Evolution of the Tropospheric Split Jet over the South Pacific Ocean during the 1986–89 ENSO Cycle

Abstract

A case study investigation into the meridional and horizontal circulation over the South Pacific Ocean is presented for the 1986–89 El Niño–Southern Oscillation (ENSO) cycle. Using the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses, annual average fields are created for the years before and after the 1987 minimum (warm phase) and 1989 maximum (cold phase) in the Southern Oscillation index. The analyses reveal a shift in the split jet stream over the south Pacific sector(180°–120°W)from a strong subtropical jet (STJ) and weak polar front jet (PFJ) during the warm phase to a weak STJ and strong PFJ during the cold phase. Analysis of the momentum budget reveals how the split jet in the upper troposphere over South Pacific Ocean evolved during the 1986–89 ENSO cycle. During the warm phase, the strong STJ is associated with advection of the mean flow momentum flux from the Australian sector, which is approximately balanced by a large negative ageostrophic term; the PFJ is primarily associated with eddy momentum convergence, which is partially counterbalanced by the ageostrophic term. During the cold phase, the weakened STJ is related to an increasingly negative ageostrophic term and a less positive mean flow momentum convergence. The strengthened PFJ is associated with an increase in the convergence of eddy momentum flux that is mainly composed of 2.5–6-day poleward momentum transport from midlatitudes and 7–30-day equatorward momentum transport from high latitudes. In general, the impacts of eddy stress on the STJ and the mean momentum divergence on the PFJ in this sector are small. The variations in the split jet may reflect the poleward propagation of the ENSO signal via the South Pacific convergence zone. The implications for the high southern latitudes are discussed as interannual variations are found in the low-level easterlies near Antarctica and the Amundsen Sea low.