Atmospheric fronts embedded in extratropical cyclones are high-impact weather phenomena, contributing significantly to midlatitude winter precipitation. The three vital characteristics of the atmospheric fronts, high wind speeds, abrupt change in wind direction, and rapid translation, force the induced surface waves to be misaligned with winds exclusively behind the cold fronts. The effects of the misaligned waves on air-sea fluxes remain undocumented. Using the multi-year in situ near-surface observations and direct covariance flux measurements from the Pioneer Array off the coast of New England, we find that the majority of the passing cold fronts generate misaligned waves behind the cold front. Once generated, the waves remain misaligned, on average, for about 8 hours. The fully-coupled model simulations indicate that the misaligned waves significantly increase wave roughness length (300%), drag coefficient (30%), and momentum flux (20%). The increased surface drag reduces the wind speeds in the surface layer. The upward turbulent heat flux is weakly decreased by the misaligned waves because of the compensating effect between the decrease in temperature and humidity scaling parameters and the increase in friction velocity. The misaligned wave effect is not accurately represented in a commonly used wave-based bulk flux algorithm. Yet, the suggested modification to the current formulation improves the overall accuracy of parameterized momentum flux estimates. The results imply that better representing a directional wind-wave coupling in the bulk formula of the numerical models may help improve the air-sea interaction simulations under the passing atmospheric fronts in the midlatitudes.

Ocean-atmosphere coupled climate models struggle to produce a single northern hemisphere intertropical convergence zone (ITCZ), and instead simulate ITCZ bands in both hemispheres. This “double ITCZ’ bias can negatively impact representations of large-scale modes of variability, such as the Madden-Julian oscillation and El Ni\ no–Southern Oscillation. A new method to estimate model fluxes that would have been obtained with the COARE3.0 bulk flux algorithm indicates that twelve of fourteen CMIP6 models overestimate surface fluxes in the ITCZ region, suggesting that biases rooted in model flux algorithms may contribute to ITCZ biases. This finding is supported by atmosphere-only simulations of two models where the original flux algorithms are replaced with the COARE3.0 algorithm. In the experiments, precipitation root mean square errors in the double ITCZ region were reduced by 26\% and 15\%, respectively. We interpret these findings through the lenses of global energy constraints and convection-boundary layer interactions.

In winter, the Northwest Tropical Atlantic Ocean can be characterized by various regimes of interactions among ocean current, surface wind, and wind waves, which are critical for accurately describing surface wind stress. In this work, coupled wave-ocean-atmosphere model simulations are conducted using two different wave roughness parameterizations within COARE3.5, including one that relies solely on wind speed and another that uses wave age and wave slope as inputs. Comparisons with the directly measured momentum fluxes during the ATOMIC/EUREC4A experiments in winter 2020 show that, for sea states dominated by short wind waves under moderate to strong winds, the wave-based formulation increases the surface roughness length by 40\% compared to the wind-speed-based approach. For sea states dominated by remotely generated swells under moderate to strong wind intensity, the wave-based formulation predicts significantly lower roughness length and surface stress (~20%), resulting in increased near-surface wind speed above the constant flux layer (~5%). Further investigation of the mixed sea states in the model and data indicates that the impact of swell on wind stress is over-emphasized in the COARE3.5 wave-based formulation, especially under moderate wind regimes. Various approaches are explored to alleviate this deficiency by either introducing directional alignment between wind and waves or using the mean wave period instead of the wave period corresponding to the spectral peak to compute the wave age.