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Karin Labitzke and Harry Van Loon

Abstract

The probable association in the northern winter between the atmosphere and the 11-yr solar cycle extends to the frequency of lows in the North American east coast trough and thus adds a synoptic aspect to the previously described atmospheric variability on the 11-yr time scale. Statistically significant correlations of sea level pressure, 700-mb height, and surface air temperature on the Northern Hemisphere in July–August with the 11-yr solar cycle are found primarily over the oceans. The few years for which data of sea level pressure at grid points are available an the Southern Hemisphere yield coherent correlation patterns in summer and winter which are especially marked in the East years of the QBO. The temperature in the lower stratosphere over the South Pole in spring is well correlated with the solar activity in the East and hardly at all in the West years of the QBO. On the Northern Hemisphere the West years in spring are as strongly correlated with the solar cycle in the stratosphere as they are in winter. The pattern of positive and negative correlations is, however, the opposite of that in winter, which we interpret as being related to the different time of occurrence of the final warming in years with or without major midwinter warmings.

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Harry Van Loon and Karin Labitzke

Abstract

Sea level pressure, surface air temperature, and 700-mb temperature and geopotential height show a probable association with the 11-year solar cycle which can be observed only if the data are divided according to the phase of the Quasi-Biennial Oscillation. The range of the response is as large as the interannual variability of the given element, and the correlations prove statistically meaningful when tested by Monte Carlo techniques. The sign of the correlations changes over the hemisphere on the spatial scale of extensive teleconnections. The correlations at 700 mb tend to be of opposite sign in the east and west years of the QBO, a result which Labitzke and van Loon also found in an analysis of the stratosphere. The pattern of correlation between the 700-mb heights on the Northern Hemisphere and the solar flux is the same as that of point-to-point correlations (teleconnections) between the 700-mb height at selected points and the heights at all other points. We interpret this similarity as a property of the atmosphere's internal dynamics, a favored resonance evoked within the atmosphere itself or by extraneous effects.

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Tsing-Chang Chen, Ming-Cheng Yen, and Harry Van Loon

Abstract

The upper-air wind data generated by the Global Data Assimilation System of the National Meteorological Center for 1986–1992 are used to depict the three-dimensional structure of the semiannual oscillation of tropical stationary eddies in terms of the eddy streamfunction. An eddy streamfunction budget analysis was also performed to disclose the cause of this semiannual oscillation. The major findings are: 1) The tropical stationary eddies exhibit a seesaw semiannual oscillation between the eastern and Western Hemisphere with a phase reversal vertically at 400–500 mb and meridionally at the equator. 2) It is inferred from the streamfunction budget analysis that the semiannual oscillation of the tropical stationary eddies is caused by the semiannual cast-west seesaw of the global divergent circulation between the areas of the Asian-Australian (AA) monsoon (60°E–120°W) and the extra-AA monsoon (120°W–60°E). This mechanism is particularly clear in the Southern Hemisphere Tropics but less well established in the Northern Hemisphere Tropics, which may be attributed to the strong effect of the land-sea contrast on the east-west interseasonal phase change of the stationary eddies in the Northern Hemisphere.

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Hans Von Storch, Harry Van Loon, and George N. Kiladis

Abstract

We show by means of a general circulation model experiment that the atmospheric circulation over the South Pacific Ocean is sensitive to sea surface temperature anomalies in the tropical and subtropical regions of the South Pacific convergence zone. The possible implications for understanding the life cycle of an extreme event in the Southern Oscillation are discussed.

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Harry Van Loon, John W. Kidson, and A. Brett Mullan

Abstract

During the 1970s and 1980s, elements of the atmospheric circulation over the Southern Hemisphere changed markedly from their state in the two previous decades. The change was evident in the zonal asymmetry, especially in the zonal harmonic wave 3 at higher latitudes, and in the semiannual wave in pressure and wind. The semiannual wave changed in the same sense in all three southern oceans, but by the largest amount in the South Pacific Ocean. The second harmonic in a 12-month series, by which we describe the semiannual wave, dominates the shape of the long-term mean annual curve of sea level pressure in mid- and high latitudes; this harmonic weakened during the period, and its midlatitude peak disappeared in the South Pacific Ocean.

As part of the low-frequency changes during the 18 years from 1972 to 1988, the central pressure in the subantarctic trough fell, the trough moved northward, and the subtropical ridge moved south. The meridional movement of the trough and the ridge, and the concurrent trend in their central pressure, weakened the semiannual wave in midlatitudes.

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David J. Stephens, Harry van Loon, Malcolm H. Lamond, and Nicola P. Telcik
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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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Gerald A. Meehl, Julie M. Arblaster, Grant Branstator, and Harry van Loon

Abstract

The 11-yr solar cycle [decadal solar oscillation (DSO)] at its peaks strengthens the climatological precipitation maxima in the tropical Pacific during northern winter. Results from two global coupled climate model ensemble simulations of twentieth-century climate that include anthropogenic (greenhouse gases, ozone, and sulfate aerosols, as well as black carbon aerosols in one of the models) and natural (volcano and solar) forcings agree with observations in the Pacific region, though the amplitude of the response in the models is about half the magnitude of the observations. These models have poorly resolved stratospheres and no 11-yr ozone variations, so the mechanism depends almost entirely on the increased solar forcing at peaks in the DSO acting on the ocean surface in clear sky areas of the equatorial and subtropical Pacific. Mainly due to geometrical considerations and cloud feedbacks, this solar forcing can be nearly an order of magnitude greater in those regions than the globally averaged solar forcing. The mechanism involves the increased solar forcing at the surface being manifested by increased latent heat flux and evaporation. The resulting moisture is carried to the convergence zones by the trade winds, thereby strengthening the intertropical convergence zone (ITCZ) and the South Pacific convergence zone (SPCZ). Once these precipitation regimes begin to intensify, an amplifying set of coupled feedbacks similar to that in cold events (or La Niña events) occurs. There is a strengthening of the trades and greater upwelling of colder water that extends the equatorial cold tongue farther west and reduces precipitation across the equatorial Pacific, while increasing precipitation even more in the ITCZ and SPCZ. Experiments with the atmosphere component from one of the coupled models are performed in which heating anomalies similar to those observed during DSO peaks are specified in the tropical Pacific. The result is an anomalous Rossby wave response in the atmosphere and consequent positive sea level pressure (SLP) anomalies in the North Pacific extending to western North America. These patterns match features that occur during DSO peak years in observations and the coupled models.

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