The Angola Current and its seasonal variability as observed at 11°S

The Angola Current

and its seasonal variability as observed at 11°S

Robert Kopte

, Peter Brandt

, Marcus Dengler

, Martin Claus

, Richard J. Greatbatch

[1] GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany

[2] Kiel University, Kiel, Germany

EGU General Assembly 2016 Vienna, Austria

April 18, 2016

0^o 5^oE 1 0^oE 1 5^oE 3 0^oS

2 5^oS 2 0^oS 1 5^oS 1 0^oS 5^oS 0^o

A N G O L A

N A M IB IA

EUC SEUC

GC

SECC AD

BC

AC

ABF

ABA region

11°S array

The Angola Current and its seasonal variability at 11°S

The Angola Current (AC) within the southeastern Atlantic upwelling region

adapted from

Rouault et al., 2007

▶ Key component for the upwelling region as the connecting element with the

tropical Atlantic

What drives the variability?

▶ Remotely generated coastal Kelvin waves

▶ Locally generated coastal Kelvin waves

▶ Advective processes?

Interannual SST variability in the upwelling region

Benguela Niño

Benguela Niña

Figure S4:Annual cycle of wind stress (top) and SLA (bottom) along

the equator (left) and along the Southern African coast (right); Hovmoller plot of 1999-2009 weekly mean climatology of: top: QuikScat wind stress (zonal component

SLA (cm) along the equator (left) and the southwest African coast (right)

Wind stress

[N/m²] (zonal along equator, meridional along coast)

The Angola Current and its seasonal variability at 11°S

▶ Bi-annual intrusion of warm tropical water into the northern Benguela upwelling region, quasi-synchronized to the seasonal cycle in sea level anomaly (SLA)

Rouault, 2012

▶ What is the associated variability of the Angola Current?

Remotely forced semi-annual waves

The Angola Current and its seasonal variability at 11°S

Historic observations of the Angola Current

▶ Hydrographic survey to enhance understanding of the circulation in the Southeast Atlantic Ocean in 1968

▶ Name change from “South Current” to “Angola Current”

20 m depth 150 m depth

(cm/s)

Based on geostrophic circulation in various depth layers:

▶ Maximum velocities >50 cm/s below the surface (9°S – 16°S)

▶ Vertical extent to 250-300m depth

Moroshkin et al., 1968

The Angola Current and its seasonal variability at 11°S

Meteor cruises M98 and M120

▶ Snapshots of alongshore velocity and hydrography on several sections along the pathway of the Angola Current

▶ Mooring array installation / maintenance

▶ High temporal variability of the alongshore flow

1 2 . 8 °E 1 3 °E 1 3 . 2°E 1 3 . 4°E

0 − 5

0 5

5 5 2 5 . 7

2 6 . 4

2 6 . 8

2 7 . 1

0 m

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

1 2 . 8 °E 1 3°E 1 3 . 2°E 1 3 . 4°E

− 2 5 − 1 5 − 5

0

0

5 1 5

1 5 2 5

2 4 . 5 2 5 . 7

2 6 . 4

2 6 . 8

2 7 . 1

( c m / s )

− 2 5

− 2 0

− 1 5

− 1 0

− 5 0 5 1 0 1 5 2 0 2 5

ADCP shield Flotec ADCP

11°S M120 10 / 2015 11°S

M98

07 / 2013

Alongshore flow

(cm/s)

First moored observations of the Angola Current

KPO_1107 KPO_1106

▶ Weak mean southward flow of 5-8 cm/s at 50 m depth

▶ Mean southward current extends to about 200 m depth Geostrophic velocities derived from AVISO dynamic heights

inshore offshoreAlongshore flow

▶ Pronounced intra-seasonal to seasonal variability, slightly stronger on the inshore mooring position

The Angola Current and its seasonal variability at 11°S

Transport estimate of the Angola Current

Method

▶ Derive patterns of variability from ship sections and / or model output (following Brandt et al. 2014)

▶ Regress main patterns on mooring time series to reconstruct 3D flow field

▶ Integrate velocities within a box representing the mean limits of the Angola Current

Ship sections OGCM

Mean transport (Sv)

-0.28 -0.39

Standard dev. (Sv)

0.51 0.66

The Angola Current and its seasonal variability at 11°S

Intra-seasonal to seasonal variability

▶ Dominance of 120-day, semi- annual, and annual oscillations

▶ Longer periods show more

baroclinic structures _{offshore inshore}

The Angola Current and its seasonal variability at 11°S

Influence of resonant equatorial basin modes?

Zonal velocity – 182-day – Baroclinic mode 2

cm/s

Meridional velocity – 182-day – Baroclinic mode 2

cm/s

Kelvin wave Rossby wave

▶ Equatorial basin modes: Low-frequency standing equatorial modes (Cane and Moore, 1981)

▶ 365-day, 182-day, and 120-day periods are close to resonance periods of baroclinic mode 4, 2, and 1

The Angola Current and its seasonal variability at 11°S

Summary and outlook

▶ First long-term observations of the AC (July 2013 – October 2015)

▶ Mean southward flow of 5-8 cm/s, AC penetrates down to ~200 m depth

▶ Weak mean southward transport of the AC of 0.28 Sv

▶ Seasonal variability is

dominated by 120-day, semi- annual, and annual oscillations

▶ How much of the observed seasonal variability can be explained by linear superposition of equatorial basin modes?

▶ Which parts of the spectrum are forced locally / remotely?