Download - Factors that influence the interannual variability of hurricane frequency in the NE Pacific
Factors that influence the Factors that influence the interannual variability of interannual variability of
hurricane frequency in the hurricane frequency in the NE PacificNE Pacific
Dr. Jennifer CollinsGeography Department
USF
May 19-21, 2008
The NE Pacific and Atlantic hurricane The NE Pacific and Atlantic hurricane linklink
Dr. Jennifer CollinsGeography Department
OutlineOutline– Other ocean basins besides the North AtlanticOther ocean basins besides the North Atlantic
– Sub-dividing the ocean basin for the analysisSub-dividing the ocean basin for the analysis
– Dynamic vs. thermodynamic local variablesDynamic vs. thermodynamic local variables
– Influence of ENSO?Influence of ENSO?
– Other non-local influences?Other non-local influences?
– Examine the relationship between N. Atlantic and NE Examine the relationship between N. Atlantic and NE Pacific frequency of tropical cyclonesPacific frequency of tropical cyclones
Why study the Northeast Pacific?Why study the Northeast Pacific?– High frequency of hurricanesHigh frequency of hurricanes
Why study the Northeast Pacific?Why study the Northeast Pacific?– High frequency of hurricanesHigh frequency of hurricanes– May hit US or MexicoMay hit US or Mexico
Hurricane Dora threatened Johnston Island, 1999Hurricane Dora threatened Johnston Island, 1999 Intense hurricanes in season prior to the start of Intense hurricanes in season prior to the start of
this workthis work
Why study the Northeast Pacific?Why study the Northeast Pacific?– High frequency of hurricanesHigh frequency of hurricanes– May hit US or MexicoMay hit US or Mexico
Hurricane Dora threatened Johnston Island, 1999Hurricane Dora threatened Johnston Island, 1999 Intense hurricanes in season prior to PhDIntense hurricanes in season prior to PhD
– Little published researchLittle published research
Why study variability of numbers from Why study variability of numbers from year to year?year to year?– NE Pacific number of stormsNE Pacific number of storms
1992 – 271992 – 27 1977 – 81977 – 8
– Forecasting potentialForecasting potential
1983 season
1994 season
Location of NE Pacific hurricanesLocation of NE Pacific hurricanes
WDR EDR
WDR EDR
WDR: Western Development Region
EDR:EasternDevelopmentRegion
Gray’s six factors necessary for tropical cyclone Gray’s six factors necessary for tropical cyclone formationformation
1/ Coriolis Force is sufficient to give initial cyclonic spin
2/ Low vertical windshear
3/ Sufficient amount of pre-existing low level vorticity
4/ high values of mid-tropospheric relative humidity
5/ SST > 26-27 oC to a depth of 60 m.
6/ Environmental lapse rate should be steep enough to allow free convection
MethodologyMethodology
Statistical study Statistical study
- Using the deviance test- Using the deviance test
Relationships between hurricane frequency and the Relationships between hurricane frequency and the environmental variables (Reduction in deviance environmental variables (Reduction in deviance
shown)shown)Variable WDR EDR
Relative Humidity 20.65 1.56
Sea Surface Temp. 13.98 1.72
Precipitable Water 18.36 1.92
Upward Longwave Radiation Flux 13.48 1.00
Relative Vorticity 3.13 1.15
Wind Shear 3.46 0.39
ENSO
(kw index) 6.28 0.83
(SOI) 10.48 1.15
(Nino1) 3.76 1.50
(Nino2) 3.84 0.91
(Nino3) 3.27 0.50
(Nino4) 12.40 1.62
QBO 1.25 1.09
Relative humidity (RH) and sea surface temperature (SST) Relative humidity (RH) and sea surface temperature (SST) for active hurricane years (black) and inactive hurricane for active hurricane years (black) and inactive hurricane
years (green).years (green).
Environmental variables are averaged over latitudes 10°N to 20 °N
Wind shear (200 mb to 850 mb) for active hurricane Wind shear (200 mb to 850 mb) for active hurricane years (black) and inactive hurricane years (green).years (black) and inactive hurricane years (green).
Wind shear values are averaged over latitudes 10°N to 20 °N
Wind shear (m/s)
Relative humidity (RH) differences between the 5 Relative humidity (RH) differences between the 5 most and 5 least active hurricane years in the most and 5 least active hurricane years in the
western development regionwestern development region
nb. White box is the western development region
-10 15RH difference(%) highlow
Sea surface temperature (SST) differences Sea surface temperature (SST) differences between the 5 most and 5 least active hurricane between the 5 most and 5 least active hurricane
years in the western development regionyears in the western development region
nb. White box is the western development region
-0.8 0.8low highSST difference
Relative HumidityRelative Humidity
What causes relative humidity to vary What causes relative humidity to vary from year to year?from year to year?
Relative humidity in active and inactive Relative humidity in active and inactive hurricane yearshurricane years
Active hurricane years Inactive hurricane years
11 66 11 66RH (%) RH (%)low high low high
Relative HumidityRelative Humidity
What causes relative humidity to vary What causes relative humidity to vary from year to year?from year to year?
onNi~El
Six warmest ENSO years Six coldest ENSO years
Sea surface temperature and surface winds at Sea surface temperature and surface winds at 10 meters in warm and cold ENSO years10 meters in warm and cold ENSO years
20 28 20 28SSTlow high SSTlow high
Six warmest ENSO years Six coldest ENSO years
Mid-tropospheric relative humidity and surface Mid-tropospheric relative humidity and surface winds at 10 meters in warm and cold ENSO winds at 10 meters in warm and cold ENSO
yearsyears
666611 11 RH (%)RH (%)low high low high
Relative HumidityRelative Humidity
What causes relative humidity to vary What causes relative humidity to vary from year to year?from year to year?
and …and …El onNi~
Relative HumidityRelative Humidity
What causes relative humidity to vary What causes relative humidity to vary from year to year?from year to year?
and …and …
a thermal low pressure centered at a thermal low pressure centered at 3030ooN and -112.5N and -112.5ooW?W?
El onNi~
Climatology of pressure (mean sea level)Climatology of pressure (mean sea level)
1995 1991
nb. Pressure values are averaged over July-September season
Pressure (mb) Pressure (mb)1009.2 1013.5 1009.2 1013.5
Relative humidityRelative humidity
Relative humidity (%) Relative humidity (%)
Six deepest thermal low years Six shallowest thermal low years
nb. Values are averaged over July-September
11 66 11 66
Surface wind differences in the six deepest minus Surface wind differences in the six deepest minus six shallowest thermal low years (wind anomalies six shallowest thermal low years (wind anomalies
shown).shown).
nb. Values are averaged over July-September.
Lower relative humidity in the western development Lower relative humidity in the western development region due to increased subsidenceregion due to increased subsidence
Latitude10°N 20°N
Sub -Tropical
High
Western development region
Increased subsidence causes inversion layer to drop and shift south simultaneously. This lowers the relative humidity in the western development region
Relative humidityRelative humidity
Relative humidity (%) Relative humidity (%)
Six deepest thermal low years Six shallowest thermal low years
nb. Values are averaged over July-September
11 66 11 66
Model to explain relative humidityModel to explain relative humidity
VariableVariable Percentage of variance Percentage of variance explainedexplained
ENSOENSO 5555
Thermal lowThermal low 4242
ENSO + Thermal lowENSO + Thermal low 7272
Relationships between N. Atlantic tropical cyclones Relationships between N. Atlantic tropical cyclones and NE Pacific tropical cyclones (western and NE Pacific tropical cyclones (western
development region and eastern development development region and eastern development region)region)
tt-statistics-statistics
NE Pacific basinNE Pacific basinTSTS
WDRWDRHH
WDRWDRIHIH
WDRWDRTS TS
EDREDRHH
EDREDRIHIH
EDREDR
N. Atlantic N. Atlantic basinbasin
TSTS -2.07-2.07 -2.18-2.18 -1.30-1.30 0.330.33 1.741.74 0.370.37
HH -3.-3.9191 -2.35-2.35 -2.45-2.45 -0.25-0.25 0.500.50 0.060.06
IHIH -3.58-3.58 -3.28-3.28 -3.03-3.03 -0.49-0.49 0.090.09 -0.11-0.11
ConclusionsConclusions It is necessary to sub-divide the NE Pacific basin to examine It is necessary to sub-divide the NE Pacific basin to examine
factors that affect hurricane formationfactors that affect hurricane formation
There are strong relationships with thermodynamic factors and There are strong relationships with thermodynamic factors and hurricane frequency for the WDRhurricane frequency for the WDR
Dynamic factors, important for the Atlantic basin, are not important Dynamic factors, important for the Atlantic basin, are not important for the NE Pacificfor the NE Pacific– The strongest relationship is with Relative HumidityThe strongest relationship is with Relative Humidity
El Nino and the thermal low have a strong influence on the El Nino and the thermal low have a strong influence on the variations of Relative Humidityvariations of Relative Humidity
There is a negative relationship between hurricane frequency in There is a negative relationship between hurricane frequency in the NE Pacific and N Atlantic oceanthe NE Pacific and N Atlantic ocean
Factors that influence the Factors that influence the interannual variability of interannual variability of
hurricane frequency in the NE hurricane frequency in the NE PacificPacific
Dr. Jennifer CollinsGeography Department
USF
May 19-21, 2008
[email protected] www.weathercenter.usf.eduwww.weathercenter.usf.edu