Tropical nights

Tropical nights in South Tyrol

1980-2022 for areas below 500 meters in altitude

Description of the results

The number of tropical nights which occur annually in South Tyrol has been increasing since 1980, especially in the last 20 years. The average values of the indicator for the current period are about 5 tropical nights more than at the beginning of the 1980s, with a statistically significant upward trend over the last 43 years. The upward trend over the last 43 years is statistically significant. 2015 was the record year with an average of more than 15 tropical nights across the province, followed by 2019 and 2003.

Method

The term “tropical night” refers to days when the minimum night-time temperature does not drop below 20 degrees Celsius. The graph depicts the indicator values from 1980 to 2022, represented as a spatial average of areas in South Tyrol situated at altitudes below 500 meters. In South Tyrol, these areas include densely populated urban areas. The graph is based on daily meteorological observations from over 80 measurement points of the Autonomous Province of Bolzano‘s Office for Meteorology and Avalanche Warning, supplemented by observational data from some locations in Switzerland and Austria near the national border. The collected data series were interpolated using a geostatistical method onto a regular grid with a resolution of 1 km for the entire provincial territory.

Prior to interpolation, all observation series were checked for measurement error and temporal homogeneity. In addition, missing daily values were reconstructed using a statistical method to maximize the temporal continuity of the series.

The interpolation allows the determination of a regional average that is more representative and stable than that based on individual monitoring stations. Only areas below 500 meters were considered, as they provide the most meaningful information, and the spatial average was calculated for them. The trend is calculated using the Theil-Sen method and significance is assessed using the Mann-Kendall test. The trend is considered significant if the resulting p-value is less than 0.05.

Due to the method used (counting tropical nights as days with a minimum temperature of at least 20°C), the information may differ slightly from the official data on tropical nights provided by the national meteorological service.

Where in South Tyrol do people live below 500 meters in altitude?

Tropical nights in Bolzano

Bolzano is particularly affected by the phenomenon of tropical nights. The observation series for Bolzano from 1961 to 2021 shows an increase of about +9 tropical nights since the beginning of the series. The upward trend in the number of tropical nights per year is statistically significant. Bolzano is particularly affected by the phenomenon of tropical nights.

Sectors affected

• Flora and Fauna

• Soil

• Natural hazard

• Ecosystem service

• Agriculture

• Tourism

• Health

Related indicators

Future scenarios

Available climate projections suggest an increase in the annual frequency of tropical nights in areas of South Tyrol below 500 meters in altitude, with the increase depending on the emission scenario considered.

By mid-century (2041 to 2070), the RCP 4.5 scenario is projected to lead to an average increase of about 13 tropical nights per year compared to the 30-year reference period (1981 to 2010), while the RCP 8.5 scenario is projected to lead to an increase of about 21 tropical nights per year. By the end of the century (2071 to 2100), the most pessimistic scenario projects an average increase of 51 tropical nights per year compared to the historical period, while the moderate RCP 4.5 scenario projects an increase of about 17 tropical nights.

The spatial distribution of the 30-year mean number of tropical nights in South Tyrol illustrates the areas of the country affected by this phenomenon. While during the historical reference period, almost everywhere there were less than four tropical nights per year, it is expected that the frequency of these events will increase in the coming decades, especially along the Etsch/Adige Valley. Furthermore, the phenomenon is expected to gradually spread to areas at the bottom of valleys, such as the Vinschgau/Val Venosta and Eisacktal/Valle Isarco regions, where tropical nights were very rare or absent during the thirty-year period from 1981 to 2010, especially under the RCP 8.5 scenario.

Method

The climate scenarios for annual and seasonal precipitation in South Tyrol were derived from EURO-CORDEX climate simulations over Europe for the two emission scenarios RCP 4.5 and RCP 8.5. RCP stands for “Representative Concentration Pathways”, which project how greenhouse gas emissions in the atmosphere will develop in the future.

RCP 4.5 is an intermediate scenario in which greenhouse gas emissions are reduced, but atmospheric concentrations continue to rise over the next 50 years and the +2°C target is not met. RCP 8.5 represents the most pessimistic scenario, in which greenhouse gas emissions continue to rise steadily and no action is taken to combat climate change.

The daily minimum temperature projections from 1971 to 2100 provided by 15 different climate models (ensembles) for the two scenarios were further processed by a downscaling procedure. This procedure transfers the simulated values from the original spatial resolution (in this case about 12 km) to a finer resolution (in this case 1 km). This step helps to reduce systematic errors in model simulations due to the limited spatial resolution of available models, which do not adequately represent local features, especially in mountainous regions with complex orography. The applied downscaling method is based on the delta quantile mapping (QDM, Cannon et al., 2015). In QDM, simulated values are compared with observations over a common reference period and corrected to match probability distributions. In addition, the corrections in the QDM method are made in such a way that the original long-term climate signal in the simulations is not altered.

In this case, the reference period is 1981 to 2010, and the correction was made based on the 1 km grid observational data set.

From the corrected simulations, the annual frost days were calculated for each model in the ensemble and for both scenarios from 1971 to 2100. The indicator values for the ensemble were then aggregated by calculating the median of the 15 model simulations for each year. The inter-quantile range, i.e., the range of values between the 25th and 75th percentiles, was used to estimate the variability of the model simulations.

Contact

Eurac Research: Alice Crespi, Center for Climate Change and Transformation

Data provided by: Office for Meteorology and Avalanche Warning of the Autonomous Province of Bolzano