Periodic Reporting for period 1 - Polar2MidLat (Polar-to-mid-latitude climate linkage effects on cold air outbreaks)
Période du rapport: 2023-02-01 au 2025-01-31
The climate of the polar regions has changed dramatically during the past decades, and as the globe continues to warm is it expected to change even further. Over the Northern Hemisphere, the largest manifestation of global warming at high latitudes has been in the form of a significant reduction in sea ice cover. This change from sea ice to liquid water has at the same time fostered a number of local and regional feedbacks in the form of an excessive amount of surface warming over the polar cap, in a phenomenon usually called “Arctic amplification” (AA). In this project, our aim was to address the role of a warming Arctic on the climate of the mid-latitudes. In particular, our focus was the climate of Europe and the changes in the frequency of cold snaps under reduced sea ice. Another focus was on a potential linkage between sea ice changes and the pathways and frequency of cut-off lows (COLs) affecting southern Europe. COLs are mid-tropospheric cyclonic perturbations that usually explain a significant portion of the annual precipitation, particularly over the Mediterranean Sea region. Therefore, any changes to such systems could have an impact on the availability of drinking water and on the risk of flooding, among others.
The main results from our project have shown that the alteration caused from sea ice loss to the atmospheric circulation of the Northern Hemisphere (via north-south migrations of the storm tracks and upper-level jet) has a significant impact on the onset of winter blocking over central and northern Europe, which are crucial for the advection of cold polar air into mid-latitudes. At the same time, the latitudinal shift of the jet is capable of modifying the pathways of COLs reaching the Mediterranean Sea: under reduced sea ice over the Arctic, there are less COLs reaching the western Mediterranean, which most clearly impact the Iberian Peninsula where a significant decrease in total and extreme precipitation is obtained. These results are of particular relevance, given that they might bring light into the underlying mechanisms sustaining the recent decline in precipitation over parts of eastern and southern Spain in recent years.
The main results from our project have shown that the alteration caused from sea ice loss to the atmospheric circulation of the Northern Hemisphere (via north-south migrations of the storm tracks and upper-level jet) has a significant impact on the onset of winter blocking over central and northern Europe, which are crucial for the advection of cold polar air into mid-latitudes. At the same time, the latitudinal shift of the jet is capable of modifying the pathways of COLs reaching the Mediterranean Sea: under reduced sea ice over the Arctic, there are less COLs reaching the western Mediterranean, which most clearly impact the Iberian Peninsula where a significant decrease in total and extreme precipitation is obtained. These results are of particular relevance, given that they might bring light into the underlying mechanisms sustaining the recent decline in precipitation over parts of eastern and southern Spain in recent years.
The work performed along the fellowship consisted in analysing several datasets of different atmospheric variables in reanalysis (ERA5) and from model outputs (mainly from the Coupled Model Intercomparison Project version 6, CMIP6, and from the Polar Amplification Model Intercomparison Project, PAMIP). These were analysed using a large set of statistical methods in order to identify potential linkages between varying conditions over the Arctic in terms of sea ice concentration and variability in the atmospheric circulation at mid-latitudes. To this aim, the most relevant variables under strutiny were the upper-level zonal winds, sea-level pressure, and low- and mod-tropospheric temperature fields.
Among the PAMIP dataset, the experiments that were particularly relevant for this fellowship were called pdSST_pdSIC, which were forced by present-day conditions of sea surface temperatures (SST) and sea ice cover (SIC), and pdSST_futArcSIC, in which SST are also from present-day climate where SIC is reduced so as to be representative of a globe 2ºC warmer than in the preindustrial period (which will likely occur towards 2040 or 2050). By comparing both datasets, the signal resulting exclusively from sea ice loss on the mid-latitude climate can be isolated: this showed a clear impact from sea ice loss on the frequency of cold air reaching central and southern Europe in winter (under less sea ice, cold snaps tend to occur less frequency over Europe), while a similar impact but on the frequency of COLs reaching the Mediterranean Sea has also been detected. Both results point at a potentially significant alteration to both cold snaps and to heavy rain-producing systems over central and southern Europe. Naturally, these results are resulted to a single forcing (sea ice loss): different impacts can arise either from natural variability or as a forced response.
In parallel, some progress could be done on the identification of the impact from the horizontal resolution on the way models simulate the polar-midlatitude linkages: preliminary results from HighResMIP and PRIMAVERA model outputs suggest that a stronger linkage could exist when the horizontal resolution is increased. However, this pattern is not consistent among the models, and, more importantly, the state-of-the-art simulations from the EERIE project were not still ready at the time of the end of this fellowship, preventing their analysis. This is expected to be done in the upcoming months, even after the fellowship has come to an end.
Among the PAMIP dataset, the experiments that were particularly relevant for this fellowship were called pdSST_pdSIC, which were forced by present-day conditions of sea surface temperatures (SST) and sea ice cover (SIC), and pdSST_futArcSIC, in which SST are also from present-day climate where SIC is reduced so as to be representative of a globe 2ºC warmer than in the preindustrial period (which will likely occur towards 2040 or 2050). By comparing both datasets, the signal resulting exclusively from sea ice loss on the mid-latitude climate can be isolated: this showed a clear impact from sea ice loss on the frequency of cold air reaching central and southern Europe in winter (under less sea ice, cold snaps tend to occur less frequency over Europe), while a similar impact but on the frequency of COLs reaching the Mediterranean Sea has also been detected. Both results point at a potentially significant alteration to both cold snaps and to heavy rain-producing systems over central and southern Europe. Naturally, these results are resulted to a single forcing (sea ice loss): different impacts can arise either from natural variability or as a forced response.
In parallel, some progress could be done on the identification of the impact from the horizontal resolution on the way models simulate the polar-midlatitude linkages: preliminary results from HighResMIP and PRIMAVERA model outputs suggest that a stronger linkage could exist when the horizontal resolution is increased. However, this pattern is not consistent among the models, and, more importantly, the state-of-the-art simulations from the EERIE project were not still ready at the time of the end of this fellowship, preventing their analysis. This is expected to be done in the upcoming months, even after the fellowship has come to an end.
It is possible that the most relevant and original result from this fellowship is related to the evidence provided on the linkage between sea ice loss and the pathways and frequency of COLs over southern Europe. This had not been identified before, and is indeed a relevant result that can shed light into some of the mechanisms responsible for the ongoing drought conditions that have been severely affecting the western Mediterranean in recent years. In particular, eastern Spain has been under a severe drought, while also showing a large reduction in the frequency of COLs. By the results found in the fellowship, some hints are provided on the plausible role of sea ice loss on the observed scarcity of COLs over the region.