Upwelling, filament and eddies in the Canary Current upwelling system during the CAIBEX cruise (Summer 2009)

Upwelling, filament and eddies in the Canary Current upwelling

system during the CAIBEX cruise (Summer 2009)

C. Troupin

, P. Sangr`a

, J. Ar´ıstegui

, B. Barreiro Gonz´alez

& E. D. Barton

29oN 30oN 31oN 32oN 33oN 10 m/s 2009−08−15 → 2009−09−05 T (°C) 17 18 19 20 21 22 23 24 10 m/s 2009−08−17 10 m/s 29oN 30oN 31oN 32oN 33oN 2009−08−20 10 m/s 2009−08−24 10 m/s 13oW 12oW 11oW 10oW 9oW 29oN 30oN 31oN 32oN 33oN 2009−08−27 10 m/s 13oW 12oW 11oW 10oW 9oW 2009−09−01

FIGURE 1: The sea surface temperature and wind images show different stages of development of the filament. The coldest and widest filaments occurred between July 20 and 24, 2009. The time-averaged image corresponding

to the period of the cruise (first image) shows the coastal upwelling, the mean position of the filament and a region of higher SST along the coast, starting at 31◦N.

1

Upwelling filaments

• Upwelling filaments = narrow, elongated structures of cold water ex-tending offshore in the ocean upper layers (Figure 1).

• They deserve a particular attention because of their role in the offshore exportation of nutrients.

• _{Their mechanism(s) of generation are not perfectly understood.}

• The filament we have studied lies off Cape Ghir (30◦38’N, Morocco).

2

CAIBEX–Cape Ghir

Canaries-Iberian marine ecosystem Exchanges cruise off Cape Ghir:

• _{Aboard R/V Sarmiento de Gamboa}

• From August 16 to September 5, 2009.

• _{Data: CTD, SeaSoar, drifting buoys and ADCP velocities.}

18oW 15oW 12oW 9oW 6oW 28oN 30oN 32oN 34oN 36oN Canary Islands Madeira C. Ghir C. Sim C. Juby C. Bedouzza AFRICA

NORTH ATLANTIC OCEAN

−4000 −3000 −2000 −1000 0 1000 2000 3000 (a) 12oW 11oW 10oW 30oN 30’ 31oN 30’ 1 2 3 4 5 6−9 T 1 T 13 C. Ghir C. Sim Seasoar CTD Drifters (b)

FIGURE 2: Numbers from 1 to 9 indicate the Seasoar meridional

transects. Transects 6–9 are repeated at different times of the cruise in order to examine the variability of the filament on a time scale of a few day. T1 and T13 indicate the positions of the northernmost and

southernmost stations of the CTD transect, respectively.

The development of the filament appeared to be correlated to the inten-sity of the wind: the period of strong winds were followed by an large offshore extension of the filament.

Aug 15 Sep 01 Sep 15 Oct 01 Oct 15

−10 −5 0

Wind speed and direction

Cruise Drifter trajectories

FIGURE 3: Wind vectors obtained by averaging the QuikSCAT

mea-surements over the domain 30◦N-31◦45’N, 11◦30’W-9◦W. Winds

stronger than 10 ms−1 are indicated by bold arrows. During most

of the cruise, wind had a stronger meridional component and was generally upwelling-favourable.

3

Structure of the filament

3.1

Horizontal maps

At 4.5 m : cool water along the coast → upwelling,

offshore extension of upwelled → filament, warmer area south to 30◦40’N → wind shelter.

At 25 m: the filament appears as an elongated structure with a mean

lat-itude corresponding to that of Cape Ghir. The persistence of the fila-ment throughout the three week observation period is reflected in the strong signal in the mean.

At 300 m: structure of relatively warm water, centred at 31◦N and close to

the coast. 30oN 20’ 40’ 31oN 20’ 4.5 m T (°C) 16 18 20 22 24 26 30oN 20’ 40’ 31oN 20’ 25 m T (°C) 17 18 19 20 21 12oW 30’ 11oW 30’ 10oW 30’ 30oN 20’ 40’ 31oN 20’ 300 m T (°C) 13.4 13.5 13.6 13.7 13.8 13.9 14 14.1 14.2

FIGURE 4: Horizontal maps of temperature obtained by spatial

in-terpolation of all the measurements made during the cruise: the ther-mosalinometer data at 4.5 m (black dots), the CTD (squares) and SeaSoar data (dots) at 25 and 300 m. .

3.2

Vertical sections

Temperature: filament between stations T9 and T10.

Salinity: signal of the filament not identified. Chlorophyll-a: highest values at 25 m.

T & S: dooming of the isotherms and isohalines below 200 m between T4

and T7 → vertical velocity?

Depth (m) 1 2 3 4 5 6 7 8 9 10 11 12 13 400 300 200 100 0 T (°C) 12 13 14 15 16 17 18 19 20 Depth (m) 1 2 3 4 5 6 7 8 9 10 11 12 13 400 300 200 100 0 S 35.7 35.8 35.9 36 36.1 36.2 36.3 36.4 36.5 Depth (m) Latitude (°N) 30°15′ 30°30′ 30°45′ 31° 31°15′ 1 2 3 4 5 6 7 8 9 10 11 12 13 100 75 50 25 0 Chl a (mg/m 3₎ 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26

FIGURE 5: Vertical sections of temperature, salinity and

chlorophyll-a (Chl_a) obtained by interpolation of the meridional CTD transect.

Transects 5, 6 and 7: downbowing of the isotherms between 200 and

400 m.

Transect 9: filament position slightly south of 30◦30’N (SST≈ 19◦C).

30°15′ 30°30′ 30°45′ 31° Depth (m) Track 5: Temperature (2009−08−19) 400 300 200 100 0 T (°C) 12 13 14 15 16 17 18 19 20 21 22 23 30°15′ 30°30′ 30°45′ 31° 31°15′ Depth (m) Track 7: Temperature (2009−08−22/23) 400 300 200 100 0 T (°C) 12 13 14 15 16 17 18 19 20 21 22 23 30° 30°15′ 30°30′ 30°45′ Latitude (°N) Depth (m) Track 9: Temperature (2009−09−02/03) 400 300 200 100 0 T (°C) 12 13 14 15 16 17 18 19 20 21 22 23

FIGURE 6: Vertical sections of temperature obtained by interpolation

of the data SeaSoar and thermosalinometer data during the SeaSoar transects. The black dots represent the trajectory of the SeaSoar ve-hicle.

3.3

ADCP velocities

Surface layer (0–25 m): max. velocity = 0.55 m s−1 at 30◦40’N.

South of 30◦30’N: flow directed toward the coast (v ≤ 0.35 m s−1).

250–350 m: north-eastward velocities in the southern part of the transect.

Negative shear between T3 and T6 → anticyclonic eddy?

SeaSoar transects, 0–25 m : highest velocities in transects 1 and 4.

Current mainly westward.

Steep changes of velocity direction → mesoscale eddies?

11oW 36’ 10oW 30oN 30’ 31oN 0.5 m s−1 T₁₃ T₁ T 5 CTD transect, layer: 0−25 m, 150 kHz Shear/f −1 0 1 11oW 36’ 10oW 30oN 30’ 31oN 0.5 m s−1 T₁₃ T₁ T 5 CTD transect, layer: 250−350 m, 75 kHz Shear/f −0.5 0 0.5 12oW 30’ 11oW 30’ 10oW 30oN 30’ 31oN 30’ 0.5 m/s

SeaSoar tracks 1−6, layer: 0−25 m, 150 kHz

FIGURE 7: ADCP velocities measured along the CTD transect in the

0–25 m layer (top), in the 250-350 m layer (middle) and along the SeaSoar transects 1–6 in the 0–25 m layer. The color scales indicate the current shear normalized by the Coriolis frequency f .

3.4

Drifters

First 10–15 days: weak displacements (calm winds).

After 15 days: north-westward motion for drifters 95854, 95855 and

95867 → entrainment by the filament.

Between 31◦N and 31◦30’N: cyclonic trajectory then south-westward

motion. Median velocity ≈ 0.35 m s−1.

300 m-drogue drifter (95864): cyclonic loops: period T ≃ 5-6 days;

ra-dius R ≃ 25–30 km; median velocity v ≈ 0.15 m s−1.

14oW 13oW 12oW 11oW 10oW 28oN 29oN 30oN 31oN = Release 95854 95855 95861 95862 95864 95867

FIGURE 8: The drifter trajectories from September 3 to October 31,

2009, show an overall north-westward displacement, followed by a cyclonic loop and a southward motion. Drifter no. 95864 (black line) was drogued at 300 m.

4

Conclusions & future

work

The main features occurring off Cape Ghir in summer:

1. the coastal upwelling, with a width of 50 km and minimal surface tem-perature of 16◦C;

2. the filament, with an extension on the order of 100 km, a depth of 50 m and surface temperature up to 4◦C lower than the open-ocean water at the same latitude;

3. the warm subsurface anticyclonic eddy, located north of the filament, at depth between 200 and 400 m;

4. the wind-sheltered area south of Cape Ghir, where the coastline under-goes a change of concavity.

Future work:

• Confirm the existence of the subsurface anticyclonic eddy and its in-teraction with the filament

• Compare with observations of other cruises in the same area at different time of the year.

5

Acknowledgements

The CAIBEX cruise was financed by the Spanish Ministry of Education and Sciences (MEC) through project CTM2007-66408-C02-01/MAR. The authors are thankful to the captain and crew of the B/O Sarmiento

de Gamboa, as well as the staff of the UTM (CSIC) for their assistance

at sea.

I. Tomazic (GHER, ULg) helped for the satellite images processing and M. Benavides Gorostegui helped with the chlorophyll data.