Intraseasonal temperature variability in a shelf-incised valley in the Southwestern Tropical Atlantic

Abstract

ABSTRACT Temperature variability in shelf-incised systems remains poorly constrained, particularly in the Southwestern Tropical Atlantic (SWTA), where boundary-current variability, remote atmospheric forcing, tides, and complex topography may interact across multiple timescales. The intraseasonal temperature variability within the Zieta shelf valley (Zsv) was investigated using a thermistor record from March to July 2017 at a depth of 82 m (near the bottom) and evaluated against potential physical drivers. The analysis combined Ensemble Empirical Mode Decomposition, Hilbert-based time-frequency diagnostics, and Time-Dependent Intrinsic Correlation, together with reanalysis fields of wind stress, sea-surface height, temperature, salinity, barotropic tidal constituents, and intermediate-depth current observations from a nearby mooring. Temperature variability exhibited a broad range (25.33-28.27 °C; standard deviation 0.53 °C) organized into six main variability scales with average periods ranging from ~2.4 to 93 days. The dominant mode (~68 days) was associated with westward-propagating sea-surface-height anomalies, consistent with large-scale oceanic variability reaching the shelf environment. A lower-frequency feature near 93 days was related to intermediate-depth current variability, although it was represented by fewer than two cycles and is therefore interpreted cautiously. At higher frequencies, the 4.9/2.4-day modes showed significant correlations with remote zonal/meridional wind-stress and subsurface current, with characteristic lags. Temperature also exhibited a significant fortnightly signal (10.3-18.6 days) consistent with spring-neap tidal modulation. Stratification over the adjacent slope remained favorable for internal-wave propagation during much of the record, but energy decreased with the deepening of the mixed layer by the end of the time series, suggesting that background water-column structure modulated the expression of these signals at 82 m. Overall, temperature variability reflected the combined influence of local and remote processes acting across multiple timescales. Because the record is short and supported by reanalysis, which does not resolve local topography, the detected relationships should be viewed as physically suggestive rather than conclusive, and representative only of the March-July seasonal window.