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Co-authors: Justin Small, Roger Samelson, Bo Qiu, Terry Joyce, and Meghan Cronin

Western Boundary Currents and Frontal Air-Sea Interaction: Gulf Stream and Kuroshio Extension Kathie Kelly University of Washington WBCWG Workshop Phoenix, January 2009. Co-authors: Justin Small, Roger Samelson, Bo Qiu, Terry Joyce, and Meghan Cronin. Motivation.

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Co-authors: Justin Small, Roger Samelson, Bo Qiu, Terry Joyce, and Meghan Cronin

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  1. Western Boundary Currents and Frontal Air-Sea Interaction: Gulf Stream and Kuroshio Extension Kathie KellyUniversity of WashingtonWBCWG WorkshopPhoenix, January 2009 Co-authors: Justin Small, Roger Samelson, Bo Qiu, Terry Joyce, and Meghan Cronin

  2. Motivation [Western boundary currents comparison paper: Kelly, Small, Samelson, Qiu, Joyce, Cronin,…] What are the key processes in WBCs that might influence air-sea interaction? What are the key differences between the GS and the KE? What processes might contribute to a coupled mode? (where is the lag or memory?)

  3. Western North Atlantic Circulation Florida Current brings warm water north Gulf Stream separates & recirculates Recirculation creates heat reservoir Heat fluxed to atmosphere

  4. Annual Mean Net Surface Heat Flux Kuroshio Extension Large heat loss balanced by poleward heat transport (advection) Latent heat flux is large relative to sensible. Gulf Stream Southhampton Oceanography Centre

  5. What is so important about little boundary regions? US 2004 election results by state

  6. What is so important about little boundary regions? Results by population Weighting by geographical area does not necessarily reflect contribution.

  7. Storm Intensification: maxima over WBCs Genesis density of synoptic storms in boreal winter Left: 850 hPa cyclonic vorticity Right: mean growth rate (day-1) (Hoskins & Hodges, 2002)

  8. Western Boundary Current Heat Budgets (a) Gulf Stream region (b) Kuroshio Extension region • Dominant balance: heat storage vs advection • Advection larger than air-sea fluxes • Advection is mostly geostrophic Dong and Kelly, JPO, 2004 Vivier et al., JPO, 2002 Seasonalcycle removed

  9. Subtropical Mode Water:What does it contribute to air-sea interaction? • WBC advection brings warm water into region • Heat is stored south of WBC (recirculation gyre) • Accumulated heat is fluxed to atmosphere • Large heat storage <=> Low STMW volume Shenfu Dong, Ph D thesis, 2004

  10. Heat Content and Air-Sea Heat Flux Gulf Stream STMW region Kuroshio Extension STMW region • Sea Surface Height (heat content) and turbulent flux: • GS SSH leads by 3 months • Heat storage forces heat loss • Negative, but not significant in KE

  11. Flux and Heat Storage Effects • Large heat and moisture fluxes from convergence in the WBCs • Storage & release of heat in STMW region: memory through re-emergence • Heat storage (SSH) leads latent heat flux by 3 months

  12. Kuroshio Extension Path Changes Monthly paths from altimeter: stable/unstable periods

  13. Gulf Stream path Gulf Stream transport Path and Transport Indices Kuroshio Extension path From altimeter data: • KE has larger (factor of 2), low frequency path shifts • KE path, transport shifts more coherent • GS transport includes contribution from NRG • path/transport correlation not sig. in KE or GS during altimeter obs [Schneider]) Kuroshio Extension transport

  14. Path/Transport Statistics Kuroshio Extension Indices of GS transport (red) & path variability* (dash) Stronger current implies more stable path 3-month std dev of path latitude (meandering) Gulf Stream

  15. Impact of Path and Transport Anomalies on SST Gulf Stream path Kuroshio Extension path • Impact of KE path and transport anomalies is similar (for 15-yr altimeter period) • GS path variations show SST dipole • GS transport variations related to changes in NRG (Joyce et al, submitted) Kuroshio Extension transport Gulf Stream transport

  16. Path/transport Effects • Different magnitudes and periods of path shifts in KE vs. GS • Strong, stable current periods may have more influence on storm track, etc • Changes in path and strength may have different effects on SST and heat content • Changes in path may be internal to ocean (DWBC)

  17. North Pacific and North Atlantic • Top: bottom topography • Center: RMS SSH (altimeter) • Bottom: Std Dev turbulent flux

  18. Sea-Air Temperature Difference North Pacific North Atlantic • Air over Siberia colder than over Canada/US • Air masses in Pacific adjust over water, before WBC • Mean winds flow along/across max. temperature difference in Atlantic/Pacific

  19. March SST and Turbulent Fluxes Gulf Stream Kuroshio Extension • GS has stronger warm core => stronger advection • KE warm core occurs upstream of separation • Implications for air-sea fluxes (recovery from storm effects)

  20. SST Product Differences: a caution (top) GHRSST misses distinct warm core (new hi-res product!) (bottom) AMSR-E microwave (thru clouds) BUT heat content (integrated) more robust (Deser et al 2003)

  21. KE and GS STMW Temperature Profiles Gulf Stream Kuroshio Extension Implications for STMW formation [based on short data records] • GS STMW volume changes from top (warm water advection, Dong et al 2007) • KE volume changes from the bottom (thermocline heave, Qiu et al 2007)

  22. Other Effects: • Upstream influence on cold, continental air is important (KE vs. GS) • Path shifts may also affect land/sea SST gradient (GS) • GS has stronger warm core (may indicate shorter recovery time from cooling, stronger advection) [importance of accurate SST product] • GS STMW: restratification limits wintertime cooling • KE STMW: small contribution to air-sea flux (thermocline anomalies and path instability) • Ocean currents affect wind stress

  23. Heat Reservoir: Subtropical Mode Water (STMW) Gulf Stream wintertime temperature sections • 1996: thick STMW layer => SST drops quickly in winter to 18C • 1999: warm surface layer => SST drops slowly in winter mode water

  24. Wind Forcing in the North Pacific PDO-related wind forcing explains changes in circulation (Qiu & Chen, 2006)

  25. Effect of Ocean Currents on Wind (Stress) WBC currents of 1-2 m/s have (negative) imprint on wind stress and wind stress curl)

  26. Summary of Relevant Ocean Processes • Large heat and moisture fluxes from convergence • Storage & release of heat in STMW region (GS forcing) • Different magnitudes and periods of path shifts • Strong, stable current periods vs. unstable periods • SST effects of changes in path and strength • Upstream influence on cold, continental air • Path shifts may affect land/sea SST gradient (GS) • Stronger warm core (more rapid recovery from cooling) • GS STMW: restratification limits wintertime cooling • KE STMW: small contribution to air-sea flux (thermocline anomalies and path instability)

  27. Candidate processes for coupling(is a time lag necessary?) • Large heat storage reservoir (STMW) with residence time of several years => forcing of interannual air-sea heat flux anomalies • Delayed response of ocean to changes in wind stress => changes in path, transport, SST, heat content • Ocean forcing atmosphere: more robust in GS • Atmosphere forcing ocean: more robust in KE

  28. Questions Observations & Data Analysis: • what observations are required to evaluate frontal processes? • what data analyses are needed to establish/refute effects? Modeling: • how well are frontal processes currently modeled? • what types of models can address which questions (high or low resolution? coupled? • there is evidence for mean effects -- what about response to decadal anomalies and trends?

  29. Air-Sea Temperature Difference CLIMODE cruise March 8-9 2007 Courtesy Terry Joyce and CLIMODE PIs

  30. The Northern Annular Mode in the Ocean NAM: 500 mb pressure (contours) (Wallace & Thompson, 2002) Heat content and SSH (color) (Kelly & Dong, 2006) Stronger westerlies => more heat in ocean

  31. Impact of Gulf Stream on Atmosphere

  32. Gulf Sream STMW: Formation and Advection Volume changes (mostly) explained by advection Formation-dominant years (1996, 1999): outcrop more important than heat flux anomalies Dong et al 2007

  33. Frequency of High Winds over Gulf Stream Wintertime climatology of high-wind (>20m/s) frequency (percent, color) and SST (contours) (Sampe and Xie, 2007)

  34. Impact of Gulf Stream on Troposphere Annual climatology of parameters connecting MABL and free atmosphere: a) upward wind, b) wind divergence c) occurrence of low OLR (Minobe et al 2008)

  35. Passing the Heat “Baton” in the Midlatitudes Heat Transport by the Atmosphere and the Ocean Total Atmos Ocean After Trenberth & Caron, 2001 Between 30-45N, ocean transport drops by 1.2 Pw -- atmospheretransport increases by 1.3 Pw => gain in atmosphere comes from the ocean

  36. CLIMODE (in Gulf Stream) CLIVAR-endorsed field program to study STMW Observations Top: ship & drifting buoy tracks Middle: air, sea temperatures Bottom: air-sea fluxes (DC & bulk)

  37. Kuroshio Extension System Study (KESS) CLIVAR-endorsed field program to study STMW STMW anomalies related to KE path stability Top: ocean temperature Middle: potential vorticity Bottom: path variations

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