Main Development Region Research Articles

Multidecadal Covariability of North Atlantic Sea Surface Temperature, African Dust, Sahel Rainfall, and Atlantic Hurricanes

Abstract Most studies of African dust and North Atlantic climate have been limited to the short time period since the satellite era (1980 onward), precluding the examination of their relationship on longer time scales. Here a new dust dataset with the record extending back to the 1950s is used to show a multidecadal covariability of North Atlantic SST and aerosol, Sahel rainfall, and Atlantic hurricanes. When the North Atlantic Ocean was cold from the late 1960s to the early 1990s, the Sahel received less rainfall and the tropical North Atlantic experienced a high concentration of dust. The opposite was true when the North Atlantic Ocean was warm before the late 1960s and after the early 1990s. This suggests a novel mechanism for North Atlantic SST variability—a positive feedback between North Atlantic SST, African dust, and Sahel rainfall on multidecadal time scales. That is, a warm (cold) North Atlantic Ocean produces a wet (dry) condition in the Sahel and thus leads to low (high) concentration of dust in the tropical North Atlantic, which in turn warms (cools) the North Atlantic Ocean. An implication of this study is that coupled climate models need to be able to simulate this aerosol-related feedback in order to correctly simulate climate variability in the North Atlantic. Additionally, it is found that dust in the tropical North Atlantic varies inversely with the number of Atlantic hurricanes on multidecadal time scales because of the multidecadal variability of both direct and indirect influences of dust on vertical wind shear in the hurricane main development region.

Impacts of non‐canonical El Niño patterns on Atlantic hurricane activity

The impact of non‐canonical El Niño patterns, typically characterized by warmer than normal sea surface temperatures (SSTs) in the central tropical Pacific, on Atlantic tropical cyclone (TC) is explored by using composites of key Atlantic TC indices and tropospheric vertical wind shear over the Atlantic main development region (MDR). The highlight of our major findings is that, while the canonical El Niño pattern has a strong suppressing influence on Atlantic TC activity, non‐canonical El Niño patterns considered in this study, namely central Pacific warming, El Niño Modoki, positive phase Trans‐Niño, and positive phase Pacific meridional mode, all have insubstantial impact on Atlantic TC activity. This result becomes more conclusive when the impact of MDR SST is removed from the Atlantic TC indices and MDR wind shear by using the method of linear regression. Further analysis suggests that the tropical Pacific SST anomalies associated with the non‐canonical El Niño patterns are not strong enough to cause a substantial warming of the tropical troposphere in the Atlantic region, which is the key factor that increases the wind shear and atmospheric static stability over the MDR. During the recent decades, the non‐canonical El Niños have been more frequent while the canonical El Niño has been less frequent. If such a trend continues in the future, it is expected that the suppressing effect of El Niño on Atlantic TC activity will diminish and thus the MDR SST will play a more important role in controlling Atlantic TC activity in the coming decades.

Large-Scale Diagnostics of Tropical Cyclogenesis Potential Using Environment Variability Metrics and Logistic Regression Models

Abstract The authors propose that inclusion of medium- to high-frequency variability information will provide improved metrics of tropical cyclogenesis (TCG) applicable to climate GCM diagnostics. Capabilities of the Community Atmosphere Model version 3.1 (CAM3.1) GCM are assessed for detecting both large-scale and localized conditions for TCG in the tropical North Atlantic by comparison with the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) and observed TCG occurrences. CAM3.1 seasonality of large-scale environmental factors conducive to TCG compares favorably with the ERA-40. It is determined that most of the TCG-related high-frequency temporal variability in the ERA-40 is explained by dynamic variability; each of the CAM3.1 ensemble members has lower variability in these dynamic fields. Seventeen environmental variables are evaluated as potential indicators of TCG activity based on daily anomalous variability with respect to a 15-day base period. Principal component analysis (PCA) is employed to synthesize these into a set of uncorrelated parameters. ERA-40 PCA composite variables are used to develop logistic and Poisson regression models for TCG detection and frequency in the North Atlantic main development region. Some skill metrics for the logistic model are promising, but the threat score and hit rate signify that further development of the logistic regression model is warranted; results from the Poisson regression model based on the same inputs are weaker, implying that weighting by TCG counts does not improve the results. These findings indicate merit in incorporating medium- to high-frequency variability in TCG metrics for diagnostics of seasonal activity and for application to climate models.

Influence of Mean Flow on the ENSO–Vertical Wind Shear Relationship over the Northern Tropical Atlantic

Abstract Vertical wind shear over the tropical Atlantic Ocean plays an important role in mediating hurricane activity. The vertical shear variability over the main development region for Atlantic hurricanes is affected by local factors as well as by the remote influence of the El Niño–Southern Oscillation (ENSO) phenomenon, as indicated by observational and climate modeling analyses. Climate model simulations of the ENSO–shear relationship are compared with observations. It is shown that there is a strong influence of background mean flow on the ENSO–shear relationship, because of the inherently nonlinear nature of vertical wind shear. In particular, the simulation of zonal flow over the tropical Atlantic is shown to play a critical role in how the remote influence of ENSO modulates the shear. Even with realistic simulations of the ENSO-induced westerly anomaly in the upper troposphere, overestimated easterly background flow in the model simulations can alter the relationship between ENSO and vertical wind shear, resulting in decreased vertical wind shear during warm ENSO events. This nonlinear superposition has important implications for the assessment of trends in large-scale environmental factors affecting tropical cyclogenesis in climate change simulations.

The ability of general circulation models to simulate tropical cyclones and their precursors over the North Atlantic main development region

The ability of General Circulation Models (GCMs) to generate Tropical Cyclones (TCs) over the North Atlantic Main Development Region (MDR; 10–20°N, 20–80°W; Goldenberg and Shapiro in J Clim 9:1169–1187, 1996) is examined through a subset of ocean–atmosphere coupled simulations from the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel data set and a high-resolution (0.5°) Sea Surface Temperature (SST)-forced simulation from the Australian Conformal-Cubic Atmospheric Model GCM. The results are compared with National Center for Environmental Prediction (NCEP-2) and European Center for Medium Range Weather Forecasts Re-Analysis (ERA-40) reanalyses over a common period from 1980 to 1998. Important biases in the representation of the TC activity are encountered over the MDR. This study emphasizes the strong link in the GCMs between African Easterly Waves (AEWs) and TC activity in this region. However, the generation of AEWs is not a sufficient condition alone for the models to produce TCs. Precipitation over the Sahel, especially rainfall over the Fouta Djallon highlands (cf. Fig. 1), is playing a role in the generation of TCs over the MDR. The influence of large-scale fields such as SST, vertical wind shear and tropospheric humidity on TC genesis is also examined. The ability of TC genesis indices, such as the Genesis Potential Index and the Convective Yearly Genesis Potential, to represent TC activity over the MDR in simulations at low to high spatial resolutions is analysed. These indices are found to be a reasonable method for comparing cyclogenesis in different models, even though other factors such as AEW activity should also be considered.

Climatological Variations in North Atlantic Tropical Cyclone Tracks

Abstract This study investigates the relationship between tropical cyclone (TC) tracks and climatological variations in large-scale environmental parameters associated with the TC steering flow. By using the Atlantic Ocean hurricane database for 1950–2010, TCs that form in the main development region (MDR) are categorized into one of three track types: straight moving, recurving landfall, or recurving ocean. As expected, the straight-moving storms are associated with a westward extension and strengthening of the subtropical high, whereas the recurving ocean storms are associated with a weakening of the high. The presence of El Niño conditions in the tropical Pacific Ocean is shown to be associated with a weakening of the high, an increase in the percentage of recurving ocean TCs, and a decrease in the percentage of recurving landfall TCs. Positive phases of the Atlantic Meridional Mode are associated with an increase in the percentage of recurving ocean TCs and a decrease in the percentage of straight-moving TCs. Synthetic tracks are simulated for each storm using a beta and advection model. Sensitivity experiments using both observed and uniformly seeded genesis locations indicate that the path of straight-moving TCs is largely a reflection of their tendency to form in the southwestern portion of the MDR rather than of differences in steering flow. These experiments also suggest that the shift in TC tracks associated with El Niño/La Niña conditions is largely attributable to changes in the steering flow, whereas the track changes associated with variations in the Atlantic Meridional Mode are due to a systematic shift in genesis location.

An Analysis of the Impact of Global Warming on Hurricane Activity in the United States

This paper discusses the influence of global warming on ocean temperatures, and explains how vertical wind shear affects hurricane formation. It explores the scientific arguments against linking global warming to increased hurricanes, that ocean warming increases vertical wind shear in the main development region for Atlantic hurricanes, and that current/recent American hurricane trends fall within the realm of natural, multidecadal oscillations. Arguments supporting the link between global warming and hurricanes are addressed with climate modeling that links sea surface temperature (SST) increase to anthropogenic causes. It discusses SST’s influencing hurricane activity globally, if not locally to the U.S., and points to the need for consensus and action.

Persistent Atmospheric and Oceanic Anomalies in the North Atlantic from Summer 2009 to Summer 2010

Abstract In this work, the authors analyze the air–sea interaction processes associated with the persistent atmospheric and oceanic anomalies in the North Atlantic Ocean during summer 2009–summer 2010 with a record-breaking positive sea surface temperature anomaly (SSTA) in the hurricane Main Development Region (MDR) in the spring and summer of 2010. Contributions to the anomalies from the El Niño–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and a long-term trend are identified. The warming in the tropical North Atlantic during summer 2009–summer 2010 represented a typical response to ENSO, preconditioned and amplified by the influence of a strong and persistent negative phase of the NAO. The long-term trends enhanced the warming in the high and low latitudes and weakened the cooling in the midlatitudes. The persistent negative phase of the NAO was associated with active thermodynamic air–sea interaction in the North Atlantic basin. Surface wind anomalies associated with the NAO altered the ocean surface heat flux and changed the SSTA, which was likely further enhanced by the positive wind speed–evaporation–SST feedback. The total heat flux was dominated by the latent and sensible heat fluxes, while the shortwave radiation contributed to the tropical SSTA to a lesser degree. Sensitivity experiments with an atmospheric general circulation model forced by observed SST in the Atlantic Ocean alone suggested that the Atlantic SSTA, which was partly forced by the NAO, had some positive contribution to the persistence of the negative phase of the NAO. Therefore, the persistent NAO condition is partly an outcome of the global climate anomalies and the ocean–atmosphere feedback within the Atlantic basin. The combination of the ENSO, NAO, and long-term trend resulted in the record-breaking positive SSTA in the MDR in the boreal spring and summer of 2010. On the basis of the statistical relationship, the SSTA pattern in the North Atlantic was reasonably well predicted by using the preceding ENSO and NAO as predictors.

Cluster analysis of tropical cyclone tracks in the Southern Hemisphere

A probabilistic clustering method is used to describe various aspects of tropical cyclone (TC) tracks in the Southern Hemisphere, for the period 1969–2008. A total of 7 clusters are examined: three in the South Indian Ocean, three in the Australian Region, and one in the South Pacific Ocean. Large-scale environmental variables related to TC genesis in each cluster are explored, including sea surface temperature, low-level relative vorticity, deep-layer vertical wind shear, outgoing longwave radiation, El Nino-Southern Oscillation (ENSO) and the Madden-Julian Oscillation (MJO). Composite maps, constructed 2 days prior to genesis, show some of these to be significant precursors to TC formation—most prominently, westerly wind anomalies equatorward of the main development regions. Clusters are also evaluated with respect to their genesis location, seasonality, mean peak intensity, track duration, landfall location, and intensity at landfall. ENSO is found to play a significant role in modulating annual frequency and mean genesis location in three of the seven clusters (two in the South Indian Ocean and one in the Pacific). The ENSO-modulating effect on genesis frequency is caused primarily by changes in low-level zonal flow between the equator and 10°S, and associated relative vorticity changes in the main development regions. ENSO also has a significant effect on mean genesis location in three clusters, with TCs forming further equatorward (poleward) during El Nino (La Nina) in addition to large shifts in mean longitude. The MJO has a strong influence on TC genesis in all clusters, though the amount modulation is found to be sensitive to the definition of the MJO.

Temporal clustering of tropical cyclones and its ecosystem impacts

Tropical cyclones have massive economic, social, and ecological impacts, and models of their occurrence influence many planning activities from setting insurance premiums to conservation planning. Most impact models allow for geographically varying cyclone rates but assume that individual storm events occur randomly with constant rate in time. This study analyzes the statistical properties of Atlantic tropical cyclones and shows that local cyclone counts vary in time, with periods of elevated activity followed by relative quiescence. Such temporal clustering is particularly strong in the Caribbean Sea, along the coasts of Belize, Honduras, Costa Rica, Jamaica, the southwest of Haiti, and in the main hurricane development region in the North Atlantic between Africa and the Caribbean. Failing to recognize this natural nonstationarity in cyclone rates can give inaccurate impact predictions. We demonstrate this by exploring cyclone impacts on coral reefs. For a given cyclone rate, we find that clustered events have a less detrimental impact than independent random events. Predictions using a standard random hurricane model were overly pessimistic, predicting reef degradation more than a decade earlier than that expected under clustered disturbance. The presence of clustering allows coral reefs more time to recover to healthier states, but the impacts of clustering will vary from one ecosystem to another.

Understanding and simulating the link between African easterly waves and Atlantic tropical cyclones using a regional climate model: the role of domain size and lateral boundary conditions

Using a suite of lateral boundary conditions, we investigate the impact of domain size and boundary conditions on the Atlantic tropical cyclone and african easterly Wave activity simulated by a regional climate model. Irrespective of boundary conditions, simulations closest to observed climatology are obtained using a domain covering both the entire tropical Atlantic and northern African region. There is a clear degradation when the high-resolution model domain is diminished to cover only part of the African continent or only the tropical Atlantic. This is found to be the result of biases in the boundary data, which for the smaller domains, have a large impact on TC activity. In this series of simulations, the large-scale Atlantic atmospheric environment appears to be the primary control on simulated TC activity. Weaker wave activity is usually accompanied by a shift in cyclogenesis location, from the MDR to the subtropics. All ERA40-driven integrations manage to capture the observed interannual variability and to reproduce most of the upward trend in tropical cyclone activity observed during that period. When driven by low-resolution global climate model (GCM) integrations, the regional climate model captures interannual variability (albeit with lower correlation coefficients) only if tropical cyclones form in sufficient numbers in the main development region. However, all GCM-driven integrations fail to capture the upward trend in Atlantic tropical cyclone activity. In most integrations, variations in Atlantic tropical cyclone activity appear uncorrelated with variations in African easterly wave activity.

A simplified Atlantic basin seasonal hurricane prediction scheme from 1 August

[1] The Tropical Meteorology Project at Colorado State University has issued seasonal forecasts for Atlantic basin hurricane activity in early August since 1984. This paper proposes a simplified scheme, using a combination of two surface predictors selected from the newly-developed Climate Forecast System Reanalysis (CFSR) as well as a dynamical forecast for El Nino-Southern Oscillation (ENSO) from the European Centre for Medium-Range Weather Forecasts (ECMWF). These three predictors in combination explain approximately 72% of the cross-validated variance in post-1 August Net Tropical Cyclone (NTC) activity over the hindcast period from 1982–2010. While uncertainties in the data grow as one goes back further in time, all three predictor correlations remain significant with NTC when tested on data from 1900–1981. These predictors are also shown to correlate with August-October physical features across the Atlantic Main Development region known to impact hurricane activity.

Multi-year predictability of the tropical Atlantic atmosphere driven by the high latitude North Atlantic Ocean

[1] Using idealised model experiments we show that the tropical Atlantic main hurricane development region (MDR) is potentially one of the most predictable regions for atmospheric variables such as precipitation and wind shear on multi-year timescales. Similarly we also find predictability for the number of tropical storms and the position of the inter-tropical convergence zone. Further experiments that withhold data in different parts of the ocean identify the North Atlantic sub-polar gyre as the key region for driving the skill in the model MDR. This further highlights the importance of observing the high-latitude North Atlantic Ocean for initialising future decadal predictions.

Estimating Annual Numbers of Atlantic Hurricanes Missing from the HURDAT Database (1878–1965) Using Ship Track Density

Abstract This study assesses the impact of imperfect sampling in the presatellite era (between 1878 and 1965) on North Atlantic hurricane activity measures and on the long-term trends in those measures. The results indicate that a substantial upward adjustment of hurricane counts may be needed prior to 1965 to account for likely “missed” hurricanes due to sparse density of reporting ship traffic. After adjusting for the estimate of missed hurricanes in the basin, the long-term (1878–2008) trend in hurricane counts changes from significantly positive to no significant change (with a nominally negative trend). The adjusted hurricane count record is more strongly connected to the difference between main development region (MDR) sea surface temperature (SST) and tropical-mean SST than with MDR SST. These results do not support the hypothesis that the warming of the tropical North Atlantic due to anthropogenic greenhouse gas emissions has caused Atlantic hurricane frequency to increase.

Future Impact of Differential Interbasin Ocean Warming on Atlantic Hurricanes

AbstractGlobal climate model simulations forced by future greenhouse warming project that the tropical North Atlantic (TNA) warms at a slower rate than the tropical Indo-Pacific in the twenty-first century, consistent with their projections of a weakened Atlantic thermohaline circulation. Here, an atmospheric general circulation model is used to advance a consistent physical rationale that the suppressed warming of the TNA increases the vertical wind shear and static stability aloft in the main development region (MDR) for Atlantic hurricanes, and thus decreases overall Atlantic hurricane activity in the twenty-first century. A carefully designed suite of model experiments illustrates that the preferential warming of the tropical Indo-Pacific induces a global average warming of the tropical troposphere, via a tropical teleconnection mechanism, and thus increases atmospheric static stability and decreases convection over the suppressed warming region of the TNA. The anomalous diabatic cooling, in turn, forces the formation of a stationary baroclinic Rossby wave northwest of the forcing region, consistent with Gill’s simple model of tropical atmospheric circulations, in such a way as to induce a secular increase of the MDR vertical wind shear. However, a further analysis indicates that the net effect of future greenhouse warming on the MDR vertical wind shear is less than the observed multidecadal swing of the MDR vertical wind shear in the twentieth century. Thus, it is likely that the Atlantic multidecadal oscillation will still play a decisive role over the greenhouse warming in the fate of Atlantic hurricane activity throughout the twenty-first century under the assumption that the twenty-first-century changes in interbasin SST difference, projected by the global climate model simulations, are accurate.

Observations of Saharan dust microphysical and optical properties from the Eastern Atlantic during NAMMA airborne field campaign

Abstract. As part of the international project entitled "African Monsoon Multidisciplinary Analysis (AMMA)", NAMMA (NASA AMMA) aimed to gain a better understanding of the relationship between the African Easterly Waves (AEWs), the Sahara Air Layer (SAL), and tropical cyclogenesis. The NAMMA airborne field campaign was based out of the Cape Verde Islands during the peak of the hurricane season, i.e., August and September 2006. Multiple Sahara dust layers were sampled during 62 encounters in the eastern portion of the hurricane main development region, covering both the eastern North Atlantic Ocean and the western Saharan desert (i.e., 5–22° N and 10–35° W). The centers of these layers were located at altitudes between 1.5 and 3.3 km and the layer thickness ranged from 0.5 to 3 km. Detailed dust microphysical and optical properties were characterized using a suite of in-situ instruments aboard the NASA DC-8 that included a particle counter, an Ultra-High Sensitivity Aerosol Spectrometer, an Aerodynamic Particle Sizer, a nephelometer, and a Particle Soot Absorption Photometer. The NAAMA sampling inlet has a size cut (i.e., 50% transmission efficiency size) of approximately 4 μm in diameter for dust particles, which limits the representativeness of the NAMMA observational findings. The NAMMA dust observations showed relatively low particle number densities, ranging from 268 to 461 cm−3, but highly elevated volume density with an average at 45 μm3 cm−3. NAMMA dust particle size distributions can be well represented by tri-modal lognormal regressions. The estimated volume median diameter (VMD) is averaged at 2.1 μm with a small range of variation regardless of the vertical and geographical sampling locations. The Ångström Exponent assessments exhibited strong wavelength dependence for absorption but a weak one for scattering. The single scattering albedo was estimated at 0.97 ± 0.02. The imaginary part of the refractive index for Sahara dust was estimated at 0.0022, with a range from 0.0015 to 0.0044. Closure analysis showed that observed scattering coefficients are highly correlated with those calculated from spherical Mie-Theory and observed dust particle size distributions. These values are generally consistent with literature values reported from studies with similar particle sampling size range.

Predictability of North Atlantic Tropical Cyclone Activity on Intraseasonal Time Scales

Abstract Recent work suggests that there may exist skill in forecasting tropical cyclones (TC) using dynamically based ensemble products, such as those obtained from the ECMWF Monthly Forecast System (ECMFS). The ECMFS features an ensemble of 51 coupled ocean–atmosphere simulations integrated to 32 days once per week. Predicted levels of TC activity in the North Atlantic Ocean with these monthly ensemble forecasts is compared with the observed variability during the months of June–October during 2008 and 2009. Results indicate that the forecast system can capture large-scale regions that have a higher or lower risk of TC activity and that it has skill above climatology for the Gulf of Mexico and the “Main Development Region” on intraseasonal time scales. Regional forecast skill is traced to the model’s ability to capture the large-scale evolution of deep-layer vertical shear, the frequency of easterly waves, and the variance in 850-hPa relative vorticity. The predictability of TC activity, along with the forecast utility of the ECMFS, is shown to be sensitive to the phase and intensity of the Madden–Julian oscillation at the time of model initialization.

Extended‐range seasonal hurricane forecasts for the North Atlantic with a hybrid dynamical‐statistical model

A hybrid forecast model for seasonal hurricane activity in the North Atlantic is developed using a combined numerical coupled ocean‐atmosphere climate and empirical prediction models. Based on a 29‐yr (1981‐2009) dataset, an empirical relationship developed between the number of seasonal hurricane and the large‐scale variables from ECMWF hindcasts. The increase of seasonal hurricane activity correlates negatively with the sea surface temperature (SST) anomaly over the tropical East Pacific, positively with the SST anomaly over the Main Development Region (MDR) and North Atlantic and the decrease of wind shear over the MDR. The North Atlantic SST and the MDR vertical wind shear are selected as predictors based on sensitivity tests. Forecasts of these predictors are made with the ECMWF climate model run in ensemble mode thus providing a probability distribution of hurricane number. The forecast skill of the hybrid model is better than or at least competitive with publicly‐available forecast models but made with a one month earlier lead‐time. The hybrid model initialized in June and July 2010 forecasts an active season with 9 hurricanes.

On the Potential Causes of the Nonstationary Correlations between West African Precipitation and Atlantic Hurricane Activity

Abstract For years, various indices of seasonal West African precipitation have served as useful predictors of the overall tropical cyclone activity in the Atlantic Ocean. Since the mid-1990s, the correlation unexpectedly deteriorated. In the present study, statistical techniques are developed to describe the nonstationary nature of the correlations between annual measures of Atlantic tropical cyclone activity and three selected West African precipitation indices (namely, western Sahelian precipitation in June–September, central Sahelian precipitation in June–September, and Guinean coastal precipitation in the preceding year’s August–November period). The correlations between these parameters are found to vary over the period from 1921 to 2007 on a range of time scales. Additionally, considerable year-to-year variability in the strength of these correlations is documented by selecting subsamples of years with respect to various meteorological factors. Broadly, in years when the environment in the main development region is generally favorable for enhanced tropical cyclogenesis (e.g., when sea surface temperatures are high, when there is relatively little wind shear through the depth of the troposphere, or when the relative vorticity in the midtroposphere is anomalously high), the correlations between indices of West African monsoon precipitation and Atlantic tropical cyclone activity are considerably weaker than in years when the overall conditions in the region are less conducive. Other more remote climate parameters, such as the phase of the Southern Oscillation, are less effective at modulating the nature of these interactions.

Atlantic tropical cyclones in the twentieth century: natural variability and secular change in cyclone count

The twentieth century record of the annual count of Atlantic tropical cyclones (TCs) is analyzed to develop consistent estimates of its natural variability and secular change components. The analysis scheme permits development of multidecadal trends from natural variability alone, reducing aliasing of the variability and change components. The scheme is rooted in recurrent variability modes of the influential SST field and cognizant of Pacific-Atlantic links. The origin of increased cyclone counts in the early 1930s, suppressed counts in 1950–1960s, and the recent increase (since 1990s) is investigated using the count data set developed by Landsea et al. (J Clim 23: 2508–2519, 2010). We show that annual TC counts can be more closely reconstructed from Pacific and Atlantic SSTs than SST of the main development region (MDR) of Atlantic TCs; the former accounting for ~60% of the decadal count variance as opposed to ~30% for MDR SST. Atlantic Multidecadal Oscillation (AMO) dominates the reconstruction, accounting for ~55% of the natural decadal count variance, followed by the ENSO Non-Canonical and Pan-Pacific decadal variability contributions. We argue for an expansive view of the domain of influential SSTs—extending much beyond the MDR. The additional accounting of count variance by SSTs outside the MDR suggests a role for remotely-forced influences over the tropical Atlantic: the Pan-Pacific decadal mode is linked with decreased westerly wind shear (200–850 hPa) in its warm phase, much as the AMO impact itself. Non-canonical ENSO variability, in contrast, exerts little influence on decadal timescales. Interestingly, the secular but non-uniform warming of the oceans is linked with increased westerly shear, leading to off-setting dynamical and thermodynamical impacts on TC activity! The early-1930s increase in smoothed counts can be partially (~50%) reconstructed from SST natural variability. The 1950–1960s decrease, in contrast, could not be reconstructed at all, leading, deductively, to the hypothesis that it results from increased aerosols in this period. The early-1990s increase is shown to arise both from the abatement of count suppression maintained by SST natural variability and the increasing SST secular trend contribution; the abatement is related to the AMO phase-change in early-1990s. Were it not for this suppression, TC counts would have risen since the early 1970s itself, tracking the secular change contribution. The analysis suggests that when SST natural variability begins to significantly augment counts in the post-1990 period—some evidence for which is present in the preceding decade—Atlantic TC counts could increase rapidly on decadal timescales unless offset by SST-unrelated effects which apparently account for a non-trivial amount (~40%) of the decadal count variance.