Seb Swart, Sandy Thomalla & Pedro Monteiro

1. Resolving the seasonal cycle of mixed layer physics and phytoplankton biomass in the SAZ using high-resolution glider data Seb Swart, Sandy Thomalla & Pedro Monteiro

2. Chl-a seasonal cycle Recent work highlights importance of seasonal to sub-seasonal forcing of ML on PP (Levy, Klein, 2009; Thomalla et al., 2011; Fauchereau et al., 2011) SAZ Thomalla et al., 2011 The overall Chl variance that is explained by the seasonal cycle (0-100%) was computed as the variance explained by the regression of Chlonto a repetition of the mean seasonal cycle. Joubertet al., submitted Complex balance between light and nutrient limitation that drives higher production in SAZ >> Sub-seasonal variability of MLD modulates this balance

3. High res in situ MLD summer progression and variability 17 transects ofXBTs to derive MLD Light - Fe threshold (Jourbert et al., submitted) SAZ Underway chl-a south of Africa

4. STZ What do gridded datasets tell us? • Well stratified, punctuated by short winter mix • Summer highly reproducible but winter not • Dominated by heat fluxes. 17% SAZ • Weak seasonal cycle = 14% • Variable MLDs & weak strat. • Assoc with high wind var= 2.5 m.s-1 14% APZ 57% • MLDs are deep (±100m) • MLD isseasonal = 57% Monthly EN3-derived Brunt-Vaisala Frequency and MLD

5. HYPOTHESIS High rates of PP in SAZ are a direct result of MLD variability at submeso-subseasonal scales (around a threshold depth) that allows for alleviation of both light and Fe limitations at appropriate time scales for phytoplankton growth Unless these time scales (sub-seasonal) are correctly defined in terms physical – biogeochemical coupling, models will not accurately reproduce the seasonal cycle and hence predict future climate states Swart et al., 2012 …At present we cannotdo this without continuously sampling autonomous platforms!

6. = Glider deployment & ship CTD station = ship based underway measurements So seasonal cycle experiment Cape Town GoodHope Line Gough/Tristan Transect Gough Is. Bathymetry (meters) STF SAF APF ±2000 nm away… September 2012 – March 2013

7. Surface – 1000m 1.4 km horiz res SG573 SG543 SG575 SG574 SG542 2532 dives = 5064 profiles 537 days of sampling + ship process study

8. FLUOR TEMP SPRING Bloom PRIMING PERIOD SUMMER Bloom Sustaining PERIOD

9. FLUOR f BVF T

10. Cyclone Submeso filament -eddy Front edge Cyclone edge Intrusion Strat. (BVF) 0-100m & 100-300m MLD Fluor Temp Poster by S. Nicholson et al: PP sensitivities to submesodyn and subseasatm forcing Density

11. FLUOR BVF

12. Strat. (BVF) Wind R=0.52 Density MLD Fluor

13. Spring – Summer MLD progression… a reminder of scales Monthly EN3 CFSR 7-day 5-hrly Glider

14. Conclusions Bloom initiations vary depending on the criteria used to define them. Different bloom initiations can be explained by different mechanisms (e.g. Sverdrup’s critical depth, Taylor and Ferrari’s turbulent convection, Mahadevan’s eddy driven stratification) >> The response of the bloom onset to interannual and climatic change will depend strongly on which mechanism prevails, eg. wind/features 2. In Spring, feature driven changes to the mixed drivesearly stratification and bloom initiations >> If climate models don’t include lateral processes they will overestimate bloom initiation dates 3. In Summer, wind driven adjustments to the mixed layer plays an important role in sustaining the summer phytoplankton bloom by relieving Fe and light limitation at the appropriate time scales >> Highlight the importance of interplay between meso-submesoscale features versus wind-buoyancy processes in characterising the ML, productivity, timing of the bloom and carbon export

15. Many thanks to the following people and collaborators! • Geoff Shilling, Craig Lee &Eric D’Asaro at APL, UW • Derek &Andre at STS / SOERDC • Grant Pitcher & Andre Du Randt, DAFF • Stewart Bernard, Marjo Krug & Andy Rabagliati, CSIR • Gavin Tutt, DEA • IMT, SANAP, UCT, DEA & DAFF