H. van Loon

The Southern Oscillation. Part V: The Anomalies in the Lower Stratosphere of the Northern Hemisphere in Winter and a Comparison with the Quasi-Biennial Oscillation

Abstract The mean anomalies of 50 mb height in the northern winter for seven Warm Events in the Southern Oscillation show a weak polar vortex and an enhanced Aleutian high. In the mean for six Cold Events the polar vortex is unusually strong and the Aleutian high is weakened and displaced far to the southwest. These anomalies are consistent with the corresponding anomalies in sea level pressure pattern. The Warm Events of 1963 and 1982 did not fit this pattern as in both years the polar vortex was cold and intense. These events happened in years when volcanoes injected large amounts of gases and aerosols into the stratosphere and the temperature of the tropical stratosphere became unusually high. In other Warm Events the temperature of the tropical stratosphere was abnormally low. The mean anomalies of the Quasi-Biennial Oscillation for the winter as a whole (west minus east phase) computed from years with no Cold or Warm Events are zonally symmetrical and shaped as four concentric regions with alternatin...

The Southern Oscillation. Part IV: The Precursors South of 15°S to the Extremes of the Oscillation

Abstract The year before a Warm Event takes place in the Southern Oscillation the trough in the westerlies at the surface over the South Pacific Ocean fails to amplify to its normal size in the latitudes north of 45°S during the southern fall and winter. There is therefore an anomalous northerly wind in these months over the Pacific Ocean between 15°S and 45°S, west of 140°W. In contrast, the troughs amplitude is above normal in the fall and winter of the following year when the Warm Event takes place, and one therefore observes an anomalous southerly wind where a northerly anomaly occurred the previous year. Consistent with the different wind anomalies, the temperature of the surface water is higher in the year before the Warm Event than in the year of the event between 15°S and 45°S, from Australia to 140°W. We propose that when the South Pacific Convergence Zone expands toward the south as usual in the southern spring of the year before a Warm Event, the convection in the Convergence Zone is enhanced ...

The Southern Oscillation. Part VI: Anomalies of Sea Level Pressure on the Southern Hemisphere and of Pacific Sea Surface Temperature during the Development of a Warm Event

Abstract The paper shows the discrete, mean three-month anomalies of sea level pressure on the Southern Hemisphere during the year before and the year of a Warm Event in the Southern Oscillation, together with associated anomalies of sea surface temperature in the South Pacific 0cean. The two sets anomalies develop in a parallel and physically logical sequence over the South Pacific Ocean in conjunction with changes in the South Pacific Convergence Zone. Nearly all of the Southern Hemisphere responds to the Southern Oscillation, but the response is largest in the Australia-South Pacific sector. Large anomalies of sea level pressure form well ahead of any on the Northern Hemisphere, and this observation together with the conspicuous anomalies in the region of Australia and the South Pacific suggest that the origin of the Southern Oscillation must be sought in this region.

The Influence of the 11-year Solar Cycle on the Stratosphere Below 30 km: a Review

The NCEP/NCAR re-analyses of the global data as high as 10hPa have made it possible to examine the influence of the 11-year sunspot cycle on the lower stratosphere over the entire globe. Previously, the signal of the solar cycle had been detected in the temperatures and heights of the stratosphere at 30hPa and below on the Northern Hemisphere by means of a data set from the Freie Universität Berlin. The global re-analyses show that the signal exists on the Southern Hemisphere too, and that it is almost a mirror image of that on the Northern Hemisphere. The largest temperature correlations with the solar cycle move from one summer hemisphere to the other, and the largest height correlations move poleward within each hemisphere from winter to summer.The correlations are weakest over the whole globe in the northern winter. If, however, one divides the data into the winters when the equatorial Quasi-Biennial Oscillation was easterly or westerly, the arctic correlations become positive and large in the west years, but insignificantly small over the rest of the earth. The correlations in the east years are negative in the Arctic but positive in the subtropics and tropics on both hemispheres.The difference between the east and west years in January-February can be ascribed to the fact that the dominant stratospheric teleconnection and the solar influence work in the same direction in the east years but oppose each other in the west years.

THE SIGNAL OF THE 11-YEAR SUNSPOT CYCLE IN THE UPPER TROPOSPHERE-LOWER STRATOSPHERE

The paper summarizes work by the authors over the past ten years on an apparent signal of the 11-year sunspot cycle in the lower stratosphere-upper troposphere. The signal appears as a basic, consistent pattern in correlations between heights of stratospheric constant-pressure levels, at least as high as 25 km, and the solar cycle in which the highest correlations are in the subtropics.The variation of the stratospheric heights in phase with the sunspot cycle are – in the areas of high correlations between the two – associated with temperature variations on the same time scale in the middle and upper troposphere. The spatial distribution of the correlations suggests that the year-to-year changes in tropical and subtropical vertical motions contain a component on the time scale of the solar cycle.In January and February the correlations with the sunspot cycle are smallest. The smallness of the correlations is owing to the fact that they are different in the east and west years of the quasi-biennial oscillation in the equatorial stratospheric winds. The correlation pattern in the east years is the same as in the other seasons and is statistically significant. In the west years the correlations are insignificant outside the arctic, and the positive correlation in the arctic in these years is related to the fact that major midwinter breakdowns of the cyclonic vortex in the west years so far have happened only at maxima in the solar cycle.Until recently reliable continuous series of analyses of the stratosphere were not available for the southern hemisphere. The U.S. National Centers for Environmental Prediction and the National Center for Atmospheric Research have now, however, issued a 23-year series of re-analyzed global data which has made it possible to detect the solar signal on the southern hemisphere. It turns out to be almost the same as that on the northern hemisphere.The correlations between total column ozone and the sunspot cycle are lowest in the equatorial regions, where ozone is produced, and in the subpolar regions, where the largest amounts are found. In the annual mean the largest correlations lie between 5° lat. and 30° lat. We suggest that this distribution of correlations is due to the fact that the subtropical heights of the constant-pressure surfaces in the ozone layer are higher in maximum than in minimum years of the sunspot cycle, and that the higher subtropical heights in the solar maxima depress the poleward transport of ozone through the subtropics and thus create an abundance of ozone.

Associations between the 11-year solar cycle, the quasi-biennial oscillation and the atmosphere: a summary of recent work

Atmospheric elements at all levels from the surface to the top of the middle atmosphere show a probable association with the 11-year solar cycle that can be observed only if the data are divided according to the phase of the quasi-biennial oscillation. In either phase the range between solar extremes is as large as the interannual variability of the given element; and the correlations are statistically meaningful when tested both by conventional and Monte Carlo techniques. The sign of the correlations changes spatially on the scale of planetary waves or teleconnections. As the correlations tend to be of opposite sign in the two phases of the quasi-biennial oscillation, correlating a full time series of an atmospheric element with the solar cycle nearly always yields negligible correlation coefficients.

The QBO effect on the solar signal in the global stratosphere in the winter of the Northern Hemisphere

Abstract This paper contains correlations between the NCEP/NCAR global stratospheric data below 10 hPa and the 11-year solar cycle. In the north summer the correlations between the stratospheric geopotential heights and the 11-year solar cycle are strong and positive on the Northern Hemisphere and as far south as 30°S, whereas they are weak in the north winter all over the globe. If the global stratospheric heights and temperatures in the north winter are stratified according to the phase of the QBO in the lower stratosphere, their correlations with the solar cycle are large and positive in the Arctic in the west years of the QBO but insignificantly small over the rest of the earth, as far as the South Pole. In the east years, however, the arctic correlations with the solar cycle are negative, but to the south they are positive and strong in the tropical and temperate regions of both hemispheres, similar to the correlations with the full series of stratospheric data in the other seasons. The influence of the solar cycle in the Arctic is stronger in the latter half of the winter. The global difference, in the northern winter, in the sign and strength of the correlations between the stratospheric heights and temperatures and the solar cycle in east and west years of the QBO can be ascribed to the fact that the dominant stratospheric teleconnection and the solar influence work in the same direction in the east years, but oppose each other in the west years.

THE SIGNAL OF THE 11-YEAR SOLAR CYCLE IN THE GLOBAL STRATOSPHERE

Abstract The search for a signal of the 11-year sunspot cycle in the heights and temperatures of the lower stratosphere was previously successfully conducted for the northern hemisphere with a data set from the Freie Universitat Berlin, covering four solar cycles. This work has been extended to the whole globe by means of the NCEP/NCAR reanalyses for the period 1968–1996. The re-analyses show that the signal exists in the southern hemisphere too, and that it is of nearly the same size and shape as on the northern hemisphere. The NCEP/NCAR reanalyses yield higher correlations with the solar cycle than do the Berlin analyses for the same period, because the interannual variability is lower in the NCEP/NCAR data. The correlations between the solar cycle and the zonally averaged temperatures at the standard levels between 200 and 10 hPa are largest between the tropopause and the 25 km level, that is, in the ozone layer. This may be partly a direct effect in this layer, because of more absorber (ozone) and more ultraviolet radiation from the sun in the peaks of the 11-year solar cycle. However, it is more likely to be mainly an indirect dynamical consequence of UV absorption by ozone in the middle and upper stratosphere. The largest temperature correlations move with the sun from one summer hemisphere to the other, and the largest height correlations move poleward from winter to summer.

Total ozone and the 11-yr sunspot cycle

Abstract The correlations between the total column ozone observed by TOMS and the 11-yr sunspot cycle are lowest in the equatorial region, where ozone is produced, and in the subpolar regions, where the largest amounts are found. In the annual mean the highest, statistically significant, correlations lie between the 5 ° and 30 ° parallels of latitude in either hemisphere—between the area of production and the areas of plenty. This position of the largest correlations suggests that the association between the Sun and the ozone is not a direct, radiative one, but that it is due to solar induced changes in the transport of ozone, that is, to changes in the atmospheric circulation. The highest tropical-subtropical correlations move with the Sun from summer hemisphere to summer hemisphere. The subtropical geopotential heights in the ozone layer are higher in the peaks than in the valleys of the 11-yr sunspot cycle. It is probable that the higher subtropical geopotentials in solar maxima depress the poleward transport of ozone through the subtropics and therefore create an abundance of ozone in the tropics relative to the solar minima. These results are based on a 15-yr series of ozone observations and may thus not necessarily be representative of a longer period.

Review of the decadal oscillation in the stratosphere of the northern hemisphere

The decadal oscillation of the temperature and geopotential height in the lower stratosphere of the northern hemisphere can be followed back to the early 1950s. During this time it was in phase with the 11-year sunspot cycle. The correlation with the solar cycle is positive and largest in the stratospheric geopotential heights of the subtropics below 10 mbar (∼31 km, which is as high as the grid point data reach), especially on the western, ocean-dominated side of the hemisphere. As expected from the hydrostatic equation, it is also evident in the temperatures of the upper troposphere in the same region. There is no large correlation at high latitudes. The correlation with the sunspot cycle is weakest in January-February, but if the data in these months are grouped according to the wind direction in the quasi-biennial oscillation (QBO) of the lower equatorial stratosphere, the positive subtropical correlations in the east years are as high as in all other months. There are, in addition large negative correlations in the Arctic in agreement with the strong teleconnection (negative correlation) between lower and higher latitudes in winter. There is no consistent sign in the weak correlations at middle and lower latitudes in the west years, but in the Arctic the correlation with the solar cycle is highly positive, because those major midwinter warmings that occur in west years of the QBO take place in solar maxima, whereas the years without major warmings are found in solar minima. There is not yet an explanation of the 10-12 year oscillation in the stratosphere.