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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.

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H. van Loon and K. Labitzke

Abstract

The authors correlate the 23-yr series of reanalyzed 30-hPa heights and temperatures and 10-hPa heights with the 11-yr solar cycle for the summers of both hemispheres and for the annual mean. The size and spatial pattern of the correlations in the Northern Hemisphere are the same as those of the correlations computed with a nearly twice as long series from the Freie Universität Berlin: a belt of correlations that encircles the hemisphere in the outer Tropics–subtropics. The correlation pattern is similar in the Southern Hemisphere.

The spatial distribution of correlations between 30-hPa temperatures and the solar cycle has the same configuration as the height correlations with the cycle. The largest temperature correlations move with the sun from one summer hemisphere to the other.

The first eigenvector in a principal component analysis of the 30-hPa heights in summer and in the annual mean has the same shape as the above-mentioned pattern in the correlations between the stratospheric data and the 11-yr solar cycle. The EOF 1 explains 77% of the variance in summer on the Northern Hemisphere and 72% on the Southern Hemisphere, and the time series of its amplitude is dominated by a decadal wave in phase with the 11-yr sunspot cycle.

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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.

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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.

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W. G. Large and H. Van Loon

Abstract

The surface response of the Southern Hemisphere's oceans to the large spatial scale, interseasonal changes in wind forcing during the FGGE year of 1979 is investigated. The primary data are the analyzed daily wind fields, and the trajectories of the FGGE drifting buoy array. The zonal wind forcing is characterized by large spatial patterns of low frequency (annual and semiannual) variability. particular attention is paid to the second harmonic, which has amplitude peaks at 35°–40° S with solstitial maxima, and amplitude peaks at 60°S with equinoctial maxima. The distinct phase change occurs at 50°S.

The analysis of the drifting buoy data is guided by the wind patterns, but first the question of the current-following characteristics of the FGGE buoys is addressed. Compared to the wind, the buoy drift has even larger spatial scales, and more low frequency contributions to its intra-annual variance. Like the wind, amplitude peaks in the second harmonic of monthly mean zonal drift are found in each ocean basin sector at 40 ± 5° S and at 55° −60° S, with a phase change at about 50° S. These wind and drift patterns extend from 30°S to antarctica, and so encompass the entire antarctic Circumpolar Current (ACC) and the poleward halves of the subtropical gyres.

The results are discussed in relation to Southern Ocean dynamics and previous studies. A simple barotropic calculation shows that interseasonal changes in buoy drifts are small enough relative to the wind forcing that neither baroclinic surface enhancement nor slip error need be invoked to explain them. Latitudinal shear in zonal drift is shown to have a great deal of temporal variability implying momentum transports across the ACC, to the center or from the center of the ACC, depending on the time of year. The observed buoy drift is consistent with the view of the ACC consisting of multiple narrow cores. Furthermore, it suggests that as the latitude of the peak in zonal wind shifts with the half-year waves, different underlying cores of the ACC are accelerated to be the one with the greatest velocity. The Seasat satellite altimetric results are interpreted as capturing a half-cycle of the second harmonic, and as showing a phase change in zonal geostrophic flow at about 50°S. A second harmonic with equinoctial maxima is found in the 500 m depth pressure difference across the Drake Passage, although we find that this area is not very representative of the ACC as a whole.

We propose that the semiannual signals in the winds and surface currents should be important diagnostics in coupled ocean-atmosphere models of the Southern Ocean. This wave is, however, faithfully represented only in products from daily analyzed pressure fields and in their climatological analyses, but not in atmospheric general circulation models nor in wind climatologies based on ship observatons.

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David J. Stephens, Michael J. Meuleners, Harry van Loon, Malcolm H. Lamond, and Nicola P. Telcik

Abstract

In this study temporal and spatial aspects of El Niño (warm event) development are explored by comparing composite sequences of sea level pressure (SLP), surface wind, and sea surface temperature (SST) anomalies leading into strong and weak events. El Niño strength is found to be related to the magnitude and spatial extent of large-scale SLP anomalies that move in a low-frequency mode. In association with this, it is also intricately linked to the amplitude and wavelength of the Rossby waves in the southern midlatitudes. The primary signature of the Southern Oscillation is a more pronounced standing wave of pressure anomalies between southeastern Australia and the central South Pacific leading into stronger events. A strong reversal in the strength of the annual cycle between these two regions causes a stronger (weaker) SLP gradient that drives southwesterly (northwesterly) wind stress forcing toward (away from) the western equatorial Pacific in austral winter–spring of year 0 (−1). Thus, pressure variations in the southwest Pacific preconditions the equatorial environment to a particular phase of ENSO and establishes the setting for greater tropical–extratropical interactions to occur in stronger events.

Maximum warming in the Niño-3 region occurs between April and July (0) when a strong South Pacific trough most influences the trade winds at both ends of the Pacific. Cool SST anomalies that form to the east of high pressure anomalies over Indo–Australia assist an eastward propogation of high pressure into the Pacific midlatitudes and the demise of El Niño. Strong events have a more pronounced eastward propogation of SST and SLP anomalies and a much more noticeable enhancement of winter hemisphere Rossby waves from May–July (−1) to November–January (+1). Weak events require an enhanced South Pacific trough to develop but have much less support from the North Pacific. They also appear more variable in their development and more difficult to predict with lead time.

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