Abstract
Abstract. Extratropical cyclones are known to generate extreme significant wave height (swh) values at the ocean surface in the western South Atlantic (wSA), which are highly influenced by intraseasonal scales. This work aims to investigate the importance of intraseasonal timescales (30–180 d) in the regional climatology of waves and its atmospheric forcing. The variability is explained by analyzing the storm track modulation due to westerly winds. These winds present timescales and spatial patterns compatible with the intraseasonal component of the Pacific South American (PSA) patterns. The analyses are made using ECMWF’s ERA5 from 1979 to 2019 and a database of extratropical cyclones based on the same reanalysis. Empirical orthogonal function (EOF) analyses of the 10 m zonal wind and swh are used to assess the regime of westerlies and waves in the wSA. The EOF1 of the 10 m zonal wind (u10) presented a core centered at 45∘ W and 40∘ S, while the EOF2 is represented by two cores organized into a seesaw pattern with a center between 30–40∘ S and another to the south of 40∘ S. Composites of cyclone genesis and track densities as well as swh fields were calculated based on the phases of both EOFs. In short, EOF phases presenting cores with a positive (negative) u10 anomaly provide a favorable (unfavorable) environment for cyclone genesis and track densities and, therefore, positive (negative) swh anomalies. The modulation of the cyclone tracks is significant for extreme values of the swh. The spatial patterns of the EOFs of u10 are physically and statistically consistent with 200 and 850 hPa geopotential height signals from the Pacific, indicating the importance of the remote influence of the PSA patterns over the wSA.
Highlights
Ocean surface gravity waves or wind waves are relevant for a number of socioeconomic activities over the coast
The western subtropical South Atlantic (wSA) seasonal wave climatology description depicts a region with wave parameters influenced by easterlies from the South Atlantic subtropical high (SASH) and frequent (3 to 5 times per month) S/SE wind associated with cold fronts (Pianca et al, 2010)
Can this intraseasonal variability be linked to Pacific South American (PSA)? Since the surface wind field forces the wave field, it is used here as a proxy of the atmospheric variability, associated with the geopotential height at 200 hPa (Z200) and 800 hPa (Z850)
Summary
Ocean surface gravity waves or wind waves ( just called “waves”) are relevant for a number of socioeconomic activities over the coast They may impact the safety of operations in ports, oil shelves and present relevant coastal impacts related to erosion and building damage (de Andrade et al, 2019). These types of waves are generated by the wind, and, extratropical cyclones (hereafter just “cyclones”) are responsible for driving most of the wave pattern at middle and high latitudes, generating extreme conditions in the western subtropical South Atlantic (wSA) (Fig. 1), where significant wave height (swh) may present values as high as 10 m (Gramcianinov et al, 2020c). In the wSA, cold fronts are synoptic features associated with cyclones, which can generate
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