Processes controlling the mean tropical Pacific precipitation pattern
Takahashi, Ken, 1975-
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In this dissertation, mechanisms responsible for the climatological large-scale distribution of precipitation in the tropical Pacific are addressed, in particular the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). This study is performed using numerical experiments with idealized ocean-atmosphere models.First, it is shown that the presence of the Andes can lead to the observed north-south asymmetry in the eastern Pacific, which features lower surface temperatures south of the equator and the intertropical convergence zone (ITCZ) in the northern hemisphere. This results from the non-linear physics that governs the zonal mean flow as it interacts with the Andes, which leads to subsidence of dry air to the west of the mountains and, therefore, to surface cooling through latent heat fluxes. The resulting low surface temperature and moisture suppresses deep convection south of the equator, which confines convection to the northern hemisphere. The orographically-induced subsidence is further enhanced by air-sea interactions. The orography in the northern hemisphere has a similar and opposing effect, but the asymmetry set by the Andes farther to the east extends westward sufficiently far to oppose the tendency for the ITCZ to move southward due to the mountains in the northern hemisphere. It is this longitudinal offset of the major mountain ranges in the western hemisphere that allows the effect of the Andes to dominate. However, the positive feedback between sea surface temperature and low-level cloud cover is an important element for the forcing by the Andes to prevail. The presence of a strong east Pacific equatorial cold tongue and, to a lesser extent, the seasonality of insolation are shown to have a weaker (symmetrizing) influence on the ITCZ.The SPCZ and the dry zone in the southeast Pacific, emerge from the idealized atmosphere-slab ocean system when the Andes are added to the otherwise symmetric aquaplanet configuration. It is shown that, in this setting, the horizontal structure of the dry zone, and therefore of the SPCZ, can be understood in terms of advection of low moist static energy air by the southeasterly trades and local air-sea interaction. According to this view, the tilt of the eastern edge of the SPCZ coincides, to zeroth order, with the direction of the trade winds. It is also shown that air-sea interaction within the SPCZ region is an important positive feedback to its organization.A general result from this study is that the mean large-scale distribution of sea surface temperature in the tropical Pacific can be explained to a large extent by the distribution of the specific humidity of the air brought into contact with the surface, which can be traced back to the forcing by orography (mainly the Andes). Hence, the mean distribution of precipitation can be understood as resulting from the organization of the atmosphere to orography forcing, with the ocean response acting as an important feedback.
- Atmospheric sciences