The response of Cabo Frio upwelling system to spatially ...
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The Cabo Frio coastal upwelling system has been focus of research due to its intensity and recurrence year-round, and its physical- biological implications. Strong seasonal signal of upwelled waters is evidenced by relatively low sea surface temperature (SST) specially along austral summer and spring, when the predominance of northeasterly (NE) wind stress forces coastal upwelling. During winter, southwesterly winds associated to atmospheric frontal systems eventually disrupt coastal upwelling reversing Ekman transport, causing advection of warmer waters shoreward (Ribeiro et al., 2011). The response of Cabo Frio upwelling system to spatially variable wind forcing 1 Brazilian Navy - IEAPM, 2 Federal University of Paraná - CEM-UFPR [email protected] [email protected] Couto, P. P. 1,2 & Soutelino, R. G. 1 Results Da Silva (2011) shows atypical positive SST anomalies during 2010 austral summer at Cabo Frio region. In situ and remote measurements have shown temperatures up to 30°C at the inner shelf in an upwelling-favorable wind scenario acting in the region (Figure 1). A similar process has been described at the northern California coastal ocean, as a conspicuous sharp tongue of warm water being advected along the coast (Figure 2), by a flux contrary to the upwelling coastal jet (Send et al., 1987). The authors attribute the cause of such phenomena to relaxation and inversions of upwelling-favorable wind, unsettling the gradient pressure field and consequently the momentum balance. Efforts have been done on process-modeling the upwelling in the region but mostly directed on the onset and development of upwelling forced by spatially homogeneous wind stress and little attention has been given to transient events regarding spatial or temporal variations in the wind field. A process-study numerical experiment was conducted using the Regional Ocean Modeling System (ROMS). A climatological temperature- salinity field (Brazilian Navy) representing austral summer was used and initial velocity field was assumed to be at rest. The one-year experiment was forced with a synoptic, spatially variable wind stress (Figure 6), from NCEP/ CFSR atmospheric reanalysis from January 2010, when the atypical event described by Da Silva (2011) occurred. During the model run an initial spin-up phase was followed by non-stationary upwelling setup until the end. In Figure 3a,d, a strong upwelling signal is noted in most of the domain, with its plume occupying the entire inner shelf up to the 100 m isobath. Positive meridional pressure gradient force seems to establish a westward alongshelf coastal jet of about 15 cm s -1 . A rearrangement of the meridional pressure gradient field is evidenced by its inversion after 15 days, generating an eastward flow (32 cm s -1 ) followed by alongshelf eastward thermal advection, shown as a narrow band of warmer waters nearshore (Figure 3b,e). After 20 days, the entire shelf is dominated by the presence of higher values of SST while in the northern region near Macaé upwelling remained almost unaffected during the whole period (Figure 3c,f). The momentum balance response seems to be related to alternations on the pressure gradient field (apparently intermitent, as in Figure 5) as a function of the sea surface elevation arrangement along the domain, due to upwelling coastal jet convergence in the presence of a positive zonal wind stress magnitude zonal gradient. References Da Silva, L. S., 2011: Estudo da variabilidade interanual da TSM no estado do Rio de Janeiro. Ribeiro, F. N. D., Soares, J., Oliveira, A. P., 2011: A coupled numerical model to investigate the air-sea interaction at the coastal upwelling area of Cabo Frio, Brazil, Environ. Fluid Mech., 11, 551-572. Send, U., Beardsley R. C., Winant, C. D., 1987: Relaxation from upwelling in the coastal ocean dynamics experiment. J. Geophys. Res. 92: 1683- 1698. Acknowledgements We thank Brazilian Navy for supporting the authors. The SIODOC Project for the in situ data, Python and ROMS community. We thank CAPES for the financial support for travel expenses. Figure 5: Hovmoller diagram of model SST field during 71 and 34 days. Near-zonal SST patch is computed along 30 m isobath and meridionally averaged within one deformation radius extension. The 34 days represents the time period showed in Figure 5, and show the abrupt advection of warm waters from Sepetiba to Rio de Janeiro. The 71 days period illustrates the apparently pulsing pattern of the balance between coastal upwelling jet and the counter-jet eventually generated. SACW SACW Final Remarks Our model results shows evidences that the dynamical response of the continental shelf flow and thermal distribution of Cabo Frio upwelling system is unsteady in the presence of spatially variable winds. The results indicates that the following scenario may be ocurring: 1) NE winds are stronger in Cabo Frio and weaker towards the south coast. 2) After Ekman and geostrophic adjustment scales, coastal upwelling and westward coastal jets develop in the along-shelf direction. 3) The weakening of the coastal jet towards west causes a substantial convergence in Rio de Janeiro south coast, leading to the development of a negative meridional pressure gradient force. 4) An eastward coastal jet develops and advects western warmer waters to the upwelling site, despite the constant action of NE winds locally. That sequence occurs periodically during the entire experiment. Additionally, recent observations (Figure 7) corroborates the model results. Introduction Objectives The main objective of this study is to numerically simulate wind-driven upwelling at Cabo Frio and assess its response to spatially variable wind stress field. Scientific Hypothesis Spatially variable wind field sets up a remote coastal jet in the opposite direction of the upwelling circulation, leading to the advection of warmer waters to the upwelling site. Methods Figure 2. SST images of the northern California coast between San Francisco and Point Arena during the Coastal Ocean Dynamics Experiment (CODE2), during May 18-27 1982 (Cortesy from SEND et al., 1987). QUIVER PLOT SEPETIBA RIO DE JANEIRO CABO FRIO o C ) ( HOVMOLLER CROSS-SHORE AVERAGED DOMAIN Figure 7: Panels (a,b,c): Mooring data time-series (SIODOC Project) at surface level representing wind, along-shelf surface flow and SST at the 50 m isobath near Cabo Frio (Figure 3e - B spot). A well-developed upwelling coastal jet (panel b, purple domain) generated as a response of the persistent and intense NE upwelling-favorable winds (a) shown at the first 7 days is suddenly interrupted (day 16) by an eastward coastal jet (green domain), despite local ocurrence of NE winds. Fast response of SST (c) is noted by its increasing and local inhibition of upwelling (day 17). Panels (d) and (e): Model time-series representing along-shelf flow and SST (Figure 3e - M2 spot) in a 32 days time spawn. Similar dynamics is noticed by the interruption of the upwelling coastal jet as an oposing flow (day 43) starts acting. Temperature rise is evident since day 62 and it remains high as the eastward coastal jet continues acting. a b c d e CW CW SACW SACW Figure 6. Constant wind field implemented in the model simulation and run-averaged SST. Note that the wind field is predominantly NE, with significant spatial variations. Figure 4. Time series of depth-averaged alongshore momentum terms (averaged over 72 hours, multiplied by 10 4 ) , alongshore barotropic velocity, free-surface elevation and SST. The virtual stations M1 and M2 are located along, approximately, the 50 m isobath, indicated by white stars (Figure 3e). Figure 3. Snapshots showing 3-day-averaged fields for meridional barotropic pressure gradient term (x10 4 m s -2 ) overlaid by barotropic velocity vectors (cm s -1 ) (a, b, c) and sea surface temperature in (°C) (d, e, f). At panels a, d a well establishment of upwelling is noted, while at panels b, e and c the upwelling seems to be interrupted at inner shelf, from Sepetiba to Cabo Frio, with the presence of warmer (up to 26°C) waters. As can be noticed near Macaé, the upwelling seems to maintain its maturity, providing lower temperatures (18°C) at the surface. a b c d e f Spetiba Cabo Frio Macaé Figure 1. Left panel: NOAA SST from march 2002 showing inner shelf advection of warmer waters to the upwelling site in a scenario of temporal variations in local wind (wind time-series above). Right panel: SST anomaly (°C) and wind time-series for january 2010 showing warmer waters at Rio de Janeiro inner shelf with the predominace of upwelling-favorable winds (modified from Da Silva, 2011).
Transcript of The response of Cabo Frio upwelling system to spatially ...
The Cabo Frio coastal upwelling system has been focus of research due to its intensity and recurrence year-round, and its physical-biological implications. Strong seasonal signal of upwelled waters is evidenced by relatively low sea surface temperature (SST) specially along austral summer and spring, when the predominance of northeasterly (NE) wind stress forces coastal upwelling. During winter, southwesterly winds associated to atmospheric frontal systems eventually disrupt coastal upwelling reversing Ekman transport, causing advection of warmer waters shoreward (Ribeiro et al., 2011).
The response of Cabo Frio upwelling system to spatially variable wind forcing
1Brazilian Navy - IEAPM, 2Federal University of Paraná - CEM-UFPR
[email protected], P. P.1,2 & Soutelino, R. G.1
Results
Da Silva (2011) shows atypical positive SST anomalies during 2010 austral summer at Cabo Frio region. In situ and remote measurements have shown temperatures up to 30°C at the inner shelf in an upwelling-favorable wind scenario acting in the region (Figure 1).
A similar process has been described at the northern California coastal ocean, as a conspicuous sharp tongue of warm water being advected along the coast (Figure 2), by a flux contrary to the upwelling coastal jet (Send et al., 1987). The authors attribute the cause of such phenomena to relaxation and inversions of upwelling-favorable wind, unsettling the gradient pressure field and consequently the momentum balance. Efforts have been done on process-modeling the upwelling in the region but mostly directed on the onset and development of upwelling forced by spatially homogeneous wind stress and little attention has been given to transient events regarding spatial or temporal variations in the wind field.
A process-study numerical experiment was conducted using the Regional Ocean Modeling System (ROMS). A climatological temperature-salinity field (Brazilian Navy) representing austral summer was used and initial velocity field was assumed to be at rest. The one-year experiment was forced with a synoptic, spatially variable wind stress (Figure 6), from NCEP/CFSR atmospheric reanalysis from January 2010, when the atypical event described by Da Silva (2011) occurred.
During the model run an initial spin-up phase was followed by non-stationary upwelling setup until the end. In Figure 3a,d, a strong upwelling signal is noted in most of the domain, with its plume occupying the entire inner shelf up to the 100 m isobath. Positive meridional pressure gradient force seems to establish a westward alongshelf coastal jet of about 15 cm s-1. A rearrangement of the meridional pressure gradient field is evidenced by its inversion after 15 days, generating an eastward flow (32 cm s-1) followed by alongshelf eastward thermal advection, shown as a narrow band of warmer waters nearshore (Figure 3b,e).
After 20 days, the entire shelf is dominated by the presence of higher values of SST while in the northern region near Macae upwelling remained almost unaffected during the whole period (Figure 3c,f). The momentum balance response seems to be related to alternations on the pressure gradient field (apparently intermitent, as in Figure 5) as a function of the sea surface elevation arrangement along the domain, due to upwelling coastal jet convergence in the presence of a positive zonal wind stress magnitude zonal gradient.
ReferencesDa Silva, L. S., 2011: Estudo da variabilidade interanual da TSM no estado do Rio de Janeiro.
Ribeiro, F. N. D., Soares, J., Oliveira, A. P., 2011: A coupled numerical model to investigate the air-sea interaction at the coastal upwelling area of Cabo Frio, Brazil, Environ. Fluid Mech., 11, 551-572.
Send, U., Beardsley R. C., Winant, C. D., 1987: Relaxation from upwelling in the coastal ocean dynamics experiment. J. Geophys. Res. 92: 1683- 1698.
AcknowledgementsWe thank Brazilian Navy for supporting the authors. The SIODOC
Project for the in situ data, Python and ROMS community. We thank CAPES for the financial support for travel expenses.
Figure 5: Hovmoller diagram of model SST field during 71 and 34 days. Near-zonal SST patch is computed along 30 m isobath and
meridionally averaged within one deformation radius extension. The 34 days represents the time period showed in Figure 5, and show the abrupt advection of warm waters from Sepetiba to Rio de Janeiro. The 71 days period illustrates the apparently pulsing pattern of the balance between
coastal upwelling jet and the counter-jet eventually generated.
SACW
SACW
Final Remarks
Our model results shows evidences that the dynamical response of the continental shelf flow and thermal distribution of Cabo Frio upwelling system is unsteady in the presence of spatially variable winds. The results indicates that the following scenario may be ocurring: 1) NE winds are stronger in Cabo Frio and weaker towards the south coast. 2) After Ekman and geostrophic adjustment scales, coastal upwelling and westward coastal jets develop in the along-shelf direction. 3) The weakening of the coastal jet towards west causes a substantial convergence in Rio de Janeiro south coast, leading to the development of a negative meridional pressure gradient force. 4) An eastward coastal jet develops and advects western warmer waters to the upwelling site, despite the constant action of NE winds locally. That sequence occurs periodically during the entire experiment. Additionally, recent observations (Figure 7) corroborates the model results.
IntroductionObjectives
The main objective of this study is to numerically simulate wind-driven upwelling at Cabo Frio and
assess its response to spatially variable wind stress field.
Scientific Hypothesis
Spatially variable wind field sets up a remote coastal jet in the opposite direction of the
upwelling circulation, leading to the advection of warmer waters to the upwelling site.
Methods
Figure 2. SST images of the northern California coast between San Francisco and Point Arena during the Coastal Ocean Dynamics Experiment (CODE2), during May 18-27 1982 (Cortesy from SEND et al., 1987).
QUIVER PLOT
SEPETIBA RIO DE JANEIRO CABO FRIO
o C )(
HOVMOLLER CROSS-SHORE AVERAGED DOMAIN
Figure 7: Panels (a,b,c): Mooring data time-series (SIODOC Project) at surface level representing wind, along-shelf surface flow and SST at the 50 m isobath near Cabo Frio
(Figure 3e - B spot). A well-developed upwelling coastal jet (panel b, purple domain) generated as a response of the persistent and intense NE upwelling-favorable winds (a)
shown at the first 7 days is suddenly interrupted (day 16) by an eastward coastal jet (green domain), despite local ocurrence of NE winds. Fast response of SST (c) is noted by its increasing and local inhibition of upwelling (day 17). Panels (d) and (e): Model time-series representing along-shelf flow and SST (Figure 3e - M2 spot) in a 32 days
time spawn. Similar dynamics is noticed by the interruption of the upwelling coastal jet as an oposing flow (day 43) starts acting. Temperature rise is evident since day 62 and it
remains high as the eastward coastal jet continues acting.
a
b
c
d
e
CW
CW
SACW
SACW
Figure 6. Constant wind field implemented in the model simulation and run-averaged SST. Note that the wind field is predominantly NE, with significant spatial variations.
Figure 4. Time series of depth-averaged alongshore momentum terms (averaged over 72 hours, multiplied by 104) , alongshore barotropic velocity,free-surface elevation and SST. The virtual stations M1 and M2 are located along, approximately, the 50 m isobath, indicated by white stars (Figure 3e).
Figure 3. Snapshots showing 3-day-averaged fields for meridional barotropic pressure gradient term (x104 m s-2) overlaid by barotropic velocity vectors (cm s -1) (a, b, c) and sea surface temperature in (°C) (d, e, f). At panels a, d a well establishment of upwelling is noted, while at panels b, e and c the upwelling seems to be interrupted at inner shelf, from Sepetiba to Cabo Frio, with the presence of warmer (up to 26°C)
waters. As can be noticed near Macae, the upwelling seems to maintain its maturity, providing lower temperatures (18°C) at the surface.
a b c
d e fSpetiba
Cabo Frio
Macaé
Figure 1. Left panel: NOAA SST from march 2002 showing inner shelf advection of warmer waters to the upwelling site in a scenario of temporal variations in local wind (wind time-series above). Right panel: SST anomaly (°C) and wind time-series for january 2010 showing
warmer waters at Rio de Janeiro inner shelf with the predominace of upwelling-favorable winds (modified from Da Silva, 2011).
