TC Activity in WNP, [email protected], Oct08
15
1 TC Activity in WNP, [email protected], Oct 33 rd Annual Climate Diagnostics and Prediction Workshop 21 October 2008 Long Term Changes in Tropical Cyclone Activity in the Western North Pacific Tom Murphree and David Meyer Naval Postgraduate School (NPS) [email protected] and [email protected] Typhoon Saomai & Tropical Storm Bopha, Western North Pacific, 08Aug06
description
Typhoon Saomai & Tropical Storm Bopha, Western North Pacific, 08Aug06. Long Term Changes in Tropical Cyclone Activity in the Western North Pacific Tom Murphree and David Meyer Naval Postgraduate School (NPS) [email protected] and [email protected]. - PowerPoint PPT Presentation
Transcript of TC Activity in WNP, [email protected], Oct08
11TC Activity in WNP, [email protected], Oct08
33rd Annual Climate Diagnostics and Prediction Workshop21 October 2008
Long Term Changes in Tropical Cyclone Activity in the Western North PacificTom Murphree and David Meyer
Naval Postgraduate School (NPS)[email protected] and [email protected]
Typhoon Saomai & Tropical Storm Bopha, Western North Pacific, 08Aug06
22
OverviewAssessment of changes in tropical cyclone (TC) formations andintensities in western North Pacific during 1970-2006:
1. Intraseasonal to multidecadal changes2. Long term net changes3. Relationships between TC changes and known large
scale environmental factors (LSEFs)4. Relationships between TC changes and global warming
(GW)
Long Term Changes in WNP Tropical Cyclone Activity
TC Activity in WNP, [email protected], Oct08
Hypothesis: Assessments of GW impacts on TCs need to account for all LSEFs that affect TCs, and do so at
time and space scales at which LSEFs affect TCs.
33
Prior Studies1. LSEFs strongly affect TC formations (e.g., McBride 1995):
a. SSTb. Vertical shear c. Vertical velocity d. Absolute vorticity e. Relative humidity
2. ENLN impacts on TC activity (e.g., Chan 2000, Ford 2000):a. Impacts on formation sites, intensities, tracks
3. GW impacts on TCs so far (e.g., Chan and Liu 2004, Emanuel 2005, Webster and Holland 2005, Klotzbach 2006, Landsea 2007):
a. Little or no increase in TC numbersb. Increase in TC intensities via SST increasesc. Impacts from interannual-multidecadal variations dominated. TC data inadequate to determine GW impactse. Limitations of prior studies:
1. Coarse resolution: basin and seasonal averages2. Focus on one LSEF: SST3. Uncertainties in TC data4. Distinction and attribution: GW impacts vs. other impacts
Long Term Changes in WNP Tropical Cyclone Activity
TC Activity in WNP, [email protected], Oct08
44
Data1. TC activity: JTWC best track data (no adjustments)2. LSEFs: NCEP reanalysis 1 and 23. Period: 1970-2006 4. Main TC season: May-December5. Region: western North Pacific (WNP): 0-40° N, 115–185°E6. Main development region (MDR): 0-20° N, 115–165°E7. Temporal resolution: one week8. Spatial resolution: 5°x5° 9. Number of TCs: 989
Long Term Changes in WNP Tropical Cyclone Activity
TC Activity in WNP, [email protected], Oct08
5
Long Term Changes in WNP Tropical Cyclone Activity
1950 1954 1958 1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002 2006Year
0
10
20
30
40
coun
t
TC Numbers, WNP1950-2006
1970Global Surface T Anomaly1850-2007
1970
From CRU/UEATC Activity in WNP, [email protected], Oct08
Selection of analysis period
66
Methods
1. Identify major temporal and spatial patterns of changes in:a. TCs: formations and accumulated cyclone energy (ACE)b. LSEFs
2. Develop regression models that relate TC formation probability and ACE to local LSEFs at weekly and 5x5 resolutions
3. Validate models using independent TC and LSEF data
4. Use models to assess sensitivity of TC formations and ACE to LSEF variations
5. Relate GW to LSEF changes, and, thus, to TC changes
6. By-product: Models for sub-seasonal forecasting of TC formations and general tropical convection
Long Term Changes in WNP Tropical Cyclone Activity
TC Activity in WNP, [email protected], Oct08
77
Number of TC Formations, 1970-2006Long Term Changes in WNP Tropical Cyclone Activity
TC Activity in WNP, [email protected], Oct08
Main development region (MDR): 0-20°N, 115-165°E
120 E 150 E 180 E
0
15 N
30 N
0 4 81216202428323640
0° N
120 E 150 E 180 E
0
15 N
30 N
0 4 81216202428323640
No. of TCs
88
Long Term Changes in WNP Tropical Cyclone Activity
1970 1973 1976 1979 1982 1985 1988 1991 1994 1997 2000 2003 2006Year
20
25
30
35
40
TCs
1970 1973 1976 1979 1982 1985 1988 1991 1994 1997 2000 2003 2006Year
1000000
2000000
3000000
4000000
5000000
AC
E
Annual TC Numbers Annual ACE
Year
TC N
umbe
r
AC
E (k
ts2 )
Year
Changes in TC Numbers and ACE, 1970-2006
TC Activity in WNP, [email protected], Oct08
1. Large interannual-multidecadal variations (linked to ENLN)2. Long term net increases in TC formations and ACE
Can changes in LSEFs explain these long term changes in TC activity?
1970 1973 1976 1979 1982 1985 1988 1991 1994 1997 2000 2003 2006Year
28.5
28.9
29.3
29.7
SST
99
LSEF Changes – SST in MDRLong Term Changes in WNP Tropical Cyclone Activity
TC Activity in WNP, [email protected], Oct08
Year
SST
(C)
10
Long Term Changes in WNP Tropical Cyclone ActivityLSEF Changes – SST
1. Overall SST increase in MDR (+0.4 °C) is favorable for increases in TC formations and intensities, and consistent with GW.
2. Some SST decreases in TC active areas of WNP.
SST increases consistent with long term increases in TC numbers and intensities.
TC Activity in WNP, [email protected], Oct08
90 E 120 E 150 E 180 E 150 W 120 W
15 S
0
15 N
30 N
45 N
60 N
SST Change, JASO, 1970-2006 (°C)
90 E 120 E 150 E 180 E 150 W 120 W
15 S
0
15 N
30 N
45 N
60 N -2-1.6-1.2-0.8-0.4 0 0.4 0.8 1.2 1.6 2
TC Unfavorable
TC Favorable
11
LSEF Changes – Vertical Velocity in MDRLong Term Changes in WNP Tropical Cyclone Activity
TC Activity in WNP, [email protected], Oct08
1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006Year
3.1
3.6
4.1
4.6
5.1
5.6
Vert
Vel
Vert
ical
Vel
ocity
(-P
a/s)
Year
90 E 120 E 150 E 180 E 150 W 120 W 90 W
15 S
0
15 N
30 N
45 N
60 N
1212
LSEF Changes – Vertical VelocityLong Term Changes in WNP Tropical Cyclone Activity
TC Activity in WNP, [email protected], Oct08
Vertical Velocity Change, JASO, 1980-2006 (-Pa/s)
90 E 120 E 150 E 180 E 150 W 120 W 90 W
15 S
0
15 N
30 N
45 N
60 N -12-10 -8 -6 -4 -2 0 2 4 6 8 10
TC unfavorable
TC favorable
Have vertical velocity decreases countered impacts of SST increases on TC formations?
1. Overall decrease in MDR is unfavorable for TC formation and ACE increases.2. Other LSEFs (shear, vorticity, RH) show no pronounced long term changes.
1313
Regression Modeling of TC – LSEF Relationships
1. TC formation probability = log(p/(1-p))= a + b·SST + c·VertVel - d·Shear – e·Shear2 + f·AbsVort + g·Equat
2. ACE = A + B·SST + C·VertVel + D·Shear - E·Shear2 + F·AbsVort + G·RH
Sensitivity to LSEF Changes: Increase in each LSEF by one standard deviationleads to following changes in formation probability and ACE:
Long Term Changes in WNP Tropical Cyclone Activity
TC Activity in WNP, [email protected], Oct08
LSEF % Increase in Formation Probability
Vertical velocity +73.68
Shear -40.51
SST +25.67
Absolute vorticity +1.68
LSEF % Increase inACE
Absolute vorticity +21.20SST +14.83Relative humidity +10.31Shear -8.63Vertical velocity +3.38
1. TC formation relatively sensitive (insensitive) to vertical velocity (SST).2. ACE moderately sensitive to SST and relatively insensitive to vertical
velocity.
1515
Validation of TC Formation Probability Model
TC Activity in WNP, [email protected], Oct08
1. Modeled TC formation probabilities for individual week at 5x5 degree resolution (calculated using independent LSEF data)
2. Developing sub-seasonal TC formation forecasts based on model
TC Formation Probability for 2006 Week 40 Thresholds not considered
120 E 150 E 180 E 150 W
15 S
0
15 N
30 N
45 N Modeled TC formation
probabilities,01-07 Oct 2006
10%
25%
40%
55%
70%
Verifying observations of TC
formations from best track data
Long Term Changes in WNP Tropical Cyclone Activity
1616
Summary1. Shortcomings of TC and LSEF data sets are problematic.2. TC formations and intensities have undergone long term increases since
1970.3. SST and vertical velocity are the only LSEFs that experienced net long term
changes in WNP during 1970-2006.4. SST (vertical velocity) changes were favorable (unfavorable) for increases
in TC formation and intensity.5. Impacts of SSTs on TC formations may have been minimized by vertical
velocity decreases.6. GW may have contributed to increases in TC formations and ACE, by way
of SST increases but mitigated by vertical velocity decreases.7. But attribution is problematic, given:
a. Shortcomings of data setsb. Large interannual to multidecadal variationsc. Uncertainties about LSEF-TC relationships and GW signatures
8. LSEFs affect both TC formations and intensities, and need to be considered at realistic time and space scales in assessing GW impacts.
Long Term Changes in WNP Tropical Cyclone Activity
TC Activity in WNP, [email protected], Oct08
