Drivers of warm water variability in Atlantic hurricane regions

Abstract

Tropical North Atlantic Ocean Heat Content has increased materially over the past 40-50 years, and has been linked to an uptick in major hurricane landfalls. This increasing damage potential has led to fears for the future from extreme events. In this thesis, the volume of water warmer than 26.5 °C is used to diagnose observed historical and modelled future changes in the underlaying fuel source available for the development of intense hurricanes. The processes driving changes in this fuel source are examined using complimentary Eulerian and Lagrangian techniques.
From the Eulerian perspective, observed month-to-month volume changes in water warmer than 26.5 °C in the North Atlantic can be explained by changes in the anomalous volume of warm water transformed across the 26.5 °C isotherm by atmospheric heat flux; this is primarily attributed to surface heat gain in some years. An inference is that ocean heat transport is more important for warm water volume anomaly development in other years. Transformed volume changes are calculated using the Water Mass Transformation Framework in temperature space. Anomalies are notably driven by latent heat flux, which is highly correlated with wind speed and cloud fraction over most of the warm water surface.
The residual warm water volume accumulated by ocean heat transport also plays a key role in heat content accumulation in the tropical North Atlantic. Lagrangian analysis is used to analyse heat flux along ocean currents into the Main Development Region (MDR) for Atlantic hurricanes. ARIANE particle tracking output highlights that a large number of particles are resident in the MDR (20-40%), and relatively fewer particles are transported into the MDR via the North Brazil Current (5-15%) or Ekman drift across 10 °N six months before the start of hurricane season, in years with high hurricane activity. The results are consistent with the view that a reduced meridional circulation is likely to lead to accumulation of warm water in the tropical North Atlantic and more active hurricane seasons.
Both Eulerian and Lagrangian analyses are applied to a high-resolution climate model, HadGEM3-GC31-HH, to examine evolving mechanisms impacting the growth of the Atlantic Warm Pool under future high anthropogenic greenhouse gas emissions. Warm water convergence due to reduced ocean transport is confirmed to be the driver of additional future heat available to fuel major hurricane development in this model. The total heat accumulation along modelled particle tracks crossing 10 °N and ending at 20 °N after 6 months, which are warmer than 26.5 °C, is 14% higher (4.2 GW) in 2041-2050 than 2001-2010. The results imply an increasing importance for ocean transport over atmospheric heat flux as the Atlantic continues warming.

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ORCID for Elizabeth Harris: orcid.org/0000-0002-0274-560X

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