The nominal Cal/Val period for the SWOT satellite has been updated to end of March – end of June 2023, during which fast-sampling phase data will be acquired for science validation. This means that satellite data available to SWOT-AdAC Consortium to compare with in situ observations correspond to spring in the Northern Hemisphere and to autumn in the Southern Hemisphere. What is the quality that can be expected for SSH maps during the fast-sampling phase? Which key oceanographic processes can be best studied in these periods? Which ones are left out? An interview with experts of the SWOT-AdAC oceanography community.
Earlier in November, NASA communicated that the nominal Cal/Val period for the SWOT satellite has been updated to end of March – end of June 2023, during which fast-sampling phase data will be acquired for science validation. This means that satellite data available to SWOT-AdAC Consortium to compare with in situ observations correspond to spring in the Northern Hemisphere and to autumn in the Southern Hemisphere.
What is the quality that can be expected for SSH maps during the fast-sampling phase? Which key oceanographic processes can be best studied in these periods? Which ones are left out?
“SAR-interferometry SWOT data will be along the swath, in 2 bands 50 km wide separated by a 20 km gap. For the open ocean, there will be a resolution of 2 km x 2 km. In the gap between of the 2 bands, there is a conventional Jason-class altimetry radar, with a ground resolution of around 10 km, with less precision in SSH height compared to the swath SAR- interferometry. The SWOT 2D swath data also measures right to the coast and into the estuaries, whereas the nadir altimeter with its larger footprint has larger noise introduced by the coast” explains Rosemary Morrow (LEGOS). SWOT swath data will be used to tune the on-board instrument parameters until the end of the commissioning phase, which is planned to last until the end of March. Delays are possible due to unforeseen interruptions but less likely according to the current progress. After this phase, the acquisitions for science exploitation will start. The first processed 1-day swath products including all the corrections will be released at the end of the year. AVISO/CMEMS multi-satellite maps on a regular grid including SWOT swath data are planned for 2024. We recall that in support to their in situ strategies, SWOT-AdAC members can request instead a set of dedicated near-real time high resolution products as well as other tools.
The fast-sampling phase is a unique opportunity for observing both the spatial and the temporal variability of the fine-scale ocean dynamics. However, part of the 10-100 km ocean dynamics, which is one of the main highlights of SWOT, is especially active during winter, not spring or autumn. Indeed, according to Baylor Fox-Kemper (Brown University), the period will impose some limitations in terms of the smaller scale that can be studied with SWOT during the fast-sampling phase. “The thing that worries me about this [timing] is that that season will not be winter in either hemisphere. That means we will not really get a good lens into the submesoscale, which is largest during the winter. The smaller submesoscale won’t be detectable by SWOT.” Fox-Kemper cites two papers that show that mixed layer instabilities (MLI) and Symmetric Instability (SI) have strong seasonality and are higher in the winter. In practice, “We will need to be extra-careful: if we don’t see a lot of submesoscale signal, that may just be because of the timing of the fast-sampling phase“.
However, there are two sides to this. In the middle latitudes, “spring and autumn are interesting periods, when change in stratification and mixed-layer depth are typically more important. We know that the submesoscales tend to oppose destratification – so that might be something to think about. Also this implies that in terms of phytoplankton, both are periods when we expect to see some temporal changes and have nice variability in ocean color which would complement the SWOT observations” observes Marina Lévy (LOCEAN-IPSL).
The situation may be different in the tropics. Around New Caledonia, a SWOT-AdAC site representative of some typical tropical conditions “we expect low N2 fixation rates in April (fixation rates are much stronger in summer), but diazotrophy is so impressive in this region that even in relatively low activity seasons, the impact is still probably high” says Mar Benavides (MIO). Her colleague Sophie Cravatte (LEGOS) adds that the seasonality of significant wave height has also to be taken into account. As in many other regions of the Southern Hemisphere, in the New Caledonia area “significant wave height will be shifting during the fast-sampling phase from a dominant regime with weak significant wave height to a season when 3m-significant waves height periods are more frequent“.
“The fact that the SWOT fast sampling phase and data acquisition will not be in winter, in my view, is not necessarily bad news” notes Francesco d’Ovidio (LOCEAN-IPSL). “SWOT will see SSH, so that the purely ageostrophic part of the submesoscale spectrum won’t be visible anyway. To me, what SWOT will provide is, rather than the submesoscale, the “small mesoscale” [eddies smaller than 70-100km] today invisible to conventional altimetry.” “We really expect a breakthrough from SWOT, in the observation of the geostrophically-balanced dynamics. Pushing the resolution in the ~10 km domain will be useful for locating precisely frontal features, distinguishing eddy cores from their peripheries, as well as for precisely estimating Lagrangian pathways. In turn, this information will be used to reconstruct physical parameters of key importance for biophysical processes, like vertical velocities or stirring patterns, and to colocalize precisely in situ biological information.” Pointing to a recent campaign, d’Ovidio remarks that in situ biophysical studies are today possible mainly for large and energetic structures like the mesoscale eddies that detach from boundary currents. However, with SWOT, it will be possible to explore much broader oceanic conditions, testing hypotheses on biodiversity and biogeochemical processes that until now could be mainly explored in models. In this regard, the spring/autumn period, with its strong middle-latitude phytoplanktonic activity, is especially suitable for these studies. Besides stronger biological activity, the signal from internal waves is stronger in spring/autumn than in winter. This can be interpreted as worse or better depending on one’s perspective. “The observability also depends on surface waves, which introduce instrument noise and happen to be smaller during summer and larger during winter. So there is another balance act between stronger submesoscale and larger noise during winter” says Jinbo Wang (JPL).
(i) Moderate activity in mixed layer instabilities and symmetric instabilities;
(ii) Transition period for mixed layer depth and significant wave height dynamics;
(iii) Strong phytoplanktonic activity in the middle latitudes, and medium to strong internal wave signal.
The SWOT fast-sampling phase welcomes you to the spring/autumn dynamics of the fine-scale realm!
If your campaign samples a crossover/SWOT swath before April, please inform us! SWOT data will be acquired before April, but the possibility of having interruption or some non-usable acquisition will be higher. If some users need these early data on specific locations, we will pass the information to the SWOT Project so that all efforts will be done to reprocess these acquisitions, where possible.