North Atlantic Oscillation

Introduction

In climate studies based on astronomical methods, static temporal and spatial definitions of seasons are used. However, seasons change with time and space. Changes in the length and timing of seasons have major impacts on nature and human society. For example, shorter winters and reduced snow cover combined with an earlier onset of spring lead to earlier and weaker spring floods and increased river discharge in winter. Changes in climatic parameters, especially snowfall and snow storage, significantly affect the seasonal water balance and the precipitation-runoff process. Furthermore, these changes are predicted to continue in the future due to further climate change. The large range of elevation changes in the Lake Urmia basin has caused the distribution of snow cover in the basin to be heterogeneous. Global warming caused by greenhouse gas emissions can affect the temporal and spatial distribution of snow cover in the Lake Urmia basin, both through changes in the timing and length of the winter season and through increased evapotranspiration, which has the largest contribution to controlling snow cover area. In this regard, this study aims to evaluate and compare the characteristics of the start, end, and length of the winter season in two recent 30-year periods (1964-1993 and 1994-2023) using a winter season definition method based on daily air temperature data. It is worth noting that the method used is based on the inclusion of spatial heterogeneity of winter season characteristics and allows for the investigation of the relationship between winter season characteristics and altitude.



Materials and Methods

In this study, data from the ERA5-Land reanalysis database were used. Daily ERA5-land data were extracted by coding in Google Earth Engine for the Lake Urmia basin and for the two recent 30-year periods, including 1964-1993 and 1994-2023. To determine the winter season, instead of calendar winter, a cell-based definition was used. In this method, winter was defined based on the 25th percentile of daily temperature (T25) based on each of the two periods. With this method, a set of days with T25 less than zero degrees Celsius was defined as winter. Therefore, the beginning and end of the winter season were defined as the first and last day of the year with T25 less than zero degrees Celsius. The start and end of the winter season were reported in terms of the Julian day-number (starting from January 1), and the length of the winter season (in days) was calculated as the interval from the start to the end of the winter season. Next, the spatial distribution and changes in the day-number of the start, the day-number of the end, and the length of the winter season were compared for the two periods. Also, the correlation of the start, end, and length of the winter season with altitude was examined. For this purpose, global SRTM digital elevation model data with a spatial resolution of 30 meters was used. The elevation corresponding to the coordinates of each ERA5-land grid cell within the basin was extracted using ArcGIS software.



Results and Discussion

The percentage of cells with an increase in the day-number of the start of the winter (46.11%) is very close to the percentage of cells with a decrease (47.96%), and the average changes in cells with increasing or decreasing changes are, respectively, 7 and -7.5 days. The spatial distribution of the difference in the day-number of start of the winter is uniform at the basin and has a median of zero (meaning no change in the start of the winter).

Regarding the end of the winter, the differences in most cells (82.45%) are negative, and the average changes in the day-number of the end of the winter in cells with increasing or decreasing changes are, respectively, 7.5 and -15 days. In general, the day-number of the end of the winter has a median of -11 days, which indicates an earlier end of the winter in the second period (1994-2023).

The length of the second period winter in most cells (73.92%) is shorter than the first period. The average change in winter length in cells with increasing or decreasing changes is -17.1 and -17.4 days, respectively. The median change in winter length is also similar to the end of winter, -11 days, indicating a decrease in winter length in the second period.

In both periods, the start, the end and the length of winter show significant negative, positive and positive correlations with altitude, respectively, and the slope of change is slightly steeper in the second period.



Conclusion

The spatial distribution of the start, end, and length of the winter season is non-uniform and follows the topography of the Lake Urmia basin; so that with increasing altitude, the start of the winter season occurs earlier and the end of the winter season occurs later. Therefore, the length of the winter season is longer at higher altitudes. The median change in the length of the winter season is -11 days, which indicates a decrease in the length of the winter season in the second period (1994-2023) compared to the first period (1964-1993). The sensitivity of the characteristics of the winter season to global warming is non-uniform at the basin and the rate of change is greater at lower altitudes. As the trend of increasing air temperature continues, it is predicted that the decrease in the length of the winter season will be even more severe in the coming decades, because according to previous studies under climate change scenarios, the length of the winter season will decrease significantly and reach less than 2 months in the middle latitudes of the Northern Hemisphere. Since global warming can affect the temporal and spatial distribution of snow cover in the Lake Urmia basin, it is recommended to predict changes in winter characteristics in the Lake Urmia basin under climate change scenarios for future studies.

High-resolution (annual to decadal) stable isotope records of oxygen and carbon are analysed from an annually laminated stalagmite from NW Scotland. The sample, which was deposited for ~1000 yrs until 1996 AD, has previously provided annual resolution climate reconstructions of local rainfall and regional winter North Atlantic Oscillation (wNAO) from variations in annual growth rate. For our stalagmite, for which modern cave monitoring demonstrates that equilibrium deposition is highly likely for δ 18 O but not for δ 13 C, stalagmite δ 13 C originally derives from soil CO 2 produced predominantly by microbial respiration, modified by degassing-related kinetic fractionation, and δ 18 O from the composition of infiltrating water during periods of infiltrating water. Both the presence of fluorescent laminae and modern drip-water monitoring demonstrate a drip hydrology that comprises both event and storage components. Over the instrumental period, no correlations between stalagmite or rainfall δ 18 O and precipitation amount or temperature are observed, but correlations are observed between rainfall δ 18 O and 500 mb height at regional IAEA monitoring stations. However, no correlations are observed between stalagmite δ 18 O and instrumental and reconstructed atmospheric circulation, preventing a simple palaeoclimate interpretation of the stalagmite δ 18 O proxy. Stalagmite δ 13 C has a stronger temporal autocorrelation than δ 18 O, indicative of soil mixing of respired CO 2 and significant variability between drips and at different times; correlations with instrumental climate data are therefore not possible. The relative timing of changes in growth rate, δ 18 O and δ 13 C are discussed, and interpretations compared with other regional climate records. We conclude that, over the last millennium at this mid-latitude cave site, neither δ 18 O nor δ 13 C cannot be interpreted as a simple paleoclimate proxy.

Our analyses partition the relative influence of progressive climate change and large-scale climate drivers that can be associated with the Quasi-Biennial Oscillation (QBO), El Niño Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), solar sunspot cycle, and multi-decadal oscillations on lake ice breakup dates for thirteen Northern Hemisphere lakes. Oscillatory dynamics explain 26 % of the total variance in the time series compared with 15 % for linear trends, leaving 60 % unexplained and likely attributable, in part, to local weather. Significant oscillatory dynamics include frequencies in 2-3 year periods (9.4 % of the total variance), 3-6 year periods (8.2 %), 10-12 year periods (1.6 %) and various multidecadal periods (0.4-1.3 %). All 13 study lakes, although widely scattered in the Northern Hemisphere, had similar oscillatory dynamics and linear trends, emphasizing that global processes influence lake ice breakup locally. We illustrate that while quasi-periodic dynamics associated with large-scale climate drivers are important, they do not mask the clear evidence for progressive climate change.

A 20 year coastal altimetry data set (X‐TRACK) is used, for the first time, to gain insight into the long‐term interannual variations of the surface circulation in the Sicily Channel. First, a spectral along with a time/space diagram analysis are applied to the monthly means. They reveal a regionally coherent current patterns from track to track with a marked interannual variability that is unequally shared between the Atlantic Tunisian Current and Atlantic Ionian Stream inflows in the Sicily Channel and the Bifurcation Tyrrhenian Current outflow northeast of Sicily. Second, an empirical altimetry‐based transport‐like technique is proposed to quantify volume budgets inside the closed boxes formed by the crossing of the altimetry tracks and coastlines over the study area. A set of hydrographic measurements is used to validate the method. The inferred altimetry transports give a well‐balanced mean eastward Atlantic Waters baroclinic flow of 0.4 Sv and standard deviations of 0.2 Sv on ...

This paper will employ a novel approach that builds upon the lasso method, utilizing it in two stages. The first stage applies to the principal components to select the important principal component and exclude the unimportant ones. This technique is effective in identifying significant principal components while attempting to eliminate bias in selecting these components over others. Additionally, it removes the ranking in determining the principal components compared to classical methods. Moreover, the second stage involves determining the effective importance within each component by zeroing out the scores loading values within each component. To compare the performance of the proposed method in principal component analysis, a simulation approach can be used. Subsequently, the performance of the proposed method is tested using real data.

SUPPORTING MATERIAL FOR: Reconstructing the rainfall series of the Greek and Balkan Peninsula since 1595 A.D based on tree-ring data and historical evidence verification

Collected documentary records (historical evidence/reference) with the relevant information for each reconstruction (Pfister indices, site, event type, year/period/sate, etc). Year/period/date Pfister Indices based on reconstructions of the present study SITE EVENT TYPE Historical evidence / Reference

A b s t r a c t . The analysis of atmospheric thaws was based on the mean daily values of air temperature recorded at 200 cm above the ground at 15 IMGW meteorological stations within the span of 1960/61-1999/2000. For the determination of the effect of atmospheric circulation on the frequency of thaws in Pomerania, Lityński's classification and also the indices of the North Atlantic Oscillation (NAO) according to Hurrell, Jones and Rogers were used. The vital role played by atmospheric circulation in determining atmospheric thaws in Pomerania was confirmed by statistically significant coefficients of correlation obtained for all of the applied indicators, the largest ones for the group of circulation directions from the western (SW, W and NW) and eastern (NE and E) sectors, including non-advective situations according to Lityński's classification. The spectrum analysis carried out showed that periodic character of the course of atmospheric thaws, with cyclic components of 5.5 and 8.1 years, is determined by similar changes of atmospheric circulation. K e y w o r d s : temperature, Lityński's classification, NAO winter indices, periodicity

A b s t r a c t . The analysis of atmospheric thaws was based on the mean daily values of air temperature recorded at 200 cm above the ground at 15 IMGW meteorological stations within the span of 1960/61-1999/2000. For the determination of the effect of atmospheric circulation on the frequency of thaws in Pomerania, Lityński's classification and also the indices of the North Atlantic Oscillation (NAO) according to Hurrell, Jones and Rogers were used. The vital role played by atmospheric circulation in determining atmospheric thaws in Pomerania was confirmed by statistically significant coefficients of correlation obtained for all of the applied indicators, the largest ones for the group of circulation directions from the western (SW, W and NW) and eastern (NE and E) sectors, including non-advective situations according to Lityński's classification. The spectrum analysis carried out showed that periodic character of the course of atmospheric thaws, with cyclic components of 5.5 and 8.1 years, is determined by similar changes of atmospheric circulation. K e y w o r d s : temperature, Lityński's classification, NAO winter indices, periodicity

South situations or days in which the South Wind (SE-S-SW) constitutes the predominant wind direction (mode) and one of the factors more determinants of the climatic conditions in terms of temperature and relative humidity in the autumn-winter period of the geographical region of southwestern Europe around the central territorial axis of the Bay of Biscay-Gascony. According to our conclusions on the official data analyzed in thirteen meteorological stations in this region of southwestern Europe, for the 1961-2010 annual series, more than 43% of the autumn-winter days with prevailing winds from the South, or South Wind, register average temperature values (T) higher than their respective autumn-winter average. Likewise, in eleven of the thirteen stations analyzed, for the same annual series, the average relative humidity (H) record corresponding to the set of autumn-winter days with predominant South Wind is lower than the respective mid autumn-winter and in the other two seasons both records are equal.

This multi-proxy study of IODP Site U1387 (Faro Drift) aims to reconstruct the history of the surface water and the Mediterranean Outflow Water (MOW) during the interval from 630 to 1100ka, i.e. the Mid-Pleistocene Transition. Surface and MOW experienced orbital-and millennial-scale variations. MOW related proxies show a tight coupling to insolation with poor ventilation during insolation maxima and contourite layers being formed by a faster flowing MOW during insolation minima.

Airborne pollen records constitute a powerful tool for different purposes. First, in studies of dispersion and source identification of pollen grains, which, incorporated into the atmosphere by anemophilous plants, produce allergic symptoms in a part of the population sensitive to it. On the other hand, aerobiological databases provide useful information for the understanding of the trends induced by the climate change. The decadal series allow the extraction of patterns with intra-annual variability (seasonality of the events and their relationship with the atmospheric transport patterns) and inter-annual variability (inter-annual trends and its possible relationship with climatic variability indexes). The longer the aerobiological monitoring series the better the understanding of the effects of climate change on pollination trends. The aim of this study is to explore the influence of climate change on the airborne pollen series recorded in Catalonia during the period 1994-2012 by ...

Climatic oscillations triggered by the atmospheric modes of the Northern Hemisphere teleconnection patterns have an important influence on the atmospheric circulation at synoptic scale in Western Mediter-44 ranean Basin. Simultaneously, this climate variability could affect a variety of ecological processes. This 45 work provides a first assessment of the effect of North Atlantic Oscillation (NAO), Arctic Oscillation (AO) 46 and Western Mediterranean Oscillation (WeMO) on the atmospheric long-range pollen transport episodes 47 in the North-Eastern Iberian Peninsula for the period 1994-2011. Alnus, Ambrosia, Betula, Corylus and Fagus 48 have been selected as allergenic pollen taxa with potential long-range transport associated to the Northern 49 Hemisphere teleconnection patterns in the Western Mediterranean Basin. The results showed an increase 50 of long range pollen transport episodes of: (1) Alnus, Corylus and Fagus from Western and Central Europe 51 during the negative phase of annual NAO and AO; (2) Ambrosia, Betula and Fagus from Europe during the 52 negative phase of winter WeMO; (3) Corylus and Fagus from Mediterranean area during the positive 53 phase of the annual AO; and (4) Ambrosia from France and Northern Europe during the positive phase of 54 winter WeMO. Conversely, the positive phase of annual NAO and AO are linked with the regional transport 55 of Alnus, Betula and Corylus from Western Iberian Peninsula. The positive phase of annual WeMO was also 56 positively correlated with regional transport of Corylus from this area.

Using 22 pollen taxa collected at 6 localities in Catalonia (N E Spain) during the period 1994-2011, we show that climate variability associated with Northern Hemisphere teleconnection patterns (North Atlantic Oscillation, Arctic Oscillation and Western Mediterranean Oscillation) affects both seasonal pollen production and the timing of the airborne pollen season. For most of the studied pollen taxa, positive phases of the 3 climatic indices were related to a decrease in the seasonal pollen index and an advance and enlargement of the pollen season. Furthermore, for most taxa, negative phases of the climatic indices were linked to higher pollen production via an increase in rainfall. A clear relationship between climatic indices and the end of the pollen season was not observed. Since pollen parameters correlate more strongly with the Western Mediterranean Oscillation index (WeMOi) than other indices, the WeMOi negative trend observed over the last decades needs to be confirmed to properly identify its effect on ecosystems and public health in the western Mediterranean basin.

Through calculating the scatter diagrams of the streamfunction ( P or T ) versus potential vorticity (PV) (q P or q T ), where P and T are the planetary-scale streamfunction and total streamfunction, respectively, and using a weakly nonlinear NAO model proposed in Part I of this paper, it is suggested that negative-and positive-phase NAO events may approximately correspond to free modes even though driven by synopticscale eddies. In a planetary-scale field, the q P ( P ) scatter diagram of an NAO event exhibits a linear multivalued functional relationship in a narrow region for the negative phase, but exhibits a linear singlevalued functional relationship during the positive phase. It was also found that there is no steepening of the slope of the main straight line in the q P ( P ) scatter diagrams for two phases of the NAO event. Instead, the slope of the straight line in the scatterplots is time independent throughout the life cycle of the NAO event. However, when synoptic-scale eddies are included in the streamfunction field, the q T ( T ) scatter diagram of the negative-phase NAO event shows a trend toward steepening during the intensification phase, and this tendency reverses during the decay phase. During the positive NAO phase the slope of the q t ( T ) scatter diagram shoals during the intensification phase and then steepens during the decay phase. Thus, it appears that the steepening and shoaling of the scatter diagrams of the streamfunction versus PV for the negativeand positive-phase NAO events are attributed to the effect of synoptic-scale eddies that force NAO events to form. Diagnostic studies using both composite and unfiltered fields of observed NAO events are presented to confirm these conclusions.

A simple theoretical model is proposed to clarify how synoptic-scale waves drive the life cycle of the North Atlantic Oscillation (NAO) with a period of nearly two weeks. This model is able to elucidate what determines the phase of the NAO and an analytical solution is presented to indicate a high similarity between the dynamical processes of the NAO and zonal index, which is not derived analytically in previous theoretical studies. It is suggested theoretically that the NAO is indeed a nonlinear initial-value problem, which is forced by both preexisting planetary-scale and synoptic-scale waves. The eddy forcing arising from the preexisting synoptic-scale waves is shown to be crucial for the growth and decay of the NAO, but the preexisting low-over-high (high-over-low) dipole planetary-scale wave must be required to match the preexisting positive-over-negative (negative-over-positive) dipole eddy forcing so as to excite a positive (negative) phase NAO event. The positive and negative feedbacks of the preexisting dipole eddy forcing depending upon the background westerly wind seem to dominate the life cycle of the NAO and its life period. An important finding in the theoretical model is that negative-phase NAO events could be excited repeatedly after the first event has decayed, but for the positive phase downstream isolated dipole blocks could be produced after the first event has decayed. This is supported by observed cases of the NAO events presented in this paper. In addition, a statistical study of the relationship between the phase of the NAO and blocking activity over Europe in terms of the seasonal mean NAO index shows that blocking events over Europe are more frequent and long-lived for strong positive-phase NAO years, indicating that the positivephase NAO favors the occurrence of European blocking events.

The El Niño-Southern Oscillation (ENSO) is a dominant source of global climate variability. The effects of this phenomenon alter the flow of heat from tropical to polar latitudes, resulting in weather and climate anomalies that are difficult to forecast. The current work quantified two components of the vertically integrated equation for the total energy content of an atmospheric column, to show the anomalous horizontal redistribution of surface heat flux anomalies. Symmetric and asymmetric components of the vertically integrated latent and sensible heat flux divergence were quantified using ERA-Interim atmospheric reanalysis output on 30 model layers between 1979 and 2016. Results indicate that asymmetry is a fundamental component of ENSO-induced weather and climate anomalies at the global scale, challenging the common assumption that each phase of ENSO is equal and opposite. In particular, a substantial asymmetric component was identified in the relationship between ENSO and patterns of extratropical climate variability that may be proportional to differences in sea surface temperature anomalies during each phase of ENSO. This work advances our understanding of the global distributions of source and sink regions, which may improve future predictions of ENSO-induced precipitation and surface temperature anomalies. Future studies should apply these methods to advance understanding and to validate predictions of ENSO-induced weather and climate anomalies.

The knowledge of local littoral processes is of utmost importance for any coastal zone management with regard to coastal protection and maintenance of navigation channels. Observations of wave conditions and nearshore current patterns have long been used to determine the sediment transport in the littoral zone. Several methods of quantifying the longshore transport rate at a particular location are applied such as a well known CERC formula and numerical models for longshore modeling along 1D coastal profiles and 2D area morphodynamic modeling. Since the results of all methods critically depend on the wave input data, careful compilation of site specific wave characteristics is essential in morphodynamic studies. We discuss longshore transport phenomena for a time span of 5 years in Gellen bay which is located at the German coast of the southern Baltic Sea. Special attention is paid to the preparation of boundary conditions for the different study methods and to the complementary inf...

Global climate change has readjusted a global-scale precipitation distribution in magnitude and timing. In mountainous areas, meteorological stations and observation data are very limited, making it difficult to accurately understand the response of precipitation to global climate change. Based on ECMWF Reanalysis v5 precipitation products, Berkeley Earth global temperature, and typical atmospheric circulation indexes, we integrated a gradient descent-nonlinear regression downscaling model, cross wavelet transform, and wavelet correlation method to analyze the precipitation response in Tianshan to global climate change. This study provides a high-resolution (90 m × 90 m) precipitation dataset in Tianshan and confirms that global warming, the North Pacific Pattern (NP), the Western Hemisphere Warm Pool (WHWP), and the Atlantic Multidecadal Oscillation (AMO) are related to the humidification of Tianshan over the past 40 years. The precipitation in Tianshan and global temperature have ...

Atlantic meridional overturning circulation (AMOC) variability is documented in the Community Climate System Model, version 4 (CCSM4) preindustrial control simulation that uses nominal 18 horizontal resolution in all its components. AMOC shows a broad spectrum of low-frequency variability covering the 50-200-yr range, contrasting sharply with the multidecadal variability seen in the T85 3 1 resolution CCSM3 presentday control simulation. Furthermore, the amplitude of variability is much reduced in CCSM4 compared to that of CCSM3. Similarities as well as differences in AMOC variability mechanisms between CCSM3 and CCSM4 are discussed. As in CCSM3, the CCSM4 AMOC variability is primarily driven by the positive density anomalies at the Labrador Sea (LS) deep-water formation site, peaking 2 yr prior to an AMOC maximum. All processes, including parameterized mesoscale and submesoscale eddies, play a role in the creation of salinity anomalies that dominate these density anomalies. High Nordic Sea densities do not necessarily lead to increased overflow transports because the overflow physics is governed by source and interior region density differences. Increased overflow transports do not lead to a higher AMOC either but instead appear to be a precursor to lower AMOC transports through enhanced stratification in LS. This has important implications for decadal prediction studies. The North Atlantic Oscillation (NAO) is significantly correlated with the positive boundary layer depth and density anomalies prior to an AMOC maximum. This suggests a role for NAO through setting the surface flux anomalies in LS and affecting the subpolar gyre circulation strength.

representation of the NAO and the utility of this approach. Finally, we note that the interannual statistics of the NAO and associated surface climate impacts are subject to uncertainty due to sampling fluctuations, even when based on a century of data.

Large changes in the wintertime atmospheric circulation have occurred over the past two decades over the ocean basins of the Northern Hemisphere, and these changes have had a profound effect on regional distributions of surface temperature and precipitation. The changes over the North Pacific have been well documented and have contributed to increases in temperatures across Alaska and much of western North America and to decreases in sea surface temperatures over the central North Pacific. The variations over the North Atlantic are related to changes in the North Atlantic Oscillation (NAO). Over the past 130 years, the NAO has exhibited considerable variability at quasibiennial and quasi-decadal time scales, and the latter have become especially pronounced the second half of this century. Since 1980, the NAO has tended to remain in one extreme phase and has accounted for a substantial part of the observed wintertime surface warming over Europe and downstream over Eurasia and cooling in the northwest Atlantic. Anomalies in precipitation, including dry wintertime conditions over southern Europe and the Mediterranean and wetter-than-normal conditions over northern Europe and Scandinavia since 1980, are also linked to the behavior of the NAO. Changes in the monthly mean flow over the Atlantic are accompanied by a northward shift in the storm tracks and associated synoptic eddy activity, and these changes help to reinforce and maintain the anomalous mean circulation in the upper troposphere. It is important that studies of trends in local climate records, such as those from high elevation sites, recognize the presence of strong regional patterns of change associated with phenomena like the NAO.

This chapter provides a broad review of the North Atlantic Oscillation (NAO) and its forcing of the North Atlantic Ocean. Of particular interest is the long, irregular amplification of the oscillation towards one extreme phase during winter over recent decades. This climatic event, which is unprecedented in the modern instrumental record of NAO behaviour, has produced a wide range of effects on North Atlantic ecosystems. Some attention will also be given to the climatic impacts of periods of atypical NAO behaviour, such as the spatial displacement of the main centres of action in some winters, or to periods when other patterns of large-scale Atlantic climate variability are more pronounced.

We present a brief commentary on the landmark study of , in which an analysis of the correlation between monthly anomalies of "polar pressure" and sea level pressure from numerous stations around the Northern Hemisphere during winter was presented. Exner's analysis still stands today as an excellent description of the spatial structure of the leading pattern of atmospheric variability over the extratropical Northern Hemisphere (NH); namely, the North Atlantic Oscillation (NAO). Moreover, Exner provided an accurate commentary on regional changes in surface air temperature driven by NAO variability. His quantitative analysis of these aspects represented a major step forward in the history of research on NH climate variability that, prior to his study, was primarily qualitative.

The failure to detect C3 in absorption in diffuse clouds (24), however, appears to rule out this attribution. Note added in proof: We recently received a manuscript by R. J. Glinski and J. A. Nuth (27) in which a vibrational band assignment of the 5800 A Red Rectangle emis- sion features to C3 is presented. Combining this assignment with the results described here leads to the conclusion that C3 is the carrier of the X=5797, X=5850, and nearby related diffuse interstellar absorption bands.

Climate influences a variety of ecological processes. These effects operate through local weather parameters such as temperature, wind, rain, snow, and ocean currents, as well as interactions among these. In the temperate zone, local variations in weather are often coupled over large geographic areas through the transient behavior of atmospheric planetary-scale waves. These variations drive temporally and spatially averaged exchanges of heat, momentum, and water vapor that ultimately determine growth, recruitment, and migration patterns. Recently, there have been several studies of the impact of large-scale climatic forcing on ecological systems. We review how two of the best-known climate phenomena—the North Atlantic Oscillation and the El Niño–Southern Oscillation—affect ecological patterns and processes in both marine and terrestrial systems.

To understand recent climate change in the North Atlantic region and to produce better climate forecasts with uncertainty estimates it is important to determine the atmospheric ‘response’ to Atlantic sea‐surface temperature (SST) forcing. There have been conflicting results regarding the strength, character and tropical‐versus‐extratropical origin of this response. For model‐based studies, this may indicate differing sensitivities to Atlantic SST, but the comparison is complicated by changes in experimental design. Here, a highly controlled experiment with five atmospheric models is undertaken. The influence of realistic (if reasonably strong) and optimally chosen North Atlantic (equator to 70°N) SST anomalies is isolated. Unexpected global agreement between the models is found (e.g. the North Atlantic Oscillation (NAO), Eurasian temperatures, rainfall over the Americas and Africa, and the Asian monsoon). The extratropical North Atlantic region response appears to be associated with...

This paper discusses large-scale climate variability for several marine ecosystems and suggests types of ecosystem responses to climate change. Our analysis of observations and model results for the Pacific and Atlantic Oceans concludes that most climate variability is accounted for by the combination of intermittent 1-2 year duration events, e.g. the cumulative effect of monthly weather anomalies or the more organized El Niño/La Niña, plus broad-band "red noise" intrinsic variability operating at decadal and longer timescales. While ocean processes such as heat storage and lags due to ocean circulation provide some multi-year memory to the climate system, basic understanding of the mechanisms resulting in observed large decadal variability is lacking and forces the adoption of a "stochastic or red noise" conceptual model of low frequency variability at the present time. Thus we conclude that decadal events with rapid shifts and major departures from climatic means will occur, but their timing cannot be forecast. The responses to climate by biological systems are diverse in character because intervening processes introduce a variety of amplifications, time lags, feedbacks, and non-linearities. Decadal ecosystem variability can involve a variety of climate to ecosystem transfer functions. These can be expected to convert red noise of the physical system to redder (lower frequency) noise of the biological response, but can also convert climatic red noise to more abrupt and discontinuous biological shifts, transient climatic disturbance to prolonged ecosystem recovery, and perhaps transient disturbance to sustained ecosystem regimes. All of these ecosystem response characteristics are likely to be active for at least some locations and time periods, leading to a mix of slow fluctuations, prolonged trends, and step-like changes in ecosystems and fish populations in response to climate change. Climate variables such as temperatures and winds can have strong teleconnections (large spatial covariability) within individual ocean basins, but between-basin teleconnections, and potential climate-driven biological synchrony over several decades, are usually much weaker and a highly intermittent function of the conditions prevailing at the time within the adjoining basins. As noted in the recent IPCC 4th Assessment Report, a warming trend of ocean surface layers and loss of regional sea ice is likely before 2030, due to addition of greenhouse gases. Combined with large continuing natural climate variability, this will stress ecosystems in ways that they have not encountered for at least 100s of years.

The climatically sensitive zone of the Arctic Ocean lies squarely within the domain of the North Atlantic oscillation (NAO), one of the most robust recurrent modes of atmospheric behavior. However, the specific response of the Arctic to annual and longer-period changes in the NAO is not well understood. Here that response is investigated using a wide range of datasets, but concentrating on the winter season when the forcing is maximal and on the postwar period, which includes the most comprehensive instrumental record. This period also contains the largest recorded low-frequency change in NAO activity-from its most persistent and extreme low index phase in the 1960s to its most persistent and extreme high index phase in the late 1980s/early 1990s. This longperiod shift between contrasting NAO extrema was accompanied, among other changes, by an intensifying storm track through the Nordic Seas, a radical increase in the atmospheric moisture flux convergence and winter precipitation in this sector, an increase in the amount and temperature of the Atlantic water inflow to the Arctic Ocean via both inflow branches (Barents Sea Throughflow and West Spitsbergen Current), a decrease in the late-winter extent of sea ice throughout the European subarctic, and (temporarily at least) an increase in the annual volume flux of ice from the Fram Strait.

The observed low-frequency winter atmospheric variability of the North Atlantic-European region and its relationship with global surface oceanic conditions is investigated based on the climate and weather regimes paradigm. Asymmetries between the two phases of the North Atlantic Oscillation (NAO) are found in the position of the Azores high and, to a weaker extent, the Icelandic low. There is a significant eastward displacement or expansion toward Europe for the NAOϩ climate regime compared to the NAOϪ regime. This barotropic signal is found in different datasets and for two quasi-independent periods of record (1900-60 and 1950-2001); hence, it appears to be intrinsic to the NAOϩ phase. Strong spatial similarities between weather and climate regimes suggest that the latter, representing long time scale variability, can be interpreted as the time-averaging signature of much shorter time scale processes. Model results from the ARPEGE atmospheric general circulation model are used to validate observed findings. They confirm in particular the eastward shift of the Atlantic centers of action for the NAOϩ phase and strongly suggest a synoptic origin as it can be extracted from daily analyses. These results bring together present-day climate variability and scenario studies where such an NAO shift was suggested, as it is shown that the last three decades are clearly dominated by the occurrence of NAOϩ regimes when concentrations of greenhouse gases are rapidly increasing. These findings highlight that the displacement of the North Atlantic centers of action should be treated as a dynamical property of the North Atlantic atmosphere and not as a mean longitudinal shift of climatological entities in response to anthropogenic forcings. The nonstationarity with time of the atmospheric variability is documented. Late-century decades differ from early ones by the predominance of NAO climate regimes versus others. In such a context, comments on the relevance of the station-based NAO index is provided. Both tropical and extratropical sea surface temperature (SST) anomalies alter the frequency distribution of the North Atlantic regimes. Evidence is presented that the so-called ridge regime is preferably excited during La Nin ˜a events, while the NAO regimes are associated with the North Atlantic SST tripole. The ARPEGE model results indicate that the tropical branch of the SST tripole affects the NAO regimes occurrence. Warm tropical SST anomalies are more efficient at exciting NAOϪ regimes than cold anomalies are at forcing NAOϩ regimes. The extratropical portion of the North Atlantic SST tripole also seems to play a significant role in the model, tending to counteract the dominant influence of the tropical Atlantic basin on NAO regimes.

The origin of the so-called summer North Atlantic ''Horseshoe'' (HS) sea surface temperature (SST) mode of variability, which is statistically linked to the next winter's North Atlantic Oscillation (NAO), is investigated from data and experiments with the CCM3 atmospheric general circulation model (AGCM). Lagged observational analyses reveal a linkage between HS and anomalous rainfall in the vicinity of the Atlantic intertropical convergence zone. Prescribing the observed anomalous convection in the model generates forced atmospheric Rossby waves that propagate into the North Atlantic sector. The accompanying perturbations in the surface turbulent and radiative fluxes are consistent with forcing the SST anomalies associated with HS. It is suggested that HS can therefore be interpreted as the remote footprint of tropical atmospheric changes. The ARPEGE AGCM is then used to test if the persistence of HS SST anomalies from summer to late fall can feed back to the atmosphere and have an impact on the next winter's North Atlantic variability. Observed HS SST patterns are imposed in the model from August to November. They generate a weak but coherent early winter response projecting onto the NAO and therefore reproduce the observed HS-NAO relationship obtained from lagged statistics. Changes in the simulated upper-level jet are associated with the anomalous HS meridional SST gradient and interact with synoptic eddy activity from October onward. The strength and position of the transients as a function of seasons are hypothesized to be of central importance to explain the nature, timing, and sign of the model response. In summary, the present study emphasizes the importance of summer oceanic and atmospheric conditions in both the Tropics and extratropics, and their persistence into early winter for explaining part of the NAO's lowfrequency variability.

The variability of Atlantic Meridional Overturning Circulation (AMOC) in the pre-industrial control experiment of the Flexible Global Ocean-Atmosphere-Land System model, Grid-point Version 2 (FGOALS-g2) was investigated using the model outputs with the most stable state in a 512-yr time window from the total 1500-yr period of the experiment. The period of AMOC in FGOALS-g2 is double peaked at 20 and 32 years according to the power spectrum, and 22 years according to an auto-correlation analysis, which shows very obvious decadal variability. Like many other coupled climate models, the decadal variability of AMOC in FGOALS-g2 is closely related to the convection that occurs in the Labrador Sea region. Deep convection in the Labrador Sea in FGOALS-g2 leads the AMOC maximum by 3-4 years. The contributions of thermal and haline effects to the variability of the convection in three different regions [the Labrador, Irminger and Greenland-Iceland-Norwegian (GIN) Seas] were analyzed for FGOALS-g2. The variability of convection in the Labrador and Irminger Seas is thermally dominant, while that in the colder GIN Seas can be mainly attributed to salinity changes due to the lower thermal expansion. By comparing the simulation results from FGOALS-g2 and 11 other models, it was found that AMOC variability can be attributed to salinity changes for longer periods (longer than 35 years) and to temperature changes for shorter periods.

Marine ecosystems are undergoing rapid change at local and global scales. To understand these changes, including the relative roles of natural variability and anthropogenic effects, and to predict the future state of marine ecosystems requires quantitative understanding of the physics, biogeochemistry and ecology of oceanic systems at mechanistic levels. Central to this understanding is the role played by dominant patterns or "modes" of atmospheric and oceanic variability, which orchestrate coherent variations in climate over large regions with profound impacts on ecosystems. We review the spatial structure of extratropical climate variability over the Northern Hemisphere and, specifically, focus on modes of climate variability over the extratropical North Atlantic. A leading pattern of weather and climate variability over the Northern Hemisphere is the North Atlantic Oscillation (NAO). The NAO refers to a redistribution of atmospheric mass between the Arctic and the subtropical Atlantic, and swings from one phase to another produce large changes in surface air temperature, winds, storminess and precipitation over the Atlantic as well as the adjacent continents. The NAO also affects the ocean through changes in heat content, gyre circulations, mixed layer depth, salinity, high latitude deep water formation and sea ice cover. Thus, indices of the NAO have become widely used to document and understand how this mode of variability alters the structure and functioning of marine ecosystems.

The question of the impact of the Atlantic on North American (NA) seasonal prediction skill and predictability is examined. Basic material is collected from the literature, a review of seasonal forecast procedures in Canada and the United States, and some fresh calculations using the NCEP–NCAR reanalysis data. The general impression is one of low predictability (due to the Atlantic) for seasonal mean surface temperature and precipitation over NA. Predictability may be slightly better in the Caribbean and the (sub)tropical Americas, even for precipitation. The NAO is widely seen as an agent making the Atlantic influence felt in NA. While the NAO is well established in most months, its prediction skill is limited. Year-round evidence for an equatorially displaced version of the NAO (named ED_NAO) carrying a good fraction of the variance is also found. In general the predictability from the Pacific is thought to dominate over that from the Atlantic sector, which explains the minimal nu...

Precipitation is one of the most important and variable climatic elements, exhibiting more complex behavior compared to other climatic factors. This element shows significant temporal and spatial variability. The aim of this research is to examine the trend of precipitation changes in the Caspian Sea basin and its relationship with teleconnection patterns. In this context, monthly precipitation data from the ERA-Interim dataset, with a resolution of 2° x 2°, covering the statistical period from 1970 to 2023, were utilized. Given the non-normality of the data, the non-parametric Mann-Kendall test was employed to calculate the trend. The precipitation slope was also determined using the Sen's slope estimator. The results indicated an increasing trend in precipitation during the months of january, february, march, november, december, and to some extent in may and october, while a decreasing trend was observed in other months. The increasing trend in precipitation was most pronounced in february, while the decreasing trend was most significant in june and august. The correlation between precipitation and teleconnection patterns showed that, temporally, correlations were observed in january, february, march, june, august, september, october, and december. The EA.WR pattern exhibited the highest correlation with the basin's precipitation compared to other patterns, followed by the TNA, AMO, and AMOS patterns. Overall, february showed the highest correlation between teleconnection patterns and precipitation, and in terms of pattern type, the EA.WR pattern had the strongest correlation with the basin's precipitation.

This study highlights an important and previously overlooked summer North Atlantic Oscillation (NAO) influence over the eastern Mediterranean. The featured analysis is based on a synergistic use of reanalysis data, satellite retrievals, and coastal and buoy meteorological observations. The physical mechanisms at play reveal a strong summer NAO involvement on the pressure fields over northern Europe and the Anatolian plateau. Especially during August, the summer NAO modulates the Anatolian low, together with the air temperature, meridional atmospheric circulation, and cloudiness over the eastern Mediterranean. Including the dominant action centers over Greenland and the Arctic, the identified modulations rank among the strongest summer NAO-related signals over the entire Northern Hemisphere.

The influence of large-scale modes of climate variability on worldwide summer and winter temperature extremes has been analyzed, namely, that of the El Niño–Southern Oscillation, the North Atlantic Oscillation, and Pacific interdecadal climate variability. Monthly indexes for temperature extremes from worldwide land areas are used describe moderate extremes, such as the number of exceedences of the 90th and 10th climatological percentiles, and more extreme events such as the annual, most extreme temperature. This study examines which extremes show a statistically significant (5%) difference between the positive and negative phases of a circulation regime. Results show that temperature extremes are substantially affected by large-scale circulation patterns, and they show distinct regional patterns of response to modes of climate variability. The effects of the El Niño–Southern Oscillation are seen throughout the world but most clearly around the Pacific Rim and throughout all of Nort...

The objective of the work was to describe the thermal and snow conditions in the winter period in the Siedlce area. The average daily air temperatures were used in addition to numbers of days with a snow cover of at least 1 cm for the years 2000-2016 obtained from the Meteorological Station in Siedlce. Dates of the beginning and end of the winter season were determined. The average temperature of the winter season was determined in addition to the degree of winter severity, according to Oskin. The average, minimum and maximum values of parameters were calculated. The probability of an occurrence of individual types of winter severity was determined. Next, principal component analysis and cluster analysis were applied, the latter to the group years in terms of the days with a given type of weather in winter. It was found that -on average -the thermal winter began on 5 December and ended on 6th March. The winter was found to have lasted for 66 days. From year to year, there was observed an increase in the average number of days with mild weather. The greatest decline was found for the days with the weather typical of slightly severe and moderately severe winter. The last study years had the highest average number of days with weather typical of mild, slightly severe and moderately severe winter, and the lowest number of the days with weather typical of severe, very severe, unusually severe and extremely severe winter.

Extreme rainfall events are a serious concern for regional hydrology and agriculture in the Ebro River Basin. Repeated anomalous rainfall in recent decades has had a devastating impact on this region, both socially and economically. Some studies developed in Italy and USA have shown that there is a change in seasonal patterns and an increasing frequency of extreme rainfall events, whereas other studies have pointed out that no global behaviour could be observed in monthly trends due to high climatic variability. The aim of this work is to test which of these scenarios is the case for the Ebro River Basin. For this purpose, 14 meteorological stations were selected based on the length of the rainfall series and the climatic classification to obtain a representative untreated dataset from the river basin. Daily rainfall series from 1957 to 2002 were obtained from each meteorological station. First, classical climatic indexes were analysed with an autoregressive test to study possible trends in rainfall. The results can be explained following the evolution of the NAO and WeMO indexes, which indicate that the initial period should be subdivided in two periods (1957-1979 and 1980-2002) to assume stationarity and to analyse the rainfall distribution functions. The general results obtained in this study for both subperiods, through the generalised Pareto distribution (GPD) parameters and the maximum expected return values, do not support the results previously obtained by other authors that affirm a positive trend in extreme rainfall indexes and point to a slight reduction indicated by others. Three extreme precipitation indexes show negative statistical significant trends. GPD-scale parameters decrease except for only one rain gauge, although this decrease is only statistically significant for two rain gauges. Another two locations show statistical significance decreased for maximum expected return values.

According to the Köppen climate classification, almost the entire area of Latvia belongs to the same climate type, Dfb, which is characterized by humid continental climates with warm (sometimes hot) summers and cold winters. In the last decades whether conditions on the western coast of Latvia more characterized by temperate maritime climates. In this area there has been a transition (and still ongoing) to the climate type Cfb.

temperatures include noise. Both factors can lead to circulation anomalies in the DA analysis that are substantially different from reality, leading to unrealistic representation of small-scale temperature variability. Moreover, we show that even if the true amplitudes of the leading MCA circulation patterns were known, there is still a large amount of unexplained local temperature variance. Based on these results, we argue that assimilating temperature reconstructions with a higher spatial resolution might improve the DA performance.

Seasonal reconstructions of the Southern Hemisphere Annular Mode (SAM) index are derived to extend the record before the reanalysis period, using station sea level pressure (SLP) data as predictors. Two reconstructions using different predictands are obtained, one (JW) based on the first principal component (PC) of extratropical SLP and the other (Fogt) on the index of Marshall (2003). A regional-based SAM index (Visbeck 2009) is also considered. These predictands agree well post 1979; correlations fall in all seasons except austral summer for the full series starting in 1958. Predictand agreement is strongest in spring and summer; hence agreement between the reconstructions is highest in these seasons. The less zonally symmetric SAM structure in winter and spring influences the strength of the SAM signal over land areas, hence the number of stations included in the reconstructions. Reconstructions from 1865 were therefore derived in summer and autumn, and from 1905 in winter and sp...

To investigate relationships between large-scale circulation and regional-scale temperatures during the last (Eemian) interglacial, a simulation with a general circulation model (GCM) under orbital forcing conditions of 125 kyr BP is compared with a simulation forced with the Late Holocene preindustrial conditions. Consistent with previous GCM simulations for the Eemian, higher northern summer 2-m temperatures are found, which are directly related to the different insolation. Differences in the mean circulation are evident such as, for instance, stronger northern winter westerlies toward Europe, which are associated with warmer temperatures in central and northeastern Europe in the Eemian simulation, while the circulation variability, analyzed by means of a principal component analysis of the sea level pressure (SLP) field, is very similar in both periods. As a consequence of the differences in the mean circulation the simulated Arctic Oscillation (AO) temperature signal in the nort...

Climate forcing in complex ecosystems can have profound implications for ecosystem sustainability and may thus challenge a precautionary ecosystem management. Climatic influences documented to affect various ecological functions on a global scale, may themselves be observed on quantitative or qualitative scales including regime shifts in complex marine ecosystems. This study investigates the potential climatic impact on the reproduction success of spring-spawning herring (Clupea harengus) in the Western Baltic Sea (WBSS herring). To test for climate effects on reproduction success, the regionally determined and scientifically well-documented spawning grounds of WBSS herring represent an ideal model system. Climate effects on herring reproduction were investigated using two global indices of atmospheric variability and sea surface temperature, represented by the North Atlantic Oscillation (NAO) and the Atlantic Multi-decadal Oscillation (AMO), respectively, and the Baltic Sea Index (BSI) which is a regional-scale atmospheric index for the Baltic Sea. Moreover, we combined a traditional approach with modern time series analysis based on a recruitment model connecting parental population components with reproduction success. Generalized transfer functions (ARIMAX models) allowed evaluating the dynamic nature of exogenous climate processes interacting with the endogenous recruitment process. Using different model selection criteria our results reveal that in contrast to NAO and AMO, the BSI shows a significant positive but delayed signal on the annual dynamics of herring recruitment. The westward influence of the Siberian high is considered strongly suppressing the influence of the NAO in this area leading to a higher explanatory power of the BSI reflecting the atmospheric pressure regime on a North-South transect between Oslo, Norway and Szczecin, Poland. We suggest incorporating climate-induced effects into stock and risk assessments and management strategies as part of the EU ecosystem approach to support sustainable herring fisheries in the Western Baltic Sea.