Interannual synchronization of the North American summer monsoon and the North Atlantic tropical cyclone genesis frequency

Variations of the North American summer monsoon (NASM) and North Atlantic tropical cyclone (NATC) activities strongly influence climate anomalies in North America, with serious potential risk to life and property. Despite the scientific importance of this topic, the possible linkage between the NASM and the NATC genesis frequency remains unexplored. Here, we aim to examine the relationship between interannual variations of the NASM intensity and the NATC genesis frequency based on observations and Coupled Model Intercomparison Project Phase 6 (CMIP6) models. Our results show a strong association between the NASM intensity and the NATC genesis frequency during the extended boreal summer, with a good synchronization between their interannual variations. In years with stronger (weaker) NASM intensity, the NATC genesis frequency tends to be higher (lower). The observed NASM–NATC synchronization may be explained by two pathways: tropical-ocean-driven pathway and monsoon-heating-driven pathway. In the tropical-ocean-driven pathway, the tropical Pacific and Atlantic interbasin sea surface temperature (SST) anomalies play a critical role in bridging the NASM and NATC, by modulating the cross-Central American wind. Simulations of the tropical Pacific–Atlantic interbasin SST anomalies are critical for CMIP6 models to capture the observed linkage between the NASM and the vertical wind shear over the NATC main development region (MDR). In the monsoon-heating-driven pathway, the heating source due to the rainfall anomalies associated with the NASM can trigger atmospheric circulation anomalies through the Gill-type response, thereby affecting the NATC by changing the vertical wind shear over the MDR. This study demonstrates a connection between interannual variations of the NASM and the NATC genesis frequency, results of which can be used to advance our understanding of the monsoon–TC relationship and increase research focus on the interannual NASM–NATC synchronization in climate prediction.


Introduction
Major variations in monsoons and tropical cyclones (TCs) can cause serious natural disasters, and the characteristics and mechanisms of anomalous monsoons and TCs have become important topics in climate variability and prediction studies (Knutson et al 2010, An et al 2015, Vishnu et al 2016, Cao et al 2021, Basconcillo and Moon 2022).Variations of the North American summer monsoon (NASM) and North Atlantic tropical cyclone (NATC) activities strongly influence climate anomalies in North America, with serious potential risk to life and property (Adams et al 1997, Higgins et al 1997, Rappaport 2000, Barlow et al 2011, Klotzbach et al 2018, Boos et al 2021).Understanding variations of the NASM and the NATC is essential for climate forecasting and disaster preparedness.
In terms of the monsoon-TC relationship, previous studies have noticed that TC activities in some ocean basins may be closely related to variations of the adjacent monsoon, including the links between the western North Pacific (WNP) TC genesis and the WNP summer monsoon (Choi et al 2016, Zhao et al 2019), the eastern North Pacific (ENP) TC genesis and the NASM (Weng et al 2022), and the NATC and the West African monsoon (Gray 1990, Landsea andGray 1992).Variations of the adjacent monsoon may modulate TC genesis activities by providing favorable or unfavorable modulations to the basic large-scale environmental characteristics (Harr and Wu 2011), thus leading to the connection between the monsoon intensity and TC genesis frequency (Choi et al 2016, Weng et al 2022).The intensified WNP summer monsoon favors the genesis of WNP TCs through local air-sea interactions and positive feedback (Choi et al 2016).Weng et al (2022) revealed a remarkable negative correlation between the intensity of the NASM and the TC genesis frequency over the ENP.This may be explained by the increased vertical wind shear over the tropical ENP during strong NASM years, which reduces the TC genesis frequency.The intensity of the West African monsoon exhibits a robust linkage with the TC genesis frequency over the North Atlantic (Landsea and Gray 1992, Chen et al 2008) because the reduction of precipitation across West Africa is associated with the weakened easterly jet over the tropical North Atlantic, which can suppress regional TC genesis.
The NATC genesis frequency largely depends on changes in related background environmental factors (Gray 1968, Goldenberg and Shapiro 1996, Goldenberg et al 2001, Latif et al 2007, Lee et al 2011, Chen et al 2021, West et al 2022, Cao et al 2023, Moharana and Swain 2023).In particular, the important role of the vertical wind shear in modulating the NATC genesis has been revealed in previous studies (Aiyyer andThorncroft 2011, Wang et al 2016).The strong vertical wind shear may inhibit the TC genesis because of the ventilation effect (Demaria et al 1996), in which heat and moisture are advected away from low-level circulations and the TC warm core structure is broken.Accompanied by changes in vertical wind shear over the tropical North Atlantic, anomalous cross-basin zonal winds tend to occur over Central America at 200 hPa (Wang et al 2016).This cross-Central American wind (CCAW) phenomenon has been extensively explored by Wang et al (2016).In addition, the NASM is characterized by the upper tropospheric anticyclonic circulation in the background mean state (Adams andComrie 1997, Vera et al 2006).The anomalous CCAW in the upper troposphere may possibly influence the upper tropospheric anticyclonic circulation of the NASM, thus further leading to the changes of the NASM intensity.Based on the above information, we speculate that the co-modulation of the CCAW may cause synchronized variations between the NASM and the NATC genesis frequency, which has not been thoroughly explored and confirmed in previous studies.
The possible linkage between the NASM and the NATC genesis frequency has not yet been directly investigated.Two related questions naturally arise: (a) Is there a link between the interannual variations of the NASM and NATC genesis frequency?(b) What physical processes and mechanisms may contribute to this NASM-NATC linkage?To fill the knowledge gap in this area, we examine the relationship between interannual variations of the NASM intensity and the NATC genesis frequency based on observations and the Coupled Model Intercomparison Project Phase 6 (CMIP6) models.

Datasets and models
In this study, several observational datasets and CMIP6 models (Eyring et al 2016) were used to examine the interannual NASM-NATC relationship and the underlying mechanisms.To depict the variations of NATC activities, we used the TC dataset from the International Best Track Archive for Climate Stewardship version v04r10 (Knapp et al 2010).The monthly-mean atmospheric fields were derived from the National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis product (Kalnay et al 1996), which was applied to quantify the variations of the NASM and the roles of the large-scale environmental conditions in TC genesis.Precipitation data from the Climate Prediction Center merged analysis of precipitation (Xie and Arkin 1997) were also utilized to characterize the precipitation of the NASM.The monthly mean sea surface temperature (SST) data were acquired from the National Oceanic and Atmospheric Administration extended reconstructed SST dataset version 5 (Huang et al 2017) and used to investigate the SST anomalies modulating the variations of the NATC and the NASM.The monthly mean outgoing longwave radiation data (Liebmann and Smith 1996) were used to examine the atmospheric convection anomalies associated with the NASM.
We also applied the monthly historical simulations produced by the 28 models taking part in the CMIP6 (table S1) to evaluate the simulations of the connection between the NASM and the NATCassociated large-scale environmental factors in these CMIP6 models.The influences of the tropical Pacific and Atlantic SST anomalies on the simulated NASM-NATC linkage in these CMIP6 models were also assessed.

Analysis methods
The following climatic indexes are used in the analysis.(1) The NASM index, which is defined by the difference in area-mean 850 hPa zonal wind anomalies between the southern region (5 Yim et al (2014), was used to quantify the intensity of the NASM.A positive (negative) NASM index corresponds to the anomalous low-level cyclonic (anticyclonic) circulation, indicating a stronger (weaker) NASM.(2) Following Wang et al (2016), the CCAW index is defined as the 200 hPa zonal wind anomalies averaged over Central America (10 Strong and weak NASM years were selected for a composite analysis, to compare the differences of NATC genesis activities and associated large-scale environmental fields between strong and weak NASM years.In this analysis, years with the normalized NASM index greater (less) than 0.55 (−0.55) were defined as strong (weak) NASM years, with the sample sizes in weak, normal, and strong monsoon years being approximately the same according to this threshold.In total, 12 strong NASM years and 14 weak NASM years were selected (table S2).To clarify the impact of co-occurring ENSO on the results, El Niño (La Niña) years were selected based on the Niño3.4index during the extended boreal summer exceeding the positive (negative) 0.8 standard deviation.Further, NASM years were categorized into two types: Type-I and Type-II (table S2).Type-I years (Type-II years) were selected as strong/weak NASM years with (without) co-occurring La Niña/ El Niño.The composite analysis was also conducted for both Type-I and Type-II years.
To reduce uncertainties in identifying weak systems, only TCs that reached tropical storm intensity were analyzed in this study.Following the definition by Daloz et al (2012), the region (10 • N-20 • N, 80 • W-20 • W) was designated as the main development region (MDR) for TCs over the North Atlantic.According to the definition method used in previous studies (He et al 2015, Choi et al 2016), the NATC genesis density was calculated as the TC genesis frequency within each 10 • × 10 • latitude-longitude grid, to compare the spatial distribution of NATC genesis frequency anomalies during different NASM years.
We employed the genesis potential index (GPI) analysis to diagnose the roles of related background environmental factors in modulating the NATC genesis.Based on Murakami et al (2011), the modified GPI was defined as follows: where ζ represents the 850 hPa absolute vorticity, H stands for the 600 hPa relative humidity, ω is the 500 hPa vertical velocity, and V pot signifies the maximum potential intensity, as defined by Bister and Emanuel (2002).V shear denotes the vertical wind shear, which is determined by calculating the magnitude of the vector difference between horizontal winds at 850 and 200 hPa.A higher (lower) GPI indicates a higher (lower) likelihood of TC genesis, with more (less) favorable environmental conditions.
To assess the relative contributions of different environmental factors on TC genesis, we further deconstructed the GPI into five distinct terms ), following Camargo et al (2007).The relative contributions of these five terms to the change in GPI can be expressed as: where the bar and prime terms represent climatology and anomaly, respectively.The first five terms of the right-hand side of equation ( 2) denote the relative contributions of each large-scale environmental factor, and the last term (F) represents the non-linear term.
The statistical analysis methods used in this study include correlation, composite, and regression analyses.To eliminate the influences of global warming, all datasets were detrended in the analysis.The statistical significance was tested by the two-tailed Student's t test.

Co-variability of the NATC genesis frequency and the NASM
During the period from 1979 to 2019, a total of 327 TCs were generated over the North Atlantic during the extended boreal summer (June-September; JJAS) (figure 1(a)).These TCs were predominantly formed within the background easterly flow in the lower troposphere (figure 1(a)).The climatological low-level atmospheric easterlies are also prevalent in the northern region used in defining the NASM index (figure 1(a)).The NASM index and the NATC genesis frequency tend to vary in-phase, with a remarkable co-variability according to their time series (figure 1(b)).In years with a stronger (weaker) NASM intensity, the genesis frequency of NATC tends to be higher (lower).On average, there are 10.00 TCs/yr (6.29 TCs/yr) in the strong (weak) NASM years, which is significantly higher (lower) than the climatological mean value of 7.98 TCs/yr during 1979-2019.The correlation coefficient (r = 0.65) indicates a significant synchronicity between the NASM intensity and the NATC genesis frequency (figure 1(c)).A similar significant positive correlation was also found between the average precipitation rate within the NASM core region (7.5 • N-22.5 • N, 110 • W-80 • W) and the NATC genesis frequency, further supporting this NASM-NATC synchronicity.The enhanced NASM precipitation and the increased NATC genesis frequency tend to co-occur on the interannual time scale.We also employed a 21-yr sliding correlation analysis (figure S1), the results of which reveal a consistently significant positive NASM-NATC correlation throughout the entire study period.After removing the signal of concurrent Niño3.4 index, the partial correlation coefficient between the NASM and NATC is still significant at the 95% confidence level (r = 0.47), suggesting that the NASM-NATC synchronization may still exist after removing the ENSO influence.
Differences in TC genesis location between the strong and weak NASM years are not statistically significant (figure S2), which suggests that the TC genesis location over the North Atlantic remains relatively stable despite the varying NASM conditions.The comparison of TC genesis frequency in two subregions (the MDR and the northern region used in defining the NASM index) reveals that the proportion of TC genesis frequency in the MDR region is higher and its correlation with the NASM index is stronger; thus, we focus on the TC genesis over the MDR in the following analysis.

Possible mechanisms linking the NASM and the NATC
The preceding statistical analysis highlighted the synchronized interannual variations between the NASM

Changes in the GPI associated with the NASM variability
We employ the GPI analysis to diagnose the roles of related background environmental factors, to explain why more (less) TCs tend to form over the MDR in years with stronger (weaker) NASM intensity.Positive (negative) GPI anomalies are observed over the MDR during strong (weak) NASM years, as shown in figure 2(a) and (b), which is consistent with the observed fact that more (less) TCs tend to form over the MDR in years with stronger (weaker) NASM intensity.This suggests that the changes in GPI could explain the observed variations of the NATC genesis frequency associated with the NASM variability.The TC genesis frequency over the MDR exhibits a significant positive correlation with the local GPI anomalies (r = 0.53), further demonstrating that the changes in GPI tend to be consistent with the changes in TC genesis frequency.We further deconstruct the differences in GPI between strong and weak NASM years according to equation (2), the results of which suggest that the vertical wind shear made the largest contribution of all studied environmental factors (figure 2(c)).While the mid-level relative humidity and the maximum potential intensity also contribute to GPI anomalies to some extent (figures 2(c), S3 and S4), their contributions are relatively smaller compared to the vertical wind shear (figure 2(c)).According to the comparisons of different environmental factors in the GPI (tables S3 and S4), the vertical wind shear may be the environmental factor that contributes the most to the NASM-NATC synchronization.
Negative (positive) anomalies of vertical wind shear prevail over the MDR during the strong (weak) NASM years (figure S5).The signals of anomalous upper-level wind fields over the MDR are more pronounced than those of the lower-level wind anomalies (figure S6), suggesting that the upper-level winds can exert a stronger influence than the lowlevel winds on the vertical wind shear.In the strong (weak) NASM years, anomalous easterlies (westerlies) prevail in the upper-level over the MDR, which can weaken (enhance) the climatological westerlies.Consequently, these changes in the upper-level winds can further favor the attenuation (enhancement) of the vertical wind shear over the MDR during strong (weak) NASM years, thereby shaping the conducive (inhibitive) conditions for TC genesis.
The NASM-associated changes in NATC genesis and GPI anomalies can be observed in both Type-I years (with co-occurring ENSO) and Type-II years (without co-occurring ENSO) (figure S7).Compared to Type-II years, the amplitude of NATC anomalies and GPI anomalies during Type-I years are larger (figure S7), indicating that the co-occurring ENSO may boost the NASM-NATC relationship.The vertical wind shear is the most important environmental factor in GPI for both Type-I years and Type-II years (figures S7(c) and (f)).The relative contributions of the mid-level relative humidity and the potential intensity differ between Type-I years and Type-II years, with the potential intensity (the mid-level relative humidity) making the second largest contributions for Type-I (Type-II) years (figures S3, S4 and S7).
During strong NASM years, wide spread rainfall and convection anomalies can be observed over the NASM region to the western tropical Atlantic Ocean (figure S8).The atmospheric heating source due to these rainfall and convection anomalies associated with the NASM can affect the atmospheric circulation anomalies (figures S9(a) and (b)) through the Gilltype response, thereby modulating the NATC genesis by regulating the vertical wind shear over the MDR (figure S9(c)).Thus, we propose a monsoon-heatingdriven pathway including the above processes, which is considered to be able to contribute to the NASM-NATC synchronization.Next, we examine the distributions of associated SST and wind anomalies based on regression analyses (figure 3).Clear tropical Pacific-Atlantic IBS anomalies are apparent in the regressed SST patterns.Favorable conditions for an increase in the CCAW index arise from the positive SST anomalies in the tropical central-eastern Pacific, coupled with the negative SST anomalies over the tropical Atlantic (figure 3(a)).Accompanied by the IBS anomalies, there are inter-basin sea level pressure dipolelike anomalies, which in turn trigger low-level atmospheric wind anomalies (figure S11).In addition, this type of SST pattern contributes to a reduction in both the NASM index (figure 3(b)) and TC genesis frequency over the MDR (figure 3(c)).These findings suggest that the tropical Pacific-Atlantic interbasin SST anomalies may play a critical role in modulating the CCAM, thus further contributing to link the NASM and NATC.

The role of tropical
In order to determine how the tropical Pacific-Atlantic interbasin SST anomalies modulate the CCAM, we examine the overall effect of this SST pattern through an IBS index.When the IBS index is in its positive phase, the tropical central-eastern Pacific (tropical Atlantic) exhibits negative (positive) SST anomalies, in conjunction with the change of the CCAW (figure S12(a)).The occurrence of El Niño conditions over the tropical central-eastern Pacific significantly influences the CCAW, partially through the teleconnection of the Pacific-North American (PNA) pattern (Wallace and Gutzler 1981, Wang et al 2016).Furthermore, the warming of the tropical North Atlantic contributes to the amplification of the CCAW, which stems from the anomalous upperlevel anticyclonic circulation driven by the Gill pattern (Gill 1980, Wang et al 2016).Moreover, the SST anomalies over the tropical central-eastern Pacific and tropical Atlantic have a collective impact on the CCAW, which is realized through the initiation of anomalous sea-level pressure gradients in the lower Based on the above results, we believe that the observed NASM-NATC synchronization may also be explained by the tropical-ocean-driven pathway.In this tropical-ocean-driven pathway, the tropical Pacific-Atlantic interbasin SST anomalies can contribute to the NASM-NATC synchronization, by bridging the NASM and NATC via modulating the CCAW.
In addition, we compare the Niño3.4index and the IBS index in terms of their correlations with the NATC/NASM (table S5) We further calculate the partial correlation coefficient between the NASM and NATC indices after removing the influence from the tropical Pacific-Atlantic interbasin SST anomalies, results of which show that the calculated partial correlation coefficient is 0.38.On the one hand, compared to the original correlation coefficient between the NASM and NATC indices (r = 0.65), the partial correlation coefficient tends to decrease obviously (reduced by approximately 42%), which can also support the important contribution of the tropical Pacific-Atlantic interbasin SST anomalies in boosting the NASM-NATC synchronization.On the other hand, the partial correlation coefficient (by removing the influence from the tropical Pacific-Atlantic interbasin SST anomalies) still remains significant, suggesting that the NASM-NATC synchronization may not solely rely on the influence of tropical SST, although the tropical Pacific-Atlantic SST anomalies is very important in promoting the NASM-NATC synchronization.The anomalous atmospheric circulation and vertical wind shear driven by the monsoon-heating associated with the NASM can still be observed after removing the influence of tropical Pacific-Atlantic interbasin SST anomalies (figure S13), suggesting that this monsoon-heating-driven pathway may still exist even without the influence from tropical Pacific-Atlantic SST anomalies.
The CMIP6 models show different simulation results in reproducing the correlation between the NASM index and the vertical wind shear over the MDR (figure S14).This may relate to differences in the models when simulating the effects of the tropical Pacific-Atlantic interbasin SST anomalies (figure S15).The patterns of the tropical Pacific-Atlantic interbasin SST anomalies are more consistent with the observations in the models that have high skill in reproducing the connection between the NASM index and the vertical wind shear (figures S15(a) and (b)) than in the models with low skill (figures S15(c) and ( d)).This suggests that the simulations of the tropical Pacific-Atlantic interbasin SST anomalies are likely to be critical for the models to capture the observed linkage between the NASM index and the vertical wind shear over the MDR.These results based on CMIP6 models also support the important role of the tropical Pacific-Atlantic IBS in shaping the NASM-NATC relationship.
The These diverse processes may work together to contribute to the formation of tropical Pacific-Atlantic IBS anomalies.The CMIP6 models may need to be able to reproduce these different air-sea coupling processes in order to better simulate the tropical Pacific-Atlantic IBS anomalies.

Related discussions 3.3.1. Implications for climate prediction
The observed NASM-NATC synchronization may also have broader implications in climate prediction, providing useful information for the prediction of the NASM and the NATC genesis frequency.Given the NASM-NATC synchronization on the interannual time scale, the NASM intensity and the NATC genesis frequency may be considered together in climate prediction.
Compared with the monsoon-heating-driven pathway, the tropical-ocean-driven pathway may provide more early forecast signals in the preceding spring (table S6).The tropical Pacific-Atlantic interbasin SST anomalies tend to have a good spring-tosummer persistence, with a high positive correlation between the springtime and summertime IBS index (figure S16).There are clear relationships between the springtime IBS anomalies and the subsequent summertime NASM intensity, as well as the NATC genesis frequency (figure S16).These findings suggest that the springtime IBS anomalies might serve as predictive indicators for the summertime NASM intensity and NATC genesis frequency.Most of the CMIP6 models are able to successfully capture the spring-to-summer persistence of the IBS anomalies (figure S17).Consequently, our study is conducive to improving the forecasting skills for the NASM and NATC activities, thus bearing important practical applications in disaster prevention and reduction.

Comparison with other monsoon-TC relationships
Monsoons and TCs are both global phenomena, and their activities can be found in various regions across the world (figure S18).As mentioned in the introduction, previous studies have noticed other monsoon-TC relationships.It is meaningful to compare the NASM-NATC relationship proposed in this study with other monsoon-TC relationships.
Compared with the WNP monsoon-TC relationship, the NASM-NATC relationship exhibits both similarities and differences.Similarly, both of these monsoon-TC relationships demonstrate a positive correlation, signifying that intensified (weakened) monsoon intensity corresponds to a higher (lower) TC genesis frequency.However, there are notable distinctions.The genesis location of the TCs over the WNP largely coincides with the spatial location of the WNP summer monsoon, with most WNP TCs originating within the monsoon trough environment of the monsoon core region.In contrast, the genesis location of the NATC, primarily the MDR, does not spatially overlap with the monsoon core region of the NASM.Furthermore, the WNP monsoon-TC relationship is largely influenced by the local air-sea positive feedback (Choi et al 2016), whereas the NASM-NATC relationship is predominantly governed by atmospheric teleconnection induced by remote SST anomalies.In addition, the WNP monsoon-TC relationship has exhibited notable interdecadal changes since the 1990s (Zhao et al 2019), while the NASM-NATC relationship remained relatively stable.
Unlike the NASM-NATC relationship, which exhibits a positive correlation, the linkage between the NASM and the ENP TC genesis frequency display a negative correlation.Prior studies have shown that the TC genesis frequency over the North Atlantic and the ENP often manifest out-of-phase variations on the interannual time scale (Wang and Lee 2009, Cao et al 2023).Consequently, the TC genesis frequency in the western hemisphere (including both the ENP and the North Atlantic), NASM, and tropical Pacific-Atlantic IBS anomalies may be closely linked within a unified climatic system.
In contrast to the NATC genesis frequency and the West African monsoon, both the NASM-NATC and the Western African monsoon-NATC relationships exhibit positive correlations.The NATC genesis frequency can be closely linked to both its eastern neighbor (the West African monsoon) and its western neighbor (the NASM).There might be a connection between the variabilities in the West African monsoon and the NASM, the possibility of which requires additional investigation.Further analysis by using numerical modelling experiments will also be necessary to reveal more differences of these two monsoons in relation to the NATC genesis.

Future research
Further exploration of the monsoon-TC relationship in the Southern Hemisphere (such as the Australian monsoon and the TC genesis frequency in the adjacent southwestern Pacific) is warranted in future research.Investigating monsoon-TC relationships in various regions worldwide contributes to a more holistic and comprehensive understanding of the global monsoon-TC relationship.
Several studies have separately examined the changes in NASM intensity and NATC genesis frequency in future warming scenarios (Lee and Wang 2014, He et al 2020, Murakami and Wang 2022, Karnauskas et al 2023).The NASM-NATC relationship under global warming may be different to that under the current climate background conditions.Further research will be needed to assess the potential change in the NASM-NATC synchronization in future scenarios.The future numerical sensitivity experimental analysis can be used to further confirm and quantify relative contributions of two possible pathways (tropical-ocean-driven pathway and monsoon-heating-driven pathway) for the NASM-NATC synchronization.

Conclusions
In this study, we performed statistical analyses using observations and CMIP6 models to exhibit the strong link between interannual variations of the NASM and NATC genesis frequency.In years with stronger (weaker) NASM intensity, the NATC genesis frequency tends to be higher (lower), which could be explained by simultaneous decreases (increases) of vertical wind shear over the MDR of NATC.Physical processes and mechanisms contributing to this NASM-NATC synchronization are investigated in this analysis.The linkage between the NASM and the frequency of NATC genesis has not yet been directly investigated in previous studies, and the findings in this study can fill the knowledge gap in this area.This research provides valuable insights into the NASM-NATC relationship, emphasizing the synchronization of these two systems.These results will also underpin better predictions of the NASM and the NATC genesis activities.
The observed NASM-NATC synchronization may be explained by two pathways: tropical-oceandriven pathway and monsoon-heating-driven pathway.Physical processes in the tropical-ocean-driven pathway are summarized in figure 4. In this tropicalocean-driven pathway, the tropical Pacific-Atlantic interbasin SST anomalies play a critical role in shaping and boosting the NASM-NATC synchronization.The tropical Pacific and Atlantic interbasin SST anomalies can bridge the NASM and NATC, by modulating the CCAW.The CCAW can serve as a bridge in linking the NASM and the NATC genesis frequency, by co-modulations of the vertical wind shear in the MDR and the anticyclones in the upper troposphere over the NASM.The tropical Pacific-Atlantic interbasin SST could be a key driver in the fluctuations of the CCAW through PNA atmospheric teleconnection and Gill-type atmospheric responses, thus further shaping the NASM-NATC synchronization.The CMIP6 models also support the important role of the tropical Pacific-Atlantic IBS anomalies in reproducing the NASM-NATC synchronization in the model simulation results.In the monsoonheating-driven pathway, the heating source due to the rainfall anomalies associated with the NASM can trigger atmospheric circulation anomalies through the Gill-type response, thereby affecting the NATC by changing the vertical wind shear over the MDR.Compared with the tropical-ocean-driven pathway, monsoon-heating-driven pathway may include more direct monsoon-to-TC causality.Compared with the monsoon-heating-driven pathway, the tropicalocean-driven pathway may provide more early forecast signals in the preceding spring season.
Overall, this study demonstrates a connection in interannual variations of the NASM and NATC genesis frequency and highlights the role of the tropical Pacific-Atlantic interbasin SST in shaping and boosting the NASM-NATC synchronization.This has important implications for climate prediction, by providing useful information for the prediction of the NASM and the NATC genesis frequency.The results of this study can be used to advance our understanding of the monsoon-TC relationship and increase to quantify variations of the anomalous cross-basin zonal winds over Central America in the upper troposphere.(3) An inter-basin SST (IBS) index is defined as the differences in the SST anomalies in the tropical Atlantic (5 • S-20 • N, 85 • W-15 • W) and tropical centraleastern Pacific (5 • S-5 • N, 170 • W-120 • W; i.e.Niño3.4 region), to quantify variations of the tropical Pacific-Atlantic interbasin SST anomalies.Because the Niño3.4region is utilized for the tropical Pacific portion in the IBS definition, the El Niño-Southern Oscillation (ENSO) information can also be included in the IBS index.

Figure 1 .
Figure 1.(a) The NATC genesis locations (black dots) and climatological 850 hPa zonal wind speed (shading) during the extended boreal summer (June-September) for the period 1979-2019.The green boxes are the northern and southern regions used to define the NASM index.The red box encompasses the MDR of the NATC.(b) The time series of NASM index and NATC genesis frequency for the period 1979-2019.(c) Scatter plot showing the relationship between the NASM index and NATC genesis frequency for the period 1979-2019.The correlation is shown in the scatter plot with an asterisk indicating that the correlation is significant at the 99% confidence level.

Figure 2 .
Figure 2. Composite GPI anomalies (shading) and NATC genesis density anomalies (contours, solid for positive and dashed for negative values) in the (a) strong NASM years and (b) weak NASM years.(c) Composite differences in the GPI anomalies and contributions of each term to GPI anomalies over the MDR according to equation (2) between the strong and weak NASM years.The dots in (a) and (b) indicate the difference is statistically significant at the 90% confidence level.The red boxes in (a) and (b) delineate the MDR.
Pacific-Atlantic IBSAccompanied by changes in vertical wind shear over the MDR, anomalous cross-basin zonal winds sometimes occur over Central America at 200 hPa (figures S6(b) and (c)).This is the CCAW phenomenon explored byWang et al (2016).The CCAW index exhibits significant correlations with the vertical wind shear and TC genesis frequency over the MDR (figures S10(a) and (c)).The correlation analysis further confirms the linkage between the vertical wind shear over the MDR and local TC genesis (figure S10(b)).The CCAW index and the NASM index also exhibit a significant correlation (r = − 0.71) (figureS10(d)).These results indicate that the CCAW can help to connect the NASM and the NATC genesis frequency.On the one hand, changes in CCAW can alter the vertical wind shear in the MDR region (figures S5 and S10(c)), thereby causing fluctuations in NATC genesis frequency (figureS10(b)).On the other hand, changes in CCAW can lead to variation in the intensity of anticyclones in the upper troposphere over the NASM region (figures S6(b) and (c)), thus influencing the NASM intensity (figureS10(d)).Therefore, changes in CCAW can be accompanied by synchronized changes in the intensity of the NASM and the NATC genesis frequency.Based on the above analysis, the CCAW can serve as a bridge in linking the NASM and the NATC genesis frequency.One related question naturally arises: what drives the interannual variations of the CCAW?The tropical SST anomalies may the important potential pacemaker for the fluctuations of the CCAW.

Figure 3 .
Figure 3. SST anomalies (shading) and 200 hPa wind anomalies (vector) obtained as regression onto the normalized (a) CCAM index, (b) NASM index, and (c) MDR TC genesis frequency.Only the values significant at the 90% confidence level or higher are shown.The purple boxes are the regions used to define the IBS index.
, results of which indicate that the IBS index can show better correlations with the NATC/NASM than the Niño3.4index.Particularly, the springtime IBS index tends to have better early forecasting signals for the following summertime NATC/NASM compared to the springtime Niño3.4 index.Only using the Niño3.4index may fail to capture non-ENSO signals and exhibit weaker predictive performance in the preceding spring season.Utilizing the IBS index provides a more comprehensive set of information compared to solely using the Niño3.4index.The need to treat tropical Pacific and Atlantic Oceans as a unified whole tends to be highlighted here, instead of only focusing on the tropical Pacific (e.g. the Niño3.4index).
formation of tropical Pacific-Atlantic IBS anomalies may involve a series of air-sea coupling processes (Wang 2006, Ham et al 2013, Wang et al 2017, Lv et al 2024).The persistence of tropical Pacific-Atlantic IBS anomalies from spring to summer may involve interactions between SST and atmospheric modes of internal variability (such as the north Atlantic oscillation) (Lv et al 2024).Tropical Pacific-Atlantic IBS anomalies can trigger positive feedback coupling with overlying atmospheric Walker circulation anomalies (Wang 2006).Spring tropical North Atlantic warming may promote the development of subsequent La Niña by triggering atmospheric Rossby waves and a series of air-sea coupling processes (Ham et al 2013, Wang et al 2017).

Figure 4 .
Figure 4.A schematic diagram showing the physical processes how the tropical Pacific-Atlantic interbasin SST anomalies contribute to shape the NASM-NATC synchronization.Cold SST anomalies over the tropical Pacific (blue shading) and warm SST anomalies over the tropical Atlantic (warm shading) are shown for the positive IBS index.The purple box in the upper level is the region applied to define the CCAW index.The blue boxes are the regions applied to define the IBS index.The black boxes are the regions used to define the NASM index.The red box delineates the MDR.
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