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Steven W. Lyons

Abstract

A zonal wind oscillation with a period of about 15–35 days is isolated over the middle latitudes of the Northern Hemisphere during winter. A systematic relationship between the zonal winds and wind departures from zonal symmetry (eddies) is found for nine prominent zonal wind oscillation cycles taken from six 120-day winters. This zonal/eddy relationship consists of an oscillation between anomalous zonal flow and anomalous meridional flow, which is strongest across North America from 150°W to 0°.

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Steven W. Lyons

Abstract

Analysis of NOAA/NESS outgoing longwave radiation (OLR) over the greater Africa region reveals a large area of low OLR (5°–20°S, 20°E–40°E) during three austral summers (November through February; 1982/83, 1983/84 and 1984/85). This low OLR area is consistent with the climatological rainy area and persistent convective activity. Using OLR as a proxy for synoptic and large-scale cloudiness and convection, OLR standard deviations are computed for the three summers. Highest OLR variability is observed across Africa along 15–17°S, which is about five degrees latitude south of the OLR minimum. Based on the region of maximum OLR standard deviations and minimum mean OLR, a box-average OLR index is derived. Time series of the OLR index for November through February indicate large (±40 W m−2), aperiodic OLR fluctuations within each of the three summers.

Outgoing longwave radiation composites are constructed for periods of large OLR changes from negative OLR anomalies (wet conditions) to positive OLR anomalies (dry conditions). Although fluctuations are noncyclic in time, OLR composites reveal propagation of OLR from south of the Cape of Good Hope toward the northeast. The origin of these OLR fluctuations appears to be the Southern Hemisphere midlatitudes. This is consistent with OLR correlation maps derived for each season. However, a large portion of the OLR changes over equatorial southern Africa are of standing character.

Circulation features associated with the large OLR fluctuations are analyzed by compositing NMC wind and temperature fields. It is found that east-northeastward propagation of midlatitude waves into the subtropical western Indian Ocean occurs prior to OLR decreases over equatorial southern Africa. Trough (ridge) intrusions into subtropical and tropical Africa from the southeast are associated with OLR decreases (increases).

The wind circulation and divergence in these equatorward penetrating troughs is strongest in the upper troposphere (300 mb), temperature perturbations are largest at 500 mb, and the wave signature can be seen down to the low levels.

A close examination of circulation features associated with one prominent OLR change indicates that individual events are similar to the composite average, however, they reveal greater temporal detail. The midlatitude upper level trough does not penetrate directly to equatorial Africa. Rather, the midlatitude trough merges with the Tropical Upper Tropospheric Trough (TUTT), which is a persistent feature at 300–200 mb over the southwest Indian Ocean. The TUTT is then instrumental in modifying circulation over equatorial southern Africa, which is favorable for OLR decreases over that region.

Based on these results it appears that a major source of OLR/convective variability over the rainy region of equatorial southern Africa during austral summer is associated with interaction between midlatitude wave disturbances embedded in the westerlies and the quasi-stationary tropical upper tropospheric trough in the vicinity of the southwest equatorial Indian Ocean.

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Carlos R. Mechoso and Steven W. Lyons

Abstract

The impact of sea surface temperature (SST) anomalies observed during the Northern Hemisphere spring of 1984, which include the growing phase of an intense Atlantic warm event on the atmospheric circulation over the tropical Atlantic and Pacific is investigated using the nine-layer, low resolution version of the UCLA general circulation model. This impact is contrasted with that for the same period during 1983, when SST anomalies include the decaying phase of the strongest Pacific El Niño on record. Results obtained in control and anomaly simulations, consisting, respectively, of extended integrations with and without the observed SST anomalies, are analyzed.

It is found that simulated anomalies in the atmospheric circulation corresponding to 1984 include low-level westerlies over the equatorial Atlantic and easterlies over the equatorial Pacific. There are centers of anomalous low-level convergence and divergence off the northeast coast of Brazil and equatorial Brazil, respectively, which are associated with positive and negative precipitation anomalies. Differences between results corresponding to 1984 and 1983 show the impact of El Niño over the Pacific. Further, positive precipitation anomalies over the equatorial Atlantic shift from generally north of the equator in 1983 to south of the equator in 1994 (dry and wet years for northeast Brazil, respectively).

These simulated anomalies and interannual differences in the atmospheric circulation are in good general agreement with those observed. This agreement strongly suggests that the atmospheric anomalies observed during the northern springs of 1984 and 1983 over the tropical Atlantic and Pacific were primarily due to the SST anomalies.

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Carlos R. Mechoso, Steven W. Lyons, and Joseph A. Spahr

Abstract

The response of the tropical atmosphere to the sea surface temperature (SST) anomalies in the Northern Hemisphere spring of 1984 is investigated. The methodology for investigation consists of comparing simulations with and without the global distribution of SST anomalies in the boundary conditions of the UCLA General Circulation Model (GCM). At low levels, the response includes weaker southeast trade winds over the Atlantic, increased precipitation off the northeast coast of Brazil, and reduced precipitation west of this region. The increased precipitation is due to enhanced convergence of moisture advected by the southeast trade winds, although the trades themselves are weaker. The results for the western equatorial Atlantic am in apparent agreement with the observed anomalous southern migration of the ITCZ in years with warm SST anomalies in the southern tropical Atlantic. There are strong anomalous trade winds over the Pacific extending east of the date line and weak wind anomalies over the maritime continent, in broad agreement with the observed.

The sensitivity of the simulated atmospheric response over an ocean basin to using the SST anomalies confined to the basin or in the global ocean is analyzed. It is found that there can be notable local differences in the results obtained using those procedures. In particular, the simulation with the SST anomalies confined to the Pacific shows weak anomalous trade winds over the western part of this ocean basin and strong westerly anomalies over the maritime continent unlike that with the anomalies in the global ocean.

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