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James K. Angell

Abstract

The mean monthly polar stereographic map analyses of the Free University of Berlin terminated at the end of 2001. This paper summarizes the changes in size of the 300-mb north circumpolar vortex, and quadrants, for the full period of record, 1963–2001, where the size has been defined by planimetering the area poleward of contours in the jet stream core. A contracted vortex has tended to be a deep vortex in winter but a shallow vortex in summer. During 1963–2001 there was a statistically significant decrease in vortex size of 1.5% per decade, the decrease in size of Western Hemisphere quadrants being twice that of Eastern Hemisphere quadrants. A significant increase in Arctic Oscillation (AO) index accompanies the significant decrease in vortex size, but since the vortex contracts appreciably in all four seasons, whereas the positive trend in the AO index is mainly in winter, the vortex cannot serve as a proxy for the AO index. The evidence for vortex contraction at the time of the 1976–77 regime shift is not conclusive, but there is good evidence for a 6% increase in vortex size due to the 1991 Pinatubo eruption. There is little change in vortex size following the 1982 El Chichon eruption, however. Because on average there is a significant 4% contraction of the vortex following an El Niño, it is proposed that the vortex expansion to be expected following the 1982 El Chichon eruption has been contravened by the contraction following the strong 1982–83 El Niño. There is little relation between vortex size and phase of the quasi-biennial oscillation (QBO), and the evidence for a contracted vortex near 11-yr sunspot maxima is tenuous because the vortex record extends through only three full sunspot cycles. There is a highly significant tendency for opposite vortex quadrants 0°–90°E and 90°W–180° to vary in size together, indicating either a pulsating polar vortex or the propagation of planetary wavenumber 2.

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James K. Angell

Abstract

Based on the 120-station North American radiosonde network, temperature trends for 100–50-mb (low stratosphere), 300–100-mb (tropopause), and 850–300-mb (troposphere) layers, and Earth’s surface, are evaluated for six 10° latitude bands extending from 20°–30°N to 70°–80°N for the 20-yr interval 1975–94. Confidence estimates are indicated by two standard errors of the least squares regression. In the average for the six latitude bands, the 100–50-mb annual temperature trend is −0.5°C decade−1 and the 850–300-mb trend is 0.2°C decade−1. In spring at 70°–80°N, the 100–50-mb and 300–100-mb layers cool by almost 2°C decade−1. The 300–100-mb layer cools by 0.7°C decade−1 relative to the 850–300-mb layer at 70°–80°N, but the two layers have the same warming trend at 20°–30°N, indicating the transition from the 300–100-mb layer being mostly in the stratosphere in polar regions to mostly in the troposphere in the northern subtropics. The surface warms much more than the troposphere at 70°–80°N (showing that surface temperature trends are not representative of tropospheric trends in polar regions) and slightly more at 20°–30°N, but surface warming is less than tropospheric warming in the 40°–70°N belt. At the surface at the radiosonde sites the 1200 UTC (morning) temperature cools relative to the 0000 UTC (evening) temperature by 0.05°C per decade on average, but the 850–300-mb temperature trends at 0000 UTC and 1200 UTC are essentially the same. The 0.7°C decade−1 cooling of low stratosphere relative to troposphere increases to 0.9°C decade−1 when adjustment is made for the stratospheric warming and tropospheric cooling following El Chichon and Pinatubo eruptions. The temperature trends obtained from 11 North American radiosonde stations in a 63-station global network agree well with the trends based on the entire 120-station network, and the latter are fairly representative of zonally averaged trends based on the 63-station network and microwave sounding unit data. Comparison with Canadian ozonesonde data shows that, in the low stratosphere and high troposphere during 1975–94, a decrease in temperature of 1°C decade−1 was associated with a decrease in ozone of about 10% decade−1.

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James K. Angell

Abstract

A 63-station radiosonde network has been used for many years to estimate temperature variations and trends at the surface and in the 850–300-, 300–100-, and 100–50-mb layers of climate zones, both hemispheres, and the globe, but with little regard for the quality of individual station data. In this paper, nine tropical radiosonde stations in this network are identified as anomalous based on unrepresentatively large standard-error-of-regression values for 300–100-mb trends for the period 1958–2000. In the Tropics the exclusion of the 9 anomalous stations from the 63-station network for 1958–2000 results in a warming of the 300–100-mb layer rather than a cooling, a doubling of the warming of the 850–300-mb layer to a value of 0.13 K decade−1, and a greater warming at 850–300-mb than at the surface. The global changes in trend are smaller, but include a change to the same warming of the surface and the 850–300-mb layer during 1958–2000. The effect of the station exclusions is much less for 1979–2000, suggesting that most of the data problems are before this time. Temperature trends based on the 63-station network are compared with the Microwave Sounding Unit (MSU) and other radiosonde trends, and agreement is better after the exclusion of the anomalous stations. There is consensus that in the Tropics the troposphere has warmed slightly more than the surface during 1958–2000, but that there has been a warming of the surface relative to the troposphere during 1979–2000. Globally, the warming of the surface and the troposphere are essentially the same during 1958–2000, but during 1979–2000 the surface warms more than the troposphere. During the latter period the radiosondes indicate considerably more low-stratospheric cooling in the Tropics than does the MSU.

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Shuntai Zhou
,
Alvin J. Miller
,
Julian Wang
, and
James K. Angell

Abstract

Dynamical links of the Northern Hemisphere stratosphere and troposphere are studied, with an emphasis on whether stratospheric changes have a direct effect on tropospheric weather and climate. In particular, downward propagation of stratospheric anomalies of polar temperature in the winter–spring season is examined based upon 22 years of NCEP–NCAR reanalysis data. It is found that the polar stratosphere is sometimes preconditioned, which allows a warm anomaly to propagate from the upper stratosphere to the troposphere, and sometimes it prohibits downward propagation. The Arctic Oscillation (AO) is more clearly seen in the former case. To understand what dynamical conditions dictate the stratospheric property of downward propagation, the upper-stratospheric warming episodes with very large anomalies (such as stratospheric sudden warming) are selected and divided into two categories according to their downward-propagating features. Eliassen–Palm (E–P) diagnostics and wave propagation theories are used to examine the characteristics of wave–mean flow interactions in the two different categories. It is found that in the propagating case the initial wave forcing is very large and the polar westerly wind is reversed. As a result, dynamically induced anomalies propagate down as the critical line descends. A positive feedback is that the dramatic change in zonal wind alters the refractive index in a way favorable for continuous poleward transport of wave energy. The second pulse of wave flux conducts polar warm anomalies farther down. Consequently, the upper-tropospheric circulations are changed, in particular, the subtropical North Atlantic jet stream shifts to the south by ∼5 degrees of latitude, and the alignment of the jet stream becomes more zonal, which is similar to the negative phase of the North Atlantic Oscillation (NAO).

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