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

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

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

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?

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

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

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

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

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?

Cold Events (Type-II)

Cold events (unsurprisingly) form at more poleward latitudes, in regions of shallower tropopause heights

Cold Events (Type-II) 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

Cold Events (Type-II) 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

Cold Events (Type-II) 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?

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

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?

Physical Mechanisms of Type-II TC Genesis Coupling Index = Θ DT – Θ e850 Lower values indicate less atmospheric stability Less stability is favorable for baroclinic growth in the Eady model

Physical Mechanisms of Type-II TC Genesis Coupling Index = Θ DT – Θ e850 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

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

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

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%

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

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

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