Frequent heat waves in the latest decade have drawn attention due to large mortality and economic losses. Urban areas are in additional danger of extreme heat due to the well‐known urban heat island ...(UHI) phenomena. Here, we investigate the effect of the North Atlantic Oscillation (NAO) in modifying urban heat in Zagreb, Croatia. Both winter (wNAO) and summer (sNAO) NAO components are analysed in terms of indirect (lagged) and direct effects on the urban heat load. The strongest increase in heat load was found when positive wNAO was followed by negative sNAO, while the opposite situation led to the strongest decrease. Situations with the same wNAO and sNAO polarity resulted in much weaker amplitude of changes due to opposing effects on climate parameters over investigated area. These findings were confirmed using ground measurements from meteorological station, as well as an urban climate model. However, not only changes in the total heat load, but also differences regarding UHI intensity are detected. It is found that soil moisture, at least partially, drives such behaviour. The combination of positive wNAO followed by negative sNAO leads to a longer drier and warmer conditions over the Zagreb area and vice versa. Hence, in situations with an extended drier period the increase in heat load over green areas is found to be stronger compared to the densely built‐up regions resulting in lower spatial gradient. This was confirmed by additional modelling experiments where the standardized precipitation evapotranspiration index (SPEI) was used to analyse the effect of drought conditions on UHI. These findings are additionally confirmed using satellite land surface temperature data. The potential applicability of this study includes UHI mitigation by irrigation of green urban areas in situations when seasonal forecasts indicate that warmer and drier conditions are expected.
Analysis of NAO impact on urban climate of Zagreb showed that both winter (wNAO) and summer (sNAO) NAO events affect summer heat load. The magnitude of that effect is not uniform over the city area and depends on the combination of NAO phases. In general, combinations related to extended drier (wetter) conditions are associated with an increase (a decrease) in urban heat load.
Atmospheric variability and predictable components over the North Atlantic–European region (NAE) were analysed in the late-winter season using a general circulation model of intermediate complexity ...(ICTP AGCM). The method of empirical orthogonal functions (EOF) analysis was applied to 200-hPa geopotential heights to extract individual modes of variability occurring in the ensemble of numerical simulations. The same variable was selected for the signal-to-noise optimal patterns method, which identifies the patterns maximising the signal-to-noise ratio, following Straus et al. (J Clim 16(22):3629–3649, 2003). Six experiments based on a 35-member ensemble of 156-year long simulations were conducted to detect the potential impact of tropical sea surface temperatures. Each experiment was forced with observed sea surface temperature anomalies prescribed in different ocean areas: globally, in the entire tropical zone, the tropical Atlantic region, the tropical Pacific area, and the tropical Indian Ocean area. In late winter, the leading EOF pattern calculated for all individual ensemble members projects onto the North Atlantic Oscillation, while the second EOF pattern projects onto the East Atlantic pattern. However, EOF modes based on the ensemble mean, which should reflect the forced component of the signal, have different spatial characteristics. Alongside the classical analysis of signal and noise, results of the signal-to-noise optimal patterns method suggest that the optimal patterns and signal-to-noise ratio are affected by the boundary forcing of the oceans. Furthermore, the resemblance between the first optimal pattern and the EOF1 pattern based on the ensemble mean points toward the vital role of the lower-boundary-forced signal in establishing potential predictability in the NAE region.
New observational evidence for variability of the atmospheric response to wintertime El Niño-Southern Oscillation (ENSO) is found. Using different approaches and datasets, a weakening in the recent ...ENSO teleconnection over the North Atlantic-European (NAE) region is demonstrated. Changes in both pattern and strength of the teleconnection indicate a turning point in the 1970s with a shift from a response resembling the North Atlantic Oscillation (NAO) to an anomaly pattern orthogonal to NAO with very weak or statistically non-significant values; and to nearly non-existent teleconnection in the most recent decades. Results shows the importance of the background sea surface temperature (SST) state and sea-ice climatology having opposite effects in modulating the ENSO-NAE teleconnection. As indicated with targeted simulations, the recent change in the SST climatology in the Atlantic and Arctic has contributed to the weakening of the ENSO effect. The findings of this study can have implications on our understanding of modulations of ENSO teleconnections and ENSO as a source of predictability in the NAE sector.
ABSTRACT
The impact of the wintertime El Niño‐Southern Oscillation (ENSO) on European late spring climate is examined using an atmospheric general circulation model of intermediate complexity. The ...analysis is focused on the response of the lower stratosphere/upper troposphere, downward propagation of the ENSO signal detected in temperature, zonal wind and geopotential height and contribution of the North Atlantic to the persistence of the associated surface anomalies. It is found that the stratosphere responds to El Niño (La Niña) events with substantial warming (cooling) occurring in the polar area accompanied by a corresponding modification of upper‐level geopotential heights and zonal winds resembling the pattern of the Northern Annular Mode. The atmospheric response is detected in the upper troposphere as well as at the surface where it interacts with the North Atlantic. In this way, the footprint of atmospheric wintertime ENSO signal is preserved in the ocean persisting until the following spring when it is transmitted back into the atmosphere through the sea–air interaction. Furthermore, it is showed here that the late springtime ENSO signal over Europe may be considered as a result of two contributing processes: one is a direct (spring‐to‐spring) ENSO influence and the other is a delayed (winter‐to‐spring) ENSO influence. Two processes enable the delayed ENSO impact: the persistence of the wintertime ENSO signal in the stratosphere and the atmosphere–ocean interaction in the North Atlantic. Numerical simulations indicate that the memory of the ocean mixed layer is stronger than that of the stratosphere. In this manner, the presented results emphasize the extratropical Atlantic as a contributing factor for climate variability linking wintertime atmospheric circulation associated with ENSO forcing and European climate during the following spring.
Characteristics and changes in minimum and maximum air temperatures and associated climate indices are analysed for Zagreb city (Croatia). Daily data from the period 1960–2019 at four meteorological ...stations (urban, suburb, airport and mountain) are used. Generally, changes in extreme temperatures showed to be the least expressed at the mountain site compared to other city stations. An increase (decrease) in the frequency of warm (cold) extremes is obtained using both stationary (applied to non-overlapping 30-year periods) and non-stationary (applied to the whole period of the analysis) generalized extreme value distribution. Likelihood ratio test revealed that inclusion of linearly time-dependent location parameter significantly improves the performance of the model, especially for maximum air temperature. The effect of a period used as a reference for the percentile-based indices estimation is analysed using three different periods: standard climatological period 1961–1990, recent climatological period 1981–2010 and the whole period of the analysis 1960–2019. Regardless of the base period used, generally significant increase (decrease) in warm (cold) indices is detected for both summer and winter season. Still, increase (decrease) in warm (cold) indices is stronger when period 1961–1990 (1981–2010) is used as a reference. The effect of different period used as a reference is more expressed for summer season. Summer trend rates showed to be stronger compared to the winter ones, especially for the right side of the distributions (warm indices). This is due to a more pronounced increasing trend in the latest 30-year period compared to the earlier period.
The obtained results imply potential increase (decrease) in risk associated with warm (cold) events which showed to be more expressed for warm summer extremes.
In this paper a potential seasonally lagged impact of the wintertime North Atlantic Oscillation (NAO) on the subsequent spring climate over the European region is explored. Supported by the ...observational indication of the wintertime NAO–spring climate connection, a modeling approach is used that employs the International Centre for Theoretical Physics (ICTP) atmospheric general circulation model (AGCM) as a stand-alone model and that is also coupled with a mixed layer ocean in the North Atlantic. Both observational and modeled data indicate a pattern of sea surface temperatures (SSTs) in North Atlantic as a possible link between wintertime NAO and climate anomalies in the following spring. The SST pattern is associated with wintertime NAO and persists through the following spring. It is argued that these SST anomalies can affect the springtime atmospheric circulation and surface conditions over Europe. The atmospheric response is recognized in observed as well as in modeled data (mean sea level pressure, temperature, and precipitation). Additionally, an impact on springtime storm activity is found as well.
It is demonstrated that the SST anomalies associated with wintertime NAO persist into the subsequent spring. These SST anomalies enable atmosphere–ocean interaction over the North Atlantic and consequently affect the climate variability over Europe. Although it has a relatively weak impact, the described mechanism provides a temporal teleconnection between the wintertime NAO and subsequent spring climate anomalies.
We investigate the Northern Hemisphere atmospheric circulation anomalies associated to the sea surface temperature (SST) anomalies that are related to the eastern-Pacific and central-Pacific El ...Nino-Southern Oscillations in the late autumn (November). This research is motivated by the need for improving understanding of the autumn climate conditions which can impact on winter climate, as well as the relative lack of study on the boreal autumn climate processes compared to winter. Using reanalysis and SST datasets available from the late nineteenth century through the recent years, we found that there are two major atmospheric responses; one is a hemispheric-wide wave number-4 pattern, another has a more annular pattern. Both of these project on the East Atlantic pattern (southward-shifted North Atlantic Oscillation) in the Atlantic sector. Which of the patterns is active is suggested to depend on the background mean flow, with the annular anomaly active in the most recent decades, while the wave-4 pattern in the decades before. This switch is associated with a change of correlation sign in the North Pacific. We discuss the robustness of this finding. The ability of two atmospheric general circulation models (ICTP-AGCM and ECHAM-AGCM) to reproduce the teleconnections is also examined. Evidence provided shows that the wave-4 pattern and the East Atlantic pattern signals can be reproduced by the models, while the shift from this to an annular response for the recent years is not found conclusively.
A limited number of meteorological stations and sparse data challenge microclimate assessment in urban areas. Therefore, it is necessary to complement these data with additional measurements to ...achieve a denser spatial coverage, enabling a detailed representation of the city’s microclimatic features. In this study, conducted in Zagreb, Croatia, mobile air temperature measurements were utilized and compared with satellite-derived land surface temperature (LST). Here, air temperature measurements were carried out using bicycles and an instrument with a GPS receiver and temperature probe during a heat wave in June 2021, capturing the spatial pattern of air temperature to highlight the city’s microclimate characteristics (i.e. urban heat load; UHL) in extremely hot weather conditions. Simultaneously, remotely sensed LST was retrieved from the Landsat-8 satellite. Air temperature measurements were compared to city-specific street type classification, while neighbourhood heat load characteristics were analysed based on local climate zones (LCZ) and LST. Results indicated significant thermal differences between surface types and urban forms and between street types and LCZs. Air temperatures reached up to 35 °C, while LST exceeded 40 °C. City parks, tree-lined streets and areas near blue infrastructure were 1.5–3 °C cooler than densely built areas. Temperature contrasts between LCZs in terms of median LST were more emphasised and reached 9 °C between some classes. These findings highlight the importance of preserving green areas to reduce UHL and enhance urban resilience. Here, exemplified by the city of Zagreb, it has been demonstrated that the use of multiple datasets allows a comprehensive understanding of temperature patterns and their implications for urban climate research.
Recent studies have indicated the importance of fall climate forcings and teleconnections in influencing the climate of the northern mid- to high latitudes. Here, we present some exploratory analyses ...using observational data and seasonal hindcasts, with the aim of highlighting the potential of the El Niño–Southern Oscillation (ENSO) as a driver of climate variability during boreal late fall and early winter (November and December) in the North Atlantic–European sector, and motivating further research on this relatively unexplored topic. The atmospheric ENSO teleconnection in November and December is reminiscent of the east Atlantic pattern and distinct from the well-known arching extratropical Rossby wave train found from January to March. Temperature and precipitation over Europe in November are positively correlated with the Niño-3.4 index, which suggests a potentially important ENSO climate impact during late fall. In particular, the ENSO-related temperature anomaly extends over a much larger area than during the subsequent winter months. We discuss the implications of these results and pose some research questions.
Urban climate is affected by weather, global climate change and urban development. However, climate change and urbanisation take place simultaneously with intertwined impacts. To analyse their ...relative contribution to the heat load of Zagreb, a modelling approach is applied to two land-use/land-cover (LULC) situations and corresponding climate conditions. The results indicate that the change in total heat load is dominantly affected by climate change (∼88%) with an average increase in the summer days for 35 days. LULC alterations have a weaker impact (∼12%), but they strongly affect heat load spatial variability. The sign of LULC related heat load change depends on the type of the change (e.g. an increase is detected in areas that have turned from green into built-up classes). Generally, LULC effect is limited to the area with the modification, however it can spread to adjacent areas due to the processes like advection and evapotranspiration. In areas with considerable LULC alterations, their impact on the heat load is comparable to that of climate change. These results highlight the potential of change in the city infrastructure for climate adaptation, as well as emphasise the importance of considering future climate conditions when assessing efficiency of climate adaptation measures.
•total heat load change is dominantly affected by climate change effect (∼ 88%).•climate change effect manifests as relatively uniform amplitude increase over the whole domain (with increase in number of summer days for ∼35 days).•land-use/land-cover alterations impact heat load spatial variability.•land-use/land-cover effect is mainly limited to the area with the modification and in certain areas comparable to the climate effect.
PDF
