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Tropical Cyclone Activity - and - North Atlantic Decadal Variability of Ocean Surface Fluxes

Tropical Cyclone Activity - and - North Atlantic Decadal Variability of Ocean Surface Fluxes. Mark A. Bourassa, Paul J. Hughes, Jeremy Rolph, and Shawn R. Smith Center for Ocean-Atmospheric Prediction Studies, and Department of Meteorology Florida State University. Objectives.

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Tropical Cyclone Activity - and - North Atlantic Decadal Variability of Ocean Surface Fluxes

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  1. Tropical Cyclone Activity - and-North Atlantic Decadal Variability of Ocean Surface Fluxes Mark A. Bourassa, Paul J. Hughes, Jeremy Rolph, and Shawn R. Smith Center for Ocean-Atmospheric Prediction Studies, andDepartment of Meteorology Florida State University

  2. Objectives • Develop a new objectively produced monthly mean 1°x1° gridded wind and surface flux product (FSU3) • Derived from in situ ship and buoy observations • To examine the spatial and temporal variability of the surface turbulent heat fluxes over the North Atlantic for 1978-2003 • Discuss how the fluxes could be related to variability in hurricane seasons The Florida State University

  3. Null Hypothesis: No Change In The Annual Number of Named Tropical Storms • Our null distribution, in more detail is Number of Atlantic tropical storms and hurricanes per year for 25 years. 1982 5 1983 41984 131985 111986 61987 71988 121989 111990 141991 81992 71993 81994 7 1995 191996 131997 81998 141999 122000 142001 152002 122003 162004 14 The Florida State University

  4. What are surface turbulent fluxes? Latent Heat Flux (E) • Vertical transport of energy associated with the phase change of water • Forced by wind speed and air/sea temperature differences Sensible Heat Flux (H) • Vertical transport of energy associated with heating, but without a phase change • Forced by wind speed and vertical moisture differences Stress () • Vertical transport of horizontal momentum • Forced by vertical momentum differences Atmosphere Sign convention E+ E- H+ H- - + Ocean Latent Heat FluxE (w10 – wsfc)(qsfc – q10) Sensible Heat FluxH  (w10 – wsfc)(qsfc – q10) Stress (U10 – Usfc)2 wscalar averaged wind speedU vector averaged wind speedq specific humidityq potential temperature The Florida State University

  5. Latent Heat Flux: January 1989 Sensible Heat Flux: January 1989 15 45 75 105 135 175 Wm-2 10 30 50 70 90 110 Wm-2 Wind Stress: January 1989 Stress Forcing the upper ocean circulation, upwelling, and downwelling Latent and Sensible heat fluxes are an important mechanism for transporting heat from the ocean to the atmosphere 0.05 0.15 0.25 0.35 0.45 0.55 Nm-2 The Florida State University

  6. 2005 Hurricane Season:Location of Genesis • The locations of tropical cyclone activity evolved with the latent heat flux pattern. • One year is a small sample. Further analysis is needed. The Florida State University

  7. Regions of Low Level Convergence • Similarly, the surface convergence (and presumably moisture convergence) is also a factor. The Florida State University

  8. i. Input data • International Comprehensive Ocean-Atmosphere Data Set (ICOADS; Woodruff et al. 1987; Worley et al. 2005) • Fields prior to and including 1997 • 1994 • National Climatic Data Center’s (NCDC) technical document Marine Surface Observations (TD-1129; NCDC 2003) • 1998 through 2003 • Input into ICOADS • GTS • 2005 • Reynolds SSTs (Reynolds 1988) • Bias corrections for ship based SSTs not well understood and vary greatly on ship to ship basis The Florida State University

  9. Average Number of Ship Observations January Average Number of Ship Observations 0 1 4 9 16 25 36 49 64 >81 August 0 1 4 9 16 25 36 49 64 >81 The Florida State University

  10. Producing the Gridded Product • Bias correction to input data • Winds • Air temperatures • SSTs (via Reynolds) • Data quality control • Objective analysis The Florida State University

  11. Quality Control • Comparison to climatology • Applied to individual observations • Excessive trimming not a problem • “Auto-flag” procedure • Applied to monthly mean gridded ship observations • Flags and removes grid points that differ too much from adjacent points • FSU3 fluxes are the first version of FSU winds to employ technique • Flux editor • Analyst visually inspects the in situ fields and subjectively removes suspect data not eliminated by the preceding quality control procedures • Very few data removed The Florida State University

  12. Cost Function • A cost function based on weighted constraints is minimized via a conjugate-gradient minimization scheme • Three constraints for vector variables • Misfit to observations • Laplacian smoothing term • Misfit of the curl • Constraints help maximize the similarity of the solution fields to the observations and minimize unrealistic spatial feature • Each constraint multiplied by a weight that is determined using cross validation (Wahba and Wendelberger 1980; Pegion et al. 2000) The Florida State University

  13. Past Studies Zhao and McBean (1986) Cayan (1992) Alexander and Scott (1997) • Examined the longer time scale basin wide variability of the turbulent heat fluxes over the North Pacific and Atlantic Oceans • Concluded that the latent and sensible heat flux respond to changes in the low level atmospheric circulation patterns, e.g., the North Atlantic Oscillation (NAO) • Showed that anomalous fluxes are organized over regions of atypical zonal and meridional flow The Florida State University

  14. EOF analysis: Latent Heat Flux -2.6 -1.6 -0.6 0.6 1.6 2.6 • Mode 1 explains ~26% of the total variance • Depicts a situation where the majority of the North Atlantic is dominated by positive latent heat flux anomalies during 1982-1997 with a shift to negative anomalies around 1998 The Florida State University

  15. Climate Modes • North Atlantic Oscillation (NAO) • Zonal bands of anomalous fluxes (Cayan1992; Alexander and Scott1997) • Mode 2 (not shown) depicts NAO-like spatial pattern • El Nino-Southern Oscillation (ENSO) • El Nino linked to anomalous warm SSTs and across tropical North Atlantic and diminished trade winds • Reduced latent heat flux (Curtis and Hastenrath 1995) • Atlantic Multidecadal Oscillation (AMO) • Characterized by SST anomalies of the same sign over the entire North Atlantic • Schlesinger and Ramankutty 1994; Kerr 2000 The Florida State University

  16. Atlantic Multidecadal Oscillation (AMO) • Thought to be forced by fluctuations in the thermohaline circulation (Delworth and Mann 2000) • Period of 65-70 years (seen in smoothed SST-based index) • Linked to anomalous precipitation patterns and North Atlantic hurricane activity (Enfield et al. 2001; Sutton and Hodson 2005; Goldenberg et al. 2001) The Florida State University

  17. Unsmoothed AMO Index (1948-2003) Smoothed AMO Index (1948-2000) Unsmoothed AMO Index (1978-2003) The Florida State University

  18. Black: PC1 Blue: AMO index Sea Surface Temperature PC 1 Mode 1 The Florida State University

  19. PC 1 Air Temperature Mode 1 Black: PC1 Blue: AMO index The Florida State University

  20. Specific Humidity (10m) PC 1 Mode 1 Black: PC1 Blue: AMO index The Florida State University

  21. Wind Speed PC 1 Mode 1 Black: PC1 Blue: AMO index The Florida State University

  22. Zonal averages: 10°S - 62°N Sensible Heat Flux (Wm-2) Latent Heat Flux (Wm-2) Black: 1978-2003 mean Blue: 1998-2003 Red: 1982-1997 • Distinction is evident between the latent and sensible heat fluxes for 1982-1997 and 1998-2003 • Greater values for 1982-1997, coinciding with a cool phase of the AMO The Florida State University

  23. Wind Speed (ms-1) Zonal averages 10°S - 62°N Black: 1978-2003 mean Blue: 1998-2003 Red: 1982-1997 qsfc – qair (kgkg-1) SST – Tair (°C) The Florida State University

  24. Wind Speed (ms-1) Zonal averages 10°S - 62°N Black: 1978-2003 mean Blue: 1998-2003 Red: 1982-1997 Greater values depicted for 1982-1997 qsfc – qair (kgkg-1) SST – Tair (°C) The Florida State University

  25. Similar Findings For Earlier Years? Wind Speed -10 0 10 20 30 40 50 60N Latitude Latent Heat Flux  -10 0 10 20 30 40 50 60N Latitude The Florida State University

  26. 1982-1997 minus 1998-2003 Latent Heat Flux (Wm-2) Sensible Heat Flux (Wm-2) -25 -20 -15 -10 -5 5 10 15 20 25 Wm-2 -18 -14 -10 -6 -2 2 6 10 14 18 Wm-2 • Predominantly positive over the entire North Atlantic, implying larger values for 1982-1997 The Florida State University

  27. Wind Speed (ms-1) 1982-1997 minus 1998-2003 -1.8 -1.0 -0.4 0.4 1.0 1.8 ms-1 qsfc – qair (kgkg-1) SST – Tair (°C) -1.0 -0.6 -0.2 0.2 0.6 1.0 °C -9x10-4 -5x10-4 5x10-4 9x10-4 kgkg-1 The Florida State University

  28. Wind Speed (ms-1) 1982-1997 minus 1998-2003 Largest differences appear to be organized around the periphery of the subtropical high -1.8 -1.0 -0.4 0.4 1.0 1.8 ms-1 qsfc – qair (kgkg-1) SST – Tair (°C) -1.0 -0.6 -0.2 0.2 0.6 1.0 °C -9x10-4 -5x10-4 5x10-4 9x10-4 kgkg-1 The Florida State University

  29. 1978-2003 Climatology Comparison of vector winds 1 2 3 4 5 6 7 8 9 ms-1 1982-1997 Anomalies (ms-1) 1998-2003 Anomalies (ms-1) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 ms-1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 ms-1 The Florida State University

  30. Implications to Tropical Cyclone Genesis • The wind and SST • Boundary-layer stability • More unstable leads to • Boundary-layer depth • Easier formation of convective systems • Less low level wind shear • Changes to the surface heat fluxes and wind forcing also modifies the ocean heat content (next speaker) • SST is probably more important for Genesis • Ocean heat content is presumably more important for development of strong hurricanes. The Florida State University

  31. 5. Summary • The spatial and temporal variability of the surface turbulent heat fluxes over the North Atlantic was examined using the new FSU3 gridded product • FSU3 product derived from in situ ship and buoy observations via a variational method • The analysis shows that the latent and sensible heat fluxes exhibit a low frequency (basin wide) mode of variability • Transition from predominantly positive to negative anomalies around 1998 • Timing of the transition along with the basin wide extent of the signal suggests a possible link to the AMO • Wind speed acting as the primary forcing mechanism • Zonal averages show a distinction between the heat fluxes and wind speed during the periods 1982-1997 and 1998-2003 • Largest latent heat flux differences occur over the tropics, Gulf Stream, and higher latitude regions of the North Atlantic • ~15 to >25 Wm-2 • Greatest wind speed differences located around the periphery of the subtropical high • Suggests a change in the large scale circulation patterns • Weakening during 1998-2003 • Implies changes to TC genesis, rainfall, and temperature extremes. The Florida State University

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