| dc.description.abstract | The Gulf Stream (GS) is a region with large mesoscale activity, as strong sea surface
temperature gradients and oceanic eddies are developed in the area. These mesoscale features
force the atmosphere by affecting the near surface wind. In this study, we investigate
this mesoscale air-sea interaction using several air-sea coupled configurations of the Global
Climate Models: ECMWF-IFS, HadGEM3 and MPI-ESM. The climatology and the internal
variability of variables such as sea surface temperature (SST), turbulent heat fluxes, precipitation
and near surface wind divergence are studied for the time period between 1951 and
2049.
The horizontal resolution of the oceanic component of the models seems to be determinant
factor for the accurate representation of the air-sea coupling. We find that the models
with eddy parameterized oceanic component are incapable of accurately describing several
atmospheric fields over this region. Also, only the eddy resolving oceanic component is
capable of describing correctly the separation of GS at Cape Hatteras, whereas the rest of
the configurations shift the GS path northwards. Even though the atmospheric resolution
does not seem to be important for the climatology, it is significant for the variability of many
atmospheric fields.
Next, we focus on the two main mechanisms which explain the wind response to the SST
fronts. These are: the Vertical Mixing (VMM) and Pressure Adjustment (PAM) mechanisms.
In accordance with the VMM, the destabilization of the atmosphere over the warmer SSTs
leads to intensification of the vertical mixing in the boundary layer. Thus, large momentum
from aloft is transported downwards resulting in acceleration of the near surface wind. On
the other hand, the PAM suggests that the differential heating of the atmosphere at each side
of the SST front generates pressure gradient forces. These pressure anomalies are responsible
for the convergence and divergence zones on the warm and cold flank of GS respectively.
The strength of the two aforementioned mechanisms is investigated for all the configurations.
For the VMM, we find that the coupling between the downwind SST gradients and
the wind divergence becomes stronger by increasing the resolution. However, the coupling
between the crosswind SST gradients and the wind curl is not statistically significant for all
the configurations.
Regarding the PAM, we find that the mean sea level pressure adjustment due to SST
forcing is stronger in the higher atmospheric resolution configurations. In contrast, the
coupling between the mean sea level pressure Laplacian and the wind convergence gets
slightly weaker by increasing the resolution. This fact is likely due to the large scale processes
that can contribute to the generation of secondary circulations.
For the strength of the air-sea coupling, both oceanic and atmospheric resolutions are
significant, as a too coarse atmospheric component is not capable of capturing the mesoscale
features produced by the oceanic one. | |
