Wind Vorticity and Upwelling along the Coast of South Africa
<p>Mean maps of: (<b>a</b>) MERRA2 975 hPa potential vorticity (10<sup>−6</sup> K·m<sup>2</sup>/kg·s) and wind vectors (largest 7 m/s) with 3 key areas (boxed), (<b>b</b>) ERA5 atmospheric boundary layer height <700 m and section lines, (<b>c</b>) satellite SST < 19 °C with topography and shelf edge, all averaged 2000–2021. Inset in (<b>a</b>), lower right, identifies the study area comprising the southern tip of Africa.</p> "> Figure 2
<p>(<b>a</b>) GODAS-SODA3 vertical entrainment (shaded m/day) and currents (largest vector 0.8 m/s) in 1–200 m layer. Depth section of cross-shore ocean circulation on (<b>b</b>) 32 S (west) and (<b>c</b>) 25.25 E (south) with longshore current dashed, and smoothed shelf profile; all averaged 2000–2021. Annual river discharge >500 m<sup>3</sup>/s is shown by blue arrows in (<b>a</b>) together with capes; vertical motions in (<b>b</b>,<b>c</b>) are exaggerated 100-fold.</p> "> Figure 3
<p>Monthly time series in each area (top to bottom: W, S, E) of MERRA2 potential vorticity (black line), the sum of daily ERA5 upwelling-favorable wind stress (thin red line), and EC offshore sea-slope representing the longshore current (thick aqua line, lower). The vertical motion over the shelf (W<sub>E</sub>) is derived from theoretical influence, with <span class="html-italic">y</span>-axis units of m/day (+upward). Legend: -vort refers to cyclonic wind stress curl, which is upwelling favorable.</p> "> Figure 4
<p>(<b>a</b>) Mean annual cycle of coastal upwelling from the sum of daily longshore wind stress per area and all-coast chlorophyll. (<b>b</b>) Lag cross-correlation of MERRA2 daily potential vorticity from one area to the next. (<b>c</b>) Lag auto-correlation of daily potential vorticity time series in each area in January 2016–December 2018, where N = 1096, 90% confidence (blue), and arrows indicate significant pulsing. (<b>d</b>) Simultaneous correlation of daily vorticity time series with S.L. air pressure field, where icons highlight marine high/coastal low per area (left–right): west, south, east.</p> "> Figure 5
<p>The sequence of cyclonic cases transiting from west to south coast. Height section of ERA5 longshore wind (shaded) and cross-shore airflow (largest vectors 2 m/s) on (<b>a</b>) 32 S west 10 January 2016, and (<b>b</b>) 25.25 E south 12 January 2016. The rotational arrow highlights cyclonic vorticity, with upstream topography overlain and vertical atmospheric motion exaggerated 30-fold. (<b>c</b>) CFS2 vertical motion W (m/day) and upwelling favorable wind +V, -U (m/s) at the two sites; with night indicated by grey shading lower and western 21:00 potential vorticity listed (10<sup>−6</sup> K·m<sup>2</sup>/kg·s).</p> "> Figure 6
<p>Cyclonic case on the south coast: (<b>a</b>) map of 975 hPa winds (black vectors) and 850 hPa airflow (red streamlines) at 18:00 3 November 2018, with icons showing eastward movement of marine high and coastal low in days before/after; (<b>b</b>) skew-T radiosonde profile of temperature (red), dewpoint (green), wind (barbs) at Port Elizabeth airport 34 S, 25.6 E, 61 m (green triangle) at 12:00 3 November 2018, and (<b>c</b>) map on same date of daily SST (shaded) and wind (thin lines, m/s) increasing from 1 inshore to 11 offshore.</p> "> Figure 7
<p>Cyclonic case on the east coast: (<b>a</b>) plan and section view of 24 hourly ensemble Hysplit back-trajectories of near-surface airflow arriving at the east area on 26 September 2016. Hourly time series on 25–28 September at 29.25 S, 31.5 E of (<b>b</b>) CFS2 vertical entrainment (averaged 40–70 m) and surface wind stick vectors (−UV upwelling favorable), (<b>c</b>) satellite dynamic topography (ƞ) inshore/offshore, and S.L. air pressure, and (<b>d</b>) CFS2 air temperature, SST, and MERRA2 potential wind vorticity (bars, cyclonic upward), with times labeled.</p> "> Figure 8
<p>(<b>a</b>–<b>c</b>) Box–whisker plots of the mean diurnal cycle of MERRA2 975 hPa potential vorticity per area. Maps of mean wind departures from all hours for (<b>d</b>) summer days 12:00–18:00, and (<b>e</b>) winter nights 00:00–06:00 h. Mean summer day (red) and winter night (blue) surface air temperature differentials along sections are listed.</p> "> Figure 9
<p>(<b>a</b>) Map of EOF-1 loading pattern for satellite chlorophyll and its monthly time score (inset). Regression of time score onto annual fields of (<b>b</b>) sea-level air pressure (Pa) and (<b>c</b>) net solar radiation (W/m<sup>2</sup>), 2000–2021.</p> "> Figure A1
<p>Static surface roughness length (CFS2) quantifying the undulating nature of South Africa’s coastal mountains that induce wind shadows. Labels refer to mean coast and shelf-edge wind (m/s).</p> "> Figure A2
<p>In situ data density of ships (grey shaded 10–100/month/degree) and automatic weather stations (blue dots) operationally reporting winds; key areas are labeled.</p> ">
Abstract
:1. Introduction
2. Data and Methods
3. Results
3.1. Study Area and Mean Maps
3.2. Temporal Characteristics and Seasonality
3.3. Intraseasonal Pulsing
3.4. Local Scenario of Cyclonic Cases
3.5. Diurnal Cycling of Wind Vorticity
3.6. Chlorophyll and Interannual Climate
4. Concluding Discussion
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
Appendix A
References
- Burk, S.D.; Thompson, W.T. The summertime low-level jet and marine boundary layer structure along the California coast. Mon. Weather Rev. 1996, 124, 668–686. [Google Scholar] [CrossRef]
- Rogerson, A.M. Trans-critical flows in the coastal marine atmospheric boundary layer. J. Atmos. Sci. 1999, 56, 2761–2779. [Google Scholar] [CrossRef]
- Edwards, K.A.; Rogerson, A.M.; Winant, C.D.; Rogers, D.P. Adjustment of the marine atmospheric boundary layer to a coastal cape. J. Atmos. Sci. 2001, 58, 1511–1528. [Google Scholar] [CrossRef]
- Perlin, N.; Skyllingstad, E.D.; Samelson, R.M. Coastal atmospheric circulation around an idealized cape during wind-driven upwelling studied from a coupled ocean–atmosphere model. Mon. Weather Rev. 2011, 139, 809–829. [Google Scholar] [CrossRef]
- Enriquez, A.G.; Friehe, C.A. Effects of wind stress and wind stress curl variability on coastal upwelling. J. Phys. Oceanogr. 1995, 25, 1651–1671. [Google Scholar] [CrossRef]
- Bielli, S.; Barbour, P.L.; Samelson, R.M.; Skyllingstad, E.; Wilczak, J. Numerical study of the diurnal cycle along the central Oregon coast during summertime northerly flow. Mon. Weather Rev. 2002, 130, 992–1008. [Google Scholar] [CrossRef]
- Castelao, R.; Barth, J.A. The role of wind stress curl in jet separation at a cape. J. Phys. Oceanogr. 2007, 37, 2652–2671. [Google Scholar] [CrossRef]
- Skyllingstad, E.D.; Samelson, R.M.; Mahrt, L.; Barbour, P. A numerical modeling study of warm offshore flow over cool water. Mon. Weather Rev. 2005, 133, 345–361. [Google Scholar] [CrossRef]
- Song, Q.; Chelton, D.B.; Esbensen, S.K.; Thum, N.; O’Neill, L.W. Coupling between sea surface temperature and low-level winds in mesoscale numerical models. J. Clim. 2009, 22, 146–164. [Google Scholar] [CrossRef]
- Jury, M.R.; Goschen, W. Inter-relationships between physical ocean-atmosphere variables over the shelf south of South Africa from reanalysis products. Cont. Shelf Res. 2020, 202, 104135. [Google Scholar] [CrossRef]
- Bakun, A.; Nelson, C.S. The seasonal cycle of wind stress curl in subtropical eastern boundary current regions. J. Phys. Oceanogr. 1991, 21, 1815–1834. [Google Scholar] [CrossRef]
- Rueda-Roa, D.T.; Muller-Karger, F.E. The southern Caribbean upwelling system: Sea surface temperature, wind forcing and chlorophyll concentration patterns. Deep Sea Res. I 2013, 78, 102–114. [Google Scholar] [CrossRef]
- Bordbar, M.H.; Mohrholz, V.; Schmidt, M. The relation of wind-driven coastal and offshore upwelling in the Benguela System. J. Phys. Oceanogr. 2021, 51, 3117–3133. [Google Scholar] [CrossRef]
- Holton, J.R. An Introduction to Dynamic Meteorology, 3rd ed.; Academic Press: New York, NY, USA, 1992; p. 511. [Google Scholar]
- Schumann, E.H.; Brink, K.H. Coastal trapped waves off the coast of South Africa: Generation, propagation and current structures. J. Phys. Oceanogr. 1990, 20, 1206–1218. [Google Scholar] [CrossRef]
- Lutjeharms, J.R.E.; Cooper, J.; Roberts, M.J. Upwelling at the inshore edge of the Agulhas Current. Cont. Shelf Res. 2000, 20, 737–761. [Google Scholar] [CrossRef]
- Lutjeharms, J.R.E. The Agulhas Current; Springer: Berlin/Heidelberg, Germany, 2006; p. 329. [Google Scholar]
- Goschen, W.S.; Bornman, T.G.; Deyzel, S.H.P.; Schumann, E.H. Coastal upwelling on the far eastern Agulhas Bank associated with large meanders in the Agulhas Current. Cont. Shelf Res. 2015, 101, 34–46. [Google Scholar] [CrossRef]
- DeRuijter, W.P.M.; VanLeeuwen, J.P.; Lutjeharms, J.R.E. Generation and evolution of Natal pulses: Solitary meanders in the Agulhas Current. J. Phys. Oceanogr. 1999, 29, 3043–3055. [Google Scholar] [CrossRef]
- Tsugawa, M.; Hasumi, H. Generation and growth mechanism of the Natal Pulse. J. Phys. Oceanogr. 2010, 40, 1597–1612. [Google Scholar] [CrossRef]
- Jury, M.R. Modulation of currents near Durban. Reg. Stud. Mar. Sci. 2018, 18, 208–218. [Google Scholar] [CrossRef]
- Behringer, D.W. The Global Ocean Data Assimilation System (GODAS) at NCEP. In Proceedings of the 11th Symposium on Integrated Observing and Assimilation Systems for the Atmosphere, Oceans, and Land Surface, San Antonio, TX, USA, 14–18 January 2007. [Google Scholar]
- Carton, J.A.; Chepurin, G.A.; Chen, L. SODA-3 a new ocean climate reanalysis. J. Clim. 2018, 31, 6967–6983. [Google Scholar] [CrossRef]
- Saha, S.; Moorthi, S.; Wu, X.; Wang, J.; Nadiga, S.; Tripp, P.; Behringer, D.; Hou, Y.-T.; Chuang, H.-Y.; Iredell, M.; et al. The NCEP climate forecast system version 2. J. Clim. 2014, 27, 2185–2208. [Google Scholar] [CrossRef]
- Hersbach, H.; Bell, B.; Berrisford, P.; Hirahara, S.; Horányi, A.; Muñoz-Sabater, J.; Nicolas, J.; Peubey, C.; Radu, R.; Schepers, D.; et al. The ERA5 global reanalysis. Q. J. R. Meteol. Soc. 2020, 146, 1999–2049. [Google Scholar] [CrossRef]
- Gelaro, R.; McCarty, W.; Suárez, M.J.; Todling, R.; Molod, A.; Takacs, L.; Randles, C.A.; Darmenov, A.; Bosilovich, M.G.; Reichle, R.; et al. The modern-era retrospective analysis for research and applications, version 2 (MERRA-2). J. Clim. 2017, 30, 5419–5454. [Google Scholar] [CrossRef] [PubMed]
- Chin, T.M.; Vazquez-Cuervo, J.; Armstrong, E.M. A multi-scale high-resolution analysis of global sea surface temperature. Remote Sens. Environ. 2017, 200, 154–169. [Google Scholar] [CrossRef]
- O’Rielly, J.E.; Werdell, P.J. Chlorophyll algorithms for ocean color sensors. Remote Sens. Environ. 2019, 229, 32–47. [Google Scholar]
- Rossby, C.G. Planetary flow patterns in the atmosphere. Q. J. R. Meteorol. Soc. 1940, 66, 68–87. [Google Scholar] [CrossRef]
- Ertel, H. A new form of hydrodynamic vorticity. Meteorol. Z. 1942, 59, 277–281. [Google Scholar]
- Thorpe, A.J.; Volkert, H. Potential vorticity: A short history of its definitions and uses. Meteorol. Z. 1997, 6, 275–280. [Google Scholar] [CrossRef]
- Cao, J.; Xu, Q. Computing hydrostatic potential vorticity in terrain-following coordinates. Mon. Weather Rev. 2011, 139, 2955–2961. [Google Scholar] [CrossRef]
- Legeais, J.-F.; Ablain, M.; Zawadzki, L.; Zuo, H.; Johannessen, J.A.; Scharffenberg, M.G.; Fenoglio-Marc, L.; Fernandes, M.J.; Andersen, O.B.; Rudenko, S.; et al. An improved and homogeneous altimeter sea level record from the ESA climate change initiative. Earth Syst. Sci. Data 2018, 10, 281–301. [Google Scholar] [CrossRef]
- Rueda-Roa, D.T.; Ezer, T.; Muller-Karger, F.E. Description and mechanisms of the mid-year upwelling in the southern Caribbean Sea from remote sensing and local data. J. Mar. Sci. Eng. 2018, 6, 36. [Google Scholar] [CrossRef]
- Jacox, M.G.; Edwards, C.A. Upwelling source depth in the presence of nearshore wind stress curl. J. Geophys. Res. 2012, 117, C05008. [Google Scholar] [CrossRef]
- Söderberg, S.; Tjernström, M. Supercritical channel flow in the coastal atmospheric boundary layer: Idealized numerical simulations. J. Geophys. Res. 2001, 106, 17811–17830. [Google Scholar] [CrossRef]
- Pielke, R.A.; Segal, M. Mesoscale circulations forced by differential terrain heating. In Mesoscale Meteorology and Forecasting; Ray, P., Ed.; AMS: Providence, RI, USA, 1986; pp. 516–548. [Google Scholar]
- Koracin, D.R.; Dorman, C.E.; Dever, E.P. Coastal perturbations of marine layer wind stress and wind stress curl along California and Baja California in June 1999. J. Phys. Ocean. 2004, 34, 1152–1173. [Google Scholar] [CrossRef]
- Bravo, L.; Ramos, M.; Astudillo, O.; DeWitte, B.; Goubanova, K. Seasonal variability of the Ekman transport and pumping in the upwelling system off central-northern Chile (30S) based on a high-resolution atmospheric regional model (WRF). Ocean Sci. 2016, 12, 1049–1065. [Google Scholar] [CrossRef]
- Capet, X.J.; Marchesiello, P.; McWilliams, J.C. Upwelling response to coastal wind profiles. Geophys. Res. Lett. 2004, 31, L13311. [Google Scholar] [CrossRef]
- Dever, E.P.; Dorman, C.E.; Largier, J.L. Surface boundary-layer variability off Northern California, during upwelling. Deep. Sea Res. II Top. Stud. Oceanogr. 2006, 53, 2887–2905. [Google Scholar] [CrossRef]
- Song, H.; Miller, A.J.; Cornuelle, B.D.; DiLorenzo, E. Changes in upwelling and its water sources in the California Current system driven by different wind forcing. Dyn. Atmos. Oceans 2011, 52, 170–191. [Google Scholar] [CrossRef]
- Brandt, P.; Bordbar, M.H.; Coelho, P.; Koungue, R.A.; Körner, M.; Lamont, T.; Lübbecke, J.F.; Mohrholz, V.; Prigent, A.; Roch, M.; et al. Physical drivers of southwest African coastal upwelling and its response to climate variability and change. In Sustainability of Southern African Ecosystems under Global Change, Ecol. Studies, 248; von Maltitz, G.P., Ed.; Springer: Berlin/Heidelberg, Germany, 2024. [Google Scholar] [CrossRef]
- Shan, S.; Sheng, J. Numerical Study of Topographic Effects on Wind-Driven Coastal Upwelling on the Scotian Shelf. J. Mar. Sci. Eng. 2022, 10, 497. [Google Scholar] [CrossRef]
- Jury, M.R. Marine climate change over the eastern Agulhas Bank of South Africa. Ocean. Sci. 2020, 16, 1529–1544. [Google Scholar] [CrossRef]
Acronym | Name Version | Horiz. Res. |
---|---|---|
CFS2 | Coupled forecast system reanalysis v2, meteorology | 25 km |
EC | European Centre satellite dynamic topography | 25 km |
ERA5 | European Centre reanalysis v5, meteorology | 25 km |
GODAS | Global Ocean Data Assimilation system (NOAA) | 50 km |
MERRA2 | Modern Era Reanalysis for Research and Applications v2 | 50 km |
MODIS | Moderate Imaging Sensor, SST and chlorophyll | 10 km |
SODA3 | Simple Ocean Data Assimilation v3 | 25 km |
West | -Vort | Ʃ Stressy | ∂η/∂x | SSTa |
---|---|---|---|---|
Ʃ stressy | 0.68 | |||
∂η/∂x | −0.04 | 0.10 | ||
SSTa | −0.22 | −0.23 | 0.07 | |
CHL | 0.29 | 0.30 | 0.01 | −0.17 |
South | -vort | Ʃ stressx | ∂η/∂y | SSTa |
Ʃ stressx | −0.43 | |||
∂η/∂y | −0.11 | 0.25 | ||
SSTa | −0.16 | −0.16 | −0.08 | |
CHL | −0.04 | 0.17 | 0.01 | −0.20 |
East | -vort | Ʃ stressn | ∂η/∂n | SSTa |
Ʃ stressn | 0.18 | |||
∂η/∂n | −0.12 | 0.05 | ||
SSTa | −0.10 | −0.33 | −0.28 | |
CHL | 0.18 | 0.01 | −0.02 | −0.03 |
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Jury, M.R. Wind Vorticity and Upwelling along the Coast of South Africa. Coasts 2024, 4, 619-637. https://doi.org/10.3390/coasts4030032
Jury MR. Wind Vorticity and Upwelling along the Coast of South Africa. Coasts. 2024; 4(3):619-637. https://doi.org/10.3390/coasts4030032
Chicago/Turabian StyleJury, Mark R. 2024. "Wind Vorticity and Upwelling along the Coast of South Africa" Coasts 4, no. 3: 619-637. https://doi.org/10.3390/coasts4030032