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

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.

Rouault et al (2003)

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

SCTR L ABFZ

Agulhas C & retroflection

S Atl HIGH S Ind Ocn HIGH

TC

BNino floods

Monsoons

MJO

COLs

L

MOC

ITCZ

L

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

Botswana High

JFM 500 hPa GPH

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

Topography Contours: starting from 500m at 250 m interval Port Elizabeth 625 mm

Durban ~1000mm

East London ~700mm

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

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

Wide shelf, Port Elizabeth

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 MCC

Cut-off low 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

Station data

TRMM rainfall

Maximum of 119 mm recorded at Uloa Agricultural Office (mostly between 21h00-01h00 UTC) Heaviest rain rate - Mbazwana Airfield = 48 mm/hr This event contributed approximately 20% to the DJF rainfall in parts of northern KZN

8

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

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

0

500

1000

1500

2000

2500

3000

3500

28 29 30 31

Longitude

Hei

ght (

m)

Control

exTOPO

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

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

Precipitation (mm) from MM5 Model Runs:

Using 1° SST data vs 18 km SST data

Control exSST

Control - exSST exTOPO exTopo

* Low level convergence, upward motion and instability

23h00 01h00 03h00

exSST exTOPO 23h00 UTC

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

SWIO SST and correlations with Oct-Mar, OND, JFM rainfall

Oct-Mar

OND

JFM

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

1950-82

1983-98

1999-08

1950-1982

1983-1998

1999-2008

JFM rainfall over SA/Zim and SST correlations

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