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Current coupled climate models contain large biases in simulating tropical cyclogenesis, reducing the confidence in tropical cyclone (TC) projection. In this study, we investigated the influence of sea surface temperature (SST) biases on TC genesis in the Coupled Model Intercomparison Project phase 6 simulations from 1979 to 2014. Positive TC genesis biases were found over the tropical central North Pacific (CNP) in most of climate models, including the high-resolution models. Compared to coupled models, TC genesis density (TCGD) simulations over CNP in uncoupled models forced by observational SST improved obviously. A warm SST bias over the tropical CNP in the coupled models is the main cause of TC genesis biases. The SST bias–induced diabatic heating leads to an anomalous Gill-type atmospheric circulation response, which contributes to a series of favorable environmental conditions for TC formation over the CNP. Numerical experiments were also performed with HiRAM to demonstrate the influence of SST biases on the TCGD simulation, further confirming our conclusion. The current results highlight the importance of improving TC simulation in state-of-the-art climate models by reducing SST simulation bias. [ABSTRACT FROM AUTHOR]

Tropical cyclone (TC) rapid intensification (RI) in the nearshore region often poses a serious threat to populated coastal regions. Although several previous studies have revealed an increased probability of RI events just before landfall in North America under global warming, it is unknown whether a similar increase has occurred along China coast. Here, we examine the long-term change of TC RI events along China coast during 1981–2020. Results show that the RI events along China coast have experienced a statistically significant increase since 1981, particularly in the latest decade. Further analysis reveals that the local sea surface temperature, maximum potential intensity, upper-level divergence, low-level vorticity, and vertical wind shear along China coast become increasingly favorable for TC RI occurrence. Among these factors, the intensified maximum potential intensity caused by the significant warming along China coast is the most important factor, most likely contributed by global warming instead of internal climate variability. This result is further confirmed by the CMIP6 model simulations. [ABSTRACT FROM AUTHOR]

Global tropical cyclone (TC) genesis frequency (TCGF) has been documented to decrease or increase linearly in a changing climate. However, our numerical experiments show that the global TCGF exhibits a parabolic relation with spatio‐uniform climate changes in sea surface temperature (SST) from −15 K to 5 K relative to the present climate, with the peak in the 5 K‐cooler climate. The parabolic relation is found in all TC basins except the eastern North Pacific where TCGF keeps increasing with the changing climate. TCGF can be expressed as the product of the frequency of TC seeds and the TC survival rate (SR). Further analysis shows that this parabolic structure in the global TCGF depends on TC seeds rather than the TC SR. The TC SR exhibits an increasing trend with the SST increase, while TC seeds show a consistent change with TCGF, which might be linked to the changes in low‐level relative humidity. Plain Language Summary: It is a hot‐debated question as to how tropical cyclone (TC) genesis frequency (TCGF) would change under global warming. To address this problem, we examine the TCGF change in response to a simplified spatio‐uniform change of sea surface temperature (SST) from −15 K to 5 K relative to the present climate. The global TCGF begins to increase with the global SST increase from −15 K, reaches peak in the 5 K‐cooler climate, and subsequently decreases with further SST warming, forming a parabolic relation between the TCGF and the SST changes. Further, we find the parabolic relationship in all TC basins except the eastern North Pacific where the TCGF keeps increasing with SST changes. TCGF can be expressed as the product of the frequency of TC seeds and the TC survival rate (SR). Further analysis shows the TC SR cannot explain the global TCGF change, while the changes in frequency of TC seeds associated with the low‐level relative humidity contribute to the parabolic change of TCGF. Key Points: The global tropical cyclone (TC) genesis frequency (TCGF) in response to uniform sea surface temperature (SST) changes shows a parabolic relation with a peak under 5 K‐cooler climate than current climateThe parabolic relation was found in all TC basins except the eastern North Pacific where the TCGF keeps increasing with SST changesSuch a parabolic relation was attributed to the change in TC seed frequency [ABSTRACT FROM AUTHOR]

Near‐term prediction of tropical cyclone (TC) activity is of great importance to formulate the long‐term plans for mitigating the potential TC‐induced damages. Here, we introduce a multi‐timescale regression model of TC genesis frequency (TCGF), which includes contributions by three interannual modes (El Niño‐Southern Oscillation, sea surface temperature anomalies over the eastern Indian Ocean and tropical North Atlantic), two interdecadal modes (the Atlantic Multidecadal Oscillation and the Interdecadal Pacific Oscillation), and a global warming mode. The model is shown to be able to capture well the present‐day multi‐timescale changes in TCGF in the major TC basins in the Northern Hemisphere. By combining the model and 100‐member simulations by the Max Planck Institute Earth System Model, we predict a robust increase in TCGF over the eastern North Pacific, an insignificant increase over the western North Pacific, and little change over the North Atlantic during 2020–2030 relative to 2009–2019. Plain Language Summary: How tropical cyclone (TC) activity will change in the next decade or so is especially important for government policymakers. However, the field on TC activity is very much in its infancy. Here we predict the near‐term changes in TC genesis frequency (TCGF) in the three main TC basins in the Northern Hemisphere, namely the western North Pacific, the eastern North Pacific, and the North Atlantic, by combining a new multi‐timescale regression model and the state‐of‐the‐art climate model projections. This regression model is constructed based on six key climate factors on different time scales, and can well reproduce the present‐day TCGF change during 1960–2019. The model predicts a significant increase in TCGF over the eastern North Pacific while insignificant change over the other two basins in the next 10 years. Key Points: We have developed a multi‐timescale regression model to represent changes in tropical cyclone (TC) genesis frequency (TCGF)The model can capture well the present‐day multi‐timescale changes in TCGF in the major TC basins in the Northern HemisphereThe model is used to conduct near‐term prediction of TCGF in the Northern Hemisphere by combining climate model projections [ABSTRACT FROM AUTHOR]

Observational and climate model analysis showed that the anomalous sea surface temperature in the north tropical Atlantic (NTA) in boreal spring can trigger El Niño–Southern Oscillation (ENSO) in the subsequent winter. Similarly, the climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) are known to reasonably simulate the NTA effect. Nevertheless, the strengths of the NTA effect on ENSO among the climate models are also diverse. In this light, we revisited the possible causes that contributed to the different NTA effects on ENSO in the CMIP5 climate models. We found that the strength of the NTA triggering ENSO in the climate model tended to be proportional to the intensity of the climatological subtropical North Pacific high system in boreal spring. The stronger climatological subtropical North Pacific high accompanied enhanced trade wind, precipitation reduction, and cold sea surface temperature over the subtropics. Under these conditions, the moist static energy feedback process, also known as the moist enthalpy advection mechanism, effectively operated around the Pacific intertropical convergence zone. That is, the NTA-induced signals in the subtropical North Pacific readily intruded into the deep tropical Pacific with the aid of the feedback processes, leading to an ENSO event. Consistent with the CMIP5 analysis results, the observed NTA effect on ENSO became stronger during the decades when the climatological North Pacific subtropical high intensified, underpinning the importance of climatology in the subtropical North Pacific in the NTA–ENSO connection. [ABSTRACT FROM AUTHOR]

The multi‐model ensemble mean sea surface temperature (SST) of CMIP5 models shows an El Niño‐like Pacific warming (PW) trend, contradictory with the observational result which manifests a La Niña‐like PW pattern. Here, we demonstrate that these two SST PWs coexist in the CMIP5 models and they are largely determined by the model's tropical Indo‐Pacific SST gradient. When the Pacific warms faster than the Indian Ocean (IO), the model tends to project an El Niño‐like PW pattern. In contrast, a La Niña‐like warming trend prevails if a more rapid IO warming is simulated. We suggest that the PW pattern in an individual model is nonstationary and may transform from a La Niña‐like to an El Niño‐like when its interbasin SST gradient changes with more robust warming in the Pacific, and vice versa. Our conclusions shed great light on the future SST PW pattern change projected by the coupled models. Plain Language Summary: Previous studies demonstrate that an El Niño‐like SST warming pattern prevails in a majority of CMIP5 models. But a La Niña‐like warming pattern is detected based on historical SST records. This inconsistency between the observations and model simulation triggers an intense debate in the climate community. In this study, we find that both El Niño‐like and La Niña‐like SST warming patterns can be identified in the CMIP5 model simulations. The warming pattern is largely determined by the SST gradient between the Indian and Pacific oceans. Distinct from previous studies, an El Niño‐like pattern in an individual model can shift to a La Niña‐like pattern when the Indian warming is stronger than the Pacific warming trend, and vice versa. Key Points: El Niño‐like and La Niña‐like Pacific sea surface temperature warming patterns coexist and are interchangeable in climate model projectionsThe tropical Pacific warming pattern is primarily determined by a zonal gradient in the Indo‐Pacific sea surface warmingThis Pacific warming pattern diversity sheds light on future sea surface temperature change under greenhouse gases concentrations [ABSTRACT FROM AUTHOR]

Using the ensemble hindcasts of the European Centre for Medium-Range Weather Forecasts (ECMWF) coupled model for the period of 1980–2005, spatiotemporal evolution in the covariability of sea surface temperature (SST) and low-level winds in the ensemble mean and spread over the tropical Atlantic is investigated with the month-reliant singular value decomposition (SVD) method, which treats the variables in a given monthly sequence as one time step. The leading mode of the ensemble mean represents a coevolution of SST and winds over the tropical Atlantic associated with a phase transition of El Niño from the peak to decay phase, while the second mode is related to a phase transition from El Niño to La Niña, indicating a precursory role of the north tropical Atlantic (NTA) SST warming in La Niña development. The leading mode of ensemble spread in SST and winds further illustrates that an NTA SST anomaly acts as a precursor for El Niño–Southern Oscillation (ENSO). A north-tropical pathway for the delayed effect of the NTA SST anomaly on the subsequent ENSO event is identified; the NTA SST warming induces the subtropical northeast Pacific SST cooling through the modulation of a zonal–vertical circulation, setting off a North Pacific meridional mode (NPMM). The coupled SST–wind anomalies migrate southwestward to the central equatorial Pacific and eventually amplify into a La Niña event in the following months due to the equatorial Bjerknes feedback. Ensemble spread greatly increases the sample size and affords insights into the interbasin interactions between the tropical Atlantic and Pacific, as demonstrated here in the NTA SST impact on ENSO. [ABSTRACT FROM AUTHOR]

Numerous studies have demonstrated that the North Pacific Meridional Mode (NPMM) plays an important role in determining El Niño‐Southern Oscillation (ENSO) events in the following winter season. However, little attention has been given to significant differences among its spatial patterns. Here we show that the NPMM exhibits a large diversity in spatial patterns, leading to distinct impacts on ENSO. Based on objective clustering analysis, two distinct spatial patterns of NPMM are detected. Cluster 1 (C1) NPMM exhibits a strong sea surface temperature dipole over the subtropical eastern Pacific and midlatitude central Pacific whereas Cluster 2 (C2) features a dipole over the subtropical eastern Pacific and equatorial cold tongue region. We find that the C1 NPMM is strongly linked to following ENSO events while the C2 NPMM has no statistically significant relation. This gives new implications for ENSO dynamics and predictions. Key Points: The North Pacific Meridional Mode (NPMM) has strong spatial diversity that can be separated into two groupsThese two types of the spring NPMM have distinct impacts on ENSO events in the following winterWe suggest an optimal pattern of the spring NPMM for ENSO prediction [ABSTRACT FROM AUTHOR]

How future tropical cyclone (TC) activity could change under global warming is enormously important to society, which has been widely assessed using state‐of‐the‐art climate models. However, these models were predominantly based on projection of an El Niño‐like warming pattern. Recent studies suggested that a La Niña‐like warming pattern is also possible. Here we compare the responses of TC track density (TCTD) over the western North Pacific and North Atlantic to the two distinct global warming patterns. We find that the La Niña‐like warming pattern reduces western North Pacific TCTD except in the South China Sea and along China coast and increases NA TCTD, while the El Niño‐like warming pattern generally reduces TCTD in both basins. This is due to different responses of large‐scale dynamic/thermodynamic conditions to the distinct zonal sea surface temperature gradients associated with the two warming patterns. These results help better understand potential future change in TC tracks. Key Points: The La Niña‐like warming reduces (increases) TC track density in the main WNP (NA), while the El Niño‐like warming reduces it in two basinsThe different responses are due to the different tropical zonal SST gradients associated with the two warming patternsChina and North America would experience more landfalling TCs in La Niña‐like warming pattern than in El Niño‐like warming pattern [ABSTRACT FROM AUTHOR]

This study attempts to evaluate quantitatively the contributions of sea surface temperature (SST) anomalies in the Indo-Pacific Ocean to the interannual variability of tropical cyclone (TC) genesis frequency (TCGF) over the western North Pacific (WNP). Three SST factors in the Indo-Pacific Ocean are found to play key roles in modulating the interannual variability of WNP TCGF. They are summer SST anomaly in the east Indian Ocean (EIO), the summer El Niño–Southern Oscillation Modoki index (EMI), and the spring SST gradient (SSTG) between the southwestern Pacific and the western Pacific warm pool. Results show that the three factors together can explain 72% of the total variance of WNP TCGF in the typhoon season for the period 1980–2015. Among them, the spring SSTG and the summer EIO contribute predominantly to the interannual variability of TCGF, followed by the summer EMI, with respective contributions being 39%, 38%, and 23%. Further analysis shows that the summer EMI was affected significantly by the spring SSTG and thus had a relatively lower contribution to the TCGF than the spring SSTG. In addition, a statistical model is constructed to predict the WNP TCGF in the typhoon season by a combination of the May EIO and the spring SSTG. The new model can reproduce well the observed WNP TCGF and shows an overall better skill than the ECMWF Seasonal Forecasting System 5 (SEAS5) hindcasts. This statistical model provides a good tool for seasonal prediction of WNP TCGF. [ABSTRACT FROM AUTHOR]

Previous studies have documented an abrupt decrease of tropical cyclone (TC) genesis frequency over the western North Pacific (WNP) since 1998. In this study, results from an objective clustering analysis demonstrated that this abrupt decrease is primarily related to the decrease in a cluster of TCs (C1) that mostly formed over the southeastern WNP, south of 15°N and east of the Philippines, and possessed long tracks. Further statistical analyses based on both best track TC data and global reanalysis data during 1980–2015 revealed that the genesis of C1 TCs was significantly modulated by the interdecadal Pacific oscillation (IPO), whose recent negative phase since 1998 corresponded to a La Niña–like sea surface temperature anomaly (SSTA) pattern, which strengthened the Walker circulation in the tropical Pacific and weakened the WNP monsoon trough, suppressing genesis of C1 TCs in the southeastern WNP and predominantly contributing to the decrease in TC genesis frequency over the entire WNP basin. These findings were further confirmed by results from similar analyses based on longer observational datasets and also the outputs from a 500-yr preindustrial general circulation model experiment using the Geophysical Fluid Dynamics Laboratory (GFDL) Coupled Model, version 3. Additional analysis indicates that the decrease in C1 TC genesis frequency in the recent period was dominated during August–October, with the largest decrease in October. [ABSTRACT FROM AUTHOR]

The sea surface temperature gradient (SSTG) between the southwest Pacific (SWP; 40°-20°S, 160°E-170°W) and the western Pacific warm pool (WWP; 0°-16°N, 125°-165°E) in boreal spring was revealed to contribute significantly to the interannual variability of tropical cyclone (TC) genesis frequency over the western North Pacific (WNP) since 1980. The present study extends the analysis back to 1951. Results show that the interannual relationship between the SSTG and WNP TC genesis frequency is statistically significant only after the mid-1970s while insignificant prior to 1974. Further analyses show that the SSTG is strongly negatively correlated with the SST anomaly (SSTA) in the central Pacific (CP) in the recent period while with the SSTA in the equatorial eastern Pacific (EP) in the prior period. In the prior period, the spring positive (negative) EP SSTA induces an anticyclonic (cyclonic) circulation over the WNP. This partly offsets the impact of the SSTG anomaly on the atmospheric circulation over the WNP. In addition, the SSTG anomaly in spring shows poor persistence through summer. As a result, the relationship between the spring SSTG anomaly and WNP TC genesis frequency is largely weakened in the prior period. In the recent period, the response of the atmospheric circulation over the WNP to the CP SSTAs enhances the atmospheric circulation response to the spring SSTG anomaly, thus strengthening the relationship between the SSTG and WNP TC genesis frequency. These results are further confirmed by the atmospheric general circulation model (AGCM) simulations. [ABSTRACT FROM AUTHOR]

Previous studies have shown that the Pacific Meridional Mode (PMM) has a crucial impact on the central Pacific El Niño–Southern Oscillation (ENSO). In this study, we redefined the PMM index used previously by focusing on the sea surface temperature in the subtropical northeastern Pacific (NEP), and found that the correlations between the PMM and ENSO are distinct on different timescales. We refer to this redefined index as PMM NEP. Results show that a positive PMM is weakly related to El Niño, while a negative PMM is more convincingly related to La Niña, specifically, the equatorial negative SSTAs possibly affecting negative PMM events. Moreover, the cause–effect relation between the PMM NEP and ENSO is much more significant on a decadal timescale than on an interannual timescale, and their interaction may be related to the equatorial zonal wind field. Our finding indicates that a La Niña-like decadal pattern is established through the combined actions of the PMM and ENSO, rather than an El Niño-like decadal pattern. • The PMM index defined in this study removes the signals of ENSO more effectively. • A positive PMM is weakly related to El Niño, while a negative PMM is more convincingly related to La Niña. • The relation between the PMM and ENSO is much more significant on a decadal timescale than on an interannual timescale. [ABSTRACT FROM AUTHOR]

Previous studies have documented the significant influences of the North Pacific subtropical sea surface temperature (SST) front (NPSTF) on the overlying atmospheric circulation. However, whether and how the NPSTF could affect remote circulations has not been well addressed. Here, we demonstrate that the enhanced NPSTF can induce a southward shift of the downstream North Atlantic westerly jet stream (NAWJS) in winter. A strong NPSTF with an increased meridional SST gradient induces a positive (negative) air temperature anomaly in its south (north) flank through the heat flux, leading to a direct thermal wind response and an indirect transient eddy feedback. The direct thermal wind response benefits a negative height anomaly south of Gulf of Alaska below 250 hPa and a positive one above 200 hPa, while the transient eddy activity could locally enhance and tilt northward with height. The kinetic energy significantly grows from the baroclinic energy conversion below 250 hPa over the NPSTF and from the barotropic energy conversion north of the NPSTF at the upper levels. This enhanced transient eddy activity gives rise to a negative height anomaly south of Gulf of Alaska through high-frequency transient eddy feedback forcing at all levels dominated by transient eddy vorticity flux. The anomalous low south of Gulf of Alaska favors an anomalous high over Canada and an anomalous low over the United States through Rossby energy propagation, causing the southward shift of the NAWJS. The numerical model further verifies the downstream influences of the NPSTF on the NAWJS in winter. • In addition to local effects, NPSTF can deliver its effect further downstream and modulate the meridional shift in NAWJS. • The height response south of Alaska Bay fosters downstream wind and height anomalies via Rossby energy propagation. • The transient eddy activity jointly induced by BTEC and BCEC dominates the local atmospheric response of NPSTF. [ABSTRACT FROM AUTHOR]