Abstract

Some of the characteristic features of the 1982/83 ENSO are described in terms of 3-month running mean anomalous winds (u″, v″) at 850 mb, outgoing longwave radiation (OLR″), and sea surface temperature (SST″) during the 6 yr period 1980–85 over the Indian and Pacific means. Climatological information on “westerly bursts” is obtained from band-pass filtered winds, which are denoted as us, and vs (1–6 day), um and vm (7–20 day), and ul and vl (30–60 day).

Atmospheric interannual u″ and OLR″ modes are characterized by typical spatial scales of zonal wavenumbers 1 and 2, and very slow eastward phase propagations of about 0.3 m s−1 all the way from the western Indian Ocean to the eastern Pacific. Oceanic interannual SST″ modes also exhibit a slow eastward movement across the Pacific.

Westerly ul bursts are sandwiched between twin cyclonic disturbances which are nearly symmetric with respect to the equator. The east-west extent of ul westerly bursts along the equator is about 10 000 km, while the corresponding extent of um bursts amounts to 6000–7000 km. The (um, vm) wind field is asymmetric with respect to the equator and is characterized by cold equatorward surges from the winter hemisphere midlatitude region. The longitudinal scale of us bursts along the equator is only 3000 km or less. Thus, the structure of westerly bursts varies significantly with different periodicities.

The relationship between synoptic-scale westerly bursts on time scales of a few days and the planetary scale interannual modes is first investigated by counting the number of days of westerlies (and/or westerly acceleration) in each season at every 20 deg of longitude from 40°E to 100°W along the equator. There exists an association between u″ and westerly bursts involving the frequency of events; namely, even though westerly bursts are occurring all the time, the frequency of westerly bursts increases after (not before) the ENSO onset. Furthermore, westerly bursts are not unique to the equatorial western Pacific.

The standard deviation of us, um, and ul is then computed for each season, followed by the removal of normal seasonal mean values. These anomaly standard deviation fields, signified as σ&Prime(us), σ&Prime(um), and σ&Prime(ul) exhibit the same time and space scales as noted for interannual u″ modes. Consequently, groups of transient disturbances can interact with interannual modes. When interannual u″ modes reached the western Pacific in July 1982, after crossing the Indian Ocean, they dramatically intensified through nonlinear interaction with all the transients (us, um, ul). This corresponded to the 1982/83 ENSO onset. Large SST″ anomalies over the eastern pacific are a result of strong local air-sea interaction on interannual lime scales.

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