It is often claimed that memory for weather events is short, resulting in limited retention of extreme events, which we tend to exaggerate. There is a prevailing belief among people of all ages that winters have changed significantly from their historical patterns. But is this belief correct? Is it true that winters are becoming shorter in Spain?
In this article we are going to analyze If winters are getting shorter in Spain.
Cold days in Spain
For many years, a lower than anticipated frequency of cold days has been documented, attributed to basic natural variability, along with generally high minimum temperatures, in line with the forecasts outlined by several climate change scenarios. One of them predicts a decrease in the duration of winters and an increase in the duration of summers. A study by César Rodríguez examined temperature records from several observatories across the country and discovered that summers are extending by 4 to 15 days per decade, depending on the regionBased on this research, a similar analysis was conducted across the country. This current study aims to conduct the same assessment specifically for winter. To learn more about how climate change affects our seasons, you can consult the impact of melting ice in Spain.
What methodology was used to perform the calculations?
To determine potential winter disturbances, the initial task is to establish a definition of winter and its duration. The analysis has been carried out using the Copernicus Climate Change Service (C5S) ERA3 reanalysis database, which provides hourly data from 1940 to the present, with a spatial resolution of 0,25⁰ for both latitude and longitude. The following calculations have been performed for each specific point:
The average daily temperature has been determined for the period from , covering the years 1991 to 2020. To avoid including periods outside winter that present minimum temperatures characteristic of that season but deviate significantly from typical winter conditions (including low maximum temperatures), the average temperature has been used instead of the minimum temperature.
Thus, Periods, often indicative of spring, which could be misleading and unnecessarily lengthen the winter season, have been effectively excluded. A threshold based on the 70th percentile of the distribution of daily mean temperatures was then established, allowing outlier days with significantly high positive anomalies to be removed. The threshold values at each location are illustrated in the image below.
For each year from 1940 to 2022, the nine-day moving average temperature was calculated and the beginning and end days of the year when the designated temperature threshold was reached or fell below were identified. To avoid the occurrence of false positives, A prolonged winter period was chosen rather than focusing on an individual spring or autumn day characterized by an unusually low temperature.The temperature of a specific day was determined by averaging the values of the four days before and the four days after, resulting in a total of nine days.
Finally, the dates of the 82 winters were examined for increasing or decreasing trends by applying the Mann-Kendall test. Although the methodology and thresholds may seem subjective, the results demonstrated consistent and comparable behavior when certain parameters (including percentiles, dates, climatological period, etc.) were modified, suggesting that In fact, the length of winter is decreasingHowever, it is essential to recognize that the findings should be considered an approximation and not definitive or rigid values.
Winter decline
Studies reveal a notable reduction in the winter season in almost all territories since the 1940s. This decline is less pronounced in the southwestern region of the peninsula, while the central and eastern areas show a more significant decrease, with many localities experiencing a reduction of more than one month. On average, The length of winters today is one month shorter than in the mid-20th century..
Here you can learn more about how Last winter was the warmest ever recorded in the history of Spain.There are regions where the trend cannot be confirmed with more than 95% certainty (p-value greater than 0,05). In percentage terms, Winter has been shortened by more than 30% in more than half of the countryIt's important to note that this significant trend is also observed in the maritime regions of the Canary Islands, where the reduction is even more pronounced and clear than on the Iberian Peninsula. Part of the explanation for this phenomenon may lie in the lower temperature ranges experienced in the archipelago; as a result, even minimal temperature variations can cause substantial changes in the seasonal calendar.
It is essential to recognize that a downward trend does not mean that every winter is shorter than its predecessor. Winters of varying length have occurred over the past ten years, but a broader time frame reveals more details about the onset of cold weather in Spain, can offer a more complete view of seasonal behavior.
Winter and Autumn
The length of winters is being reduced as a result of the "theft" of both spring and autumn. This gradual lengthening It is not uniform, generally showing a more significant increase in spring than in autumn.The lengthening of spring, known as "invernavera," is especially evident in the southern, central, and eastern regions of the Iberian Peninsula, where in many areas there is an increase of more than three weeks. This trend is also observed throughout much of the territory. A similar pattern is observed in the Canary Islands. Thus, it can be concluded that the shortening of spring due to the lengthening of summer is offset by its lengthening at the expense of winter.
As for the phenomenon of winter reduction by autumn, called "inverotoño", the magnitude is minimal. In the southern regions and some eastern areas, the reduction is remarkably low, with a level of certainty that does not exceed 95%. In other regions, however, the trend is longer than ten days and the certainty is quite clear. In particular, in the Canary Islands, autumn is longer in the western areas, lasting more than three weeks. The results of this analysis invite speculation about the potential duration of winter in 82 years; however, such projections must be approached with great caution.
The gradual decline in winter is characterized by nonlinear and nonmonotonic patterns, showing periods of pronounced trends, stagnation, and even slight increases. In general, a downward trend has been observed on the Peninsula since the 1980s and 1990s, which corresponds to the second half of the study period. However, it is plausible that the reduction in winter is accelerating.
The evidence is consistent with the view that winters are becoming shorter, a trend that is in line with projections related to climate change. This decrease in winter length is somewhat offset by a lengthening of spring, while the effects observed in autumn are less significant.
