# Evaluation of GlobCurrent surface ocean current products: A case study in Australia

Mathilde Cancet, David Griffin, Madeleine Cahill, Bertrand Chapron, Johnny

Johannessen, Craig Donlon

► To cite this version:

Mathilde Cancet, David Griffin, Madeleine Cahill, Bertrand Chapron, Johnny Johannessen, et al. Evaluation of GlobCurrent surface ocean current products: A case study in Australia. Remote Sensing of Environment, 2019, 220, pp.71-93. 10.1016/j.rse.2018.10.029. hal-04202201

HAL Id: hal-04202201

https://hal.science/hal-04202201v1

Submitted on 11 Aug 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
1

Evaluation of GlobCurrent surface ocean current products: a case study in Australia

2

Mathilde Cancet(1), David Griffin(2), Madeleine Cahill(2), Bertrand Chapron(3), Johnny

3

Johannessen(4), Craig Donlon(5)

4

(1) NOVELTIS

5

(2) CSIRO

6

(3) IFREMER

7

(4) NERSC

8

(5) ESA/ESTEC

9

Corresponding author: Mathilde Cancet, mathilde.cancet@noveltis.fr

10

NOVELTIS, 153 rue du Lac, 31670 Labège, France

11

**Abstract**

12

13

14

15

16

17

18

Australia's marginal seas include a wide range of ocean current regimes ranging from tide-dominated in the North-West where the continental shelf is wide, to boundary current-and eddy-dominated in the South-East, where the shelf is narrow. Here, we take the opportunity to test the GlobCurrent surface ocean current products against in-situ observations in these two contrasting regimes. Observations by Acoustic Doppler Current Profiler (ADCP) instruments of Australia's Integrated Marine Observing System (IMOS) and drifting buoys of the Global Drifting Programme (GDP) are used.

19

20

21

22

The monthly-timescale variability of the GlobCurrent alongshore current component is in moderately good agreement with the observations on the continental shelf in the South-East but neither the shorter period variability nor the long-term mean are representative of the ADCP observations. While the observed tidal currents are negligibly small, the wind-driven
signals are not. But these are evidently too transient to be adequately sampled by altimetry.

The inclusion of an Ekman component does not represent these signals because the
interaction of the Ekman transport with the coastal boundary condition is not included.

Similarly, the error of the time-mean velocity, which is the dominant error, is because that
product is not designed to represent the highly anisotropic nature of the sea level gradients
over the continental shelf nor the constraints on the flow field that are imposed by the
topography. We thus conclude that the GlobCurrent product needs improvements before it
can be described as very suitable for applications on this, and probably other, narrow
continental shelf. Off the continental shelf, in contrast, the GlobCurrent products compare
quite well with the trajectories of drifting buoys, confirming that the products are quite
suitable for blue-water applications.

In contrast to the South-East, the tides are very strong in the North-West region of
Australia. The sub-tidal variability is weak, in both relative and absolute senses.
Consequently, the removal of the tidal signal from the sea level observations needs to be very
complete for the residual error to be smaller than the true sub-tidal signal. Transient wind
forced signals are also occasionally large so this step of the de-aliasing also needs to be very
accurate. Unfortunately, it appears that more work is required before accurate estimates of
sub-tidal variability are available from GlobCurrent: the magnitude of the GlobCurrent
estimates of sub-tidal current variability far exceed the magnitude of, and are uncorrelated
with, the detided ADCP data.

44 **Keywords**

Surface ocean currents; GlobCurrent assessment; East Australian Current; tidal currents; in situ measurements
## 1. Introduction

There is currently no instrument onboard satellites that can directly provide the total surface ocean currents worldwide. Some satellite missions with this objective are under study (SEASTAR ex-WAVEMILL (Martin and Gommenginger, 2017), SKIM (Ardhuin et al, 2017)) but, today, the estimation of the total surface ocean currents from remote sensing observations can only be made through the combination of observations provided by various instruments and after application of approximate models. The geostrophic currents are derived from the satellite altimetry observations and the Ekman drift is derived from the wind observations. This method, classically used by several ocean current products providers (AVISO (Le Traon and Dibarboure, 2002), OSCAR (Bonjean and Lagerloef, 2002), GEKCO (Sudre et al, 2013), GlobCurrent (Rio et al, 2014; Rio, 2015)), is based on the assumption that the dominant signals in the surface ocean currents are the geostrophic component and the Ekman drift. This approximation is adequate in much of the open ocean (away from the equator), especially for time scales longer than the inertial period. On the continental shelves, other ocean dynamics processes such as the tidal currents can dominate the ocean circulation. Ignoring this contribution to the total surface ocean currents can lead to underestimating dramatically the currents in regions with large tidal amplitudes.

This paper presents an assessment of the GlobCurrent surface ocean currents products produced and distributed in the framework of a project funded by the European Space Agency (ESA). This assessment focuses on Australia, a region characterized by a variety of ocean dynamics processes including large geostrophic structures such as the East Australian Current (EAC) and wide shelves where the tidal currents are predominant, in particular on the northwest coast.

A set of Acoustic Doppler Current Profiler (ADCP) stations as well as drifting buoys are used to evaluate the physical content of the GlobCurrent products in the East Australian
72 Current (EAC) region and to quantify the omission error made on the macro-tidal shelf of the
73 Joseph Bonaparte Gulf.

74

75 **2. Data**

76 **2.1. GlobCurrent products**

77 The GlobCurrent surface ocean currents products (version 3.0) are provided on a
78 1/4°grid, over the 1993-2016 period. The geostrophic currents and the Ekman currents are
79 provided separately, on daily and 3-hourly bases, respectively, and also combined in a “total
80 surface currents” product, which is the sum of the two products and is distributed on a 3-
81 hourly basis (Rio, 2015, http://www.globcurrent.org/).

82 The GlobCurrent geostrophic currents are derived from maps of satellite altimetry sea
83 level anomalies (SLA), combined with the most recent Mean Dynamic Topography (MDT)
84 field (CNES-CLS-2013 MDT, Rio et al, 2013), which provides an estimate of the time-mean
85 geostrophic current. The spatiotemporal coverage of the satellite altimetry missions and the
86 correlation scales used in the optimal interpolation process that generates the SLA and MDT
87 maps imply that the intrinsic temporal resolution of the geostrophic currents products is about
88 10 days and the spatial resolution is in the order of 100-150 km (Ubelman et al, 2015).

89 The GlobCurrent Ekman currents products are based on the Ekman model of Rio et al
90 (2014). The Ekman currents are provided at 15 m below the surface and at the surface (0 m),
91 by estimating the Ekman model parameters using the velocities derived from the trajectories
92 of the drifting buoys drogued at 15 m on the one hand and the velocities derived for the Argo
93 float trajectories on the other hand (Rio et al, 2014). To tune the Ekman model parameters,
94 the drifting buoys data are filtered using an inertial-period-to-20-days band-pass filter, which
95 corresponds to the frequencies of maximum coherency between the wind stress and the
96 Ekman currents (Rio et al, 2014). The Ekman model is then applied to the 3-hourly ECMWF
97 wind stress fields (80 km of spatial resolution) to produce the GlobCurrent Ekman current
98 products (Rio, 2015).

## 2.2. In situ data

100 For more than 10 years, Australia has been maintaining a network of about 50 ADCP
101 instruments all around the country (Australian National Mooring Network facility),
102 principally through the government-supported Integrated Marine Observing System (IMOS).
103 Time series of ocean current velocities are available over periods of several years, at various
104 depths in the water column and with temporal sampling of 5 to 15 minutes, depending on the
105 stations. For details, see http://oceancurrent.imos.org.au/timeseries/.

106 For this study, we considered four ADCP stations along the New South Wales coast,
107 where the East Australian Current flows: NRSNSI (North Stradbroke Island, off Brisbane),
108 CH100 (Coffs Harbour), SYD100 and SYD140 (Sydney). ADCP velocities considered in this
109 study were all chosen close to 15-m depth, to avoid the zone of side-lobe interference between
110 the surface and approximately 12% of the depth of the ADCP. The locations of these ADCP
111 stations were checked in order to be sure that they were not mainly representative of some
112 very local dynamics that would not be sampled in the grid of the GlobCurrent products.

113 Three other ADCP stations were considered on the macro-tidal shelf of the Joseph
114 Bonaparte Gulf: the Margaret Harries Banks (ITFMHB) station, the Flat Top Banks
115 (ITFFTB) station and the Joseph Bonaparte Gulf (ITFJBG) station. These three ADCP
116 stations were specifically processed to extract the tidal current signal (see section 3.2). Given
117 the temporal sampling of the ADCP observations, they contain much more high frequency
118 signals than the GlobCurrent products and temporal filtering is needed to compare both
119 datasets, as discussed in the next section.

120 Tens of drifters datasets are also available in the EAC region during the 1992-2015
121 period covered by the GlobCurrent products. The drifting buoys velocity observations come
from the Global Drifter Program and were downloaded from the NOAA/AOML web-service.

The eastward and northward ocean current components are provided at 6-hour temporal
sampling for each buoy. In this paper, we focused on four of these drifting buoys, during the
period 2009-2015. These four drifting buoys were chosen for their proximity to the New
South Wales coast and the ADCP stations, and for the structures they described in the open
ocean (EAC path and eddies of different sizes). The analysis was done during the “drogued
period” of the drifters, i.e. before they lost their drogues, so that the velocities derived from
the drifters trajectories are representative of the ocean currents at 15 m and can be compared
with the GlobCurrent total ocean currents at 15-m depth (Lumpkin et al, 2013).

In general, the ADCP stations and the drifter buoys were chosen with the objective to
ensure fair comparison. Indeed, some ADCP stations and drifters are so coastal that they fall
out of the GlobCurrent 0.25° grid. Some ADCP stations are also installed in sites with very
specific dynamics that have almost no chance to be reproduced in the GlobCurrent products
because of their coarser spatiotemporal resolution.

136

137 **3. Results**

138 **3.1. Assessment in a geostrophic structure: the East Australian Current**

139 **3.1.1. Coastal comparison with ADCP data**

The East Australian Current (EAC) is a complex western boundary system off the
eastern coast of Australia, whose main pattern flows southward (Fig. 1). In terms of mean
flow, the EAC is about 30 - 100 km wide and 200 - 500 m deep, with rather weak mean flow
compared to the other western boundary currents (Suthers et al, 2011). However, the EAC is
strongly dynamic and highly energetic, as it is dominated by highly variable mesoscale
eddies, which results in the fact that its variability is often stronger than the mean flow
(Bowen et al, 2005; Mata et al, 2006). The mean current flows along the steep continental shelf slope, close to the 200-m isobaths, as shown in Fig. 1 (100-m and 200-m isobaths are plotted in gray lines). The ADCP stations of NRSNSI (in 65m of water) and CH100 are located close to the EAC mean path, in the region where the mean current is strongest. The two stations off Sydney (SYD100 and SYD140) are located in an area where the mean current is weaker and the variability of the current is larger, with the production of mesoscale eddies and the separation of the current in several branches (Ridgway and Hill, 2009). Such a configuration provides good conditions to assess the GlobCurrent ocean surface current products, both offshore and near the coast.

<figure><img src="image_2.png" /><figcaption>Figure 1: Mean geostrophic velocities in cm/s derived from the CNES-CLS13 ¼° Mean Dynamic Topography off New South Wales. The velocity magnitude is in color and the arrows give the direction and the amplitude of</figcaption></figure>
158

the current. The locations of the 4 ADCP stations are shown with black triangles. The light gray lines show the

159

100m and 200m isobaths.

160

The GlobCurrent ocean surface currents were extracted at the locations of the four ADCP stations for the geostrophic currents (daily) and the total currents at 15 m (3-hourly).

161

162

The GlobCurrent total currents at 15 m are expected to show more consistent physical content with the ADCP observations at 15 m, as they contain more ocean dynamics (geostrophic +

163

164

Ekman currents) than the GlobCurrent geostrophic currents only. The spectral analysis of the GlobCurrent geostrophic currents, computed with the Fast Fourier Transform (FFT) method

165

166

shows that there is very little signal at the location of the ADCP station at periods shorter than 25 to 30 days (Fig. 2 for the CH100 ADCP station in Coffs Harbour, similar results obtained

167

168

for the three other stations, not shown here). In the meantime, the ADCP observations, with a temporal sampling of 5 to 15 minutes depending on the station, contain much high-frequency

169

170

signal (in particular diurnal and semi-diurnal signals) as shown in Fig. 2. The spectral analysis of the GlobCurrent total currents at 15 m (not shown here) exhibits the same spectrum as the

171

172

GlobCurrent geostrophic currents, with two additional diurnal and semi-diurnal energy peaks, due to the Ekman contribution. The comparison of the high frequency signals (under 48

173

174

hours) contained in the ADCP observations and in the GlobCurrent total surface currents at 15 m does not show any significant correlation at these ADCP stations. At Coffs Harbour

175

176

(CH100), the amplitude of the high frequency signals in the ADCP currents is about two to three times the amplitude of the GlobCurrent total surface currents at 15 m. Both datasets

177

178

contain strong diurnal signals, generally in phase. However, it is difficult to draw any conclusions from the comparison at this temporal scale, as the ADCP observations contain

179

180

more physical processes, such as the ocean tides (even if they are rather weak in this region) and Coastal Trapped Waves (Griffin and Middleton, 1991), while the GlobCurrent total

181

182

currents products only contain the Ekman currents at this temporal scale. Given these
183

considerations, the ADCP observations and the GlobCurrent products were filtered with a 30-
day low-pass filter for the comparisons presented hereafter.

184

<figure><img src="image_3.png" /></figure>

185

<caption>Figure 2 : Spectral analysis of the CH100 ADCP currents at 15 m (in black) and of the GlobCurrent geostrophic currents (in blue) extracted at the location of the CH100 ADCP station in Coffs Harbour, in each current direction. The single-sided amplitude spectrum was computed using an FFT method.</caption>

186

187

188

The current roses (Fig. 3) plotted at each of the four ADCP stations show the number of occurrences in the observations for each angle of the ocean current direction (0° is eastward, 90° is northward). To build these plots, the ADCP observations were interpolated at the GlobCurrent geostrophic currents dates, over the common time period. In addition, Tab. 1 gives some statistics computed on the monthly means of the ADCP and GlobCurrent current estimates, for each of the four ADCP stations.

189

190

191

192

193

194

195

At the NRSNSI station, Fig. 3 shows that the GlobCurrent geostrophic current estimates are strongly variable, whereas the ADCP sees a very stable southeastward flow (the

196
GlobCurrent total ocean surface currents at 15m were not plotted at the NRSNSI station because the station is off the GlobCurrent total current grid). The ADCP currents main direction is parallel to the orientation of the 100m and 200m isobaths, which means that the EAC probably follows the isobaths more closely than it is described in the CNES-CLS13 MDT product (see zoom in on the area on Fig. 4). In addition, the MDT shows a northward current all along the coast in this region, with amplitudes of about 30 cm/s, which is not realistic. In this coastal region, the satellite altimetry data used to build the MDT and the GlobCurrent products are certainly less accurate than in the open ocean, which leads to large errors in the time-mean and daily geostrophic current estimates. These poor results are confirmed by the statistics given in Tab. 1, where the correlations between the NRSNSI ADCP data and the GlobCurrent products are very low, and there is a noticeable mean bias of about 20 cm/s on the norm of the currents between the two datasets.

At the two stations off Sydney (SYD100 and SYD140), the axis of variability of the ADCP and GlobCurrent currents are more consistent, but the directions of the mean differ (Fig. 3). In particular, at the SYD100 station, the ADCP currents are mainly oriented southwestward while the mean GlobCurrent velocity is northeastward. At the SYD140 station, the GlobCurrent currents are almost equally distributed between the southwestward and the northeastward directions, while the ADCP data clearly sees a southwestward flow. A zoom in on the mean geostrophic currents derived from the CNES-CLS13 MDT in the region off Sydney (Fig. 5) shows that, close to the SYD100 station, there is a mean northeastward current of about 15 cm/s that flows along the coast in the CNES-CLS13 product, which is the basis of the GlobCurrent currents products. As for the SYD140 station, it appears to be located at a convergence between northeastward and southwestward flows in the MDT product, which can explain the large spatial variability of the GlobCurrent ocean current estimates at this point. Tab. 1 shows that the correlations in the separate current directions are
higher at the SYD100 and SYD140 stations (0.7 on the eastward component) than at the other stations. However, when considering the correlations on the current norms (including both directions at once), the statistics are very poor at the two stations off Sydney (less than 0.2), which is explained by the fact that the ADCP and GlobCurrent systems show the same large variations in the currents estimates but with very different amplitudes (see the time series of the monthly means of the currents for the SYD100 station in Fig. 6).

The Coffs Harbour station (CH100) is the closest to the main stable structure of the EAC and the comparison of the 30-day low-pass filtered observations shows the more consistent results. It is all the more true when considering monthly means of the current estimates, as shown in Fig. 7. Except for some periods when the ADCP and the GlobCurrent systems give opposite currents directions, a general agreement can be observed between all the datasets, with consistent amplitudes and correlations of 0.7 in the meridional direction, as shown in Tab. 1. The seasonal variations of the EAC are also clearly visible, with intensification during the southern summer (December to February). In the zonal direction, the correlations are lower (about 0.50), because of the higher variability and lower amplitude of the currents in this direction, which is probably more difficult to capture with the current altimetry data processing strategies. The correlations on the current norms are also rather good (about 0.60, which is easily the highest score obtained among the four ADCP stations), and the RMS differences and biases computed on the current norms are low (a few cm/s), as summarized in Tab. 1.

This comparison with ADCP data shows that, in this region, the GlobCurrent products are poorly consistent with the coastal in situ observations, even at long temporal scales (more than 30 days), except in the case of the Coffs Harbour 100m ADCP station, which is located closest to the EAC path and observes some ocean dynamics that is largely influenced by the open ocean.
247

<figure><img src="image_4.png" /><figcaption>Directions of the current at NRSNSI station
30-day filtered GlobCurrent (geostrophic) data</figcaption></figure>

<figure><img src="image_5.png" /></figure>

248

<figure><img src="image_6.png" /><figcaption>Directions of the current at CH100 station
30-day filtered GlobCurrent (geostrophic) data</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Directions of the current at CH100 station
30-day filtered GlobCurrent (total at 15m) data</figcaption></figure>

<figure><img src="image_8.png" /></figure>

249

<figure><img src="image_9.png" /><figcaption>Directions of the current at SYD100 station
30-day filtered GlobCurrent (geostrophic) data</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Directions of the current at SYD100 station
30-day filtered GlobCurrent (total at 15m) data</figcaption></figure>

<figure><img src="image_11.png" /></figure>

250
Directions of the current at SYD140 station
30-day filtered ADCP data

<figure><img src="image_12.png" /></figure>

Directions of the current at SYD140 station
30-day filtered GlobCurrent (geostrophic) data

<figure><img src="image_13.png" /></figure>

Directions of the current at SYD140 station
30-day filtered GlobCurrent (total at 15m) data

<figure><img src="image_14.png" /></figure>

251

Figure 3: Ocean current roses of the 30-day low-pass filtered ADCP observations and GlobCurrent products (geostrophic currents and total surface currents at 15 m) for each of the four ADCP stations. The numbers in blue indicate the angle of the ocean current direction (0° is eastward, 90° is northward). The numbers in black and the concentric circles give the number of occurrences in the observations for each angle. The NSRNSI ADCP station is located out of the grid of the GlobCurrent total currents and no current rose was computed in this case.

252

253

254

255

256

257

258

Table 1: Statistics computed on the monthly means of the ADCP currents and the GlobCurrent geostrophic and total surface currents. The correlations are computed for each current direction separately (U – eastward, V – northward) and on the current norm. The RMS differences and the biases are computed on the current norm only, following the convention (GlobCurrent data - ADCP data). The NSRNSI ADCP station is located out of the grid of the GlobCurrent total currents and no statistics were computed in this case.

259

260

261

262

263

<table><thead><tr><th rowspan="3">ADCP<br>station</th><th colspan="6">GlobCurrent geostrophic currents</th><th colspan="6">GlobCurrent total currents at 15 m</th></tr><tr><th colspan="3">Correlation</th><th rowspan="2">RMS diff.<br>(m/s)</th><th rowspan="2">Bias<br>(m/s)</th><th colspan="3">Correlation</th><th rowspan="2">RMS diff.<br>(m/s)</th><th rowspan="2">Bias<br>(m/s)</th></tr><tr><th>U</th><th>V</th><th>Norm</th><th>U</th><th>V</th><th>Norm</th></tr></thead><tbody><tr><td>NRSNSI</td><td>-0.02</td><td>0.24</td><td>-0.02</td><td>0.17</td><td>-0.19</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>CH100</td><td>0.49</td><td>0.68</td><td>0.59</td><td>0.14</td><td>0.06</td><td>0.52</td><td>0.69</td><td>0.60</td><td>0.15</td><td>0.07</td></tr><tr><td>SYD100</td><td>0.73</td><td>0.51</td><td>-0.09</td><td>0.11</td><td>0.03</td><td>0.72</td><td>0.54</td><td>-0.10</td><td>0.11</td><td>0.03</td></tr><tr><td>SYD140</td><td>0.68</td><td>0.65</td><td>0.18</td><td>0.11</td><td>0.04</td><td>0.70</td><td>0.67</td><td>0.19</td><td>0.11</td><td>0.04</td></tr></tbody></table>

264
<figure><img src="image_15.png" /><figcaption>Figure 4: Mean geostrophic velocities from the CNES-CLS13 ¼° Mean Dynamic Topography off Brisbane. The location of the NRSNSI ADCP station is shown with a black triangle. The light gray lines show the 100m and 200m isobaths.</figcaption></figure>
Mean geostrophic velocity (cm/s) from MDT CNES-CLS13 off New South Wales

<figure><img src="image_16.png" /><figcaption>Figure 5 : Mean geostrophic velocities from the CNES-CLS13 ¼° Mean Dynamic Topography off New South Wales. The locations of the ADCP stations off Sydney are shown with black triangles. The light gray lines show the 100m and 200m isobaths.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Figure 6: Monthly means of the ADCP currents (in black) and the GlobCurrent geostrophic currents (in blue) and total surface currents at 15 m (in red) at the SYD100 ADCP station.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>Figure 7: Monthly means of the ADCP currents (in black) and the GlobCurrent geostrophic currents (in blue) and total surface currents at 15 m (in red) at the CH100 ADCP station.</figcaption></figure>

### 3.1.2. Offshore comparison with drifting buoys data

To complete the coastal analysis and assess the performance of the GlobCurrent products in the open ocean, offshore comparisons were made with four drifting buoys in the EAC region. Because the GlobCurrent 3-hourly total ocean currents products are designed for offshore applications, the aim of this offshore comparison was to put them to the test of the total ocean currents derived from the drifter trajectories. Thus no temporal filtering was applied to any of the datasets in order to keep the 3-hourly Ekman currents contribution in the GlobCurrent products (mainly diurnal and semi-diurnal signals as shown previously) for the comparison.

First, the velocities derived from the drifters trajectories were compared with the GlobCurrent total ocean currents at 15 m, extracted at the positions and dates of the drifters,
every 6 hours (Fig. 8). Note that the blue arrows of the GlobCurrent products are sometimes missing on the figures, when the drifter positions are out of the GlobCurrent grid. The non-filtered ADCP velocities (in black in Fig. 8) were also extracted for the closest positions of the drifters. The very good agreement between the drifters and the ADCP data at all the ADCP stations confirms that the GlobCurrent products are generally not correct close to the Sydney stations in particular, as it was shown in the previous section.

For the four considered drifting buoys, Fig. 8 shows a general good agreement between the drifters currents and the GlobCurrent currents, in terms of directions. In particular, the large eddy structures described by drifters 101921 (Fig. 8 a) and 122612 (Fig. 8 b) are well represented in the GlobCurrent products. However, the GlobCurrent products generally lack energy, as the amplitude of the drifter currents is generally larger.
<figure><img src="image_19.png" /><figcaption>Drifter 122612 trajectory and GlobCurrent total currents at 15 m</figcaption></figure>

<figure><img src="image_20.png" /></figure>
<figure><img src="image_21.png" /></figure>

Drifter 139659 trajectory and GlobCurrent total currents at 15 m

<figure><img src="image_22.png" /></figure>
308

Figure 8: Drifters trajectories and velocities (in red) and velocities of the GlobCurrent total currents at 15 m at the drifters positions (in blue). The ADCP velocities at the closest positions of the drifters are also shown (in black).

309

310

311

In order to characterize the consistency between the drifters and the GlobCurrent products with quantitative metrics, the skill scores of the GlobCurrent products were computed using the methodology proposed in Liu and Weisberg, 2011, and used in the Gulf of Mexico for several ocean current products in Liu et al, 2014. This non-dimensional skill score indicates the relative performance of the GlobCurrent products compared to the drifters observations and, as it is only based on the trajectories, no prior knowledge of the ocean circulation is needed.

312

313

314

315

316

317

318

For each position of the real drifters, the trajectories of pseudo-drifters were computed as if they were advected by the GlobCurrent products, either by the geostrophic currents alone or by the total ocean currents at 15 m. As the GlobCurrent total currents at 15 m are provided 3-hourly, the computation of the pseudo-drifters advected by these fields was done using all the maps, at 3-hour temporal sampling. Indeed, even if the real drifter data are available every 6 hours, the real drifter undergoes ocean dynamics variations at higher frequency and it is fair to compare the pseudo-drifter trajectories using the full temporal resolution of the GlobCurrent products. On the other hand, the GlobCurrent geostrophic currents are provided daily and the position of the pseudo-drifter can only be computed for one point of four of the real drifter trajectory. Thus, the pseudo-drifters advected by the GlobCurrent geostrophic current fields are submitted to the same velocity and direction of the current during 24 hours, while the pseudo-drifters advected by the GlobCurrent total current fields are potentially submitted to changes in the velocity and direction of the current every 3 hours.

319

320

321

322

323

324

325

326

327

328

329

330

331

The skill score (s) is defined by Liu and Weisberg (2011) as:

332

$$ \begin{cases} s = 1 - \frac{c}{n} & (c \le n) \\ s = 0 & (c > n) \end{cases} \qquad (\text{Eq. 1}) $$
where

$$c = \frac{\sum_{i=1}^{N} d_i}{\sum_{k=1}^{M} l_k} \quad (\text{Eq. 2})$$

Starting from a given point of the real drifter trajectory, N is the number of GlobCurrent dates during the 3-day period following the date of the considered point (N=3 for the daily GlobCurrent geostrophic currents products and N=3*8=24 for the 3-hourly GlobCurrent total currents products), M is the number of 6-hourly drifter dates during the same 3-day period (M=3*4=12) and c is the cumulative Lagrangian separation distance (d) divided by the cumulative length of the observed trajectory (l). The tolerance threshold n is set to 1 as suggested by Liu and Weisberg, 2011. Thus, the skill score can be written as follows:

$$\begin{cases} s = 1 - c & (c \le 1) \\ s = 0 & (c > 1) \end{cases} \quad (\text{Eq. 3})$$

Pseudo-drifters with c>1 correspond to a cumulative separation distance larger than the cumulative length of the observed trajectory. When s=1, the test dataset is considered to be perfectly skilled.

Fig. 9 to 12 show the skill scores in color scale during a 3-day period for each of the four drifters considered in this work. The trajectories of drifter 101921 (Fig. 9) and drifter 122612 (Fig. 10) are of particular interest as they present a large variety of structures, flowing along the EAC and then curling around eddies. For these two cases, the skill scores were computed for both GlobCurrent products, the geostrophic currents ((a) plots) and the total currents at 15 m ((b) plots). For the two other drifters (Fig. 11 and Fig. 12), the skill scores are only shown for the GlobCurrent total currents. On each plot, the observed drifter trajectory is shown with a black dotted line and the light gray lines show the trajectories of the pseudo-drifters during the 3-day period following their launch.
356

As expected, the skill scores are very low or equal to zero when the drifters are close to the New South Wales coast. In most of these cases, the pseudo-drifters end up on the shore or fall out of the GlobCurrent grid. For example, the pseudo-drifters launched at about 33.5°S on the path of drifter 101921 are systematically pushed to the shore whereas the real drifter continues its way southward, following the EAC stream (Fig. 9 (a) and (b)). This is consistent with the findings of the previous section, where the comparisons to the ADCP data along this coast revealed that the GlobCurrent products, and the MDT on which they are based, show unrealistic counter-currents in this area.

357

358

359

360

361

362

363

364

When the drifters stay in the EAC stream relatively far from the coast, especially between 30°S and 34°S, the skill scores increase to 0.6 or 0.7 as the pseudo-drifters are mainly advected by the EAC (drifter 101921 between 32°S and 33°S (Fig. 9), drifter 122612 between 33°S and 34°S (Fig. 10) and drifter 139659 between 31°S and 32°S (Fig. 12)). The comparison between the GlobCurrent geostrophic currents and the GlobCurrent total currents at 15 m ((c) plots on Fig. 9 and Fig. 10) shows that the skill scores in the EAC stream are generally slightly better when considering the GlobCurrent geostrophic currents only. This is probably mainly due to the lower temporal resolution of the GlobCurrent geostrophic-only currents, which induces that the pseudo-drifters are advected along the EAC main stream with the same current amplitude and direction over a 24-hour period instead of 3 hours for the total currents estimates, with less chance to be pushed to the shore. The 3-hourly GlobCurrent Ekman current contribution does not improve the description of the EAC in this region, mainly because of its coarse spatial resolution close to the coast.

365

366

367

368

369

370

371

372

373

374

375

376

377

Regarding the other structures that are described by the four drifters in the region, the skill scores of the GlobCurrent products are rather satisfactory (ranging between 0.6 and 0.9) on large eddy structures, such as for drifter 101921 (Fig. 9) and drifter 122612 (Fig. 10). The scores are generally lower in the areas of rapid direction change or in regions of divergences,

378

379

380
where the pseudo-drifters sometimes catch streamlines that are completely different from the real drifter trajectory. This is the case for drifter 139659 (Fig. 12), when the pseudo-drifters continue to curl around the cyclonic eddy structure located between 28.5°S and 30°S instead of entering the EAC southward path like the real drifter. In the same way, the last observed positions, between 32°S and 32.5°S, show that this drifter turns to the West and then heads to the North following a retroflection along the coast that is clearly not present in the GlobCurrent products as all the pseudo-drifters continue their ways southward.

As already mentioned above, the trajectory of drifter 101921 is of particular interest as it describes a variety of oceanic features of different sizes. This is thus a good test-bed for the GlobCurrent products. Fig. 13 highlights in colored boxes the four features of interest that are analyzed hereafter:

392 A: the East Australian Current path;

393 B and C: two large (300-km diameter) anti-cyclonic eddies;

394 D: a smaller (100-km diameter) cyclonic eddy.

In order to assess the performance of the GlobCurrent products on these four oceanic features, Fig. 14 shows the GlobCurrent fields and the positions of seven pseudo-drifters advected by these current fields (one color for each pseudo-drifter) for a selection of dates. The positions of the real drifter are shown in black. The left plots show the pseudo-drifters advected by the GlobCurrent geostrophic currents and the right plots show the pseudo-drifters advected by the GlobCurrent total currents.

The first maps on Fig. 14 (20/02/2013) show that the red pseudo-drifter, launched in the EAC (feature A on Fig. 13) on the 14/02/2013, is maintained a bit farther from the coast by the GlobCurrent streamlines as it flows along the EAC path about 30 km East from the real drifter. In terms of speed, the GlobCurrent currents are very consistent with the real drifter in the EAC path, whatever the products, down to 34°S.
There, the velocity of the real drifter increases while it curls around the large anti-cyclonic eddy (feature B on Fig. 13). In the GlobCurrent products, the diameter of this eddy feature is quite consistent with the real drifter trajectory. However, in terms of position, the eddy appears to be a little shifted southwestward in the GlobCurrent products, as the real drifter trajectory goes through its most southwestern part, at 152°E. The pseudo-drifter advected by the GlobCurrent total currents sticks better to the real drifter trajectory around the eddy feature B, thanks to the Ekman current contribution and to the higher temporal resolution of these fields. However, the velocities of both GlobCurrent products are clearly underestimated from 34°S, in the eddy feature B, as the pseudo-drifters advected by the GlobCurrent total currents and by the GlobCurrent geostrophic currents are respectively 200 km and 300 km behind the real drifter on the 20/02/2013. These observations are in agreement with the low skill scores (0.3 or 0.4) obtained in this region, as shown on Fig. 9.

The orange pseudo-drifter on Fig. 14 was launched a few hours before the real drifter left the eddy feature B to enter the eddy feature C (Fig. 13). In this region, the GlobCurrent total current streamlines are quite consistent with the real drifter trajectory, including the sharp 90°-turn that occurs on the 24/02/2013 at 155°E and 36.5°S (Fig. 14). There, the skill scores of the GlobCurrent total current product are rather good, ranging between 0.6 and 0.8 (Fig. 9). This is clearly due to the Ekman current contribution (shown on Fig. 15), that drives the two (red and orange) pseudo-drifters in the southward direction, whereas the two pseudo-drifters advected only by the geostrophic currents heads northeastward, following a completely different path, far from the real drifter trajectory (Fig. 14, maps of 26/02/2013).

The GlobCurrent total currents map on the 08/03/2013 (Fig. 14) shows that the two pseudo-drifters then turned to the West, like the real drifter. However, in the case of the pseudo-drifters, this turn happened at 37°S, about 50 km more North than the turn of the real drifter. Then the pseudo-drifters caught strong eastward streamlines and left the path of the
431

real drifter. On the other hand, the real drifter stayed a few days in this area of divergence and described a small loop that is not so well represented in the GlobCurrent products, as confirmed by the low skill scores on Fig. 9.

432

433

434

The yellow pseudo-drifter and the green pseudo-drifter (Fig. 14, maps of 26/03/2013) were launched while the real drifter was curling around the eddy feature C indicated on Fig. 13. For this large eddy feature, like previously shown for eddy feature B, the paths of the pseudo-drifters advected by the GlobCurrent fields are quite consistent with the real drifter trajectory, in terms of position, diameter and velocity, whatever the GlobCurrent fields. The large skill scores in the region confirm these observations (Fig. 9). Local decreases in the skill scores are due to some rapid changes in direction of the real drifter trajectory. One can also note that while the drifter heads North along the eddy feature C, between 38.5°S and 37.5°S, it is about 75 km behind the green pseudo-drifter on the 26/03/2013 (Fig. 14), whatever the GlobCurrent fields, which means that the GlobCurrent velocities are overestimated by about 20 cm/s.

435

436

437

438

439

440

441

442

443

444

445

Finally, the smaller eddy feature D indicated on Fig. 13 is present in both GlobCurrent products, as shown on the maps of 03/05/2013 (Fig. 14). The real drifter curls twice around this eddy. In the meantime, the pseudo-drifters launched in this area describe one loop and a half around the eddy. The GlobCurrent velocities are consequently underestimated over this eddy but the position and diameter are quite consistent with the real drifter trajectory. The paths of the pseudo-drifters advected by the GlobCurrent total currents are closer to the real drifter path, as they benefit from higher frequency updates of their positions and velocities (every 3 hours) than the pseudo-drifters advected by the GlobCurrent geostrophic currents (once a day). Again the skill score results shown on Fig. 9 confirm these observations, as well as the analysis of the skill scores for the three other drifters (Fig. 10, Fig. 11, Fig. 12): the

446

447

448

449

450

451

452

453

454
smaller the ocean dynamics structures, the lower the skill scores, which is in good agreement

with the intrinsic spatiotemporal resolution of the GlobCurrent products.

457
<figure><img src="image_23.png" /><figcaption>Figure 9: Skill scores (in color) after 3 days of advection with the GlobCurrent products ((a) geostrophic currents and (b) total currents at 15 m). The black dotted line shows the positions of the real drifter (101921).</figcaption></figure>

The light gray lines show the positions of the pseudo-drifters launched at each position of the real drifter and after 3 days of advection by the GlobCurrent maps. Below (c), time series of the skill scores along the drifter trajectory, for the GlobCurrent geostrophic currents in blue and the GlobCurrent total currents at 15 m in red.

The black dashed vertical line shows the moment when the drifter leaves the East Australian Current (EAC).
<figure><img src="image_24.png" /><figcaption>Figure 10: Skill scores (in color) after 3 days of advection with the GlobCurrent products ((a) geostrophic currents and (b) total currents at 15 m). The black dotted line shows the positions of the real drifter (122612). The light gray lines show the positions of the pseudo-drifters launched at each position of the real drifter and after 3 days of advection by the GlobCurrent maps. Below (c), time series of the skill scores along the drifter trajectory, for the GlobCurrent geostrophic currents in blue and the GlobCurrent total currents at 15 m in red. The black dashed vertical line shows the moment when the drifter leaves the East Australian Current (EAC).</figcaption></figure>
<figure><img src="image_25.png" /></figure>

Figure 11: Skill scores (in color) after 3 days of advection with the GlobCurrent total currents at 15 m. The black dotted line shows the positions of the real drifter (101749). The light gray lines show the positions of the pseudo-drifters launched at each position of the real drifter and after 3 days of advection by the GlobCurrent maps.
<figure><img src="image_26.png" /></figure>

Figure 12: Skill scores (in color) after 3 days of advection with the GlobCurrent total currents at 15 m. The black dotted line shows the positions of the real drifter (139659). The light gray lines show the positions of the pseudo-drifters launched at each position of the real drifter and after 3 days of advection by the GlobCurrent maps.
<figure><img src="image_27.png" /><figcaption>Figure 13: GlobCurrent total current field (blue arrows) on the 20/02/2013 at 00:00 and positions of drifter 101921(black dots) from the 13/02/2013 to the 16/05/2013. The four boxes in color (A, B, C and D) show the features of interest for the analysis of the GlobCurrent products.</figcaption></figure>
485

GlobCurrent geostrophic - 20/02/2013

<figure><img src="image_28.png" /></figure>

GlobCurrent geostrophic - 26/02/2013

<figure><img src="image_29.png" /></figure>

GlobCurrent total at 15 m - 20/02/2013

<figure><img src="image_30.png" /></figure>

GlobCurrent total at 15 m - 26/02/2013

<figure><img src="image_31.png" /></figure>

486
GlobCurrent geostrophic - 08/03/2013

<figure><img src="image_32.png" /></figure>

GlobCurrent total at 15 m - 08/03/2013

<figure><img src="image_33.png" /></figure>

GlobCurrent geostrophic - 26/03/2013

<figure><img src="image_34.png" /></figure>

GlobCurrent total at 15 m - 26/03/2013

<figure><img src="image_35.png" /></figure>
<figure><img src="image_36.png" /><figcaption>Figure 14: Trajectories over the time of pseudo-drifters (in color) advected by the fields of GlobCurrent total surface currents at 15 m (right plot) or GlobCurrent geostrophic currents (left plot). The positions of the real drifter (101921) are in black. The GlobCurrent velocity fields used to advect the pseudo-drifters are shown with gray arrows.</figcaption></figure>
<figure><img src="image_37.png" /><figcaption>Figure 15: GlobCurrent Ekman current field (blue arrows) on the 24/02/2013 at 00:00 and path of drifter 101921(black dotted line) from the 13/02/2013 to the 24/02/2013.</figcaption></figure>

495

496

497

498

499

## 3.2. Assessment in a macro-tidal region: the Joseph Bonaparte Gulf

500

501

502

503

The Joseph Bonaparte Gulf is located on a 500-km wide shelf in the northwest part of Australia, close to Darwin, between the Timor and Arafura Seas (Fig. 16). The ocean tides in this region reach some of the largest amplitudes that can be observed around Australia, with about 10 m range. The tidal currents are consequently strong in this macro-tidal region and
504

they can reach more than 1 m/s. The tidal currents are not provided in the GlobCurrent products (v3.0). Here we quantify the error associated with this missing component in the GlobCurrent products as well as we assess the accuracy of the GlobCurrent estimates of sub-tidal variability.

505

506

507

508

The three ADCP stations considered in this study are the Margaret Harries Banks (ITFMHB) station, the Flat Top Banks (ITFFTB) station and the Joseph Bonaparte Gulf (ITFJBG) station. They are located in the Joseph Bonaparte Gulf, at about 200m, 100m and 50m depth, respectively. The tidal currents at the ADCP stations were computed by applying tidal harmonic analysis to the ADCP raw currents time series at 15m depth, for both current directions separately. The five main tidal components in the region (M2, K1, S2, O1 and N2) were extracted in the tidal harmonic analysis, as the amplitudes of the other tidal components were in the data noise level. The GlobCurrent total currents at 15 m were extracted at each ADCP station location. In this region, the mean geostrophic current is very low, as shown in Fig. 16, and the tidal currents are dominant, reaching more than 50 cm/s, as shown by the tidal current ellipses in Fig. 17 and by the time series extract at the ITFFTB station in Fig. 18. This is confirmed by the very low correlations between the GlobCurrent total currents and the ADCP total currents (Tab. 2).

509

510

511

512

513

514

515

516

517

518

519

520

521

As the GlobCurrent total current products do not include the tidal currents, they are potentially comparable to the detided ADCP observations. However, even after removing the main tidal currents, the ADCP data still contain high-frequency ocean dynamics signals such as inertial currents and remaining tidal currents from tidal components with low signal to noise ratio. Both datasets were consequently filtered with a 30-day low-pass filter, as previously done in the EAC region.

522

523

524

525

526

527

Filtering the GlobCurrent total currents and the ADCP detided currents slightly improves the correlations at the ITFFTB and ITFMHB stations, mainly in the eastward and in

528
the northward directions, respectively (Tab. 2). However, there is still no clear agreement between the two datasets, whatever the direction of the current and the ADCP station (Fig. 19). At the ITFJBG station, the filtered GlobCurrent products globally show larger amplitudes than the ADCP data, with some differences of about 30 cm/s between the two datasets, in both directions. In addition, the GlobCurrent products seem to contain some signal of roughly one-month period, which is not visible in the ITFJBG ADCP current time series. Maps of the GlobCurrent geostrophic currents confirm that they contain strong unrealistic current features on the shelf (Fig. 20), with larger amplitudes even than the tidal signal.

537

538

<figure><img src="image_38.png" /><figcaption>Figure 16: Mean geostrophic velocities from the CNES-CLS13 ¼° Mean Dynamic Topography in the Joseph Bonaparte Gulf. The location of the three ADCP stations are shown with black triangles. The light gray lines show the 100m and 200m isobaths.</figcaption></figure>

539

540

541

542
<figure><img src="image_39.png" /><figcaption>Figure 17: Tidal current ellipses for the four main tidal components (M2, S2, K1 and N2) extracted from the ADCP current time series at 15-m depth. The location of the three ADCP stations are shown with black triangles. The light gray lines show the 100m and 200m isobaths.</figcaption></figure>

<table><caption>Table 2: Statistics computed for the comparison between the GlobCurrent total surface currents and the ADCP observations in the Joseph Bonaparte Gulf. The correlations are computed for each current direction separately (U – eastward, V – northward) and on the current norm. The RMS differences and the biases are computed on the current norm only, following the convention (GlobCurrent data - ADCP data).</caption><thead><tr><th rowspan="3">ADCP station</th><th colspan="4">Non-filtered total currents</th><th colspan="4">30-day filtered detided currents</th></tr><tr><th colspan="3">Correlation</th><th rowspan="2">RMS diff. (m/s)</th><th rowspan="2">Bias (m/s)</th><th colspan="3">Correlation</th><th rowspan="2">RMS diff. (m/s)</th><th rowspan="2">Bias (m/s)</th></tr><tr><th>U</th><th>V</th><th>Norm</th><th>U</th><th>V</th><th>Norm</th></tr></thead><tbody><tr><td>ITFJBG</td><td>0.03</td><td>0.00</td><td>0.10</td><td>0.31</td><td>0.08</td><td>0.16</td><td>-0.03</td><td>0.05</td><td>0.18</td><td>0.30</td></tr><tr><td>ITFFTB</td><td>0.06</td><td>0.06</td><td>0.07</td><td>0.16</td><td>-0.11</td><td>0.42</td><td>0.27</td><td>0.11</td><td>0.06</td><td>0.06</td></tr><tr><td>ITFMBH</td><td>0.15</td><td>0.22</td><td>0.10</td><td>0.11</td><td>-0.03</td><td>0.24</td><td>0.48</td><td>0.12</td><td>0.07</td><td>0.03</td></tr></tbody></table>
<figure><img src="image_40.png" /></figure>

Figure 18: Extract of the time series of the eastward and northward currents at the ITFFTB ADCP station, for the raw data at 15-m depth (in black), the tidal signal extracted from these raw data by harmonic analysis (in blue) and the GlobCurrent total currents at 15-m depth extracted at the ADCP location (in red). Jan.-Feb. 2013.
557

Eastward detided ADCP currents and GlobCurrent total currents - 30-day low-pass filter - Station ITFJBG

<figure><img src="image_41.png" /></figure>

30-day filtered detided ADCP

30-day filtered GlobCurrent total currents

Northward detided ADCP currents and GlobCurrent total currents - 30-day low-pass filter - Station ITFJBG

<figure><img src="image_42.png" /></figure>

(a)

Eastward detided ADCP currents and GlobCurrent total currents - 30-day low-pass filter - Station ITFFTB

<figure><img src="image_43.png" /></figure>

30-day filtered detided ADCP

30-day filtered GlobCurrent total currents

Northward detided ADCP currents and GlobCurrent total currents - 30-day low-pass filter - Station ITFFTB

<figure><img src="image_44.png" /></figure>

(b)

558
<figure><img src="image_45.png" /><figcaption>Figure 19: Time series of the eastward and northward ADCP residual currents at 15-m depth (raw currents – tidal currents), non-filtered data in gray, 30-day low-pass filtered data in blue, and 30-day low-pass filtered GlobCurrent total currents at 15 m in red, at the three ADCP stations: ITFJBG (a), ITFFTB (b) and ITFMHB (c).</figcaption></figure>
<figure><img src="image_46.png" /></figure>

Figure 20: GlobCurrent geostrophic currents on the 15/05/2014 in the Joseph Bonaparte Gulf. The location of the three ADCP stations are shown with black triangles. The light gray lines show the 100m and 200m isobaths.

# 4. Discussion and conclusions

In this paper, we have analyzed the consistency of the GlobCurrent ocean surface current products with in situ data (ADCP and drifting buoys velocities) in two specific regions off Australia: the South-East, where the East Australian Current predominates, and the Joseph Bonaparte Gulf, where tides and wind-driven currents predominate.

The results obtained for the on-shelf portion of South-East region, using the ADCP stations of SYD100, SYD140 and NRSNSI, indicate clearly that the GlobCurrent ocean surface currents are not fitted for shelf applications, at least in this and probably other similar regions. These current fields strongly rely on the accuracy of the MDT, whose spatial
resolution is not sufficient in the area, and on the satellite altimetry data coverage and quality, which both decrease near the coasts. The spatial resolution of the GlobCurrent products on the continental shelf is certainly too coarse to be able to represent the coastal processes, such as the upwelling which is known to occur in the region with the combination of encroachments of the EAC and southward winds (Roughan and Middleton, 2004). The coast of New South Wales and southern Queensland is also known to act as a wave-guide for coastal-trapped waves, which have moderately strong currents and a coastal sea level signal at scales that are smaller than the spatiotemporal scales of the GlobCurrent products, but are certainly seen by the ADCP instruments.

The offshore pseudo-drifter exercise gives a very good overview of the performance of the GlobCurrent products in the EAC region. It shows that the large and main structures of the ocean dynamics (EAC and eddies with diameters larger than 100 km) are rather well located on the maps, with spatial extensions consistent with the structures that are described by the real drifters trajectories. The smaller structures may not be always well located because of the inter-track distance of the satellite altimetry missions used to map the geostrophic currents. Indeed, these small structures (< 100 km) can occur in these inter-track intervals and never be sampled by the satellites. The Ekman current contribution in the GlobCurrent total currents product also clearly brings valuable information to the total surface current estimates. The pseudo-drifters advected by the GlobCurrent total current fields generally agree better with the real drifter trajectory than the pseudo-drifters advected by the GlobCurrent geostrophic current fields. However, the GlobCurrent products sometimes lack energy, probably because of the way they are constructed (optimal interpolation of satellite altimetry data and Ekman component derived from wind maps).

For the tidally-dominated North-West region, it is obviously important to take tides into consideration to obtain useful estimates of the total speed, so users will need to add tidal
predictions to the GlobCurrent estimates of the lower-frequency velocity, otherwise there is an ‘omission error’ that can reach 0.5m/s or more. For the non-tidal component of the current velocity, we find that the GlobCurrent products do not provide useful estimates, showing large unrealistic features on the shelf. These features are presumably due to incomplete de-aliasing of tide and wind-driven sea level variability in the altimetry sea surface anomalies that are used to compute the geostrophic current maps. Indeed, the global models used to remove these signals from the altimetry sea surface heights probably lack resolution in this wide shelf region to provide accurate corrections. Also, the mean sea surface used to compute the altimetry sea level anomalies probably contains large errors in this shelf region. All those errors result in artificial features in the GlobCurrent geostrophic currents.

Therefore, the GlobCurrent products should be used very carefully in such wide-shelf regions, especially if the tides are large and the high-frequency variability is not well modelled by the global models used for de-aliasing altimetry data in order to estimate the low frequency variability from the sparse data produced by the present constellation of (nadir-sampling) altimeters.

More generally, the GlobCurrent products could benefit from current efforts in many ways. Much progress has been made in coastal altimetry processing for these last ten years, including coastal waveforms retracking techniques, coastal corrections processing strategies and high resolution regional tidal models. The new generation of satellite altimeters, such as Sentinel-3, measures the sea surface height in SAR mode, which brings higher along-track resolution. The geodetic or quasi-geodetic missions, such as Jason-1 and CryoSat-2, are also of great relevance to improve the mean sea surface and the mean dynamic topography close to the coasts. Work is also on-going to optimize the parameters of the optimal interpolation technique used to grid the altimetry data, with very promising results regarding the location, amplitude and lifespan of the ocean dynamics pattern with size ranging between 50 and 100
628

km (Ubelmann et al, 2015). All these new methods and datasets should improve the
GlobCurrent-style products in terms of geostrophic currents.

629

630

The quality of the Ekman current component in the GlobCurrent products could
probably be increased by using higher spatiotemporal resolution wind fields.

631

632

Finally, it should be mentioned that global tidal atlases could be potentially used to
complete the GlobCurrent suite of products. For example, the FES2014 (Carrère et al, 2015)
and TPXO (Egbert and Erofeeva, 2002) global models are distributed not only with the tidal
elevations but also with the tidal currents. Even if these tidal models are based on barotropic
hydrodynamic assumptions, they would bring a first approximation of the main tidal signals
in this kind of regions, or could be replaced with regional 3-D models if available.

633

634

635

636

637

638

639

Acknowledgements

640

This work was supported by the Data User Element program of the European Space Agency.

641

The GlobCurrent data (version 3) were downloaded from the GlobCurrent portal on the
09/03/2017. The ADCP data are processed and provided by IMOS. They were downloaded
from the IMOS portal on the 07/01/2017. The drifting buoys data were processed and
provided by NOAA/AOML. They were downloaded from AOML service on the 12/04/2017.

642

643

644

645

646

References

647

Ardhuin, F., Y. Aksenov, A. Benetazzo, L. Bertino, P. Brandt, E. Caubet, B. Chapron, F. Collard, S. Cravatte, F. Dias, et al. (2017), Measuring currents, ice drift, and waves from space: the Sea Surface Kinematics Multiscale monitoring (SKIM) concept, Ocean Sci. Discuss., https://doi.org/10.5194/os-2017-65, in review

648

649

650
Bonjean, F., and G. S. E. Lagerloef, 2002. Diagnostic model and analysis of the surface currents in the tropical Pacific Ocean. J. Phys. Oceanogr., vol. 32, pg. 2938-2954.

Bowen, M. M., J. L. Wilkin, and W. J. Emery (2005), Variability and forcing of the East Australian Current, J. Geophys. Res., 110, C03019, doi: 10.1029/2004JC002533.

Carrère, L., Lyard, F., Cancet, M., Guillot, A., Picot, N., Dupuy, S. (2015), FES2014: A new global tidal model, presented at the Ocean Surface Topography Science Team meeting, Reston, USA.

Egbert G. D. and Erofeeva S. Y. (2002), Efficient inverse modeling of barotropic ocean tides. J. Atmos. Oceanic Technol., 19 (2), 183-204.

Griffin, D.A. and Middleton, J.H. (1991). Local and remote wind forcing of New South Wales inner shelf currents and sea level. J. Phys. Oceanogr., 21, 304-322.

Le Traon, P.Y. and G. Dibarboure, 2002 Velocity mapping capabilities of present and future altimeter missions: the role of high frequency signals. J. Atmos. Oceanic Technol., 19, 2077-2088.

Liu, Y., and R. H. Weisberg (2011), Evaluation of trajectory modeling in different dynamic regions using normalized cumulative Lagrangian separation, J. Geophys. Res., 116, C09013, doi:10.1029/2010JC006837

Liu, Y., R. H. Weisberg, S. Vignudelli, and G. T. Mitchum (2014), Evaluation of altimetry-derived surface current products using Lagrangian drifter trajectories in the eastern Gulf of Mexico, J. Geophys. Res. Oceans, 119, 2827-2842, doi:10.1002/2013JC009710.

Lumpkin, R., Semyon A. Grodsky, Luca Centurioni, Marie-Helene Rio, James A. Carton, and Dongkyu Lee (2013), Removing Spurious Low-Frequency Variability in Drifter Velocities, J. Atmos. Oceanic Technol., 30, 353-360, doi: https://doi.org/10.1175/JTECH-D-12-00139.1
Martin, A. C. H., and C. Gommenginger (2017), Towards wide-swath high-resolution mapping of total ocean surface current vectors from space: Airborne proof-of-concept and validation, Remote Sensing of Environment, Volume 197, August 2017, Pages 58-71, doi: 10.1016/j.rse.2017.05.020

Mata, M. M., S. E. Wijffels, J. A. Church, and M. Tomczak (2006), Eddy shedding and energy conversions in the East Australian Current, J. Geophys. Res., 111, C09034, doi:10.1029/2006JC003592.

Ridgway, K. and Hill, K. (2009), The East Australian Current. In A Marine Climate Change Impacts and Adaptation Report Card for Australia 2009 (Eds. E.S. Poloczanska, A.J. Hobday and A.J. Richardson), NCCARF Publication 05/09, ISBN 978-1-921609-03-9

Rio, M-H, S. Mulet, and N. Picot (2013): New global Mean Dynamic Topography from a GOCE geoid model, altimeter measurements and oceanographic in-situ data. Proceedings of the ESA Living Planet Symposium, Edinburg, September 2013.

Rio, M.-H., S. Mulet, and N. Picot (2014), Beyond GOCE for the ocean circulation estimate: Synergetic use of altimetry, gravimetry, and in situ data provides new insight into geostrophic and Ekman currents, Geophys. Res. Lett., 41, doi:10.1002/2014GL061773

Rio, M.-H. (2015), Globcurrent Product Data Handbook: the combined geostrophy + Ekman currents
(ftp://ftp.ifremer.fr/ifremer/cersat/documentation/gridded/globcurrent/GlobCurrent_D-280_Product_Handbook_L4combined_mhr.pdf)

Roughan, M., and J. H. Middleton (2004), On the East Australian Current: Variability, encroachment, and upwelling, J. Geophys. Res., 109, C07003, doi:10.1029/2003JC001833
696 Sudre, J., Maes, C., and V. Garçon (2013), On the global estimates of geostrophic and Ekman
697 surface currents, Limnology and Oceanography: Fluids and Environments, 3, pp 1–20, DOI
698 10.1215/21573689-2071927

699 Suthers, I. M., J. W. Young, M. E. Baird, M. Roughan, J. D. Everett, G. B. Brassington, M.
700 Byrne, S. A. Condie, J. R. Hartog, C. S. Hassler, et al, (2011), Editorial: The strengthening
701 East Australian Current, its eddies and biological effects – an introduction and overview,
702 Deep-Sea Research II, 58, pp: 538-546. doi:10.1016/j.dsr2.2010.09.029

703 Ubelmann C, Klein P, Fu L L (2015), Dynamic interpolation of sea surface height and
704 potential applications for future high-resolution altimetry mapping. J Atmos Ocean Technol.
705 doi:10.1175/jtech-d-14-00152.1

706

707

708

709

710

711 LIST OF FIGURE CAPTIONS

712 Figure 1: Mean geostrophic velocities in cm/s derived from the CNES-CLS13 ¼° Mean
713 Dynamic Topography off New South Wales. The velocity magnitude is in color and the
714 arrows give the direction and the amplitude of the current. The locations of the 4 ADCP
715 stations are shown with black triangles. The light gray lines show the 100m and 200m
716 isobaths.

717
Figure 2: Spectral analysis of the CH100 ADCP currents at 15 m (in black) and of the GlobCurrent geostrophic currents (in blue) extracted at the location of the CH100 ADCP station in Coffs Harbour, in each current direction. The single-sided amplitude spectrum was computed using an FFT method.

722

Figure 3: Ocean current roses of the 30-day low-pass filtered ADCP observations and GlobCurrent products (geostrophic currents and total surface currents at 15m) for each of the four ADCP stations. The numbers in blue indicate the angle of the ocean current direction (0° is eastward, 90° is northward). The numbers in black and the concentric circles give the number of occurrences in the observations for each angle. The NSRNSI ADCP station is located out of the grid of the GlobCurrent total currents and no current rose was computed in this case.

730

Figure 4: Mean geostrophic velocities from the CNES-CLS13 ¼° Mean Dynamic Topography off Brisbane. The location of the NRSNSI ADCP station is shown with a black triangle. The light gray lines show the 100m and 200m isobaths.

734

Figure 5: Mean geostrophic velocities from the CNES-CLS13 ¼° Mean Dynamic Topography off New South Wales. The locations of the ADCP stations off Sydney are shown with black triangles. The light gray lines show the 100m and 200m isobaths.

738

Figure 6: Monthly means of the ADCP currents (in black) and the GlobCurrent geostrophic currents (in blue) and total surface currents at 15 m (in red) at the SYD100 ADCP station.

740
741

742

Figure 7: Monthly means of the ADCP currents (in black) and the GlobCurrent geostrophic currents (in blue) and total surface currents at 15 m (in red) at the CH100 ADCP station.

743

744

745

Figure 8: Drifters trajectories and velocities (in red) and velocities of the GlobCurrent total currents at 15 m at the drifters positions (in blue). The ADCP velocities at the closest positions of the drifters are also shown (in black).

746

747

748

749

Figure 9: Skill scores (in color) after 3 days of advection with the GobCurrent products (a) geostrophic currents and (b) total currents at 15 m). The black dotted line shows the positions of the real drifter (101921). The light gray lines show the positions of the pseudo-drifters launched at each position of the real drifter and after 3 days of advection by the GlobCurrent maps. Below (c), time series of the skill scores along the drifter trajectory, for the GlobCurrent geostrophic currents in blue and the GlobCurrent total currents at 15 m in red. The black dashed vertical line shows the moment when the drifter leaves the East Australian Current (EAC).

750

751

752

753

754

755

756

757

758

Figure 10: Skill scores (in color) after 3 days of advection with the GobCurrent products (a) geostrophic currents and (b) total currents at 15 m). The black dotted line shows the positions of the real drifter (122612). The light gray lines show the positions of the pseudo-drifters launched at each position of the real drifter and after 3 days of advection by the GlobCurrent maps. Below (c), time series of the skill scores along the drifter trajectory, for the GlobCurrent geostrophic currents in blue and the GlobCurrent total currents at 15 m in red.

759

760

761

762

763
764

The black dashed vertical line shows the moment when the drifter leaves the East Australian Current (EAC).

765

766

767

Figure 11: Skill scores (in color) after 3 days of advection with the GlobCurrent total currents at 15 m. The black dotted line shows the positions of the real drifter (101749). The light gray lines show the positions of the pseudo-drifters launched at each position of the real drifter and after 3 days of advection by the GlobCurrent maps.

768

769

770

771

772

Figure 12: Skill scores (in color) after 3 days of advection with the GlobCurrent total currents at 15 m. The black dotted line shows the positions of the real drifter (139659). The light gray lines show the positions of the pseudo-drifters launched at each position of the real drifter and after 3 days of advection by the GlobCurrent maps.

773

774

775

776

777

Figure 13: GlobCurrent total current field (blue arrows) on the 20/02/2013 at 00:00 and positions of drifter 101921 (black dots) from the 13/02/2013 to the 16/05/2013. The four boxes in color (A, B, C and D) show the features of interest for the analysis of the GlobCurrent products.

778

779

780

781

782

Figure 14: Trajectories over the time of pseudo-drifters (in color) advected by the fields of GlobCurrent total surface currents at 15 m (right plot) or GlobCurrent geostrophic currents (left plot). The positions of the real drifter (101921) are in black. The GlobCurrent velocity fields used to advect the pseudo-drifters are shown with gray arrows.

783

784

785

786
787 Figure 15: GlobCurrent Ekman current field (blue arrows) on the 24/02/2013 at 00:00 and
788 path of drifter 101921(black dotted line) from the 13/02/2013 to the 24/02/2013.

789

790 Figure 16: Mean geostrophic velocities from the CNES-CLS13 ¼° Mean Dynamic
791 Topography in the Joseph Bonaparte Gulf. The location of the three ADCP stations are shown
792 with black triangles. The light gray lines show the 100m and 200m isobaths.

793

794 Figure 17: Tidal current ellipses for the four main tidal components (M2, S2, K1 and N2)
795 extracted from the ADCP current time series at 15-m depth. The location of the three ADCP
796 stations are shown with black triangles. The light gray lines show the 100m and 200m
797 isobaths.

798

799 Figure 18: Extract of the time series of the eastward and northward currents at the ITFFTB
800 ADCP station, for the raw data at 15-m depth (in black), the tidal signal extracted from these
801 raw data by harmonic analysis (in blue) and the GlobCurrent total currents at 15-m depth
802 extracted at the ADCP location (in red). Jan.-Feb. 2013.

803

804 Figure 19: Time series of the eastward and northward ADCP residual currents at 15-m depth
805 (raw currents – tidal currents), non-filtered data in gray, 30-day low-pass filtered data in blue,
806 and 30-day low-pass filtered GlobCurrent total currents at 15 m in red, at the three ADCP
807 stations: ITFJBG (a), ITFFTB (b) and ITFMHB (c).

808
809 Figure 20: GlobCurrent geostrophic currents on the 15/05/2014 in the Joseph Bonaparte Gulf.

810 The location of the three ADCP stations are shown with black triangles. The light gray lines

811 show the 100m and 200m isobaths.