| 1. |
- Datseris, George, et al.
(author)
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Minimal Recipes for Global Cloudiness
- 2022
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 49:20
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Journal article (peer-reviewed)abstract
- Clouds are primary modulators of Earth's energy balance. It is thus important to understand the links connecting variabilities in cloudiness to variabilities in other state variables of the climate system, and also describe how these links would change in a changing climate. A conceptual model of global cloudiness can help elucidate these points. In this work we derive simple representations of cloudiness, that can be useful in creating a theory of global cloudiness. These representations illustrate how both spatial and temporal variability of cloudiness can be expressed in terms of basic state variables. Specifically, cloud albedo is captured by a nonlinear combination of pressure velocity and a measure of the low-level stability, and cloud longwave effect is captured by surface temperature, pressure velocity, and standard deviation of pressure velocity. We conclude with a short discussion on the usefulness of this work in the context of global warming response studies.
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| 2. |
- Hanley, John, 1982-, et al.
(author)
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The role of large-scale atmospheric flow and Rossby wave breaking in the evolution of extreme windstorms over Europe
- 2012
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 39, s. L21708-
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Journal article (peer-reviewed)abstract
- We investigate the relationship between large-scale atmospheric flow and the evolution of the most extreme windstorms affecting Western Continental Europe. The 25 most destructive Western Continental European wind storms are selected from a 43-year climatology. 22 of these storms are grouped as having a similar trajectory and evolution. We show that these storms typically occur during particularly strong and persistent positive NAO anomalies which peak approximately 2 days before the storms' peak intensity; the NAO pattern then shifts eastward to a position over the European continent when the storms strike Europe. A temporal composite of potential temperature on the 2-PVU surface suggests that this NAO shift is the result of simultaneous cyclonic and anticyclonic wave breaking penetrating further to the east than during a typical high-NAO event. This creates an extremely intense, zonally-orientated jet over the North Atlantic whose baroclinicity favours explosive intensification of storms while steering them into Western Continental Europe.
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| 3. |
- Harnik, Nili, et al.
(author)
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The Circumglobal North American wave pattern and its relation to cold events in eastern North America
- 2016
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 43:20, s. 11015-11023
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Journal article (peer-reviewed)abstract
- Extreme large-scale North American cold events are associated with strong undulations in the tropospheric jet stream which bring cold polar air southward over the continent. Here we propose that these jet undulations are associated with the North American part of the Circumglobal Teleconnection Patterna pair of zonally oriented waves of zonal wave number 5 which are in zonal quadrature with each other. While the Pacific/North American pattern is associated with the first circumglobal wave pattern, North American extreme cold events are associated with the second pattern. The 300hPa meridional wind and surface temperature anomalies associated with the Circumglobal North American wave packet are similar to those associated with the strongest eastern U.S. cold events. Both types of events are associated with a wave packet propagating all the way from Asia across the Pacific and across North America, with cold temperature anomalies spreading southeastward from Canada over the continent.
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| 4. |
- Lee, Sukyoung, et al.
(author)
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Relation Between Arctic Moisture Flux and Tropical Temperature Biases in CMIP5 Simulations and Its Fingerprint in RCP8.5 Projections
- 2019
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 46:2, s. 1088-1096
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Journal article (peer-reviewed)abstract
- Arctic moisture intrusions have played an important role in warming the Arctic over the past few decades. A prior study found that Coupled Model Intercomparison Project Phase 5 (CMIP5) models exhibit large regional biases in the moisture flux across 70 degrees N. It is shown here that the systematic misrepresentation of the moisture flux is related to the models' overprediction of zonal wavenumber k = 2 contribution and underprediction of k = 1 contribution to the flux. Models with a warmer tropical upper troposphere and El-Nino-like tropical surface temperature tend to simulate stronger k = 2 flux, while k = 1 flux is uncorrelated with tropical upper tropospheric temperature and is associated with La-Nina-like surface temperature. The models also overpredict the transient eddy moisture flux while underpredicting the stationary eddy flux. Moreover, future projections in Representative Concentration Pathway 8.5 (RCP8.5) simulations show trends in moisture flux that is consistent with biases in historical simulations, suggesting that these CMIP5 projections reflect the same error(s) that cause the model biases. Plain Language Summary The Arctic is the region where climate change has been most rapid. A growing body of work indicates that moisture intrusions into Arctic have played an important role in warming the Arctic over the past decades. Coupled Model Intercomparison Project Phase 5 (CMIP5) models have served as a critical tool for projecting future climate changes. Therefore, it is imperative to evaluate whether the physical processes governing moisture intrusions are accurately represented by the models. It is shown here that there is a systematic misrepresentation of the moisture flux into the Arctic related to the models' biased representation of tropical temperatures. Moreover, future projections in Representative Concentration Pathway 8.5 (RCP8.5) simulations show moisture flux trends that are consistent with biases in historical simulations, suggesting that these CMIP5 projections reflect the same error(s) that cause the model biases. It is common practice to regard averages across climate model as being the true response to climate forcing. The findings here question this widespread assumption and underscore the need to pay close attention to model biases and their causes.
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| 5. |
- Messori, Gabriele, et al.
(author)
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A dynamical systems approach to studying midlatitude weather extremes
- 2017
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 44:7, s. 3346-3354
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Journal article (peer-reviewed)abstract
- Extreme weather occurrences carry enormous social and economic costs and routinely garner widespread scientific and media coverage. The ability to predict these events is therefore a topic of crucial importance. Here we propose a novel predictability pathway for extreme events, by building upon recent advances in dynamical systems theory. We show that simple dynamical systems metrics can be used to identify sets of large-scale atmospheric flow patterns with similar spatial structure and temporal evolution on time scales of several days to a week. In regions where these patterns favor extreme weather, they afford a particularly good predictability of the extremes. We specifically test this technique on the atmospheric circulation in the North Atlantic region, where it provides predictability of large-scale wintertime surface temperature extremes in Europe up to 1week in advance. Plain Language Summary Extreme weather occurrences carry enormous social and economic costs and routinely garner widespread scientific and media coverage. The ability to predict these events is therefore a topic of crucial importance. Here we propose a novel analysis technique for improving the prediction of extreme events, which identifies the large-scale atmospheric circulation configurations affording the best predictability. We specifically test our technique on the atmospheric circulation in the North Atlantic region, where it provides predictability of large-scale wintertime surface temperature extremes in Europe up to 1week in advance.
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| 6. |
- Messori, Gabriele, et al.
(author)
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On cold spells in North America and storminess in western Europe
- 2016
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 43:12, s. 6620-6628
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Journal article (peer-reviewed)abstract
- We discuss the dynamical and statistical links between cold extremes over eastern North America and storminess over western Europe, with a focus on the midlatitude jet stream, the North Atlantic Oscillation (NAO) and the Pacific-North American Pattern (PNA). The analysis is performed on the European Centre for Medium-Range Weather Forecasts 20th Century Reanalysis. The large-scale circulation associated with the cold spells corresponds to advection of cold air from the Arctic region into North America and to a very zonal and intense North Atlantic jet, shifted persistently south of its climatological location. These features of the Atlantic jet are conducive to destructive windstorms and intense precipitation over a large part of southern and continental Europe and the British Isles. The cold spells are preceded by a negative NAO and followed by a positive PNA; however, we interpret the associated circulation anomalies as being distinct from these standard modes of climate variability.
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| 7. |
- Monteiro, Joy Merwin, et al.
(author)
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Characterization of Extreme Wet-Bulb Temperature Events in Southern Pakistan
- 2019
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 46:17-18, s. 10659-10668
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Journal article (peer-reviewed)abstract
- We study extreme wet-bulb temperature (T-w) events in a regional hot spot of high T-w values in southwest Pakistan's Indus Valley. We focus on the May-June premonsoon months and employ a combination of station data, ERA-Interim reanalysis, and trajectory calculations to analyze the extreme events. We find that the events are associated with anomalous influx of marine air from the Arabian Sea. The air flows in a shallow layer below 950 hPa and is heated and moistened by diabatic processes over land, arriving at the hot spot region with very high T-w. We surmise that surface evaporation within the strip of well-irrigated land flanking the Indus River plays a major role in generating these extreme T-w values. Our results imply that studies of extreme T-w events in future, warmer climates must pay close attention to fine-scale diabatic processes within the boundary layer, especially evaporation over land.
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| 8. |
- Pausata, Francesco S. R., et al.
(author)
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ENSO response to high-latitude volcanic eruptions in the Northern Hemisphere : The role of the initial conditions
- 2016
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 43:16, s. 8694-8702
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Journal article (peer-reviewed)abstract
- A large ensemble of Earth System Model simulations is analyzed to show that high-latitude Northern Hemisphere eruptions give rise to El Nino-like anomalies in the winter following the eruption, the amplitude of which depends on the state of the tropical Pacific at the time of the eruption. The El Nino-like anomalies are almost three times larger when the eruption occurs during an incipient La Nina or during a neutral state compared to an incipient El Nino. The differential response results from stronger atmosphere-ocean coupling and extra-tropical feedbacks during an incipient La Nina compared to El Nino. Differences in the response continue through the second and third years following the eruption. When the eruption happens in a year of an incipient El Nino, a large cold (La Nina-like) anomaly develops in year 2; if the eruption occurs in a year of an incipient La Nina, no anomalies are simulated in year 2 and a La Nina-like response appears in year 3. After the El Nino-like anomaly in the first winter, the overall tendency of ENSO in the following 2years is toward a La Nina state. Our results highlight the high sensitivity of tropical Pacific dynamics under volcanic forcing to the ENSO initial state and lay the groundwork for improved predictions of the global climatic response to high-latitude volcanic eruptions.
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| 9. |
- Pausata, Francesco S. R., et al.
(author)
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Revisiting the Mechanisms of ENSO Response to Tropical Volcanic Eruptions
- 2023
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 50:3
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Journal article (peer-reviewed)abstract
- Stratospheric volcanic aerosol can have major impacts on global climate. Despite a consensus among studies on an El Niño-like response in the first or second post-eruption year, the mechanisms that trigger a change in the state of El Niño-Southern Oscillation (ENSO) following volcanic eruptions are still debated. Here, we shed light on the processes that govern the ENSO response to tropical volcanic eruptions through a series of sensitivity experiments with an Earth System Model where a uniform stratospheric volcanic aerosol loading is imposed over different parts of the tropics. Three tropical mechanisms are tested: the “ocean dynamical thermostat” (ODT); the cooling of the Maritime Continent; and the cooling of tropical northern Africa (NAFR). We find that the NAFR mechanism plays the largest role, while the ODT mechanism is absent in our simulations as La Niña-like rather than El-Niño-like conditions develop following a uniform radiative forcing over the equatorial Pacific.
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| 10. |
- Woods, Cian, et al.
(author)
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Large-scale circulation associated with moisture intrusions into the Arctic during winter
- 2013
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In: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 40:17, s. 4717-4721
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Journal article (peer-reviewed)abstract
- We examine the poleward transport of water vapor across 70 degrees N during boreal winter in the ERA-Interim reanalysis product, focusing on intense moisture intrusion events. We analyze the large-scale circulation patterns associated with these intrusions and the impacts they have at the surface. A total of 298 events are identified between 1990 and 2010, an average of 14 per season, accounting for 28% of the total poleward transport of moisture across 70 degrees N. They are concentrated over the main ocean basins at that latitude in the Labrador Sea, North Atlantic, Barents/Kara Sea, and Pacific. Composites of sea level pressure and potential temperature on the 2 potential vorticity unit surface during intrusions show a large-scale blocking pattern to the east of each basin, deflecting midlatitude cyclones and their associated moisture poleward. The interannual variability of intrusions is strongly correlated with variability in winter-mean surface downward longwave radiation and skin temperature averaged over the Arctic.
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