Explore the vast range of titles available.

For over 60 years, an approximation involving spherical geopotentials has underlain the representation of gravity in global numerical models of Earth's atmosphere and oceans. This article explores how departures from sphericity can be allowed by assuming spheroidal geopotentials instead. A route to more accurate model formulation is indicated, and the theoretical basis of the classical spherical‐geopotential approximation is illuminated too. An overview—from the time of Newton to the present—is given of the development of zeroth‐ and first‐order approximations to the geopotential field in terms of two small non‐dimensional parameters. The importance of using geopotential coordinate systems in atmospheric and oceanic models is emphasized. Early suggestions for such systems using non‐spherical coordinates involved qualitatively inappropriate choices of ellipsoids derived from families of confocal ellipses. Specific examples of appropriate ellipsoid choices are considered before presentation of the recently developed Geophysically Realistic, Ellipsoidal, Analytically Tractable (GREAT) system. This is based on a suitably constructed geopotential field approximation, first order accurate, from which—without further approximation—may be analytically derived equations for geopotential surfaces and surfaces orthogonal to them. GREAT coordinates satisfy stated desiderata for geopotential coordinate systems and are applicable both above and below Earth's geoid (assumed to coincide with the WGS 84 [2004, https://gis‐lab.info/docs/nima‐tr8350.2‐wgs84fin.pdf] reference ellipsoid to an excellent approximation). GREAT‐coordinate analysis provides justification for the classical spherical‐geopotential approximation: It is revealed as a mathematical limit, but not a physically realizable one. Attention is drawn to a certain partially spherical limit that is realizable physically. Plain Language Summary Gravity is by far the dominant external force in the equations of motion for Earth's atmosphere and oceans. It is crucially important that it be adequately represented in global atmospheric and oceanic models for climate and weather prediction. In principle, one simply applies Newton's inverse square law of gravitational attraction to represent gravity in the equations of motion. In practice, this is far too complicated to do exactly—due principally to the rotating Earth being closer to spheroidal than spherical in shape, with an inhomogeneous mass distribution—thereby necessitating approximation. The dominant nature of gravity means that forecast accuracy is enhanced if one constructs an orthogonal coordinate system to integrate the equations of motion, whereby gravity only acts in the vertical and not in the horizontal. This is termed a geopotential coordinate system and, to further complicate matters, its construction is intrinsically coupled to a suitable approximation of gravity. We first review the basic concepts to represent gravity in global atmospheric and oceanic models. Next, we discuss the importance and principles of geopotential coordinates for modeling purposes. Various geopotential coordinate systems of varying accuracy are then compared. Finally, we outline some possible developments for representing gravity in future models. Key Points We review the derivation from first principles of geopotential approximations to represent gravity in global atmospheric and oceanic models Various geopotential coordinate systems of varying accuracy are compared, leading to one that satisfies all desiderata for such a system The low‐order, ubiquitous, classical, spherical‐geopotential approximation is shown to be the asymptotic limit of a more accurate one

A prolonged drought affected Western Europe and the Mediterranean region in 2022 producing large socio-ecological impacts. The role of anthropogenic climate change (ACC) in exacerbating this drought has been often invoked in the public debate, but the link between atmospheric circulation and ACC has not received much attention so far. Here we address this question by applying the method of circulation analogs, which allows us to identify atmospheric patterns in the period 1836-2021 very similar to those occurred in 2022. By comparing the circulation analogs when global warming was absent (1836-1915) with those occurred recently (1942-2021), and by excluding interannual and interdecadal variability as possible drivers, we identify the contribution of ACC. The 2022 drought was associated with a persistent anticyclonic anomaly over Western Europe. Circulation analogs of this atmospheric pattern in 1941-2021 feature 500 hPa geopotential height anomalies larger in both extent and magnitude, and higher temperatures at the surface, relative to those in 1836-1915. Both factors exacerbated the drought, by increasing the area affected and enhancing soil drying through evapotranspiration. While the occurrence of the atmospheric circulation associated with the 2022 drought has not become more frequent in recent decades, the influence of the Atlantic Multidecadal oscillation cannot be ruled-out.

The responses of atmospheric variability to Tibetan Plateau (TP) snow cover (TPSC) at seasonal, interannual and decadal time scales have been extensively investigated. However, the atmospheric response to faster subseasonal variability of TPSC has been largely ignored. Here, we show that the subseasonal variability of TPSC, as revealed by daily data, is closely related to the subsequent East Asian atmospheric circulation at medium-range time scales (approximately 3–8 days later) during wintertime. TPSC acts as an elevated cooling source in the middle troposphere during wintertime and rapidly modulates the land surface thermal conditions over the TP. When TPSC is high, the upper-level geopotential height is lower, and the East Asia upper-level westerly jet stream is stronger. This finding improves our understanding of the influence of TPSC at multiple time scales. Furthermore, our work highlights the need to understand how atmospheric variability is rapidly modulated by fast snow cover changes. The atmospheric response to subseasonal variability of Tibetan Plateau snow cover has been largely ignored. Here the authors show that the fast subseasonal variability of Tibetan Plateau snow cover is closely related to the subsequent East Asian atmospheric circulation at medium-range time scales.

In summer, convective activity over the Tibetan Plateau (TP) is vigorous, with some of it moving eastward and vacating the plateau [defined as the eastward-moving type (EMT)]. Although the EMT only accounts for a small proportion, it is closely related to heavy precipitation east of the TP. This study investigates EMT impacts based on a series of composite semi-idealized simulations and piecewise potential vorticity (PV) inversion. The main results are as follows. (i) An EMT begins to affect downstream precipitation before it vacates the TP. A weaker EMT tends to cause the main downstream rainband to reduce in intensity and move southward. (ii) The EMT contributes to the formation of an eastward-moving plateau vortex (PLV) by enhancing convergence-induced stretching. Over the TP, the PLV mainly enhances/maintains the EMT, whereas during the vacating stage, the PLV dissipates (since convergence decreases rapidly when sensible heating from the TP reduces), which substantially reduces the intensity of the EMT. (iii) After PLV dissipation, a southwest vortex (SWV) forms around the Sichuan basin mainly due to convergence-induced stretching, convection-related tilting, and background transport. Piecewise PV inversion indicates that an EMT can directly contribute to SWV formation via lowering geopotential height and enhancing cyclonic wind perturbations around the Sichuan basin (even before its vacating stage), while neither of them governs the SWV formation. Sensitivity runs show that an EMT is not necessary for SWV formation, but can modify the SWV formation time and location, as well as its displacement, which significantly affects downstream precipitation.

The performance of a new historical reanalysis, the NOAA–CIRES–DOE Twentieth Century Reanalysis version 3 (20CRv3), is evaluated via comparisons with other reanalyses and independent observations. This dataset provides global, 3-hourly estimates of the atmosphere from 1806 to 2015 by assimilating only surface pressure observations and prescribing sea surface temperature, sea ice concentration, and radiative forcings. Comparisons with independent observations, other reanalyses, and satellite products suggest that 20CRv3 can reliably produce atmospheric estimates on scales ranging from weather events to long-term climatic trends. Not only does 20CRv3 recreate a ‘‘best estimate’’ of the weather, including extreme events, it also provides an estimate of its confidence through the use of an ensemble. Surface pressure statistics suggest that these confidence estimates are reliable. Comparisons with independent upper-air observations in the Northern Hemisphere demonstrate that 20CRv3 has skill throughout the twentieth century. Upper-air fields from 20CRv3 in the late twentieth century and early twenty-first century correlate well with full-input reanalyses, and the correlation is predicted by the confidence fields from 20CRv3. The skill of analyzed 500-hPa geopotential heights from 20CRv3 for 1979–2015 is comparable to that of modern operational 3–4-day forecasts. Finally, 20CRv3 performs well on climate time scales. Long time series and multidecadal averages of mass, circulation, and precipitation fields agree well with modern reanalyses and station- and satellite-based products. 20CRv3 is also able to capture trends in tropospheric-layer temperatures that correlate well with independent products in the twentieth century, placing recent trends in a longer historical context.

We have analyzed the record-breaking drought that affected western and central Europe from July 2016 to June 2017. It caused widespread impacts on water supplies, agriculture, and hydroelectric power production, and was associated with forest fires in Iberia. Unlike common continental-scale droughts, this event displayed a highly unusual spatial pattern affecting both northern and southern European regions. Drought conditions were observed over 90% of central-western Europe, hitting record-breaking values (with respect to 1979–2017) in 25% of the area. Therefore, the event can be considered as the most severe European drought at the continental scale since at least 1979. The main dynamical forcing of the drought was the consecutive occurrence of blocking and subtropical ridges, sometimes displaced from their typical locations. This led to latitudinal shifts of the jet stream and record-breaking positive geopotential height anomalies over most of the continent. The reduction in moisture transport from the Atlantic was relevant in the northern part of the region, where decreased precipitation and increased sunshine duration were the main contributors to the drought. On the other hand, thermodynamic processes, mostly associated with high temperatures and the resulting increase in atmospheric evaporative demand, were more important in the south. Finally, using flow circulation analogs we show that this drought was more severe than it would have been in the early past.

The Last Millennium Reanalysis (LMR) utilizes an ensemble methodology to assimilate paleoclimate data for the production of annually resolved climate field reconstructions of the Common Era. Two key elements are the focus of this work: the set of assimilated proxy records and the forward models that map climate variables to proxy measurements. Results based on an updated proxy database and seasonal regression-based forward models are compared to the LMR prototype, which was based on a smaller set of proxy records and simpler proxy models formulated as univariate linear regressions against annual temperature. Validation against various instrumental-era gridded analyses shows that the new reconstructions of surface air temperature and 500 hPa geopotential height are significantly improved (from 10 % to more than 100 %), while improvements in reconstruction of the Palmer Drought Severity Index are more modest. Additional experiments designed to isolate the sources of improvement reveal the importance of the updated proxy records, including coral records for improving tropical reconstructions, and tree-ring density records for temperature reconstructions, particularly in high northern latitudes. Proxy forward models that account for seasonal responses, and dependence on both temperature and moisture for tree-ring width, also contribute to improvements in reconstructed thermodynamic and hydroclimate variables in midlatitudes. The variability of temperature at multidecadal to centennial scales is also shown to be sensitive to the set of assimilated proxies, especially to the inclusion of primarily moisture-sensitive tree-ring-width records.

The El Niño/Southern Oscillation (ENSO) has a profound influence on global climate and ecosystems. Determining how the ENSO responds to greenhouse warming is a crucial issue in climate science. Despite recent progress in understanding, the responses of important ENSO characteristics, such as air temperature and atmospheric circulation, are still unknown. Here, we use a suite of global climate model projections to show that greenhouse warming drives a robust intensification of ENSO-driven variability in boreal winter tropical upper tropospheric temperature and geopotential height, tropical humidity, subtropical jets and tropical Pacific rainfall. These robust changes are primarily due to the Clausius–Clapeyron relationship, whereby saturation vapour pressure increases nearly exponentially with increasing temperature. Therefore, the vapour response to temperature variability is larger under a warmer climate. As a result, under global warming, even if the ENSO’s sea surface temperature remains unchanged, the response of tropical lower tropospheric humidity to the ENSO amplifies, which in turn results in major reorganization of atmospheric temperature, circulation and rainfall. These findings provide a novel theoretical constraint for ENSO changes and reduce uncertainty in the ENSO response to greenhouse warming. Greenhouse gas-induced warming intensifies atmospheric variability associated with the El Niño/Southern Oscillation, according to an analysis of global climate model projections.

For the newly implemented Global Ensemble Forecast System, version 12 (GEFSv12), a 31-yr (1989–2019) ensemble reforecast dataset has been generated at the National Centers for Environmental Prediction (NCEP). The reforecast system is based on NCEP’s Global Forecast System, version 15.1, and GEFSv12, which uses the Finite Volume 3 dynamical core. The resolution of the forecast system is ∼25 km with 64 vertical hybrid levels. The Climate Forecast System (CFS) reanalysis and GEFSv12 reanalysis serve as initial conditions for the Phase 1 (1989–99) and Phase 2 (2000–19) reforecasts, respectively. The perturbations were produced using breeding vectors and ensemble transforms with a rescaling technique for Phase 1 and ensemble Kalman filter 6-h forecasts for Phase 2. The reforecasts were initialized at 0000 (0300) UTC once per day out to 16 days with 5 ensemble members for Phase 1 (Phase 2), except on Wednesdays when the integrations were extended to 35 days with 11 members. The reforecast dataset was produced on NOAA’s Weather and Climate Operational Supercomputing System at NCEP. This study summarizes the configuration and dataset of the GEFSv12 reforecast and presents some preliminary evaluations of 500-hPa geopotential height, tropical storm track, precipitation, 2-m temperature, and MJO forecasts. The results were also compared with GEFSv10 or GEFS Subseasonal Experiment reforecasts. In addition to supporting calibration and validation for the National Water Center, NCEP Climate Prediction Center, and other National Weather Service stakeholders, this high-resolution subseasonal dataset also serves as a useful tool for the broader research community in different applications.

The summer drought of 2015 affected a large portion of continental Europe and was one of the most severe droughts in the region since summer 2003. The summer of 2015 was characterized by exceptionally high temperatures in many parts of central and eastern Europe, with daily maximum temperatures 2 °C higher than the seasonal mean (1971–2000) over most of western Europe, and more than 3 °C higher in the east. It was the hottest and climatologically driest summer over the 1950–2015 study period for an area stretching from the eastern Czech Republic to Ukraine. For Europe, as a whole, it is among the six hottest and driest summers since 1950. High evapotranspiration rates combined with a lack of precipitation affected soil moisture and vegetation and led to record low river flows in several major rivers, even beyond the drought-hit region. The 2015 drought developed rather rapidly over the Iberian Peninsula, France, southern Benelux and central Germany in May and reached peak intensity and spatial extent by August, affecting especially the eastern part of Europe. Over the summer period, there were four heat wave episodes, all associated with persistent blocking events. Upper-level atmospheric circulation over Europe was characterized by positive 500 hPa geopotential height anomalies flanked by a large negative anomaly to the north and west (i.e., over the central North Atlantic Ocean extending to northern Fennoscandia) and another center of positive geopotential height anomalies over Greenland and northern Canada. Simultaneously, the summer sea surface temperatures (SSTs) were characterized by large negative anomalies in the central North Atlantic Ocean and large positive anomalies in the Mediterranean basin. Composite analysis shows that the western Mediterranean SST is strongly related to the occurrence of dry and hot summers over the last 66 years (especially over the eastern part of Europe). The lagged relationship between the Mediterranean SST and summer drought conditions established in this study can provide valuable skill for the prediction of drought conditions over Europe on interannual to decadal timescales.