Figure 2: Fine scales of
synthetic mean heights, all reference depths averaged by 1/8° by 1/8°
boxes in cm, equivalent to mean synthetic heights minus first guess
(input to optimal analysis).
2.3 Computation of the synthetic mean
velocities
The objective of this estimation is to process the velocities estimated
from drifters (section 3) and surface Argo float drifts to obtain the
physical content of the geostrophic currents associated with the MDT.
This is achieved by removing from the drifter velocities the
ageostrophic components of the current, as well as the temporal
variability of the geostrophic component of the velocities:
\begin{equation}
U_{\text{synth}}=U_{\text{drifter}}-U_{\text{Ekman}}-U_{\text{Stokes}}-U_{\text{inertial}}-U_{\text{tidal}}-U_{ageo-hf}-U_{\text{slippage}}-U_{\text{alti}}^{{}^{\prime}}\nonumber \\
\end{equation}
This method, the estimation of the wind-driven component of the current
and slippage, and the filtering applied are fully described in Mulet et
al. (2021, section 5) and build on previous work by Rio and Hernandez
(2004), Rio et al. (2007, 2011 and 2014a).
For this new MDT, we are also using velocities data estimated from High
Frequency radars located in the Mid Atlantic Bight area, on the East
Coast of the USA, from Cape Hatteras to Cape Cod. In the same way as for
drifter data, the aim is to process these velocities to obtain synthetic
velocities with the physical content of geostrophic MDT velocities. We
used the cleaned, detited, high-frequency signal-filtered and mean
currents over the period 2006-2016, processed by Rutgers University
(Roarty et al. 2020). Then the mean wind-driven currents over the same
period (\(U_{\text{Ekman}}\) taken from the Copernicus-Globcurrent
product MULTIOBS_GLO_PHY_REP_015_004) were removed and finally
these mean currents were re-referenced to the 1993-2012 reference
period.
Figure 3 shows these mean synthetic velocities estimated from (a)
drifters (at 1/8° resolution) and (b) HF radar data, over the Mid
Atlantic Bight area off New Jersey and Delaware (USA). The figure also
shows the 100m and 2000m isobaths that mark the limit of the continental
shelf. The average synthetic velocities estimated by drifters are much
noisier, with more intense currents, than those estimated from HF
radars. However, both maps show a recirculation current to the
south-east at the 100m isobath. The drifters (Figure 3 a) show this
current as narrow and intense (15 to 20 cm/s), whereas the HF radars
(Figure 3 b) show a broad current of between 5 and 10 cm/s. Very close
to the coast, velocities are generally low (below 5 cm/s), but currents
can be perpendicular to the coast (e.g. Figure 3 b at 74.5°, 75° and
75.5°W). Between 74.9° and 75°W, the outflow of the Delaware River could
explain these cross-shore currents.
These differences can be explained by the very different sampling of the
two types of data:
- HF radar: at least 50% of data every hour over 10 years
- Drifters (1 or 2 drifters per box near the coast).
Moreover, it is likely that drifters tend to converge at the center of
the current due to convergence and subduction, resulting in a sampling
bias in favor of a narrow jet.