Links between Agulhas Current variability and shelf upwelling off Port Alfred, South Africa
Neil Malan1,2([email protected]), Mike Roberts3, Isabelle Ansorge1!
1 Dept. Oceanography, University of Cape Town,!
2 South African Environmental Observation Network!
3 Oceans and Coasts, Dept. of Environmental Affairs, Cape Town, South Africa
The nature of upwelling off Port Alfred
temperature (oC)
Introduction
Background!
Topographically driven upwelling inshore of the Agulhas Current, centred
around Port Alfred (33.6°S, 26.9°E), has been identified as an important source
of cold, nutrient-rich water for the shelf regions of the Agulhas Bank
(Lutjeharms et. al. 2000, Lutjeharms 2006). However little is known of the
variability of this upwelling activity and its sensitivity to changes in the nature of
the Agulhas Current itself. Recent studies suggest that the Agulhas Current
core and variability are changing in response to basin-scale wind changes
(Backeberg et. al. 2012) and that this may be causing an intensification of
upwelling at Port Alfred (Rouault et. al. 2010). This study uses in-situ and
remote sensing measurements to explore the variability of upwelling activity off
Port Alfred.!
20
15
• MIdshelf temperature
10
5
May
Jun
Jul
Aug
Sep
Oct
Nov
time)
Dec
Jan
Feb
Mar
Apr
May
1.8
1.6
1.4
Power Spectral Density
Case Studies: Natal Pulse effects
1.2
•
1
0.8
0.6
0.4
0.2
0 −2
10
−1
10
0
1
10
2
10
spectra reveal a dominant
mode of 50-80 days, similar to
that of Natal Pulses.!
Peaks are also seen at 5-7
days and 10-12 days when
comparing spectra from the 3
mooring sites.
3
10
10
Period (days)
!
Data!
ADCP Moorings!
!
• Inshore: 31m depth, 3.6 km
•
•
from coast !
Midshelf: 76m depth, 23 km
from coast!
Offshore: 175m depth, 42km
from coast!
An analysis of depth-binned currents, bottom temperatures and
along-shelf current profile at the Offshore mooring during a Natal
Pulse event. Note the double reversal of currents.
!
!
!
!
!
inshore bottom temperature (oC) 31m
25
20
Data from 3 hydrographic cruises provides insight into the nature of these
upwelling events.!
15
!
10
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Midshelf bottom temperature (oC) 76m
20
!
b) September 9th: Current touching shelf (marked by salinity max), no shelf
upwelling, in the wake of meander event. Shelf water from previous upwelling
event outcropping at surface. !
15
10
5
May
a) May 5th: Current in offshore position (small meander), limited shelf
upwelling, 12°C water on shelf up to 50m, little outcropping at the surface.!
!
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Offshore bottom temperature (oC) 175m
20
c) April 10th: Current visible inshore but not touching shelf, large bolus of !
< 12°C water upwelled onto the shelf, little surface outcropping.!
!
15
Port Alfred Airport
windrose (m.s-1)
NORTH
10
15%
5
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
10%
May
5%
Bottom temperatures recorded at the inshore, midshelf and offshore mooring
sites show 5 main cold events (shaded grey), where upwelled water intrudes
all the way to the inshore mooring site. These events happen roughly every 2
months over the 11-month record.!
WEST
EAST
12 − 14
10 − 12
8 − 10
!
Water below 16°C (indicative of South Indian Central Water upwelled from
below 400m) was present at the midshelf mooring site 85% of the time.!
6−8
4−6
2−4
SOUTH
The authors would like to acknowledge additional travel funding from SAEON and the US Office of Naval Research
0−2
The effect of wind is
predominately downwelling,
with SW winds dominant.
Short sharp periods of
upwelling (NE) wind stress
appear to have an important
role in outcropping and
inshore temperatures, but are
not long term enough to
account for the semipermanent nature of upwelling
The same event at the Midshelf mooring shows more barotropic
current reversals, possibly due to coastal trapped waves (Schumann
& Brink 1990)
Conclusions
• Shelf upwelling is more prevalent than previous studies suggest.!
• The main mode of variability appears to be the Natal Pulse, however
the shelf’s interaction with these is complex.!
• Upwelling wind events are important for the outcropping of upwelled
water.!
• There is evidence of the passage of coastal trapped waves on the
Midshelf and Inshore sites.!
• The position of the Agulhas Current in relation to the shelf appears to
be important for the activation or cessation of shelf upwelling in the
bottom layers.
References: Backeberg B. Penven C. & M. Rouault (2012). Impact of intensified Indian Ocean winds on mesoscale variability in the Agulhas system. Nature Clim. Change, Vol 2.
Lutjeharms, J. R. E., Cooper, J., and Roberts, M. (2000). Upwelling at the inshore edge of the Agulhas Current. Continental Shelf Research , 20.
Lutjeharms, J. (2006). The Agulhas Current. Springer. Rouault M., B. Pohl & P. Penven (2010). Coastal oceanic climate change and variability from 1982 to 2009 around South Africa. SA J. Mar. Sci. 32(2).
Schumann EH. & KH Brink (1990). Coastal-Trapped Waves off the Coast of South Africa: Generation, Propagation and Current Structures. J. Phys. Oceanography. Vol 20.