Changing Trends in Rainfall Extremes in the Metropolitan Area of São Paulo: Causes and Impacts

Jose A Marengo,Lincoln M Alves,Naurinete J C Barreto,Tercio Ambrizzi,Andrea M Ramos,Michelle Simões Reboita

doi:10.3389/fclim.2020.00003

Abstract

This study analyses observed trends in extreme rainfall events in the Metropolitan Area of São Paulo (MASP). Rainfall data sets with more than 60 years of record in MASP are used. In MASP, extreme rainfall events represent hydro meteorological hazards that trigger flash floods and landslides. Changes in rainfall extremes can be partly due to natural climate variability. In addition, it can also be related to global warming and/or urbanization. Total annual precipitation and the number of days with precipitation of 20 mm exhibit the largest significant increase during 1930-2019. This is better noticed during summer. This tendency is also noticed in the number of days with precipitation of 100 mm or more. Therefore, the positive trend in annual precipitation is mainly due to an increase in the frequency of extreme precipitation events. On the other hand, our analysis shows that the number of consecutive dry days increased. Though these results appear to be contradictory, they indicate an important climate change in recent times. Intense precipitation is concentrated in few days, separated by longer dry spells. The focus is on how atmospheric circulation variations are contributing to these changes. During 1960-2019 the South Atlantic Subtropical Anticyclone has intensified and slightly moved southwestward of its normal position. This change influences the transport of humidity and therefore impact precipitation. This can explain the increase in the precipitation extremes in the MASP. However, other atmospheric systems may also be important.

Highlights

Episodes of heavy rainfall and wet and dry spells impact urban and rural areas worldwide
In this study we provide updated information on trends in total and extreme rainfall events over the Metropolitan Area of São Paulo (MASP) until 2019
This study presents an updates analysis of trend of annual precipitation and rainfall extreme indices for two meteorological stations in the MASP region

Summary

Introduction

Episodes of heavy rainfall and wet and dry spells impact urban and rural areas worldwide These extremes trigger natural disasters such as floods, flash floods, or droughts. Extremes of weather and climate affect mainly risk areas where vulnerable people live (Bouwer, 2019), like urban areas, where most of the population is concentrated. More people live in urban areas (54%) than in rural ones, and by 2030, it is projected to the world to have 41 mega-cities with 10 million inhabitants or more (UN-Habitat, 2015). This is not going to improve with urbanization and occupation of risk areas under climate change. Megacities in Latin America would be among the most affected (Vergara, 2008; Magrin et al, 2014)

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