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Jefery C. Rogers and Harry Van Loon

Abstract

Description of the seesaw in wintertime climate between Greenland and northern Europe is continued in terms of variations in long waves, frequencies of highs and lows, zonal geostrophic winds, precipitation, sea ice and sea surface temperatures. The monthly variations in four circulation modes are described. Significant spatial correlations exist between the zonal. component of the geostrophic wind in the area of the strongest westerlies in the North Atlantic Ocean and the zonal geostrophic wind elsewhere north of 20°N in winter. Long waves 1 and 2 change substantially from one phase of the seesaw to the other at 63°N. At 45°N only wave 2 changes appreciably between phases.

Large, statisticaly significant anomalies occur during and after seesaw winters in the atmosphere-ocean-ice system of the North Atlantic Ocean and its periphery, as well as in the North Pacific Ocean. Anomalies of sea surface temperature which develop during seesaw winters tend to persist through the subsequent spring and summer. In addition, the seesaw modes are characterized by significantly different ice conditions in Davis Strait, near Newfoundland, and in the Baltic Sea in winter and during the following spring.

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Harry van Loon and Jeffery C. Rogers

Abstract

We have investigated the well-known tendency for winter temperatures to be low over northern Europe when they are high over Greenland and the Canadian Arctic, and conversely. Well-defined pressure anomalies over most of the Northern Hemisphere are associated with this regional seesaw in temperature, and these pressure anomalies are so distributed that the pressure in the region of the Icelandic low is negatively correlated with the pressure over the North Pacific Ocean and over the area south of 50°N in the North Atlantic Ocean, Mediterranean and Middle East, but positively correlated with the pressure over the Rocky Mountains. The composite patterns of pressure anomalies in the seesaw are almost identical to the fist eigenvector in the monthly mean pressure, but the standard deviations of pressure anomalies in seesaw mouths are as large as the standard deviations of monthly means in general. Since 1840 the seesaw, as defined by temperatures in Scandinavia and Greenland, occurred in more than 40% of the winter months and the occurrences are seemingly not randomly distributed in time as one anomaly pattern would be more frequent than the other for several decades. For this reason the circulation anomalies in the seesaw come to play an important part in deciding the level of regional mean temperatures in winter and thus in deciding the long-term temperature trends. These regional temperature trends are then closely associated with change in frequency of atmospheric circulation types, and it is therefore unlikely that the trends are caused directly by changes in insolation or in atmospheric constituents and aerosols.

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Harry van Loon and Roland A. Madden

Abstract

We describe the global correlations between a measure of the Southern Oscillation and sea level pressure and surface air temperature in the northern winter. The stability of these correlations were tested on the Northern Hemisphere for an 80-year period, and it turned out that most stable correlation coefficients were found over India, the North Pacific Ocean, the Rocky Mountains, and the central and western North Atlantic Ocean. On the Southern Hemisphere most records are too short for a similar test, but the following may tentatively be said about the Southern Oscillation in middle and high southern latitudes: when pressure is low in lower latitudes over the South Pacific Ocean it tends to be high at higher latitudes of that ocean, high over East Antarctica and low in the belt of westerlies in the Indian and South Atlantic oceans. In the zonal average on both hemispheres the pressure gradients in this extreme of the oscillation tend to be steeper at lower latitudes and flatter at higher latitudes than in the other extreme. The apparent large-scale sympathetic variations between the SO and temperature are shown to occur over the relatively wide range of periods dust have been attributed to the SO itself.

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Gerald A. Meehl and Harry Van Loon

Abstract

Tropical teleconnections to the seesaw in winter mean temperatures between Greenland and northern Europe are described in terms of the trade winds, African precipitation, sea surface temperatures and Gulf Stream intensity as indicated by changes in relative sea levels. There is a statistically significant correlation between the strength of the northeast and southeast trades in the Atlantic during seesaw winters, but not in other winters. Latitudinal differences in the position of the ITCZ over Africa, as defined by the belt of heaviest precipitation, are seen between the two modes of the seesaw but not in other circulation types.

Relative sea levels on either side of the Gulf Stream show that geostrophic velocity of the surface current is out of phase with the strength of the trades in the North Atlantic Ocean during seesaw winters, which presumably indicates that in this region steric effects on the current prevail over the influence of the wind. Small-magnitude pressure anomaly patterns of the type seen during seesaw winters are present in autumns preceding those winters, and sea surface temperature anomalies associated with the seesaw occur at all latitudes in summers and autumns preceding, and springs following seesaw winters.

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Harry Van Loon, John W. Kidson, and A. Brett Mullan

Abstract

During the 1970s and 1980s, elements of the atmospheric circulation over the Southern Hemisphere changed markedly from their state in the two previous decades. The change was evident in the zonal asymmetry, especially in the zonal harmonic wave 3 at higher latitudes, and in the semiannual wave in pressure and wind. The semiannual wave changed in the same sense in all three southern oceans, but by the largest amount in the South Pacific Ocean. The second harmonic in a 12-month series, by which we describe the semiannual wave, dominates the shape of the long-term mean annual curve of sea level pressure in mid- and high latitudes; this harmonic weakened during the period, and its midlatitude peak disappeared in the South Pacific Ocean.

As part of the low-frequency changes during the 18 years from 1972 to 1988, the central pressure in the subantarctic trough fell, the trough moved northward, and the subtropical ridge moved south. The meridional movement of the trough and the ridge, and the concurrent trend in their central pressure, weakened the semiannual wave in midlatitudes.

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Tsing-Chang Chen, Ming-Cheng Yen, and Harry Van Loon

Abstract

The upper-air wind data generated by the Global Data Assimilation System of the National Meteorological Center for 1986–1992 are used to depict the three-dimensional structure of the semiannual oscillation of tropical stationary eddies in terms of the eddy streamfunction. An eddy streamfunction budget analysis was also performed to disclose the cause of this semiannual oscillation. The major findings are: 1) The tropical stationary eddies exhibit a seesaw semiannual oscillation between the eastern and Western Hemisphere with a phase reversal vertically at 400–500 mb and meridionally at the equator. 2) It is inferred from the streamfunction budget analysis that the semiannual oscillation of the tropical stationary eddies is caused by the semiannual cast-west seesaw of the global divergent circulation between the areas of the Asian-Australian (AA) monsoon (60°E–120°W) and the extra-AA monsoon (120°W–60°E). This mechanism is particularly clear in the Southern Hemisphere Tropics but less well established in the Northern Hemisphere Tropics, which may be attributed to the strong effect of the land-sea contrast on the east-west interseasonal phase change of the stationary eddies in the Northern Hemisphere.

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Hans Von Storch, Harry Van Loon, and George N. Kiladis

Abstract

We show by means of a general circulation model experiment that the atmospheric circulation over the South Pacific Ocean is sensitive to sea surface temperature anomalies in the tropical and subtropical regions of the South Pacific convergence zone. The possible implications for understanding the life cycle of an extreme event in the Southern Oscillation are discussed.

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Harry van Loon, Karin Labitzke, and Roy L. Jenne

Abstract

This note deals with the standard deviations of 24-hr changes in 10-mb temperatures and heights. The standard deviations are differently distributed in disturbed and in quiet winter months. In the disturbed months their largest values form a coherent area at high latitudes; in the quiet months they surround the polar region as a ring with its center on the Atlantic side.

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Harry Van Loon, Roland A. Madden, and Roy L. Jenne

Abstract

Two patterns dominate changes of monthly mean temperature and pressure-height in the stratosphere. In the one, the middle latitudes vary oppositely to low and high latitudes, and in the other the changes at higher latitudes are out of phase with those at lower latitudes.

A shorter trend consisting of opposite changes at middle and high latitudes is superposed on the above variations which a cross-spectrum analysis shows has a preferred time scale of one to three weeks. The contrast between middle and high latitudes thus undergoes a series of corresponding fluctuations and we show that these are associated with amplitude changes in waves 1 and 2 in that the meridional contrast decreases when the amplitude of one or both waves is large, and vice versa.

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Gerald A. Meehl, Julie M. Arblaster, Grant Branstator, and Harry van Loon

Abstract

The 11-yr solar cycle [decadal solar oscillation (DSO)] at its peaks strengthens the climatological precipitation maxima in the tropical Pacific during northern winter. Results from two global coupled climate model ensemble simulations of twentieth-century climate that include anthropogenic (greenhouse gases, ozone, and sulfate aerosols, as well as black carbon aerosols in one of the models) and natural (volcano and solar) forcings agree with observations in the Pacific region, though the amplitude of the response in the models is about half the magnitude of the observations. These models have poorly resolved stratospheres and no 11-yr ozone variations, so the mechanism depends almost entirely on the increased solar forcing at peaks in the DSO acting on the ocean surface in clear sky areas of the equatorial and subtropical Pacific. Mainly due to geometrical considerations and cloud feedbacks, this solar forcing can be nearly an order of magnitude greater in those regions than the globally averaged solar forcing. The mechanism involves the increased solar forcing at the surface being manifested by increased latent heat flux and evaporation. The resulting moisture is carried to the convergence zones by the trade winds, thereby strengthening the intertropical convergence zone (ITCZ) and the South Pacific convergence zone (SPCZ). Once these precipitation regimes begin to intensify, an amplifying set of coupled feedbacks similar to that in cold events (or La Niña events) occurs. There is a strengthening of the trades and greater upwelling of colder water that extends the equatorial cold tongue farther west and reduces precipitation across the equatorial Pacific, while increasing precipitation even more in the ITCZ and SPCZ. Experiments with the atmosphere component from one of the coupled models are performed in which heating anomalies similar to those observed during DSO peaks are specified in the tropical Pacific. The result is an anomalous Rossby wave response in the atmosphere and consequent positive sea level pressure (SLP) anomalies in the North Pacific extending to western North America. These patterns match features that occur during DSO peak years in observations and the coupled models.

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