Jerome Namias

Multiple Causes of the North American Abnormal Winter 1976–77

Abstract The severe 1976–77 winter over eastern North America and the drought in the west are related to contemporary and antecedent atmospheric, oceanic and cryospheric factors. Although greatly amplified, the atmospheric flow pattern was in phase with the normal winter pattern so that the seasonal forcing by mountains, coastlines, etc., did not oppose the anomalous pattern. The autumn pattern of southerly air flow over the eastern North Pacific reduced heat losses from the ocean, induced advection of warmer waters, and reduced coastal upwelling. Meanwhile, the cold sea surface temperatures generated in spring and summer over the central North Pacific persisted. These cold waters seem to have been generated by the persistently strong Aleutian low which, following Bjerknes’ hypothesis, was associated with a persistent El Nino which began several months before and lasted through the abnormal winter. The atmospheric baroclinicity from the strong sea surface temperature gradient near 140°W strengthened front...

THE INDEX CYCLE AND ITS ROLE IN THE GENERAL CIRCULATION

Abstract On the basis of accumulated aerological data on a hemisphere-wide scale, a reexamination is made of the problem of hemispheric zonal-index variations first raised in the late 1930s. These data lead to a theory explaining the important problem of how and why during each winter the zonal westerlies gradually fall to low strength and subsequently recover—the period of this “index cycle” consuming some four to six weeks. The theory postulates a mechanism for containment of air composing the polar and sub-polar cap by means of strong mid-troposphere westerlies created by large-scale confluence. In this manner cold air is produced and stored in northern latitudes, the atmospheric circulation operating as a vast condenser. The discharge of the condenser in the form of cold air outbreaks, long recognized as necessary for the atmospheric heat balance, is effected by a certain cellular-type blocking, which reaches maximum effectiveness in producing an extended index cycle only when the supply of abnormall...

SEASONAL INTERACTIONS BETWEEN THE NORTH PACIFIC OCEAN AND THE ATMOSPHERE DURING THE 1960's

Abstract An attempt is made to explain the large-scale seasonal ocean-atmosphere interactions that occurred from 1961 through the winter of 1967–68 and that led to a special “climatic regime” over the North Pacific and North America. This regime was characterized by anomalously warm water over the central ocean, which helped generate atmospheric circulations of a kind favoring the persistence of the warm water, especially in the nonsummer seasons. The prevailing atmospheric circulation generated by the warm water during winter consisted of strong and southward-displaced cyclogenesis that produced a standing (Rossby) long wave downstream, resulting in climatic cooling over the eastern two-thirds of the United States. A theory is advanced for the evolution of atmospheric pressure and sea-surface temperature patterns in other seasons based on theoretical and empirical considerations. The inception of the regime came abruptly in the late summer and fall of 1961, following several years when the eastern North ...

Some Empirical Evidence for the Influence of Snow Cover on Temperature and Precipitation

Abstract A case study is made of the effect of snow cover or lack of snow cover on the surface temperature and precipitation in the coterminous United States during winter 1983/1984. Extensive snow occurred in the eastern half of the country early in December 1983; this snow cover appears to have produced substantial reduction in the observed surface temperature, especially during January 1984. This diminution was especially pronounced in the daytime maximum temperatures, presumably due to increase surface albedo. The effect involving anomalous snow cover also shows up in the Great Basin area during February 1984. A quantitative evaluation of the net effect of snow cover is made using errors of temperature specifications from 700 mb patterns formed from stepwise multiple regression equations. Temperature differences as great as 5°C were found over the temperature expected from the large-scale circulation alone. The concomitant effect of increased static stability near the surface may have played a role in...

CONFLUENCE THEORY OF THE HIGH TROPOSPHERIC JET STREAM

Abstract In 1947 a suggestion was made by one of the authors that the existence of fast narrow currents in the upper troposphere was due to the bringing together, or confluence, of warm and cold air masses from northerly and southerly regions. In the present paper an attempt is made to extend this theory by a somewhat more detailed examination of the dynamics of confluence. It is suggested that the energy of the jet is derived from a direct transformation of potential into kinetic energy. This source of energy is independent of stability in the usual sense, a fact which may be of some importance in understanding the general circulation. A comparison is made between this theory and the known behavior of the jet stream. Here extensive use is made of recent synoptic studies by Palmen and his co-workers and of charts showing the normal appearance of the hemispheric jet stream (which are shown for the first time). It is concluded that these observational studies tend to confirm the theory of confluence.

Spring and Summer 1988 Drought over the Contiguous United States—Causes and Prediction

Abstract This paper deals primarily with the 1988 summer drought over much of the contiguous United States and its generation from conditions during the preceding spring. Both the spring and summer environment are described in terms of hemispheric flow patterns in midtroposphere, temperature and precipitation anomalies, and sea surface temperature anomalies. Conditions in March were especially indicative of the ensuing drought, since a model routinely employed in long-range forecasting showed that the March circulation would most likely be followed by a hot dry April, May, and June over much of the nation—a pattern which persisted into early summer. This result suggests that the initiation of the drought was rooted in extratropical climate variations, an alternative hypothesis to one which attributes the persistent drought-producing circulation to oceanic and atmospheric conditions in the tropics. In many respects the summer drought of 1988 was similar to earlier great droughts of the Great Plains, althou...

Persistence of North Pacific Sea Surface Temperature and Atmospheric Flow Patterns

Abstract North Pacific monthly sea surface temperature (SST) anomalies are more persistent than a first-order Markev process, often lasting for more than 5 months. Sea surface temperature persistence undergoes an annual cycle that is attributable to the depth of the surface mixed layer and to the annual cycle of focing. For a given lag, the pattern correlation is minimum when it involves SST during the summer months and maximum when it involves SST during the winter months. Average winter SST anomalies that have exhibited greatest persistence during the last four decades have been negative in the central North Pacific and positive along the West Coast but antipersistent SST anomalies have not confermed to a repeated pattern. The atmospheric 700 mb height anomalies associated with high persistence SST cases indicate that strong SST persistence is associated with long-lasting atmospheric anomaly patterns. For highly persistent January SST anamalies, 700 mb anomalies often last from December through February...

North American Influences on the Circulation and Climate of the North Atlantic Sector

Abstract During the post-war period the pressure field at Greenland has been characterized by long-sustained winter regimes of alternating high and low pressure, with important effects on the winter climate of Europe. Although these alternations of pressure anomaly at Greenland may be shown to be associated with periods when the pattern of long waves in the upper westerlies showed a general reversal over much of the Northern Hemisphere, it is also suggested that within this hemispheric pattern of change, contemporary variations of winter climate along the Atlantic seaboard of North America have exerted an important influence on the pressure field at Greenland and, through teleconnections, elsewhere (e.g., the North Atlantic and Europe). Comparing months of extreme winter warmth and cold over the southeastern United States it is shown that changes in the strength of the baroclinic field at the coast are associated with major changes in the distribution of winter storms. More specifically, during winters of...

Anatomy of Great Plains Protracted Heat Waves (especially the 1980 U.S. summer drought)

Abstract The protracted heat wave and drought of the Great Plains during summer 1980 was a manifestation of an abnormal form of the general circulation. An upper-level continental high developed rapidly over the Southern Plains in late May and persisted with only small changes throughout the summer. The associated subsidence, dry soil, lack of cloud and high insolation led to unrelenting heat. Descriptions of several interlocking features are given followed by an attempt to ascribe the development and maintenance of the drought-producing cell to several physical factors. Strong low-latitude westerlies (expanded circumpolar vortex) over the North Pacific, North America and North Atlantic during spring 1980 led to latitudinally depressed storm tracks relative to normal. Combined with normal seasonal forcing this anomalous pattern was stable in spring but unstable in summer after the climatologically dependable northward march of the westerlies. Moreover, normal spring to summer mid-tropospheric height chang...

Large-Scale Changes in North Pacific and North American Weather Patterns in Recent Decades

Abstract This paper compares long-term seasonal means of atmospheric and oceanic fields for the recent 1969–80 period, relative to the earlier 1947–66 base period. Difference maps of North Pacific sea surface temperature (SST), 700 mb height, and United States surface temperature and precipitation are examined. An appreciable cooling of the North Pacific SST field during the 1969–80 period was found to exhibit large coherent cold pools with the maximum average cooling over the period being ∼1°C for the coldest locations. The center of maximum cooling varied with season; its position shifted eastward from winter to fall. The pattern of SST cooling corresponds well to the patterns of difference of 700 mb height over the North Pacific for the 1969–80 period, relative to 1947–66. The areas of cooling have generally occurred in regions of strengthened northerly wind components and/or low heights, a fact which can be rationalized by physical arguments. The winter and spring seasonal averages for 1969–80 are cha...