The temperature vs growth rate model (Fig. 1) seems consistent with the common observations of chlorophyll bloom-concentrations across the PF. The circulation model’s cluster simulation analysis suggests that the physical-biological interaction of exposing growing diatoms at near optimum ambient temperatures by the wind-driven advection process is an important, if not a primary driver for bloom development in the PF in the absence of iron.
Earlier investigations suggested  that the Antarctic Surface Water (ASW) could be transported together with diatom cells northwards across the PF thermal gradient, and the hydrodynamics of this transport has been confirmed . Our model demonstrates this might happen in all regions of the circumpolar PF belt. As a validation of the model results, chlorophyll concentration for January of 2019 obtained from NASA Earth Observation site shows the high surface chlorophyll concentrations match well with the northern edge of the PF (Fig. 5), agreeing with the simulated results of our model (Fig. 1).
Results from the numerical modeling sensitivity experiments can be summarized as follows. For all regions, increased winds increased the distance the particles traveled moving them into warmer waters and therefore increasing the growth rate. Some regions, particularly regions C, E, and F showed stronger influence of increased winds. This can be seen in figure (Fig. 4a-f) by the increased difference in growth rates between experiments. Region D (Eastern Pacific sector) showed the largest northward transport in all experiments, including the control, showing large geostrophic flows in this region pushing the particles into warmer waters.
In four of the regions, A, D, E and F, (all in the Eastern Pacific through Atlantic-Indian sectors) the winds pushed the parcels into temperature zones over the max threshold of 5°C (Fig. 4a-f), corresponding to a growth rate of K=0.62 d-1 given by Eq 1. For regions A and D the threshold is exceeded in all three experiments. In regions E and F this threshold in only reached in some of the experiments, double and triple wind for E and all experiments apart from the control in F. This indicates a window of opportunity, where growth rate can be maximized before being pushed to unfavorable conditions by strong winds. In region F the growth rate plateaus right above the threshold for the wind experiments and remains just below the threshold for the control experiment with no wind. This indicates that in region F phytoplankton drifting across the front would remain at near optimal temperature for most of their journey. This matches well with what is seen in the surface chlorophyll in Figure 5 with region F (near Drake Passage) being an area of high primary productivity. These regions also have the narrowest band of the PF, which might set up ideal conditions for cells to transverse across the PF to a growth zone of higher temperature.
A limitation of this modelling is that these simulations are done in an idealized manner with average winds and January (summer) conditions only. Future studies can include similar experiments with synoptic realistic winds (which would be much stronger than average winds) and for other months in spring and summer. Additionally, in our model seed locations were selected to avoid the eddies within the geostrophic flow field. These mesoscale eddies are very prevalent within the polar font and have the potential to prevent the particles northward movement. Depending on the temperature of these eddies, this can either be detrimental or beneficial to the phytoplankton growth. In one scenario an eddy trapped a particle at 5°C, the optimum growth rate, potentially allowing for a bloom to from. In other case the eddies trapped the particles in around 2°C, not ideal for temperatures for growth.
Finally, we would like to reiterate the idea of the Polar Front blooms and their mechanistic linkages to the physical and temperature-dependent growth rate in the light of above results and discussion of simulations. Maximum growth rates of the polar diatoms have been reported to occur above the ambient temperature [22-24, 27], meaning molecular and genetic evolution for temperature-dependent growth rate adaptation is still in progress in polar waters. Most shipboard iron-enrichment incubations and field fertilizations demonstrated changes in the phytoplankton composition towards larger macro-sized cells (mostly diatoms), enhanced physiological performance of the cells and the consequent increase of phytoplankton biomass [17, 21, 37, 38]. However, culture enrichments with iron did not always give the expected results [21, 39]. Therefore, a more consistent growth rate trend of polar diatoms as a function of temperature and iron availability still needs to be achieved. It is worth noting that the sole addition of iron does not always double the growth rate of polar diatoms . In contrast, temperature does allow for growth, even if increased by just a few degrees above the ambient conditions [23, 24, 38, 40], and in iron-limited conditions [26, 38]. The enhancement of polar diatom growth rates with increasing temperatures has been reported within the usual temperature range of the PF zone [24, 27, 26]. Early investigations calculated up to a 50-100% increase of photosynthetic rates of natural Antarctic phytoplankton from ambient sub-zero temperatures up to 4-7⁰C [22, 23]. Batch cultured diatoms collected in the Weddell Sea displayed 200% increased of maxima growth rates when temperatures rise from -1.6⁰C (ambient) to 1⁰C .
High chlorophyll concentrations up to 2 mg.m-3 under limiting iron conditions of 0.15-0.45 nM has been reported in the PF of the Atlantic sector justifying that past higher iron availability could have already been incorporated by phytoplankton prior to their sampling survey . The authors  also neglected the effect of a small temperature range of just 1.2⁰C on biomass differences across the front. In fact, this could be the case regarding iron, yet polar diatoms are very sensitive to even slight increases of temperature [22-26, 40] and our model predicts that under that temperature range growth rates could have been increased by 0.13 d-1, which would be enough to double cell density in just five days. In addition, it has been reported that Antarctic phytoplankton growth rates in coastal, presumably non-iron deficient, and offshore iron-limited waters tend to be similar if temperature is held constant . A difference of 40% between the growth rate of iron-replete and iron-deplete cultures was recently reported, yet the effect of temperature increase on the growth of both iron-deplete and iron-replete diatoms was remarkably higher (60-100%) [26, 38] meaning the effect of temperature on the growth of polar diatoms can be higher than the isolated effect of iron.
Despite a seasonal decrease of ca 1 nM of iron stock over the upper 100 m layers across the PF region , the left-over of 1-1.5 nM of iron was still above the minimum requirement of 0.2-1.0 nM of oceanic and coastal phytoplankton [41, 42] and within the range of the half-saturation constant (Km) of 0.59-1.12 nM for iron uptake by polar diatoms  and at least 5x times higher than the Km of 0.022 nM and 0.027 nM found in the northern and southern side of the PF, respectively . In addition, the necessity of iron for the synthesis of the enzyme nitrate reductase increases at lower temperatures , suggesting that iron deficiency is alleviated in the warmer northern bound of the PF. High concentrations of chlorophyll in cyclonic eddies formed in the PF have been associated with cross-front transport of iron from the south to the north and heat from the north to the south . The authors argued that heat exchange increases the physical stratification of the euphotic zone, reducing simultaneously iron and light limitations, hence favoring phytoplankton growth. Our model simulations suggest that besides iron and non-light limited condition, the increase of mean temperature outside the eddy is an additional driver for the higher phytoplankton growth rates and chlorophyll accumulation in the Antarctic region. As stated earlier, phytoplankton growth needs the simultaneous interaction of multiple drivers [30-33] to balance losses and increase net biomass in any natural environment. Yet for a bloom to develop in the PF growth rates must increase to a level that temperature by itself may provide, independent of iron availability . Our model predicts that phytoplankton bloom development in the PF depends on the wind-driven Ekman transport and the mixing of cold south waters with the warm north waters of the front, providing cells a mean ambient temperature near the optimum levels for growth rates necessary to develop blooms. We demonstrate that the wind field, combined with geostrophic flow in the PF zone, is the main driver exposing diatoms to optimal temperatures on the northern side of the front, enhancing their growth rates over that observed in the colder ASW south of the front. This does not contradict iron limitation on bloom development along the Southern Ocean, particularly along the PF where usual dissolved iron concentration is in fact exceptionally low . It simply demonstrates that the advection of cells across the front turns temperature into an additional factor that may even be more effective than iron for growth rate enhancement, when and where iron is not limiting.