Positive North Atlantic Oscillation Phase Research Articles

Impact of the North Atlantic Oscillation on the Decadal Variability of the Upper Subtropical‐Tropical Atlantic Ocean

AbstractIn the northeastern tropical Atlantic, a region of high potential vorticity (PV) determines the size of the exchange window for the interior thermocline flow of the subtropical cell via its variations in strength and extent. Variability of this PV barrier has the potential to impact the ventilation of the tropical Atlantic on decadal timescales. Here, the impact of the North Atlantic Oscillation (NAO) on the PV barrier related to isopycnals within the thermocline of the subtropical‐tropical Atlantic Ocean is assessed from Argo observations for the time period of 2006–2022. Relative to the negative NAO phase (2009–2010), during the positive NAO phase (2014–2019), the North Atlantic subtropical high and the northeast trades are intensified. Satellite‐derived wind stress curl shows increased upwelling/downwelling on the equatorward/poleward side of the trade wind zone, respectively. In the subtropical‐tropical Atlantic, a symmetric pattern of isopycnal heave is observed: rising isopycnals within 20°N and 20°S and sinking poleward of that. With rising isopycnals, the PV barrier in the northeastern tropical Atlantic becomes stronger. Analyses of geostrophic velocities and the Sverdrup streamfunction show that during the positive NAO phase there are increased equatorward velocities at thermocline level along the western boundary and reduced velocities through the interior as a result of intensified northeast trades and therefore a strengthened PV barrier. Intensified trades lead to enhanced subduction of thermocline waters and, independent of that, to a strengthened Equatorial Undercurrent transport as observed at the mooring site at 0°, 23°W, likely via the pulling effect of the subtropical cells.

Revealing the spatiotemporal patterns of water vapor and its link to North Atlantic Oscillation over Greenland using GPS and ERA5 data

Precipitation plays an important role in the interannual mass variations of Greenland Ice Sheet (GrIS) and is highly influenced by atmospheric circulation change. The relationship between precipitation and North Atlantic Oscillation (NAO) has been revealed by many studies, but the role of water vapor transportation in the NAO-precipitation relationship was rarely investigated. Therefore, to fill the knowledge gap of how water vapor changes and responds to NAO in space and time, we applied Multichannel Singular Spectral Analysis (MSSA) to the Global Positioning System (GPS) and the fifth-generation reanalysis dataset of the European Center for Medium-Range Weather Forecasting (ERA5) Precipitable Water Vapor (PWV) data to extract the interannual PWV signals in Greenland. Results show that the interannual PWV signals overall increased in 2008–2011, decreased in 2011–2015, and increased in 2015–2021. The amplitudes of the interannual signals derived from both the GPS PWV and ERA5 basin-averaged PWV exhibited an overall southwest-northeast decreasing gradient. We also found anticorrelation between the interannual PWV signals and the NAO signal over Greenland but the correlation coefficients are not statistically significant, and the correlation coefficients in most cases were less than −0.65, indicating that positive (negative) NAO phase decreased (increased) the water vapor content. The Fast Fourier Transform (FFT) results illustrated that the interannual signals derived from both the GPS site-dependent and the ERA5 basin-averaged PWV had similar dominant frequencies to that of the NAO signal, reinforcing their correlations. This study reveals the spatiotemporal pattern of the interannual water vapor and its linkage to the NAO, providing a new perspective for understanding the climate change on Greenland.

A climatology of Arctic fog along the coast of East Greenland

AbstractThis study presents a comprehensive climatology of coastal fog from four synoptic weather stations operated by the Danish Meteorological Institute along the entire East Greenland coast between 1958 and 2016. Elements investigated include fog frequency, daily timing, temperature, wind, visibility and radiosonde profiles during fog. The spatiotemporal patterns in fog from the low‐ to high‐Arctic locations were related to varying regional seasonal temperatures, surface and upper‐air wind and sea ice conditions, and to correlations with the North Atlantic Oscillation (NAO) and the Greenland Blocking Index (GBI). Results indicate that ˜70–80% of East Greenland fog occurs in summer (MJJA), and yearly fog onset is near‐coincident with the start of sea ice break‐up. This warm‐season fog has the typical characteristics of advection fog, as shown in the radiosonde profiles and the association with a gentle sea breeze. More than 95% of warm‐season fog is warmer than −10°C, and peaks close to 0°C and, therefore, consists of liquid or supercooled water droplets. In the cold season, mixed‐phase fog prevails in the high‐Arctic locations, accounting for ˜70% of observations. Ice fog (T < −30°C) occurs in only 2% of observations and is limited to Northeast Greenland during the cold season. The cold‐season composite radiosonde fog profiles in the high‐Arctic locations are characterized by deep (˜1000 m) and strong (˜6°C) surface‐based temperature inversions. Visibility during most fog conditions is lowest during the warm season (<500 m) and highest during the cold season (<800 m). In Northeast Greenland, visibility during warm‐season fog has decreased by ˜50 m·dec−1 between 1981 and 2016. In Southeast Greenland, fog visibility is high during low GBI and a positive phase of NAO, but no other correlations with climate indices were found.

A quantitative assessment of air–sea heat flux trends from ERA5 since 1950 in the North Atlantic basin

Abstract. This work aims to investigate the temporal stability and reliability of trends in air–sea heat fluxes from ERA5 forecasts over the North Atlantic basin for the period 1950–2019. Driving forces of the trends are investigated using analyzed state quantities from ERA5. Estimating trends from reanalysis data can be challenging as changes in the observing system may introduce temporal inconsistencies. To this end, the impact of analysis increments is discussed. For individual sub-regions in the North Atlantic basin, parametrization formulas for latent and sensible heat fluxes are linearized to quantitatively attribute trends to long-term changes in wind speed, moisture, and temperature. Our results suggest good temporal stability and reliability of air–sea heat fluxes from ERA5 forecasts on sub-basin scales and below. Regional averages show that trends are largely driven by changes in the skin temperature and atmospheric advection (e.g., of warmer or drier air masses). The influence of modes of climate variability, such as the North Atlantic Oscillation (NAO) and Atlantic Multidecadal Oscillation, on the patterns found is discussed as well. Results indicate a significant impact on trends in the Irminger and Labrador seas associated with more positive NAO phases during the past 4 decades. Finally, we use basin-wide trends of air–sea heat fluxes in combination with an observational ocean heat content estimate to provide an energy-budget-based trend estimate of the Atlantic Meridional Overturning Circulation (AMOC). A decrease in area-averaged air–sea heat fluxes in the North Atlantic basin suggests a decline in the AMOC over the study period. However, basin-wide flux trends are deemed partially artificial, as indicated by temporally varying moisture increments. Thus, the exact magnitude of change is uncertain, but its sign appears robust and adds complementary evidence that the AMOC has weakened over the past 70 years.

Connection of Compound Extremes of Air Temperature and Precipitation with Atmospheric Circulation Patterns in Eastern Europe

Recent studies show an increase in the frequency of compound extremes in air temperature and precipitation in many parts of the world, especially under dry and hot conditions. Compound extremes have a significant impact on all spheres of human activity, such as health, agriculture, and energy. Features of atmospheric circulation are closely related to the occurrence of anomalies in air temperature and precipitation. The article analyzes the relationship of atmospheric circulation modes with compound extremes that have had the greatest impact on the Atlantic–European region over the territory of Eastern Europe over the past 60 years on extreme air temperature and precipitation. Combinations of extreme temperature and humidity conditions (indices)—cold-dry (CD), cold-wet (CW), warm-dry (WD) and warm-wet (WW)—were used as compound extremes. Indices of compound extremes were calculated according to the E-OBS reanalysis data. Estimates of the relationship between two time series were carried out using standard correlation and composite analyses, as well as cross wavelet analysis. Phase relationships and time intervals for different climatic indices were different. The period of most fluctuations in the indices of compound extremes was from 4 to 12 years and was observed during 1970–2000. The coherent fluctuations in the time series of the WD and WW indices and the North Atlantic oscillation (NAO) index occurred rather in phase, those in the time series of the CD and WD indices and the Arctic oscillation (AO) index occurred in antiphase, and those in the time series of the WD and WW indices and the Scandinavia pattern (SCAND) index occurred in antiphase. Statistically significant increase in the number of warm compound extremes was found for the northern parts of the study region in the winter season with positive NAO and AO phases.

The role of teleconnection in the occurrence probability of extreme precipitation over China based on extreme value theory

Non-stationarity of extreme climate events has been reported worldwide in recent decades, and traditional stationary analysis methods are no longer sufficient to properly reveal the occurrence probability of climate extremes. Based on the 0.25°C × 0.25°C gridded precipitation data (i.e., CN05.1), stationary and non-stationary models of generalized extreme value (GEV) and generalized Pareto (GP) distributions are adopted to estimate the occurrence probability of extreme precipitation over China during 1961–2018. Low-frequency oscillation (LFO) indices, such as El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), North Atlantic Oscillation (NAO), Southern Annular Mode (SAM), and Pacific Decadal Oscillation (PDO), are included as time-varying covariates in the non-stationary GEV and GP models. Results illustrate that the occurrence probability of extreme precipitation estimated from the stationary GEV and GP distributions shows a significant increasing trend in northwestern and southeastern China, and the opposite trend in southwestern, central, and northeastern China. In comparison with stationary model, the fitness of extreme precipitation series is improved for both the GEV and GP distributions if these LFO indices are used as time-varying covariates. Positive ENSO, IOD and PDO tend to cause negative anomalies in the occurrence probability of extreme precipitation in northeastern China and Tibet Plateau, and positive anomalies in southern China. Positive NAO and SAM phases mainly tend to cause positive anomalies in southern China. The circulation patterns of extreme precipitation anomalies associated with these LFO indices are discussed from aspects of precipitable water, vertical integrated moisture transport, 500-hPa geopotential height and 850-hPa wind field.

Shift of the storm surge season in Europe due to climate variability

Along the European coasts, changes in the timing of the storm surge season are analyzed. Using 10 long-term tide gauges located in western Europe, a consistent spatio-temporal shift emerged in the storm surge season between 1950 and 2000. Temporal shifts are positive (later events) in the North, negative (earlier events) in the South. Extreme surge events occurred about 4 days/decade later in northern Europe, and 5 days/decade earlier in southern Europe. Such a tendency is similar to the one already reported for European river floods between 1960 and 2010. In northern Europe, extreme surges are known to occur during the positive North Atlantic Oscillation phase (NAO+). Identified spatio-temporal shifts likely trace that NAO+ storms tend to occur later between 1950 and 2000. A new index measuring the timing of the NAO+ and NAO− persistent situations is shown to help capture this spatial distribution in the timing of the storm surge seasons.

ВЛИЯНИЕ СЕВЕРОАТЛАНТИЧЕСКОГО И ВОСТОЧНО-АТЛАНТИЧЕСКОГО КОЛЕБАНИЙ НА ЗАВИХРЕННОСТЬ КАСАТЕЛЬНОГО НАПРЯЖЕНИЯ ТРЕНИЯ ВЕТРА НАД ЧЕРНЫМ МОРЕМ

The influence of two climate modes (North Atlantic Oscillation (NAO) and East Atlantic pattern (EA)) on the wind stress curl (WSC) over the Black Sea, which determines the intensity of water circulation, is investigated using the MERRA2 reanalysis wind data and NAO and EA indices for 1980-2019. The composite method taking into account the nonstationarity of the time series of the NAO and EA indices is used. It is shown that in some months of the summer-autumn period during the positive phase of NAO, there is a significant increase in the cyclonic WSC over the eastern part of the sea. The highest influence of the EA on the WSC over the Black Sea is observed in November to April: during the negative EA phase, the cyclonic WSC increases over most of the Black Sea area. Such increase should lead to an intensification of the Black Sea water circulation. It is confirmed that the EA has a greater effect on the WSC over the Black Sea than the NAO.

Asymmetrical Trends of Burned Area Between Eastern and Western Siberia Regulated by Atmospheric Oscillation

AbstractWildfires in Siberian cause organic carbon released from Arctic ecosystems to the atmosphere, which further enhances greenhouse effects. However, the trends of Siberian wildfires and their mechanisms are not well understood. We show that the burned fraction trends were opposite in the eastern and western Siberia. The contrasting changes are explained by vapor pressure deficit and soil moisture inferring atmospheric and ground aridity conditions, respectively. The difference in aridity change in the eastern and western Siberia is regulated by increased positive phase of Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO), which leads to reduced moisture transport to the east but increased transport to the west from the North Atlantic and Arctic Ocean mainly during May–August. The growing trend of positive phase of AO and NAO will continuously lead to the asymmetrical trends of aridity condition, which could enhance wildfire in the eastern Siberia.

Hydrological drought variability and its teleconnections with climate indices

In this study, the spatial–temporal variability of hydrological drought in Meriç Basin, Turkey, has been examined, and its relationship with global climate oscillations has been explored. Prior to drought analysis, streamflow data gaps were filled, and series homogeneity was examined. The standardized streamflow index (SSFI) was used to detect drought periods from monthly streamflow data of nine stations well spread over the study area. The hydrological drought intensity, magnitude, and duration were investigated using the SSFI time series. The drought variability was assessed by performing trend analysis and wavelet transforms. Trend analysis showed an increase in drought events in the Northern region and a decrease in the Southern region of the basin. Wavelet analysis reveals that the 4–10 year scales explicate the drought series pattern. The identification of climate patterns that are most associated with drought variability is performed via correlation analysis. The investigation showed that distinct climate patterns simultaneously influence drought events. Significantly, the influence of El Nino Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), and Atlantic Multidecadal Oscillation (AMO) is notable. The analysis demonstrated a remarkable correlation between ENSO and SSFI series at the interannual scale (2–8 years). The obtained phase difference suggests that the maximum dry (wet) conditions coincide with La Nina (El Nino) events. In addition, positive NAO phases and negative AMO phases lead to drought occurrence.

Interdecadal Linkage Between the Winter Northern Hemisphere Climate and Arctic Sea Ice of Diverse Location and Seasonality

During the past few decades, Arctic sea-ice has declined rapidly in both autumn and winter, which is likely to link extreme weather and climate events across the Northern Hemisphere midlatitudes. Here, we use reanalysis data to investigate the possible linkage between mid–high-latitude atmospheric circulation and Arctic sea-ice loss in different geographical locations and seasons and associated impacts on wintertime climate on interdecadal timescales. Four critical sea-ice subregions are analyzed in this study—namely, the Pan-Arctic, Barents–Kara–Laptev Seas (BKL), East Siberia–Chukchi–Beaufort Seas (EsCB), and Bering Sea (Ber). Results suggest that interdecadal reduction of autumn sea-ice, irrespective of geographical location, is dynamically associated with the negative phase of the North Atlantic Oscillation (NAO) in the subsequent winter via stratospheric pathways. Specifically, autumn sea-ice loss appears to cause a weakened stratospheric polar vortex that propagates to the troposphere in the ensuing months, leading to lower surface air temperature and a deficit in precipitation over Siberia and northeastern North America. Meanwhile, an anomalous cyclone over Europe favors excessive precipitation over southern Europe. For wintertime sea-ice loss in the Pan-Arctic and BKL, a weak positive NAO phase, with a dipole pressure pattern over Greenland–northeastern North America and North Atlantic, and a shrunken Siberian high over Eurasia are observed over mid–high-latitudes. The former results in excessive precipitation over northwestern and southeastern North America, whilst the latter leads to less precipitation and mild winter over Siberia. In contrast, Ber sea-ice loss is associated with a circumglobal wave train downstream of the Bering Sea, leading to extensive warming over Eurasia. The anomalous dipole cyclone and anticyclone over the Bering Sea transport more Pacific and Arctic water vapor to North America, and the anomalous cyclone over the Barents Sea results in abundant precipitation in Siberia. Such midlatitude anomaly is dynamically linked to winter sea-ice loss, mainly through tropospheric rather than stratospheric pathways. These results have important implications for future seasonal and interdecadal forecasts in the context of ongoing sea-ice decline.

Climate-driven variability in the context of the water-energy nexus: A case study in southern Portugal

Research on climate-driven variability in the water and energy sectors is required to drive adaptative policies to climate change and boost cross-sectorial synergies. This study addresses the role of the North Atlantic Oscillation (NAO) and East Atlantic pattern (EA) on the water-energy nexus in southern Portugal (Algarve region) from the point of view of water demand, instead of the usual point of view of hydropower production. Water at surface reservoirs and aquifers and solar and wind energy potentials (SP and WP, respectively) do not share the same dominant variability scales, but their interrelationships have implications for leveraging the use of renewable energy in the water sector, particularly through water pumping efficiency gains. Water availability is dominated by interannual fluctuations (70% of the total variance), whereas SP and WP are characterized by seasonal variability scales (98% and 41% of the total variance, respectively). At interannual scales NAO is the main driver of low-frequency variability governing cycles in the 6–8 -year band, whereas fluctuations in the 2–4 -year band are mainly associated with EA. Coupling or synchronizations between opposite phases of NAO and EA correspond to extremes in water availability. Minimum water levels in the summer and during droughts, corresponding to maximum energy demand in the water sector, are clearly connected to synchronized positive NAO and negative EA phases in the preceding winter. Recent advances in the seasonal and long-term predictability of NAO and EA climate patterns can help to improve drought resilience and groundwater sustainability and have huge potential benefits for the water-energy nexus in the Algarve region. Finally, to decarbonize freshwater supply in the Algarve, policy instruments will need to account for unregulated pumping which enables conditions for groundwater depletion, encourage energy and water management integration, and explore innovative energy investments.

Hydroclimate variability during the last 2700 years based on stalagmite multi-proxy records in the central-western Mediterranean

This study presents the first high-resolution speleothem-based hydrological reconstruction for much of the last 2.7 kyr in the central-western Mediterranean. The paleohydrological information comes from a combination of five U-Th dated stalagmites from two Mallorca island caves. Interpretations are based on high-resolution records of δ18O, δ13C and trace element analyses combined with information from mineralogical X-ray diffraction, fabrics and morphological features, and cave monitoring data.None of the studied stalagmites cover the whole 2.7 kyr period but they provide sufficient overlap to replicate most of the discussed climatic intervals with the exception of the Medieval Climate Anomaly (MCA), which is represented by a hiatus. Taking into account the results of five years farmed calcite collected in glass plates and cave environmental parameters, we argue that main patterns in the stalagmite geochemical records are mostly controlled by changing rates of prior calcite precipitation (PCP) that respond to hydrological changes in the region. We apply a principal component analysis to the stalagmite geochemical data set and a composite δ18O record to obtain a robust regional hydrological record.This record supports wet conditions for the early Roman Period (RP), the first half of the Early Middle Ages (EMA) and the Little Ice Age (LIA), and drier conditions for the late RP, the late EMA and the entire MCA. These results are discussed in the context of other climatic and oceanographic records from the region including paleo North Atlantic Oscillation (NAO) records. This ocean-atmosphere approach suggests complex non-stationary climate patterns for the last 2.7 kyr, including the occurrence of both wet-warm and wet-cold intervals and underlying the complex interaction of factors controlling climate evolution in the region. Overall, positive (negative) NAO phases appear coincident with drier (wetter) conditions for all the examined period at decadal time-scales.

North Atlantic gyre circulation in PRIMAVERA models

We study the impact of horizontal resolution in setting the North Atlantic gyre circulation and representing the ocean–atmosphere interactions that modulate the low-frequency variability in the region. Simulations from five state-of-the-art climate models performed at standard and high-resolution as part of the High-Resolution Model Inter-comparison Project (HighResMIP) were analysed. In some models, the resolution is enhanced in the atmospheric and oceanic components whereas, in some other models, the resolution is increased only in the atmosphere. Enhancing the horizontal resolution from non-eddy to eddy-permitting ocean produces stronger barotropic mass transports inside the subpolar and subtropical gyres. The first mode of inter-annual variability is associated with the North Atlantic Oscillation (NAO) in all the cases. The rapid ocean response to it consists of a shift in the position of the inter-gyre zone and it is better captured by the non-eddy models. The delayed ocean response consists of an intensification of the subpolar gyre (SPG) after around 3 years of a positive phase of NAO and it is better represented by the eddy-permitting oceans. A lagged relationship between the intensity of the SPG and the Atlantic Meridional Overturning Circulation (AMOC) is stronger in the cases of the non-eddy ocean. Then, the SPG is more tightly coupled to the AMOC in low-resolution models.

Impacts of the North Atlantic Oscillation on winter precipitations and storm track variability in southeast Canada and the northeast United States

Abstract. The North Atlantic Oscillation (NAO) affects atmospheric variability from eastern North America to Europe. Although the link between the NAO and winter precipitations in eastern North America has been the focus of previous work, only few studies have considered extreme precipitation and hitherto provided clear physical explanations on these relationships. In this study we revisit and extend the analysis of the effect of the NAO on mean and heavy winter precipitations over a large domain covering southeast Canada and the northeastern United States. Furthermore, we use the recent ERA5 reanalysis dataset (1979–2018), which currently has the highest available horizontal resolution for a global reanalysis (0.25∘), to track extratropical cyclones to delve into the physical processes behind the relationship between NAO and precipitation, snowfall, snowfall-to-precipitation ratio (S∕P), and snow cover depth anomalies in the region. In particular, our results show that positive NAO phases are associated with less snowfall over a wide region covering Nova Scotia, New England and the Mid-Atlantic of the United States relative to negative NAO phases. Over the same area, the analysis of heavy snowfall revealed that there are up to twice as many heavy snowfall events during negative phases compared to positive phases. Therefore, a significant negative correlation is also seen between S∕P and the NAO over this region. This is due to a decrease (increase) in cyclogenesis of coastal storms near the United States east coast during positive (negative) NAO phases, as well as a northward (southward) displacement of the mean storm track over North America.

Linking hydrological variations at local scales to regional climate teleconnection patterns

AbstractClimate patterns over preceding years affect seasonal water and moisture conditions. The linkage between regional climate and local hydrology is challenging due to scale differences, both spatially and temporally. In this study, variance, correlation, and singular spectrum analyses were conducted to identify multiple hydroclimatic phases during which climate teleconnection patterns were related to hydrology of a small headwater basin in Idaho, USA. Combined field observations and simulations from a physically based hydrological model were used for this purpose. Results showed statistically significant relations between climate teleconnection patterns and hydrological fluxes in the basin, and climate indices explained up to 58% of hydrological variations. Antarctic Oscillation (AAO), North Atlantic Oscillation (NAO), and Pacific North America (PNA) patterns affected mountain hydrology, in that order, by decreasing annual runoff and rain on snow (ROS) runoff by 43% and 26% during a positive phase of NAO and 25% and 9% during a positive phase of PNA. AAO showed a significant association with the rainfall‐to‐precipitation ratio and explained 49% of its interannual variation. The runoff response was affected by the phase of climate variability indices and the legacy of past atmospheric conditions. Specifically, a switch in the phase of the teleconnection patterns of NAO and PNA caused a transition from wet to dry conditions in the basin. Positive AAO showed no relation with peak snow water equivalent and ROS runoff in the same year, but AAO in the preceding year explained 24 and 25% (p < 0.05) of their variations, suggesting that the past atmospheric patterns are equally important as the present conditions in affecting local hydrology. Areas sheltered from the wind and acted as a source for snow transport showed the lowest (40% below normal) ROS runoff generation, which was associated with positive NAO that explained 33% (p < 0.01) of its variation. The findings of this research highlighted the importance of hydroclimatic phases and multiple year variations that must be considered in hydrological forecasts, climate projections, and water resources planning.

‘HESS-BREZOWSKY’ ATMOSPHERIC CLASSIFICATION TYPES ASSOCIATED WITH NAO AND AO PHASES IN THE NORTHERN PART OF MOLDOVA

The North Atlantic Oscillation (NAO) and that Artic (AO), respectively, are by considerable importance to make the short-time weather forecast (until three days), followed by those of middle range and long time (until two weeks). To analyzing the ‘Hess-Brezowsky’ weather patterns associated with the NAO and AO during 19 years (2000 to 2018), has highlighted some aspects over the weather kind in the north part of Moldova. The zonal air-masses’ movements such as northwestern and western, both of them into the anti-cyclonic and cyclonic regime, were deployed in a predominately positive phase of NAO and AO. In the case of NWA type, NAO was 0.17 and AO 0.60, and for WA, NAO was 0.12 and AO 0.71. From NWZ, the NAO shown values around of 0.03 and AO 0.08. Exceptionally, NAO was into a negative phase in the case of WZ, with a value of around -0.13. Instead, for WZ, the AO was positive, with a yearly average of 0.08. Such air-masses movements are associated with a mild weather pattern, especially during the wintertime. Then, over the southern part of Europe, it’s set up the ‘Omega’ ridge. In doing so, the amounts of the precipitations are low at the studied area’s level. By a more significant interest is the southwesterly air-masses movement into a cyclonic regime (SWZ) who shown values of NAO of -0.16 and 00.26 for AO. Also, negative values for NAO were in the case of TRM (Trough over Central Europe), with -0.04 and HNFZ (High over Scandinavia-Iceland, Trough over Central Europe), with -0.64. For the same synoptically contexts (TRM and HNFZ), the AO was of -0.23, respectively, -1.31. Associated with the negative phases of NAO and AO, the planetary waves have shown an undulatory tendency. In such a case, there are development areas with extreme weather. For the studied region, the negative phases of NAO and AO were associated with severe weather events, such as blizzards, storms, and floods.

The Enhancement of the Impact of the Wintertime North Atlantic Oscillation on the Subsequent Sea Surface Temperature over the Tropical Atlantic since the Middle 1990s

AbstractThe impact of the wintertime North Atlantic Oscillation (NAO) on the subsequent sea surface temperature (SST) anomalies over the tropical Atlantic has experienced obvious interdecadal changes during 1950–2015. During 1995–2015, the negative (positive) phase of the wintertime NAO favors positive (negative) SST anomalies over the tropical Atlantic in the subsequent spring–summer, whereas the NAO–SST connection is insignificant during 1970–94 and is confined to the northern tropical Atlantic (NTA) during 1950–69. Compared to 1970–94, the much stronger influence on the NTA SST during 1995–2015 and 1950–69 is associated with a southward shift of the southern boundary of the NAO. During 1995–2015, the inverted NAO-related warming of the tropical Atlantic consists of three stages: 1) the pronounced increase in SST over the subtropical North Atlantic (SNA) and the tropical South Atlantic (TSA) during December–January, 2) the pronounced increase in SST over the northwestern tropical Atlantic (NWTA) during February–April, and 3) the persistent warming over the tropical Atlantic during May–August. The increases in SST over the SNA and the TSA are attributed to significant positive latent heat flux anomalies via the wind–evaporation effect, which are connected by the suppressed regional Hadley circulation. Afterward, the associated anomalous downward motion over the NWTA persists into February–April, which induces more incoming shortwave radiation and results in a significant increase in the local SST via the cloud–radiation effect. In contrast, during 1950–69, due to the decreased interannual variability of the vertical motion over the NWTA, the anomalous downward branch aloft and the low-level cross-equatorial northwesterly winds associated with the inverted NAO are not evident, and thus the regions with an increase in SST are confined to the Northern Hemisphere.

Strengthened Linkage between November/December North Atlantic Oscillation and Subsequent January European Precipitation after the Late 1980s

AbstractThis work investigates the nonsynchronous relationship between the North Atlantic Oscillation (NAO) and winter European precipitation. The results indicate that the linkage between early-winter (November and December) NAO and the following January precipitation and atmospheric circulation over the North Atlantic and European sectors became statistically significant after the late 1980s. Before the late 1980s, January precipitation and atmospheric circulation are weakly correlated with early-winter NAO. After the late 1980s, by contrast, the positive phase of the early-winter NAO is generally followed by an anomalous meridional dipole pattern with barotropic structure over the North Atlantic, which provides conditions for more (less) precipitation south of Iceland (east of the Azores). Further analysis elucidates that this regime shift may be partly attributed to the change of early-winter NAO, which is concurrent with significant change in the intensity of the synoptic and low-frequency eddy interaction over the Atlantic–European sectors. Anomalous positive sea level pressure and geopotential height, along with zonal wind anomalies associated with a positive early-winter NAO over the North Atlantic, are more significant and extend more northeastward after the late 1980s, which may be induced by an intensified transient eddy feedback after the late 1980s, as well as the enhanced storm-track activity over the North Atlantic. Thus, early-winter NAO can induce significant ocean temperature anomalies in the North Atlantic after the late 1980s, which extend downward into the middle parts of the thermocline and persist until the following January to trigger NAO-like atmospheric circulation patterns. Analyses from the Community Earth System Model large ensemble simulations indicate the effects of internal climate variability on such a strengthened linkage.

Forest stocks control long‐term climatic mortality risks in Scots pine dry‐edge forests

AbstractForest research has addressed the importance of an improved understanding of drought–stocks interactions in the dry edge of tree species range. Nonetheless, more efforts are still critically needed to link up the multiple ways by which climatic stressors can trigger tree mortality, including population‐level determinants and management. Here, we analyze the interactive effects of North Atlantic Oscillation (NAO), a surrogate of climatic variability in southwestern Europe, and forest stocks on tree mortality in dry‐edge populations of the most widespread Eurasian tree species, Pinus sylvestris L., in the forest of Valsaín (central Spain). Specifically, we use tree mortality data gathered since 1941 in six multiannual periods. Results suggest that the main mortality risks in these forests can occur either in positive or negative NAO phases, but that their relative impacts are critically mediated by forest structure. In NAO+ periods, commonly associated with warm–dry conditions in the Iberian Peninsula, a peak of mortality was found in closed forest sections, whereas the second peak, found in open forest sections, was related to NAO‐ periods, correlated with temperate‐rainy weather conditions. This finding reinforces the key role of management—through its control on forest structure—as a driver of forest vulnerability to climate. Accounting for the multiple ways in which stocks modulate tree responses to different risks emerges as a critical element when it comes to the design of efficient adaptation measures in managed dry‐edge forests.