The Role of Upper Ocean Heat Content and Sea Surface Temperature on Northeast Pacific Hurricane Evolution during Average and Active Years

Abstract

Upon comparison to typical neutral-ENSO conditions in the Northeast Pacific Ocean, the 2014 hurricane season has been identified as highly anomalous in both tropical cyclone frequency and intensity. This thesis seeks to investigate the influence of sea surface temperatures (SSTs) and upper ocean heat content (UOHC), defined as the excess of heat present above 26°C, upon the upper ocean thermal structure, mesoscale features, and anomalies that led to an active hurricane season in the Northeast Pacific. The 2012 Northeast Pacific hurricane season was selected as a ‘normal’ season to fully quantify the anomalous 2014 hurricane season. Data sets utilized in this work included optimally interpolated SSTs, UOHC, and the National Hurricane Center’s post-storm observation database. Oceanic variables were extracted nearest-pixel to the hurricane path and were scrutinized through the use of along-track time series analysis, gridded UOHC fields, and a complex scheme of linear regression models. In order to quantify intensity modulation throughout a hurricane, enthalpy flux was calculated along-track for the duration of the storm with atmospheric model data, as well as from in-situ dropsonde observations. Results suggest that variable SSTs and UOHC were critical in tropical cyclone genesis, duration, and maximum intensity. A minimum requirement of 30 kJ cm-2 of UOHC was found at genesis for all storms. At least 7 of the major hurricanes interacted with a warm oceanic mesoscale feature either at genesis or along-track that induced rapid intensification periods, including one high-end Category-5 hurricane. Cool wake signatures from deep upwelling along-track were detected within gridded UOHC and were also found to be influential in changes of subsequent hurricane trajectories. Linear regressions suggested that storm-specific models, and not season-specific models, were optimal for estimating the influence of oceanic and environmental parameters within the intensification phase. Specifically, regressions for Hurricanes Emilia (2012) and Odile (2014) performed extremely well, indicating that a combination of oceanic parameters, storm position, and storm translation speed could explain 98% of along-track intensity variability. Along-track enthalpy fluxes peaked at 1234 W m-2 as the hurricane attained maximum intensity, while dropsonde-derived enthalpy emphasized stronger enthalpy fluxes within the northeast quadrants of the hurricane.