{"id":"fe164296-11f9-49fa-8515-ecb05a2c902b","arxiv_id":"2501.12920","paper_version":2,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Upwelling cells in the Canary and Benguela systems have subregion-dependent structures, and future vertical transport trends are mostly insignificant but weakly support a poleward shift.","lead":"Using vertical ocean transport from five CMIP6 simulations, this study maps the upwelling cells off Northwest Africa and southwest Africa and shows that their depth, width, and distance from the coast vary between subregions. Future trends under a high-emission scenario are mostly not statistically significant but point weakly toward a poleward shift, and the subregional view helps explain why earlier studies disagreed.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The trend results depend on a post hoc choice of index depth and averaging period; the depth (50 m) was selected in part for maximizing significance, so the reported significant trends and the rejection of Bakun are not robustly established.","rationale":"The reader's weakest assumption identifies exactly the fragile point: the index choice was tuned to maximize significance. I agree. The paper is otherwise an honest diagnostic: it validates seasonal cycles against CCI/ERA5, displays consistent cell structures across models, and openly acknowledges single members and high variability. Those descriptive contributions would survive even if the trend inference is weakened. However, the central explanation of contradictory prior trends is a trend-sign claim, so the post hoc depth/time-scale selection directly affects the main interpretive conclusion. Because the authors disclose the selection and provide enough information for alternative definitions to be tested, a conditional verdict is appropriate rather than rejection. The concrete test above would settle whether the significant trends are robust or artifacts of metric choice.","tokens_in":21342,"tokens_out":3811,"duration_ms":44618,"concrete_test":"Recompute the Fig. 8 trends for all four models used in the 1995-2100 analysis with VT defined at z = 30, 50, 75, 100 and zmax, coastal widths of 0.5°, 1° and 2°, and for both annual and seasonal means, reporting sign and 90% CI for every combination. Then apply a multiple-comparison correction (e.g., Benjamini-Hochberg) over the full grid of definitions. If the significant NCUC/CCUC increases and NBUC decrease do not persist across the plausible range (or lose significance after correction), the trend-based conclusions and the rejection of Bakun should be withdrawn or explicitly labeled as exploratory.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central trend claims rest on the Vertical Transport Index defined at a fixed depth z=50 m within 1° of the coast (Eq. 4) and annual means. Section 3.4 reports that 'some tests' were done on depth and time scale, and 50 m was chosen because it was 'closest to the Ekman index and showing the most significance.' This is post hoc selection of the metric that produces the headline result. With only four simulations used for trends (HH ends in 2050), and with the autocorrelation-based effective sample size only ~106/12≈9 independent years, the 90% confidence intervals in Fig. 8 are already fragile. If the depth or averaging period had been fixed a priori at another plausible value, the sign or significance of the trends in the NCUC/CCUC and NBUC could change; the paper does not show the full set of tested definitions or apply any correction for multiple testing. Because the explanation of the Jing/Chang contradiction relies on the sign pattern of these trends, this selection effect is load-bearing. The descriptive cell-structure results (depth and distance of VT maxima) are less affected by this concern, since they are based on VT(z) fields rather than a single significance-maximizing index.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper uses the vertical water mass transport (wmo) from five CMIP6 simulations (two model families, three ocean resolutions) to characterize the upwelling cells of the Canary and Benguela systems during 1995–2014 and their future evolution under SSP5-8.5 until 2100. It defines a Vertical Transport Index at a fixed depth of 50 m within a 1° coastal band, compares its seasonal cycle with SST and Ekman indices, maps the cross-shore and vertical structure of upwelling cells in six subregions, and computes linear trends of the index over 1995–2100. The central claims are that the two Atlantic upwelling systems show different cell structures in terms of depth and distance to the coast of maximum vertical transport, that subregions within each system have different future trend signs, and that these subregional, depth-resolved patterns help explain contradictory trends reported by Jing et al. (2023) and Chang et al. (2023).","tokens_in":21577,"tokens_out":4599,"duration_ms":46103,"significance":"If the descriptive claims hold, the paper makes a useful methodological contribution: the vertical transport diagnostic integrates all upwelling-driving processes, is directly comparable to the Ekman transport, and reveals spatial structures that are lost in region-wide indices. The explicit subregional decomposition and the inclusion of eddy-rich (1/12°) and eddy-permitting (1/4°) models are strengths, as is the use of a common ocean component (NEMO3.6) across the two model families. However, the trend-related conclusions are weakened by a post hoc selection of the index depth and averaging period (Section 3.4), which makes the reported significance levels and the rejection of the Bakun hypothesis not fully robust. The descriptive cell-structure results are less affected and are supported by the VT(z) fields shown in the figures.","major_comments":[{"comment":"The choice of the vertical transport index definition is load-bearing for the trend results. The paper states that \"some tests\" were done on depth and time scale and that 50 m and annual means were selected because they gave the most significant results and closest agreement with the Ekman index. Because the same data are used to select the metric and to compute the significance of the resulting trends, the p-values reported in Fig. 8 and the statement \"we can reject the Bakun hypothesis\" are not confirmatory. Please report the full set of tested depths and averaging periods, show the trend sign and significance for each, and either apply a multiple-comparison correction or reframe the trend section explicitly as exploratory. The descriptive cell-structure analysis in Section 3.3, which uses the full VT(z) field, is not affected by this issue.","section":"Section 3.4, Eq. (4)"},{"comment":"The significance levels are fragile given the effective sample size: with a 12-year autocorrelation lag and 106 years of data, the effective number of independent years is about 9, and the trends are computed from only four simulations (the eddy-rich HH simulation is excluded because it ends in 2050). Under these conditions, selecting the index definition that maximizes significance can easily produce false positives. Please provide robustness tests (for example, trends computed at 30 m, 75 m, and 100 m; seasonally averaged series; and sensitivity to subregion boundaries) and report how many tested configurations give significant trends of each sign. Without such a sensitivity analysis, the claim of a \"significant increase in the northern and central Canary cell\" and a \"significant decrease in the NBUC\" is not robustly established.","section":"Section 3.4, Fig. 8"},{"comment":"The paper argues that the depth of maximum upwelling differs by subregion and is ecologically important (Section 3.3), yet the VTI used for trends is evaluated at the fixed depth of 50 m. If the depth of maximum transport varies among subregions, a fixed-depth index may not represent the regional upwelling signal equally well in all cells, and the inter-subregion differences in trend could be partly an artifact of the fixed depth. Please justify the fixed depth against the diagnosed z_max VT fields, or show that the trends are insensitive to choosing z_max VT instead of 50 m.","section":"Section 2.3.2, Eq. (4)"}],"minor_comments":[{"comment":"There is a typo in the sentence \"as the the index becomes lower\" describing delayed upwelling onset in the Benguela; please correct it.","section":"Section 3.2"},{"comment":"The description of the SST index is ambiguous: the coastal reference point is said to be on the 100 m isobath, and the offshore SST is taken 5° offshore, but the exact distance from the coast for the offshore point is not stated; please clarify.","section":"Section 2.3.1"},{"comment":"The subregion boundaries shown as black boxes in Fig. 1 are central to the analysis, but the latitude and longitude ranges are only given in prose; a table listing the exact boundaries for the six subregions would improve reproducibility.","section":"Fig. 1"},{"comment":"The sentence \"The depth of 50 m having provided results closest to the Ekman index and showing the most significance, this is used for the calculation of the index for the future\" is grammatically awkward and should be rewritten; please also clarify whether \"most significance\" refers to significance of the seasonal cycle or of the trends.","section":"Section 3.4"},{"comment":"The notation z_max VT is used in Eq. (4) but only defined in the following sentence; please define it before or at the point of first use.","section":"Eq. (4)"}],"recommendation":"major_revision","confidential_remarks":"The post hoc selection issue identified during review is genuine and directly evidenced by the text in Section 3.4. It is, however, a fixable problem: the authors could rerun the trend analysis over a range of depths and averaging periods and either demonstrate robustness or soften the claims. The descriptive analysis and the subregional comparison are valuable and within the scope of the journal. I would not reject the manuscript, but the trend section needs substantial revision before publication."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: the descriptive core is solid, the trend section is fragile because of a post hoc index choice.\n\nThe new thing here is the systematic subregional treatment of upwelling cells in the Canary and Benguela systems, using vertical transport from two CMIP6 model families at 1°, 1/4°, and 1/12° ocean resolutions. The authors map where the vertical transport maximum sits in depth and offshore distance, and they show that the structure varies by subregion. That is a real contribution, and the validation against CCI SST and ERA5 is standard and careful. The finding that the vertical transport index tracks the Ekman index closely except in the equatorward subregions, where ocean circulation matters more, is useful.\n\nThe soft spot is the future-trend analysis. Section 3.4 says they tested different depths and time scales and selected 50 m and annual means because they gave the most significant results and the closest match to the Ekman index. That is post hoc selection, and it is load-bearing for the trend claims. With only four simulations available for trends (the 1/12° run ends in 2050 and is excluded) and an effective sample size around nine independent years, the 90% confidence intervals are not robust to that choice. The paper does not show results for other depths or apply any multiple-testing correction. Also, the statement that the Bakun hypothesis can be rejected rests on a significant decrease in the northern Benguela cell in two HadGEM simulations, with no significant trend in the CNRM model. That is a thin basis for a strong claim, even though the authors are appropriately cautious elsewhere.\n\nThe descriptive cell-structure results are not affected by the selection problem, because they use full vertical-transport fields rather than a significance-maximizing index. The authors are transparent about the selection and about the high interannual variability, and they call for more ensemble members. So this is honest work with a genuine methodological caveat.\n\nThe paper is for upwelling researchers and CMIP6 model evaluators. It is not a new theory, and it doesn't settle the Bakun question. A serious referee should be able to extract value from the structural analysis and request robustness checks for the trends. I would send it to review, with the expectation that the trend section needs revision: either show the sensitivity to depth and averaging period, or soften the Bakun rejection.","headline":"Useful subregional mapping of upwelling cell structure, but the future-trend claims rest on a post hoc index choice and need robustness checks before they can be taken at face value.","tokens_in":22134,"tokens_out":3659,"would_cite":false,"duration_ms":37376,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Future upwelling trends in the Atlantic depend on the subregion and depth at which they are measured, and resolving that structure reconciles contradictory earlier results.","keywords":["Eastern Boundary Upwelling Systems","vertical transport index","CMIP6","Canary Upwelling","Benguela Upwelling","future trends","Bakun hypothesis","poleward migration"],"falsifier":"Compute the same vertical-transport trends using depths of 25, 100, and 150 m for the Canary and Benguela subregions; if the significant increase in the northern and central Canary and the decrease in the northern Benguela do not persist at other depths, the fixed-depth index is not a robust carrier of the claimed trends. A second check is to repeat the analysis for the full CMIP6 multi-model ensemble and for multiple ensemble members, since the paper uses one realization per model, and to test in the same model whether the 50-km versus 200-km offshore extents reverse the Benguela sign as the paper suggests.","tokens_in":27,"feed_emoji":"🌊","tokens_out":14161,"duration_ms":185634,"temperature":0.7,"pith_summary":"Coastal upwelling along West Africa and Namibia is usually summarised by a single index per region, which has produced contradictory future projections. This paper claims the contradictions come from treating each upwelling system as one block, and that measuring the actual vertical transport of water, mapped by depth and distance from shore and split into the seasonally distinct northern, central, and southern cells, resolves them. Using five simulations from two CMIP6 model families, it shows the cells differ in the depth and offshore position of their strongest upwelling, and that future trends under a high-emission scenario follow these subregional boundaries: a significant increase in the northern and central Canary, a significant decrease in the northern Benguela, and no significant wind-driven trends. If correct, future upwelling assessments need depth-resolved, subregional indices rather than single regional values.","feed_headline":"Coastal upwelling trends flip sign by latitude and depth","feed_subtitle":"Vertical-transport mapping in two climate models shows why earlier upwelling studies disagreed.","key_machinery":"The vertical transport index (VTI): the depth- and coast-integrated vertical water mass flux $w_{mo}$ per unit meridional distance, evaluated within a 1° coastal band at a fixed depth of 50 m, expressed in the same units as the Ekman transport. Its full cross-shore transects $VT(X,y,z)$ are what reveal the depth and offshore position of maximum upwelling in each cell. The other piece is the subregional partition: each system is split into three cells whose seasonal upwelling windows are distinct, so trends are computed per cell rather than for the system as a whole.","core_discovery":"The central discovery is that the upwelling cell — represented by the vertical transport $VT(X,y,z)=\\frac{1}{\\rho_0}\\sum_{i_{\\mathrm{coast}}}^{i_X} w_{mo}(x_i,y,z)/\\delta y$ — has a three-dimensional structure that differs between subregions of the same upwelling system. In the Canary system the maximum transport sits close to shore and just below the mixed layer in the northern and central cells, while the southern cell has a wider offshore extent; in the Benguela the central cell, containing the Lüderitz cell, is the strongest and deepest, and the southern cell peaks farther from shore despite weaker winds. Over the 1995–2100 period under SSP5-8.5, the Vertical Transport Index $VTI$, defined as $VT$ at 1° offshore width and 50 m depth, shows significant positive trends in the northern and central Canary cells and a significant negative trend in the northern Benguela cell, while the wind-based Ekman index shows no significant trends. The authors conclude that the Bakun hypothesis of uniform intensification is not supported and that the pattern is instead consistent with a poleward migration, and they argue that the opposite signs found in the two earlier studies of the Benguela arise from their different latitudinal and longitudinal sampling choices.","pith_inferences":["Editorial inference: because the paper admits the depth and time-scale of the index were chosen to maximise statistical significance, a robustness exercise at other depths (e.g., 25, 100, 150 m) is the natural next test before any policy weight is placed on the trend signs.","Editorial inference: the biological stakes of these systems imply that the depth of upwelling matters as much as its surface intensity; if the cells really sit at different depths, marine-ecosystem projections should track the source depth of upwelled nutrients rather than the surface wind index.","Editorial inference: the same subregional, depth-resolved treatment could be applied to the California and Humboldt eastern boundary systems, where similar contradictions in future wind trends have been reported, to test whether their disputes also dissolve once cell geometry is resolved.","Editorial inference: the paper's claim that geostrophic flows matter most near the equator suggests a testable prediction — models with more realistic Angola and Guinea Dome circulations should show an even lower correlation between wind and vertical transport indices at low latitudes, and this could be checked with the planned multi-model ensembles."],"forward_implications":["Future projections of the Atlantic upwelling systems depend on which subregion is examined: the northern and central Canary cells show a significant strengthening of vertical transport in the medium-resolution models, while the northern Benguela cell weakens significantly.","Wind-based Ekman and wind-stress-curl indices do not show significant trends, so ocean processes other than local wind forcing are needed to explain the vertical transport changes.","The conflicting signs in the two earlier Benguela studies are attributable to their different choices of latitude and longitude boundaries, since the vertical transport trends can be opposite close to the coast versus further offshore.","Under the high-emission scenario, detectable trends appear only in the second half of the century, and the highest-resolution model run (ending in 2050) cannot contribute to the trend statistics."],"supporting_citations":[{"why":"Defines the Bakun hypothesis of globally intensified upwelling, which the paper tests and rejects based on subregional trends.","marker":"(Bakun, 1990)"},{"why":"Proposes the poleward migration of upwelling-favorable winds that the paper's trend pattern is compared against.","marker":"(Rykaczewski et al., 2015)"},{"why":"Supplies the IPCC assessment that rejects the Bakun hypothesis and confirms Hadley-cell expansion, setting up the two hypotheses examined here.","marker":"(Gulev et al., 2021)"},{"why":"One of the two contradictory Benguela trend studies; its positive vertical-transport trends are reinterpreted through the subregional lens.","marker":"(Jing et al., 2023)"},{"why":"The other contradictory Benguela study, with negative vertical-velocity trends and positive Ekman changes; its longitudinal choice is invoked to explain the discrepancy.","marker":"(Chang et al., 2023)"},{"why":"Defines the wmo vertical water mass transport diagnostic from which the vertical transport index is built.","marker":"(Griffies et al., 2016)"},{"why":"Supplies the 1° offshore coastal band used to define the vertical transport index.","marker":"(Tim et al., 2015)"},{"why":"Provides the subregional seasonal definitions for the Canary system and earlier CMIP5 evidence that trend results depend on region boundaries.","marker":"(Sylla et al., 2019)"},{"why":"Explains geostrophic convergence and divergence in the Benguela, used to interpret equatorward differences between wind and vertical transport indices.","marker":"(Veitch et al., 2010)"}],"fun_headline_variants":["Upwelling trends flip with depth and latitude","3D structure explains conflicting upwelling trends","Atlantic upwelling shifts poleward in high-emission future","Model shows upwelling cells differ by region and depth","Vertical flux reveals upwelling's true spatial variation"],"cache_read_input_tokens":24320,"weakest_assumption_plain":"The results rest on measuring upwelling at one fixed depth (50 m) inside a one-degree band along the coast; that depth was chosen partly because it gave statistically significant trends, so a different choice could change the findings.","fun_headline_variants_meta":{"raw":{"variants":["Upwelling trends flip with depth and latitude","3D structure explains conflicting upwelling trends","Atlantic upwelling shifts poleward in high-emission future","Model shows upwelling cells differ by region and depth","Vertical flux reveals upwelling's true spatial variation"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000235,"raw_usage":{"total_tokens":1569,"prompt_tokens":1085,"completion_tokens":484,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":701,"completion_tokens_details":{"reasoning_tokens":412}},"tokens_in":701,"tokens_out":484,"duration_ms":5227,"temperature":1.0,"reasoning_tokens":412,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T16:37:06.027897+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Compute the same vertical-transport trends using depths of 25, 100, and 150 m for the Canary and Benguela subregions; if the significant increase in the northern and central Canary and the decrease in the northern Benguela do not persist at other depths, the fixed-depth index is not a robust carrier of the claimed trends. A second check is to repeat the analysis for the full CMIP6 multi-model ensemble and for multiple ensemble members, since the paper uses one realization per model, and to test in the same model whether the 50-km versus 200-km offshore extents reverse the Benguela sign as the paper suggests.","supporting_citations":[],"review_version":1}