mardi 26 juin 2018

Climactualités - juin 2018

Le 6 juin 2018 - A global slowdown of tropical-cyclone translation speed
https://www.nature.com/articles/s41586-018-0158-3#author-information

Abstract.
As the Earth’s atmosphere warms, the atmospheric circulation changes. These changes vary by region and time of year, but there is evidence that anthropogenic warming causes a general weakening of summertime tropical circulation1,2,3,4,5,6,7,8. Because tropical cyclones are carried along within their ambient environmental wind, there is a plausible a priori expectation that the translation speed of tropical cyclones has slowed with warming. In addition to circulation changes, anthropogenic warming causes increases in atmospheric water-vapour capacity, which are generally expected to increase precipitation rates9. Rain rates near the centres of tropical cyclones are also expected to increase with increasing global temperatures10,11,12. The amount of tropical-cyclone-related rainfall that any given local area will experience is proportional to the rain rates and inversely proportional to the translation speeds of tropical cyclones. Here I show that tropical-cyclone translation speed has decreased globally by 10 per cent over the period 1949–2016, which is very likely to have compounded, and possibly dominated, any increases in local rainfall totals that may have occurred as a result of increased tropical-cyclone rain rates. The magnitude of the slowdown varies substantially by region and by latitude, but is generally consistent with expected changes in atmospheric circulation forced by anthropogenic emissions. Of particular importance is the slowdown of 30 per cent and 20 per cent over land areas affected by western North Pacific and North Atlantic tropical cyclones, respectively, and the slowdown of 19 per cent over land areas in the Australian region. The unprecedented rainfall totals associated with the ‘stall’ of Hurricane Harvey13,14,15 over Texas in 2017 provide a notable example of the relationship between regional rainfall amounts and tropical-cyclone translation speed. Any systematic past or future change in the translation speed of tropical cyclones, particularly over land, is therefore highly relevant when considering potential changes in local rainfall totals.

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Le 13 juin 2018 : Trends and connections across the Antarctic cryosphere
https://www.nature.com/articles/s41586-018-0171-6

Abstract.
Satellite observations have transformed our understanding of the Antarctic cryosphere. The continent holds the vast majority of Earth’s fresh water, and blankets swathes of the Southern Hemisphere in ice. Reductions in the thickness and extent of floating ice shelves have disturbed inland ice, triggering retreat, acceleration and drawdown of marine-terminating glaciers. The waxing and waning of Antarctic sea ice is one of Earth’s greatest seasonal habitat changes, and although the maximum extent of the sea ice has increased modestly since the 1970s, inter-annual variability is high, and there is evidence of longer-term decline in its extent.
Around the Antarctic Peninsula, where air temperatures have risen sharply, more than 13,000 square miles of ice shelf area has been lost since the 1950s, according to the study. (source insideclimatenews)

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Le 13 juin 2018 : Extensive retreat and re-advance of the West Antarctic Ice Sheet during the Holocene
https://www.nature.com/articles/s41586-018-0208-x

Abstract.
To predict the future contributions of the Antarctic ice sheets to sea-level rise, numerical models use reconstructions of past ice-sheet retreat after the Last Glacial Maximum to tune model parameters1. Reconstructions of the West Antarctic Ice Sheet have assumed that it retreated progressively throughout the Holocene epoch (the past 11,500 years or so)2,3,4. Here we show, however, that over this period the grounding line of the West Antarctic Ice Sheet (which marks the point at which it is no longer in contact with the ground and becomes a floating ice shelf) retreated several hundred kilometres inland of today’s grounding line, before isostatic rebound caused it to re-advance to its present position. Our evidence includes, first, radiocarbon dating of sediment cores recovered from beneath the ice streams of the Ross Sea sector, indicating widespread Holocene marine exposure; and second, ice-penetrating radar observations of englacial structure in the Weddell Sea sector, indicating ice-shelf grounding. We explore the implications of these findings with an ice-sheet model. Modelled re-advance of the grounding line in the Holocene requires ice-shelf grounding caused by isostatic rebound. Our findings overturn the assumption of progressive retreat of the grounding line during the Holocene in West Antarctica, and corroborate previous suggestions of ice-sheet re-advance5. Rebound-driven stabilizing processes were apparently able to halt and reverse climate-initiated ice loss. Whether these processes can reverse present-day ice loss6 on millennial timescales will depend on bedrock topography and mantle viscosity—parameters that are difficult to measure and to incorporate into ice-sheet models.

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Le 13 juin 2018 : Basal channels drive active surface hydrology and transverse ice shelf fracture
http://advances.sciencemag.org/content/4/6/eaao7212

Abstract.
Ice shelves control sea-level rise through frictional resistance, which slows the seaward flow of grounded glacial ice. Evidence from around Antarctica indicates that ice shelves are thinning and weakening, primarily driven by warm ocean water entering into the shelf cavities. We have identified a mechanism for ice shelf destabilization where basal channels underneath the shelves cause ice thinning that drives fracture perpendicular to flow. These channels also result in ice surface deformation, which diverts supraglacial rivers into the transverse fractures. We report direct evidence that a major 2016 calving event at Nansen Ice Shelf in the Ross Sea was the result of fracture driven by such channelized thinning and demonstrate that similar basal channel–driven transverse fractures occur elsewhere in Greenland and Antarctica. In the event of increased basal and surface melt resulting from rising ocean and air temperatures, ice shelves will become increasingly vulnerable to these tandem effects of basal channel destabilization.


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ENSO
Le 26/06/2018 : climate.gov/enso
ENSO-neutral conditions are favored through Northern Hemisphere summer 2018, with the chance for El Niño rising to 50% during fall, and ~65% by winter 2018-19.


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GISS L-OTI anomalies de températures vs 1951-1980
26/06/2018 : data.giss.nasa.gov
Note: Gray areas signify missing data.
Note: Ocean data are not used over land nor within 100km of a reporting land station.

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Coral Reef Watch
NOAA Coral Reef Watch's most recent Four-Month Coral Bleaching Heat Stress Outlook is below. 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.

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Climate Prediction Center
26/06/2018 : cpc.ncep.noaa.gov


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Polar Science Center
26/06/2018 : psc.apl.uw.edu

Average Arctic sea ice volume in May 2018 was 22,400 km3. This value is the 4th lowest on record about 1600 km3 above the May record that was set in 2017 with 19,800 km3 . Ice volume was 34% below the maximum in 1979 and 20% below the mean value for 1979-2017. May 2018 ice volume sits just slightly above the long term trend line.

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 2017 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 2010-2018. Shaded areas indicate one and two standard deviations from the mean.

Fig.3 Monthly Sea Ice Volume from PIOMAS for April and Sep.
Fig 8 Comparison of Daily Sea Ice Volume Anomalies relative to 1979-2016.

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Arctic Data archive system (ADS)
26/06/2018 : ads.nipr.ac.jp

Service non disponible à la date du 26 juin (j'actualiserai l'information dès que possible)

Edit : apparemment les problèmes informatiques rencontrés par le site ont été résolus, voici les deux graphiques habituels à la date du 3 juillet :

Arctic Sea Ice Extent [million km2]

Antarctic Sea Ice Extent [million km2]

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C'est si vrai, en riant avec What on earth? comics !

Certains vont même jusqu'à dire que ce qu'il dit sort de son autre orifice…

Anthony Watts en plein travail.

C'est comme si.


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