Factors that influence the interannual variability of hurricane frequency in the NE Pacific Dr. Jennifer Collins Geography Department USF May 19-21, 2008

The NE Pacific and Atlantic hurricane link Dr. Jennifer Collins Geography Department USF

 Outline –Other ocean basins besides the North Atlantic –Sub-dividing the ocean basin for the analysis –Dynamic vs. thermodynamic local variables –Influence of ENSO? –Other non-local influences? –Examine the relationship between N. Atlantic and NE Pacific frequency of tropical cyclones

 Why study the Northeast Pacific? –High frequency of hurricanes

 Why study the Northeast Pacific? –High frequency of hurricanes –May hit US or Mexico  Hurricane Dora threatened Johnston Island, 1999  Intense hurricanes in season prior to the start of this work

 Why study the Northeast Pacific? –High frequency of hurricanes –May hit US or Mexico  Hurricane Dora threatened Johnston Island, 1999  Intense hurricanes in season prior to PhD –Little published research

 Why study variability of numbers from year to year? –NE Pacific number of storms 1992 – – 8 –Forecasting potential

1983 season 1994 season Location of NE Pacific hurricanes WDREDR WDREDR WDR: Western Development Region EDR: Eastern Development Region

Gray’s six factors necessary for tropical cyclone formation 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 > o C to a depth of 60 m. 6/ Environmental lapse rate should be steep enough to allow free convection

Methodology  Statistical study - Using the deviance test

Relationships between hurricane frequency and the environmental variables (Reduction in deviance shown) VariableWDREDR Relative Humidity Sea Surface Temp Precipitable Water Upward Longwave Radiation Flux Relative Vorticity Wind Shear ENSO (kw index) (SOI) (Nino1) (Nino2) (Nino3) (Nino4) QBO

Relative humidity (RH) and sea surface temperature (SST) for active hurricane years (black) and inactive hurricane years (green). Environmental variables are averaged over latitudes 10°N to 20 °N

Wind shear (200 mb to 850 mb) for active hurricane 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 most and 5 least active hurricane years in the western development region nb. White box is the western development region RH difference(%)highlow

Sea surface temperature (SST) differences between the 5 most and 5 least active hurricane years in the western development region nb. White box is the western development region lowhighSST difference

Relative Humidity  What causes relative humidity to vary from year to year?

Relative humidity in active and inactive hurricane years Active hurricane yearsInactive hurricane years RH (%) lowhigh lowhigh

Relative Humidity  What causes relative humidity to vary from year to year?

Six warmest ENSO yearsSix coldest ENSO years Sea surface temperature and surface winds at 10 meters in warm and cold ENSO years SSTlowhighSSTlowhigh

Six warmest ENSO yearsSix coldest ENSO years Mid-tropospheric relative humidity and surface winds at 10 meters in warm and cold ENSO years RH (%) lowhigh low high

Relative Humidity  What causes relative humidity to vary from year to year? and … and …

Relative Humidity  What causes relative humidity to vary from year to year? and … and … a thermal low pressure centered at 30 o N and o W?

Climatology of pressure (mean sea level) nb. Pressure values are averaged over July-September season Pressure (mb)

Relative humidity Relative humidity (%) Six deepest thermal low yearsSix shallowest thermal low years nb. Values are averaged over July-September

Surface wind differences in the six deepest minus six shallowest thermal low years (wind anomalies shown). nb. Values are averaged over July-September.

Lower relative humidity in the western development region due to increased subsidence Latitude 10°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 humidity Relative humidity (%) Six deepest thermal low yearsSix shallowest thermal low years nb. Values are averaged over July-September

Model to explain relative humidity VariablePercentage of variance explained ENSO55 Thermal low42 ENSO + Thermal low72

Relationships between N. Atlantic tropical cyclones and NE Pacific tropical cyclones (western development region and eastern development region) t-statistics t-statistics NE Pacific basin TSWDRHWDRIHWDRTSEDRHEDRIHEDR N. Atlantic basin TS H IH

Conclusions  It is necessary to sub-divide the NE Pacific basin to examine factors that affect hurricane formation  There are strong relationships with thermodynamic factors and hurricane frequency for the WDR  Dynamic factors, important for the Atlantic basin, are not important for the NE Pacific –The strongest relationship is with Relative Humidity  El Nino and the thermal low have a strong influence on the variations of Relative Humidity  There is a negative relationship between hurricane frequency in the NE Pacific and N Atlantic ocean

Factors that influence the interannual variability of hurricane frequency in the NE Pacific Dr. Jennifer Collins Geography Department USF May 19-21,