Climate dynamics of the South Pacific convergence zone and similarities with other subtropical convergence zones in the southern hemisphere

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 convergence zones, with less intensity, exist in the South Atlantic and South Indian oceans. Convection in each basin is most pronounced during the austral summer season, especially in the SPCZ, which is the focus here. During this period, storminess veers away from equatorial convection over the warm pool and often appears to connect with the baroclinic zone of the extratropical South Pacific. Numerous studies have examined these diagonal cloud bands and occasionally described the convergence zones as regions where cold fronts become quasi-stationary, yet a dynamical explanation of these features is lacking. We propose a general hypothesis, based on the kinematic behavior of synoptic waves, which explains why convection veers southward into the higher latitudes of each basin.;We examine in detail the South Pacific climate using a combination of satellite observations and reanalysis data, prior to analyzing simulations of the South Pacific climate in two atmospheric general circulation models (GCMs). One model is coupled and the other one is uncoupled with the ocean. In the equatorial region of the SPCZ, convection slowly varies on intraseasonal timescales, while higher frequency variability is observed poleward of 20°S where synoptic disturbances provide most of the convective variance. The subtropical jet stream transects the South Pacific, but upper tropospheric (200 hPa) zonal winds decelerate around 150°W in a region just southwest of the maximum zonal sea surface temperature (SST) gradient. Decelerating zonal winds are collocated with the deepest convection at that latitude (20°S-35°S).;Many global climate models are unable to simulate the poleward veering of convection in the SPCZ for a variety of reasons; however, the two atmospheric models examined here both depict the salient properties of the South Pacific climate, although subtle biases exist. In the coupled ocean-atmosphere GCM, a spurious horizontal band of convection extends east from the tropical SPCZ towards the equatorial East Pacific where the meridional SST gradient is too large. The uncoupled GCM is forced by fixed monthly varying SSTs and convection biases are much smaller in the East Pacific; however, the simulated SPCZ appears weaker and more zonally oriented than observed. In spite of these biases, each model simulates convection extending poleward in a diagonal band collocated with decelerating 200 hPa westerly winds. We further utilize the uncoupled GCM, due to its inherent simplicity, in a series of controlled numerical experiments which examine the long wave circulation response to altered boundary conditions, such as a prescribed SST gradient.;Based on the long wave circulation patterns identified in observations and simulations, we explain the physical mechanisms that promote the diagonal orientation of the SPCZ and the processes that determine the timescales of its variability. Previous studies have argued that slowly varying SST patterns produce upper tropospheric wind fields that vary substantially in longitude. In regions where 200 hPa zonal winds decrease with longitude (i.e., negative zonal stretching deformation, or ∂U/∂x < 0); the group speed and scale of eastward propagating synoptic (3-6 day period) Rossby waves is reduced and the wave energy density locally increases. We find that such a region of wave accumulation occurs in the vicinity of the SPCZ where the subtropical jet decelerates, 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, ∂ U/∂x = 0 demarks the boundary of the graveyard while regions where ∂U/∂x < 0 denote the graveyard itself.;We use a series of idealized numerical experiments to determine factors that control the Southern Hemisphere circulation patterns which produce negative zonal stretching deformation in the SPCZ region and then use statistical analyses of observations to provide evidence that wave energy accumulation occurs there. The experiments confirm that the Pacific zonal SST gradient forces a region of ∂U/∂x < 0 that is collocated with the deepest convection in the subtropics. Observations of the large-scale circulation response during El Nino, when the zonal SST gradient is typically small, support the modeling results. Composites of the life cycles of synoptic disturbances confirm the hypothesis that wave propagation slows and storms become more energetic in the ∂U/∂x < 0 region. The vertical structure of these disturbances also becomes enhanced as wave energy accumulates, further explaining why convection increases in the SPCZ.;From the wave energy accumulation results and 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. The South Atlantic basin shares many climate features with the South Pacific and wave energy accumulation appears to also occur in the South Atlantic Convergence Zone (SACZ), although zonal SST gradients are smaller and land surface heating more directly forces the basic state circulation near South America. Fewer similarities are evident in the Indian Ocean. Remaining questions concern how the SPCZ may feedback onto the underlying SST pattern and planetary wave structure. Interaction between convection and the long wave circulation also suggests teleconnection avenues between each of the Southern Hemisphere convergence zones.