Chroniques du têtard mouillé: Climactualités

lundi 30 octobre 2017

Climactualités - Octobre 2017

Actualité climatique du mois passé dans laquelle j'entrepose pêle-mêle les articles que j'ai trouvés intéressants (mais j'ai pu, et dû, en louper un certain nombre) ; comme je n'ai pas toute la journée à dédier à la tenue de ce blog je me dispenserai de traduire les articles en français, à chacun donc de se débrouiller avec la langue de Shakespeare en fonction de ses capacités (il y a au demeurant des outils de traduction en ligne assez performants...)

Comme je ne ferai aucun commentaire (sauf pour les dessins humoristiques), me contentant de reprendre quelques extraits ou graphiques des articles en question, les lecteurs qui m'accuseraient de cherry-picking verraient leur prose automatiquement envoyée à la poubelle sans forcément une explication de ma part ; je donnerai à chaque fois les liens donc toute personne n'ayant pas de poil dans la main sera capable d'aller consulter les sources dans leur totalité.

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Le 27/09/2017 : journals.ametsoc
More-Persistent Weak Stratospheric Polar Vortex States Linked to Cold Extremes

Abstract

Over the last decades, the stratospheric polar vortex has shifted towards more frequent weak states which can explain Eurasian cooling trends in boreal winter in the era of Arctic amplification.

The extra-tropical stratosphere in boreal winter is characterized by a strong circumpolar westerly jet, confining the coldest temperatures at high latitudes. The jet, referred to as the stratospheric polar vortex, is predominantly zonal and centered around the pole; however, it does exhibit large variability in wind speed and location. Previous studies showed that a weak stratospheric polar vortex can lead to cold-air outbreaks in the mid-latitudes but the exact relationships and mechanisms are unclear. Particularly, it is unclear whether stratospheric variability has contributed to the observed anomalous cooling trends in mid-latitude Eurasia. Using hierarchical clustering, we show that over the last 37 years, the frequency of weak vortex states in mid to late winter (January and February) has increased which were accompanied by subsequent cold extremes in mid-latitude Eurasia. For this region 60% of the observed cooling in the era of Arctic amplification, i.e. since 1990, can be explained by the increased frequency of weak stratospheric polar vortex states, a number which increases to almost 80% when El Niño/Southern Oscillation (ENSO) variability is included as well.

A strong polar vortex (left, from December 2013) is centered over the Arctic. A weakened polar vortex (right, from January 2014) allows cold air to dip farther south. Credit: NOAA (source insideclimatenews)

The arrows show movement of the polar vortex winds. The size of the arrows reflects the strength of the flow. Credit: NASA (source insideclimatenews)

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Le 25/09/2017 : journals.ametsoc
Seasonal sensitivity of the Northern Hemisphere jet-streams to Arctic temperatures on subseasonal timescales

Abstract

Near-surface Arctic warming has been shown to impact the midlatitude jet-streams through the use of carefully designed model simulations with and without Arctic sea ice loss. In this work, we instead take a Granger causality regression approach to quantify the response of the zonal wind to variability of near-surface Arctic temperatures on subseasonal timescales across the CMIP5 models. Using this technique, we demonstrate a robust influence of regional Arctic warming on the North Atlantic and North Pacific jet-stream positions, speeds and zonal winds. However, Arctic temperatures only explain an additional 3-5% of the variance of the winds after accounting for the variance associated with the persistence of the wind anomalies from previous weeks.

In terms of the jet-stream response, the North Pacific and North Atlantic jet-streams consistently shift equatorward in response to Arctic warming, but also strengthen, rather than weaken, during most months of the year. Furthermore, the sensitivity of the jet-stream position and strength to Arctic warming is shown to be a strong function of season. Specifically, in both ocean basins, the jets shift furthest equatorward in the summer months. We argue that this seasonal sensitivity is due to the Arctic-warming-induced wind anomalies remaining relatively fixed in latitude, while the climatological jet migrates in and out of the anomalies throughout the annual cycle. Based on these results, we go on to demonstrate that model differences in the climatological jet-stream position lead to differences in the jet-stream position’s sensitivity to Arctic warming.

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Le 5/10/2017 : nature

Delta progradation in Greenland driven by increasing glacial mass loss

Climate changes are pronounced in Arctic regions and increase the vulnerability of the Arctic coastal zone. For example, increases in melting of the Greenland Ice Sheet and reductions in sea ice and permafrost distribution are likely to alter coastal morphodynamics. The deltas of Greenland are largely unaffected by human activity, but increased freshwater runoff and sediment fluxes may increase the size of the deltas, whereas increased wave activity in ice-free periods could reduce their size, with the net impact being unclear until now. Here we show that southwestern Greenland deltas were largely stable from the 1940s to 1980s, but prograded (that is, sediment deposition extended the delta into the sea) in a warming Arctic from the 1980s to 2010s. Our results are based on the areal changes of 121 deltas since the 1940s, assessed using newly discovered aerial photographs and remotely sensed imagery. We find that delta progradation was driven by high freshwater runoff from the Greenland Ice Sheet coinciding with periods of open water. Progradation was controlled by the local initial environmental conditions (that is, accumulated air temperatures above 0 °C per year, freshwater runoff and sea ice in the 1980s) rather than by local changes in these conditions from the 1980s to 2010s at each delta. This is in contrast to a dominantly eroding trend of Arctic sedimentary coasts along the coastal plains of Alaska, Siberia and western Canada, and to the spatially variable patterns of erosion and accretion along the large deltas of the main rivers in the Arctic. Our results improve the understanding of Arctic coastal evolution in a changing climate, and reveal the impacts on coastal areas of increasing ice mass loss and the associated freshwater runoff and lengthening of open-water periods.

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Le 26/10/2017 : iopscience

Linking sea level rise and socioeconomic indicators under the Shared Socioeconomic Pathways

Abstract

In order to assess future sea level rise and its societal impacts, we need to study climate change pathways combined with different scenarios of socioeconomic development. Here, we present sea level rise (SLR) projections for the Shared Socioeconomic Pathway (SSP) storylines and different year-2100 radiative forcing targets (FTs). Future SLR is estimated with a comprehensive SLR emulator that accounts for Antarctic rapid discharge from hydrofracturing and ice cliff instability. Across all baseline scenario realizations (no dedicated climate mitigation), we find 2100 median SLR relative to 1986–2005 of 89 cm (likely range: 57–130 cm) for SSP1, 105 cm (73–150 cm) for SSP2, 105 cm (75–147 cm) for SSP3, 93 cm (63–133 cm) for SSP4, and 132 cm (95–189 cm) for SSP5. The 2100 sea level responses for combined SSP-FT scenarios are dominated by the mitigation targets and yield median estimates of 52 cm (34–75 cm) for FT 2.6 Wm⁻², 62 cm (40–96 cm) for FT 3.4 Wm⁻², 75 cm (47–113 cm) for FT 4.5 Wm⁻², and 91 cm (61–132 cm) for FT 6.0 Wm⁻². Average 2081–2100 annual SLR rates are 5 mm yr⁻¹ and 19 mm yr⁻¹ for FT 2.6 Wm⁻² and the baseline scenarios, respectively. Our model setup allows linking scenario-specific emission and socioeconomic indicators to projected SLR. We find that 2100 median SSP SLR projections could be limited to around 50 cm if 2050 cumulative CO₂ emissions since pre-industrial stay below 850 GtC, with a global coal phase-out nearly completed by that time. For SSP mitigation scenarios, a 2050 carbon price of 100 US$2005 tCO₂ ⁻¹ would correspond to a median 2100 SLR of around 65 cm. Our results confirm that rapid and early emission reductions are essential for limiting 2100 SLR.

Source : insideclimatenews

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Le 3/10/2017 : pnas
Impact of climate change on New York City’s coastal flood hazard: Increasing flood heights from the preindustrial to 2300 CE

Abstract

The flood hazard in New York City depends on both storm surges and rising sea levels. We combine modeled storm surges with probabilistic sea-level rise projections to assess future coastal inundation in New York City from the preindustrial era through 2300 CE. The storm surges are derived from large sets of synthetic tropical cyclones, downscaled from RCP8.5 simulations from three CMIP5 models. The sea-level rise projections account for potential partial collapse of the Antarctic ice sheet in assessing future coastal inundation. CMIP5 models indicate that there will be minimal change in storm-surge heights from 2010 to 2100 or 2300, because the predicted strengthening of the strongest storms will be compensated by storm tracks moving offshore at the latitude of New York City. However, projected sea-level rise causes overall flood heights associated with tropical cyclones in New York City in coming centuries to increase greatly compared with preindustrial or modern flood heights. For the various sea-level rise scenarios we consider, the 1-in-500-y flood event increases from 3.4 m above mean tidal level during 1970–2005 to 4.0–5.1 m above mean tidal level by 2080–2100 and ranges from 5.0–15.4 m above mean tidal level by 2280–2300. Further, we find that the return period of a 2.25-m flood has decreased from ∼500 y before 1800 to ∼25 y during 1970–2005 and further decreases to ∼5 y by 2030–2045 in 95% of our simulations. The 2.25-m flood height is permanently exceeded by 2280–2300 for scenarios that include Antarctica’s potential partial collapse.

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ENSO

12/10/2017 : climate.gov/enso
The atmosphere over the tropical Pacific was La Niña-like in September, but the required cooling of the ocean surface was interrupted in the second half of the month. However, the deeper waters in the east cooled further, and forecasters say the odds of at least a weak La Niña by late fall or winter are 55-65%. The next update will be on November 9.

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Coral Reef Watch

30/10/2017 : coralreefwatch.noaa.gov

This figure shows the regions currently experiencing high levels of heat stress that can cause coral bleaching.

This figure shows the distribution of the lowest heat stress levels predicted by at least 60% of the model ensemble members. In other words, there is a 60% chance that the displayed heat stress levels will occur.

NOAA Coral Reef Watch's satellite Coral Bleaching Alert Area below shows the maximum heat stress during the Third Global Coral Bleaching Event. Regions that experienced the high heat stress that can cause coral bleaching, from June 1, 2014 to May 31, 2017, are displayed. Alert Level 2 heat stress indicates widespread coral bleaching and significant mortality. Alert Level 1 heat stress indicates significant coral bleaching. Lower levels of stress may have caused some bleaching as well. More than 70% of coral reefs around the world experienced the heat stress that can cause bleaching and/or mortality during the three-year long global event.

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Climate Prediction Center

30/10/2017 : cpc.ncep.noaa.gov

Global Tropics Benefits/Hazards

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Polar Science Center

30/10/2017 : psc.apl.uw.edu
Average Arctic sea ice volume through September 2017 was 4700 km³ a 1000 km³ above the record of 2012 ( 3700 km³) and almost the same as 2010, 2011, and 2016 with differences between those years well below the uncertainty of the estimate. September 2017 volume was 72% below the maximum August ice volume in 1979, 58% below the 1979-2016 mean, and very close to the long term trend line. While 2017 started well below prior years and remained so through May, ice loss during June through September was less than previous years with July and August accounting for most of the “catch up”. This is shown in Fig 8 which compares daily ice volume anomalies for several recent years (base period 1979-2016). The difference between 2012 (the previous record) is notable. While 2017 started out with much lower sea ice volume, 2012 had a much more rapid sea ice loss through May and June. Both 2012 and 2017 have very similar anomaly progression through July. August 2017 by comparison was a month of reprieve relative to 2012.
Average ice thickness in September 2017 over the PIOMAS domain is equivalent to the lowest on record (Fig 4.) Average ice thickness from PIOMAS is a bit thicker than 2016 and 2010 few years and about 1 m thinner than in 1980. Note that the interpretation of average ice thickness needs to take into account that only areas with ice thickness greater than 15 cm are included so that years with less total volume can have a greater ice thickness. That’s why the average ice thickness can increase late in the year as thin regrown sea ice is added into the average.

Fig 8 Comparison of Daily Sea Ice Volume Anomalies relative to 1979-2016

Fig 4.Average Arctic sea ice thickness over the ice-covered regions from PIOMAS for a selection of years. The average thickness is calculated for the PIOMAS domain by only including locations where ice is thicker than .15 m

Fig.1 Arctic sea ice volume anomaly from PIOMAS updated once a month. Daily Sea Ice volume anomalies for each day are computed relative to the 1979 to 2016 average for that day of the year. Tickmarks on time axis refer to 1st day of year. The trend for the period 1979- present is shown in blue. Shaded areas show one and two standard deviations from the trend. Error bars indicate the uncertainty of the monthly anomaly plotted once per year.

Fig. 2 Total Arctic sea ice volume from PIOMAS showing the volume of the mean annual cycle, and from 2010-2017. Shaded areas indicate one and two standard deviations from the mean.