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Tsing-Chang Chen
and
Harry van Loon

Abstract

The 200-mb tropical wind fields analyzed at Florida State University for 1965–74 and the 200 and 700 mb tropical wind fields from the National Meteorological Center for 1979–82 were used to explore the mechanism for the interannual variation of the tropical easterly jet. This jet is generally weaker during the summers of Warm Events (dry summers) in the Southern Oscillation when anomalously warm surface water appears over the eastern and central equatorial Pacific and drought occurs over the Indian subcontinent. It is observed that divergence (convergence) exists on the upstream (downstream) side of the jet. The tropical divergent circulations, i.e., the east-west Walker and the local Hadley circulations, during such summers are weakened and shifted eastward. Therefore, divergence anomalies appear in the upper troposphere over equatorial Africa or the cast coast of Africa, whole convergence anomalies exist over the Indian subcontinent or the Arabian Sea. These changes of the tropical divergent circulations may cause the change in the energetics maintenance of the tropical easterly jet. Our analysis shows that the divergence anomalies of the divergent kinetic energy flux appear over the east coast of Africa, and the convergence anomalies of divergent kinetic energy flux appear over the Indian subcontinent. It is inferred from these anomalies of kinetic energy flux that the kinetic energy generation and destruction associated with the tropical easterly jet are less in dry summers.

Based upon these changes in the upper-level tropical circulations during dry summers, a suggestion is altered that relates the anomalously warm surface water over the eastern and central equatorial Pacific to the weakening of the low-level monsoon circulation and the tropical easterly jet.

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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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Harry van Loon
and
Jeffery C. Rogers

Abstract

We have investigated the relationship between the extremes of the Southern Oscillation and the following quantities at 700 mb in winter, 1948/1949 to 1978/1979: eddy transfer of sensible heat, temperature, geopotential height and geostrophic wind. In the phase of the Southern Oscillation when pressures are high over the tropical South Indian Ocean and low over the tropical South Pacific Ocean, in contrast with the opposite pressure distribution, the zonal mean poleward flux of sensible heat in the quasistationary waves tends to be higher in middle latitudes; the temperatures and heights tend to be lower between 30 and 60°N with the maximum difference at 45°N; the geostrophic wind tends to be stronger south of 45°N and weaker to the north; and the transfer of sensible heat by the transient waves tends to be stronger south of 45°S, and weaker to the north.

In this extreme of the Southern Oscillation the zonal mean geostrophic wind on both hemispheres is stronger in the subtropics and weaker at higher latitudes than in the other extreme when pressures are high over the tropical South Pacific and low in the tropical South Indian Ocean.

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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
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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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

The circulation anomalies over the whole Southern Hemisphere in the First GARP Global Experiment (FGGE) were not those that one would expect in either extreme of the Southern Oscillation; examples of the anomalies in such extremes are given. The zonally averaged pressure gradients between 50 and 65°S in FGGE when compared with those of several other years turned out to be abnormally strong in winter (2.5σ above the mean), and moderately strong in summer (1.4σ. above the mean). The 500 mb heights were above normal in middle latitudes and below normal at high latitudes when compared with station data from series 16–29 years long. As the computations are based on operational analyses they are not final, although the conclusions are unlikely to be changed by the use of the complete FGGE data set.

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Harry van Loon
and
Jill Williams

Abstract

During 24 years when the 700 mb winter mean temperature dropped over most of the hemisphere north of 20°N, the biggest falls were in the belt of strongest baroclinity and were simultaneous with a southward movement and strengthening of the peak in total meridional eddy transport of sensible heat. These changes were accompanied by a southward displacement of the region of most frequent storm tracks at the surface and by compatible trends in surface mean temperature and sea level pressure. At middle latitudes the layer between surface and 700 mb destabilized, while in the arctic it stabilized as the surface temperature over a large part of the polar cap fell more than the 700 mb temperature.

A comparison with the Southern Hemisphere showed that local temperature trends in the antarctic also take place on the scale of long waves, that they are as large as those in the Northern Hemisphere and that a zonally averaged trend is not necessary the same in summer and winter. The net transport of sensible heat by stationary waves is much smaller in the Southern than in the Northern Hemisphere, and changes in stationary wave transport in the Southern Hemisphere are therefore not likely to contribute much to large changes in the net poleward transport of sensible heat by waves. This is connected with the observation that the stationary waves in temperature and pressure are nearly in phase over the almost continuous water surface in southern temperate latitudes.

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Harry Van Loon
and
Jill Williams

Abstract

The local temperature trends in summer are not so obviously associated with advection changes as are those in winter. This appears to be due to weaker temperature contrasts at middle and high latitudes in summer combined with a smaller amplitude of the mean waves. A larger share of the total variance in the trend of sea level pressure is accounted for by the shorter waves than in winter. Local temperature changes are as big in summer as in winter in many places at middle latitudes, whereas in the arctic they are appreciably smaller. The zonally averaged trends in summer are larger at middle than at high latitudes, which is the reverse of winter. The sign of the zonally averaged temperature changes differs from one latitude belt to another as in winter, and the sign at a given latitude is not necessarily the same in both seasons. In contrast with winter, the sensible heat transport by mean waves in the sea level pressure in summer plays an insignificant part in causing trends in the zonally averaged temperature.

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Jill Williams
and
Harry Van Loon

Abstract

For each grid point (5° latitude by 5° longitude) and each season, the long-term mean sea-level pressure (1899–1972) and its standard deviation were found, using a data set compiled by NCAR. Individual deviations from the mean greater than three standard deviations were compared with nearby station data from World Whether Records. Some deviations were found in the sea-level pressure data and not in the station pressure data. Comparison was made between the NCAR sea-level pressure data set and the United Kingdom Meteorological Office data set; large differences are found since 1940 when the data acts started using different sources.

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