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

jcollins@cas.usf.edu

The NE Pacific and Atlantic hurricane The NE Pacific and Atlantic hurricane linklink

Dr. Jennifer CollinsGeography Department

USFjcollins@cas.usf.edu

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

jcollins@cas.usf.edu www.weathercenter.usf.eduwww.weathercenter.usf.edu