Regional Ocean-Atmosphere Intera by pengtt


									Regional Ocean-Atmosphere Interactions
         in the Eastern Pacific:
TIW’s, Mesoscale Eddies and Gap Winds
                  Arthur J. Miller
        Scripps Institution of Oceanography
        University of California, San Diego

        Based on the Ph.D. Dissertation of
            Mr. Hyodae Seo (SIO)

           Including collaboration with
                John Roads (SIO)
          Ragu Murtugudde (Maryland)
             Markus Jochum (NCAR)

       Woods Hole Oceanographic Institution
        Ocean Engineering Seminar Series
                  April 26, 2006
• Background
• Regional Ocean-Atmosphere Coupled Model
• Research Topics
   TIWs and Air-Sea Interaction
     Atmospheric Response to TIWs
      - Stability adjustment of ABL  Thermal and dynamic response
     Effect of Atmospheric Feedback on TIWs frequency-wavenumber
   California Current Eddies and Air-Sea Interaction
   Gap Winds and Air-Sea Interaction
     Wind-induced forcing  Thermocline doming
      Suppression of atmospheric deep convection
• Summary
• Air-sea interaction in the Eastern Pacific
                                      • Important component of large-scale
                                         atmospheric and oceanic circulation
                                      • Atmospheric deep convection over the
                                         eastern Pacific warm pool and Equatorial
                                         Current system
                                      • Coastal upwelling and equatorial cold
                                      • Equatorial SST front and TIWs
                                      • Influence by land and coastline
                                      • Different cloud response to SSTs
 Shallow               Deep Cumulus
 Stratocumulus                          All involve interactions among air, sea
                                         and land. Studying the nature of such
                                         coupling is important for regional climate,
                                         and large-scale climate as well.

                                        Consider a new high-resolution
                                       regional coupled model….
Scripps Coupled Ocean-Atmosphere Regional
(SCOAR) Model
   Schematic of Ocean-Atmosphere Coupled Model                  • Bulk formulae or
                                                                RSM physics in ABL
  Atmosphere                                   Ocean
                       Boundary Layer
                                                                for momentum, heat
                                                                and fresh-water fluxes
 Regional Spectral
      Model           Bulk Formulae        Regional Ocean
                                           Modeling System
                                                                • Wind stress relative
      (RSM)            or RSM BL
                         physics              (ROMS)            to ocean currents:


                                                                • Sequential Coupling
 IC and Lateral BC:                Lateral BC: Ocean Analysis
NCEP/DOE Reanalysis               (JPL/ECCO) or Climatology      3 hourly or daily

      Seo, Miller and Roads (2006) J. Climate, sub judice
Eastern equatorial Pacific domain example
   Evolving SST and wind-stress vector in 1999-2000
              45 km ROMS + 50 km RSM
                                                   Coupled
                        Tehuantepec                ITCZ / Eastern
                                                  Pacific Warm Pool
                                                   Cross-equatorial
                                                  trade winds
                                                   Gap Winds
                                                   Tropical
                                                  Depressions and
                                                   Equatorial and
                                                  Coastal Upwelling
                                                   Tropical
                                                  Instability Waves
Model domains
in the eastern
Pacific sector
(a) Eastern Tropical
  Pacific: TIW’s
(b) California Current
  System: Eddies
(c) Central American
  Coast: Gap Winds
     Tropical Instability Waves:
How do Feedbacks Between SST and
  the Atmospheric Boundary Layer
 Affect TIW stability characteristics?
TIW Domain in the Eastern Tropical Pacific
            Tropical Instability Waves
           20 km ROMS + 30 km RSM

                                Galapagos Is.

          RSM: 28 layers (6 below 900mb)
          ROMS: 30 layers (9 in top 100m)
                                        EOF from September
EOF analysis of SST                     to December, 1999
                                        over 1S-6N, 130W-
 CLM EOF 1; 34.2%     PC 1

                             • 1st and 2nd EOFs and
                               PCs are paired and
 CLM EOF 2; 30.5%
                               directly related to TIWs,
                      PC 2
                               explaining more than 60%
                               of the total variance.

  CLM EOF 3; 9.3%     PC 3   • We are interested in....
                             1) from EOFs, changes in
                               amplitude and
                               wavelength of zonal
 CLM EOF 4; 6.6%               temperature fluctuations
                      PC 4
                               by TIWs.
                             2) from PCs, changes in
                               frequency of TIWs.
Stability Changes in ABL due to SST
    17(15) warm(cold) phases during 2-4 Sep. 1999
Atmospheric Temperature               U-Wind

                                         Stronger shear
                                                           Weaker
                          Weaker shear
                                                              stratification of
                                                              ABL over warm
   Ocean Temperature             Ocean Temp. Profile          phase of TIWs.
                                                          •   Stronger
                                                              surface winds
                                                              over warmer
   Modification of heat and momentum flux
                                                  Change in dynamic state
    Change in thermal state

    CEOF 1 of SST and Latent Heat Flux
                                                   Coupling of SST and Wind stress


 LH anomaly : 20W / m2
 Div and curl anomaly :  2N/m2 per 100km
• Turbulent heat flux damps the SST; a negative feedback
• Feedback from wind stress perturbation remains largely unknown
                       Observed: -40-50 W/m2/K

Comparable to observed values
 Effects of atmospheric feedbacks on TIW’s
 How do the perturbation heat fluxes and wind stresses affect the
     characteristics of TIW’s?
 Additional experiments: sensitivity test for the year of 1999

          wind stress      heat flux
  CPL         coupled        coupled      fully coupled
 DYNM         coupled     smoothed* CPL
                                                          *: temporally
  THER                                     thermally
                                                          smoothed using 120-
           smoothed CPL      coupled
   M                                        coupled
                                                          day moving mean
  CLM      smoothed CPL   smoothed CPL     uncoupled

• Analysis using the first two EOFs and PCs of ocean temperature
• We are interested in....
1) EOFs: changes in wavelength of zonal temperature fluctuations by TIWs.
2) PCs: changes in frequency of the TIWs.
 Changes in amplitude of SST fluctuations
           CPL EOF-1 37% (2nd: 26%, Total 63%)                         mean of 1st and 2nd modes
                                                                  std of SST    Reduction wrt
                                                                     (C)         CLM (%)
                                                         CPL         1.1               35
           DYNM EOF-1 31% (2nd 19%, Total 50%)
                                                        DYNM         1.3               23
                                                        THERM        1.5               11
                                                         CLM         1.7                -

           THERM EOF-1 37% (2nd 30%, Total 67%)    • TIWs occur under climatological
                                                    •   Heat flux coupling damps the
                                                        fluctuations of SST by TIWs.
           CLM EOF-1 34% (2nd 30%, Total 64%)      •   Wind coupling yields a stronger

                                                        damping; also increases wavelength.
                                                        (cf. Pezzi et al., 2002)
                                                    •   Full-coupling results in weakest
                                                        fluctuations of SST over the TIW
                             Longitude                  region.
EOF from September to December, 1999 over 1S-6N, 130W-100W.
    Changes in vertical distribution
                                                                                       Average over 1N-6N
            CPL EOF-1 40% (2nd 31%,Total  71%)
                                                                     Zonal STD of temperature

                                                         Depth (m)
            DYNM EOF-1 32% (2nd 28%, Total  60%)

                                                                      Zonal STD of temperature (C)
            THERM EOF-1 40% (2nd 37%, Total  77%)
                                                     • Heat flux coupling : thermal
                                                       damping increases baroclinicity in
                                                       the mixed layer
                                                     • Wind coupling: damping + increase
            CLM EOF-1 41% (2nd 35%, Total  76%)
Depth (m)

                                                       in wavelength.
                                                     • Full-coupling: mixture of effects
                                                       from wind and heat feedback

                              Changes in wavenumber and frequency
                                            Wavenumber spectra                                                        Frequency spectra
Spectral density [(C2//cplong.]

                                    ~0.07cycle / long.

                                                                               Spectral density [(C)2/cpd]
                                                                                                              ~ 0.03 cycle / day

                                      ~ 0.1 cycle /  long.                                                              ~ 0.04 cycle / day

                                         Wavenumber (cycle per long.)                                             Frequency (cycle per day)

             Average of 1st and 2nd PCs
             Average of 1N-6N
                                                                                                                            perio         phase
  Coupling increases the                                                                                                     d         speed (m s-
                                                                                                    h ( long.)                             1)
 period of waves.                                                                                                           (day)
                                                                         CPL                                  12              36               0.4
  Dynamic coupling
                           DYNM                                                                               18              36               0.6
 increases the wavelength
 of the wave.             THERM                                                                               12              36               0.4
Air-Sea Coupling in the California Current Region

               SST & WS

                                     • Similar coupling
                                     of SST with
                                     dynamics and
                                     of ABL is also
                                     seen in CCS
                                     region over
RSM:                                 various spatial
                                     and temporal
16 km            WSD                 scales.
                                     •But model
                                     coupling strength
ROMS:                                in midlatitudes is
                                     3 - 5 times weaker
 7 km                                than observed
        Gap Winds and Air-Sea Interactions
         OBS; Chelton et al., 2000
                                                • Gap winds are driven by
                                                  pressure gradient across
            Tehuantepec                           narrow gaps or by intrinsic
                                                  variability of the trades.
                                                       AVHRR Satellite SST Image; Jan 1999


• Gap Winds produce cold tongues
due to evaporative cooling and
entrainment, plus windstress curl
• Affect the atmospheric deep
convection and precipitation.
  Wind Stress and Ekman Pumping Velocity
             OBSERVATION       MODEL: 1999-2003
                                                   • Ekman Pumping
                                                   Velocity Unit :

                                                  • Low-level wind
                   Summer         95W   85W       jets through
                                                  mountain gaps

                                                  • Wind-induced
                                                  vorticity forcing leads
                                                  to dynamic response
                                  95W   85W
Xie et al., 2005
                                                  in the ocean

                             RSM: 27 km       ROMS: 25 km
Thermocline Doming by Ekman Forcing;
Costa Rica Dome
OBSERVATION       MODEL: 1999-2003
 Along 8.5°N           Along 8.5°N

                                                95W      85W

                                     • Ekman pumping (above)
                                     causes thermocline shoaling
                                     (left), which further cools SST
                                     and supports a productive
               Costa                 • MLD is ~10 m and
               Rica    Along 90°W    thermocline is ~30 m deep
  Along 90°W
               Dome                  over Costa Rica Dome, both in
                                     obs and model.
SST: Response to Gap Winds
• Cold tongues off the major   MODEL: 1999-2003
mountain gaps (due to wind-
induced mixing, evaporative
cooling, and Ekman pumping)



                                Cold bias in CRD
Rainfall: Suppression of Precipitation by Eddies

                                    • Costa Rica Dome and
                                    cold tongues by gap
                                    winds suppress
               Summer               atmospheric deep
                                    convection and
                                    precipitation, shifting
                                    the ITCZ southward
                                    (Xu et al., 2005)

                                   Region of rain
 Xie et al., 2005                      deficit
                                    within ITCZ
    Summary of TIW feedbacks
• Coupled model simulates the observed atmospheric response
     to TIWs - Evolving SST induces ABL stability adjustment and
     changes in heat flux and wind stress.

• Series of fully coupled, partially coupled, and uncoupled
    experiment show that ...
•   1) as expected, heat flux feedback suppresses amplitude of
    SST fluctuation by TIWs; a negative feedback
•   2) dynamic feedback provides even stronger damping to SST
    fluctuation (cf. Pezzi et al., 2002)
•   3) surface damping of temperature by heat flux results in
    stronger baroclinicity of zonal temperature fluctuation.
•   4) dynamic feedback also increases the wavelength of TIW
 Summary of Gap Winds Feedbacks

• Coupled model simulates observed mean structure and
  seasonal variability of gap winds and their influences on
  upper ocean hydrography (Xie et al. 2005).

• Shoaling of thermocline and colder SST over Costa Rica
  Dome results in suppression and displacement of
  atmospheric deep convection and rainfall (Xie et al. 2005; Xu
  et al. 2005).
Future work….

 1) Bering Sea
        - Add Sea Ice
        - Bering Ecosystem Study (BEST)
        - Peru-Humboldt Upwelling – SST - Stratocumulus
 3) North Pacific Decadal Variability
        - KOE – Aleutian Low feedbacks 1948-2005

To top