The North Atlantic experienced a significant decrease in hurricane activity during the 1970s and 1980s, a period known as the "hurricane drought." Understanding the causes of this drought is crucial for predicting future hurricane activity and preparing for associated hazards. The prevailing explanation, that anthropogenic aerosol radiative forcing locally cooled sea surface temperatures (SSTs), is insufficient to account for the observed magnitude of the activity decrease. This paper investigates the hypothesis that the effect of anthropogenic sulfate aerosols was amplified by a positive feedback mechanism involving Saharan dust. The study's importance stems from the need to differentiate between natural and anthropogenic contributions to hurricane variability. If the drought were natural, similar events might be expected in the future. However, if anthropogenic factors played a significant role, the recent high hurricane activity might represent a new normal, requiring adjustments in preparedness strategies. Hurricane activity, as measured by the Power Dissipation Index (PDI) and Accumulated Cyclone Energy (ACE), is strongly correlated with SSTs in the tropical North Atlantic. Previous research suggests a link between anthropogenic sulfate aerosols and reduced SSTs, but climate models have struggled to replicate the magnitude of the observed SST changes and the resulting hurricane drought. This discrepancy suggests a possible role for unmodeled positive feedback mechanisms.

A considerable body of research points to the influence of anthropogenic sulfate aerosol radiative effects on Atlantic SSTs during the hurricane drought. Increases in SO2 emissions, peaking in the 1970s and 1980s, are strongly correlated with reduced hurricane activity. However, climate model simulations, even when accounting for sulfate forcing, fail to reproduce the observed magnitude of SST variations and the associated decrease in hurricane activity. This gap between statistical correlations and model simulations highlights the need to explore other factors, such as positive feedback mechanisms, that might amplify the effects of sulfate aerosols. Existing literature also establishes a link between Saharan dust and SSTs in the tropical North Atlantic, with dust acting to depress SST and reduce hurricane activity. However, a detailed understanding of the mechanisms driving dust variability remains limited, potentially contributing to the inadequacy of climate models in simulating the 1970s-1980s hurricane drought.

The study uses multiple data sources and analytical techniques to investigate the relationship between sulfate aerosols, Saharan dust, SSTs, and hurricane activity. First, the magnitude of the hurricane drought is quantified by comparing the PDI during the 1970s-1980s with that of the broader period from 1960 to 2017. The analysis separates PDI into its three components: storm number, intensity, and duration, showing that the reduction in hurricane activity resulted from fewer, weaker, and shorter-lived storms. The average SST anomaly in the main development region during the drought was found to be -0.13 K. A strong correlation between SSTs and the PDI is presented, supporting the use of SST as a predictor of hurricane activity at decadal-multidecadal timescales. To reconstruct dust variations, the study utilizes Sahel precipitation as a proxy, reflecting the strong correlation between Sahel drought and dust emissions and transport. The reconstructed dust optical depth is validated against other relevant datasets, including Barbados dust measurements and AVHRR satellite observations. To assess the impact of dust on SSTs, single-column model (SCM) simulations are conducted under a weak temperature gradient constraint, varying dust optical depth to determine its effect on SST and the thermodynamic component of the Genesis Potential Index (GPI). The GPI, indicative of storm formation, was shown to decrease with increasing dust concentrations. Finally, the study employs Low-Frequency Component Analysis to objectively identify modes of SST variability associated with sulfate aerosols and dust, which allows for a separation of spatial patterns and temporal changes. Parseval's theorem helps to partition SST variance across different timescales, quantifying the multidecadal contribution of the dust-sulfate mode.

The study's key findings include: (1) A significant decrease in hurricane activity (55% lower PDI) during the 1970s-1980s, primarily due to fewer, weaker, and shorter-lived storms; (2) A strong correlation between summertime tropical Atlantic SSTs and the PDI; (3) A robust reconstruction of dust optical depth over the tropical North Atlantic during the 20th century, showing a peak in the 1970s and 1980s correlating with Sahel drought and sulfate aerosol optical depth asymmetry; (4) A linear relationship between dust optical depth and SST in SCM simulations, indicating that dust variations can account for a substantial fraction (46-77%, depending on uncertainty estimates) of the observed SST depression during the hurricane drought; (5) A 15% reduction in the simulated GPI due to dust variations, consistent with the observed reduction in hurricane numbers; and (6) Identification of a "dust-sulfate mode" of SST variability, explaining 88% of the local SST variance at multidecadal timescales in the main development region, and accounting for -0.14 K SST anomaly during the 1970s and 1980s.

The results suggest that the 1970s-1980s hurricane drought was significantly influenced by a positive feedback loop between anthropogenic sulfate aerosols and Saharan dust. Sulfate aerosols, originating from Europe and North America, led to reduced precipitation in the Sahel, enhancing dust emissions and transport over the Atlantic. This increased dust further cooled the SSTs, suppressing hurricane formation. The study's findings demonstrate that the effects of sulfate aerosols were amplified by this dust feedback, explaining the discrepancy between observed and modeled SST changes. The dust-sulfate mode identified in the analysis is consistent with the hypothesis of a coupled mechanism, suggesting that a large fraction of the sulfate aerosol's impact on SST is indirect, through its effect on dust. This highlights the importance of including dust variability in climate models to accurately simulate Atlantic hurricane activity. The study's findings have significant implications for understanding past and future hurricane activity, suggesting that the currently high levels of activity may persist, rather than reverting to a pattern like the 1970s-1980s drought.

This research presents a novel mechanism linking anthropogenic sulfate aerosol forcing to reduced hurricane activity during the 1970s-1980s through an amplified feedback involving Saharan dust. The study emphasizes the importance of considering this dust-mediated feedback in climate models to improve predictions of future hurricane activity. Future research could focus on improving the representation of dust variability in climate models and investigating the role of wind and cloud feedbacks in modulating the amplitude of tropical North Atlantic variability.

The study acknowledges several limitations. The dust reconstruction relies on Sahel precipitation as a proxy, which could introduce uncertainties. The SCM simulations do not include cloud feedback, which could influence the amplitude of SST variations. Furthermore, the effects of wind shear on hurricane activity are not explicitly considered. Finally, the study focuses primarily on the multidecadal timescale, and higher-frequency natural variability could also play a role in hurricane activity.