Predictability of weather and climate

Abstract:

Posit numbers, a recently proposed alternative to floating-point numbers, claim to have smaller arithmetic rounding errors in many applications. By studying weather and climate models of low and medium complexity (the Lorenz system and a shallow water model) we present benefits of posits compared to floats at 16 bit. As a standardised posit processor does not exist yet, we emulate posit arithmetic on a conventional CPU. Using a shallow water model, forecasts based on 16-bit posits with 1 or 2 exponent bits are clearly more accurate than half precision floats. We therefore propose 16 bit with 2 exponent bits as a standard posit format, as its wide dynamic range of 32 orders of magnitude provides a great potential for many weather and climate models. Although the focus is on geophysical fluid simulations, the results are also meaningful and promising for reduced precision posit arithmetic in the wider field of computational fluid dynamics.

Abstract:

Southern Hemisphere (SH) circulation and surface climate changes are emerging in recent decades. For example, in most seasons, the SH Hadley cell edge and the eddy‐driven jet stream have shifted polewards, and the Southern Annular Mode (SAM) has trended towards its positive phase. However, attributing these changes to specific external forcings, such as greenhouse gas (GHG) increases or stratospheric ozone depletion, has been hindered by small ensemble sizes and lack of coherent forcing methodology across multiple models. In this study, we analyzed simulations from 10 models within the Large Ensemble Single Forcing Model Intercomparison Project (LESFMIP) to isolate the long‐term (1850–2014) SH climate response to individual forcings (GHGs, aerosols, and ozone). We found that long‐term SH climate trends are dominated by GHGs across all seasons. Over this time period, stratospheric ozone depletion exerts an influence comparable to GHGs but is restricted to austral spring and summer. Although anthropogenic aerosols show weaker effects on climate trends that oppose those induced by GHG forcing and ozone depletion, they have a noticeable impact. Notably, ozone recovery following the Montreal Protocol is currently weakening or reversing spring and summer trends in key circulation metrics. Finally, we quantify the degree to which the observed sea surface cooling of the Southern Ocean and Antarctic sea ice increase from 1980 to 2014 fall outside the models' ensemble spread. Our results highlight the need to better understand the roles of different forcings, model differences, and discrepancies between models and observations to constrain projections of future Southern Hemisphere climate change. Atmospheric and oceanic circulation changes are emerging in the Southern Hemisphere (SH). To better understand the drivers of these changes, in this study we use a new multi‐model large ensemble data set to look at how specific factors (forcings) have individually influenced the long‐term SH climate trends from 1850 to 2014. We find that greenhouse gases are the main driver of long‐term changes in the SH since the preindustrial era. The impact of stratospheric ozone depletion is comparable to greenhouse gases, but only during austral spring and summer. On the other hand, anthropogenic aerosols partially counteract greenhouse gas effects on the circulation. The models are able to simulate the recent weakening or even reversal of some of the trends in key atmospheric circulation metrics due to the recovery of the ozone layer following the enactment of the Montreal Protocol and its amendments. However, the disagreement between model simulations and real‐world observations for recent trends in Southern Ocean sea surface temperatures and Antarctic sea ice highlights the need for more research to reliably project future climate change in this crucial region. Greenhouse gases drive Southern Hemisphere circulation changes year‐round, while ozone is a key driver during austral spring and summer Although weaker, anthropogenic aerosol effects are substantial and should be accounted for in attribution studies Persistent model‐observational discrepancies exist for recent trends in Southern Ocean sea surface temperature and sea ice extent Greenhouse gases drive Southern Hemisphere circulation changes year‐round, while ozone is a key driver during austral spring and summer Although weaker, anthropogenic aerosol effects are substantial and should be accounted for in attribution studies Persistent model‐observational discrepancies exist for recent trends in Southern Ocean sea surface temperature and sea ice extent

Abstract:

Abstract Storm Eunice was a severe windstorm that impacted Central Europe in February 2022. The meteorology and synoptic dynamics of Eunice have been studied in depth in several studies examining features of the storm such as its sting jet. The contribution of climate change to the storm dynamics and severity was examined in previous work, which found that in counterfactual weather forecasts - given an identical initial synoptic setup - climate change had measurably increased the severity of the storm. Here we move beyond meteorological attribution and quantify the role of climate change in the insured losses incurred during Eunice in, to the best of our knowledge, the first impact attribution of its kind for a European windstorm event. We combine the same counterfactual weather forecasts with three loss models, including two state-of-the-art commercial models, finding that the increases in meteorological severity do translate through to significant increases in estimated loss. We estimate a conditional increase in insured loss of nearly €2 bn between pre-industrial and present-day climates. Of particular note is the existence of several members within the forecast ensembles whose losses are far greater than what unfolded in reality. This includes one realisation, simulated in a warmer “future” climate, in which the estimated loss could reach over 10x the realised loss during Eunice. The plausible existence of such a catastrophic loss is of considerable relevance to a wide variety of stakeholders across adaptation planning and the financial sector. We suggest that our results practically demonstrate not only the utility of counterfactual weather forecasts in quantifying impacts attributable to climate change, but also the value of academic - private partnerships in which the two sectors are able to bring different areas of expertise.