Regional coupled modeling of eddy-wind interaction in the California Current System Eddy kinetic energy and Ekman pumping

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Hole Oceanographic Institution Art Miller & Joel Norris Scripps

1 Regional coupled modeling of eddy-wind interaction in the California Current System Eddy kinetic energy and Ekman pumping Hyodae Seo Woods Hole Oceanographic Institution Art Miller & Joel Norris Scripps Institution of Oceanography PICES-2014 Annual Meeting Yeosu, Korea, October 21, 2014

2 Eddy-wind interaction: wind stress τ=ρcd(ua Uo) Ua Uo

3 Eddy-wind interaction: wind stress τ=ρcd(ua Uo) Ua Uo Increased wind over warm SST 10m wind Ua= Uab + UaSST Wallace et al (1998) Correlation (SST and wind speed): high-passed Xie 2004

4 Eddy-wind interaction: wind stress τ=ρcd(ua Uo) Ua Uo Increased wind over warm SST Correlation (SST and wind speed): high-passed Xie 2004 a Surface temperature SST and height SSH m wind Ua= Uab + UaSST Wallace et al (1998) Uniform eastward wind over an anticyclonic eddy in the Southern Ocean (Chelton 2013) Dipole Ekman velocity surface temperature τ U D Ekman pumping anomaly 90 out of phase with SSH propagation of an eddy h contour interval = 0.5 cm da

5 Eddy-wind interaction: wind stress τ=ρcd(ua Uo) Ua Uo a Surface SST temperature and SSH and height surface current We=τ/[ρ(f+ζ)] Monopole Ekman velocity τ τ surface currents U h contour interval = 0.5 cm da Upwelling at the center of an anticyclonic eddy: damping of an eddy surface current Uo=Uob + Uoe (Uob Uoe)

6 Eddy-wind interaction: wind stress τ=ρcd(ua Uo) Ua Uo a Surface SST temperature and SSH and height surface current We=τ/[ρ(f+ζ)] Monopole Ekman velocity τ τ surface currents U h contour interval = 0.5 cm da Upwelling at the center of an anticyclonic eddy: damping of an eddy surface current Uo=Uob + Uoe (Uob Uoe) Dipole Ekman velocity surface temperature τ D U Feedback to ocean would be different! h contour interval = 0.5 cm da

7 Eddy-wind interaction: wind stress τ=ρcd(ua Uo) Ua Uo 10m wind Ua= Uab + UaSST surface current resulting wind stress Uo=Uob + Uoe (Uob Uoe) τ τb +τsst + τoe

8 Eddy-wind interaction: wind stress τ=ρcd(ua Uo) Ua Uo 10m wind Ua= Uab + UaSST surface current resulting wind stress Uo=Uob + Uoe (Uob Uoe) τ τb +τsst + τoe Effects of τsst and τcur on the ocean? EKE and Ekman pumping

9 Result from previous studies and the goal of this study Previous studies considered either SST or Uo in τ formulation in ocean-only models and saw weakened eddy variability. JOURNAL OF PHYSICAL OCEANOGRAPHY uncoupled SST SST-τ coupled SST C05023 VOLUME 39 SST-τ coupling: Jin et al. (2009) FIG. 4. SST distribution on day 60: (left) uncoupled and (right) coupled. d from measurements that the summer-mean hore wind stress over the shelf off Bodega Bay, rnia, decreases from 0.14 N m22 at 25 km offshore N m22 at 2 km. Perlin et al. (2007) found that the tress decreases from 0.14 to N m22 near the after 72 h in a coupled mesoscale atmosphere model. The mechanism for the broad nearshore of strong wind stress curl in the CCS resulting ST wind coupling was hypothesized by Chelton 2007a). sensitivity experiments that double and halve the cal coupling coefficients show modest impacts on nd stress changes (Fig. 7), although the SST changes ger. Thus, the overall effect of the coupling is on day 60 (Fig. 4) where the fluctuations are at an earlier phase in their unstable development. In addition, in the equilibrium phase (days ), the energy is smaller because of the coupling. The mean stratification and circulation (Figs. 5, 6) show coupling influences through a weaker thermocline tilt resulting from weaker nearshore wind stress. They have an increased poleward transport, especially in the undercurrent, which is consistent with increased coastal wind stress curl and Sverdrup balance. The Ekman circulation in the zonal plane has weaker upwelling right at the boundary with stronger upwelling offshore (Fig. 8). In the faroffshore region away from the upwelling circulation and eddies, the zonal transport in the surface layer approaches EDEN AND DIETZE: EFFECTS OF EDDY/WIND INT uncoupled EKE Uo-τ coupled EKE Figure kinetic energy the upper 50 m) in 20 Uo3.-τ Eddy coupling: Eden(EKE) and (average Dietzeof(2009) (a) the reference experiment, (b) WINDFEED, and (c) difference (WIND in cm2 s&2. This study examines the between relative importance of SST and both for the year Besides a small eddy signal where p denotes in the Gulf model. Stream/North Atlantic Current system, there are fluctuations and w usfc in a fully coupled regional only minor systematic changes in the mean circulation of The EKE budget the model. This applies for the subsequent years as well (Figure 4). The only systematic effect of including the ocean currents in the formulation of the wind stress forcing is a reduction of the mean South Equatorial Current (SEC) and 0 average of the scal perturbation u0o wi the primitive equat (3) describe chang

2007, 2014 An input-output based coupler; portable & flexible 7 km O-A resolutions & matching mask 6-yr

10 Regional coupled model Scripps Coupled Ocean-Atmosphere Regional Model Atmosphere WRF WRF or bulk physics τ (Q & FW) 6-h coupling SST & Usfc Ocean ROMS 6-h NCEP FNL monthly SODA Seo et al. 2007, 2014 An input-output based coupler; portable & flexible 7 km O-A resolutions & matching mask 6-yr integration ( ) Ttot Tb Te Smoothing of mesoscale SST and Uo (Putrasahan et al. 2013) Utot Ub Ue 5 loess smoothing (~3 boxcar smoothing)

11 Experiments τ=ρcd(ua-uo) Ua-Uo Ttot = Tb + Te Utot = Ub+ Ue 5 loess filtering ( 3 boxcar smoothing) Experiments τ formulation includes CTL Tb Te Ub Ue note Tb Te Ub Ue noue Tb Te Ub Ue noteue Tb Te Ub Ue noutot Tb Te Ub Ue

CTL = 171 note = 174 noue = 231 noteue =

noteue noutot JAS 2005-2010 Te no impact

12 CTL = 171 note = 174 noue = 231 noteue = 230 noutot = 247 Summer surface eddy kinetic energy EKE time-series 25-30% EKE difference CTL note NoTeUe cm 2 s -2 noue noteue noutot JAS Te no impact 25% weaker EKE with Ue 30% weaker EKE with Ub+Ue

13 Eddy kinetic energy budget!!!!!!!! Ke + U Ke + u# Ke + ( u # p #) = t g ρ " "! w + ρ ( " o! u ( " advection by mean and eddy current (offshore)!!! u U))+ " u! τ " +ε Pe Ke baroclinic conversion (BC) Km Ke barotropic conversion (BT) wind work (P) if positive (eddy drag if negative) Upper 100 m average H~fL/N, where f=10-4, L=10 4 m, N=10-2 H=10 2 m

14 EKE budget: CTL BT BC P Significant difference in only P u τx v τy P a primary source of EKE. - Wind work from v τy - Eddy damping by u τx u τx τy v 150 m average

15 EKE budget: CTL BT BC P Significant difference in only P u τx v τy P a primary source of EKE. - Wind work from v τy - Eddy damping by u τx u τx τy v 150 m average

16 [cm 2 s -2 ] Cross-shore distribution of EKE and P cross-shore distance (km) EKE CTL note noue P and BC maximum near the coast (20-30 km). [10-5 kgs -1 m -3 ] P noue CTL: P decreases by 20% cross-shore distance (km)

17 [10-5 kgs -1 m -3 ] [10-5 kgs -1 m -3 ] Eddy drag and wind work u τx = eddy drag eddy drag CTL=-0.47 note=-0.53 noue= % stronger eddy drag v τy = wind work wind work CTL=1.74 note=1.86 noue= % weaker wind work Ue: increases the eddy drag and weakens the wind work

18 W tot = 1 o r W tot = W cur + W SST background wind stress r 1 = o (f + ) {z } o (f + ) 2 @y {z } W c W W Wlin Wζ Wβ WSST To the extent that the high-pass filtered curl of the surface r Curl-induced linear Ekman pumping Ekman pumping velocity (f + ) apple r r Stern 1965; Gaube et al. (2014) Vorticity gradient-induced nonlinear Ekman pumping x o (f + ) 2 {z } + r 0 SST o (f + ) {z } W SST investigated here is, to β Ekman pumping (negligible) SST induced Ekman pumping (Chelton et al. 2004) W SST = τ# SST ρ o ( f +ζ ). α c c SST ρ o f +ζ ( )

19 Wind stress curl and cross-wind SST gradient W SST = # τ SST ( ) ρ o f +ζ α c c SST ρ o f +ζ ( ) OBS CTL Wind stress curl [Nm -2 per 10 7 m] note αc=0.8 noue αc=0.6 αc=0.1 αc=0.6 Cross-wind SST gradient [ C per 100km] JAS ; QuikSCAT wind stress and TRMM SST

20 Ekman pumping velocity JAS climatology OBS Wsst Wlin Wζ Wtot CTL Wsst Wlin Wζ Wtot m/day JAS

21 Ekman pumping velocity JAS climatology note Wsst Wlin Wζ Wtot noue Wsst Wlin Wζ Wtot m/day JAS

Long-term effect of SST and vorticity on Ekman pumping velocity Wek from

r=-0.06 SST and vorticity induce the Wek response of comparable magnitudes

indicative of different feedback processes Wek: CTL-NoUe Wek [mday -1 ]

22 Long-term effect of SST and vorticity on Ekman pumping velocity Wek from CTL Wek: CTL-noTe Wek vs crosswind SST gradient Wctl-WnoTe Wek [mday -1 ] r=-0.06 SST and vorticity induce the Wek response of comparable magnitudes but of different spatial pattern. indicative of different feedback processes Wek: CTL-NoUe Wek [mday -1 ] crosswind SST gradient [ C per 100km] Wek vs surface vorticity Wctl-WnoUe r=-0.3 JAS surface vorticity [day -1 ]

23 Summary Examined the relative importance of τsst vs τcur in EKE and Ekman pumping velocity in the CCS using a regional coupled model. Surface EKE is weakened almost entirely due to mesoscale current. - SST has no impact. EKE budget: enhanced eddy drag and reduced wind work. WSST reflects the crosswind SST gradient, while Wζ surface vorticity - Associated patterns of change imply different feedback processes. - Further investigation on the mechanisms for feedback is underway.

24 Thanks!

25 Summertime climatology: coastal upwelling CTL yields reasonable representation of the observed summertime upwelling condition in CCS. JAS

26 CTL Change SST and surface current CTL-NoTe CTL-NoUe CTL CTL-NoTe CTL-NoUe Change in SST pattern reflects the change in surface current: advection by mean and eddies. CTL-NoUe

27 CTL EKE Cross-shore vs depth EKE CTL-noTe CTL-noUe CTL-noTeUe m CTL-noUtot cm 2 s 2 41N 34N alongshore averages

28 NOAA OI SST Change in JAS SST CTL CTL-NoTe CTL-NoUe CTL-NoTeUe CTL-NoUtot

29 Overlaid with contours for SST difference Surface currents show both alongshore and offshore component (Ekman current). Change JAS Surface current CTL CTL-NoTe Change in offshore (onshore) temperature advection by mean current mainly responsible for the change in SST CTL-NoUe CTL-NoTeUe CTL-NoUtot

30 wind speed (and also stress) is ENHANCED (REDUCED) over warm (cold) SST. It is a response to change in SST, damping the SST anomaly.

Observations and Modeling of Coupled Ocean-Atmosphere Interaction over the California Current System

Observations and Modeling of Coupled Ocean-Atmosphere Interaction over the California Current System Cape Blanco Dudley Chelton 1, Xin Jin 2, Jim McWilliams 2 & Tracy Haack 3 1 Oregon State University