1. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 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

2. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The 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

3. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 3 Null Hypothesis: No Change In The Annual Number of Named Tropical Storms 1995 19 1996 13 1997 8 1998 14 1999 12 2000 14 2001 15 2002 12 2003 16 2004 14 1982 5 1983 4 1984 13 1985 11 1986 6 1987 7 1988 12 1989 11 1990 14 1991 8 1992 7 1993 8 1994 7 Number of Atlantic tropical storms and hurricanes per year for 25 years.  Our null distribution, in more detail is

4. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 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 E+E+ E-E-H+H+H-H- -- ++ Ocean Atmosphere Latent Heat Flux E  (w 10 – w sfc )(q sfc – q 10 ) Sensible Heat Flux H  (w 10 – w sfc )(  sfc –  10 ) Stress   (U 10 – U sfc ) 2 Sign convention w scalar averaged wind speed U vector averaged wind speed q specific humidity  potential temperature

5. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 5 15 45 75 105 135 175 Wm -2 10 30 50 70 90 110 Wm -2 Latent Heat Flux: January 1989Sensible Heat Flux: January 1989 Wind Stress: January 1989 0.05 0.15 0.25 0.35 0.45 0.55 Nm -2 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

6. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 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.

7. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 7 Regions of Low Level Convergence  Similarly, the surface convergence (and presumably moisture convergence) is also a factor.

8. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 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

9. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 9 0 1 4 9 16 25 36 49 64 >81 January August Average Number of Ship Observations

10. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 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

11. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 11 Quality Control 1) Comparison to climatology  Applied to individual observations  Excessive trimming not a problem 2) “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 3) 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

12. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 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)

13. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 13 Past Studies 1) Zhao and McBean (1986) 2) Cayan (1992) 3) 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

14. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 14 EOF analysis: Latent Heat Flux  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 -2.6 -1.6 -0.6 0.6 1.6 2.6

15. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 15 Climate Modes 1)North Atlantic Oscillation (NAO)  Zonal bands of anomalous fluxes (Cayan 1992; Alexander and Scott 1997)  Mode 2 (not shown) depicts NAO-like spatial pattern 2)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) 3)Atlantic Multidecadal Oscillation (AMO)  Characterized by SST anomalies of the same sign over the entire North Atlantic  Schlesinger and Ramankutty 1994; Kerr 2000

16. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 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)

17. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 17 Unsmoothed AMO Index (1948-2003) Smoothed AMO Index (1948-2000) Unsmoothed AMO Index (1978-2003)

18. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 18 Sea Surface Temperature Mode 1 PC 1 Black: PC1 Blue: AMO index

19. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 19 Mode 1 Air Temperature PC 1 Black: PC1 Blue: AMO index

20. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 20 Specific Humidity (10m) Mode 1 PC 1 Black: PC1 Blue: AMO index

21. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 21 Wind Speed Mode 1 PC 1 Black: PC1 Blue: AMO index

22. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 22 Zonal averages: 10°S - 62°N Latent Heat Flux (Wm -2 ) Sensible 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

23. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 23 Zonal averages 10°S - 62°N q sfc – q air (kgkg -1 )SST – Tair (°C) Wind Speed (ms -1 ) Black: 1978-2003 mean Blue: 1998-2003 Red: 1982-1997

24. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 24 q sfc – q air (kgkg -1 )SST – Tair (°C) Wind Speed (ms -1 ) Black: 1978-2003 mean Blue: 1998-2003 Red: 1982-1997 Greater values depicted for 1982-1997 Zonal averages 10°S - 62°N

25. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 25 Similar Findings For Earlier Years?  Wind Speed Latent Heat Flux  -10 0 10 20 30 40 50 60N Latitude

26. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 26 -25 -20 -15 -10 -5 5 10 15 20 25 Wm -2 -18 -14 -10 -6 -2 2 6 10 14 18 Wm -2 1982-1997 minus 1998-2003  Predominantly positive over the entire North Atlantic, implying larger values for 1982-1997 Latent Heat Flux (Wm -2 )Sensible Heat Flux (Wm -2 )

27. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 27 1982-1997 minus 1998-2003 -1.8 -1.0 -0.4 0.4 1.0 1.8 ms -1 -1.0 -0.6 -0.2 0.2 0.6 1.0 °C -9x10 -4 -5x10 -4 5x10 -4 9x10 -4 kgkg -1 Wind Speed (ms -1 ) q sfc – q air (kgkg -1 )SST – Tair (°C)

28. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 28 1982-1997 minus 1998-2003 -1.8 -1.0 -0.4 0.4 1.0 1.8 ms -1 -1.0 -0.6 -0.2 0.2 0.6 1.0 °C -9x10 -4 -5x10 -4 5x10 -4 9x10 -4 kgkg -1 Largest differences appear to be organized around the periphery of the subtropical high Wind Speed (ms -1 ) q sfc – q air (kgkg -1 )SST – Tair (°C)

29. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 The Florida State University 29 1 2 3 4 5 6 7 8 9 ms -1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 ms -1 Comparison of vector winds 1978-2003 Climatology 1982-1997 Anomalies (ms -1 )1998-2003 Anomalies (ms -1 )

30. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 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.

31. bourassa@met.fsu.edu smith@coaps.fsu.edu OCO Review 2006 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.