Regional OceanAtmosphere Interactions in the Eastern Pacific: TIWs, Mesoscale Eddies and Gap Winds

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Regional Ocean-Atmosphere Interactions in the
Eastern Pacific TIWs, 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)
Sete, France July 27, 2006
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Awards Associated with Hyodae Seos Research
Best Student Paper Award Winner 2006 Ocean
Science Meeting Honolulu, Hawaii Air-Sea Coupling
Associated with the TIWs High-Resolution
Coupled Model Study 2006 Frieman Prize
Winner Best Student Paper at Scripps Seo, H., M.
Jochum, R. Murtugudde and A. J. Miller, 2006
Effect of ocean mesoscale variability on the
mean state of tropical Atlantic climate.
Geophysical Research Letters, 33, L09606
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Outline

• 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

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Background

• 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 tongue
• Equatorial SST front and TIWs
• Influence by land and coastline
• Different cloud response to SSTs
• ? 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.
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Scripps Coupled Ocean-Atmosphere Regional (SCOAR)
Model

• Bulk formulae or RSM physics in ABL for
  momentum, heat and fresh-water fluxes

Atmosphere
Ocean

• Wind stress relative to ocean currents

• Sequential Coupling
• ? 3 hourly or daily coupling

Seo, Miller and Roads (2006) J. Climate, in press
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Eastern equatorial Pacific domain example
Evolving SST and wind-stress vector in 1999-2000
45 km ROMS 50 km RSM

• Coupled system
• ITCZ / Eastern Pacific Warm Pool
• Cross-equatorial trade winds
• Gap Winds
• Tropical Depressions and Hurricanes
• Equatorial and Coastal Upwelling
• Tropical Instability Waves

Tehuantepec
Papagayo
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Model domains in the eastern Pacific sector

• (a) Eastern Tropical Pacific TIWs
• (b) California Current System Eddies
• (c) Central American Coast Gap Winds

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Tropical Instability WavesHow do Feedbacks
Between SST and the Atmospheric Boundary
LayerAffect TIW stability characteristics?
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TIW Domain in the Eastern Tropical Pacific
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EOF analysis of SST
EOF from September to December, 1999 over
1?S-6?N, 130?W-100?W.
CLM EOF 1 34.2
PC 1

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

CLM EOF 2 30.5
PC 2
CLM EOF 3 9.3

• We are interested in....
• from EOFs, changes in amplitude and wavelength
  of zonal temperature fluctuations by TIWs.
• from PCs, changes in frequency of TIWs.

PC 3
CLM EOF 4 6.6
PC 4
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Stability Changes in ABL due to SST
17(15) warm(cold) phases during 2-4 Sep. 1999
Atmospheric Temperature
U-Wind
Stronger shear

• ? Weaker stratification of ABL over warm phase of
  TIWs.
• ?
• Stronger surface winds over warmer

Weaker shear
Ocean Temp. Profile
Ocean Temperature
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Modification of heat and momentum flux
Change in dynamic state
Change in thermal state
Coupling of SST and Wind stress
WSD
WSC
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

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Observed -40-50 W/m2/K
Comparable to observed values
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Effects of atmospheric feedbacks on TIWs
? How do the perturbation heat fluxes and wind
stresses affect the characteristics of TIWs? ?
Additional experiments sensitivity test for the
year of 1999
temporally smoothed using 120-day moving mean

• Analysis using the first two EOFs and PCs of
  ocean temperature
• We are interested in....
• EOFs changes in wavelength of zonal temperature
  fluctuations by TIWs.
• PCs changes in frequency of the TIWs.

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Changes in amplitude of SST fluctuations
mean of 1st and 2nd modes

• TIWs occur under climatological forcing.
• Heat flux coupling damps the fluctuations of SST
  by TIWs.
• 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 region.

EOF from September to December, 1999 over
1?S-6?N, 130?W-100?W.
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Changes in vertical distribution
Average over 1?N-6?N
Zonal STD of temperature
Depth (m)
Zonal STD of temperature (?C)

• Heat flux coupling thermal damping increases
  baroclinicity in the mixed layer
• Wind coupling damping increase in wavelength.
• Full-coupling mixture of effects from wind and
  heat feedback

CLM EOF-1 41 (2nd 35, Total ? 76)
Depth (m)
Longitude
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Changes in wavenumber and frequency
characteristics
Average of 1st and 2nd PCs Average of 1?N-6?N

• Coupling increases the period of waves.
• Dynamic coupling increases the wavelength of the
  wave.

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Air-Sea Coupling in the California Current Region

• Similar coupling of SST with dynamics and
  thermodynamics of ABL is also seen in CCS region
  over various spatial and temporal scales.
• But model coupling strength in midlatitudes is 3
  - 5 times weaker than observed

RSM 16 km ROMS 7 km
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Gap Winds and Air-Sea Interactions

• Gap winds are driven by pressure gradient across
  narrow gaps or by intrinsic variability of the
  trades.

• Gap Winds produce cold tongues due to
  evaporative cooling and entrainment, plus
  windstress curl forcing.
• Affect the atmospheric deep convection and
  precipitation.

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Wind Stress and Ekman Pumping Velocity

• Ekman Pumping Velocity Unit
• 10-6m/s

• Low-level wind jets through mountain gaps

95W 85W

• Wind-induced vorticity forcing leads to dynamic
  response in the ocean thermocline.

95W 85W
RSM 27 km ROMS 25 km
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Thermocline Doming by Ekman Forcing Costa Rica
Dome
95W 85W

• Ekman pumping (above) causes thermocline
  shoaling (left), which further cools SST and
  supports a productive ecosystem.
• MLD is 10 m and thermocline is 30 m deep over
  Costa Rica Dome, both in obs and model.

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SST Response to Gap Winds

• Cold tongues off the major mountain gaps (due
  to wind-induced mixing, evaporative cooling, and
  Ekman pumping)

Cold bias in CRD
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Rainfall Suppression of Precipitation by Eddies

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

Region of rain deficit within ITCZ
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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

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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).

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Future work.

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

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Thanks!