Intraseasonal Disturbance Activity before, during and after the 1982–83 ENSO

Abstract

The longitude-time cross section of outgoing longwave radiation and zonal winds at the equator indicates a regular eastward propagation of interannual time-scale perturbations all the way from the Indian Ocean, across the maritime continent, to as far east as the eastern Pacific during the 5 years of 1979?83. These interannual perturbations also exhibit standing wave character. Associated with this are phase changes from the pre anti-ENSOF, in January 1982, via the mid-ENSO in January 1983, to the post anti-ENSO of December 1983. Both anti-ENSO and mid-ENSO are phase locked with the seasonal cycle. Anti-ENSO synoptics are the manifestation of anomalous enhancement of the normal winter circulation, whereas the mid-ENSO patterns reflect an anomalous weakening of the normal winter circulation. Thus, the mid-ENSO exhibits an approximate reversal of synoptics (anomaly) from the anti-ENSO phase. Over the eastern North and South Pacific between about the dateline and 120°W, a key area for the 1982?83 ENSO, the main characteristics of the anti-ENSO and mid-ENSO phases are summarized as follows: Anti-ENSO is characterized by 1) an anomalously intensified indirect N?S vertical overturning with below normal equatorial convection in contrast to above normal rainfall over the extratropics, 2) an unusual intensification of upper oceanic troughs over both the North and South Pacific with prominent equatorial westerlies between them, and 3) substantial intraseasonal time-scale 200 mb disturbance activity. Mid-ENSO is characterized by 1) a direct N?S vertical overturning (anomaly) accompanied by equatorial convection and extratropical dry spells, 2) an unusual weakening of upper oceanic troughs (or anomalous upper anticyclones) with equatorial easterlies inhibiting intraseasonal disturbance activity, and 3) enhanced midlatitude westerlies poleward of twin anomalous anticyclones, facilitating above normal baroclinic disturbance activity. The Japan?maritime continent?Australia region (100°?160°E) is another key area for a well-defined anomalous circulation reversal. Here, circulation changes are out of phase with those over the eastern North and South Pacific region. Less organized circulation reversals also take place over both the Afghanistan?Indian Ocean (40°?80°E) and Central America?South America (80°?40°W) regions.