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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
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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
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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
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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
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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
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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.
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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.
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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
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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
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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
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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
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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)
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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
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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
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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
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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)
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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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 )
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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)
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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)
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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 )
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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.
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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.
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