Cold spells in the Amazon: How friagem events influence atmospheric chemistry

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.

Os eventos da Friagem trazem muitas nuvens e muita chuva com eles. © Steffen Schmidt / MPI-BGC
Os eventos da Friagem trazem muitas nuvens e muita chuva com eles. © Steffen Schmidt / MPI-BGC

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.

Figure altered from Camarinha-Neto et al. (2021). It shows the radiation (top), ozone concentration (middle) and CO2 concentration (bottom) in the days leading up to and during the Friagem event.
Figure altered from Camarinha-Neto et al. (2021). It shows the radiation (top), ozone concentration (middle) and CO2 concentration (bottom) in the days leading up to and during the Friagem event.

Similar articles

A starry night at ATTO. © Andrew Crozier / MPI-C

Polari Corrêa and his co-authors analyzed the atmospheric dynamics in and above the forest canopy during one particular night at ATTO. Those conditions changed throughout the night. Turbulence was followed by the formation of a gravity wave and a low-level jet. It was likely formed due to the breeze from the Uatumã River and the hilly terrain. The study highlights the complex dynamics and mechanisms in the atmosphere above a dense forest.

Air inlet tube at the top of the ATTO tall tower. © Eva Pfannerstill / MPI-C

Biogenic volatile organic compounds remove OH from the atmosphere through chemical reactions, which affects processes such as cloud formation. In a new study, Pfannerstill et al. reveal the important contributions of previously not-considered BVOCs species and underestimated OVOCs to the total OH reactivity.

The majority of global precipitation is formed through the pathway of ice nucleation, but we’re facing large knowledge gaps that include the distribution, seasonal variations and sources of ice-nucleating particles. To fill some of those knowledge gaps, Jann Schrod and his co-authors produced a record of long-term measurements of INPs. They collected data for nearly two years at four different locations. One of those sites was ATTO.

The shadow and the light of a low sun highlight the gentle topography under the Amazon forest, which affects atmospheric turbulence. © Paulo Brando

Chamecki and his co-authors analyzed if the gentle topography underneath the Amazon rainforest impacts atmospheric turbulence. They published their results Open Access in the Journal of the Atmospheric Science.

convective storm over the forest

Convective storms often occur in the tropics and have the potential to disturb the lower part of the atmosphere. They might even improve the venting of trace gases out of the forest canopy into the atmosphere above. To better understand these processes, Maurício Oliveira and co-authors used the infrastructure at ATTO to study storm outflows during nighttime. They published the results in a new paper in the Open Access Journal Atmospheric Chemistry and Physics.

The footprint region of ATTO shifts from pristine northeast to more human influenced southeast

Christopher Pöhlker and co-authors published an extensive new paper, characterizing the footprint region of ATTO. They hope that fellow researchers in the Amazon region can use this publication as resource and reference work to embed ATTO observations into a larger context of Amazonian deforestation and land-use change. Pöhlker et al. published the paper Open Access in Atmospheric Chemistry and Physics Volume 19.

The Amazon rainforest interacts with the atmosphere by exchanging many substances. Many of these, such as carbon dioxide, methane, ozone, and organic compounds, are produced by the vegetation. They are very influential in both the regional and global climates. Until now, the estimates of their emission and absorption rates are based on classical theories. But those were developed over relatively short vegetation and are valid for the so-called “inertial sublayer.”

Aquino et al. published a new study in Agricultural and Forest Meteorology about the characteristics of turbulence within the forest canopy at two Amazonian sites. They found that the air layer close to ground is largly decouples from the air layer in the upper canopy and above.