Icelandic Low and Azores High Migrations Impact Florida Current Transport in Winter

Previous work to find an association between variations of annually averaged Florida Current transport and the North Atlantic Oscillation (NAO) have yielded negative results (Meinen et al. 2010). Here we show that Florida current in winter is impacted by displacements in the positions of the Azores High and the Icelandic Low, the constituent pressure centers of the NAO. As a one-dimensional representation of North Atlantic atmospheric circulation, the NAO index does not distinguish displacements of the pressure centers from fluctuations in their intensity. Florida Current transport is significantly correlated with Icelandic Low longitude with a lag of less than one season. We carried out perturbation experiments in the ECCOv4 model to investigate these correlations. These experiments reveal that east-west shifts of the Icelandic Low perturb the wind stress in mid-latitudes adjacent to the American coast, driving downwelling (through longshore winds) and offshore sea level anomalies (through wind stress curl) which travel to the Florida Straits within the same season. Florida Current transport is also correlated with the latitude variations of both the Icelandic Low and the Azores High with a lag of four years. Regression analysis shows that latitude variations of the Icelandic Low and the Azores High are associated with positive wind stress curl anomalies over extended regions in the ocean east of Florida. Rossby wave propagation from this region to the Florida Straits has been suggested as a mechanism for perturbing FCT transport in several previous studies (DiNezio et al. 2009; Czeschel et al. 2012; Frajka-Williams et al. 2013; Domingues et al. 2016, 2019).

Relationship between Azores High and Indian summer monsoon

A dominant mode of interannual variability of Indian summer monsoon rainfall shows west-east dipole pattern with above normal rainfall towards west and central India and subdued rainfall towards the east and northeast India, and is related to the vigorous Azores High. The vigorous Azores High is accompanied by enhanced subsidence resulting in well-built widespread upper-troposphere convergence. This forms the meridional vorticity dipole consisting of anomalous cyclonic and anti-cyclonic circulation at 30°N and 50°N, respectively, and boosts the Rossby wave source. The cascading down Rossby wave train imposes successive negative, positive and negative Geopotential height (GPH) anomalies over north Mediterranean, northwest and northeast of India, respectively. The negative GPH anomaly at the north Mediterranean increases the Asian jet towards the Caspian Sea, strengthening the monsoon circulation through the ‘silk-road’ pattern. While, the dipole GPH anomaly north of India shift the Tibetan High westwards, triggering monsoon activity towards the west.

Air-traffic restrictions at the Madeira International Airport due to adverse winds: links to synoptic-scale patterns and mountain wave phenomena

<p>The Madeira International Airport (MIA) lies on the island south-eastern coast and it is known to be exposed to wind hazards. A link between these adverse winds at MIA and the synoptic-scale circulation is established using a weather type (WT) classification. From April to September (summer period), five WTs prevail, cumulatively representing nearly 70% of days. These WTs reflect the presence of well-established Azores high, with some variations on location and strength. Although with a low frequency of occurrence (<5%), this anticyclone occasionally strengthens and extends towards Iberia, inducing anomalously strong NNE/NE up to 3-5 km over Madeira. The most severe and longer-lasting wind conditions at the MIA, with a higher frequency of gusts above 35 kt, are driven by this synoptic-scale pattern and are more common in summer. An episode of adverse winds at the MIA is analyzed, illustrating the occurrence of upstream stagnation, flow splitting, and lee wake formation. The upstream conditions include a low-level inversion, strong NNE/NE winds near and above the inversion and a Froude number less than 1. AROME model predicted the occurrence of downslope winds, in association with a large-amplitude mountain wave. At this time, the strongest wind gusts were registered and a missed approach occurred. The wind regime in different places of the island suggests that these conditions are relatively frequent, mostly in summer. Lastly, this study provides an objective verification of the AROME wind forecasts, for a 3-year period and from June to August.</p>

Air-Traffic Restrictions at the Madeira International Airport Due to Adverse Winds: Links to Synoptic-Scale Patterns and Orographic Effects

The Madeira International Airport (MIA) lies on the island’s south-eastern coast and it is known to be exposed to wind hazards. A link between these adverse winds at MIA and the synoptic-scale circulation is established using a weather type (WT) classification. From April to September (summer period), five WTs prevail, cumulatively representing nearly 70% of days. These WTs reflect the presence of well-established Azores high, with some variations on location and strength. Although with a low frequency of occurrence (<5%), this anticyclone occasionally strengthens and extends towards Iberia, inducing anomalously strong NNE/NE up to 3–5 km over Madeira. The most severe and longer-lasting wind conditions at the MIA, with a higher frequency of gusts above 35 kt, are driven by this synoptic-scale pattern and are more common in summer. An episode of adverse winds at the MIA is analysed, illustrating the occurrence of upstream stagnation, flow splitting, and lee wake formation. The upstream conditions include a low-level inversion, strong NNE/NE winds near and above the inversion and a Froude number less than 1. The AROME (Application of Research to Operations at Mesoscale) model predicted the occurrence of downslope winds, in association with a large-amplitude mountain wave. At this time, the strongest wind gusts were registered and one aircraft executed a missed approach. The wind regime in different places of the island suggests that these conditions are relatively frequent, mostly in summer. Finally, objective verification of AROME wind forecast, for a three-year period and from June to August, is discussed.

The 2018 summer heatwaves over northwestern Europe and its extended-range prediction

This study investigated the drivers and extended-range prediction of the mid-July to early August 2018 heatwaves over northwestern Europe, focusing on regional heatwave events over Scandinavia (SC) and Western Europe (WE). The persistent blocking regime (BL) was the most influential contributor for the 2018 heatwave over SC, and both the Atlantic Low regime (AL) and North Atlantic Oscillation (NAO) were secondary contributors for the heatwave, but with different effect directions. The major contributor to the heatwave over WE was AL. These causal relationships remained valid when the evolution of warm spells was considered. A multi-model ensemble of real-time forecasts from the subseasonal to seasonal (S2S) database captured the evolution of the warm spells over SC and WE up to 3 weeks in advance. However, the predictions of heatwave occurrence and significance for the two regions are unsatisfactory. BL and AL can be predicted 2 weeks in advance, resulting in the successful predictions of warm spells over SC and WE. Although variations in Azores High and NAO were captured in the forecasts, their contribution to the warm spells remains unclear.

Characteristics of North European winter lightning related to a high positive North Atlantic Oscillation index

&lt;p&gt;The variability of winter climate in the North Atlantic region is predominantly driven by a large scale alternation of atmospheric masses between the Icelandic Low and Azores High pressure systems called the North Atlantic Oscillation (NAO) and characterized by the NAO index. The calculation of the NAO index is based on the difference between sea-level pressure strengths of the Azores High and the Icelandic Low. Unusually high positive values of the NAO index were observed to manifest themselves by above-average precipitation and severe winter storms over British Isles and other parts of northwestern and northern Europe.&lt;/p&gt;&lt;p&gt;In the last two decades, the winter season 2014/2015 exhibited the highest positive monthly NAO indexes. During this winter, newspapers in the UK, Germany, Poland, and Scandinavia reported extremely strong storms which caused huge power outages, damages of buildings, and collapses of traffic which paralyzed the daily life. As winter thunderstorms are also characterized by a higher production of very energetic lightning, we use the World Wide Lightning Location Network (WWLLN) data and investigate properties of lightning which occurred in the north European region from October 2014 to March 2015.&amp;#160; The dataset consists of more than 90 thousand lightning detections. We focus on spatial and temporal distribution of lightning strokes, their energies and multiplicity.&lt;/p&gt;&lt;p&gt;We have found that the diurnal distribution of lightning was random from November till February, while the afternoon peak typical for summer storms was noticeable only in October and March. The median energy of lightning strokes observed in October, November and March reached only about 10-20% of the median energy of strokes detected in December, January and February. The most energetic strokes were concentrated above the ocean close to the western coastal areas and appeared exclusively at night and in the morning hours.&lt;/p&gt;

Unprecedented Expansion of the Azores High due to Anthropogenic Climate Change

&lt;p&gt;The Azores High is a subtropical high-pressure ridge in the North Atlantic. During boreal winters, anticyclonic winds rotate around the Azores High, transporting moisture to Western Europe. Variability in the size and intensity of the Azores High thus corresponds to variability in hydroclimate across Western Europe. We use the Last Millennium Ensemble (LME), which is run using the Community Earth System Model (CESM) and features thirteen transient simulations covering the period 850 to 2005 A.D. with prescribed external forcing (e.g. greenhouse gas, solar, volcanic, land use, orbital, and aerosol). The LME is shown to accurately simulate the variability and trends in the Azores High when compared to observational records from the 20&lt;sup&gt;th&lt;/sup&gt; century. The Azores High has grown in size during the Industrial Era. This growth is most dramatic when observing the frequency of winters during which the Azores High is extremely large. The LME shows more winters with an extremely large Azores High in the past 100 years than any other 100-year period in the last millennium. Using LME as well as other simulations from the Paleoclimate Modelling Intercomparison Project Phase III, the recent expansion of the Azores High is shown to be well outside the range of natural variability since 850 A.D. Individual forcing simulations within the LME provide smaller ensembles in which only one external forcing is varied. These experiments attribute Azores High expansion to the recent increase in atmospheric greenhouse gas concentrations. Recent hydroclimatic signals across Western Europe consistent with the Azores High variability are also discussed.&lt;/p&gt;

Variability of the Azores High and regional hydroclimate over the past millennium

&lt;p&gt;The subtropical dry zones, including the broader Mediterranean region, are likely to experience considerable changes in hydroclimate in a warming climate. An expansion of the atmosphere&amp;#8217;s meridional overturning circulation, the Hadley circulation, over recent decades has been reported, with implications for regional hydroclimate. Yet, there exists considerable disagreement in magnitude and even sign of these trends among different metrics that measure various aspects of the Hadley circulation, as well as discrepancies in trends between different analysis periods and reanalysis products during the 20&lt;sup&gt;th&lt;/sup&gt; century. In light of these uncertainties, it is therefore of interest to explore variability and trends in subtropical hydroclimate and its dominant driver, the Hadley Circulation. We focus on the North Atlantic sector and explore variability in the Azores High, the manifestation of the Hadley Circulation&amp;#8217;s downward branch, and hydroclimate across the Iberian Peninsula using a combination of observational/reanalysis products, state-of-the-art climate model simulations, and hydroclimatically-sensitive stalagmite records over the past 1200 yr. The Last Millennium Ensemble (LME) with the Community Earth System Model provides thirteen transient simulations covering the period 850 to 2005 A.D. with prescribed external forcing (e.g. greenhouse gas, solar, volcanic, land use, orbital, and aerosol) and smaller subsets with individual forcing only. The LME is shown to accurately simulate the variability and trends in the Azores High when compared to observational records from the 20&lt;sup&gt;th&lt;/sup&gt; century. We evaluate variability in the Azores High (e.g., size, intensity, position) in relation to other key metrics that measure different aspects of the Hadley circulation throughout the course of the last millennium, as well as during key periods, such as the Little Ice Age or Medieval Climate Anomaly. The smaller subsets of LME simulations with individual forcing factors (e.g., solar, volcanic) allow for an attribution of past changes in regional hydroclimate to external drivers. Results from the climate model simulations are compared with hydroclimate reconstructed from stalagmites from Portuguese caves.&lt;/p&gt;

Implications of Winter NAO Flavors on Present and Future European Climate

The North Atlantic Oscillation (NAO), a basic variability mode in the Northern Hemisphere, undergoes changes in its temporal and spatial characteristics, with significant implications on European climate. In this paper, different NAO flavors are distinguished for winter in simulations of a Coupled Atmosphere-Ocean GCM, using Self-Organizing Maps, a topology preserving clustering algorithm. These flavors refer to various sub-forms of the NAO pattern, reflecting the range of positions occupied by its action centers, the Icelandic Low and the Azores High. After having defined the NAO flavors, composites of winter temperature and precipitation over Europe are created for each one of them. The results reveal significant differences between NAO flavors in terms of their effects on the European climate. Generally, the eastwardly shifted NAO patterns induce a stronger than average influence on European temperatures. In contrast, the effects of NAO flavors on European precipitation anomalies are less coherent, with various areas responding differently. These results confirm that not only the temporal, but also the spatial variability of NAO is important in regulating European climate.