1. McTaggart-Cowan et al. (2015)
Revisiting the 26.5°C Sea Surface Temperature Threshold for Tropical Cyclone Development
McTaggart-Cowan et al. (2015)

2. Motivation
Typically, five environmental criteria need to be satisfied for TC genesis to occur
Warm sea surface temperatures
Reduced tropospheric stability
High relative humidity in lower/mid troposphere
Low vertical wind shear
Sufficient ambient vorticity

3. Motivation
Typically, five environmental criteria need to be satisfied for TC genesis to occur
Warm sea surface temperatures
Reduced tropospheric stability
High relative humidity in lower/mid troposphere
Low vertical wind shear
Sufficient ambient vorticity

4. Motivation
How warm do sea surface temperatures have to be in order to result in TC genesis?
A value of 26.5°C has commonly been accepted, even in textbooks
Dare and McBride (2011) found 7% of all TC genesis events occur over SSTs below the 26.5°C threshold
What happens during these events?

5. Datasets ERA-Interim reanalysis used for atmospheric conditions
Reynolds et al. (2007) SST dataset  available daily with 0.25° resolution
IBTrACS used for TC position and intensity
Analyze global TCs forming between
Focus on “self-sustaining” vortices with an intensity of 35 kt used for genesis
Excluded subtropical/extratropical storms

6. SST Distribution
Sea surface temperatures were averaged within 2° of the TC genesis location

7. SST Distribution
Sea surface temperatures were averaged within 2° of the TC genesis location
There is a tail in the distribution shifted toward lower SST values

8. Genesis Pathway
Storms can be classified following the methods of McTaggart-Cowan et al. (2013)
Genesis pathway dependent upon QG forcing for ascent (Q Metric) and low-level baroclinicity (Th Metric)

9. SST at TC Genesis Divided all genesis events into two types
Type-I (warm events) form over sea surface temperatures ≥ 26.5°C
Type-II (cold events) form over sea surface temperatures < 26.5°C
These are seen in the tail of the distribution
What allows these TCs to develop?

10. Cold Events (Type-II)

11. Distribution of Genesis Pathways
Cold events (unsurprisingly) form at more poleward latitudes, in regions of shallower tropopause heights

12. Distribution of Genesis Pathways
Cold events (unsurprisingly) form at more poleward latitudes, in regions of shallower tropopause heights

13. Distribution of DT Pressures
The higher dynamic tropopause pressures in cold events are primarily associated with TT (tropical transition) events

14. Distribution of DT Pressures
The higher dynamic tropopause pressures in cold events are primarily associated with TT (tropical transition) events

15. Distribution of DT Pressures
The higher dynamic tropopause pressures in cold events are primarily associated with TT (tropical transition) events
TT events account for ~16% of global TC formations, but ~45% of all cold events

16. Genesis Pathways and SST
Weak TT and Non-TT events display similar distributions of SST
Strong TT events have a large shift in the distribution toward lower SST values

17. Genesis Pathways and SST
Weak TT and Non-TT events display similar distributions of SST
Strong TT events have a large shift in the distribution toward lower SST values
Thoughts on why Weak TT distribution is so similar to Non-TT?

18. Comparisons to Atlantic Events
Simpler criteria can be applied to the genesis pathway
When focusing on only Atlantic genesis events, the presence of an upper-tropospheric PV anomaly shows a shift in the distribution toward cooler SSTs

19. Physical Mechanisms of Type-II TC Genesis
TT events, especially Strong TT events, display a correlation between lower SSTs and higher DT pressures
Why would higher DT pressures assist in TC genesis over colder SSTs?

20. Physical Mechanisms of Type-II TC Genesis
Coupling Index = ΘDT – Θe850
Lower values indicate less atmospheric stability
Less stability is favorable for baroclinic growth in the Eady model

21. Physical Mechanisms of Type-II TC Genesis
Coupling Index = ΘDT – Θe850
Lower values indicate less atmospheric stability
Less stability is favorable for baroclinic growth in the Eady model
TT events display a sharper onset of decreasing coupling index values and same threshold

22. Physical Mechanisms of Type-II TC Genesis
Upper level trough (+PV anomaly) provides QG forcing for ascent
Wind Shear Direction

23. Physical Mechanisms of Type-II TC Genesis
Upper level trough (+PV anomaly) provides QG forcing for ascent
Circulation is stretched, mid-levels moistened
Region is favorable for sustained convection, provided a sufficient coupling index
Wind Shear Direction

24. Thermodynamic Thresholds for TC Genesis
When a SST threshold is used for non-TT events and a coupling index threshold is used for TT events, the error rate for all genesis events is only 3%

25. Thermodynamic Thresholds for TC Genesis
When a SST threshold is used for non-TT events and a coupling index threshold is used for TT events, the error rate for all genesis events is only 3%
Big implications for basins where TT events are common  Atlantic

26. Favorable regions for TT events
A “Goldilocks zone” of low coupling index values
Warm enough SSTs
Cold upper troposphere

27. Implications
The thermodynamic thresholds for TT events are important since these storms affect regions not accustomed to the effects of TCs
Criteria can be applied to subtropical/hybrid storms with low error rates (~10%)
Seasonal forecasting guide
Implications on forecasting TC activity in a changing climate