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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, andDepartment of Meteorology

Florida State University

The Florida State University 2 [email protected]@coaps.fsu.edu

OCO Review 2006

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


OCO Review 2006

Null Hypothesis: No Change In The Annual Number of Named Tropical Storms

1995 191996 131997 81998 141999 122000 142001 152002 122003 162004 14

1982 5 1983 41984 131985 111986 61987 71988 121989 111990 141991 81992 71993 81994 7

Number of Atlantic tropical storms and hurricanes per year for 25 years.

Our null distribution, in more detail is


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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- H+ H- - +

Ocean

Atmosphere

Latent Heat Flux E (w10 – wsfc)(qsfc – q10)

Sensible Heat Flux H (w10 – wsfc)(sfc – 10)

Stress (U10 – Usfc)2

Sign convention

w scalar averaged wind speedU vector averaged wind speedq specific humidity potential temperature


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15 45 75 105 135 175 Wm-2

10 30 50 70 90 110 Wm-2

Latent Heat Flux: January 1989 Sensible 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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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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Regions of Low Level Convergence

Similarly, the surface convergence (and presumably moisture convergence) is also a factor.


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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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0 1 4 9 16 25 36 49 64 >81

0 1 4 9 16 25 36 49 64 >81

January

August

Average Number of Ship Observations

Average Number of Ship Observations


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Producing the Gridded Product

Bias correction to input data Winds Air temperatures SSTs (via Reynolds)

Data quality control Objective analysis


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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

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

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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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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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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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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Unsmoothed AMO Index (1948-2003)

Smoothed AMO Index (1948-2000)

Unsmoothed AMO Index (1978-2003)


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Sea Surface Temperature

Mode 1

PC 1

Black: PC1

Blue: AMO index


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Mode 1

Air Temperature PC 1

Black: PC1

Blue: AMO index


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Specific Humidity (10m)

Mode 1

PC 1

Black: PC1

Blue: AMO index


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Wind Speed

Mode 1

PC 1

Black: PC1

Blue: AMO index


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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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Zonal averages 10°S - 62°N

qsfc – qair (kgkg-1) SST – Tair (°C)

Wind Speed (ms-1)


Blue: 1998-2003

Red: 1982-1997


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Wind Speed (ms-1)


Blue: 1998-2003

Red: 1982-1997

Greater values depicted for 1982-1997

Zonal averages 10°S - 62°N


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Similar Findings For Earlier Years?

Wind Speed

Latent Heat Flux

-10 0 10 20 30 40 50 60N Latitude

-10 0 10 20 30 40 50 60N Latitude


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-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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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)



OCO Review 2006

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)



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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 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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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

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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