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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