CONICET | Buscador de Institutos y Recursos Humanos

Clouds cover about two-thirds of the Earth and are a key element of the climate system, playing an important role in the Earth's energy balance and water cycle at both global and local scales. Clouds can also have a major impact on, for example, agricultural production. Although a slow but steady decrease in global total cloud cover (TCC) has been observed, with most of the decrease occurring in mid-latitude regions, few studies have examined the specifics of TCC changes in South America (SA), and none in the densely populated, highly productive sub-region of Southeastern South America (SESA). Since the impacts of climate change are generally experienced at local and regional scales, and thus climate information at these spatial scales is considered to be more impact and risk-relevant climate change information, the objective of the present study is to present the climatology and the observed long-term changes and variability of TCC in Northeastern Argentina (NEA), a SESA sub-region of high economic and demographic importance, as it concentrates most of the country's agricultural production and population. In this study, we present the climatology and observed long-term changes in TCC in the NEA. The analyses are based on two independent datasets: TCC ground-based (GB) observations and satellite-based estimates from the International Satellite Cloud Climatology Project (ISCCP). The datasets cover a common period, from December 1983 to November 2016 (satellite period), while the GB TCC observations extend further, from March 1961 to February 2021 (GB period). To facilitate a more comprehensive study of GB TCC, we introduce a novel cloud index (CI), expressed in a familiar unit (%), which allows the study of its temporal variations. The TCC shows a distinct annual cycle and considerable spatial variability over the NEA, which is consistently evident in both satellite and GB datasets. Although spatially heterogeneous, negative seasonal trends for CI are observed throughout the satellite period, with the largest decreases in the months of March to April (MAM), followed by December to February (DJF). For the GB period, negative seasonal trends for CI are also observed, but the largest decreases are in December to February (DJF) followed by June to August (JJA). During the satellite period, CI seasonal trends did not show statistical significance for any season. We ensured the reliability of our results by comparing the two datasets, which showed similar temporal variability, although the ISCCP cloudiness values were larger than the GB TCC. The observed decrease in total cloud cover (TCC) in this study is consistent with the equatorward shift of the Hadley cell over South America over the last 60 years, which has led to an increase in the sinking motion over the subtropical latitudes of South America (SSA), including northeastern Argentina (NEA) (Saurral et al., 2017). Furthermore, the weakening of summer northerly winds over NEA in recent decades (Zilli et al., 2018) is also consistent with the decreasing trend in TCC observed in our study. Variations in local conditions (e.g. changes in land use) could also explain the TCC trends studied here. In addition, our study examines the interannual variability of the Cloud Index (CI) over the NEA, both annually and seasonally. For this purpose, we estimated the linear regression between the CI and several ERA5 reanalysis variables over SA, including temperature, total precipitation, outgoing longwave radiation flux, horizontal moisture flux at 850hPa, vertical integrated moisture divergence, and geopotential height at 850hPa. The regression maps show that cloudiness variability in the NEA is mainly related to the regional dipole between the SESA and the South Atlantic Convergence Zone (SACZ), which has been previously documented for other variables in the region (Vera et al., 2006; Grimm and Zilli, 2008). This signature is detected annually and in all seasons except the austral winter. In addition, we investigate the remote influence of SST anomalies on CI in the NEA, disentangling the influence of El Niño events on CI dynamics. This influence is found to be more important in the austral spring, in agreement with Grimm and Zilli (2008).Overall, our results provide evidence for a consistent decrease in cloudiness over the NEA over the last six decades, based on ground-based and satellite information, and provide valuable insights into the complex interplay between cloudiness and regional climate dynamics. All this provides a basis for informed decision-making in a region of economic and agricultural importance in the context of a changing climate.

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