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Vb cyclones typically emerge in the Western Mediterranean and propagate to the Northeast into Central Europe. This paper explores the temporal characteristics of Vb cyclone occurrence based on cyclone tracks identified at the atmospheric levels of Z700 and sea level pressure, using JRA‐55 reanalysis data for the period 1959–2015. The risk of Vb occurrence was significantly high in the 1960s and has remained at a lower level since then. Vb cyclones do not occur fully randomly according to a Poisson point process. Eleven well‐separated and distinct clusters as well as 11 hiatus periods are identified, with average occurrence rates of 21.5 and 5.2 yrea−1, respectively. During the event of Vb, the large‐scale atmospheric circulation is changed into a state favoring the development of successive Vb cyclones. Clustering is very prominent in the case of Genoan Vb cyclones in summer as well as those Vb cyclones developing over the Iberian Peninsula or the North African Coast in winter. Superposition of the polar and the subtropical jet stream over the Western Mediterranean is identified as a main feature at the onset of Vb cyclones. Vb cyclone occurrence appears to be synchronized with the Northern Atlantic Oscillation (NAO; at Z500) and Arctic Oscillation (AO; at Z1000). Clusters have occurred when both NAO and AO were negative. This relation applies to Western Mediterranean cyclones not following a Vb track as well, however to a much weaker extent. In contrast, Vb cyclone frequency was particularly low from 1988 to 1997 during a sustained positive phase of both NAO and AO., Key Points In the past 50 years, the frequency of Vb cyclones was high in the 1960s and lower since thenVb cyclones were strongly synchronized with NAO and AO; high frequency clusters occurred when NAO and AO were negativeThe coupling of the polar and the subtropical jet stream over the Western Mediterranean is a driving mechanism of the onset of Vb cyclones

Californian hydroclimate is strongly seasonal and prone to severe water shortages. Recent changes in climate trends have induced shifts in seasonality, thus exacerbating droughts, wildfires, and adverse water shortage effects on the environment and economy. Previous studies have examined the timing of the seasonal cycle shifts mainly as temperature driven earlier onset of the spring season. In this paper, we address quantitative changes in the onset, amounts, and termination of the precipitation season over the past 6 decades, as well as the large-scale atmospheric circulation underpinning the seasonal cycle changes. We discover that the onset of the rainy season has been progressively delayed since the 1960s, and as a result the precipitation season has become shorter and sharper in California. The progressively later onset of the rainy season is shown to be related to the summer circulation pattern extending into autumn across the North Pacific, in particular, a delay in the strengthening of the Aleutian Low and later southward displacement of the North Pacific westerlies. Plain Language Summary The rainy season over California is projected to show a distinct sharpening of the mean seasonal cycle, with winter precipitation increasing, and both autumn and spring precipitation decreasing. Our analysis of the past 6 decades of data for California suggests autumn decrease is already underway. A delayed start of the rainy season of 27 days since 1960s can exacerbate seasonal droughts and prolong the wildfire season. This delay occurs due to a number of conditions that controls precipitation: the summer circulation pattern has been extending throughout November across the North Pacific, and the wintertime strengthening of the Aleutian Low is delayed. Accordingly, the southward migration of the North Pacific jet stream as well as extratropical storm tracks, which marks the start of the California rainy season, are delayed. More work, using climate models, will be needed to provide a better understanding of atmospheric conditions across the North America and the North Pacific. However, our findings provide observational evidence for the projected rainfall change over California and inform ongoing discussion about the drying/wetting tendencies of the rainy season in California.

This paper examines the characteristics of the heatwave that affected western and central Europe in June 2017. Using a novel algorithm, we show that its extension, intensity, and persistence were comparable to those of other European mega‐heatwaves, but it occurred earlier in the summer. The most affected area was Iberia, which experienced devastating forest fires with human casualties and the warmest temperatures of the reanalysis period from daily to seasonal scales. The peak of the mega‐heatwave displayed an unprecedented warm air intrusion due to a record‐breaking subtropical ridge with signatures closer to those of July and August. The atmospheric circulation was the main triggering factor of the event. However, thermodynamical changes of the last decades made a substantial contribution to the event, by increasing the likelihood of surpassing high‐temperature thresholds. This episode could be a good example of a coming future, with high‐summer mega‐heatwaves occurring earlier., This work was supported by the Ministerio de Economía y Competitividad through the PALEOSTRAT (CGL2015‐69699‐R) and ISIPEDIA (ERA4CS) (PCIN‐2017‐046) projects. Ricardo M. Trigo and Pedro M. Sousa are supported by project IMDROFLOOD: Improving Drought and Flood Early Warning, Forecast and Mitigation using real‐time hydroclimatic indicators, JPND‐WaterJPI/0004/2014, financed by FCT/MCTES (PIDDAC). A. Sánchez‐Benítez was funded by grant FPU15/03958 from the Ministerio de Educación, Cultura y Deporte (MECD).

[1] This paper investigates the effect of Last Glacial Maximum (LGM) versus high CO2 world boundary condition on the Southern Hemisphere atmospheric circulation, in particular on the strength and latitudinal position of the near surface Southern Westerly Winds (SWW). PMIP2 and PMIP3 experiments, as well as the “abrupt 4 × CO2” simulations from CMIP5, were analyzed. Robust findings include poleward expansion of the Mean Meridional Circulation (MMC) and intensified and poleward-shifted SWW in the 4 × CO2 simulations (consistent with recent observations and 21st century climate change projections); and for the LGM, stronger and southward shifted northern hemisphere MMC, and weakened southern Hadley cell. However, six of the eight LGM simulations show a decrease in the SWW, the other two models simulate the opposite. A critical difference between the models is strong coupling between sea-ice extent, surface temperature gradients, SWW, and Ferrel cell in the two models with stronger and poleward-shifted SWW.

In this paper we present a detailed study of methanesulfonic acid (MSA) and non-sea-salt sulfate (nss-SO4) content in recent south polar precipitation. These two species are major oxidation products of dimethylsulfide (DMS) which is mainly produced by marine biota. The nss-SO4 content of south polar snow layers deposited over the 1922–1984 time period remained rather stable except short-term increases after the Mt Agung (1963) and the El Chichon (1982) eruptions. At the opposite, the MSA profile shows over the same time period several sporadic, 0.5 to 2 years, increases (2 to 10 times background level). Taking into account an uncertainty of 1–3 years in the dating of snow layers, all these MSA events seem to be correlated to major ENSO (El Nino-Southern oscillation) events having occured over the last sixty years (1925–26, 1941, 1957–58, 1972–73 and 1982–83). The relatively high MSA/nssSO4 weight ratios (R) observed in the corresponding snow layers suggest that these events reflect enhanced DMS emissions from the antarctic ocean. This suggested connection between ENSO and high DMS marine emissions at high southern latitudes is discussed in terms of atmospheric and oceanic circulation.

[1] Arctic Oscillation (AO) is a seesaw pattern in which sea level pressure (SLP) at the polar and middle latitudes in North Hemisphere fluctuates between positive and negative phases. It has been used as a representative atmospheric circulation index to express climate change. The purpose of this paper is to reveal the regional difference of SLP variation. By calculating the correlation coefficients of AO Index with all the gridded SLPs, we have uncovered a special region named as Arctic Oscillation Core Region (AOCR), where the running correlation coefficients (RCC) between gridded SLP and AO index are all negative. The averaged SLP of this region correlates significantly with the AO index. The correlations between local SLPs and AO index outside of AOCR are weaker than those inside. RCC analysis reveals several strong discrepant events different from AO. These events occurred in the years of 1954, 1955, 1962, 1971, 1982/83, 1995, 1996/97, 1998, and 1999. A comparison of these events and the ENSO or PDO indices suggests that the events in 1982/83 and 1998 are probably associated with the ENSO processes. Events centered in other years are likely connected with PDO, which reached their minima in the years of 1950, 1955, 1962 and 1971. The result in this study provides an alternative insight to look at the mechanism of the variation of Arctic Oscillation.

The increasing demand by the scientific community, policy makers, and the public for realistic projections of possible regional impacts of future climate changes has rendered the issue of regional climate simulation critically important. The problem of projecting regional climate changes can be identified as that of representing effects of atmospheric forcings on two different spatial scales: large-scale forcings, i.e., forcings which modify the general circulation and determine the sequence of weather events which characterize the climate regime of a given region (for example, greenhouse gas abundance), and mesoscale forcings, i.e., forcings which modify the local circulations, thereby regulating the regional distribution of climatic variables (for example, complex mountainous systems). General circulation models (GCMs) are the main tools available today for climate simulation. However, they are run and will likely be run for the next several years at resolutions which are too coarse to adequately describe mesoscale forcings and yield accurate regional climate detail. This paper presents a review of these approaches. They can be divided in three broad categories: (1) Purely empirical approaches, in which the forcings are not explicitly accounted for, but regional climate scenarios are constructed by using instrumental data records or paleoclimatic analogues; (2) semiempirical approaches, in which GCMs are used to describe the atmospheric response to large-scale forcings of relevance to climate changes, and empirical techniques account for the effect of mesoscale forcings; and (3) modeling approaches, in which mesoscale forcings are described by increasing the model resolution only over areas of interest. Since they are computationally inexpensive, empirical and semiempirical techniques have been so far more widely used. Their application to regional climate change projection is, however, limited by their own empiricism and by the availability of data sets of adequate quality. More recently, a nested GCM-limited area model methodology for regional climate simulation has been developed, with encouraging preliminary results. As it is physically, rather than empirically, based, the nested modeling framework has a wide range of applications.

Atmospheric CO2 measurements made with a nondispersive infrared analyzer during 1974–1985 at Mauna Loa Observatory, Hawaii, are described, with emphasis on the measurement methodology, calibrations, and data accuracy. Monthly mean CO2 data, representative of global background conditions, are presented for the period of record. The monthly means were derived from an all-data base of CO2 hourly averages archived at the National Oceanic and Atmospheric Administration (NOAA) Geophysical Monitoring for Climatic Change (GMCC) facility in Boulder, Colorado; at the Carbon Dioxide Information Analysis Center (CDIAC) in Oak Ridge, Tennessee; and in the microfiche version of this paper. Flags in the all-data base identify CO2 hourly averages that have been deemed unreliable because of sampling and analysis problems or that are unrepresentative of clean background air because of influences of the local environment, for example, CO2 uptake by nearby vegetation or contamination and pollution effects. The select NOAA GMCC monthly mean data are compared with similar data obtained independently at Mauna Loa Observatory by the Scripps Institution of Oceanography. The average difference of corresponding monthly mean CO2 values for the two data sets is 0.15±0.18 ppm, where the indicated variability is the standard deviation. Careful scrutiny of the NOAA GMCC measurement, calibration, and data processing procedures that might have caused the small bias in the data has revealed no unusual errors.

Experiments with general circulation models with no mountains, half mountains and full (modern) mountains show the sensitivity of atmospheric circulation patterns to progressive amounts of uplift. The experiments were motivated by geologic evidence for large, kilometer-scale uplift in the late Cenozoic in southern Asia and the American west, particularly within the last 10 m.y. In January the amplitude of the mid-latitude, upper tropospheric planetary waves increased with uplift, and the low-level winds were progressively blocked or diverted around the topographic features. In July the progressive uplift caused monsoonlike circulations to develop in the vicinity of the Colorado and Tibetan plateaus. Atmospheric heating rates, midtroposphere vertical motion, and upper-tropospheric planetary wave amplitudes varied approximately linearly with progressive uplift. These results suggest that geologically recent kilometer-scale uplift may have had climatic consequences comparable in magnitude and pattern to those of earlier stages of uplift. Comparison of the simulated climatic effects of uplift with the geologic evidence for late Cenozoic climatic change, especially for the last 10 m.y., is given in accompanying papers.