# The Combined Effects of SST and the North Atlantic Subtropical High-Pressure System on the Atlantic Basin Tropical Cyclone Interannual Variability

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Dataset

#### 2.2. Methodology

_{ij}]] is a matrix (n_data x n_features) of the independent variables, and b is the intercept. Further information of the GLS model is provided by Safi and Saif [62] and Davidson and MacKinnon [63].

#### Kernel Density Estimation

_{i}are the data points; f(x) is the smooth estimate; h is the selected bandwidth; and n is the length of the dataset. The KDE smooths each data point x

_{i}into a small density bumps and then sums all these small bumps together to obtain the final density estimate. The kernel function K(x) must commonly satisfy the condition $\int}K\left(x\right)dx=1$, but may be considered arbitrary. The Gaussian kernel (Equation (3)) is a common choice [64]. Here, we used the classical KDE [65].

## 3. Results and Discussion

#### 3.1. TC Genesis

_{1}, purple) that represents 14.9% of the total genesis counts in the study period, the Caribbean Sea (G

_{2}, red, 11.7%), the Lesser Antilles arc (G

_{3}, green, 13.1%), the coast of West Africa (G

_{4}, orange, 18.2%), the Central North Atlantic (G

_{5}, yellow, 6.2%), the northeast of the Bahamian archipelago (G

_{6}, brown, 21.3%), and the Gulf of Mexico (G

_{7}, blue, 14.6%), as shown in Figure 2 (top). The monthly seasonality of TCs formed on each cluster is represented in Figure 2 (bottom). Genesis at the beginning of the hurricane season generally occurs at high latitudes due to favorable thermodynamic conditions and in the Gulf of Mexico, while at the end of the season they are more frequent in the western Caribbean Sea and the central North Atlantic. The regions G

_{1}and G

_{4}exhibit the highest frequency of genesis in August and September, coinciding with the peak of the cyclonic season. These G

_{1,4}TCs are frequently originated from African easterly waves.

_{5}y G

_{7}regions.

_{1}and G

_{4}regions, as shown in Table 1. In G

_{2}, G

_{3}, and G

_{5}, the correlation coefficient ranged between 0.13 and 0.30, however, it was not statistically significant. The inverse correlation (p > 0.05) observed in the G

_{6}and G

_{7}clusters is curious, and constitutes a strong evidence of the complexity of the physical processes for TC genesis. In these two regions, the other preconditions described by Gray [66] seem to be more important. The NASH latitude center is significantly correlated with TC genesis frequency (r = 0.36) in the whole basin, while G

_{3}(r = 0.36) is significantly correlated, too. No significant correlations were found between TC genesis and NASH intensity and NASH longitude center (Table 1).

_{N}regions (N = 1, 2, 3, 4, 5, 7), the genesis frequency increases with increasing SST, while an inverse pattern is observed in G

_{6}, as shown in Figure 4. However, SST only explains 12% (p < 0.05) of the genesis frequency variance throughout the basin and 9%, 15.1%, and 9.1% in G

_{1}, G

_{4}, and G

_{5}regions, respectively. The northward latitudinal movement of the NASH center favors the genesis processes, explaining 12.9% (p < 0.05) of the variance in the entire basin. By regions, the latitude center of the NASH explains only the variability of the G

_{3}region, with 13.1% (p < 0.05). It is notable that the intensity of NASH individually does not play an important role in the genesis of TCs. These results fully support the linear relationship obtained from Pearson’s correlation coefficient.

_{4}region is more dependent on the mean SST, while G

_{3}is dependent on the latitudinal variation of the NASH center.

#### 3.2. Landfall

_{1}, purple) with 15.1% of the recorded landfalling; the Florida Peninsula, the east coast of the United States up to 40°N latitude, and the coast of the Gulf of Mexico up to 87°W longitude (L

_{2}, red, 26.4%); the coasts of the Gulf of Mexico from longitude 87°W (L

_{3}, green, 24.4%); the western region of Cuba, the Yucatan Peninsula and Central America (L

_{4}, orange, 18.1%), and the arc of the Lesser Antilles and Puerto Rico (L

_{5}, blue, 15.8%), as shown in Figure 5. In this study, landfalling on the east coast of the United States from north of 40°N latitude and those that occurred on the islands located in the central North Atlantic were not considered.

_{2}and L

_{3}, secondary maximums are observed in June and July. The secondary maximums are a consequence of the TCs genesis in the Gulf of Mexico at the beginning of the TC season, while in the region L

_{4}, a secondary maximum is observed in October, mainly because of the TCs that originate in the Western Caribbean Sea at the end of the hurricane season. For the entire NATL basin, the highest frequency of landfalling is observed in August and September. The landfalling frequency shows a similar monthly distribution to genesis frequency.

_{1}and L

_{3}, L

_{2}and L

_{4}, and are greatly different in L

_{5}. The increase of annual landfalling in the latter part of the record is also evident, particularly in L

_{1}and L

_{2}.

_{1}+…+ L

_{5}) landfalling count of 0.52 is statistically significant at 95% of the significance level, as well as in regions L

_{1}, L

_{3}, and L

_{4}with r = 0.44, r = 0.46, and r = 0.33, respectively. However, it is notable that the Pearson correlation between SST and landfalling is low in L

_{2}and L

_{5}(r = 0.30), but statistically significant at 90% of the significance level. For the whole basin, SST and landfalling counts exhibit a strong correlation (r = 0.52), statistically significant at 95% of the significance level. No statistically significant correlation was found between NASH intensity and TC landfalling counts. However, the correlation between the frequency of landfalling and the latitude NASH center in a whole basin is statistically significant (r = 0.42), as shown in Table 3. A significant correlation is also observed in L

_{1}(r = 0.34) and L

_{4}(r = 0.38) landfalling regions with the latitudinal variations of the NASH center. Moreover, L

_{1}exhibits a strong Pearson correlation (0.40, p < 0.05) with the longitude of the NASH center.

_{1}, but explaining only 33.4% of the variance. The SST mostly explains the variance (p < 0.05) of the L

_{3}and L

_{4}regions with 26.4% and 24.1%, respectively. In the case of the L

_{2}and L

_{5}regions, the landfalling events do not appear to have a significant modulation of the SST and the position of the NASH. In the same way, as for genesis, no dependence was observed between the landfalling events and the intensity of the NASH, as shown in Table 4.

_{1}are distributed among all the L

_{N}regions (N = 1, 2, 3, 4, 5), although the highest frequency is observed in L

_{5}. Similarly, G

_{3}and G

_{4}also exhibit high frequencies of landfalling in L

_{5}, as shown in Figure 7. We assume that these distributions are mainly due to the wide range of trajectories for these TCs. Additionally, G

_{3}has the highest frequency of landfalling in the central−eastern region of Cuba, the archipelago of the Bahamas, and the west of La Española (L

_{1}), and another secondary maximum in L

_{2}. TCs formed in the western Caribbean Sea frequently make landfall on the coasts of Central America, the Yucatan Peninsula, and the western region of Cuba (L

_{4}). In L

_{2}, TCs that form in G

_{4}and G

_{6}frequently make landfall, while those originating in the Gulf of Mexico (G

_{7}) generally struck the coastline in L

_{3}.

_{1}and L

_{2}, the anticylonic ridge is less pronounced, while for L

_{3}and L

_{4}, it penetrates to the Florida Peninsula. For the L

_{5}region, the structure of the NASH shows a pattern similar to that observed for L

_{1}and L

_{2}, although more contracted, suggesting that TCs that make landfall in L

_{5}have a high probability to make landfall again in L

_{1}or L

_{2}. The NASH structure directly influences the distribution of the probability density tracks of TC that make landfall. The effect is different depending on the genesis location. TCs that develop in the eastern part of the main development region (G

_{4}) tend to have a low probability of landfalling than those that develop in the western part (G

_{1}and G

_{3}), due to the longer time over the tropical ocean. These results fully support the findings of Colbert and Soden [36], who pointed out that the evolution of NASH throughout a hurricane season affects the TCs trajectory in the NATL basin. When the NASH is strong and centered to the west, TCs are directed south toward the Gulf of Mexico, increasing the landfalling probability.

_{1}, L

_{2}, and L

_{5}regions, while an NASH center low in latitude modulates the movement of the TCs through the Caribbean Sea towards the Gulf of Mexico, favoring landfalling in Central America, the western region of Cuba, and the Mexican coast.

_{2}in 2004 (Table 5), there were 10 landfallings with negative SST anomalies in the northeast of the Gulf of Mexico, the Straits of Florida, and the seas surrounding the archipelago of the Bahamas, which is evidence of the strong modulating role of steering flow imposed by the NASH.

_{1}region; however, the dependence is only significant (p < 0.05) for the SST and the NASH center longitude. In the case of region L

_{4}, the covariates explain 25.1%, but it is statistically significant for the NASH center latitude. No statistically significant relationships were found for the other regions.

## 4. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**(

**a**) Centennial Time Scale (COBE) sea surface temperature (SST) anomalies time-series; (

**b**) location of the genesis (blue points) and peak of maximum intensity (the color bar represents the maximum surface wind) of (tropical cyclones TCs) formed in the North Atlantic (NATL) basin. The genesis point is defined as the first reported location in the Atlantic hurricane database (HURDAT2) database. The black dashed line represents the interest area for the SST analysis. Period: June–November from 1980 to 2019.

**Figure 2.**Map representation of TC genesis clusters (

**top**) and monthly proportion of genesis for each cluster (

**bottom**). The monthly proportion was computed as the proportion of genesis count in a month for a cluster and the genesis counts in the whole basin in the study period (1980–2019).

**Figure 3.**Annual TC genesis counts for the whole NATL basin and for each cluster. The bold line shows the time series filtered with a centered 5-yr moving window. G

_{N}(N = 1, 2, 3, 4, 5, 6, 7) represents each genesis cluster.

**Figure 4.**Frequency of TC genesis adjusted from the generalized least squares (GLS) linear regression model using the SST, NASH intensity, and NASH position as single covariate. “All clusters” represents the whole basin and G

_{N}(N = 1, 2, 3, 4, 5, 6, 7) represents each genesis cluster.

**Figure 5.**Map representation of TC landfalling regions (

**top**) and monthly proportion of landfalling for each cluster (

**bottom**). The monthly proportion was computed as the proportion of landfalling count in a month for a cluster and the sum of total landfalling (L

_{1}+…+ L

_{5}) (1980–2019). Note that landfalling on the east coast of the United States from 40°N latitude and those that occurred on the islands located in the central North Atlantic were not considered.

**Figure 6.**Annual landfalling counts for the whole NATL basin and each cluster. The bold line shows the time series filtered with a centered 5-yr moving window. L

_{N}(N = 1, 2, 3, 4, 5) represents each landfalling cluster.

**Figure 7.**Landfalling counts of TCs formed in each genesis region from 1980 to 2019 (

**left panel**) and frequency of landfalling in each landfall region of each genesis region (

**right panel**).

**Figure 8.**Kernel density estimation (KDE) for the trajectory of TCs that make landfall in each region. Contour lines represent the composite of mean sea level pressure for a specific month and specific year that landfalling events occurred. The red point and L

_{N}(N = 1, 2, 3, 4, 5) represents the NASH center and each landfalling cluster, respectively.

**Figure 9.**Composite of NASH position and structure (contour) for months with landfalling events in the year of maximum landfalling frequency for the whole basin and each region. The shaded plots represent the mean SST anomalies for the same months and years of NASH position and structure composite. The black star point represents the NASH center, and cyan points represent landfalling events. “All clusters” represents the whole basin and L

_{N}(N = 1, 2, 3, 4, 5) represents each landfalling cluster.

**Figure 10.**Probability density of intensities at TC landfalling for the wide NATL (the sum of landfalling counts in all regions) and each cluster.

**Table 1.**Pearson’s correlation coefficients between annual time series of mean SST, the North Atlantic subtropical high-pressure system (NASH) intensity and NASH position (June to November) with the number of tropical cyclones (TCs) genesis in the whole NATL and each cluster. Statistical significance is marked with the bold text (p < 0.05). NASH latitude and NASH longitude represent the NASH center latitude and NASH center longitude, respectively.

All Clusters | G_{1} | G_{2} | G_{3} | G_{4} | G_{5} | G_{6} | G_{7} | |
---|---|---|---|---|---|---|---|---|

SST | 0.35 | 0.36 | 0.13 | 0.21 | 0.39 | 0.30 | −0.14 | −0.004 |

NASH intensity | −0.12 | −0.15 | 0.04 | −0.10 | −0.17 | −0.04 | −0.03 | 0.07 |

NASH latitude | 0.36 | 0.26 | 0.20 | 0.36 | 0.03 | 0.26 | 0.01 | 0.16 |

NASH longitude | 0.18 | 0.11 | 0.05 | 0.10 | −0.14 | 0.25 | 0.05 | 0.21 |

**Table 2.**Generalized least squares (GLS) multiple linear regression of annual NATL genesis frequency (for the whole basin and for each cluster) onto the SST and NASH (intensity and position). The SST and NASH values are based on June–November. Statistical significance is marked with bold text (p < 0.05). SST is in °C and NASH intensity in hPa. NASH latitude and NASH longitude represent the NASH center latitude and NASH center longitude, respectively.

All Clusters | G_{1} | G_{2} | G_{3} | G_{4} | G_{5} | G_{6} | G_{7} | ||
---|---|---|---|---|---|---|---|---|---|

R^{2} | 0.227 | 0.151 | 0.055 | 0.186 | 0.191 | 0.153 | 0.036 | 0.05 | |

Intercept | Estimated | 706.27 | 205.67 | −78.67 | 231.85 | −24.29 | 62.03 | 328.4 | −18.75 |

Std error | 918.01 | 267.66 | 324.98 | 254.97 | 295.13 | 0.80 | 473.2 | 301.95 | |

p value | 0.45 | 0.447 | 0.810 | 0.369 | 0.935 | 0.250 | 0.492 | 0.95 | |

SST | Coefficient | 3.71 | 0.905 | 0.55 | 0.597 | 1.93 | 0.96 | −1.372 | 0.35 |

Std error | 2.53 | 0.738 | 0.89 | 0.53 | 0.814 | 0.683 | 1.305 | 0.83 | |

p value | 0.15 | 0.228 | 0.615 | 0.70 | 0.023 | 0.168 | 0.301 | 0.68 | |

NASH intensity | Coefficient | −0.79 | −0.23 | 0.059 | −0.24 | −0.023 | −0.08 | −0.285 | 0.013 |

Std error | 0.87 | 0.256 | 0.310 | 0.243 | 0.282 | 0.237 | 0.452 | 0.28 | |

p value | 0.37 | 0.382 | 0.85 | 0.324 | 0.93 | 0.723 | 0.533 | 0.96 | |

NASH latitude | Coefficient | 0.82 | 0.171 | 0.165 | 0.27 | 0.060 | 0.09 | 0.018 | 0.034 |

Std error | 0.43 | 0.125 | 0.152 | 0.119 | 0.138 | 0.116 | 0.221 | 0.141 | |

p value | 0.065 | 0.18 | 0.28 | 0.026 | 0.665 | 0.436 | 0.935 | 0.809 | |

NASH longitude | Coefficient | 0.008 | −0.001 | −0.025 | −0.018 | −0.059 | 0.033 | 0.036 | 0.041 |

Std error | 0.148 | 0.043 | 0.052 | 0.041 | 0.048 | 0.040 | 0.076 | 0.049 | |

p value | 0.960 | 0.976 | 0.64 | 0.65 | 0.23 | 0.410 | 0.638 | 0.404 |

**Table 3.**Pearson’s correlation coefficients between annual time series of mean SST, the NASH intensity, and NASH position (center latitude and longitude) (June to November) with the landfalling counts in the whole NATL (L

_{1}+ ... + L

_{5}) and each cluster. Statistical significance is marked with bold text (p < 0.05). NASH latitude and NASH longitude represent the NASH center latitude and NASH center longitude, respectively.

All Clusters | L_{1} | L_{2} | L_{3} | L_{4} | L_{5} | |
---|---|---|---|---|---|---|

SST | 0.52 | 0.44 | 0.30 | 0.46 | 0.33 | 0.30 |

NASH intensity | −0.04 | −0.05 | −0.15 | −0.08 | 0.17 | −0.03 |

NASH latitude | 0.42 | 0.34 | 0.26 | 0.30 | 0.38 | 0.25 |

NASH longitude | 0.29 | 0.40 | 0.05 | 0.21 | 0.22 | 0.18 |

**Table 4.**Generalized least squares (GLS) multiple linear regression of annual NATL landfalling frequency (for the whole basin and each cluster) onto the SST and NASH (intensity and position). The SST and NASH values are based on June–November. Statistical significance is marked with the bold text (p < 0.05). SST is in °C and NASH intensity in hPa. NASH latitude and NASH longitude represent the NASH center latitude and NASH center longitude, respectively.

All Clusters | L_{1} | L_{2} | L_{3} | L_{4} | L_{5} | ||
---|---|---|---|---|---|---|---|

R^{2} | 0.387 | 0.334 | 0.177 | 0.264 | 0.241 | 0.118 | |

Intercept | Estimated | −85.9351 | 125.97 | 292.02 | 47.2604 | −529.73 | −21.45 |

Std error | 1286.201 | 354.64 | 445.55 | 417.389 | 404.789 | 447.52 | |

p value | 0.947 | 0.725 | 0.516 | 0.910 | 0.199 | 0.962 | |

SST | Coefficient | 11.061 | 2.3545 | 1.605 | 2.9128 | 2.2941 | 1.8939 |

Std error | 3.548 | 0.978 | 1.222 | 1.151 | 1.117 | 1.235 | |

p value | 0.004 | 0.022 | 0.200 | 0.016 | 0.047 | 0.134 | |

NASH intensity | Coefficient | −0.2033 | −0.178 | −0.336 | −0.1195 | 0.4554 | −0.0251 |

Std error | 1.228 | 0.339 | 0.425 | 0.399 | 0.386 | 0.427 | |

p value | 0.869 | 0.603 | 0.435 | 0.766 | 0.247 | 0.953 | |

NASH latitude | Coefficient | 1.109 | 0.1120 | 0.382 | 0.2169 | 0.2869 | 0.1119 |

Std error | 0.601 | 0.166 | 0.208 | 0.195 | 0.189 | 0.209 | |

p value | 0.073 | 0.504 | 0.075 | 0.274 | 0.139 | 0.596 | |

NASH longitude | Coefficient | 0.1063 | 0.111 | −0.064 | 0.0296 | −0.0069 | 0.0363 |

Std error | 0.208 | 0.057 | 0.072 | 0.067 | 0.065 | 0.072 | |

p value | 0.612 | 0.061 | 0.383 | 0.662 | 0.917 | 0.618 |

All Clusters | L_{1} | L_{2} | L_{3} | L_{4} | L_{5} | |
---|---|---|---|---|---|---|

Year | 2008 | 2008 | 2004 | 2005 | 2005 | 2017 |

Landfall counts | 29 | 10 | 10 | 10 | 8 | 9 |

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## Share and Cite

**MDPI and ACS Style**

Pérez-Alarcón, A.; Fernández-Alvarez, J.C.; Sorí, R.; Nieto, R.; Gimeno, L. The Combined Effects of SST and the North Atlantic Subtropical High-Pressure System on the Atlantic Basin Tropical Cyclone Interannual Variability. *Atmosphere* **2021**, *12*, 329.
https://doi.org/10.3390/atmos12030329

**AMA Style**

Pérez-Alarcón A, Fernández-Alvarez JC, Sorí R, Nieto R, Gimeno L. The Combined Effects of SST and the North Atlantic Subtropical High-Pressure System on the Atlantic Basin Tropical Cyclone Interannual Variability. *Atmosphere*. 2021; 12(3):329.
https://doi.org/10.3390/atmos12030329

**Chicago/Turabian Style**

Pérez-Alarcón, Albenis, José C. Fernández-Alvarez, Rogert Sorí, Raquel Nieto, and Luis Gimeno. 2021. "The Combined Effects of SST and the North Atlantic Subtropical High-Pressure System on the Atlantic Basin Tropical Cyclone Interannual Variability" *Atmosphere* 12, no. 3: 329.
https://doi.org/10.3390/atmos12030329