CVD mortality are inconsistent. METHODS: A time series study in Hunan province, central of China, from 2018 to 2022, was conducted to test the relationship between heatwave, cold spell and CVD mortality. According to different percentiles of daily mean temperatures and exposure duration, we built 9 kind of definitions for heatwave and cold spell. Distributed lag non-linear model was used to analyze the associations between heatwave, cold spell and CVD mortality, and the attributable fraction (AF) were estimated. RESULTS: The relative risks of CVD mortality associated with heatwave and cold spell varied depending on the definitions, ranging from 1.154 (95% CI: 1.148–1.160) to 1.229 (95% CI: 1.215–1.243) for heatwaves, and from 1.196 (95% CI: 1.192–1.201) to 1.290 (95% CI: 1.282–1.297) for cold spells. Under the definition of 95th percentile with ≥ 4-d duration (P95_4d), the total AF of CVD mortality attributable to heatwave was the largest at 8.43 (95% CI: 7.92–8.94). For the definition of 5th percentile with ≥ 3-d duration (P5_3d), the total AF attributable to cold spell was the largest at 12.96 (95% CI: 12.64–13.28). For heatwave and cold spell, higher CVD mortality risks were observed in females and the elderly over 75 years than males and young people. DISCUSSION: We found that both heatwave and cold spell could increase the mortality risk of CVD. The results highlight the importance of implementing warning systems for extreme temperature.

With climate change posing increasing threats and aging populations, understanding the complex relationship between extreme temperatures, PM 2.5 pollution, and respiratory health among the elderly is crucial. While some research exists, there remains a significant gap in studying the combined effects of heat waves, cold spells, and PM 2.5 on elderly respiratory health in high-altitude regions. We collected data from Xining (2016–2021), including respiratory disease outpatient visits, meteorological, and pollutant data. Employing a case-crossover design and conditional Poisson regression analysis, we investigated the individual and interactive impacts of heat wave, cold spell, and PM 2.5 on outpatient visits for respiratory disease among the elderly. We used the relative excess odds due to interaction (REOI), proportion attributable to interaction (AP), and synergy index (S) as quantitative indicators of interaction. Our analysis revealed significant associations between heat wave, cold spell, PM 2.5 exposure, and outpatient visits for respiratory disease among the elderly, with odds ratios of 1.10 (95%CI: 1.06, 1.15) and 1.16 (95%CI: 1.13, 1.20), respectively. Moreover, a synergistic effect between cold spell and PM 2.5 was observed, particularly affecting vulnerable groups such as female and those aged ≥ 80. The combined exposure to cold spell and elevated PM 2.5 levels was estimated to contribute to up to 0.18 (95%CI: 0.17, 0.27) of respiratory outpatient visits. This study underscores the need for urgent interventions, such as reducing PM 2.5 exposure and enhancing extreme weather warning systems, to protect the respiratory health of the elderly, especially in high-altitude regions.

Extreme water temperatures impact the ecological and economic value of freshwater systems. They disrupt fisheries habitat, trigger harmful algal blooms, and stress coastal infrastructure. This study examines the spatiotemporal patterns of heatwaves and cold‐spells in the Great Lakes using 82 years of simulated surface temperature data. Significant increasing trends in heatwave duration were observed in Lake Superior and Lake Michigan‐Huron, while cold‐spell duration increased on all lakes except Ontario. Temperature anomalies during these events varied from the climatological mean by as much as ±10° ^{\\circ}$C, but did not change significantly over time. Analysis revealed substantial spatial variability in heatwaves and cold‐spells, both within and across lakes, with differences driven by air temperature and ice cover anomalies as well as associated climate teleconnections (i.e., the East Pacific/North Pacific and Atlantic Multidecadal Oscillation). These findings highlight the importance of both climatic and lake processes in shaping extreme temperature events.

This study analyzes heat waves (HWs), cold spells (CSs), and mean temperature trends in Türkiye’s three major metropolises (Istanbul, Ankara, and Izmir) using long-term station data. HW and CS events were defined via a percentile-based threshold approach, utilizing daily maximum (Tmax) and minimum (Tmin) temperature data from a total of 15 meteorological stations. Temporal trends in annual and seasonal wave frequencies, alongside mean temperature series, were evaluated using the Mann–Kendall test and Sen’s slope estimator. The findings indicate that HW frequencies have significantly increased across the majority of stations, whereas CS frequencies have decreased at most locations. It was determined that while HWs predominantly concentrate in summer and CSs in winter, heat extremes can extend into transitional seasons. Mean temperatures exhibit a statistically significant upward trend across all stations. Furthermore, HWs have become more prominent and CSs have dissipated more rapidly in urban and coastal stations. These results reveal that the risk of heat extremes is escalating while cold extreme events are weakening in Türkiye’s major cities due to warming climate conditions.