Spatial and temporal variability of mode-1 and mode-2 internal solitary waves from MODIS/TERRA sunglint off the Amazon shelf

Abstract. The Amazon shelf is a key region for intense internal tides (ITs) and nonlinear internal solitary waves (ISWs) generation associated with them. The region shows well-marked seasonal variability (boreal Summer/fall ASOND vs spring MAMJJ) of the circulation and stratification, which can both induce changes in the ISWs physical characteristics. The description of the seasonal and neap-spring tidal variability of the ISWs off the Amazon shelf is performed for the first time using a meaningful data set composed of more than a hundred MODIS/TERRA imagery from 2005 to 2021, where more than 500 ISW signatures were identified in the sun glint region. Previous studies have documented the existence of mode-1 ISW, but the region appears as a newly described hotspot for mode-2 ISWs. ISWs packets separated by typical mode-1 (95–170 km; 2.1–3.8 m s-1) and mode-2 (46–85 km; 1.0–1.9 m s-1) ITs wavelengths have been identified and mapped coming from sites A, B, and F. Site A likely shows a higher ISW activity, after two patches of reflection (first - 150 km from IT generation point on the shelf break, second - 260 km from shelf break), because waves emanating from site D are focusing on the same propagation path (third patch - 380 km from the shelf break). Patches of higher occurrence of ISWs appear separated by typical mode-1 wavelength likely corresponding to the reflection beams at the surface. A fourth patch structured as a tail with finer scales might indicate some region of instability, a transfer to higher modes or dissipation. The range and values of mode-1 and mode-2 propagation velocities/wavelengths do not show significant differences according to areas A and B. Mode-2/mode-1 ratio is larger for site B likely linked to shallower pycnocline with higher maximum values when compared to area A. The wave activity is higher during spring tides than neap tides (for both A and B sites). During ASOND, mode-1 ISWs from A exhibit higher wave propagation velocities/wavelengths than MAMJJ. In contrast, no seasonal variation of mode-2 propagation velocities/wavelengths was found. During ASOND in area A, the reinforcement of the North Equatorial Counter Current appears to play a role in deviating the waves towards the northeast, increasing their phase velocities and their eastern traveling direction component which gives them an extra offshore acceleration. The impact of the circulation on the propagation velocities/wavelength is even more evident for the shorter-scale waves. During ASOND, when the circulation has higher small-scale variability the ISWs propagate in a wider pathway and have a higher diversity of propagation velocities. Calculations of the IT velocities using the Taylor-Goldstein equation supported our results of the presence of mode-2 ISWs associated with mode-2 IT wavelengths in the study area and additionally into the ISW/IT seasonal variability in terms of waves with higher diversity and higher mean values of wavelength during ASOND.