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
Dec 14, 2015
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
Chl-a seasonal cycle
Thom
alla
et
al., 2
011
Complex balance between light and nutrient limitation that drives higher production in SAZ
>> Sub-seasonal variability of MLD modulates this balance
Joub
ert
et
al.,
subm
itte
d
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)
The overall Chl variance that is explained by the seasonal cycle (0-100%) was computed as the variance explained by the regression of Chl onto a
repetition of the mean seasonal cycle.
SAZ
High res in situ MLD summer progression
and variability
17 transects of XBTs to derive
MLD
Light - Fe threshold(Jourbert et al.,
submitted)
SA
Z
Underway chl-a south of Africa
Well stratified, punctuated by short winter mix
Summer highly reproducible but winter not
Dominated by heat fluxes.
STZ
APZ MLDs are deep (±100m)
MLD is seasonal = 57%
57%
17%
What do gridded datasets tell us?
Monthly EN3-derived Brunt-Vaisala Frequency and MLD
14% Weak seasonal cycle = 14% Variable MLDs & weak strat. Assoc with high wind var =
2.5 m.s-1
SAZ
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
Sw
art
et
al.,
2012
…At present we cannot do this without continuously sampling autonomous platforms!
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
Gough Is.
STF
SAF
APF
Gough/Tristan Transect
Goo
dHop
e Li
ne
Cape Town
Bathym
etry (meters)
= Glider deployment & ship CTD station
= ship based underway measurements
±2000 nm away…
SO SEASONAL CYCLE EXPERIMENT
September 2012 – March 2013
SG573
SG574
SG543
SG575
SG542
Surface – 1000m1.4 km horiz res
2532 dives = 5064 profiles537 days of sampling + ship process study
Cycl
one
Cycl
one
edge
Fron
t ed
ge
Intrus
ion
Subm
eso
fila
men
t
-edd
y
Strat. (BVF)
0-100m & 100-300m
MLD
Fluor
Temp
DensityPoster by S. Nicholson et al:
PP sensitivities to submeso dyn and subseas atm forcing
1. 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 drives early 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
Conclusions