Search Results

You are looking at 1 - 5 of 5 items for :

  • Author or Editor: H. Van Loon x
  • Monthly Weather Review x
  • Refine by Access: All Content x
Clear All Modify Search
H. van Loon and K. Labitzke

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 alternating sign, the polar vortex being centered near the North Pole and strong in the west phase of the QBO. The anomalies of the Warm Events in the Southern Oscillation, the years of which are almost equally distributed between years of west and east phase of the QBO, are in contrast not zonally symmetrical for the winter as a whole as the influence of the SO is to strengthen substantially the Aleutian high in the stratosphere.

Full access
H. van Loon and D. J. Shea

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 trough's 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 over the warmer water, and that this contributes to lowering the pressure over large parts of the tropical and subtropical South Pacific Ocean.

We demonstrate furthermore that a Cold Event, which is the opposite extreme of the Southern Oscillation, develops in a manner opposite to that of a Warm Event with an enhanced trough and weak trades in the year before the Cold Event, and a depressed trough and strong trades in the year of the event. The surface water over the area of interest south of 15°S therefore becomes colder than normal in the southern winter and spring of the year before the Cold Event. The colder water presumably depresses convection in the South Pacific Convergence Zone, and thus contributes to raising the pressure over large parts of the tropical and subtropical South Pacific Ocean.

Full access
H. Van Loon and Sandra L. Henry

Abstract

In a narrow bell across southern Asia stretching from central India to the Philippines there seems to be no unique association between rainfall and Warm Events in the Southern Oscillation. In this area there are marked long term trends in the rainfall, and the Warm Events an inclined to fall in wet years when the period is generally wet and in dry years when the period is generally dry.

Full access
H. van Loon and D. J. Shea

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.

Full access
H. van Loon, C. S. Zerefos, and C. C. Repapis

Abstract

The signal of the Southern Oscillation in the lower half of the Northern Hemisphere stratosphere in winter appears to be as follows: In the extreme of the Southern Oscillation when the trade winds are comparatively weak in the South Pacific Ocean, stratospheric geopotential heights and temperatures tend to be higher over the Arctic and lower in middle latitudes than in the opposite extreme. At the same time, the polar-night stratospheric jetstream tends to be weaker and the subtropical westerlies to be stronger. The conclusions are based on 11 extremes within a 15-year period and on data at standard pressure levels as high as 10 mb.

Full access