Icelandic Low Research Articles

Interannual Variability of the Mass-Weighted Isentropic Zonal Mean Meridional Circulation in the Northern Hemisphere Winter

Abstract The atmospheric general circulation diagnosed by the mass-weighted isentropic zonal mean exhibits an extratropical direct (ETD) circulation driven by eddy momentum transport. We investigated regional atmospheric modulations associated with the year-to-year variability of ETD circulation in boreal winter using a reanalysis dataset. Composite analyses showed that the interannual variability of ETD circulation accompanies a seesaw-like variation between the Aleutian low (AL) and Icelandic low (IL), which coincides with the Pacific–North American teleconnection pattern and North Atlantic dipole anomaly. A diagnosis using isentropic airmass fluxes representing the geographical Lagrangian mean motion indicated that the significant intensification of upper-tropospheric poleward warm airflow over the eastern North Pacific is responsible for the enhanced ETD circulation, together with the intensified equatorward lower cold airflow over East Asia and eastern North America. The resulting enhanced upward propagation of the stationary planetary waves corresponds to the intensification of the divergence of the Eliassen–Palm flux, which explains the modulated ETD circulation in view of the balance between the Coriolis force and eddy momentum flux convergence. These results suggest that the AL–IL seesaw promotes variations in the global general circulation, and the anomalous ETD circulation interacts with several teleconnection patterns involving the modulation of planetary waves through anomalous meridional heat transport by the basinwide-scale eddies.

Change of the CP ENSO’s role in the occurrence frequency of Arctic daily warming events triggered by Atlantic storms

Arctic daily warming is gradually garnering the attention of academics. Here we discuss an interdecadal change around the mid-1980s in the role of the Central Pacific El Niño-Southern Oscillation (CP ENSO) in the occurrence frequency of Arctic daily warming events triggered by Atlantic storms in winter (called the Atlantic pattern-Arctic rapid tropospheric daily warming (A-RTDW) event) and the possible mechanism. Before the mid-1980s, the Central Pacific El Niño/La Niña events weakened/strengthened the Iceland Low (IL); the resulting anomalous northerly/southerly at the east of the IL prevented/favored the A-RTDW event occurrence by leading Atlantic storms away from/into the Arctic. Thus, the CP ENSO could affect the occurrence frequency of A-RTDW events by the CP ENSO/IL teleconnection. In contrast, this role hardly exists after the mid-1980s. Before the mid-1980s, the CP ENSO could affect the polar vortex by planetary wave propagation upwards into the stratosphere to create the CP ENSO/IL teleconnection; thereby, the CP ENSO and A-RTDW could establish a connection. However, after the mid-1980s, the planetary wave associated with CP ENSO could not propagate upwards into the stratosphere; thus, the ENSO/IL teleconnection disappears, resulting in CP ENSO having no effect on the occurrence frequency of A-RTDW events.

The separate roles played by the two geographical poles of the NAO in influencing winter precipitation over Spain

In this article, the spatiotemporal relationship between winter (DJF) precipitation over Spain and sea level pressure fields is studied using the Center of Action (COA) approach. The COAs which make up the North Atlantic Oscillation (NAO), i.e., the Azores High (AH) and the Icelandic Low (IL), are considered separately, and we find that interannual variations in the strength and position of the AH explain a very high fraction of the variance of Spain winter precipitation. The correlation of AH properties with winter precipitation is strongest in the northwest of the country. A regression model was developed that explains 83 percent of the variance of the Spain precipitation over 1981–2011. We found that the positive extreme values of AH indices are related to droughts, and negative extremes to heavy rain. The westward and southward shifts of the AH encourage the presence of low pressure system over the Iberian Peninsula and high precipitation in Spain occurs due to the moisture transport off the North Atlantic impinging upon Peninsula Spain and France.

INTERACTION OF CIRCULATION OBJECTS AND THEIR CLIMATE DYNAMICS BY THE EXAMPLE OF ATMOSPHERIC ACTION CENTERS IN THE NORTHERN HEMISPHERE

The article deals with the centers of action of the atmosphere of the Northern Hemisphere - the Icelandic and Aleutian lows, the Azores and Hawaiian highs, the Arctic and winter Asian anticyclones. The change in their characteristics in the annual course is considered. As such characteristics, the geographical coordinates of each center of action and the pressure in its center are taken. It is noted that the most mobile is the Arctic anticyclone. The winter Asian anticyclone practically does not change its position from month to month. The Icelandic Low is more mobile than the Azores High. The displacements of the Aleutian cyclone and the Hawaiian anticyclone are commensurate with each other. The winter Asian anticyclone affects the intensity of the centers of action of the atmosphere in the North Atlantic and the North Pacific. Before the appearance of the Asian winter anticyclone, pressure changes in the centers of the Atlantic and Pacific AAC occur in concert. During the formation of the winter Asian anticyclone, a mismatch occurs in the course of pressure in these centers of action. The degree of influence of the winter Asian anticyclone on pressure fluctuations in the centers of baric formations was made by the method of dispersion analysis. The degree of influence of the winter Asian anticyclone on the course of pressure in the centers of action of the North Atlantic atmosphere is estimated at 87%, in the centers of action of the Pacific Ocean - at 71 %.

Projected Changes to Mean and Extreme Surface Wind Speeds for North America Based on Regional Climate Model Simulations

This study evaluates projected changes to surface wind characteristics for the 2071–2100 period over North America (NA), using four Global Environmental Multiscale regional climate model simulations, driven by two global climate models (GCMs) for two Representative Concentration Pathway scenarios. For the current climate, the model simulates well the climatology of mean sea level pressure (MSLP) and associated wind direction over NA. Future simulations suggest increases in mean wind speed for northern and eastern parts of Canada, associated with decreases in future MSLP, which results in more intense low-pressure systems situated in those regions such as the Aleutian and Icelandic Lows. Projected changes to annual maximum 3-hourly wind speed show more spatial variability compared to seasonal and annual mean wind speed, indicating that extreme wind speeds are influenced by regional level features associated with instantaneous surface temperature and air pressure gradients. The simulations also suggest some increases in the future 50-year return levels of 3-hourly wind speed and hourly wind gusts, mainly due to increases in the inter-annual variability of annual maximum values. The variability of projected changes to both extreme wind speed and gusts indicate the need for a larger set of projections, including those from other regional models driven by many GCMs to better quantify uncertainties in future wind extremes and their characteristics.

Impact of the North Atlantic Oscillation on winter climate of Germany

Correlations of the North Atlantic Oscillation (NAO) with climate variables in Europe are strong in the north and the south of Europe but there is a zone of statistically not significant correlations in between. This paper presents evidence that the impact of NAO can be ascertained more clearly by decomposing it into its constituent centres of action (COA), the Azores High (AZH) and the Icelandic Low (IL), each of which varies continuously in intensity as well as position. Monthly values of intensity and latitude and longitude positions for each COA are estimated from sea level pressure data. These are used to analyse their impact on interannual variations of winter precipitation and climate of Germany. Correlation of NAO with winter precipitation is weak but statistically significant in north Germany and not significant in south Germany. By separately calculating correlations with the AZH and the IL, it is found that the primary influence on the interannual variations of winter precipitation and surface temperature over both north and south Germany is the zonal migration of the IL. When the IL is situated east of its mean location, there is above normal precipitation and warmer temperatures over Germany and in winters when the IL is situated west of its mean longitudinal position, drier and colder conditions occur over Germany. The impact of this teleconnection is greater in north Germany than in the south, presumably because of the mountainous topography in the south which hinders wind flow.

The Icelandic Low as a Predictor of the Gulf Stream North Wall Position

AbstractThe Gulf Stream’s north wall east of Cape Hatteras marks the abrupt change in velocity and water properties between the slope sea to the north and the Gulf Stream itself. An index of the north wall position constructed by Taylor and Stephens, called Gulf Stream north wall (GSNW), is analyzed in terms of interannual changes in the Icelandic low (IL) pressure anomaly and longitudinal displacement. Sea surface temperature (SST) composites suggest that when IL pressure is anomalously low, there are lower temperatures in the Labrador Sea and south of the Grand Banks. Two years later, warm SST anomalies are seen over the Northern Recirculation Gyre and a northward shift in the GSNW occurs. Similar changes in SSTs occur during winters in which the IL is anomalously west, resulting in a northward displacement of the GSNW 3 years later. Although time lags of 2 and 3 years between the IL and the GSNW are used in the calculations, it is shown that lags with respect to each atmospheric variable are statistically significant at the 5% level over a range of years. Utilizing the appropriate time lags between the GSNW index and the IL pressure and longitude, as well as the Southern Oscillation index, a regression prediction scheme is developed for forecasting the GSNW with a lead time of 1 year. This scheme, which uses only prior information, was used to forecast the GSNW from 1994 to 2015. The correlation between the observed and forecasted values for 1994–2014 was 0.60, significant at the 1% level. The predicted value for 2015 indicates a small northward shift of the GSNW from its 2014 position.

Decadal Variation of the Aleutian Low-Icelandic Low Seesaw Simulated by a Climate System Model (CAS-ESM-C)

AbstractBased on a simulation using a newly developed climate system model (Chinese Academy of Sciences-Earth System Model-Climate system component, CAS-ESM-C), the author investigated the Aleutian Low-Icelandic Low Seesaw (AIS) and its decadal variation. Results showed that the CAS-ESM-C can reasonably reproduce not only the spatial distribution of the climatology of sea level pressure (SLP) in winter, but also the AIS and its decadal variation. The period 496–535 of the integration by this model was divided into two sub-periods: 496–515 (PI) and 516–535 (P2) to further investigate the decadal weakening of the AIS. It was shown that this decadal variation of the AIS is mainly due to the phase transition of the Pacific Decadal Oscillation (PDO), from its positive phase to its negative phase. This transition of the PDO causes the sea surface temperature (SST) in the equatorial eastern (northern) Pacific to cool (warm), resulting in the decadal weakening of mid-latitude westerlies over the North Pacific and...

Multi-decadal modulations in the Aleutian-Icelandic Low seesaw and the axial symmetry of the Arctic Oscillation signature, as revealed in the 20th century reanalysis

Seesaw relationship in intensity between the surface Aleutian and Icelandic Lows (AIS) is a manifestation of atmospheric teleconnection that bridges the interannual variability over the Pacific and Atlantic in particular winter months. Analysis of the 20th Century Reanalysis data reveals that the strength and timing of AIS have undergone multi-decadal modulations in conjunction with those in structure of the Arctic Oscillation (AO) signature, extracted in the leading mode of interannual sea-level pressure (SLP) variability over the extratropical Northern Hemisphere. Specifically, events of what may be called ‘pure AO’, in which SLP anomalies exhibit a high degree of axial symmetry in association with in-phase SLP variability between the midlatitude Atlantic and Pacific, tended to occur during multi-decadal periods in which the inter-basin teleconnection through AIS was active under the enhanced interannual variability of the Aleutian Low. In contrast, the axial symmetry of the AO pattern was apparently reduced during a multi-decadal period in which the AIS teleconnection was inactive under the weakened interannual variability of the Aleutian Low. In this period, the leading mode of interannual SLP variability represented a meridional seesaw between the Atlantic and Arctic, which resembles SLP anomaly pattern associated with the cold-ocean/warm-land (COWL) temperature pattern. These multi-decadal modulations in interannual AIS signal and the axial symmetry of the interannual AO pattern occurred under multi-decadal changes in the background state that also represented the polarity changes of the COWL-like anomaly pattern.

A multiscale analysis of drought and pluvial mechanisms for the Southeastern United States

The Southeast (SE) U.S. has experienced several severe droughts over the past 30 years, with the most recent drought during 2006–2008 causing agricultural impacts of $1 billion. However, the mechanisms that lead to droughts over the region and their persistence have been poorly understood due to the region's humid coastal environment and its complex climate. In this study, we carry out a multiscale analysis of drought mechanisms for the SE U.S. over 1979–2008 using the North American Regional Reanalysis (NARR) to identify conditions associated with drought and contrast with those associated with pluvials. These conditions include land surface drought propagation, land-atmosphere feedbacks, regional moisture sources, persistent atmospheric patterns, and larger-scale oceanic conditions. Typical conditions for SE U.S. droughts (pluvials) are identified as follows: (1) weaker (stronger) southerly meridional fluxes and weaker (stronger) westerly zonal fluxes, (2) strong moisture flux divergence (convergence) by transient eddies, and (3) strong (weak) coupling between the land surface and atmosphere. The NARR demonstrates that historic SE droughts are mainly derived from a combination of a strong North Atlantic subtropical high (NASH) and Icelandic Low (IL) during summer and winter, respectively, which peak 1 month earlier than the onset of the drought. The land surface plays a moderate role in drought occurrence over the SE via recycling of precipitation, and the oceans show an asymmetric influence on droughts and pluvials depending on the season. This study suggests that the NASH and IL can be used as a predictor for SE droughts at 1 month lead despite the overall that it represents an atmospheric forcing.

Spatial distribution of air‐sea heat fluxes over the sub‐polar North Atlantic Ocean

On a variety of spatial and temporal scales, the energy transferred by air‐sea heat and moisture fluxes plays an important role in both atmospheric and oceanic circulations. This is particularly true in the sub‐polar North Atlantic Ocean, where these fluxes drive water‐mass transformations that are an integral component of the Atlantic Meridional Overturning Circulation (AMOC). Here we use the ECMWF Interim Reanalysis to provide a high‐resolution view of the spatial structure of the air‐sea turbulent heat fluxes over the sub‐polar North Atlantic Ocean. As has been previously recognized, the Labrador and Greenland Seas are areas where these fluxes are large during the winter months. Our particular focus is on the Iceland Sea region where, despite the fact that water‐mass transformation occurs, the winter‐time air‐sea heat fluxes are smaller than anywhere else in the sub‐polar domain. We attribute this minimum to a saddle point in the sea‐level pressure field, that results in a reduction in mean surface wind speed, as well as colder sea surface temperatures associated with the regional ocean circulation. The magnitude of the heat fluxes in this region are modulated by the relative strength of the Icelandic and Lofoten Lows, and this leads to periods of ocean cooling and even ocean warming when, intriguingly, the sensible and latent heat fluxes are of opposite sign. This suggests that the air‐sea forcing in this area has large‐scale impacts for climate, and that even modest shifts in the atmospheric circulation could potentially impact the AMOC.

The Siberian High: teleconnections, extremes and association with the Icelandic Low

AbstractA 60 year minimum temperature record of 11 stations in inner Eurasia enabled the characterization of the Siberian High (SH) intensity. The decline in the SH intensity is observed in tandem with the positive mode of the Arctic Oscillation (AO), both increasing in recent years. The coldest 1968–1969 winter in the 60 year period corresponds with the lowest AO annual index value. Spatial correlation analyses indicate that enhanced cyclogenetic conditions over the eastern flank of the Icelandic Low (IL) are associated with a milder SH. Seasonal composite analyses of the circulation pattern during the coldest 1968–1969 winter are characterized by a retreat of the IL, allowing a westward expansion of the SH cold core. A robust methodology, assuring an adequate representation of extremely cold spells in both their extent and duration, was developed. This methodology yielded three exceptional events, the most severe one lasting 10 days and affecting all stations in the SH domain. Analysing this event on a fine temporal resolution enabled the detection of short‐term synoptic scale processes, such as the polar air mass penetration, resulting in a mean minimum temperature of − 40 °C over the whole domain. This short‐term polar air incursion and its termination featuring this spell, as modulated by the location of the IL, point at its important role in modifying the SH. Copyright © 2012 Royal Meteorological Society

Interannual variations of the blocking high over the Ural Mountains and its association with the AO/NAO in boreal winter

This paper analyzes interannual variations of the blocking high over the Ural Mountains in the boreal winter and their association with the Arctic Oscillation/North Atlantic Oscillation (AO/NAO). In January, the relationship between the Ural blocking high (UR) and the AO index is statistically significant. The UR tends to occur more frequently and with greater strength during negative AO periods. Some strong URs also occur during positive AO phases (positive UR-AO events), as in January 2008. This paper discusses the characteristics of atmospheric circulation in the cases of positive UR-AO events and contrast cases (negative UR-AO events). The eastward extending of the Icelandic Low (IL) center and the associated NAO dipole anomaly pattern in the upstream region may play a more important role for the UR-AO events. When the center of the IL shifts eastward to 30°W, the amplitude of zonal wavenumber 2 (wavenumber 3) is intensified in the positive (negative) UR-AO events, which favors positive (negative) height anomalies over the Urals. Further analyses indicate that the intensified zonal wind in high latitudes and weakened zonal wind in midlatitudes over the North Atlantic Ocean render the eastward shift of the IL and the NAO dipole anomaly pattern. The Ural blocking in January 2008 bears similar characteristics to the positive UR-AO events.

Sea-level pressure–air temperature teleconnections during northern hemisphere winter

The relationship between sea-level pressure (SLP) and 1,000 hPa air temperature (AT) in winter is investigated over the northern hemisphere, and a statistical forecasting of one of the two parameters from the other is attempted on a monthly basis. Mean monthly SLP and 1,000 hPa level AT values at 563 grid points over the northern hemisphere are utilized for January, February and March, for the period 1949–2002. At first, factor analysis is applied to the data sets as a dimensionality reduction tool. Then, canonical correlation analysis is applied to the resultant factor scores time series for the five SLP–AT pairs: SLP(J)–AT(J), SLP(J)–AT(F), SLP(J)–AT(M), AT(J)–SLP(F) and AT(J)–SLP(M), and the synchronous and time-lag connections between the two parameters are investigated. The areas characterized by a satisfactory monthly or/and bi-monthly forecasting ability are detected. The most satisfactory results refer to the areas affected by the Southern Oscillation. It is found that the SLP teleconnection between the areas of the eastern and the western Pacific in January is related to the regime of AT in the central Pacific Ocean, in both February and March. Also, SLP over the Aleutian and Icelandic lows in January is related to AT over their southwestern and southeastern neighbouring areas in February and March. Finally, it appears that there is also ability for monthly/bi-monthly statistical prediction for some areas affected by the well-known oscillations of North Atlantic Oscillation and Pacific/North American Oscillation. A validation of the statistical prediction methodology is carried out, using real-time series of AT and SLP parameters for some characteristic cases. The results show that the statistical prediction presents a remarkable success. The success rate varies from 67% to 83% for the analysis period 1949–2002 and from 71% to 86% for the recent period 2003–2009.

Autumn atmospheric response to the 2007 low Arctic sea ice extent in coupled ocean–atmosphere hindcasts

The autumn and early winter atmospheric response to the record-low Arctic sea ice extent at the end of summer 2007 is examined in ensemble hindcasts with prescribed sea ice extent, made with the European Centre for Medium-Range Weather Forecasts state-of-the-art coupled ocean–atmosphere seasonal forecast model. Robust, warm anomalies over the Pacific and Siberian sectors of the Arctic, as high as 10°C at the surface, are found in October and November. A regime change occurs by December, characterized by weaker temperatures anomalies extending through the troposphere. Geopotential anomalies extend from the surface up to the stratosphere, associated to deeper Aleutian and Icelandic Lows. While the upper-level jet is weakened and shifted southward over the continents, it is intensified over both oceanic sectors, especially over the Pacific Ocean. On the American and Eurasian continents, intensified surface Highs are associated with anomalous advection of cold (warm) polar air on their eastern (western) sides, bringing cooler temperatures along the Pacific coast of Asia and Northeastern North America. Transient eddy activity is reduced over Eurasia, intensified over the entrance and exit regions of the Pacific and Atlantic storm tracks, in broad qualitative agreement with the upper-level wind anomalies. Potential predictability calculations indicate a strong influence of sea ice upon surface temperatures over the Arctic in autumn, but also along the Pacific coast of Asia in December. When the observed sea ice extent from 2007 is prescribed throughout the autumn, a higher correlation of surface temperatures with meteorological re-analyses is found at high latitudes from October until mid-November. This further emphasises the relevance of sea ice for seasonal forecasting in the Arctic region, in the autumn.

Response of Arctic 1000 hPa circulation to changes in horizontal resolution and sea ice forcing in the Community Atmospheric Model

Arctic winter surface atmospheric circulation biases inherent to the Community Atmospheric Model version 3 (CAM3) are examined with emphasis on the influence of horizontal resolution (T42, ∼2.8° versus T85, ∼1.4°). Using geopotential height at 1000 hPa (Z1000) to represent surface circulation, the self‐organizing map technique is used to better understand these biases from a synoptic climatology perspective. The previously documented low sea level pressure (SLP) bias over the Beaufort Sea, Canadian Archipelago, Greenland Sea, Norwegian Sea, and northern Europe and high SLP bias over the northern Pacific and Atlantic oceans are shown to result from an underprediction of weak Aleutian and Icelandic low patterns and an overprediction of strong Arctic low patterns, strong Icelandic low patterns, and patterns with pronounced ridges spanning the Arctic Ocean. Resolution‐caused differences are shown to be the result of CAM3 at T85 resolution predicting fewer strong Icelandic lows, North Atlantic lows, and strong high‐pressure ridges across the Arctic relative to CAM3 at T42 resolution. In addition, CAM3 at T85 resolution predicts more strong Arctic low patterns relative to CAM3 at T42 resolution. The ability of CAM3 at both resolutions to resolve mesoscale features such as polar lows near the sea ice edge is also explored. Despite these biases, CAM3 can still be used experimentally to examine atmospheric response to changing climatic conditions, particularly if results are viewed in the framework of a sensitivity study. With this in mind, the impact of future projections of sea ice on winter Arctic circulation is examined at both model resolutions. It is found that weak Arctic low patterns and strong Icelandic low patterns increase in frequency in the winter, while Aleutian/Icelandic low patterns and North Atlantic low/Pacific high patterns decrease in frequency in the winter with future projections of sea ice.

Spatial Analysis of the Factors Contributing to the Relationship between the Transient, Meridional Eddy Sensible, and Latent Heat Flux in the Southern Hemisphere

In this paper principal component analysis (PCA) and singular value decomposition (SVD) are used to defne the importance of the variables contributing to the relationship between the transient latent and sensible heat fluxes and to show their temporal and spatial variation. SVD is ofered as an alternative means of isolating spatial and temporal structures in data with the advantage that it can depict simultaneous space-time variations that are aggregates of the results produced by PCA. Both methods of analysis produced two vey important uncorrelated modes of variability in Janua y and July, indicating that the transient heat fluxes are injuenced by fm controlling factors. We suggest that these modes of variability represent the influences of the meridional temperature gradient, atmospheric moisture, and activity within the source and sink regions of the transient heat fluxes. The physical relationships between the heat fluxes that appear to represented by the statistical modes of variability are discussed. The latitudinal imbalance of energy that arises from unequal radiative heating is in part redressed by the poleward transport of energy by the atmosphere. Poleward airflow transfers warmer, more moist air toward colder, drier regions. Much of this transport takes place during winter when the equator-pole gradient of temperature (and energy) is maximized. The flux of energy is therefore necessary to maintain existing global temperature distributions. Much of the energy transported is in the form of latent and sensible heat, and the main agents of transfer in the extratropics are the quasi-stationary eddies (semipermanent features of the general circulation, for example, the Icelandic and Aleutian Lows) and transient eddies (for example, baroclinic eddies, midlatitude wave This work was completed while Marilyn Raphael was on sabbatical at the National Center for Atmospheric Research, where Steve Cherry was a postdoctoral student. The authors thank the CAS group at NCAR for the funding and data that made this work possible. They also thank the three anonymous reviewers for their helpful comments on earlier drafts of this paper. Steve Cherry was supported by the NSF through the Geophysical Statistics Project at NCAR.

Long‐term variability in precipitation and streamflow in Iceland and relations to atmospheric circulation

AbstractHow the variability of the atmospheric circulation affects precipitation in Iceland is not completely understood. Also, the sea surface temperature (SST) has a strong influence on the temperature over the country, and thereby, snow and glacial melt. This study, therefore, aims at explaining how atmospheric circulation and sea surface temperature influence seasonal and annual precipitation, and, consequently, runoff in Iceland.Empirical orthogonal function (EOF) analysis is performed on annual and seasonal time series of precipitation and discharge to identify their key modes of variability during the period 1966–2004. The correlation between the time series of each EOF mode with individual time series of sea level pressure (SLP), air temperature and SST was then evaluated.The analyses evidenced how large‐scale climate variables are connected to the regional precipitation and runoff in Iceland. They showed that the strength of the polar vortex may be, at least, as important for the precipitation in some areas of Iceland as the strength of the Icelandic Low (IL). Moreover, the location of the semi‐permanent IL often defines the predominant wind direction over the country and, as such, the regions of preferred precipitation.Since the watersheds act as large precipitation gauges with response patterns depending on the geology and glaciers, the variability of the annual discharge closely resembles the variability of precipitation, except for the glacial rivers. Glacial melt is highly correlated to air temperature and SST, and the spring discharge is affected by winter and spring temperatures.The results also revealed that Icelandic hydrological conditions in the spring can be forecasted by precipitation and temperature of the autumn and winter seasons, as well as by the general prevalent circulation patterns. Additionally, a potential for seasonal forecast of precipitation, and river discharge in other seasons was identified, particularly if seasonal forecast of SLP is available. Copyright © 2008 Royal Meteorological Society

Two types of the surface pressure oscillations in the extra-tropical zone of the Northern Hemisphere depending on long-term variations of solar activity

The increased solar cycle activity is followed by long-term anomalies in the surface air pressure that take place in the extra-tropical zone of the Northern Hemisphere in winter. The most important peculiarity is an opposite change in the pressure in the regions that tend to the Icelandic and Aleutian lows. During the 16th-23rd solar activity cycles, two scenarios of pressure change in the high latitudes are observed: (1) an increase at the border between the Arctic and Atlantic Ocean and a decrease at the border with the Pacific; (2) a decrease at the Arctic border with the Atlantic Ocean and an increase at its border with the Pacific. Spatial-temporal peculiarities of natural oscillations adapted to large-scale inhomogeneity in land and ocean distribution in the temperate and high latitudes and, therefore, the Arctic hydrometeorological conditions can be considered as important indicators of solar-terrestrial relations.

Characteristics of winter cyclone activity in the northern North Atlantic: Insights from observations and regional modeling

Fields from the NCEP/NCAR reanalysis are used in conjunction with simulations from Polar MM5 to examine regional processes contributing to cyclone development in the sub‐Arctic/Arctic sector of the North Atlantic. Strong deepening of systems (>6 hPa/6 hours) is especially common south of the tip of Greenland and in the vicinity of the mean Icelandic Low (IL). A secondary maximum is located just south of Svalbard. Composite analyses of strongly deepening systems in these areas point to the influence of pronounced preexisting low‐level temperature gradients linked to ocean currents, the sea ice margin and ocean‐land contrasts. The area of preferred deepening near Svalbard corresponds to the northernmost penetration of open water in the North Atlantic. Impacts of Greenland's orography are prominent in the synoptic complexity of the region. Typical synoptic situations, examined as case studies, include splitting, or “bifurcation” of cyclones at the southern tip of Greenland; orographic cyclogenesis in the lee of Greenland, at the location of the mean IL; and deepening of preexisting systems near the IL. Orographic influences are clearly captured in Polar MM5 simulations for which control runs of the three cases are compared to runs for which the orography of Greenland is removed.