Convection and Heat Transfer in Island (Warm) Wakes

Abstract

The interaction between the incoming winds with high mountainous islands produces a wind-sheltered area in the leeward side, known as the atmospheric wake. In addition to weaker winds, the wake is also characterized by a clearing of clouds, resulting in intense solar radiation reaching the sea surface. As a consequence, a warm oceanic wake forms on the leeward side. This phenomenon detectable from space can extend 100 km offshore of Madeira, where the sea surface temperature can be 4⁰C higher than the surrounding oceanic waters. This study considers in-situ, remote sensing, and ocean circulation model data, to investigate the effects of the warm wake in the vertical structure of the upper ocean. To characterize the convective layer (25-70m) developing within the oceanic wake, 200 vertical profiles of temperature, salinity and turbulence were considered, together with the computation of the Density Ratio and Turner-angle. In comparison to the open-ocean water column, wake waters are strongly stratified with respect to temperature although highly unstable. The vertical profiles of salinity show distinct water parcels that sink and/or rise as a response to the intense heat fluxes. During the night, the ocean surface cools, leading to the stretching of the mixed layer which was replicated by the ocean circulation model. In exposed, non-wake regions however, particularly in the southeast and north coast of the island, the stretching of the mixed layer is not detectable.

Impact of Flash Flood Events on the Coastal Waters Around Madeira Island: The “Land Mass Effect”

“…The accuracy of the modeled surface wind was also assessed at three specific locations (points 1, 2, and 3 represented in Figure 3B), and are presented in Table 2 for both descending and ascending satellite passages. Points 1 and 3 are located close to the regions of the atmospheric jets (Alves et al, 2020(Alves et al, , 2021, and point 2 is in the southern region characterized by the frequent occurrence of warm wakes (Caldeira et al, 2002(Caldeira et al, , 2016Couvelard et al, 2012;Grubišic et al, 2015;Azevedo et al, 2021). In general, correlation coefficients were high (0.78-0.96) and RMSE values were low (mostly < 1.6 m s −1 ; Table 2), showing the strong performance of the model with respect to representing the wind variability.…”

“…Previous studies in this region focused on atmospheric and oceanic island processes, combining the use of mediumhigh resolution numerical models (Caldeira and Sangrà, 2012;Couvelard et al, 2012;Caldeira and Tomé, 2013;Nunalee and Basu, 2014;Pullen et al, 2017b;Alves et al, 2021;Miranda et al, 2021), with in situ and satellite data (Caldeira et al, 2001(Caldeira et al, , 2002(Caldeira et al, , 2014Grubišic et al, 2015;Alves et al, 2020;Azevedo et al, 2021). These studies revealed the complexity and variety of oceanic and atmospheric interactive processes which occur around the Data was estimated based on the drainage network computed from the Digital Terrain Model (described in section "MOHID-Land, the Hydrological Model") using QGIS version 3.14.1 (www.qgis.org).…”

“…The exchange of variables between the models that comprise the COAWST modeling system occurred every 30 min, in which: (i) the atmospheric model WRF provided 10 m surface winds to ROMS and SWAN and atmospheric pressure, air relative humidity, 2 m atmospheric surface temperature, cloud fraction, precipitation and short-wave and long-wave net radiative fluxes to ROMS; (ii) the oceanic model ROMS provided SST to WRF and surface currents, free surface elevation and bathymetry to SWAN; and (iii) the wave model SWAN provided significant wave height and wavelength to WRF and ROMS, and wave direction, surface and bottom wave periods, percent of breaking waves, wave energy dissipation and bottom orbital velocity to ROMS (Warner et al, 2010). This numerical framework was recently applied to the Madeira Archipelago (Pullen et al, 2017a;Alves et al, 2020;Azevedo et al, 2021), demonstrating that the coupling between the atmosphere and the ocean significantly improved model results. A recent study employing the COAWST demonstrated improved performance with respect to the replication of heavy precipitation events compared with stand-alone atmospheric models (Ricchi et al, 2021).…”

Seasonal variation in microplastics and zooplankton abundances and characteristics: The ecological vulnerability of an oceanic island system

“…Furthermore, a high mountain ridge (ca. 1800 m) in the island's interior obstructs the dominant northeast trade winds, leading to warmer and sheltered waters in the south of Madeira (Caldeira et al, 2002;Caldeira and Sangrà, 2012;Azevedo et al, 2021).…”

Vortex streets to the lee of Madeira in a km-resolution regional climate model

“…Their study highlights the necessity of considering heterogeneous atmospheres (especially in the presence of vortex streets) for optical wave propagation studies. With a focus on atmosphere-ocean interactions, Azevedo et al (2020) studied the detailed responses of upper oceanic profiles of temperature, salinity, and turbulence to the Madeira warm wakes, which mainly result from intense solar radiation in cloud-free conditions.…”