the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Oct 2024
Two different mechanisms cause severe hot droughts in the western Amazon
Abstract. In the last few decades, the Amazon basin has experienced several periods of simultaneous extreme drought and heat. It is crucial to gain a better causal understanding of these compound events, which can severely damage the rain forest ecosystem. Here, we study the role of atmospheric mass, moisture and heat transport to the western Amazon, an area where pristine tropical rain forests can still be found. To investigate how anomalous the atmospheric transport during extreme events is compared to the climatological mean, we use a Lagrangian reanalysis dataset created with the particle dispersion model FLEXPART driven with meteorological input data from the ERA5 reanalysis. For the period 1979–2021, air masses over the western Amazon are selected every three hours and traced 10 days backward in time. We compared the overall transport climatology for the end of the dry season (August–September) with the transport for 21 extreme compound events that were selected for their extremity in terms of high air temperature and low soil water content. We find that extreme events over the western Amazon that happen during El Niño events have very different causes from those that take place under La Niña or neutral ENSO (El Niño Southern Oscillation) conditions. For the five extreme events that occurred under El Niño conditions, around 50 % of the air, as compared to 30 % for the climatology, is located over the tropical Atlantic Ocean 10 days prior to arrival. Air that is already anomalously dry and warm is transported from this region towards the western Amazon, highlighting the role of long-range transport for these extremes. The other 16 extreme events occurred under La Niña or neutral ENSO conditions and show similar transport patterns to the overall transport climatology until 3 days prior to arrival. During the last 3 days, however, air is preferentially transported over the southern Amazon, where already dry soil conditions cause a malfunctioning of moisture recycling, resulting in the propagation of extreme conditions to the western Amazon. Air arriving in the western Amazon during these events is travelling over areas heavily affected by deforestation. Therefore, we expect that landuse changes over the southern Amazon have a stronger impact on compound drought and heat extremes in the western Amazon under La Niña or neutral ENSO conditions than during El Niño events.
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RC1: 'Comment on egusphere-2024-2801', Anonymous Referee #1, 05 Nov 2024
In the present manuscript, the authors analyzed the source and path of air masses related to compound hot and dry events over the western Amazon. The topic is very relevant, given the recent droughts in the region. The proposed analysis is robust and provides valuable insights into the impacts of different circulations on the development of heat waves over the region. However, the categorization of the events into ENSO phases may not be the best option here. There is no clear difference in the characteristics of the events during El Nino compared to those during non-El Nino years. This weakens the conclusions of the manuscript. Thus, I recommend the rejection of the manuscript but strongly encourage its re-submission after restructuring the analysis. See more details below.
Major Comments
- My main issue with the current manuscript is the categorization of the drought events into ENSO phases. The selected events are clearly different from the climatology (outside the one standard deviation around the climatological mean), but there is no clear distinction between E+ and E0- events. In most of the graphics (Figs 3 to 5 and A1 to A3), the red lines indicating E+ events are within the envelope of the lines related to E0-. This is also evident when comparing the values of the variables listed in Table 1 and Table A1. On the other hand, there is a large variability in the E0- events, resulting in the median (dashed blue lines) being quite close to the climatology in some cases. In fact, some of the most interesting events are classified as E0-, as described in different parts of the manuscript. This makes me wonder if using ENSO events to classify the drought events is the best option here. I agree that there is a clear difference in the amount of air coming from southern Atlantic Ocean (SAT) and tropical Atlantic Ocean (TAT), as seen in Table A1 and Figures 2c, A3c and A4. Thus, maybe a better option is to categorize the events according to the source of the air masses and then verify if there is any influence of ENSO or other modes of variability on the events. Particularly, the western Amazon responds to anomalies in the northern tropical Atlantic Ocean and the ITCZ behaviour (as mentioned in the introduction but not explored in the manuscript), which, in turn, respond to the Southern Annular mode and the South Atlantic Dipole, for example. Thus, separating the events using a source criterium may lead to clearer results.
- In the conclusions, the authors mentioned the importance of the SST anomalies over SAT as a source of warm and dry air to the region. I agree that the Atlantic ocean plays an important role here (as mentioned in the previous point). However, the relationship between the SST anomalies and the characteristics of the air masses (temperature and humidity) is not clear. Most (but not all) E0- cases are related to hot and dry air masses from the Southern Atlantic, with warm SST anomalies. However, if the South Atlantic is warmer than normal, I would expect more moisture in the air, not less. This relationship should be better explained. Also, what is the source of the SST data? This is not mentioned anywhere in the manuscript.
- The information from the soil moisture could also be used to better understand the events. The dip in soil moisture anomalies in Fig 5e is quite relevant and shows that, over WAM, the soil moisture seems to be responding to the atmosphere. However, this is not true for all events, with some maintaining very low soil moisture throughout the 20-day period. Here, it is important to also the into account the land-atmosphere coupling and the characteristics of the previous months, not just 20 days, because of the large memory of the soil moisture. Other points related to the soil moisture data:
- How reliable is the soil moisture data in ERA5? This region has very few measurements so it would be interesting to have some reference about it.
- How different from climatology are the anomalies in Fig 5d and e? It would be helpful to add a measurement of standard deviation or significance to the graphic.
Minor comments
- Figure 1a and Eastern Amazon region (EAM). This region is in Northeastern Brazil, which is not part of the Amazon. This region is a semi-arid region, characterised by prolonged droughts. Thus, naming it Eastern Amazon is incorrect and misleading.
- Page 8, lines 144-146: “Even the air arriving in the lowest 1 km of the WAMt originates from mid-tropospheric levels (mostly over the TAT region), likely air that has previously been lifted by convection over the eastern part of the TAT region or Africa.” Is there any reference that supports the statement in the second part of the sentence?
- Page 9, lines 197-199: “Overall, there seems to exist a stronger surface-atmosphere coupling over the continent during the last few days of transport, and a tendency for some cases for this to occur over the SAM region.”. Which results corroborate this statement?
- Figure 6: the blue region labelled +SSTA over the Southern Atlantic Ocean is located south of 30S, while the results in Fig 1b and A4 indicate that the air masses into WAMt are originating over tropical Atlantic (north of 20S in the majority of the cases). Together with the blue arrow pointing into the region, this figure can be misinterpreted as indicating some influence of extratropical air masses into the region.
- Table A1: what is the whole Amazon region AMZ in the 5th column? Is it the combination of SAM, WAM and EAM?
Citation: https://doi.org/10.5194/egusphere-2024-2801-RC1 -
RC2: 'Comment on egusphere-2024-2801 - to editor', murilo rev lemes, 11 Nov 2024
Caro Editor
Primeiramente, gostaria de agradecer pelo envio do artigo. O artigo apresenta interesse científico significativo, especialmente para o estudo do transporte de umidade e calor para a Amazônia e também regionalmente. Acredito que meus comentários são específicos, visando principalmente melhorar a discussão e a qualidade do material. Todas as observações estão no arquivo .zip que enviei. Portanto, recomendo sua publicação após abordar os comentários.
Obrigado!
Melhor,
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