The authors propose a new climatic drought index: the standardized precipitation evapotranspiration index (SPEI). The SPEI is based on precipitation and temperature data, and it has the advantage of combining multiscalar character with the capacity to include the effects of temperature variability on drought assessment. The procedure to calculate the index is detailed and involves a climatic water balance, the accumulation of deficit/surplus at different time scales, and adjustment to a log-logistic probability distribution. Mathematically, the SPEI is similar to the standardized precipitation index (SPI), but it includes the role of temperature. Because the SPEI is based on a water balance, it can be compared to the self-calibrated Palmer drought severity index (sc-PDSI). Time series of the three indices were compared for a set of observatories with different climate characteristics, located in different parts of the world. Under global warming conditions, only the sc-PDSI and SPEI identified an...

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A Multiscalar Drought Index Sensitive to Global Warming: The Standardized Precipitation Evapotranspiration Index

Cause, conseguenze e strategie di mitigazione Proponiamo il primo di una serie di articoli in cui affronteremo l’attuale problema dei mutamenti climatici. Presentiamo il documento redatto, votato e pubblicato dall’Ipcc - Comitato intergovernativo sui cambiamenti climatici - che illustra la sintesi delle ricerche svolte su questo tema rilevante.

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Climate Change 2007: The Physical Science Basis.

The Intergovernmental Panel on Climate Change (IPCC) Technical Paper Climate Change and Water draws together and evaluates the information in IPCC Assessment and Special Reports concerning the impacts of climate change on hydrological processes and regimes, and on freshwater resources – their availability, quality, use and management. It takes into account current and projected regional key vulnerabilities, prospects for adaptation, and the relationships between climate change mitigation and water. Its objectives are:

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Climate change and water.

Climate change 2014 - Synthesis Report

The purpose of this paper is to provide a comprehensive presentation and interpretation of the Ensemble Kalman Filter (EnKF) and its numerical implementation. The EnKF has a large user group, and numerous publications have discussed applications and theoretical aspects of it. This paper reviews the important results from these studies and also presents new ideas and alternative interpretations which further explain the success of the EnKF. In addition to providing the theoretical framework needed for using the EnKF, there is also a focus on the algorithmic formulation and optimal numerical implementation. A program listing is given for some of the key subroutines. The paper also touches upon specific issues such as the use of nonlinear measurements, in situ profiles of temperature and salinity, and data which are available with high frequency in time. An ensemble based optimal interpolation (EnOI) scheme is presented as a cost-effective approach which may serve as an alternative to the EnKF in some applications. A fairly extensive discussion is devoted to the use of time correlated model errors and the estimation of model bias.

The Ensemble Kalman Filter: Theoretical formulation and practical implementation

The Filchner Trough on the continental shelf in the southern Weddell Sea is a region of great importance for the water mass exchange between the open ocean and the Filchner Ronne Ice Shelf cavity. Observations of the last 20 years and modelling studies show seasonal variations and longer lasting pulses of warm water intruding into the trough and reaching the Filchner Ice Shelf front. In this study, we evaluate the evolution of these intrusions in four climate scenarios defined for CMIP6 and simulated with the AWI Climate Model. We show that a warming climate will lead to more frequent pulses in the mitigation scenarios SSP1-2.6 and SSP2-4.5. For the high emission scenarios SSP3-7.0 and SSP5-8.5, hydrography in Filchner Trough will shift to a substantially warmer state during the second half of the 21st century with a temperature rise of 2&amp;#176;C in the trough until 2100. We demonstrate that the system&amp;#8216;s tipping into a warmer state is primarily caused by changes in the local sea ice formation and the depth of the Antarctic Slope Front. Our results show that a regime shift can be avoided by reaching the 2&amp;#176;C climate goal. 

Evolution of warm water intrusions in the Filchner Trough, Antarctica

We present intermediate results as well as ongoing work on improving the post-CMIP6 climate model of the Alfred Wegener Institute, AWI-CM3. A baseline version of the model was completed in 2021 with an above average performance when compared to other CMIP6 models. Close investigation of surfaces fluxes revealed that v3.0 can be further improved in a number of ways which will also benefit other climate models.As a first step we reevaluated old coupling simplifications and assumptions made years ago. At the air-sea ice interface we corrected the gradient of surface sensible heat flux / wind speed vs. (2m air temp - sea ice surf temp), by using a nudged version of AWI-CM3 and evaluating against in situ data from the MOSAiC-Expedition. We added the coupling of ocean current feedback, as well as new latent and sensible heat fluxes resulting from precipitation entering the ocean with a different temperature and state than the ocean surface. As a precursor to subsequent Earth System Model (ESM) development we included the coupling of mass and heat fluxes of snow falling on ice-sheets. The resulting AWI-CM3 v3.1 shows increased ability to represent the current climate and historic climate change.Furthermore we present ongoing work towards AWI-CM3 v3.2, where among other improvements we will include the coupling of sub atmospheric gridscale information from the higher resolution ocean grid, via a stochastic method. Finally we give a brief outlook on our efforts to link up with the EC-Earth climate and earth system model community for common cryosphere, vegetation and isotope developments towards two comprehensive ESMs. 

A set of deterministic and stochastic model improvements for the AWI-CM3 climate model

. The question to what extent Arctic sea ice loss is able to affect atmospheric dynamics and climate extremes over mid-latitudes still remains a highly debated topic. In this study we assess the impact of future Arctic sea ice retreat on occurrence probabilities of wintertime circulation regimes and link these dynamical changes to frequency changes in European winter temperature extremes. For this reason, we analyze ECHAM6 sea ice sensitivity model simulations from the Polar Ampliﬁcation Intercomparison Project and compare experiments with future sea ice loss prescribed over the entire Arctic, as well as only 5 locally over the Barent/Karasea with a present day reference experiment. We ﬁrst show how these imposed future Arctic sea ice reductions affect large-scale atmospheric dynamics in terms of occurrence frequency changes of ﬁve computed Euro-Atlantic winter circulation regimes. Both sensitivity experiments show similar regime frequency changes, such as more frequent occurrences of a Scandinavian blocking pattern in midwinter under reduced sea ice conditions. Afterwards we demonstrate how the Scandinavian blocking regime, but also a regime that resembles the negative phase of the North Atlantic Oscillation can be 10 linked to favored occurrences of European winter cold extremes. In contrast, winter warm extreme occurrences are typically associated with an anticyclonic regime over the eastern Atlantic and a regime similar to the positive state of the North Atlantic Oscillation. Based on these links between temperature extremes and circulation regimes, as well as on the previously detected regime frequency changes we employ a framework of conditional extreme event attribution. This enables us to decompose sea ice induced frequency changes of European temperature extremes into two different contributions: one term that is related 15 to dynamical changes in regime occurrence frequencies, and another more thermodynamically motivated contribution that assumes ﬁxed atmospheric dynamics in terms of circulation regimes. By employing this decomposition procedure we show how the overall thermodynamical warming effect, but also the previously detected increased Scandinavian blocking pattern frequency under future sea ice reductions can dominate and shape the overall response signal of European cold extremes in midwinter. We also demonstrate how for instance a decreased occurrence frequency of the anticyclonic regime over the eastern 20 Atlantic counteracts the thermodynamical warming response and results in no signiﬁcant changes in overall January warm extreme occurrences. However, when compared to other characteristics of future climate change, such as the thermodynamical impact of globally increased sea surface temperatures, we argue that the detected effects on European temperature extremes related to Arctic sea ice loss are of secondary relevance.

On the linkage between future Arctic sea ice retreat, Euro-Atlantic circulation regimes and temperature extremes over Europe

The coupled global climate model EC-Earth has been used to create an ensemble of climate simulations for 1850 to 2100. For 1850 to 2005 observed greenhouse gas and aerosol concentrations including observed volcanic eruptions have been prescribed while for 2006 to 2100 two different scenarios, the RCP 4.5 and RCP 8.5 scenarios developed for CMIP5, the Coupled Model Intercomparison Project 5, have been applied. Global and European mean temperature as well as extreme cold and hot events for Europe have been compared to a range of observation data and analyzed for the future. The global and European mean temperature development for 1850 to 2005 is well captured in the EC-Earth simulations. The intensity of extreme cold and extreme hot events, defined as 1% percentile of daily minimum temperature for winter and 99% percentile of daily maximum temperature for summer, is generally fairly well captured for continental regions while underestimated by up to 5°C in maritime regions. For the future, a decrease in the intensity of extreme cold events and an increase in the intensity of extreme hot events is simulated. According to the stronger RCP 8.5 scenario the 1% percentile of daily minimum temperature for winter increases by up to 25°C in the Barents Sea region by the end of this century while the 99% percentile of daily maximum temperature for summer increases by up to 8°C over Southern Europe.

Evaluation of Surface Temperature Simulated by EC-Earth Global Model for the European Region in Different Climate Change Scenarios

11 Several cost-efficient, high-resolution modeling approaches are applied to simulations of the 12 Southern Ocean in past, present, and future climates. The results are compared with an ensemble of 13 medium-resolution, eddy-present simulations and evaluated based on their ability to reproduce observed 14 mesoscale activity and to reveal a response to climate change distinct from natural variability. The high- 15 resolution simulations reproduce the observed magnitude of Southern Ocean eddy kinetic energy (EKE) 16 well, but differences remain in local magnitudes and the spatial distribution of EKE. The coarser, eddy- 17 present ensemble simulates a similar pattern of EKE but underrepresents observed levels by 50%. Five 18 years of simulated data in each time period is found to produce consistent results when evaluating mean 19 conditions and assessing change in the region as a whole. At 1 °C of warming, the high-resolution 20 simulations produce no change in overall EKE, in contrast to the increase projected by the eddy- 21 permitting ensemble and despite full ensemble agreement. At 4 °C of warming, both datasets produce 22 consistent levels of EKE rise in relative terms, although not absolute magnitudes, as well as an increase in 23 EKE variability. Simulated EKE rise is concentrated where flow interacts with topographic features in 24 regions already known to be eddy-rich. Regional EKE change in the high-resolution simulations is 25 consistent with changes seen in at least four of five eddy-permitting ensemble members at 1 °C of 26 warming, and all ensemble members at 4 °C. However, substantial noise would make these changes 27 difficult to distinguish from natural variability without an ensemble.

Tido Semmler

Papers

Evolution of warm water intrusions in the Filchner Trough, Antarctica

A set of deterministic and stochastic model improvements for the AWI-CM3 climate model

On the linkage between future Arctic sea ice retreat, Euro-Atlantic circulation regimes and temperature extremes over Europe

Evaluation of Surface Temperature Simulated by EC-Earth Global Model for the European Region in Different Climate Change Scenarios

Resolving the mesoscale at reduced computational cost with FESOM 2.5: efficient modeling 1 approaches applied to the Southern Ocean 2