Interactive comment on “ Environmental controls on marine productivity near Cape St Francis , South Africa ” by Mark R . Jury

This exploratory study uses reanalysis products to investigate the ocean and its atmospheric drivers off the Eastern Cape province of South Africa. The influence of these environmental variables on productivity and fisheries are further investigated. The study is novel for the region, in that it uses freely available reanalysis products instead of the traditional in situ measurements or standard regional numerical models. Some in situ measurements made in the study area are assimilated into the reanalysis products, but not many. Apart from remote sensing products, the region is not widely covered in terms of in situ measurements. This makes the use of reanalysis products more useful and in future they will most likely become a necessary component of any large scale study in the region. This study shows the way for local researchers to use a


General Comments
This exploratory study uses reanalysis products to investigate the ocean and its atmospheric drivers off the Eastern Cape province of South Africa. The influence of these environmental variables on productivity and fisheries are further investigated. The study is novel for the region, in that it uses freely available reanalysis products instead of the traditional in situ measurements or standard regional numerical models. Some in situ measurements made in the study area are assimilated into the reanalysis products, but not many. Apart from remote sensing products, the region is not widely covered in terms of in situ measurements. This makes the use of reanalysis products more useful and in future they will most likely become a necessary component of any large scale study in the region. This study shows the way for local researchers to use a C1 new tool to investigate atmospheric-ocean interactions in an area short on large-scale in situ measurements.
The main suggestion of the study, as seen by the reviewer, is that the mid-Atlantic high pressure ridge underpins marine productivity in the area. It is this high pressure ridge moving anti-clockwise around southern Africa that is responsible for the easterly winds that drive coastal upwelling and hence productivity. Apart from wind, and rain, the high pressure is also responsible for the sea surface height gradient that contributes towards shelf and coastal ocean processes. The Agulhas Current also plays a big role through pressure gradients, nearshore currents and both cold and warm water intrusions. These all play a role in determining the marine productivity of the area.
The interpretation of the chlorophyll distribution between dry and wet conditions needs further clarification. Chlorophyll and currents change drastically over a period of days, while the change from dry to wet conditions happened over months. So, the author is comparing processes that occur over different time scales, and this could lead to an incorrect interpretation.
The movement of low salinity water south-westward from the Fish River needs to be checked. Can fresh water from the Fish River actually move that far across the shelf in that amount of time, and without dispersing? What about the counter coastal current, towards the north, that could hamper the south-westward movement of water? To the reviewer, the lower salinity structure shown in Fig 4 looks very similar to a structure that is observed after extensive, strong coastal upwelling when high chlorophyll is observed along the open-ocean side of joined upwelling plumes that originated at the capes.
A few improvements could be made to the paper. Figures are small, making them difficult to read, but the figures could be fine in the published paper so maybe it is not important. References are missing. There could be more references in the discussion, and even the results, if references in the results are acceptable. I recommend that the paper be published. 148-9: The plume off Cape Recife is associated with wind-driven upwelling, not Agulhas Driven upwelling.
163-168: The time difference from dry conditions (Aug 2010) to wet condition (Mar 2011) had a span of approx. 7 months, as you state. Both coastal chlorophyll and currents (shelf and the Agulhas Current) change over periods of days or several days, or maybe even weeks in the case of the Current. To try correlate events fluctuating (or happening) at vastly different timescales, as shown by the 2 matching figures (Fig3e and 3f) as compared to rainfall, can lead to the wrong interpretation. The chlorophyll and currents could, and probably did, change many times over that 7 month period and it may just be that the day snapshots that you chose were biased towards your reasoning.
170: Should use "high/er salinity" and not "salty". 169-177: Anyway, if the Fish River position were known, then, yes, it would appear from your plots that lower salinity water filled the mid-shelf region with its northern extremity around the Fish River. What concerns me is this: would lower salinity water move westward and fill the whole mid-shelf area (distance of ∼300km) over a period of 5-6 days (Fig4a to Fig 4b) without mixing and dispersing, and could it even do that from C3 theory? If it were possible, then, yes, your reasoning would be considered sound. 210: Yes, I imagine that high offshore sea pressure could contribute to enhanced westward flow near at the coast and hence contribute positively towards upwelling. Actually, just that has been found from in situ data during Natal Pulses by Goschen et al (2015).
217: Agreed, large scale weather systems moving eastward over southern Africa produce easterly winds (among other phenomena), which are upwelling favourable.
Technical Errors 220 and 222: Should that not be "chlorophyll and fluorescence"? Are they not two different ocean variables? C4