Seasonal Tropical Cyclone Activity and its Impact on the Location of the Subtropical High

1. Seasonal Tropical Cyclone Activity and its Impact on the Location of the Subtropical High An Analysis Based Upon NCEP/NCAR Reanalysis Data By Jeremy Halland

2. Outline of Presentation • What is the purpose of this analysis? • Data Acquisition and Analysis Procedures • Years Analyzed and Selected • Variables Used • Results • Geopotential Height Anomalies • 500 hPa Wind Vectors • Monthly and Monthly Anomalies • Similarities to El Nino • Conclusions

3. Purpose of the Analysis • Tropical cyclones are latent heat engines, whose main purpose is to help return balance the latitudinal temperature gradient. • A single tropical cyclone moving from tropics to mid-latitudes is estimated to decrease the pole-equator temperature gradient by ~1% • Thus, tropical cyclones serve a useful purpose, and annual differences in total TCs that both form and recurve has an impact on the following year’s weather.

4. Hurricane Andrew Courtesy of National Oceanic & Atmospheric Adminstration (NOAA) http://www.photolib.noaa.gov/historic/nws/wea00531.htm

5. Purpose of the Analysis • By examining years where the number of storms that develop below 25°N and either stay below 25°N or recurve past 40°N, I hope to find a correlation with: • the subtropical high intensity and location • trade wind strength and direction • How does the circulation in the winter months compare with circulation patterns during an El Nino event?

6. Data Acquisition and Analysis: Years Selected • NHC best track data from January 1950-February 2006 • Above = Years with • 1954,1955,1957,1958,1959,1960,1962,1972,1976,1995,1996,1997,1998 • Average: 1972.2 • Below = Years with • 1956,1968,1970,1971,1977,1978,1980,1982,1983,1984,1986,1992,2002 • Average: 1979.0 • Note that ‘Post’ corresponds to the year after in the group of years we are talking about (ie, year = 1956, post_year = 1957).

7. Data Acquisition and Analysis: Variables Used • NCEP/NCAR Monthly Reanalysis Data • Monthly Mean = or • Done for all 12 months of each year used • Variables Analyzed: • 500 hPa Geopotential Height • Mean Sea Level Pressure • 500 hPa Zonal and Meridional Wind • Surface Zonal Wind **AYM = Above Year month, and BYM = Below Year Month

8. 500 hPa Geopotential Height Differences • Notice similar pattern between ‘actual’ and ‘post’ years, except during winter months. • Post year has dominant lower heights in Jan. & Feb. while the actual year has dominant higher heights.

9. 500 hPa Wind • The general wind pattern is observed in both ‘Above’ and ‘Below’ years. • Weakening of trades in the winter, strengthening of them in the summer. • Strengthening of mid-latitude winds in winter, weakening in the summer

10. 500 hPa Wind Vector Differences • Reversal of dominant wind pattern occurs during winter and spring months. • Post November has cyclonic anomalies straddling the equator.. La Nina indicator?

11. El Nino Similarities: Geopotential Height • Notice the similar pattern and intensity of the high and low geopotential heights to La Nina. • Since this is Above – Below years, if we took below minus above we would have a plot similar to El Nino. Courtesy CDC: http://www.cdc.noaa.gov/ENSO/enso.description.html

12. El Nino Similarities: 500 hPa Wind Vectors • Note a switch in the high over low circulation in the ‘actual’ year to a low over high circulation in the ‘post’ year.

13. El Nino Similarities: Surface Zonal Wind • A strengthening of the easterly zonal wind in the western tropical Pacific in the ‘post’ years. • Thus, there is a more westerly zonal wind during the month of February in ‘below’ years.

14. Conclusions…. • There is a shift in location of the Pacific subtropical high towards the south, and also to form a more dominant presence in March instead of February. • There appears to be a significant correlation between years with less than 16% of its total TCs that form below 25°N recurve past 40°N and El Nino. • Forecasting El Nino?

15. Questions.. ?

16. References Chu, Pao-Shin. ‘Large-Scale Circulation Features Associated with Decadal Variations of Tropical Cyclone Activity over the Central North Pacific.’ Journ. of Climate. 15. 2678-2689. Hart, Bob. ‘Class Notes 2006.’