Hurricane‐forced upwelling and chlorophyll <i>a</i> enhancement within cold‐core cyclones in the Gulf of Mexico

TLDR

Clear skies after Hurricane Ivan in September 2004 offered a unique opportunity to study upper‑ocean responses to a major hurricane. The hurricane’s strong winds (59–62 m s⁻¹) intensified cyclonic circulation, producing 50–65 m isothermal displacements that ventilated thermoclines and nutriclines, maximized upwelling and surface cooling (3–7 °C), and triggered phytoplankton blooms peaking 3–4 days later. Cool waters (20–26 °C) along Ivan’s track provided immediate negative feedback to hurricane intensity, and the study shows that mesoscale cyclones, alongside warm anticyclones, can significantly influence along‑track hurricane intensity changes.

Abstract

Clear skies, subsequent to Hurricane Ivan's passage across the Gulf of Mexico in September 2004, provided a unique opportunity to investigate upper ocean responses to a major hurricane. Oceanic cyclonic circulation was rapidly intensified by the hurricane's wind field (59–62 m s −1 ), maximizing upwelling and surface cooling (3–7°C) in two large areas along Ivan's track. Upward isothermal displacements of 50–65 m, computed from wind stress and sea surface height changes, caused rapid ventilation of thermoclines and nutriclines, leading to phytoplankton blooms with peak concentrations 3–4 days later. Wind speed changes along Ivan's track demonstrated that the cool waters (20–26°C) provided immediate negative feedback to the hurricane's intensity. Although our study focused on a relatively small ocean area, it revealed that mesoscale cyclones, in addition to warm anticyclones, may play an important role in producing along‐track hurricane intensity changes.