Low confidence in multi-decadal trends of wind-driven upwelling across the Benguela upwelling system due to internal climate variability
Abstract. Like other Eastern Boundary Upwelling Systems, the upwelling near the southwest African coasts is primarily alongshore-wind-driven, whereas it is controlled mainly by the wind stress curl farther offshore. The surface wind regime across the Benguela Upwelling System is strongly related to the South Atlantic Anticyclone that is believed to migrate poleward in response to anthropogenic global warming. Here, we investigate multi-decadal changes of the South Atlantic Anticyclone and their impacts on the cross-shore integral of wind-driven coastal upwelling, the wind stress curl-driven, and total upwelling across the Benguela Upwelling System. Even though the detailed structure of surface wind over the coastal zone matters for both local wind-driven coastal upwelling and wind stress curl-driven upwelling, we show that it is not of major importance for the total amount of upwelled water. We found a robust connection between the Anticyclone intensity and the integrated wind stress curl-driven and total upwelling. However, this connection for the wind-driven coastal upwelling is weak. With more signatures during austral winter, the upwelling in the equatorward portion of the Benguela Upwelling System is significantly affected by the anticyclone intensity. In contrast, the poleward portion is also influenced by the meridional position of the anticyclone. The multi-decadal trend in the sea level pressure across the South Atlantic renders a considerable heterogeneity in space. However, this trend features a small signal-to-noise ratio and can be obscured by internal climate variability. This view is further supported by a multi-decadal trend in the integrated coastal and wind-stress-curl-driven upwelling in several upwelling cells, which hardly depict any significant systematic changes.
Mohammad Hadi Bordbar et al.
Status: final response (author comments only)
- RC1: 'Comment on esd-2023-10', Anonymous Referee #1, 22 May 2023
- RC2: 'Comment on esd-2023-10', Fabien Desbiolles, 01 Jun 2023
Mohammad Hadi Bordbar et al.
Mohammad Hadi Bordbar et al.
Viewed (geographical distribution)
Review of “Low confidence in multi-decadal trends of wind-driven upwelling across the Benguela upwelling system due to internal climate variability” by Bordbar et al.
The present study analyses the coastal upwelling regime at the eastern boundary of the South Atlantic. The focus is on multi-decadal variability of the wind forced total upwelling composed of coastal upwelling due to alongshore winds and wind-stress-curl-driven upwelling. The manuscript analyses a specific reanalysis dataset of the global climate, ERA5. This dataset is somehow validated against few other available datasets. Total upwelling and wind-curl-driven upwelling is found to be strongly related to the strength and position of the South Atlantic Anticyclone (SAA). Long-term trends identified in the ERA5 dataset are mostly weak, showing considerable heterogeneity, and have only a small signal-to-noise ratio due to enhanced interannual climate variability. My major concerns are:
1) ERA5 is a reanalysis dataset that includes information about uncertainties for all variables. I think it would be necessary to evaluate this uncertainty with regard to possible trends. Particularly trends in the wind forcing are still highly uncertain and a clear description what are the uncertainties provided with the dataset is necessary.
2) There is a validation of the ERA5 dataset with respect to the satellite-derived ASCAT wind dataset and to sea level pressure in St. Helena. This validation focuses on short-term or seasonal variability. However, to strengthen to topic of the paper it would be important to look at the comparison of multi-decadal variability. How good do ERA5 represent long-term changes identified in ASCAT or St. Helena sea level pressure?
3) Inherent to the discussion in the manuscript is that long-term changes are associated with human-induced global warming and shorter-term (interannual to decadal) variability to internal variations of the climate system. This is an assumption that cannot be proven with the current dataset. I would suggest to more carefully discuss multi-decadal, decadal and interannual variability pointing to possible mechanisms that could be associated with internal climate variability or global warming.
This is an overall well-written manuscript that contribute to our understanding of the wind-driven upwelling, its variability and its long-term changes. It might be acceptable after revising the raised major and specific point.
Specific points, comments and suggestions:
Abstract: In the abstract should be mentioned that the study is based on ERA5 reanalysis.
L11: "cross-shore integral of wind-driven coastal upwelling": What is this? Do you mean coastal upwelling driven by alongshore winds? The different upwelling terms must be clearly defined and understandable. Also L15: “integrated wind stress curl-driven and total upwelling”: again term is not well defined and there should be a simpler way to introduce the three terms: coastal upwelling driven by alongshore winds, wind stress curl driven upwelling in the near coastal band and total upwelling as the sum of both.
L16: “more signatures” is unclear
L20: Sentence “However, …” implies that internal climate variability exists only on shorter timescales and that trends are necessarily due to other forcing. Would it be better to write “… can be obscured by interannual to decadal climate variability”?
L28: These modes result …
L28: “nonlinear climate dynamics” Why nonlinear? There are many linear climate feedbacks. Maybe: coupled ocean-atmosphere dynamics.
L31: at the eastern margin
L37: “Farther offshore, …” brackets are not logical: divergence is associated with upward, convergence with downward velocity. I would suggest to remove the terms in the brackets.
L48: “Based on …”. There is a rich literature on the local and remote forcing of Benguela Niños and Niñas. This sentence do not reflect our current understanding of these anomalous coastal warm and cold events.
L55: “equatorward part of the BUS” requires a definition, e.g. by a latitude range.
L58: “poleward portion”, same as before. Please provide a definition of the region.
L59-61: At least there is some uncertaintiy about the role of southward migration of the SAA. The southward movement is also suggested to change the upwelling in the northern portion of the nBUS (Jarre et al. 2015).
L106: In the Supplementary material the seasonal cycle etc. is analyzed. Is there any support from independent data that ERA5 reliably describe trends in the region? As the paper is dealing with long-term variability, the provided validation is not that meaningful. Please provide an additional comparison of trends and decadal variability between ERA5, ASCAT and SLP. It is well known that such long-term changes are particular uncertain and this study could contribute to assess these uncertainties.
L118: volumes transport per unit length (m2/s)
L140: remove comma after coast
L149-150: unclear: upwelled (or downwelled) … transported offshore (or onshore) ?
L151: maybe include: i.e., where the meridional wind stress is maximum
L154: I do not understand what the authors want to say. What is the reason for the simplifications? How good are the made assumptions? How large is the error, e.g., neglecting the onshore geostrophic transport? It would be good to give some more insight.
Table 1: “alongshore-driven” should be “alongshore-wind-driven”
L216: Last sentence is not necessary and distract.
L231: It would be good to provide separate correlations for correlations on interannual and decadal timescales. Which timescale dominate the correlation?
Fig. 2a-c: Please show the complete functions for positive and negative correlations and provide the significance.
L280: (used boxes are marked in Fig. 2e)
L292: “For example, the years 1997 and 2006 are characterized by persistent eastward SAA displacements”: I cannot see this behavior in Fig. 3b.
L306: “changes in the global heat budget”: Do you mean global warming or net radiative imbalance at the top of the atmosphere? Why are 1990s important the increase started earlier.
L309: “due to the enhanced radiative forcing”: what is meant with radiative forcing? of the ocean? Do you mean global warming?
L313: “positive trend is more prominent over higher latitudes”: Is this associated with poleward migration of the center of the SAA?
L365: “Based on equation 5, …” In equation 5, upwelling velocity depend on x. Do you mean maximum upwelling velocity calculated using R1 given in Fig. S5?
… corresponds to a maximum upwelling velocity …
L397: the spatial resolution
L408-413: this section must be improved: possible there are multidecadal changes or trends and there are interannual to decadal variability. It is not obvious from the analysed data what is due to global warming or what is internal climate variability. Used terms such as “historical changes” or “historical trends” are unclear. These conclusions must be formulated more carefully.