On the location and orientation of the South Pacific Convergence Zone

Abstract

Three semi-permanent cloud bands exist in the Southern Hemisphere extending southeastward from the equator, through the tropics, and into the subtropics. The most prominent of these features occurs in the South Pacific and is referred to as the South Pacific Convergence Zone (SPCZ). Similar bands, with less intensity, exist in the South Indian and Atlantic oceans. We attempt to explain the physical mechanisms that promote the diagonal orientation of the SPCZ and the processes that determine the timescales of its variability. It is argued that the slowly varying sea surface temperature patterns produce upper tropospheric wind fields that vary substantially in longitude. Regions where 200 hPa zonal winds decrease with longitude (i.e., negative zonal stretching deformation, or $$ {{\partial \overline{U} } \mathord{\left/ {\vphantom {{\partial \overline{U} } {\partial x}}} \right. \kern-\nulldelimiterspace} {\partial x}} < 0 $$ ) reduce the group speed of the eastward propagating synoptic (3–6 day period) Rossby waves and locally increase the wave energy density. Such a region of wave accumulation occurs in the vicinity of the SPCZ, thus providing a physical basis for the diagonal orientation and earlier observations that the zone acts as a “graveyard” of propagating synoptic disturbances. In essence, $$ {{\partial \overline{U} } \mathord{\left/ {\vphantom {{\partial \overline{U} } {\partial x}}} \right. \kern-\nulldelimiterspace} {\partial x}} = 0 $$ demarks the boundary of the graveyard while regions where $$ {{\partial \overline{U} } \mathord{\left/ {\vphantom {{\partial \overline{U} } {\partial x}}} \right. \kern-\nulldelimiterspace} {\partial x}} < 0 $$ denote the graveyard itself. Composites of the life cycles of synoptic waves confirm this hypothesis. From the graveyard hypothesis comes a more general theory accounting for the SPCZ’s spatial orientation and its longer term variability influenced by the El Nino-Southern Oscillation (ENSO), or alternatively, the changing background SST associated with different phases of ENSO.