Juerg Luterbacher

Monthly mean pressure reconstructions for Europe for the 1780–1995 period

Monthly grid-point pressure data are reconstructed from station records of pressure for Europe since 1780. The region encompasses 35-70°N to 30°W-40°E. The reconstructions are based on a principal components regression technique, which relates surface pressure patterns to those of the station pressure data. The relationships are derived over a calibration period (1936-1995) and the results tested with independent data (the verification period, 1881-1935). The reconstructions are of excellent quality, although this is slightly lower for regions with poor station coverage in the early years, particularly during the summer months. The reconstructions are compared with other monthly mean pressure maps produced by Lamb and Johnson (1966) for the years 1780-1872 and by Kington (1980, 1988) for 1781-1785. Both of these map series show systematic biases relative to the present reconstructions.

The International Atmospheric Circulation Reconstructions over the Earth (ACRE) Initiative

In 2006, climate applications scientists in Queensland, Australia, asked the lead author if a longer and more complete historical weather record could be created and fed directly into various crop, pasture, and production models. Existing dynamical reanalyses were steps toward such a product, but they spanned only the last six decades and had well-known shortcomings. To meet the needs of application scientists, new reanalyses would have to extend much further back in time while maintaining accuracy with limited observations. They would also need to be disseminated in a way that is easy to use directly and to downscale to small regions.

Temperature variation through 2000 years in China: An uncertainty analysis of reconstruction and regional difference

Twenty-three published proxy temperature series over China spanning the last 2000 years were selected for an uncertainty analysis in five climate regions. Results indicated that, although large uncertainties are found for the period prior to the 16th century, high level of consistency were identified in all regions during the recent 500-years, highlighted by the two cold periods 1620s-1710s and 1800s-1860s, and the warming during the 20th century. The latter started in Tibet, Northwest and Northeast, and migrated to Central East and Southeast. The analysis also indicates that the warming during the 10-14th centuries in some regions might be comparable in magnitude to the warming of the last few decades of the 20th century which was unprecedented within the past 500 years. Citation: Ge, Q.-S., J.-Y. Zheng, Z.-X. Hao, X.-M. Shao, W.-C. Wang, and J. Luterbacher (2010), Temperature variation through 2000 years in China: An uncertainty analysis of reconstruction and regional difference, Geophys. Res. Lett., 37, L03703, doi: 10.1029/2009GL041281.

Tambora 1815 as a test case for high impact volcanic eruptions: Earth system effects

The eruption of Tambora (Indonesia) in April 1815 had substantial effects on global climate and led to the ‘Year Without a Summer’ of 1816 in Europe and North America. Although a tragic event—tens of thousands of people lost their lives—the eruption also was an ‘experiment of nature’ from which science has learned until today. The aim of this study is to summarize our current understanding of the Tambora eruption and its effects on climate as expressed in early instrumental observations, climate proxies and geological evidence, climate reconstructions, and model simulations. Progress has been made with respect to our understanding of the eruption process and estimated amount of SO2 injected into the atmosphere, although large uncertainties still exist with respect to altitude and hemispheric distribution of Tambora aerosols. With respect to climate effects, the global and Northern Hemispheric cooling are well constrained by proxies whereas there is no strong signal in Southern Hemisphere proxies. Newly recovered early instrumental information for Western Europe and parts of North America, regions with particularly strong climate effects, allow Tamboras effect on the weather systems to be addressed. Climate models respond to prescribed Tambora‐like forcing with a strengthening of the wintertime stratospheric polar vortex, global cooling and a slowdown of the water cycle, weakening of the summer monsoon circulations, a strengthening of the Atlantic Meridional Overturning Circulation, and a decrease of atmospheric CO2. Combining observations, climate proxies, and model simulations for the case of Tambora, a better understanding of climate processes has emerged. WIREs Clim Change 2016, 7:569–589. doi: 10.1002/wcc.407 This article is categorized under: 1 Paleoclimates and Current Trends > Paleoclimate

Northern Hemispheric Trends of Pressure Indices and Atmospheric Circulation Patterns in Observations, Reconstructions, and Coupled GCM Simulations

Abstract The decadal trend behavior of the Northern Hemisphere atmospheric circulation is investigated utilizing long-term simulations with different state-of-the-art coupled general circulation models (GCMs) for present-day climate conditions (1990), reconstructions of the past 500 yr, and observations. The multimodel simulations show that strong positive winter North Atlantic Oscillation (NAO) trends are connected with the underlying sea surface temperature (SST) and exhibit an SST tripole trend pattern and a northward shift of the storm-track tail. Strong negative winter trends of the Aleutian low are associated with SST changes in the El Nino–Southern Oscillation (ENSO) region and a westward shift of the storm track in the North Pacific. The observed simultaneous appearance of strong positive NAO and negative Aleutian low trends is very unlikely to occur by chance in the unforced simulations and reconstructions. The positive winter NAO trend of the last 50 yr is not statistically different from the le...

The Etesians: from observations to reanalysis

AbstractThe Etesians are among the most persistent regional scale wind systems in the lower troposphere that blow over the Aegean Sea during the extended summer season. In this study we evaluate the performance of three different reanalysis products (the twentieth century reanalysis, 20CR; the 40-year European Centre for Medium-Range Weather Forecasts, ECMWF, Re-Analysis, ERA40; and the recently released ECMWF reanalysis ERA-20C) in capturing the Etesian wind system. Three-hourly data from 24 stations over Greece are used and compared with reanalysis outputs for the extended summer season (May–September) from 1971 to 2000. An objective classification of Etesians based on the pressure difference over the Aegean is provided. Classified Etesian days are then investigated as well as the associated large scale atmospheric circulation. Results highlight the ability of the investigated reanalyses to adequately describe the Etesian meteorological regimes. Intense Etesians are associated with stronger geopotential height anomalies over western-central Europe and the Eastern Mediterranean and with pronounced changes in the mean position of the jet streams. Finally, station time series provide evidence for less frequent intense Etesian days at the end of the extended summer season.

Winter amplification of the European Little Ice Age cooling by the subpolar gyre

Climate reconstructions reveal a strong winter amplification of the cooling over central and northern continental Europe during the Little Ice Age period (LIA, here defined as c. 16th–18th centuries) via persistent, blocked atmospheric conditions. Although various potential drivers have been suggested to explain the LIA cooling, no coherent mechanism has yet been proposed for this seasonal contrast. Here we demonstrate that such exceptional wintertime conditions arose from sea ice expansion and reduced ocean heat losses in the Nordic and Barents seas, driven by a multicentennial reduction in the northward heat transport by the subpolar gyre (SPG). However, these anomalous oceanic conditions were largely decoupled from the European atmospheric variability in summer. Our novel dynamical explanation is derived from analysis of an ensemble of last millennium climate simulations, and is supported by reconstructions of European temperatures and atmospheric circulation variability and North Atlantic/Arctic paleoceanographic conditions. We conclude that SPG-related internal climate feedbacks were responsible for the winter amplification of the European LIA cooling. Thus, characterization of SPG dynamics is essential for understanding multicentennial variations of the seasonal cycle in the European/North Atlantic sector.

Mediterranean circulation perturbations over the last five centuries: Relevance to past Eastern Mediterranean Transient-type events

The Eastern Mediterranean Transient (EMT) occurred in the Aegean Sea from 1988 to 1995 and is the most significant intermediate-to-deep Mediterranean overturning perturbation reported by instrumental records. The EMT was likely caused by accumulation of high salinity waters in the Levantine and enhanced heat loss in the Aegean Sea, coupled with surface water freshening in the Sicily Channel. It is still unknown whether similar transients occurred in the past and, if so, what their forcing processes were. In this study, sediments from the Sicily Channel document surface water freshening (SCFR) at 1910 ± 12, 1812 ± 18, 1725 ± 25 and 1580 ± 30 CE. A regional ocean hindcast links SCFR to enhanced deep-water production and in turn to strengthened Mediterranean thermohaline circulation. Independent evidence collected in the Aegean Sea supports this reconstruction, showing that enhanced bottom water ventilation in the Eastern Mediterranean was associated with each SCFR event. Comparison between the records and multi-decadal atmospheric circulation patterns and climatic external forcings indicates that Mediterranean circulation destabilisation occurs during positive North Atlantic Oscillation (NAO) and negative Atlantic Multidecadal Oscillation (AMO) phases, reduced solar activity and strong tropical volcanic eruptions. They may have recurrently produced favourable deep-water formation conditions, both increasing salinity and reducing temperature on multi-decadal time scales.

Changes in the annual cycle of heavy precipitation across the British Isles within the 21st century

We investigate future changes in the annual cycle of heavy daily precipitation events across the British Isles in the periods 2021–2060 and 2061–2100, relative to present day climate. Twelve combinations of regional and global climate models forced with the A1B scenario are used. The annual cycle is modelled as an inhomogeneous Poisson process with sinusoidal models for location and scale parameters of the generalized extreme value distribution. Although the peak times of the annual cycle vary considerably between projections for the 2061–2100 period, a robust shift towards later peak times is found for the south-east, while in the north-west there is evidence for a shift towards earlier peak times. In the remaining parts of the British Isles no changes in the peak times are projected. For 2021–2060 this signal is weak. The annual cycle’s relative amplitude shows no robust signal, where differences in projected changes are dominated by global climate model differences. The relative contribution of anthropogenic forcing and internal climate variability to changes in the relative amplitude cannot be identified with the available ensemble. The results might be relevant for the development of adequate risk-reduction strategies, for insurance companies and for the management and planning of water resources

Simulating Extreme Etesians over the Aegean and Implications for Wind Energy Production in Southeastern Europe

AbstractEpisodes of extremely strong northerly winds (known as etesians) during boreal summer can cause hazardous conditions over the Aegean Archipelago (Greece) and represent a threat for the safe design, construction, and operation of wind energy turbines. Here, these extremes are characterized by employing a peak-over-threshold approach in the extended summer season (May–September) from 1989 to 2008. Twelve meteorological stations in the Aegean are used, and results are compared with 6-hourly wind speed data from five ERA-Interim–driven regional climate model (RCM) simulations from the European domain of the Coordinated Regional Climate Downscaling Experiment (EURO-CORDEX). The main findings show that, in the range of wind speeds for the maximum power output of the turbine, the most etesian-exposed stations could operate 90% at a hub height of 80 m. The central and northern Aegean are identified as areas prone to wind hazards, where medium- to high-wind (class II or I according to the International Ele...