Although located in the tropics, the Amazon sporadically experiences incursions of cold waves from the high latitudes of the Southern Hemisphere. These incursions are called friagem events. They significantly impact the weather patterns during the time they occur, causing for example a temperature drop and increased cloudiness. Therefore, it seems likely that these events might also impact the chemistry of the atmosphere and its trace gas concentrations.
Friagem events and the weather
To find out if that is indeed the case, Guilherme Camarinha-Neto and his colleagues studied a friagem event at ATTO. They also collected data at the airports of Porto Velho in the Southwest of the Amazon basin and of Manaus for comparison. The event took place in July 2014. Friagem events are generally more common in the dry season, from July to September, and 2-3 events for the year are not unusual.
First of all, they found the same weather pattern for this friagem: the temperature dropped significantly compared to the average July values in all locations. However, the cooling was somewhat more pronounced in Porto Velho. When the event arrived in Manaus and at ATTO a few days later the temperature dropped by 4°C, instead of the 7°C recorded at the edge of the Amazon basin. They also noticed that the predominant wind direction changes from the common East to South-East to West to South-West. Finally, the team also observed that the interaction between the friagem air mass and the trade winds caused strong convection. This resulted in cloud formation and record rain for the usually rather dry month of July.
Friagem events and atmospheric chemistry
In addition to the meteorological parameters, the team also analyzed the chemistry of the atmosphere across the event. Specifically, they looked at ozone and CO2. Before the friagem arrived at ATTO, they measured values close to the monthly averages. However, during the cold spell, the ozone values dropped, and the CO2 values increased.
The reason is probably the cloudiness associated with the event. The forest plants, which require sunlight for photosynthesis, were less effective on this cloudy and rainy day. Consequently, they could take up less CO2, so it accumulated in the atmosphere. Clouds and variations in radiation are also known to affect the daily cycle of ozone.
This was likely amplified by strong vertical stratification of the air column. By combining observations with computer simulations, the scientist concluded the friagem only affected the lower 500 meters of the atmosphere. You can think of it as a thin tongue of cold air above the forest, which prevents vertical mixing with the air above.
Camarinha-Neto et al. published the study “The friagem event in the central Amazon and its influence on micrometeorological variables and atmospheric chemistry” Open Access in Atmos. Chem. Phys. Doi: https://doi.org/10.5194/acp-21-339-2021
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