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The purpose of this study was to assess trends in the intensity of the North American monsoon anticyclone over multiple decades from 1948 to 2010 during July and August, with a focus on the Lower Colorado River Basin (LCRB). The methodology included a 500 hPa geopotential-height regionalization of the monsoon-anticyclone domain (i.e. a large portion of the western United States and northern Mexico), a typing of 500 hPa circulation patterns over the LCRB, and an examination of multi-decadal trends as well as inter-epochal differences in geopotential heights and frequencies of synoptic types. Three regions (i.e. Northwest, Northeast, and South) were revealed that differed based on inter-annual variations in 500 hPa geopotential heights. The Northwest and South regions had significant increases in geopotential heights from 1948–1978 to 1980–2010. The synoptic types reflected the location of the monsoon anticyclone over the LCRB. The monsoon anticyclone intensified primarily over the northwestern region, which includes the LCRB, since the mid- to late 1970s. The anticyclone has thus been expanding over the LCRB. The anticyclone has been in a northern position, specifically a north-central position, over the basin more frequently over the past 30 years; conversely, the anticyclone has been in southern and eastern positions over the basin less frequently.

This study investigates the relationship between North Atlantic sea surface temperatures (SST) and persistent drought in North America using modern observations, proxy paleo-data, and simulations from multiple climate models. The observational results show that persistent droughts in the Great Plains and the southwest North America are closely related to multidecadal variations of North Atlantic SST (Atlantic Multidecadal Oscillations, AMO). During the AMO warm (cold) phases, most of North America is dry (wet). This relationship is persistent since at least 1567 AD, as based on proxy SST for the North Atlantic and the reconstructed drought index for North America. On centennial timescales, proxy SST records from the North Atlantic and proxy drought records for North America suggest that major periods of AMO-like warm (cold) SST anomalies during the last 7.0 ka correspond to dry (wet) conditions in the Great Plains. The influence of North Atlantic SST on North American droughts is examined using simulations made by five global climate models. When forced by warm North Atlantic SST anomalies, all models captured significant drying over North America, despite some regional differences. Specifically, dry summers in the Great Plains and the southwest North America are simulated by all models. The precipitation response to a cold North Atlantic is much weaker and contains greater disagreement among the models. Overall, the ensemble of the five models could well reproduce the statistical relationship between the dry/wet fluctuations in the North America and North Atlantic SST anomalies. Our results suggest that North Atlantic SSTs are likely a major driver of decadal and centennial timescale circulation, including droughts, in North America. Possible mechanisms that connect North Atlantic SST with North American drought, as well as interactions between North Atlantic and tropical Pacific SST and their relative roles on drought are also discussed.

This study revisits the question posed by Hoskins on why the Northern Hemisphere Pacific sea level pressure (SLP) anticyclone is strongest and maximally extended in summer when the Hadley cell descent in the northern subtropics is the weakest. The paradoxical evolution is revisited because anticyclone buildup to the majestic summer structure is gradual, spread evenly over the preceding 4–6 months, and not just confined to the monsoon-onset period, which is interesting, as monsoons are posited to be the cause of the summer vigor of the anticyclone. Anticyclone buildup is moreover found focused in the extratropics, not the subtropics, where SLP seasonality is shown to be much weaker, generating a related paradox within the context of the Hadley cell’s striking seasonality. Showing this seasonality to arise from, and thus represent, remarkable descent variations in the Asian monsoon sector, but not over the central-eastern ocean basins, leads to the resolution of this paradox. Evolution of other prominent anticyclones is analyzed to critique the development mechanisms: the Azores high evolves like the Pacific one, but without a monsoon to its immediate west. The Mascarene high evolves differently, peaking in austral winter. Monsoons are not implicated in both cases. Diagnostic modeling of seasonal circulation development in the Pacific sector concludes this inquiry. Of the three forcing regions examined, the Pacific midlatitudes are found to be the most influential, accounting for over two-thirds of the winter-to-summer SLP development in the extratropics (6–8 hPa), with the bulk coming from the abatement of winter storm-track heating and transients. The Asian monsoon contribution (2–3 hPa) is dominant in the Pacific (and Atlantic) subtropics. The modeling results resonate with observational findings and attest to the demise of winter storm tracks as the principal cause of the summer vigor of the Pacific anticyclone.

The Arctic region in the northern hemisphere presented humans with dietary and temperature challenges and required a maritime sea mammal hunting or sea-ice adaptation. The colonization of this frigid and florally restricted environment occurred earlier than suspected, during the late Pleistocene c .27 000 years ago in northern Eurasia. In geographic terms, only northern Greenland, the coastal margins of Scandinavia, the Taymyr, and Alaska lie beyond the Arctic Circle. By contrast, sustained settlement of the North American High Arctic and Greenland did not occur until late in the Holocene, c .5000 years ago. Complex whaling societies developed in Bering Strait by 2000 years ago. For the most part, Arctic archaeology is synonymous with a single Mongoloid population, the Inuit (formerly Eskimo) who still dominate the region from Alaska to Greenland. A brief incursion between AD 900 and 1400 by the European Norse was renewed by Danish colonists in the early eighteenth century and continues to the present.

A new and significant correlation is found between the amplitude of mean-winter atmospheric anomalies over the North Atlantic Ocean and those over the North Pacific Ocean and western North America three winters later. It is as strong as the previously known and much discussed correlation between the ocean temperature in the eastern tropical Pacific (El Nino region) and the North Pacific winter atmospheric anomalies. No physical explanation is given at this time for this correlation. A statistical prediction model for mean-winter conditions is constructed based on the above new and previously known correlations. The prediction model is tested on 36 winters of the past. The model predictions are shown to be of remarkable quality over the North Pacific and western North America.

The National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) monthly mean reanalysis dataset has been used to analyze spatial variations of summertime subtropical anticyclones over the Asia–Africa region. The geopotential height and zonal wind components of 1000, 500, 200, and 100 hPa in a 30-year period (1971–2000) have been used to determine the spatial and temporal variations of the anticyclone centres, their monthly frequency and latitudinal axis variations during April–October. The results revealed that there is a clear difference in the location of the summer anticyclone centres in lower, middle and upper levels of the troposphere. In the lower levels, the Azores subtropical anticyclone is located at the east of North Atlantic. In the middle levels, the frequencies of anticyclone centre are concentrated over the northwest of Africa, Arabian Peninsula and Iranian Plateau. In the upper troposphere, the geographical location of the anticyclone centres and their frequencies in the summer season exhibit a scattered pattern from south of China up to western Iran at 200 hPa, and a bimodal pattern over the Tibetan and the Iranian Plateaus at 100 hPa. In fact, in the entire study domain, the Iranian Plateau is a preferable location of the middle and upper troposphere anticyclones. The highest observed latitude of the subtropical anticyclone at 100, 200 and 500 hPa levels have been seen over north of Tibetan plateau, a large area from east to west of Asia and Iran during August, July–August and July, respectively. The maximum monthly variation in the latitude of the ridgeline is seen at 500, 200, and 100 hPa from June to July which goes even up to 10 degrees at some longitudes. Copyright © 2009 Royal Meteorological Society

The North Atlantic is one of the world’s most utilised ocean bodies, most strategic militarily and commercially, and one of the most marine-resource rich. The North Pacifiic is probably its closest rival in these respects. The North Atlantic has a rich history of political and economic development both among its littoral states and in comparison with other ocean regions around the world. With the break-up of the Soviet Union and the growing economic integration of eastern and central Europe, the geopolitical structure of the North Atlantic is changing somewhat; however, its status as an economic force is still great.

The most recent paleoecological data indicate that the last glacial maximum in North America occurred about 20,000 to 18,000 years ago. At this time, the continental ice sheets covered all of present-day Canada and extended a short distance into the northern tier of the Plains states in the western United States to about 48° north latitude. South of the continental ice sheets, there were valley glaciers in the higher elevations of the Rocky Mountains that reached their maximum advance at the same time as the continental glaciers. The Yellowstone Plateau in northwest Wyoming, southwest Montana, and southeast Idaho contained the largest mass of ice south of the continental ice sheets. Consequently, periglacial conditions affected climates, faunal, and floral resources over the entire Northern Plains area (see Porter 1988).

Climate simulations for the Last Glacial Maximum using general circulation models typically show a glacial anticyclone that produced an easterly wind anomaly south of the North American ice sheets. Evidence of this phenomenon has not been found in eolian sedimentary deposits that record surface wind patterns. Luminescence ages of loess and accompanying paleoecologic records from opal phytoliths and paleosol units across the Columbia Plateau, Washington State, United States, document a decrease, up to fivefold, in regional dust production and accumulation from 35 ka to 15 ka. This interval corresponds to simulations indicating that a glacial anticyclone weakened prevailing south-southwesterly winds that have shaped the eolian landscape of the Columbia Plateau since at least 75 ka. At the same time that the glacial anticyclone suppressed dust production and accumulation, enhanced cold and dry conditions resulted in soil formation dominated by intense bioturbation in periglacial steppe accompanied by shallow calcium carbonate precipitation. This is the first evidence from eolian deposits in North America verifying that the glacial anticyclone altered surface wind patterns and affected eolian systems during the Last Glacial Maximum.

Monthly North American anticyclone climatology for the period 1950-79 is examined. Preferred tracks are evident for each of the 12 calendar months, and three general types can be recognized. The winter pattern is oriented southeastward from northwestern Canada into the contiguous United States; the spring pattern is similar but is displaced eastward, extending southward across western Hudson Bay into the Great Lakes area. The summer pattern develops abruptly in July and has a more zonal orientation east-southeastward from British Columbia along the U.S.-Canadian border. The monthly total of individual anticyclone paths in the study area declined significantly through the period.

Two hypotheses for the orientation of autumnal migration over the western North Atlantic Ocean, proposed by Williams and Richardson, were tested by simulating the flight of birds from North to South America through calculated wind fields in the western North Atlantic Ocean. Use of a constant compass heading proved to be a tenable strategy for birds departing from the North American coast as far north as Nova Scotia. The range of successful headings, however, became increasingly restricted at northern latitudes. Airspeed, heading, altitude, and point of departure all had major effects on migratory success. Time required for successful non-stop migration was found to lie between 70 and 100 h for passerines and 40 to 60 h for shorebirds.

In this paper the so-called extention of the Azores' anticyclone towards the Balkans and Greece is studied by means of the synoptic evolution of mean charts for m.s.l., thickness, 500 mb and their anomalies. The period under consideration is the twelve years from 1961 through 1972 for the two month interval July-August, which is the main season of the occurrence of the Etesian winds over the eastern Greek seas having their existing in the combination of the Balkans high pressure and the permanent low pressure system over the eastern Mediterranean Sea. The synoptic evolution of the above mentioned charts proved that the mobile anticyclones or the wedges of the Greek area, seemingly form one common system with the Azores' anticyclone but their generating factors are different from these of the Azores' permanent anticyclone which implies that the term “extension” should not be used.

During the summer monsoon the upper tropospheric subtropical anticyclone of Asia is centred over SE Tibet (when it is called the Tibetan anticyclone). Further, the equatorward outflow from this anticyclone gains easterly angular momentum and therefore it appears as an easterly jet stream over SE Asia south of 20N between 150 mb and 100 mb. On finding these current concepts questionable, this study offers an alternative explanation for the migration of the upper tropospheric anticyclone to the Tibetan Plateau and also for the development of the tropical easterly jet. In summer the Bay of Bengal is cold compared to its adjoining continental plains in the north. Therefore in the beginning of summer the lower levels of the anticyclone migrate from their winter position in the Bay of Bengal to the warm plains in the north. As they reach the plains by about June, the upper levels of the anticyclone above 150 mb extend north over the Tibetan Plateau irrespective of whether the Plateau is a warm source or cold source because the upper levels of the subtropical anticyclone have a characteristic poleward slope in all seasons. By about July, when the lower levels of the anticyclone migrate from the plains to still warmer areas in the north over the Plateau, the upper levels which are already over the Plateau continue to remain there throughout the season. The zonal component of the equatorward outflow from the Tibetan anticyclone computed from the law of conservation of angular momentum does not bear any comparison with the observed winds in the upper troposphere over India. On the other hand the winds computed from a thermal gradient show a reasonable agreement with the observed winds indicating thereby that the upper tropospheric high winds are thermally generated. These high winds have been found as a unique phenomenon distinct from a jet stream and therefore it is considered appropriate to call them Tropical Strong Easterlies (TSE) rather than as a tropical easterly jet stream. Some of the characteristic features of the TSE are discussed and they are ascribed to the peculiar temperature distribution in the atmosphere between 200 mb and 60 mb mainly brought about by the vertical motion associated with the summer monsoon.

EXPLOSIVE eruptions, which inject large quantities of volcanic dust into the earth's upper atmosphere, are believed to be important factors in climatic change. Theoretical considerations suggest that the greatest climatic effect of a stratospheric dust veil would be at high latitudes during summer months, when solar radiation passes through the greatest depth of atmosphere and the surface is illuminated continously1. Furthermore, the residence time of volcanic dust is greatest at high latitudes, where it may remain in the upper atmosphere for a decade or more, depending on particle size and initial injection height2. Here we present evidence that the eruption of Mount Agung (8 °S, 115 °E) in March 1963, was responsible for a marked change in the climate of the North American High Arctic and that this change has had a significant impact on glacier mass balance in the region.

Two 10,000-year periods of Northern Hemisphere continental ice-sheet growth stand out prominently within the last full interglacial-to-glacial cycle. During the first half of each rapid ice-growth phase, the subpolar North Atlantic from 40 degrees N to 60 degrees N maintained warm sea-surface temperatures comparable to those of today's ocean. The juxtaposition at latitudes 50 degrees N to 60 degrees N of an "interglacial" ocean along-side a "glacial" land mass, particularly along eastern North America, is regarded as an optimal configuration for delivering moisture to the growing ice sheets.

WHILE the interest attaching to Sir William Dawson's Presidential address at Birmingham is still fresh, I wish to be allowed to offer a few observations on that part of it which deals with the geological age of the North Atlantic Ocean. The President in referring to those writers who, like Mr. Crosby in America, Mr. Mellard Reade and myself in Britain, maintain that the North Atlantic and the American continent have in the main changed places in Palaeozoic times, makes the following statement. Admitting the correctness of the facts as to the swelling out of the Palaeozoic sediments in the direction of the Atlantic seaboard, he endeavours to account for these very striking phenomena thus: “I prefer, with Hall, to consider these belts of sediment as in the main the deposits of northern currents, and derived from Arctic land, and that, like the great banks of the American coast at the present day, which are being built up by the present Arctic current, they had little to do with any direct drainage from the adjacent shore.” Now, in reading this passage it occurs to me that Sir W. Dawson must have felt he had a very questionable case when he attempted to support it by such an hypothesis. To liken the great sheets of sediment which spread themselves sometimes over half the North American continent south of the Great Lakes to the banks heaped up along the Atlantic coast is a point of analogy in which, probably, he will find few to concur. The Palaeozoic sediments are certainly not banks, but sheets originally spread over the sea-bed, and distributed according to certain recognised laws of increase and decrease of thickness.