On the Role of Sea Surface Temperature in the Clustering of Global Tropical Cyclone Formation

<p>Global warming influences tropical cyclones (TC) and their impacts in different ways. Warmer sea surface temperatures (SST) are expected to lead to stronger intensification, the increased water holding capacity of warmer air increases the precipitation brought by TCs. These are thermodynamic changes that are rather well understood.</p><p>When it comes to the influence of circulation changes on tropical cyclone activity open questions remain: Will there be more or less TCs in a warmer world? And what would be the physical mechanism for a change in TC frequencies?</p><p>TC formation and intensification not only depends on the available energy but also on the large-scale atmospheric circulation. For instance, TC development is strongly hampered when the vertical wind shear (difference between upper and lower level wind speeds) is high.</p><p>Here we present a tropical cyclone season emulator for the Atlantic basin that produces TCs based on SSTs averaged over the Atlantic main development region and daily time series of weather patterns obtained from a self-organizing map clustering. The emulator is based on probabilities for storm genesis, storm length and intensity changes that were empirically assessed using the ERA5 reanalysis and IBTrACS TC observations.</p><p>We see different applications for this emulator: <br>1) While most global circulation models (GCM) fail to adequately simulate TCs, their projections for SSTs and large-scale weather patterns contains valuable information. Using our emulator, we could indirectly analyse TC activity projections for all available GCMs. <br>2) In the emulator thermodynamic (SSTs) and dynamic influences (weather patterns) are distinct inputs. This allows us to construct different counterfactuals to attribute changes in TC activity to thermodynamic or dynamic changes. For example, the emulator could be used to simulate TC seasons with large scale circulation as observed in 2017 but with preindustrial SSTs, so as to analyse the extent to which warming of the ocean surface had contributed to the extreme hurricane season of 2017.</p>

Global tropical cyclone (TC) activity is influenced by the tropical Pacific mean state, which is modulated by the El Niño–Southern Oscillation (ENSO) phenomenon at interannual time scales. In recent decades, observations show a La Niña–like trend in the tropical Pacific, while most global climate models simulate a more El Niño–like trend. The cause of this inconsistency remains under debate, but it is likely quite consequential for global TC activity. This study examines the response of global TC activity to sea surface temperature (SST) warming patterns using historical simulations from the High Resolution Model Intercomparison Project (HighResMIP): atmosphere-only simulations forced by observed SSTs, which exhibit a La Niña–like SST trend in the tropical Pacific; and coupled simulations, showing a weaker La Niña–like SST trend. TC activity is compared between the two sets of simulations using multiple statistics of TC activity, along with TC-relevant large-scale environmental fields. The TC genesis index (TCGI) is used to quantify the contributions of each environmental field to changes in TC activity. Results show that the forced global TC response is sensitive to the tropical Pacific mean state and varies by basin. In SST-forced simulations, TC activity’s trend patterns resemble the TC anomalies during La Niña events. In those simulations, thermodynamic environmental fields, namely, potential intensity and column-relative humidity, induce zonal variations in TC frequency, while dynamical fields, namely, vertical wind shear and absolute vorticity, induce meridional variations in TC frequency. In contrast, coupled simulations show no consistent TC response across HighResMIP models. Significance Statement Global tropical cyclone (TC) activity is influenced by the sea surface temperature pattern in the tropical Pacific. In recent decades, observations show strengthening zonal (west minus east) and meridional (off-equator minus equator) sea surface temperature gradients, whereas most climate models simulate the opposite trends. This study examines how global TC activity responds to the sea surface warming patterns using climate model simulations. Results show that TC activity is sensitive to changes in the tropical Pacific mean state, with basin-dependent responses. In models forced with strengthening gradients, TC activity changes resemble those during La Niña events, featuring anomalously positive zonal and meridional gradients in sea surface temperature. However, no consistent signals for TC activity have been found in models with weakening gradients.

This study examines the role of tropical dynamics in the formation of global tropical cyclone (TC) clusters. Using theoretical analyses and idealized simulations, it is found that global TC clusters can be produced by the internal dynamics of the tropical atmosphere, even in the absence of all landmass surface and zonal sea surface temperature (SST) anomalies. Theoretical analyses of a two-dimensional InterTropical Convergence Zone (ITCZ) model reveal indeed some large-scale stationary waves whose zonal and meridional structures could support the formation of TC clusters at the global scale. Additional idealized simulations using the Weather Research and Forecasting (WRF) model confirm these results for a range of experiments. Specifically, the examination of two common tropical wave types including the equatorial Rossby (ER) wave and the equatorial Kelvin (EK) wave shows that ER waves could develop a stationary structure for a range of zonal wavenumbers $m\in[5-11]$, while EK waves do not. This modeling result is consistent with the ITCZ analytical model and suggests that large-scale ER waves could support stationary "hot spots" for global TC formation without any zonal SST anomalies. The findings in this study offer different insights into the importance of tropical waves in producing global TC clusters beyond the traditional explanation based on zonal SST variability.   

Whereas some studies linked the enhanced tropical cyclone (TC) formation in the North Atlantic basin to the ongoing global warming, other studies attributed it to the warm phase of the Atlantic multidecadal oscillation (AMO). Using the National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory (ESRL) Twentieth Century Reanalysis (20CR) dataset, the present study reveals the distinctive spatial patterns associated with the influences of the AMO and global warming on TC formation in the North Atlantic basin.Two leading empirical orthogonal function (EOF) patterns are identified in the climate change of TC formation on time scales longer than interannual. The first pattern is associated with the AMO and its spatial pattern shows the basin-scale enhancement of TC formation during the AMO positive phase. The second pattern is associated with global warming, showing enhanced TC formation in the east tropical Atlantic (5°–20°N, 15°–40°W) and reduced TC formation from the southeast coast of the United States extending southward to the Caribbean Sea. In the warm AMO phase, the basinwide decrease in vertical wind shear and increases in midlevel relative humidity and maximum potential intensity (MPI) favor the basinwide enhancement of TC formation. Global warming suppresses TC formation from the southeast coast of the United States extending southward to the Caribbean Sea through enhancing vertical wind shear and reducing midlevel relative humidity and MPI. The enhanced TC formation in the east tropical Atlantic is due mainly to a local increase in MPI or sea surface temperature (SST), leading to a close relationship between the Atlantic SST and TC activity over the past decades.

Despite the observed high correlation between the Atlantic sea surface temperature (SST) and the Atlantic tropical cyclone (TC) activity, interpretation of this relationship remains uncertain. This study suggests that the tropical Atlantic sea surface warming induces a pair of anomalous low-level cyclones on each side of the equator, providing favorable conditions for enhancing TC formation east of 45°W, while the effect of SST warming in the tropical Indian Ocean and Pacific Ocean tends to suppress the TC formation. Over the past 30 years (1978–2007), the TC activity in the Atlantic basin is characterized with significant enhancement of TC formation east of 45°W, where the total TC number increased significantly compared to the period 1948–77. Despite the possible undercount of TCs, this study shows that the recently enhanced TC formation may not be totally accounted for by the poor TC observing network prior to the satellite era. The Atlantic sea surface warming that occurred in recent decades might have allowed more TCs to form, to form earlier, and to take a longer track, while the effect is partially offset by the SST warming in Indian and Pacific Oceans. This study suggests that the close relationship between the Atlantic SST and TC activity over the past 30 years, including basinwide increases in the average lifetime, annual frequency, proportion of intense hurricanes, and annual accumulated power dissipation index (PDI), as reported in previous studies, is mainly a result of the SST warming in the tropical Atlantic exceeding that in the tropical Indian and Pacific Oceans. The results agree with recent argument that the relative Atlantic SST change or the SST difference between the tropical Atlantic and other oceans play an important role in controlling long-term TC activity in the Atlantic basin.

Sea surface temperature (SST) anomalies in the Pacific, Indian and Atlantic oceans were suggested to explain inter-annual variability of tropical cyclone (TC) activity over the western North Pacific (WNP). Here we show that the influences of these “trans-basin” SST anomalies in the three oceans can be collectively understood via two leading modes of variability of WNP subtropical high (WNPSH). The first mode, which is forced by SST anomalies in the eastern-central Pacific and tropical Atlantic, can shift TC formation locations southeastward/northwestward, but has insignificant influence on the total TC genesis number, albeit affects the TC tracks, total number of tropical storm days, and power dissipation index (PDI). The second mode, which is a coupled ocean–atmosphere mode associated with a dipole SST anomaly in the Indo-Pacific warm pool, has a significant control on the total TC genesis number. A set of physics-based empirical models is built to predict the two WNPSH modes and TC activity (genesis number, tropical storm days and PDI) in the peak TC season (July–September) with preceding season trans-basin SST predictors. The predictions capture very well the inter-annual variabilities of the WNPSH and reasonably well the variability of WNP TC activity. These results thus establish a unified framework to understand and forecast the inter-annual variability in TC activity over the WNP.

This study examines the teleconnection between sea surface temperature (SST) in different ocean regions and tropical cyclone (TC) activity affecting Vietnam’s coastal region. Using spatial correlation and principal component analyses, it is found that the variability of TCs affecting Vietnam during 1982–2018 is remotely connected with SST in the Indian Ocean, the southwestern Pacific Ocean, and the northern Philippine Sea. Among the three regions, SST in the northern Philippine Sea displays the most significant inverse relationship with TC activity in the South China Sea (SCS), with lower June–November TC accumulated energy (ACE) for warmer northern Philippine Sea SST. Further analyses of large-scale atmospheric circulations show that this teleconnection between the northern Philippine Sea SST and TC activity in the SCS is linked to the East Asian subtropical jet (EASJ). Principal component analyses of the 200-hPa zonal wind associated with EASJ capture indeed a strong relationship between the second principal component, which characterizes the EASJ intensity, and ACE. Specifically, higher EASJ intensity corresponding to colder northern Philippine Sea SST would enhance large-scale ascending motion and low-level cyclonic anomalies in the SCS, which are favorable for TC formation and result in an overall increased ACE. Examination of the correlation between this second principal component and the northern Philippine Sea SST confirms that this correlation is statistically significant at a 95% confidence level. In this regard, these results support the Pacific–Japan teleconnection between the northern Philippine Sea SST and TC activity in the SCS.

Mining geophysical parameters through decision-tree analysis to determine correlation with tropical cyclone development

<p>Tropical cyclone activity over the western North Pacific (WNP) is subjected to impacts of sea surface temperature (SST) anomalies in the three tropical oceans. In this talk, the interannual variations in the tropical cyclone (TC) over the WNP and the influences of regional SST anomalies are documented by separating the WNP into four quadrants considering SST-induced non-uniform environmental changes. It will be shown that the TC variations in the northwest and southeast quadrants are related to both equatorial central-eastern Pacific (EPO) and tropical Indian Ocean (TIO) SST anomalies. The TC variation in the northeast quadrant is mainly related to tropical North Atlantic Ocean (TNA) SST anomalies. The main environmental variables differ for the TC variations in the four quadrants. Low-level (850-hPa) vorticity is important for the TC variations in the northwest, southwest and southeast quadrants. Mid-level (700-hPa) humidity contributes to the TC variations in the northwest, northeast and southeast quadrants. The vertical shear has a supplementary contribution to the TC variation in the southeast quadrant. The potential intensity negatively affects the TC variations in the southwest and southeast quadrants. The remote SST anomalies modulate different environmental variables over the WNP. The TIO SST influence is manifested in the low-level vorticity and vertical motion. The TNA SST impact occurs through the low-level vorticity change. The EPO SST effect occurs via changing the low-level vorticity and vertical motion as well as the mid-level moisture and vertical shear. The environmental variables experience more prominent changes when SST anomalies coexist in two remote regions. Numerical experiments confirm the EPO and TIO SST anomaly impacts on the environmental conditions affecting the WNP TC variations.</p>

Previous studies found that tropical cyclone (TC) formation is generally suppressed over the western North Pacific (WNP) following strong El Niño events. The 2015/2016 event is identified as one of the three major El Niño events since 1950. However, a climatological average of 26 named TCs occurred over the WNP in 2016. The plausible causes for this inconsistency are investigated in this study.By examining the historical records, we also found that 28 named TCs occurred over the WNP following 1991/1992 El Niño. For most strong El Niño cases, the suppressed TC formation in the ensuing early season (January–June) can persist to the peak season and lead to the negative TC frequency anomalies. However, TC formation turns to be active during August–October in 1992 and 2016, offsetting the suppressed TC formation in the early season and thus resulting in the climatological annual TC counts. It is found that anomalous sea surface temperature (SST) cooling over the north Indian Ocean and SST warming over the tropical North Pacific contribute to the enhanced TC formation in 1992 by stimulating an anomalous cyclonic circulation over the WNP, while the tri‐polar SST pattern across the tropical Indo‐western Pacific Ocean and the related convergence zone over 130°–160°E are responsible for the enhanced TC formation in 2016. The results indicate the crucial role of SST evolution over the north Indian Ocean and tropical Pacific in TC formation following strong El Niño events, which has important implication for the seasonal forecasting of TC activity over the WNP.

The impact of climate change on tropical cyclone (TC) activity is often assessed by various downscaling approaches, statistical–dynamical frameworks, and high-resolution global climate models using the projected changes of environmental factors. Uncertainty in simulating and projecting TC-relevant, large-scale circulation is closely linked to the projection of TC activity in a warming climate. Based on the model output in phase 6 of the Coupled Model Intercomparison Project (CMIP6), this study examines the intermodel biases in simulating the western North Pacific monsoon trough (MT), which is one of the most important large-scale circulation systems for TC activity, especially TC formation. It is found that most CMIP6 models can successfully simulate the climatological mean structure of the MT, although considerable biases remain in its exact location and its simulated historical changes. The mean latitude of the simulated MT spreads between 10° and 20°N, with noticeable differences in its orientation. The multimodel ensemble mean indicates that the MT exhibits no significant long-term zonal and poleward shifts in the future scenarios, consistent with the projection in the selected models in which the simulated MT resembles the observed spatiotemporal characteristics of the counterpart. Further analysis suggests that the intermodel bias in the simulated MT location is closely related to the east–west contrast of sea surface temperature (SST) anomalies in the tropical Pacific. More attention is required on improving the simulation of the basinwide SST distribution and its associated MT to reduce the uncertainty in predicting the future location of TC formation.

Previous study detected an intensified impact of the El Niño‐Southern Oscillation (ENSO) Modoki sea surface temperature (SST) anomalies on the tropical cyclone (TC) activity over the western North Pacific (WNP) after the early 1990s and attributed it to an expansion in areal coverage of the equatorial central Pacific (ECP) SST anomalies. This study identifies the contribution of SST anomalies in several other regions to this inter‐decadal change in the relationship between ENSO Modoki SST and the WNP TC genesis. Before the early 1990s, the positive ECP SST anomalies induce an anomalous lower‐level cyclone and consequently an increase in the TC genesis frequency over the southern part of the WNP, and the positive ECP and tropical Indian Ocean SST anomalies together induce an anomalous lower‐level anticyclone and accordingly a decrease in the TC genesis frequency over the northern part of the WNP. As such, the relationship between the ECP SST and the WNP TC genesis frequency is weak. After the early 1990s, the positive ECP SST anomalies with a large areal coverage induce a large anomalous lower‐level cyclone covering most of the WNP and thus an increase in the TC genesis frequency over the WNP. Meantime, the tropical northern Atlantic and western South Pacific SST anomalies during spring enhance the succeeding summer–autumn WNP atmospheric circulation response to the ECP SST anomalies through an Atlantic–Pacific teleconnection and a wind‐evaporation positive feedback, respectively. This strengthens the relationship between the ECP SST and the WNP TC genesis frequency.

While the Atlantic basin experienced the busiest hurricane season in 2020, the typhoon activity over the western North Pacific (WNP) was also record‐setting with no tropical cyclone (TC) formation in July 2020, which is the first time in available historical records. The unprecedented absence of TC formation is consistent with the extremely unfavorable large‐scale conditions that are linked to an anomalous anticyclonic circulation over the WNP, which results mainly from sea surface temperature (SST) anomalies across the tropical oceans. Numerical experiments suggest that the tropical Indian Ocean SST anomalies, which were warmest in July 2020 since 1965, play a dominant role in the anomalous WNP anticyclonic circulation, while the increased zonal SST gradient across the tropical Pacific and the Atlantic SST warming also have important contributions. This study suggests that the configuration of SST anomalies across the tropical basins is quite important to the WNP TC activity.

Global tropical cyclone (TC) activity is simulated by the Geophysical Fluid Dynamics Laboratory (GFDL) Climate Model, version 2.5 (CM2.5), which is a fully coupled global climate model with a horizontal resolution of about 50 km for the atmosphere and 25 km for the ocean. The present climate simulation shows a fairly realistic global TC frequency, seasonal cycle, and geographical distribution. The model has some notable biases in regional TC activity, including simulating too few TCs in the North Atlantic. The regional biases in TC activity are associated with simulation biases in the large-scale environment such as sea surface temperature, vertical wind shear, and vertical velocity. Despite these biases, the model simulates the large-scale variations of TC activity induced by El Niño–Southern Oscillation fairly realistically. The response of TC activity in the model to global warming is investigated by comparing the present climate with a CO2 doubling experiment. Globally, TC frequency decreases (−19%) while the intensity increases (+2.7%) in response to CO2 doubling, consistent with previous studies. The average TC lifetime decreases by −4.6%, while the TC size and rainfall increase by about 3% and 12%, respectively. These changes are generally reproduced across the different basins in terms of the sign of the change, although the percent changes vary from basin to basin and within individual basins. For the Atlantic basin, although there is an overall reduction in frequency from CO2 doubling, the warmed climate exhibits increased interannual hurricane frequency variability so that the simulated Atlantic TC activity is enhanced more during unusually warm years in the CO2-warmed climate relative to that in unusually warm years in the control climate.

Tropical cyclones are atmospheric phenomena that occur in warm ocean areas, including the northern and southern regions of Indonesia. Although rarely formed directly in Indonesia, tropical cyclones from the Pacific and Indian Oceans often have a significant impact on weather conditions in Indonesia. This study was conducted to identify the influence of Sea Surface Temperature (SST) on the formation and development of tropical cyclones using a machine learning approach with a Random Forest model. The data used was in the form of reanalysis of SST data sourced from ECMWF (European Center for Medium - Range Weather Forecast) and analyzed for six tropical cyclone events representing the northern and southern regions of Indonesia. In addition, this study also aims to evaluate the effectiveness of machine learning-based prediction models in predicting SST parameters by using evaluation metrics such as RMSE, MAE, and R² to ensure prediction accuracy. The results showed that the SST values that supported the formation of tropical cyclones ranged from 31–33°C, which corresponds to the minimum temperature criteria for the formation of tropical cyclone systems. The Random Forest model showed excellent performance with low RMSE and MAE scores, and an R² value close to 1 in all cases tested with Tropical Cyclone Dahlia being the best case with the highest prediction accuracy. This study shows that the Random Forest model is able to effectively capture complex patterns of SST and provide accurate predictions, potentially as an instrument to understand and mitigate risks associated with tropical cyclone events in Indonesia.