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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 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 Background • 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 tongue • 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…. Latitude Scripps Coupled Ocean-Atmosphere Regional (SCOAR) Model Schematic of Ocean-Atmosphere Coupled Model • Bulk formulae or RSM physics in ABL Atmosphere Ocean Boundary Layer Variables 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: SST • Sequential Coupling IC and Lateral BC: Lateral BC: Ocean Analysis NCEP/DOE Reanalysis (JPL/ECCO) or Climatology 3 hourly or daily coupling 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 system Tehuantepec ITCZ / Eastern Pacific Warm Pool Papagayo Cross-equatorial trade winds Gap Winds Tropical Depressions and Hurricanes 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 1S-6N, 130W- 100W. 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 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 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 dynamically DYNM coupled smoothed* CPL coupled *: 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 forcing. • Heat flux coupling damps the fluctuations of SST by TIWs. CLM EOF-1 34% (2nd 30%, Total 64%) • Wind coupling yields a stronger Latitude 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 1S-6N, 130W-100W. Changes in vertical distribution Average over 1N-6N 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 Longitude Changes in wavenumber and frequency characteristics Wavenumber spectra Frequency spectra Spectral density [(C2//cplong.] ~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 1N-6N perio phase wavelengt 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 LH SST & WS • Similar coupling of SST with dynamics and thermodynamics of ABL is also seen in CCS region over RSM: various spatial and temporal 16 km WSD scales. WSC •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. Papagayo AVHRR Satellite SST Image; Jan 1999 Panama • Gap Winds produce cold tongues due to evaporative cooling and entrainment, plus windstress curl forcing. • Affect the atmospheric deep convection and precipitation. Wind Stress and Ekman Pumping Velocity OBSERVATION MODEL: 1999-2003 Winter Winter • Ekman Pumping Velocity Unit : 10-6m/s • Low-level wind Summer 95W 85W jets through Summer mountain gaps • Wind-induced vorticity forcing leads to dynamic response 95W 85W Xie et al., 2005 in the ocean thermocline. 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 ecosystem. 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 Winter mountain gaps (due to wind- induced mixing, evaporative cooling, and Ekman pumping) Winter Costa Summer Rica Dome OBSERVATION Cold bias in CRD Rainfall: Suppression of Precipitation by Eddies OBSERVATION Winter MODEL Winter • Costa Rica Dome and cold tongues by gap winds suppress Summer atmospheric deep Summer 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) 2) VOCALS - Peru-Humboldt Upwelling – SST - Stratocumulus 3) North Pacific Decadal Variability - KOE – Aleutian Low feedbacks 1948-2005 Thanks!
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