Genesis parameters (GPIs) are equations that relate variations in observed atmospheric data to changes in the TC formation rate. The parameters in the equations are based on the best statistical fit to the observed TC formation rate variations and provide a succinct way of encapsulating the effect of atmospheric and oceanic climate on TC formation. These are required because we do not have a full understanding of TC genesis and there is no complete theory for genesis. Three different genesis parameters were calculated in this study. These are the GP (here, EN so as not to confuse with the general acronym GPI) by Emanuel and Nolan (27), the CGI by Bruyère et al. (28), and the TCS (here, TIP) by Tippett et al. (29) defined below. Because these indices were developed for different datasets and time periods, we scaled the indices to the mean number of TCs observed in the North Atlantic (12.2 per year for all TCs and 9.0 per year for long-lived TCs). The EN index is defined as followsEmbedded Image(1)where η is the absolute vorticity at 850 hPa (s−1), H is the relative humidity at 700 hPa (%), Vpot is the potential intensity (m s−1) calculated using a routine provided by Emanuel (ftp://texmex.mit.edu/pub/emanuel/TCMAX/), and Vshear is the vertical shear from 850 to 200 hPa (m s−1). The CGI index is defined asEmbedded Image(2)This uses just two components from the EN (Vpot and Vshear). Bruyère et al. (28) found that this formulation results in improved correlations of interannual variability and trends compared to the original EN and updated GPI (27, 30). Because the CGI does not include a vorticity term to remove TCs that form close to the equator, its values are set to zero between 0° and 5°N, as done in the original paper. The TIP index is defined asEmbedded Image(3)where η is the clipped absolute vorticity at 850 hPa in 105 s−1 (η = min(η, 3.7)), T is the Embedded Image in °C, ϕ is the latitude, H is the relative humidity at 600 hPa (%), and V is the vertical shear from 850 to 200 hPa (m s−1). The constants used are those from line 6 of table 1 of Tippett et al. (29) and as used by Menkes et al. (31)Embedded ImageThe annual cycle from both the GPIs calculated in the reanalyses and models is shown in fig. S1 when scaled to the full TC dataset. The calculated GPIs all do a reasonable job of reproducing the observed annual cycle of North Atlantic TC formation, but values in all GPIs are too high outside of the peak North Atlantic TC season of August to October (ASO), resulting in lower values during the peak ASO period. The similarities in the GPIs calculated in the reanalyses and the models are clear, with TIP giving the lowest out-of-season values and CGI the highest. The year-to-year (interannual) variability in the reanalysis-calculated GPIs and observed number of TCs is shown in Fig. 3, with r2 values for the correlations included. The EN index has the poorest correlation, consistent with the results of Bruyère et al. (28), and this is poor in particular for the NCEP data (r2 = 0.01). The CGI and TIP indices are able to capture the relationship between year-to-year climate variations and TC formation, with r2 values above 0.5. It is worth noting here that the CGI was tested and developed for the Atlantic region only; hence, it is no surprise that it performs well over this region. However, the TIP index is a globally developed index and performs comparably well.

When the GPIs were rescaled for the long-lived TCs, the annual cycle (fig. S2) and interannual variability (fig. S6) were generally slightly poorer when compared to the observed data, and there was a slight decrease in correlations in all indices except the CGI calculated in ERA. This is due to the data originally used to formulate these GPIs including these short-lived TCs.

For the MRI model data, in addition to a GPI analysis, we also detected TCs directly in the four-time daily output of the model, using the method of Walsh (32) and Horn et al. (33). The detection wind speed threshold was chosen to be 16 m s−1, almost identical to the threshold for observed TCs and appropriate for the resolution of the model (34). For comparison with the long-lived TCs, systems that were tracked for 2.0 days or less were removed.

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