Extreme warm spells may facilitate the new temperature record in 2023


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Record-breaking mean annual temperatures (MAT) have been reported frequently in China since the 21st century.According to the observations of the China Meteorological Administration (CMA), all the 17 years with nationally averaged MAT above 10 • C since 1961 were all concentrated in 1998-2023.The MAT shows a more stable linear increasing trend compared to seasonal temperatures, resulting in a new normal of frequent records.In recent years, more severe impacts of higher MAT have been found in various sectors, such as agricultural yield, water resources, energy demand, disease risk and ecosystem vulnerability [1].
Previous climatic studies on record-breaking temperatures have mainly focused on the seasonal time scale, such as the warmest winter in 2016 [2], the hottest summer in 2018 [3], and the exceptional summer temperatures in 2022 [4].Mechanism diagnoses of the external forcing and internal atmospheric circulations of these record-breaking temperatures have also been focused on the seasonal scale.However, uncertainties remain for the record-breaking temperatures on the annual scale, due to the different durations of the forcing factors.For example, the World Meteorological Organization (WMO) announced in November that 2023 would almost certainly be the warmest year on record due to the return of the El Niño, while MAT will keep rising to make 2024 even warmer.However, 2021 was the second warmest year for both China and the world, and 2021 is in the middle of a triple La Nina process.This means that the record-breaking MAT can hardly be explained by a single factor.
Recent studies also suggest that internal atmospheric variability, in particular the increasing warm whiplash or warm spell, may have a significant climate impact on the anomalous temperature accumulation [3,5].In contrast to a cold surge or cold spell, a warm spell refers to a short-term weather phenomenon in which the air temperature rises sharply.However, compared to the research on cold spells that was conducted in China as early as in the 1930s [6], there has been relatively little research on warm spells.Studies have explored the far-reaching effects of warm spells on many aspects.For example, extreme warm spells in Europe can significantly affect agriculture and ecosystems [7].The natural environment is also affected, particularly through rapid melting of glaciers or snow cover [8].They can also cause compound hazards when they overlap with other meteorological events [9].Ding et al [3] recently found that both nationwide extreme cold and warm spells in China have increased rapidly since the 21st century.
The CMA recently announced that 2023 was the warmest year in China since 1961, with a new MAT record of 10.72 • C. Thirteen provinces in China also experienced their own new MAT records.Supplementary figure 1 shows the distribution of MAT anomalies for the year, as well as the stations ranked 1st, 2nd and 3rd during 1961-2023.It can be observed that for most regions in China, the MAT is higher than the average climate temperature, with 928 stations experiencing positive anomalies above 1 • C. By contrast, 2021 and 2022 were the second and third warmest years, with 608 and 479 stations respectively.In 2023, 28.8%, 17.9% and 11.5% of stations had temperatures ranked 1st, 2nd and 3rd, respectively.The major contributors to make 2023 the warmest year on record were the MAT anomalies from the southern part of North China to the Huanghuai River and from the western part of South China to the eastern part of the southwestern China.Meanwhile, the temperature increase in 2023 was the highest recorded at 0.18 • C, significantly higher than the range of The extreme warm spells in 2023 may make it the warmest year.Figure 1 shows the different frequency distributions of daily MAT anomalies in each 0.1 • C increments for both the 1961-2022 mean and 2023.The distribution curve for 2023 is remarkable shifted to the right side, indicating a higher frequency of warm anomalies compared to the 1961-2022 mean.Conversely, the frequencies or probabilities of cold anomalies are notably lower than the 1961-2022 mean.Based on the distribution, it appears that there is a significant increase in warm spell days and a decrease in cold spell days in 2023.Generally, more extreme warm spells and less cold spells could prompt a higher MAT.To further explore the roles of the warm spells, figure 1 also shows the number of extreme warm and cold spell days, as well as the accumulation of MAT anomalies during these spell days.Here the extreme warm (cold) day is defined as the day with the temperature anomaly above (below) the 5th (95th) percentile threshold of positive (negative) anomalies.In 2023, there were 41 extreme warm spell days, among which 40 d appeared in January-February-March and in October-November-December. The temperature anomaly accumulation of the 41 d was +173.9 • C. Both of the days and the accumulation were much higher than any previous year.There were 5 d of extreme cold and the temperature anomaly accumulation was −19.2 • C.However, these values were not the lowest during the research period.
To illustrate the role of extreme warm spells in facilitating the new temperature record, the daily temperature anomalies from the lowest to the highest based on the climatological mean are calculated to obtain the MAT in each year (figure 2).It could be seen that during the cold spell days of 2023 (left part of the curve), the accumulated temperature was not the highest.However, the accumulated temperature increased rapidly to be the highest (right part of the distribution curve), indicating that the record was mainly caused by the warm spell days, as shown in figure 1.With the increase of temperature anomaly, the difference of MAT accumulation is becoming more and more remarkably.Figure 2 also shows the scatter points between MAT and extreme warm and cold spell days.It could been found that in the lowest MAT years, such as 1967 and 1969, the MAT is determined by the cold spells.Conversely, in the highest years, such as 2021, the MAT is mainly caused by the warm spells.During the study period, correlation coefficients between the MAT and the accumulated temperatures of the first ten warm spell days was 0.76, also indicating the great contribution of extreme warm spells.
Results also indicate that the occurrences of both the warm spells and the cold spells are independent of ENSO (figure not shown).There is not close relationship between ENSO phases and them.Both the warm spell and the cold spell frequencies show significant linear trends.Even in super El Nino years such in 1998, the extreme warm spell is not quite frequent.Therefore, atmospheric internal variation, particularly the extreme warm spells, may contribute to the warmest year in 2023.But further analyses are still needed about the roles of the extreme warm spells.For instance, WMO confirms that 2023 smashes global temperature record.Is this conclusion feasible in other regions?Besides, as we mentioned before, the extreme warm spells occur mainly in winter season.But in 2023, many regions also suffered unprecedented summer heatwaves.Therefore, the contributions of the heatwaves to the record should also be studied and compared next.

Figure 1 .
Figure 1.Frequency distribution of daily temperature anomalies in each 0.1 • C degree (abscissa) in 1961-2022 mean (blue bar) and in 2023 (orange bar).Number of extreme warm (cold) spell days and the accumulation of temperature anomalies in the spell days in each year are displayed by orange (blue) scatter points in the right (left) upper corner, respectively.Deep orange (blue) point indicates the year of 2023.

Figure 2 .
Figure 2. Accumulation of daily temperatures in 365 d from the lowest temperature anomaly day to the highest temperature anomaly day in each year of 1961-2023 (thin grey lines: 1961-2022; red thick line: 2023).The annual temperature (dots), the extreme warm spell days (abscissa) and the extreme cold spell days (ordinate) are displayed in the lower right corner.