CCS dynamics: sensitivity to wind forcing, heat flux and boundary conditions

CCS dynamics: sensitivity to wind forcing, heat flux and boundary conditions

Roms workshop, 2003

X. Capet, P. Penven, P. Marchesiello, J. McWilliams

UCLA

Thanks: J. Kindle, M. Pickett, R. Samelson

MODELING KEYS

Nested (one-way) grid hierarchy : - USWC (15 km)- Central California coast (5km)- Monterey bay area (1.66 km)(part of the NOPP/SCOPE program)

L67

Floats :- neutrally buoyant - compatible with the nesting- Online floats trajectory computation compatible with nested grids (AB4-AM4 predictor corrector, linear interpolation)

wind sensitivity results

Essentially too cold offshore and too warm nearshore. The latter bias is stronger with COAMPS and NCEP. Daily forcing do not solve this issue.

• COADS (44 years of marine data,monthly climatology, resolution: 0.5o and 1o)

• NCEP (global atmospheric model,40 years of simulation,resolution: 1.875o

• COAMPS (regional atmospheric model,2 years of simulation: from October 1998 to September 2000, 3 levels of embedding, resolution: 81km – 27km – 9 km• QuickSCAT(satellite scatterometer data,gridded at 0.5° resolution,from October 1999 to March 2002)

COAMPS/QuikSCAT wind properties

QuikSCAT

COAMPS winds 9km resolution (NRL, J. Kindle, atmospheric model with

assimilation)

Small scale wind structures:

-expansion fans, [Dorman and Winant, 1987].

-Uniform nearshore wind drop off.

Both features induce wind curls reaching 0.5 Pa/100km.

Wind measured from space. shortcoming: no wind can be provided on land and close to the coast (25km)

=> reconstruction e.g. through objective analysis (smooth).

Max curls around 0.03 Pa/100 km

COAMPS/QuikSCAT wind properties

Profile of southward wind averaged in the longshore direction

• Coastal upwelling intensity• Continuity of the poleward

undercurrent and surface EKE

Presentation plan

COMPARISON OF LONG TERM SOLUTIONS OBTAINED WITH THESE 2 DIFFERERENT WIND PRODUCTS:NEARSHORE WIND PROFILE DOES MATTER FOR THE COASTAL DYNAMICS AND ALSO FOR THE INTERIOR

Ekman pumping/coastal divergence compensation ???

Why is it not so clear that the nearshore wind structure is important ???

Z

X

Ekmanpumping

Weak coastal divergence

Large coastal divergence

Upwelling : summer mean temperature (1)

• QuikSCAT coastal temperatures are colder and the upwelling is more marked.

• COAMPS solution has a wide doming corresponding to Ekman pumping.

Temperature is not well-suited to describe the upwelling in near-surface (not passive tracer)

Upwelling from a Lagrangian perspective.

Massive float release over the third grid level at different times in summer and different depths. Floats released at 50m (100m). Some reach the upper 20m (50m) within two weeks after they are released.

- QuikSCAT fluid parcels experience much more upwelling- COAMPS and QuikSCAT have comparable cross-shore patterns for the upwelling location

50m ->20m

100m ->50m

3139 (33%)1632 (17%)

1359 (14%)571 (6%)

Continuity of the poleward undercurrent

floats released nearshore at 200m depth south of Pt Sur at different times (500 floats)

more or less remain at moderate distances of the coast despite recirculations (coamps wind) Tend to leave the coast after possible complex recirculations (QuikSCAT)

COAMPS

Mean cross shore distance between the floats and the 200 m isobath

COAMPS

Dispersion

QuikSCAT floats experience continuous seaward motion COAMPS floats are more efficiently trapped nearshore and have overall less dispersive behavior

Continuity of the poleward undercurrent (2)

This is likely related to the nearshore currents instability (QuikSCAT has a more marked front that should be more unstable). Quantification is required.

Spring surface eddy kinetic energy

Coamps Coamps with reduced nearshore drop off Difference

COAMPS modified: signals of high eke spread over the domain, originating from the main upwelling centers and following pathways consistent with the mean circulation (standing eddies) COAMPS: 40% less eke

Conclusion: influence of the coastal wind profile

There is no doubt that the California Current System is strongly shaped by its own intrinsic response to the large scale winds (e.g. Marchesiello et al., JPO, 2003). Small scale of the wind would therefore be unessential.

However, we have shown that there are potential regional effects of the small scale nearshore wind patterns (upscaling). More precise quantification is still required.

The small scale of the wind does matter as we get close to the shore with adequate resolutions (upwelling, primary production). Ekman pumping associated with coastal wind drop off does not compensate for the lack of coastal divergence

QuikSCAT is very a favorable wind in terms of coastal upwelling strength. The fact that temperatures are still too warm at the coast in our solutions suggest that our generally weak upwelling (relative to observations) comes from other factors (boundary conditions ???).

Local influence

Remote influence

No upwelled float in the region of intense Ekman pumping

Upwelling from a Lagrangian viewpoint (2).

Upwelling from a Lagrangian viewpoint (3).

Surface currents (3) Blended wind versus QuikSCAT solution

Blended wind solution has intense permanent poleward currents off Pt Sur. Monterey bay unsheltered

Temp,10meters depth, August, 5 (blended)

T,10meters depth, July, 15, QuikSCAT

Main wind curl effect: barotropic meridional velocity The longshore poleward is reinforced by the wind curl at local scales (Sverdrup balance).

vbar summer mean (QuikSCAT)vbar summer mean (Blended)

Comparisons with data from Collins et al. (2000). (1)

Annual mean meridional velocities off Pt Sur

Countercurrent at the coast, thinner and shallower in roms. Equatorward current farther offshore, shallower and weaker in roms.

Comparisons with measurements from Tisch et al. (1992).

Anticyclonic structure

Measurements show that poleward current can be present even during the period of intense upwelling.

Comparisons with measurements from Collins et al. (2000).

Model (black) and Pegasus stations (red) upper 1000m mean velocities

Countercurrent off Pt Sur Complex circulation off the Bay with possible convergence

Summer mean poleward currents off Point Sur

• Blended wind solution exhibits more intense poleward currents with no surface southward jet.

• QuikSCAT exhibits a surface coastal upwelling jet, a deeper and slower undercurrent core

Filamentation and cross-shore transport : conceptual views (Strub et al., 1991)

Filamentation and cross-shore transport : conceptual views (Strub et al., 1991)

COAMPSQuickSCAT

Filamentation and cross-shore transport : Lagrangian view in roms (1)

near-surface trajectories and corresponding SSHcoamps quickscat

Filamentation and cross-shore transport : Lagrangian view in roms (2)

subsurface trajectories and corresponding SSH

coamps quickscat