# Presentation: 060638

## 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*

**Notes:**
MORE PICTURES LESS WORDS !!!!!!!!!!!!!!!!!!!!

## 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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## 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***** ******** (*****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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## Stress

**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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**Notes:**
Data Section: (1) Mention that no clusters of data removed during QC.  (2) Picture of flux editor using winds example

## January

**   ****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

**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

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## 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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## 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

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**Notes:**
Studies do not include same time period.  Do not include recent years

## 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

**North Atlantic Oscillation (NAO)**
- Zonal bands of anomalous fluxes (*Cayan** *1992; *Alexander and Scott** *1997)
- 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 ** **

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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

**Sea Surface Temperature**

- Mode 1

- PC 1

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## Air Temperature

- Mode 1

**Air Temperature**

- PC 1

- Black: PC1
- Blue: AMO index

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## Specific Humidity (10m) 

**Specific Humidity (10m) **

- Mode 1

- PC 1

- Black: PC1
- Blue: AMO index

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## Wind Speed 

**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

-      _Zonal averages 10__°S - 62°N_

- qsfc – qair (kgkg-1)

- SST – Tair (°C)

- Wind Speed (ms-1)

**Black: 1978-2003 mean**
**Blue: 1998-2003**
**Red: 1982-1997**

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##      Zonal averages 10°S - 62°N

- qsfc – qair (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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## 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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##      1982-1997 minus 1998-2003

**-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

-      _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)

- qsfc – qair (kgkg-1)

- SST – Tair (°C)

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##      1982-1997 minus 1998-2003

-      _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)

- qsfc – qair (kgkg-1)

- SST – Tair (°C)

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##    Comparison of vector winds

** ****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.

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## 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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