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

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  Discuss how the fluxes could be related to variability in hurricane seasons

OCO Review 2006 The Florida State University 3 Null Hypothesis: No Change In The Annual Number of Named Tropical Storms Number of Atlantic tropical storms and hurricanes per year for 25 years.  Our null distribution, in more detail is

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

OCO Review 2006 The Florida State University Wm Wm -2 Latent Heat Flux: January 1989Sensible Heat Flux: January 1989 Wind Stress: January 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

OCO Review 2006 The Florida State University 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.

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.

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

OCO Review 2006 The Florida State University >81 January August Average Number of Ship Observations

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

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

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)

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

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 with a shift to negative anomalies around

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

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 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)

OCO Review 2006 The Florida State University 17 Unsmoothed AMO Index ( ) Smoothed AMO Index ( ) Unsmoothed AMO Index ( )

OCO Review 2006 The Florida State University 18 Sea Surface Temperature Mode 1 PC 1 Black: PC1 Blue: AMO index

OCO Review 2006 The Florida State University 19 Mode 1 Air Temperature PC 1 Black: PC1 Blue: AMO index

OCO Review 2006 The Florida State University 20 Specific Humidity (10m) Mode 1 PC 1 Black: PC1 Blue: AMO index

OCO Review 2006 The Florida State University 21 Wind Speed Mode 1 PC 1 Black: PC1 Blue: AMO index

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: mean Blue: Red:  Distinction is evident between the latent and sensible heat fluxes for and  Greater values for , coinciding with a cool phase of the AMO

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: mean Blue: Red:

OCO Review 2006 The Florida State University 24 q sfc – q air (kgkg -1 )SST – Tair (°C) Wind Speed (ms -1 ) Black: mean Blue: Red: Greater values depicted for Zonal averages 10°S - 62°N

OCO Review 2006 The Florida State University 25 Similar Findings For Earlier Years?  Wind Speed Latent Heat Flux  N Latitude

OCO Review 2006 The Florida State University Wm Wm minus  Predominantly positive over the entire North Atlantic, implying larger values for Latent Heat Flux (Wm -2 )Sensible Heat Flux (Wm -2 )

OCO Review 2006 The Florida State University minus ms °C -9x x x x10 -4 kgkg -1 Wind Speed (ms -1 ) q sfc – q air (kgkg -1 )SST – Tair (°C)

OCO Review 2006 The Florida State University minus ms °C -9x x x x10 -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)

OCO Review 2006 The Florida State University ms ms -1 Comparison of vector winds Climatology Anomalies (ms -1 ) Anomalies (ms -1 )

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.

OCO Review 2006 The Florida State University 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 and  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  Implies changes to TC genesis, rainfall, and temperature extremes.