Flood

Ring of storms around the Bolivian High

10 January 2007 00:00 UTC

Flood
Flood

In the early weeks of 2007, heavy rain caused many deaths in Rio de Janeiro and Sao Paulo resulting from severe flooding and related accidents.

Last Updated

25 May 2022

Published on

09 January 2007

By Jose Prieto (EUMETSAT) and Marcelo Seluchi (CPTEC/INPE)

The intense rain appears to be a response to a reactivation of the El Niño phenomenon.

The summer situation depicted below for 10 January 2007 shows a marked high pressure circulation centered over Paraguay and Bolivia at high levels (see synoptic chart of 250hPa, 10 January 2007, 00:00 UTC). This 'Bolivian High' (BH) reinforces through its southern boundary the subtropical jet over the border of Argentina and Brazil, and generates some convection in southern Brazil. The stronger convection over Bolivia and Peru results from low level winds meeting the Andes.

A second convergence zone is the cloud band extending from the east of Bolivia into the Atlantic Ocean, crossing Sao Paulo and Rio. A high-level trough with highly zonal axis favours instability in the SE of Brazil (see loop for 9 January, 00:00–23:45 UTC). The subtropical jet generates strong divergence north of Buenos Aires.

The polar jet cuts the Atlantic Ocean between 56 and 61 degrees south. In the 500hPa analysis (not shown) the northern part of Chile is affected by a cyclonic vortex, and the west of Patagonia by a trough. Both systems favour the transport of humidity over the Andes and the instability in the north of Argentina, Uruguay and Paraguay. At the same time, there is an active Inter-Tropical Convergence Zone (ITCZ) between 30 and 42 W, and associated 'pulses' of humidity reach the northeast and north of Brazil.

Convection conditions

An example of generalised convection in Northern Brazil is presented for 11 January 2007 (see loop for 11 January, 10:15–20:00 UTC). On that day, higher temperatures and more moisture over eastern Brazil resulted in increased convection. Generally, precipitation in the Nordeste (northeast semi-arid region of Brazil) is controlled by the location of the convergence band over the Atlantic and the temperature of the equatorial surface waters (see zoomed loop for 11 January, 10:15–20:00 UTC).

Ring of storms around the Bolivian High
Figure 1: Meteosat-8 RGB Composite, 10 January 2007, 20:30 UTC. Interpretation. Animation (09:00–19:00 UTC)