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Vernon E. Kousky

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Vernon E. Kousky

Abstract

During February–March 1981, striking contrasts existed in the rainfall distribution over most of Brazil and in the atmospheric circulation features over South America and the North and South Atlantic Oceans. Drier than normal conditions prevailed during February in the eastern and northeastern sections of Brazil. This was followed by an excessively wet March. The onset of the rainfall was associated with a low level equatorward propagating convergence zone that originated as a cold front at higher latitudes in the Southern Hemisphere. During the period of strong convection, when observed rainfall rates in many areas exceeded 20 mm d−1, a strong anticyclonic circulation developed in the upper troposphere to the west-southwest of the maximum rainfall rates. Another anticyclonic center developed north of the equator forming a couplet which is strikingly similar to the theoretical upper level flow pattern associated with a tropical heat source. The Northern Hemisphere midlatitude circulation changes over the Atlantic are quite similar to those associated with the recently studied 30–60 day oscillation, and to the North Atlantic Oscillation.

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Vernon E. Kousky

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Vernon E. Kousky

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Vernon E. Kousky

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Vernon E. Kousky

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The diurnal rainfall variation in northeast Brazil is investigated for the period 1961–70. Most coastal areas are found to experience a nocturnal maximum in rainfall activity. probably due to convergence between the mean onshore flow and the offshore land breeze. Areas 100–300 km inland experience a daytime maximum, associated with the development and inland advance of the sea breeze. The diurnal rainfall variability, at most interior locations, seems to be, due to mountain-valley breezes.

Seasonal variations am noted for areas of the north coast of Brazil. Soure, located near the mouth of the Amazon River, experiences a nocturnal maximum between January and May, and an afternoon maximum between June and September. Evidence is presented which shows that this seasonal variation is linked to the seasonal variation in the low-level mean now.

Seasonal variations in the mew monthly rainfall for eastern coastal areas am shown to correlate well with the intensity of the land breeze, which is represented by the difference between the mean sea surface temperature and the mean air temperature over land.

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Vernon E. Kousky

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Surface observations for the 10-year period 1961–70 are analyzed for occurrences of fronts in North-east Brazil. Cold fronts, or their remains, are found to enter the Northeast throughout the year. The southern part of the region (interior Bahia) receives much of its precipitation in the period November-February associated with frontal systems. Coastal areas. which receive maximum rainfall during the months of May–July, experience a marked increase in rainfall associated with the approach and passage of cold fronts. Even precipitation events in the extreme northern part of the region (Ceará) sometimes appear to be associated with systems which are cold-frontal in origin.

Pressure fluctuations are felt nearly simultaneously throughout the region with greatest amplitudes occurring in the south. These fluctuations are also shown to occur over a fairly broad range of longitude within tropical Brazil and may have a significant effect on the position of the equatorial trough zone over the Atlantic.

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Vernon E. Kousky and Ants Leetmaa

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The evolution of oceanic and atmospheric anomaly fields for the period prior to and during the 1986–87 El Niño/Southem Oscillation (ENSO) is presented. A comparison is made between the 1986–87 ENSO and other warm episodes that occurred during the last 20 yr. In addition, for the first time, an ocean general circulation model was run in a real-time diagnostic mode. The model output provided detailed information about the evolution of subsurface features throughout the Pacific basin.

A slow trend towards warm episode (ENSO) conditions in the Pacific was evident throughout the period 1985–86 in certain atmospheric and oceanic variables. Atmospheric and oceanic fields changed much more rapidly during late 1986 as enhanced atmospheric convection developed in the equatorial Pacific near the date line. Thermocline depths rapidly increased (decreased) in the eastern (western) equatorial Pacific as low-level westerlies developed in the western portion of the basin. A remote response to those westerlies was felt along the west coast of South America in early 1987 as sea surface temperatures (SSTs) increased 3°–5°C above normal. Conditions remained anomalous in the tropical Pacific throughout 1987, but began a rapid return towards normal late in the year as low-level easterlies increased in strength. By the northern spring 1988, below normal SSTs were observed throughout the equatorial Pacific east of the date line.

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Vernon E. Kousky and S. Srivatsangam

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The annual cycle occupies a unique position in the spectra of meteorological time series. This cycle and its first three harmonics are extracted from the series as a seasonal cycle. The distributions of the annual and seasonal cycles are studied for the United States and its immediate surroundings, using four years of data.

The results show that the annual and seasonal cycles are extremely important contributors to be observed temperature and wind fields at the 850, 500 and 250 mb levels. In winter, the annual cycle establishes a west wind maximum in the subtropics, quite akin to the winter-average jet stream. The annual cycle in horizontal wind constitutes a trough over the central parts of the United States in winter and a ridge in the same area in summer.

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Vernon E. Kousky and Mary T. Kayano

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Principal modes of anomalous upper-tropospheric circulation and outgoing longwave radiation (OLR) are determined for the South American sector. A combined EOF analysis is performed using anomalous 250-mb zonal and meridional wind components plus anomalous OLR. The data are filtered so that modes on both intraseasonal and interannual timescales can be studied separately. The resulting patterns show a consistent relationship between anomalous OLR and anomalous upper-tropospheric circulation features.

The first and second interannual modes contain many atmospheric features related to extremes in the Southern Oscillation. These include 1) variations in the pattern of tropical convection, and 2) changes in the intensity of upper-tropospheric zonal flow in the equatorial band and in the subtropics. The first intraseasonal mode has its greatest loadings in the Tropics and is most active during the southern summer season. For positive (negative) amplitudes of this mode, anomalous upper-tropospheric westerly (easterly) flow dominates the Tropics throughout the region, and positive (negative) OLR anomalies are observed over northeastern South America and over the eastern equatorial Pacific. These features are associated with the 30–60 day (Madden-Julian) oscillation.

An extended combined EOF analysis was performed to study the evolution of intraseasonal oscillations in the South American sector. The first rotated combined extended EOF mode describes a sequence of patterns in which anomalous equatorial westerlies and positive OLR anomalies over tropical Brazil gradually weaken and then reverse. The largest loadings throughout the evolution of the patterns contained in the first mode remain in the Tropics and subtropics. These results provide information on the combined relationship between atmospheric circulation and convection over the South American sector and on the evolution of patterns associated with 30–60 day (intraseasonal) oscillations. By projecting the combined fields of anomalous OLR and 250-mb zonal and meridional wind onto the individual patterns contained in the first rotated combined extended EOF mode, one obtains indices that should enhance real-time monitoring efforts and may lead to improved monthly forecasts for the South American sector.

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