# Presentation: 369406

## Long Term Changes in Tropical Cyclone Activity 

- TC Activity in WNP, murphree@nps.edu, Oct08

**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)**
- _[murphree@nps.edu](mailto:murphree@nps.edu)_**  and **_[dwmeyer@nps.edu](mailto:dwmeyer@nps.edu)_

## Long Term Changes in WNP Tropical Cyclone Activity

**Overview**
**Assessment of changes in tropical cyclone (TC) formations and**
**intensities in western North Pacific during 1970-2006:**
**Intraseasonal to multidecadal changes**
**Long term net changes**
**Relationships between TC changes and known large scale environmental factors (LSEFs)**
**Relationships between TC changes and global warming (GW)**

**Long Term Changes in WNP Tropical Cyclone Activity**

- TC Activity in WNP, murphree@nps.edu, 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.**

## Long Term Changes in WNP Tropical Cyclone Activity

**Prior Studies**
**LSEFs strongly affect TC formations (e.g., McBride 1995):**
**SST**
**Vertical shear **
**Vertical velocity **
**Absolute vorticity **
**Relative humidity**
**ENLN impacts on TC activity (e.g., Chan 2000, Ford 2000):**
**Impacts on formation sites, intensities, tracks**
**GW impacts on TCs so far (e.g., Chan and Liu 2004,** **Emanuel 2005, Webster and Holland 2005, Klotzbach 2006, Landsea 2007):**
**Little or no increase in TC numbers**
**Increase in TC intensities via SST increases**
**Impacts from interannual-multidecadal variations dominate**
**TC data inadequate to determine GW impacts**
**Limitations of prior studies:**
**Coarse resolution:  basin and seasonal averages**
**Focus on one LSEF: SST**
**Uncertainties in TC data**
**Distinction and attribution: GW impacts vs. other impacts**

**Long Term Changes in WNP Tropical Cyclone Activity**

- TC Activity in WNP, murphree@nps.edu, Oct08

## Long Term Changes in WNP Tropical Cyclone Activity

**Data**
**TC activity:  JTWC best track data (no adjustments)**
**LSEFs:  NCEP reanalysis 1 and 2**
**Period:  1970-2006 **
**Main TC season:  May-December**
**Region:  western North Pacific (WNP):  0-40****° N, 115–185°E**
**Main development region (MDR):  ****0-20° N, 115–165°E**
**Temporal resolution:  one week**
**Spatial resolution:  ****5°x5°**
**Number of TCs:  989**

**Long Term Changes in WNP Tropical Cyclone Activity**

- TC Activity in WNP, murphree@nps.edu, Oct08

## Long Term Changes in WNP Tropical Cyclone Activity

**Long Term Changes in WNP Tropical Cyclone Activity**

- TC Activity in WNP, murphree@nps.edu, Oct08

**Selection of analysis period**

## Long Term Changes in WNP Tropical Cyclone Activity

**Methods**
**Identify major temporal and spatial patterns of changes in:**
**TCs: formations and accumulated cyclone energy (ACE)**
**LSEFs**
**Develop regression models that relate TC formation probability and ACE to local LSEFs at weekly and 5x5 resolutions**
**Validate models using independent TC and LSEF data**
**Use models to assess sensitivity of TC formations and ACE to LSEF variations**
**Relate GW to LSEF changes, and, thus, to TC changes**
**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, murphree@nps.edu, Oct08

**Notes:**
Accumulated cyclone energy (ACE):  Sum of maximum one-minute sustained surface wind speed squared at six-hourly intervals for all periods when the TC wind speed is greater than or equal to 34 kts

## Long Term Changes in WNP Tropical Cyclone Activity

**Number of TC  Formations, 1970-2006**

**Long Term Changes in WNP Tropical Cyclone Activity**

- TC Activity in WNP, murphree@nps.edu, Oct08

**Main development region (MDR):  0-20****°N, 115-165°E**

## Long Term Changes in WNP Tropical Cyclone Activity

**Long Term Changes in WNP Tropical Cyclone Activity**

**Changes in TC Numbers and ACE, 1970-2006**

- TC Activity in WNP, murphree@nps.edu, Oct08

**Large interannual-multidecadal variations (linked to ENLN)**
**Long term net increases in TC formations and ACE **

**Can changes in LSEFs explain these long term changes in TC activity?**

**Notes:**
 Large interannual-multidecadal variations
 Overall increasing trend in annual TC formations and intensities (+4 and +7x105 kts2)
When weighted by number of TCs, intensity has increased over time with a 90% confidence level.  Likewise for TC numbers.  But these results (especially the TC numbers result) depend a lot on the period being considered.  

## Long Term Changes in WNP Tropical Cyclone Activity

**LSEF Changes – SST in MDR**

**Long Term Changes in WNP Tropical Cyclone Activity**

- TC Activity in WNP, murphree@nps.edu, Oct08

**Notes:**
In the “heart” of the main formation region, SST is actually declining over the past couple of years

## Long Term Changes in WNP Tropical Cyclone Activity

**Long Term Changes in WNP Tropical Cyclone Activity**

**LSEF Changes – SST**

**Overall SST increase in MDR (+0.4 °C) is favorable for increases in TC formations and intensities, and consistent with GW.**
**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, murphree@nps.edu, Oct08

**Notes:**
Calculated by averaging SST for JASO each year.  Change then calculated from the least squares fitted line for each 5x5 block for 1970-2006.

## Long Term Changes in WNP Tropical Cyclone Activity

**LSEF Changes – Vertical Velocity in MDR**

**Long Term Changes in WNP Tropical Cyclone Activity**

- TC Activity in WNP, murphree@nps.edu, Oct08

**Notes:**
Formation is highly sensitive to Vertical velocity. This is a significant counter to TC formation increase (high sensitivity) and may explain noticeable ACE increase (intensity sensitive to SST and not to vertical velocity) without as noticeable TC count increase

## Long Term Changes in WNP Tropical Cyclone Activity

**LSEF Changes – Vertical Velocity**

**Long Term Changes in WNP Tropical Cyclone Activity**

- TC Activity in WNP, murphree@nps.edu, Oct08

**Vertical Velocity Change, JASO, 1980-2006 (-Pa/s)**

**Have vertical velocity decreases countered impacts of SST increases on TC formations?**

**Overall decrease in MDR is unfavorable for TC formation and ACE increases.**
**Other LSEFs (shear, vorticity, RH) show no pronounced long term changes.**

## Long Term Changes in WNP Tropical Cyclone Activity

**Regression Modeling of TC – LSEF Relationships**
**1. ****TC formation probability**** = log(p/(1-p))**
**	****= a + b·SST + c·VertVel - d·Shear – e·Shear****2**** + f·AbsVort + g·Equat**
**2.   ****ACE**** = A + B·SST + C·VertVel + D·Shear - E·Shear****2**** + F·AbsVort + G·RH***** ***
**Sensitivity to LSEF Changes:****  ****Increase in each LSEF by one standard deviation**
**leads to following changes in formation probability and ACE:**

**Long Term Changes in WNP Tropical Cyclone Activity**

- TC Activity in WNP, murphree@nps.edu, Oct08

| LSEF | % Increase in 
Formation Probability |
| --- | --- |
| Vertical velocity | +73.68 |
| Shear | -40.51 |
| SST | +25.67 |
| Absolute vorticity | +1.68 |

| LSEF | % Increase in
ACE |
| --- | --- |
| Absolute vorticity | +21.20 |
| SST | +14.83 |
| Relative humidity | +10.31 |
| Shear |    -8.63 |
| Vertical velocity |   +3.38 |

**TC formation relatively sensitive (insensitive) to vertical velocity (SST).**
**ACE moderately sensitive to SST and relatively insensitive to vertical velocity.**

**Notes:**
Note: Shear squared terms are necessary to adequately capture shear magnitude 

## Long Term Changes in WNP Tropical Cyclone Activity

**Long Term Changes in WNP Tropical Cyclone Activity**

- TC Activity in WNP, murphree@nps.edu, Oct08

**Validation of TC Formation Probability Model**

**Week-block LSEF values used to calculate ACE, given a TC had formed or was present in that week-block.**
**If model ACE within 25% of analyzed scored as a “hit”, then model hit over 90% of the time, including the  data from the 1970s and 2006 which were not used in model formation.**

## Long Term Changes in WNP Tropical Cyclone Activity

**Validation of TC Formation Probability Model**

- TC Activity in WNP, murphree@nps.edu, Oct08

**Modeled TC formation probabilities for individual week at 5x5 degree resolution (calculated using independent LSEF data)**
**Developing sub-seasonal TC formation forecasts based on model**

**Long Term Changes in WNP Tropical Cyclone Activity**

**Notes:**
The NPS Smart Climatology Program is developing a system for predicting TC formation sites in the western North Pacific, at lead times of one week to several months, and at weekly, 5x5 degree resolution.  This work is being conducted by Tom Murphree (NPS Meteorology Dept) and David Meyer (NPS Operations Research Dept).  The above figure shows an example of a probabilistic TC formation hindcast from our prediction system.  Details on this particular case are given below.  The close correspondence between the predicted formation locations (within the probability contours) and the actual formation sites for the two TCs shown in the figure indicates a relatively high skill. 

Our prediction system is centered around a regression model based on the relationships between the large scale environmental factors (LSEFs) that govern TC formation and observed TC formations.  The regression model was built using: (1) LSEF data from the NCEP reanalysis data set; and (2) TC formation site data from the JTWC best track data set.  Both the LSEF and the TC data were from a 26-year period, 1980-2005.  The model output (hindcasts and forecasts) is in the form of probabilities of TC formation for individual 5x5 degree latitude-longitude boxes spanning the western North Pacific and for one week periods spanning the main TC season (mid-May through Dec).   

To test the model, we ran it in hindcast mode, using as input data LSEF reanalysis data for years outside the 26-year period.  So, for example, we used LSEF data from 2006 as inputs to produce weekly hindcasts of TC formations throughout the 2006 TC season. The above figure is an example from one of those weeks, 01-07 October 2006.  

The hindcasts have relatively high skill.  For example, the model had an overall probability of detection (POD) of 75% for the 2006 season.  Specifically, 75% of all the TCs that formed during mid-May 2006 through Dec 2006 did so at a time and location for which a formation probability of 10% or greater was predicted.  This sort of skill makes us optimistic that the actual predictions we will be producing as this work evolves will also have relatively high skill scores (i.e., high skill for long lead predictions) and will be operationally useful.  The forecasts should at least be a big improvement over existing alternatives (e.g., over long term mean formation data to predict formation sites).  

One important additional feature of the model is that it also shows considerable promise for long range predictions of tropical convection.  We have also developed a similar model for making long lead forecasts of TC intensity, and are developing plans for making long lead predictions of TC tracks. 

## Long Term Changes in WNP Tropical Cyclone Activity

**Summary**
**Shortcomings of TC and LSEF data sets are problematic.**
**TC formations and intensities have undergone long term increases since 1970.**
**SST and vertical velocity are the only LSEFs that experienced net long term changes in WNP during 1970-2006.**
**SST (vertical velocity) changes were favorable (unfavorable) for increases in TC formation and intensity.**
**Impacts of SSTs on TC formations may have been minimized by vertical velocity decreases.**
**GW may have contributed to increases in TC formations and ACE, by way of SST increases but mitigated by vertical velocity decreases.**
**But attribution is problematic, given:**
**Shortcomings of data sets**
**Large interannual to multidecadal variations**
**Uncertainties about LSEF-TC relationships and GW signatures**
**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, murphree@nps.edu, Oct08