Hemisphere Circulation Patterns Research Articles

Linkage Between a Dominant Mode in the Lower Stratosphere and the Western Hemisphere Circulation Pattern

AbstractBased on the statistical analyses using ERA data sets, this study reveals a significant linkage between the tropospheric Western Hemisphere (WH) circulation pattern and the stratospheric EOF1 mode (North America [NA] pattern) of the wintertime daily height field at 50 hPa, which exhibits the displacement of the polar vortex to NA. The results of composite analysis indicate that the WH pattern leads the NA pattern at about 5 days. This time‐lagged linkage between the two patterns is further confirmed by the results of time‐lagged singular value decomposition (SVD) analyses. Through the linear interference with climatological waves, the WH pattern contributes to the enhanced (suppressed) wave‐2 (wave‐1) activity propagating into the stratosphere. The characteristics of planetary waves, with opposite sign of anomalous wave‐1 and wave‐2 activity, lead to the formation of the stratospheric NA pattern. We suggest the need for caution in interpreting the stratospheric influences on cold air outbreaks over NA.

Energetics of the Western Hemisphere Circulation Pattern

Abstract The Western Hemisphere (WH) circulation pattern, identified by self-organizing maps cluster analysis, is a low-frequency atmospheric regime that influences the fluctuations of large-scale circulation over the North Pacific–North American–North Atlantic areas. The reanalysis datasets from ECMWF are used to estimate the energetics of the WH pattern in this study. The composite results based on monthly WH events reveal that the kinetic energy (KE) associated with the WH pattern is maintained through the barotropic conversion from the climatological-mean westerlies, mainly in the Atlantic jet exit regions. The KE could also be gained through the barotropic feedback forcing from transient eddies. The corresponding baroclinic conversion of available potential energy (APE) from the climatological-mean state, which contributes most efficiently to the energy maintenance of the WH pattern, is obvious in the middle and lower troposphere, owing to the thermal contrast of the colder continent and warmer ocean over the North America–North Atlantic sector. The baroclinic conversion associated with the heat flux on the climatological temperature gradient is consistent with the southwestward-tilting height anomalies from 850 to 500 hPa. The baroclinic feedback from transient eddies contributes negatively to the energy conversion and destroys the maintenance of the WH pattern.

Linear and nonlinear links of winter European precipitation to Northern Hemisphere circulation patterns

The connections between European rainfall variability in winter (from January to March) and Northern Hemisphere circulation patterns, as represented by 500-hPa geopotential height, are investigated in the period 1900–2014. Initially, three statistically significant pairs of linearly-related circulation and precipitation patterns, explaining 45% of the variance in the latter field, are identified. The first two essentially represent the North Atlantic Oscillation (NAO) and the East Atlantic (EA) patterns, while the third corresponds to a structure described in recent literature as a blend of the Pacific North America (PNA) pattern and the so-called Asia Bering North America (ABNA) pattern. The residual precipitation field is then examined for patterns that can introduce non-linear modulations into the circulation-precipitation links. It is so found that the NAO impact on European rainfall is modulated by North American and Pacific factors controlling cyclogenesis over Newfoundland. The positive-phase EA rainfall anomalies over Central Europe and the British Isles seem to be markedly affected by the NAO phase. Finally, a possible signature from the Circumglobal Teleconnection Pattern disrupting the NAO influence on East Mediterranean precipitation anomalies is detected.

The Role of Synoptic Waves in the Formation and Maintenance of the Western Hemisphere Circulation Pattern

Previous studies have demonstrated that the NAO, the leading mode of atmospheric low-frequency variability over the North Atlantic, could be linked to northeast Pacific climate variability via the downstream propagation of synoptic waves. In those studies, the NAO and the northeast Pacific climate variability are considered as two separate modes that explain the variance over the North Atlantic sector and the east Pacific–North American sector, respectively. A newly identified low-frequency atmospheric regime—the Western Hemisphere (WH) circulation pattern—provides a unique example of a mode of variability that accounts for variance over the whole North Atlantic–North American–North Pacific sector. The role of synoptic waves in the formation and maintenance of the WH pattern is investigated using the ECMWF reanalysis datasets. Persistent WH events are characterized by the propagation of quasi-stationary Rossby waves across the North Pacific–North American–North Atlantic regions and by associated storm-track anomalies. The eddy-induced low-frequency height anomalies maintain the anomalous low-frequency ridge over the Gulf of Alaska, which induces more equatorward propagation of synoptic waves on its downstream side. The eddy forcing favors the strengthening of the midlatitude jet and the deepening of the mid-to-high-latitude trough over the North Atlantic, whereas the deepening of the trough over eastern North America mostly arises from the quasi-stationary waves propagating from the North Pacific. A case study for the 2013/14 winter is examined to illustrate the downstream development of synoptic waves. The roles of synoptic waves in the formation and maintenance of the WH pattern and in linking the northeast Pacific ridge anomaly with the NAO are discussed.

Classifying the tropospheric precursor patterns of sudden stratospheric warmings

AbstractClassifying the tropospheric precursor patterns of sudden stratospheric warmings (SSWs) may provide insight into the different physical mechanisms of SSWs. Based on 37 major SSWs during the 1958–2014 winters in the ERA reanalysis data sets, the self‐organizing maps method is used to classify the tropospheric precursor patterns of SSWs. The cluster analysis indicates that one of the precursor patterns appears as a mixed pattern consisting of the negative‐signed Western Hemisphere circulation pattern and the positive phase of the Pacific‐North America pattern. The mixed pattern exhibits higher statistical significance as a precursor pattern of SSWs than other previously identified precursors such as the subpolar North Pacific low, Atlantic blocking, and the western Pacific pattern. Other clusters confirm northern European blocking and Gulf of Alaska blocking as precursors of SSWs. Linear interference with the climatological planetary waves provides a simple interpretation for the precursors. The relationship between the classified precursor patterns of SSWs and ENSO phases as well as the types of SSWs is discussed.

Climatology and trends of the Adriatic Sea wind waves: analysis of a 37‐year long instrumental data set

ABSTRACTThis study concerns the analysis of a 37‐year long directional wave time series recorded since 1979 at the CNR‐ISMAR oceanographic research tower, located in the Northern Adriatic Sea. The length of the time series allows to describe the wave climate in the North Adriatic and to identify trends and links with large‐scale climate patterns. The northern part of the Adriatic Sea is characterized by two main wind and correspondingly wave regimes, strongly forced by the regional orography: the long‐fetch south‐easterly ‘sirocco’ and the short‐fetch strong north‐easterly ‘bora’ wind generated waves, blowing along the major and minor axis of the basin, respectively. Bora is the most frequent regime, but high waves are mostly associated to sirocco. A clear decrease of the significant wave height 99th percentile is evident, paralleled by a smaller, but distributed along the annual cycle, increase of the 50th and 75th ones. The estimated trend of the frequency of events above a certain threshold confirms an increase of the average storm frequency, with a shift from both the lower and the higher to the central part of the distribution, paralleled by a decrease of the maximum Hs values. In particular, a distributed decrease of the bora significant wave height is recognized. For sirocco the tendency is less pronounced, but with an evident increase of the frequency of the mean values. The high sensitivity of this particular area to even small variations of large‐scale climate allows to explore possible links of the local wave, hence wind, activity with large‐scale Northern Hemisphere circulation patterns or weather regimes. Our main interest is on the storm tracks and jet stream location/intensity over vast areas, which led us to choose four reference patterns: the North Atlantic Oscillation and the East Atlantic (EA), EA/Western Russia and Scandinavian patterns.

Seasonal climate summary southern hemisphere (summer 2015-16): strong El Niño peaks and begins to weaken

Southern hemisphere circulation patterns and associated anomalies for austral summer 2015-16 are reviewed, with an emphasis on the tropical Pacific as well as Australian rainfall and temperatures. Following the peak of El Nino in November 2015, summer 2015-16 featured continued near-record El Nino conditions in the tropical Pacific but saw the emergence of cooler subsurface waters in the equatorial Pacific. A moderate Madden Julian Oscillation (MJO) pulse and positive Southern Annular Mode (SAM) ontributed to average to above average rainfall across much of Australia, while the Maritime Continent and parts of far northern Australia saw continued below average rainfall. Sea surface temperatures during summer 2015-16 were the warmest on record for the southern hemisphere oceans, with very warm ocean temperatures in the Indian Ocean and Australian region, including the warmest summer sea surface temperatures on record around Tasmania. Air temperatures were also warmer than normal across Australia throughout the season, with a significant heatwave in southeast Australia during December.

The cold climate geomorphology of the Eastern Cape Drakensberg: A reevaluation of past climatic conditions during the last glacial cycle in Southern Africa

Southern Africa is located in a unique setting for investigating past cold climate geomorphology over glacial-interglacial timescales. It lies at the junction of three of the world's major oceans and is affected by subtropical and temperate circulation systems, therefore recording changes in Southern Hemisphere circulation patterns. Cold climate landforms are very sensitive to changes in climate and thus provide an opportunity to investigate past changes in this region. The proposed existence of glaciers in the high Eastern Cape Drakensberg mountains, together with possible rock glaciers, has led to the suggestion that temperatures in this region were as much as 10–17°C lower than present. Such large temperature depressions are inconsistent with many other palaeoclimatic proxies in Southern Africa. This paper presents new field observations and cosmogenic nuclide exposure ages from putative cold climate landforms. We discuss alternative interpretations for the formation of the landforms and confirm that glaciers were absent in the Eastern Cape Drakensberg during the last glaciation. However, we find widespread evidence for periglacial activity down to an elevation of ~1700m asl, as illustrated by extensive solifluction deposits, blockstreams, and stone garlands. These periglacial deposits suggest that the climate was significantly colder (~6°C) during the Last Glacial Maximum, in keeping with other climate proxy records from the region, but not cold enough to initiate or sustain glaciers or rock glaciers.

Seasonal climate summary southern hemisphere (spring 2015): El Niño nears its peak

Southern hemisphere circulation patterns and associated anomalies for the austral spring 2015 are reviewed, with an emphasis on Pacific climate indicators and Australian rainfall and temperature patterns. A strong El Nino persisted in the tropical Pacific Ocean with sea-surface temperature anomalies in excess of +2 °C in central and eastern parts, strongly negative outgoing longwave radiation near the Date Line, and the Southern Oscillation Index showing large negative departures. The positive Indian Ocean Dipole that had established in winter dissipated in late November, but was particularly influential on Australia's climate during the months of September and October. Australia’s spring rainfall was below average in the first two months, but improved later in the season: the northern half of Western Australia recorded above average November rainfall. Nevertheless, area-averaged rainfall in spring was below average for the country as a whole. For Australia, October was the warmest on record and had the highest mean temperature anomaly on record for any month since 1910. Spring temperatures were above average and Australia recorded its second-warmest spring on record, behind the record set in the previous year.

Seasonal climate summary southern hemisphere (summer 2015-16): strong El Niño peaks and begins to weaken

Southern hemisphere circulation patterns and associated anomalies for austral summer 2015-16 are reviewed, with an emphasis on the tropical Pacific as well as Australian rainfall and temperatures. Following the peak of El Niño in November 2015, summer 2015-16 featured continued near-record El Niño conditions in the tropical Pacific but saw the emergence of cooler subsurface waters in the equatorial Pacific. A moderate Madden Julian Oscillation (MJO) pulse and positive Southern Annular Mode (SAM) ontributed to average to above average rainfall across much of Australia, while the Maritime Continent and parts of far northern Australia saw continued below average rainfall.Sea surface temperatures during summer 2015-16 were the warmest on record for the southern hemisphere oceans, with very warm ocean temperatures in the Indian Ocean and Australian region, including the warmest summer sea surface temperatures on record around Tasmania. Air temperatures were also warmer than normal across Australia throughout the season, with a significant heatwave in southeast Australia during December.

Seasonal climate summary southern hemisphere (spring 2015): El Niño nears its peak

Southern hemisphere circulation patterns and associated anomalies for the austral spring 2015 are reviewed, with an emphasis on Pacific climate indicators and Australian rainfall and temperature patterns. A strong El Niño persisted in the tropical Pacific Ocean with sea-surface temperature anomalies in excess of +2 °C in central and eastern parts, strongly negative outgoing longwave radiation near the Date Line, and the Southern Oscillation Index showing large negative departures. The positive Indian Ocean Dipole that had established in winter dissipated in late November, but was particularly influential on Australia's climate during the months of September and October.Australia’s spring rainfall was below average in the first two months, but improved later in the season: the northern half of Western Australia recorded above average November rainfall. Nevertheless, area-averaged rainfall in spring was below average for the country as a whole. For Australia, October was the warmest on record and had the highest mean temperature anomaly on record for any month since 1910. Spring temperatures were above average and Australia recorded its second-warmest spring on record, behind the record set in the previous year.

Seasonal climate summary southern hemisphere (spring 2014): El Niño continues to try to break through, and Australia has its warmest spring on record (again!)

Southern hemisphere circulation patterns and associated anomalies for the aus-tral spring 2014 are reviewed, with emphasis given to the Pacific Basin climate indicators and Australian rainfall and temperature patterns. Equatorial sea-surface temperatures in the Pacific warmed through the season, and the South-ern Oscillation just exceeded El Nino thresholds. However, although trade wind strength weakened at times, particularly late in the season, tropical cloudiness near the Date Line and trade wind strength were more often close to average, suggesting the atmosphere was still not firmly linked with the warmer ocean be-low. The negative Indian Ocean Dipole values of winter flipped to neutral or weakly positive values through spring. Pressures in the Australian and Maritime Continent regions were high. Around Antarctica, sea ice reached a record extent in September, with expansion most prominent in the Indian Ocean sector and north of the Ross Sea. Winds to the north of these regions were strong. The Antarctic ozone hole was of moderate severity, and of similar size to ozone holes in the early 1990s. Over Australia, it was the warmest spring on record, surpassing the record warm spring of 2013. A strong anomaly in the 200 hPa geopotential height persisted over the south-east of the country through October and November. Australia was particularly dry in the east through spring, but Western Australia received good rainfall. Column ozone levels in the Australian region were slightly above average and consistent with increased ozone transport to mid-latitudes associat-ed with the phase of the Quasi-Biennial Oscillation

The Cenozoic Cooling – continental signals from the Atlantic and Pacific side of Eurasia

The evolution of Cenozoic continental climate signals from the Atlantic and Pacific side of Eurasia can be assessed for the first time by comparing climate records obtained for two mid-latitudinal regions. For the West, a detailed climate record over the past 45 Ma, based on palaeofloras from two Northern German Cenozoic basins (Mosbrugger et al., 2005) revealed major trends and shorter-term events throughout the Cenozoic Cooling, thus testifying the close correlation of continental and marine temperature evolution as derived from oxygen isotopes (Zachos et al., 2008). Using the same methodology, we analyze a total of 14 floral horizons originating from continental strata of Southern Primory'e (Russia) in order to study the evolution at the eastern side of the continent. The Primory'e record spans the middle Eocene to early Pleistocene. As the coeval record for the Atlantic side, it reflects major global signals of Cenozoic climate change such as the temperature decline throughout the late Eocene, coinciding with the growth of Antarctic Ice-sheets, warming during the Mid-Miocene Climatic Optimum, and step-wise cooling throughout the later Neogene. The comparison of both records reveals differing regional patterns. The considerable longitudinal temperature gradient, currently existing between both study areas, already began to evolve during the Aquitanian, and was very significant during the Mid-Miocene Climatic Optimum. The temperature offset between East and West is likely attributable to an effective North Atlantic Current, already operational from the late early Miocene onwards bringing about mild winters and low seasonality in Western Europe, while in Primory'e, seasonality steadily increased from the late Oligocene on. The strong late Pliocene decline of cold month mean temperatures recorded in Primory'e is supposed to coincide with the establishment of the Siberian High as semi-permanent structure of the Northern Hemisphere circulation pattern. When comparing the precipitation records obtained for both study areas, an unexpected co-variability at the longer-term (in the order of 5–20 Ma) is noted, pointing to continent-wide hydrological changes. The steady decline of mean annual precipitation in the Primory'e record, beginning in the Bartonian and culminating in the Aquitanian, coincides with an aridity increase reported from coeval Chinese inland localities of the mid-latitudes. The seasonality patterns of rainfall point to progressive intensification of the East Asian Summer Monsoon in Primory'e since the later Tortonian while the post-Zanclean decline of the precipitation of the dry season can be related to an increasing impact of the winter monsoon.

Seasonal climate summary southern hemisphere (spring 2011): La Niña returns

The southern hemisphere circulation patterns and associated anomalies for the austral spring 2011 are reviewed, with emphasis on the Pacific Basin climate indicators and Australian rainfall and temperature. Near neutral sea surface temperatures across the equatorial Pacific Ocean in winter began to cool in the spring, with the onset of La Nina conditions and below average subsurface temperatures. The Indian Ocean Dipole was positive, with cooler waters off the Indonesian coast but generally warmer than average temperatures over the Indian Ocean. Moderate positive values of the Southern Oscillation Index and negative values of the 5VAR index occurred, although not as strong as in spring 2010. Australia had much above average rainfall during spring 2011 (the ninth highest on record), especially in Western Australia, the Northern Territory and parts of South Australia, Queensland and New South Wales. Although below average rainfall fell in September over Australia, above average rainfall occurred in October and November, with every State receiving above average rainfall for November. Maximum and minimum temperatures were slightly above average over most of Australia, especially in Victoria, Tasmania, South Australia and New South Wales, and below average in the Northern Territory and Western Australia, where rainfall was higher.

Seasonal climate summary southern hemisphere (summer 2012-13): Australia's hottest summer on record and extreme east coast rainfall

Southern hemisphere circulation patterns and associated anomalies for austral summer 2012−13 are reviewed, with an emphasis on Pacific Basin climate indicators and Australian rainfall and temperature. Summer 2012−13 was a neutral ENSO period but saw an active burst of the Madden-Julian oscillation in January and February. The summer saw record-breaking temperatures across the country, with a widespread and persistent heatwave in January affecting all States and Territories and contributing to the warmest summer on record for Australia as a whole. Summer rainfall was below average Australia-wide, however extreme rainfall from tropical cyclone Oswald caused widespread record flooding in eastern Queensland and New South Wales at the end of January. In contrast, rainfall was below average in central and northern regions, particularly in the Northern Territory, due to the late onset of a weak Australian monsoon.

Role of stratospheric dynamics in the ozone–carbon connection in the Southern Hemisphere

The role of stratospheric dynamics in past and projected future long-term changes of the Southern Hemisphere climate is examined with a special regard to the oceanic carbon uptake, by comparing results from two sets of simulations performed with the high-top version and the low-top version of the CMCC-Carbon Earth System Model. An improved description of the stratospheric dynamics results in weakened (~20 to 25 %) annual-mean Southern Ocean air-to-sea carbon fluxes in the 1990–2005 period, with implications for the global ocean carbon uptake. Simulated changes in the Southern Hemisphere climate are reproduced in both model simulations and are consistent with numerous previous studies. However, the low-top model is unable to fully capture the observed stratospheric cooling, because the component associated with the changes in stratospheric circulation is missing. Smaller trend of the stratospheric polar vortex found in the low-top model (in response to stratospheric ozone and greenhouse gas changes) is followed by a smaller trend of the poleward-shifted tropospheric jet in the troposphere. The latter implies smaller (~10 %) wind stress increase in the November to February season and a smaller projection on Sea Level Pressure changes. Our results point out the importance of including a proper representation of stratospheric dynamics, at least with a certain degree of detail, in order to obtain more reliable long-term climate simulations and projections in the Southern Hemisphere circulation patterns and air-sea fluxes.

Precipitation climate of Central Asia and the large-scale atmospheric circulation

The precipitation climate in the larger Tian Shan region of Central Asia is described in terms of the climatological seasonal moisture fluxes and background circulation based on the ERA-40 reanalysis data and a precipitation reanalysis. The study area is partitioned into (1) the Tarim river basin, (2) bordering regions of China, Kyrgyzstan and Kazakhstan, and (3) Northwestern China. Moisture supply to these areas is primarily due to the midlatitude westerlies with contributions from higher latitudes. In addition, moisture from the Indian Ocean is notably imported into the Tarim drainage area. Monthly interannual precipitation variability relates to the variability of hemispheric circulation patterns. Extreme precipitation above and below normal in Western China and Central Asia is analyzed using the standardized precipitation index. Related circulation composites show that, despite regional and seasonal differences, episodes of extreme and severe dryness are dominated by various upstream standing wave patterns from the North Atlantic to Central Asia. These features extend further downstream to the North Pacific. Non-symmetry between wet and dry composites is noted upstream and in regional moisture flux composites.

Seasonal climate summary southern hemisphere (winter 2009): a developing El Niño and an exceptionally warm winter for much of Australia

Southern hemisphere circulation patterns and associated anomalies for the austral winter 2009 are reviewed, with emphasis given to the Pacific Basin climate indices and Australian rainfall and temperature patterns. Sea-surface temperatures remained significantly higher than the long-term average across most of the tropical Pacific Ocean in winter, exceeding El Nino thresholds from west of the date-line to the South American coast. Subsurface waters cooled through July and August, but a large volume of the subsurface still remained significantly warmer than the long-term average. The SOI was nearneutral for the season, not indicating an El Nino state. Trade winds were generally weaker than normal across the tropical Pacific. Although cloudiness near the date-line increased, it did not show a strong trend towards El Nino-like values. The season was exceptionally warm over much of Australia. Average maximum temperatures over the country were the highest on record, while seasonal mean temperatures fell only just short of the 1996 record. It was also a rather dry season over much of the continent, particularly the north and east.

Seasonal climate summary southern hemisphere (autumn 2009): signs of an El Niño in the Pacific

Southern hemisphere circulation patterns and associated anomalies for the austral autumn 2009 are reviewed, with emphasis given to Pacific Basin climate indicators and Australian rainfall and temperature patterns. Autumn 2009 saw neutral or near-neutral conditions in the Pacific following from the La Nina conditions experienced since late 2008. By the end of May 2009, the equatorial Pacific showed consistent warming, and the Southern Oscillation Index (SOI) fell, reaching −5.1 in May. In the Indian Ocean, a positive Indian Ocean Dipole sea-surface temperature pattern started to establish itself during May. In the Australian region, autumn was drier than normal, with maximum temperatures above average over most of Australia.

Snow cover variability in Bulgarian mountainous regions, 1931–2000

AbstractPrincipal component (PC) analysis was carried out on annual snow cover series from 15 mountain climate stations in Bulgaria to characterize the spatial and temporal variability in snow cover over the 1931–2000 period. Three distinct snow cover response regions were identified: (1) high elevation sites above 1500 m in the Vitosha and Rila Mountains; (2) low elevation sites below 1000 m in the eastern Rhodope and southern Pirin Mountains and (3) a mid‐mountain zone of 1000–1500 m elevation including stations from several mountain ranges. Snow cover exhibited evidence of significant decadal‐scale variability over the 1931–2000 period but little evidence of long‐term trends linked to climate warming. Over the more recent 1971–2000 period, the mid‐mountain zone has increased in importance and shows a significant trend toward a later start to the snow cover season.Composite analysis of high and low snow accumulation winters showed high accumulation years associated with a negative North Atlantic Oscillation (NAO)/East Atlantic (EA)‐like pattern that favored increased winter precipitation over the Mediterranean and Balkans and anomalous easterly flow across the Black Sea into Bulgaria. Low snow accumulation years were associated with the positive mode of a Scandinavian (SCA)‐like pattern that generated fall warming, a delay to the start of the snow cover season and reduced winter precipitation. Correlation of annual snow cover series with these modes of atmospheric variability revealed few significant correlations but evidence of considerable secular variability in the strength and sign of the relationships. This reflects the complex climate regime of Bulgaria, which is located at the cross‐roads of Mediterranean and Continental climate influences and on the periphery of the main nodes of influence of Northern Hemisphere circulation patterns that play important roles in European snow cover variability. Copyright © 2007 Royal Meteorological Society