Some influences of the Agulhas Current on regional weather and climate Chris Reason, University of Cape Town

• Local air-sea interaction • Severe weather • Seasonal to interannual variability • Low frequency variability

How can the Agulhas Current influence regional weather & climate? • Local air-sea interaction and intensification of weather systems • Modification of regional atmospheric circulation and storm tracks • Ocean advection of tropical signals into the midlatitudes • Modulation of climate mode impacts (e.g., Benguela Ninos, SIOD)

M. Cronin

Rouault et al (2003) 18 km resolution AVHRR Pathfinder SST (Reynolds and Smith, 1994) averaged over the period 23-30 April 1995 during which the ACASEX field expedition took place. The ACASEX cruise track is shown in blue. The core of the Agulhas Current has SST > 22oC. White areas over the ocean correspond to those for which there was persistent cloud cover during the cruise

Rouault et al (2003)

Comparison between latent heat flux estimated during the ACASEX cruise (black) and ECMWF reanalysis 4 daily latent heat (red) (upper panel) and NCEP (lower panel). ECMWF/NCEP data were interpolated at the location of the measurements. Comparison is for SST greater than 23oC when the ship was above the core of the current.

Regional ocean areas important for southern African climate • Agulhas Current and its retroflection • Seychelles-Chagos thermocline ridge in the tropical SWIO • Angola – Benguela frontal zone in the tropical SE Atlantic

Xie et al 2002

Monsoons BNino floods

ABFZ

MOC

S Atl HIGH

MJO

L

COLs

Agulhas C & retroflection

L

SCTR ITCZ

TC

S Ind Ocn HIGH

L

Moisture flux and moisture flux convergence (+ve) at 850 hPa - JFM

JFM 500 hPa GPH

Botswana High

JAS 850 hPa GPH

1. Evidence for influence of Agulhas Current on SA weather and climate

• Jury et al (1993) showed that mean summer rainfall along east coast was related to typical distance of station from the core of the current

Durban ~1000mm

East London ~700mm

Port Elizabeth 625 mm

Topography Contours: starting from 500m at 250 m interval

34S – PE 625mm

34S – Syd 1213mm

Why is Port Elizabeth so dry compared to Buenos Aires, Sydney (east coast cities at same latitude)? The Agulhas Current moves away from the coast upstream from PE & there is inshore upwelling

Wide shelf, Port Elizabeth

Secondary mesoscale circulations and proximity of current to the coast (Jury et al, 1993)

Narrow shelf, Durban

2. Evidence for influence of Agulhas Current on SA weather and climate

• Strength of weather systems like tropical-extratropical cloudbands, MCCs, and cut off lows and their rainfall appears related to SST in SW Indian Ocean, assuming that atmospheric conditions are favourable for these systems to form in the first place

Severe weather

TTT Cloud band

Cut-off low

MCC

Trop Cyclone

TC intensification TC Favio 2007

Cluster analysis of OLR anomalies (convective cloud) Fauchereau et al (2009)

Correlations between OLR clusters and SST Fauchereau et al (2009)

MCC - Blamey PhD (2012)

MCCs over southern Africa:

 Between 8-9 systems per season

 Most frequent during

November and December

 Mainly found along the eastern coastal regions

 Predominantly exhibit a nocturnal lifecycle

 Average duration 9.5 hours

 Average size ±121,100 km² Blamey and Reason (2012) Blamey 2012 – 10thICSHMO

Grid-point frequency

MM5 regional atmospheric modelling of a February 2005 MCC

 Maximum of 119 mm recorded at Uloa Agricultural Office (mostly between 21h00-01h00 UTC)  Heaviest rain rate - Mbazwana Airfield = 48 mm/hr TRMM rainfall

Station data

 This event contributed approximately 20% to the DJF rainfall in parts of northern KZN

1000 hPa Geopotential Height (m) and Wind (m sˉ¹)

8

Low level moisture flux (g kgˉ¹m sˉ¹) and convergence (g kgˉ¹sˉ¹) (derived from NCEP data) Blamey & Reason (2009)

850 hPa Moisture Flux

Moisture Flux along 30°S

1. SST Sensitivity Test (exSST OISST vs AVHRR SST)

2. Topography Sensitivity Test (exTOPO) Blamey and Reason (2009) 3500

Control

3000

exTOPO

2000 1500 1000 500

Longitude

31

30

29

0

28

Height (m)

2500

Control

exSST

Precipitation (mm) from MM5 Model Runs: Using 1° SST data vs 18 km SST data

Control - exSST

exTOPO

exTopo

23h00

01h00

03h00

* Low level convergence, upward motion and instability

exSST

23h00 UTC

exTOPO

7am-9pm 24 Mar (3 Hour Interval Singleton and Reason (2007)

Mesolow

Coastal mountains

Model rain with full physics

Model rain with effects of surface Latent heat fluxes switched off

This model result plus several others and observation analyses provide evidence that the Agulhas Current can influence severe weather phenomena (e.g., MCCs, cut-off lows) over southern and eastern parts of South Africa

3. Evidence for influence of Agulhas Current on SA weather and climate

• AGCM expts show that rainfall is related to SST in the Agulhas Current region and that it can modulate Benguela Nino rainfall associations

Seasonal scales Difference in AGCM response between run with Agulhas Current smoothed out and run with climatological SST

Summer precipitation differences

Interaction with Benguela Nino rainfall associations

Hansingo PhD

Difference in response of HadAM3 GCM forced with idealised Benguela Niño and warm SSTA in SW Ind Ocn minus response to idealised Benguela Niño only: Late summer rainfall (top) and moisture flux / convergence (bottom)

4. Evidence for influence of Agulhas Current on SA weather and climate

• Interannual rainfall variability in southeastern Africa related to SSTa in SW Indian Ocean

Oct-Mar

OND

SWIO SST and correlations with OctMar, OND, JFM rainfall

JFM

1950-82

SWIO SST and correlations with JFM rainfall for 1950-82, 83-98, 99-2008 83-98 decade had strong Indian Ocean Dipole activity

1983-98

1999-08

JFM rainfall over SA/Zim and SST correlations

1950-1982

1983-1998

1999-2008

Kruger (2006)

Kruger (2006)

Evidence for influence of Agulhas Current on SA weather and climate • Jury et al (1993) showed that mean summer rainfall along east coast was related to typical distance of station from the core of the current • Strength of weather systems like tropical-extratropical cloudbands, MCCs, and cut off lows and their rainfall appears related to SST in SW Indian Ocean • AGCM expts show that rainfall is related to SST in the Agulhas Current region and that it can modulate Benguela Nino rainfall associations • Interannual rainfall variability in southeastern Africa related to SSTa in SW Indian Ocean