Modulation of equatorial subseasonal convective episodes by
tropical-extratropical interaction in the Indian and Pacific
Ocean regions
Gerald Meehl, George Kiladis, Klaus Weickmann, Matthew Wheeler, David Gutzler, and Gilbert Compo
1996: J. Geophys. Res., 101, 15033-15049.
Abstract
Composite relationships among outgoing longwave radiation, sea level
pressure, surface winds, and upper tropospheric circulation are examined for
northern winter during subseasonal episodes of eastward progression of
convection from the Indian Ocean to the western Pacific. This evolution often
culminates with westerly wind burst events and strong air-sea interaction
associated with regional-scale convective blowups in the western equatorial
Pacific. We first document some of these interactions in the composites for two
timescales, the submonthly (6-30 days) and that of the Madden-Julian
Oscillation (MJO) timescale (30-70 days). We then analyze the December 1992
period during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere
Response Experiment (TOGA COARE) to illustrate how these composite
relationships are manifested in a case study. Convection in the Indian Ocean
for the composites is shown to be associated with a northern hemisphere wave
train at 200 hmbar, arcing through the midlatitudes, that can contribute
to convective blowups farther east on the submonthly (6-30 days) timescale
in the Intertropical Convergence Zone (ITCZ) in the eastern Pacific. The
eastern Asian trough that is part of this wave train is associated with
pressure surges from the northern hemisphere and subsequent convection over
Southeast Asia. As the MJO convective envelope moves east to Australasia,
midlatitude wave trains in either hemisphere include upper level troughs
east of Asia and Australia and pressure surges from either hemisphere
that contribute to pressure rises over the Indonesian region and a subsequent
shift of the convective envelope to the western Pacific. The vertical wind
structure for the December 1992 case study is consistent with the composite
surface and upper level winds and also shows strong vertical wind shear
in the boundary layer, a sharply defined westerly maximum near 700 mbar and
an intensification of the upper level easterlies near 100 mbar. Very deep
westerlies (to 200 mbar) are confined to shorter timescales. The case
study illustrates the various time and space scale interactions noted
in the composites. Reciprocal interactions between the tropics and the
midlatitudes on the submonthly and MJO timescales in both the composites
and the case study involve pressure surges and wave interaction that
influence subsequent convection as the convective envelope migrates
eastward from the tropical Indian to Pacific Ocean region.