Spatial Variability of Surface Waves and Nearshore Currents Induced by Hurricane Harvey along the Southern Texas Coast

Extreme weather events such as hurricanes are expected to become more severe with the human-induced increase in average global temperatures, exacerbating the risk of major damage. Efforts to predict these events typically require detailed hydrodynamic data that are difficult to collect in the field. Here, nearshore data collected with three ADCP moorings were used to describe the hydrodynamics induced by Hurricane Harvey along the southern Texas coast. Wave spectra and nearshore current variations were analyzed along the hurricane’s trajectory and compared to other offshore locations. The results indicate that winds intensified along the coast as Harvey approached the Port Aransas coastline. Southerly wind stresses of ~−0.9 Nm−2 generated ~2 ms−1 depth-averaged flows towards the southwest close to landfall in the north, while flows of ~1 ms−1 and −1 were measured in the center and the south of the study site, respectively. The hydrodynamics induced by the hurricane were compared to those induced by an intense synoptic-scale cold front (CF). Both events generated southward-directed alongshore wind stresses of similar magnitudes (τy ~−0.4 Nm−2) that caused similar depth-averaged flows (0.5 to 0.7 ms−1) and wave energy conditions (Hs of ~4 m) in the south. Harvey caused extremely energetic conditions close to landfall in the north compared to the CF; depth-averaged flows and Hs of 2 ms−1 and 10 m were induced by Harvey, as opposed to 0.6 ms−1 and 4 m by the CF, respectively. While intense currents (>1 ms−1) and waves (Hs > 4 m) lasted for less than a day during Harvey, these persisted a few days longer during the CF. This study highlights the relevant role of synoptic-scale cold fronts in modulating the nearshore hydrodynamics, which occur more frequently than tropical cyclones in the northwestern Gulf of Mexico

Sensitivity of the Wave Field to High Time-Space Resolution Winds during a Tropical Cyclone

The impact of the high space-temporal variability of the wind field during the moderate and intense storm stages of a tropical cyclone on the wave field as computed by the numerical model WaveWatch III is investigated in this work. The realistic wind fields are generated by a high-resolution implementation of the HWRF model in the Gulf of Mexico and stored over 15 min intervals. The spatial structure of the wind field computed by HWRF is highly variable in space and time, although its mean structure is very similar to that described for parametric hurricanes already specified in the previous studies. The resulting storm-generated wave fields have a persistent structure, with wave maxima present in the forward quadrants of the storm and in the rear quadrant II. This structure is determined by the strong winds and the extended fetch condition in quadrants I and II, as well as by the translation speed of the storm. When a shorter time interval is analyzed (e.g., a 3 h period, when the storm becomes a category 1 hurricane), the structure of the mean wind field may differ greatly from the mean field calculated with a sufficiently longer period; however, the spatial distribution of the wave field around the hurricane tends to maintain its typical spatial structure. The use of wind fields with reduced time variability (e.g., with a 3 h moving average) does not change the structure of the mean wave field, but reduces the mean wave height values by up to 10%

Comparing GlobCurrent dataset with numerical results from a high-resolution implementation of the POLCOMS-WAM coupled system under a strong gap wind over the Gulf of Tehuantepec

GlobCurrent provides a variety of datasets aiming to describe global ocean circulation, especially when dealing with large-scale phenomena. It includes surface Stokes drift and geostrophic, Ekman, and total (geostrophic plus Ekman) currents. GlobCurrent uses the CNES-CLS13 mean dynamic topography estimation as well as data from sea surface drifters and wind reanalysis to improve the computation of ocean currents from altimetry data, which represents a significant advance in describing the total ocean current. The aim of this work is to compare the surface GlobCurrent estimates with a coupled ocean–wave numerical simulation (POLCOMS-WAM), drifting buoys, and altimeter observations when dealing with a Tehuano event, i.e., intense (larger than 20 m s −1) and short duration (around 3–5 days) low-level winds blowing over the Gulf of Tehuantepec, Mexico. There is a good agreement between the wind-driven currents (Ekman currents plus Stokes drift) field from GlobCurrent and that estimated by POLCOMS–WAM, with the largest magnitudes ∼0.8 m s−1 in the region influenced by the highest winds’ speed. The geostrophic circulation patterns in the Gulf of Tehuantepec are similarly reproduced by GlobCurrent and POLCOMS-WAM. However, some differences were observed in the presence of an anticyclonic eddy located in the western part of the study area. Numerical results exhibit a more symmetrical eddy with geostrophic current speeds that, in agreement with along-track observations, exceed the 1 m s−1. Instead, the geostrophic eddy in GlobCurrent shows velocities of about 0.8 m s−1. As observed through drifting buoys in 2000, numerical results show that the anticyclonic eddy west of the GoT has strong ageostrophic currents related to a cyclogeostrophic balance, which is not included in GlobCurrent. This regional case study provides a guideline for future improvements of GlobCurrent products, in particular for the estimation of geostrophic and total currents

A New Coupled Ocean-Waves-Atmosphere Model Designed for Tropical Storm Studies: Example of Tropical Cyclone Bejisa (2013-2014) in the South-West Indian Ocean

International audienceOcean-Waves-Atmosphere (OWA) exchanges are not well represented in current Numerical Weather Prediction (NWP) systems, which can lead to large uncertainties in tropical cyclone track and intensity forecasts. In order to explore and better understand the impact of OWA interactions on tropical cyclone modeling, a fully coupled OWA system based on the atmospheric model Meso-NH, the oceanic model CROCO, and the wave model WW3 and called MSWC was designed and applied to the case of tropical cyclone Bejisa (2013–2014). The fully coupled OWA simulation shows good agreement with the literature and available observations. In particular, simulated significant wave height is within 30 cm of measurements made with buoys and altimeters. Short-term (< 2 days) sensitivity experiments used to highlight the effect of oceanic waves coupling show limited impact on the track, the intensity evolution, and the turbulent surface fluxes of the tropical cyclone. However, it is also shown that using a fully coupled OWA system is essential to obtain consistent sea salt emissions. Spatial and temporal coherence of the sea state with the 10 m wind speed are necessary to produce sea salt aerosol emissions in the right place (in the eyewall of the tropical cyclone) and with the right size distribution, which is critical for cloud microphysics