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The increasing frequency of heatwaves over Southeast Asia (SEA) is impacting human health, infrastructure and economies. Understanding the relationship between large‐scale circulation patterns and heatwaves is crucial for improving predictions and reducing damages. In this study, four distinct circulation patterns conducive to SEA heatwaves are identified by the self‐organizing map. Three circulation patterns are associated with high‐pressure systems over the mid‐latitude Western Pacific and SEA, significantly influenced by El Niño–Southern Oscillation (ENSO). In addition to these common high‐pressure patterns, a low‐pressure dominant pattern is identified, characterized by two enhanced low‐pressure systems over the Tibetan Plateau and mid‐latitude Western Pacific, which deplete a considerable amount of moisture, resulting in diminished cloud cover and rapid warming across continental SEA. Further analysis reveals that all patterns are significantly modulated by Madden Jullian Oscillation (MJO) activities, each showing distinct peak occurrences in different MJO phases, with further links to ENSO and the Indian Ocean Dipole. Plain Language Summary Heatwaves in Southeast Asia are becoming more frequent, damaging infrastructure and hurting the economy. Understanding how large weather patterns contribute to heatwaves can improve prediction and reduce their impacts. This study identifies four specific weather patterns that lead to heatwaves in Southeast Asia using the self‐organizing map method. Of these, three patterns are associated with the high‐pressure systems over the mid‐latitude Western Pacific and Southeast Asia, which contribute to 85.4% of total regional heatwave events and are affected by the El Niño–Southern Oscillation. These patterns can be summarized as the “uniform high” pattern, the “southern high and northern low” pattern and the “southern low and northern high” pattern, respectively. Besides the common high‐pressure dominant patterns, we also find a low‐pressure dominant pattern that causes heatwaves. This pattern involves two strong low‐pressure systems over the Tibetan Plateau and the mid‐latitude Western Pacific, which reduce moisture and cloud cover, leading to rapid warming in continental Southeast Asia. This low‐pressure pattern mostly happens during active phases 3–4 of the Madden‐Julian Oscillation and is modulated by the Indian Ocean Dipole. This work offers important insights into the physical mechanisms behind heatwaves and holds potential values for improving the forecasting of heatwaves in Southeast Asia. Key Points Four distinct circulation patterns conducive to Southeast Asia heatwaves are identified by self‐organizing map Three circulation patterns are linked to high‐pressure dominance, influenced by ENSO and could trigger extensive heatwaves over entire SEA A low‐pressure dominant pattern is also identified, modulated by Indian Ocean Dipole and Madden Jullian Oscillation, and tends to trigger intense heatwaves over continental SEA

Along with the amplified warming and dramatic sea ice decline, the Arctic has experienced regionally and seasonally variable moistening of the atmosphere. Based on reanalysis data, this study demonstrates that the regional moistening patterns during the last four decades, 1979–2018, were predominantly shaped by the strong trends in horizontal moisture transport. Our results suggest that the trends in moisture transport were largely driven by changes in atmospheric circulation. Trends in evaporation in the Arctic had a smaller role in shaping the moistening patterns. Both horizontal moisture transport and local evaporation have been affected by the diminishing sea ice cover during the cold seasons from autumn to spring. Increases in evaporation have been restricted to the vicinity of the sea ice margin over a limited period during the local sea ice decline. For the first time we demonstrate that, after the sea ice has disappeared from a region, evaporation over the open sea has had negative trends due to the effect of horizontal moisture transport to suppress evaporation. Near the sea ice margin, the trends in moisture transport and evaporation and the cloud response to those have been circulation dependent. The future moisture and cloud distributions in the Arctic are expected to respond to changes in atmospheric pressure patterns; circulation and moisture transport will also control where and when efficient surface evaporation can occur.

This study identifies diverse synoptic weather patterns of warm-season heavy rainfall events (HREs) in South Korea. The HREs not directly connected to tropical cyclones (TCs) (81.1%) are typically associated with a midlatitude cyclone from eastern China, the expanded North Pacific high, and strong southwesterly moisture transport in between. They are frequent both in the first (early summer) and second rainy periods (late summer) with impacts on the south coast and west of the mountainous region. In contrast, the HREs resulting from TCs (18.9%) are caused by the synergetic interaction between the TC and meandering midlatitude flow, especially in the second rainy period. The strong south-southeasterly moisture transport makes the southern and eastern coastal regions prone to the TC-driven HREs. By applying a self-organizing map algorithm to the non-TC HREs, their surface weather patterns are further classified into six clusters. Clusters 1 and 3 exhibit a frontal boundary between the low and high with differing relative strengths. Clusters 2 and 5 feature an extratropical cyclone migrating from eastern China under different background sea level pressure patterns. Cluster 4 is characterized by the expanded North Pacific high with no organized negative sea level pressure anomaly, and cluster 6 displays a development of a moisture pathway between the continental and oceanic highs. Each cluster exhibits a distinct spatiotemporal occurrence distribution. The result provides useful guidance for HRE prediction by depicting important factors to be differently considered depending on their synoptic categorization.

This paper showed the frequency of local-scale heavy winter snowfall in Hokkaido, Japan, its historical change, and its response to global warming using self-organizing maps (SOM) of synoptic-scale sea level pressure anomaly. Heavy snowfall days were here defined as days on which the snowfall exceeded 10mm in water equivalent. It was shown that the SOMs can be grouped into three categories for heavy snowfall days: 1) a passage of extratropical cyclones to the south of Hokkaido, 2) a pressure pattern between the Siberian high and the Aleutian low, and 3) a low pressure anomaly just to the east of Hokkaido. Groups 1 and 2 were associated with heavy snowfall in Hiroo (located in southeastern Hokkaido) and in Iwamizawa (western Hokkaido), respectively, and heavy snowfall in Sapporo (western Hokkaido) was related to group 3. The large-ensemble historical simulation reproduced the observed increasing trend in group 2, and future projections revealed that group 2 was related to a negative phase of the western Pacific pattern and that the frequency of this group would increase in the future. Heavy snowfall days associated with SOM group 2 would also increase as a result of the increase in water vapor and preferable weather patterns in a globally warming climate, in contrast to the decrease of heavy snowfall days at other sites associated with SOM group 1.

Using clustering analysis for the sea level pressure field of the ERA-Interim reanalysis between 1979 and 2016, five synoptic pressure patterns have been obtained for the Drake area and Antarctic Peninsula (AP) region (45°–75°S, 20°–120°W), and the resulting daily series has been made available to the scientific community. The five patterns have been named according to their most important features as follows: low over the Weddell Sea (LWS), low over the Amundsen and Bellingshausen Seas (LAB), low over the Drake Passage (LDP), zonal flow over the Drake Passage (ZDP), and ridge over the Antarctic Peninsula (RAP). Each atmospheric pattern is described after analyzing its development and evolution. A frequency analysis shows that the five atmospheric patterns present a similar annual frequency but a large seasonal variability. The transitions from one pattern to another tend to follow a cycle in which synoptic atmospheric waves are displaced eastward by a quarter wavelength. Four of the five atmospheric patterns (all except RAP) are very influenced by the southern annular mode (SAM); however, only LAB and LWS are influenced to some degree by ENSO. The occurrence of the LAB pattern presents a positive trend showing agreement with other studies that indicate an enhancement of the Amundsen–Bellingshausen Seas low. Finally, atmospheric circulation patterns have been related to the airmass advection and precipitation in Livingston Island, showing the potential application for studying the changes in the surface mass balance on the AP cryosphere.

The timing of melt onset in the Arctic plays a key role in the evolution of sea ice throughout Spring, Summer and Autumn. A major catalyst of early melt onset is increased downwelling longwave radiation, associated with increased levels of moisture in the atmosphere. Determining the atmospheric moisture pathways that are tied to increased downwelling longwave radiation and melt onset is therefore of keen interest. We employed Self Organizing Maps (SOM) on the daily sea level pressure for the period 1979–2018 over the Arctic during the melt season (April–July) and identified distinct circulation patterns. Melt onset dates were mapped on to these SOM patterns. The dominant moisture transport to much of the Arctic is enabled by a broad low pressure region stretching over Siberia and a high pressure over northern North America and Greenland. This configuration, which is reminiscent of the North American-Eurasian Arctic dipole pattern, funnels moisture from lower latitudes and through the Bering and Chukchi Seas. Other leading patterns are variations of this which transport moisture from North America and the Atlantic to the Central Arctic and Canadian Arctic Archipelago. Our analysis further indicates that most of the early and late melt onset timings in the Arctic are strongly related to the strong and weak emergence of these preferred circulation patterns, respectively.

The IANOS Medicane was one of the most severe storms that have formed in the Mediterranean Sea with Category 2 Hurricane characteristics. The storm induced a significant increase in sea-level elevation along its pathway and caused storm surges at the central Ionian Sea with consequent impacts on coastal regions of the Ionian Islands and western Greece. An integrated approach, based on hydrodynamic ocean simulations, coupled to meteorological and coastal flooding simulations, is used in combination with field and satellite observations to analyze the marine weather conditions, the storm surge characteristics, and the coastal inundation characteristics due to the impact of IANOS Medicane in September 2020. The evolution of the Medicane and the respective storm surge in the ocean have been successfully recorded by the met-ocean simulations, part of an active public-access operational forecast system. Both wind and atmospheric pressure patterns affected the storm surge variability over the Ionian Sea. The direct intrusion of the Medicane from the central Mediterranean Sea toward the Ionian Sea formed storm surges over several coastal areas, even before the storm’s landfall, due to the accompanying onshore currents. Storm surges in the order of 30 cm generated extensive flooding over lowland coastal areas, as confirmed by both satellite (Normalized Difference Water Index, NDWI) and numerical (coastal inundation modeling) data. Satellite-derived and simulated estimations showed that, in specific coastal regions, the run-up of seawater extended up to 200 m inland, depending on the hydraulic connectivity between the lowland areas, which determined the inundation extents during the storm surge.

The Mistral and Tramontane are important wind phenomena that occur over southern France and the northwestern Mediterranean Sea. Both winds travel through constricting valleys before flowing out towards the Mediterranean Sea. The Mistral and Tramontane are thus interesting phenomena, and represent an opportunity to study channeling effects, as well as the interactions between the atmosphere and land/ocean surfaces. This study investigates Mistral and Tramontane simulations using five regional climate models with grid spacing of about 50 km and smaller. All simulations are driven by ERA-Interim reanalysis data. Spatial patterns of surface wind, as well as wind development and error propagation along the wind tracks from inland France to offshore during Mistral and Tramontane events, are presented and discussed. To disentangle the results from large-scale error sources in Mistral and Tramontane simulations, only days with well simulated large-scale sea level pressure field patterns are evaluated. Comparisons with the observations show that the large-scale pressure patterns are well simulated by the considered models, but the orographic modifications to the wind systems are not well simulated by the coarse-grid simulations (with a grid spacing of about 50 km), and are reproduced slightly better by the higher resolution simulations. On days with Mistral and/or Tramontane events, most simulations underestimate (by 13 % on average) the wind speed over the Mediterranean Sea. This effect is strongest at the lateral borders of the main flow—the flow width is underestimated. All simulations of this study show a clockwise wind direction bias over the sea during Mistral and Tramontane events. Simulations with smaller grid spacing show smaller biases than their coarse-grid counterparts.

Interactions between upper plate deformation and plate interface seismicity in subduction zones remain poorly understood, but growing evidence indicates that fluid flow along splay faults modulates upper‐plate faulting. Field observations from two exhumed splay faults define end‐member scenarios where impermeable faults trap fluids in their footwall, whereas permeable faults channel fluids along them. Using finite element poroelastic models, with slip mode inferred solely from stress–pore‐fluid pressure patterns, we define two end‐member behaviors: (a) Impermeable, clay‐rich, mature splay faults favor footwall fluid flow, promoting low differential stress, dilation, and vein formation in the upper plate, while reducing pore‐fluid pressure and coupling the megathrust downdip of the intersection. (b) Permeable, less mature faults allow distributed upper‐plate fluid flow, increasing fluid flux, and differential stress, while maintaining plate interface overpressure and promoting creep. These models provide a framework for prism‐scale effects of splay fault permeability on shallow subduction zone deformation and seismicity.

Spatially dense surface pressure observations from personal weather stations (PWSs) are able to describe pressure patterns at the surface, such as those associated with convective events, in more detail than with standard weather stations (SWSs) only. In this study, the benefit of assimilating PWS observations with the 3DVar and the 3DEnVar data assimilation schemes of the AROME-France model is evaluated over a 1-month period and during a heavy precipitation event in the South of France. Observations of surface pressure from PWSs are bias-corrected, quality-controlled, and thinned with a spacing equal to the horizontal dimension of an AROME-France grid cell. Over France, almost half of the 55 187 available PWS observations are assimilated, which is 129 times more than the number of assimilated SWS observations. Despite the limited advantages found from their assimilation with the 3DVar assimilation scheme, the 3DEnVar assimilation scheme shows systematic improvement and reduces by 10.3 % the root-mean-square deviation in surface pressure between 1 h model forecasts and SWS observations over France. Significant improvement is observed over the first 9 h of the forecasts in mean sea level pressure. Finally, when PWS observations are assimilated with the 3DEnVar assimilation scheme, a surface pressure anomaly generated by a mesoscale convective system – observed by PWSs and not visible without them – is successfully assimilated. In that case, the forecasts of location and temporal evolution of the mesoscale convective system as well as rainfall are closer to the observations when PWS observations are assimilated.