Agulhas Group meeting notes, 2016-06-29

Andrew Hoell, Chris Funk, Jens Zinke, Laura Harrison: Modulation of the Southern Africa precipitation response to the El Niño Southern Oscillation by the subtropical Indian Ocean Dipole

What scientific questions are the authors addressing?
How the phase of the SIOD and therefore the SST anomaly expression of the Indian Ocean, modulates the Southern Africa precipitation response to ENSO?

What methods do the authors use to address the questions?

• SA Land area bound by the region 15S-35S; 12.5E-42.5E
• Observation: Global Precipitation Climatology Project version 2.2 (GPCP) 1979 to 2014, 2.5 x 2.5 deg resolution. Hadley-NOAA Optimum Interpolation dataset (OISST) 1.0x1.0 deg
• Three SST modes and their indices: ENSO (Nino3.4 >0.5K, table 2), IOD, Subtropical-IOD (Fig. 1)
• Two experiments: (1) Forced by observed global SST (2) Forced by the leading pattern of glabal time-varying monthly SST anomaly added to the monthly climatology, to isolate the atmospheric response to ENSO. (EOF1, Fig. 4)
• Each experiment has two sets of ensembles from two different models:(1) 30 members of ECHAM5.4, 50 members of GFS2, on 1x1 deg horizonal grid.
• Test whether the SIOD modifies the relationship between SA precipitation and ENSO through an examination of SA precipitation as a function of SIOD phase in two forced simulations.

What are the authors’ main conclusions?

• El Niño is associated with reduced precipitation in the Southern Africa, and La Niña tends to occur with abundant precipitation in the area. Both are further enhanced by out-of-phase SIOD, and disrupted by in-phase SIOD.
• For the in-phase SIOD and ENSO during December-March, SA precipitation is associated with an SST anomaly over the Agulhas Current region.
• ENSO is related to an equivalent barotropic Rossby wave over Southern Africa, that modifies the regional mid-tropospheric vertical motions and precipitation.
• The EOF1 forced case, either phase of SIOD does not modulate the SA precipitation response to ENSO (indistinguishable Fig. 10 and 11)

What are the implications of the results?

ENSO has been an important parameter for rainy season SA precipitation prediction, as SST playing a critical role in the regional climate. The modulation relationship between SIOD and ENSO presented here can help to make more informed forecasts.

Why did you choose this paper? looking into papers about South African precipitation variability.

Who read this paper? Yu Cheng