South Atlantic abyssal temperature variability and trends at 34.5°S

Maurício R. Rocha , Edmo J. D. Campos, Alberto R. Piola, Mathias C. van Caspel, Paulo Simionatto Polito, Cesar B. Rocha

Warming of abyssal waters in the South Atlantic has been reported in recent years. However, the variability of bottom temperatures and its implications for assessing linear trends have received limited attention. In addition to confirming positive temperature trends, this study documents substantial bottom-temperature variability, from tidal to annual time scales, captured by bottom moorings deployed at two previously unobserved sites at 34.5°S: Site E (31.13°W, 4007 m) and Site F (18.82°W, 3389 m). The record spans January 2019 to January 2023, with measurements collected every 30 minutes. At each site, temperature sensors were placed approximately 50 m above the seafloor. Both sites exhibit bottom-temperature variance concentrated in the 3–12-month band, consistent with a semiannual cycle, and a dominant semi-diurnal tidal peak. Site E displays an unexpectedly strong signal at ~4-day periods, accounting for ~10% of the total variance, and a larger warming trend (13.0±0.4 m°C/yr) compared to Site F (4.7±0.2 m°C/yr). Differences in the temperature variance spectra between sites, along with a wavelet coherence analysis, suggest that local processes (e.g., topographic eddies and internal waves) play a significant role in driving bottom-temperature variability over 2 to 8 days. These results underscore the importance of sustained deep-ocean monitoring to distinguish long-term trends from short-term fluctuations and to elucidate the mechanisms driving bottom-temperature changes in the South Atlantic.