chapter - Atmospheric and Oceanic Science

Background on other regional aspects: Land use change, aerosols and trace gases
10.3.2 Urban effects
Some features related with the urbanization effect over urban heat island
(UHI) development have been clearly demonstrated over large urban areas in the
La Plata Basin. In particular, observational studies in the Metropolitan Area of São
Paulo (MASP) have indicated statistically significant impacts of the urbanization.
Xavier et al. (1994) suggest a possible relationship between precipitation in São
Paulo and the heat island effect. With increasing minimum temperature during the
night, the probability of saturation during the night decreases. Furthermore, pollution
increases the number of cloud condensation nuclei (CCN). The available water
vapour is distributed among a larger number of CCN's which tend to remain in suspension
in view of their smaller size and resulting decrease in the droplet fall velocity.
Thus, the number of days with precipitation below 2mm has gradually
decreased during the XX century. For intense precipitation (daily accumulation
greater than 30mm), the effect is reversed: the increased thermal instability and the
plausible effect associated with increased number of ice nuclei (associated with the
urban pollution) tend to increase the probability of heavy precipitation. More recent
results by Freitas and Silva Dias (2004) separate the radiative effect of the urban
aerosol plume and the heat island forcing. It is shown that the radiative effect of the
aerosols is significantly large, comparable to the thermodynamical forcing of the
surface thermal forcing associated with the heat island effect
Although changes in the MASP climate have been attributed to the surface
forcing and air pollution impact in cloud microphysics, remote effects associated
with long term changes in the SST patterns can also be responsible for the observed
changes. A diagnosis of the winter climatology cold temperature extremes in the
Metropolitan Area of São Paulo (MASP) is presented in Goncalves et al. (2002). The
diagnosis is based on temperature data at the Meteorological Station of Parque
Estadual das Fontes do Ipiranga (IAG/USP) from 1950 to 2000. The persistence of
synoptical and climatological patterns has been studied through principal component
(PC) analysis and the results are compared to monthly anomalies in sea surface temperature
(SST) of the Eastern Pacific and South Atlantic. The extreme cold air temperatures,
on monthly bases, have shown no significant change since 1950. On the
other hand, the mean monthly air temperatures have shown a slight warming trend,
in agreement to the South Atlantic Ocean warming trend. The PC indicates significant
loadings of two SST anomaly types: the cold anomaly of the South Atlantic
Ocean, and the warm anomaly of the Southern Brazilian coast. The latter could also
be responsible for some extreme cold events (for daily minimum temperatures) in
the MASP, also presenting dominant westerly wind direction (SW to NW). Both the
cold events and the westerly wind direction were evidenced in such winters as 1953,
1975, 1978, 1981 and 1994. On the other hand, the cold mean monthly temperatures
are highly correlated to a broad cold pool anomaly in the South Atlantic near 25 to
35°S and 15 to 55°W. Thus, SST anomalies in the South Atlantic Ocean have a dominant
effect on the S. Paulo winter temperature climatology.
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