Barbara Naujokat

An Update of the Observed Quasi-Biennial Oscillation of the Stratospheric Winds over the Tropics

Abstract A survey of the zonal wind in the stratosphere near the equator is made for the 32-year period 1953–84. The monthly mean zonal wind components have been calculated from daily observations at Canton Island, Gan, and Singapore at levels between 70 and 10 mb. A frequency distribution of the zonal wind at 30 mb has been derived from these data and a long-term mean quasi-biennial oscillation (QBO) has been constructed from the 14 complete cycles included in the period.

The cold winters of the middle 1990s in the northern lower stratosphere

Lower stratospheric temperatures in the northern winters of 1994/1995, 1995/1996, and 1996/1997 were low enough to support polar stratospheric cloud (PSC) formation for prolonged periods. While the seasonal evolution of each winter was quite different, there are some common characteristics: notably, the occurrence of extremely cold periods of long duration and the coldness of the late winter in each year. Comparison with observations over more than three decades indicate the stratosphere was atypically cold in these three years, with the largest anomalies occurring in the late winter and spring. In January and February the coldness seems to be determined by the interannual variability of the circulation, while in March the persistence of the polar vortex dominated the circulation in these three years. This may be related to the lack of major midwinter warmings in those years. Comparison with other winters shows that although the persistence of the polar vortex well into the spring is not unprecedented, this did not occur frequently in the previous two decades. Further, there is a general temperature decrease in the northern lower stratosphere which contributed to the coldness of the three winters. Comparison of the late winter and spring of 1997 with 1967, both of which were forced only weakly by dynamics, supports the idea that this is due to a change in the radiative balance (with equilibrium at a lower temperature), although there are many caveats to this conclusion.

The unusual midwinter warming in the Southern Hemisphere stratosphere 2002 : A comparison to Northern Hemisphere phenomena

A strong midwinter warming occurred in the Southern Hemisphere (SH) stratosphere in September 2002. Based on experiences from the Northern Hemisphere (NH), this event can be defined as a major warming with a breakdown of the polar vortex in midwinter, which has never been detected so far in the SH since observations began at the earliest in the 1940s. Minor midwinter warmings occasionally occurred in the SH, but a strong interannual variability, as is present in winter and spring in the NH, has been explicitly associated with the spring reversals. A detailed analysis of this winter reveals the dominant role of eastward-traveling waves and their interaction with quasi-stationary planetary waves forced in the troposphere. Such wave forcing, finally leading to the sudden breakdown of the vortex, is a familiar feature of the northern winter stratosphere. Therefore, the unusual development of this Antarctic winter is described in the context of more than 50 Arctic winters, concentrating on winters with similar wave perturbations. The relevance of preconditioning of major warmings by traveling and quasi-stationary planetary waves is discussed for both hemispheres.

Correlation between stratospheric temperature, total ozone, and tropospheric weather systems

The unusual dynamical situation over Europe in the winter 1991/92 caused an unusual behaviour of the total ozone. The strongest negative deviations from the long-term monthly means occured in January. A very cold middle stratosphere developed at the edge of the polar vortex above a warm anticyclonic block in the troposphere. The corresponding low temperature in the tropopause region was located just beneath the coldest air in the stratosphere. A high positive correlation between the temperature and ozone partial pressure was derived at the edge of the polar stratospheric vortex for the tropopause region and for the middle stratosphere. The physical background was vertical motions changing temperature as well as the ozone content. That means unusually low ozone values can be expected when an elevated tropopause is combined with the adiabatically cooled end of the upwelling branch of an enhanced planetary wave in the middle stratosphere. It is shown that in February 1990, when extremely low total ozone was also observed over Scandinavia during a short anticyclonic blocking, this event was caused by the same process. However, during transient events the tropopause temperature is often anticorrelated with the middle stratospheric temperature (a tropospheric ridge reaching into the stratosphere has a cold high tropopause, but a warm middle stratosphere). The forced vertical motions result in extreme ozone columns only when they are in the same direction in both layers. The enhanced wave activity was connected with a strong polar warming in the upper stratosphere in both winters. At 30 hPa in the middle stratosphere the warm center was situated over Eastern Siberia and the coldest part shifted towards Northern Europe. The same situation was available in 11 cases during the fourteen years series of TOMS data (1979–1992), when total ozone reached values below 225 DU at the edge of the stratospheric polar vortex: enhanced wave activity in the middle stratosphere, polar warming in the upper stratosphere, shift of the coldest part of the polar vortex towards northern Europe over a cold high tropopause of a tropospheric anticyclone.

Simulations of Dynamics and Transport during the September 2002 Antarctic Major Warming

Abstract A mechanistic model simulation initialized on 14 September 2002, forced by 100-hPa geopotential heights from Met Office analyses, reproduced the dynamical features of the 2002 Antarctic major warming. The vortex split on ∼25 September; recovery after the warming, westward and equatorward tilting vortices, and strong baroclinic zones in temperature associated with a dipole pattern of upward and downward vertical velocities were all captured in the simulation. Model results and analyses show a pattern of strong upward wave propagation throughout the warming, with zonal wind deceleration throughout the stratosphere at high latitudes before the vortex split, continuing in the middle and upper stratosphere and spreading to lower latitudes after the split. Three-dimensional Eliassen–Palm fluxes show the largest upward and poleward wave propagation in the 0°–90°E sector prior to the vortex split (coincident with the location of strongest cyclogenesis at the model’s lower boundary), with an additional re...

The cold stratospheric winters 1994/1995 and 1995/1996

The stratospheric winters 1994/1995 and 1995/1996 were both extremely cold. The temperature (T) fell below that necessary for type 1 polar stratospheric cloud formation (195 K at 50 hPa) on many consecutive days in each winter and occasionally reached values low enough for ice cloud formation. The meteorological situation each winter was quite different. Both seasons were cold throughout Dec and Jan. In 1995 a dynamical warming event increased T in the latter part of Feb but a further cold spell occurred in early Mar. In contrast, the polar vortex remained well developed until early Mar 1996, when T increased rapidly. While both winters were extremely cold at 50 hPa, the polar vortex was deeper in Feb 1996 when extremely low T occurred over large areas at 30 hPa.

On the polar stratospheric cloud formation potential of the northern stratosphere

A 29-year daily temperature record at 30 hPa in the northern hemisphere stratosphere has been used to estimate the potential of the atmosphere to support polar stratospheric cloud (PSC) formation. Under certain caveats, type 1 PSC formation is assumed to occur at temperatures of 192 K (195 K) or lower at 30 hPa (50 hPa). At 30 hPa in January at least one point in the northern hemisphere was colder than 192 K on 34% of the days in the record. The PSC season begins in late November and ends in middle March ; the maximum likelihood is attained in January. The PSC formation potential is defined as the fractional area of the northern hemisphere cold enough (on the synoptic scale) to support PSC formation. In the mean it exceeds 1% between late December and early February at 30 hPa and is generally larger at 50 hPa. There is considerable interannual variability connected with the synoptic evolution of the stratospheric winter, so that up to 4% of the northern hemisphere is cold enough for PSC formation for shorter periods in individual years. Temperatures low enough for H 2 O ice cloud formation can occur in January and February ; these are generally associated with adiabatic ascent in amplifying planetary waves. The location of the cold areas relative to the vortex center is examined and a measure introduced to quantify this.

The early major warming in December 2001 – exceptional?

The early major warming in December 2001 is described and compared to the two other December major warmings in 1998 and 1987, showing a strong tropospheric-stratospheric coupling in all three cases. We argue that the occurrence of free westward propagating Rossby waves interacting with a forced quasi-stationary wave number 1 led to these three early events. The possible excitation of these waves is discussed with respect to the tropospheric circulation, which showed strong blockings over the northern Atlantic prior to the early major warmings.

Diagnostic Comparison of Meteorological Analyses during the 2002 Antarctic Winter

Abstract Several meteorological datasets, including U.K. Met Office (MetO), European Centre for Medium-Range Weather Forecasts (ECMWF), National Centers for Environmental Prediction (NCEP), and NASA’s Goddard Earth Observation System (GEOS-4) analyses, are being used in studies of the 2002 Southern Hemisphere (SH) stratospheric winter and Antarctic major warming. Diagnostics are compared to assess how these studies may be affected by the meteorological data used. While the overall structure and evolution of temperatures, winds, and wave diagnostics in the different analyses provide a consistent picture of the large-scale dynamics of the SH 2002 winter, several significant differences may affect detailed studies. The NCEP–NCAR reanalysis (REAN) and NCEP–Department of Energy (DOE) reanalysis-2 (REAN-2) datasets are not recommended for detailed studies, especially those related to polar processing, because of lower-stratospheric temperature biases that result in underestimates of polar processing potential, ...

A note on record‐high temperatures at the northern polar stratopause in winter 1997/98

A series of lidar temperature soundings from the ALOMAR observatory in northern Norway indicated an extreme warming of a descending stratopause in February 1998. The maximum temperature recorded during this event was +49°C at 40 km altitude. This stratospheric warming is described by means of SSU satellite radiance data and of stratospheric analyses from the Free University Berlin. Comparisons are made to a number of historical events with similar temperature observations from rocket soundings and to results from the Berlin general circulation model. It turns out that in all cases the highest stratopause temperatures occur close to the 40 km altitude level.