Mesoscale and submesoscale mechanisms behind asymmetric cooling and phytoplankton blooms induced by hurricanes: a comparison between an open ocean case and a continental shelf sea case

Abstract

Right-side bias in both sea surface cooling and phytoplankton blooms is often observed in the wake of hurricanes in the Northern Hemisphere. This idealized hurricane modeling study uses a coupled biological-physical model to understand the underlying mechanisms behind hurricane-induced cooling and phytoplankton bloom asymmetry. Both a deep ocean case and a continental shelf sea case are considered and contrasted. Model analyses show that while right-side asymmetric mixing due to inertial oscillations and restratification from strong right-side recirculation cells contributes to bloom asymmetry in the open ocean, the well-mixed condition in the continental shelf sea inhibits formation of recirculation cells, and the convergence of water onto the shelf is a more important process for bloom asymmetry.

APPENDIX

1.1 Dynamical equations of Powell biological model

The dynamical equations of the Powell biological model are shown below. This model calculates four state variables: dissolved nitrogen (nitrate, N), particulate nitrogen or detritus (D), phytoplankton (P), and zooplankton (Z). Each equation has the following form: the left side contains a local time derivative and an advection term, while the right contains a vertical mixing term plus the biological mechanisms. The biological mechanisms include grazing on phytoplankton by zooplankton (G), nitrogen uptake by phytoplankton (U), plankton removal (σ_d and ς_d, for phytoplankton and zooplankton respectively), sinking (w_d), and remineralization (δ).

$$ {\displaystyle \begin{array}{c}\frac{\partial N}{\partial t}+u\cdot \mathit{\nabla N}=\delta D+{\gamma}_n GZ- UP+\frac{\mathit{\partial}}{\mathit{\partial z}}\left({k}_v\frac{\partial N}{\partial z}\right)\\ {}\frac{\partial P}{\partial t}+u\cdot \nabla P= UP- GZ-{\sigma}_dP+\frac{\partial }{\partial z}\left({k}_v\frac{\partial P}{\partial z}\right)\\ {}\frac{\partial Z}{\partial t}+u\cdot \nabla Z=\left(1-{\gamma}_n\right) GZ-{\zeta}_dZ+\frac{\partial }{\partial z}\left({k}_v\frac{\partial Z}{\partial t}\right)\\ {}\frac{\partial D}{\partial t}+u\cdot \mathit{\nabla D}={\sigma}_dP+{\zeta}_dZ-\delta D+{w}_d\frac{\partial D}{dz}+\frac{\partial }{\partial z}\left({k}_v\frac{\partial D}{\partial z}\right)\\ {}G={R}_m\left(1-{e}^{-\varLambda P}\right)\\ {}I={I}_0\mathit{\exp}\Big({k}_zz+{k}_p{\int}_0^zP\left({z}^{\prime}\right)d{z}^{\prime}\\ {}U=\frac{V_mN}{k_N+N}\frac{\alpha I}{\sqrt{V_m^2+{\alpha}^2{I}^2}}.\end{array}} $$

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R_m is the zooplankton grazing rate, is the Ivlev constant, I is the intensity of light, I₀ is the surface irradiance, k_z is the light extinction coefficient, k_p is the self-shading coefficient, V_m is the nitrate uptake rate, k_N is the uptake half saturation number, and α is the initial slope of the photosynthesis-irradiance curve. The parameter values used in this model (shown below in Table 1) are the same as those in Spitz et al. 2003 and Powell et al. 2006.

Table 1 Parameter values used in the Powell biological model. Values are the same as those in Spitz et al. 2003 and Powell et al. 2006

1.2 Model initial conditions

The initial vertical profiles of temperature, nitrate, and phytoplankton are from Huang and Oey (2015), and the equations have been reproduced below. Temperature is a function of depth, and is modeled by the set of equations:

$$ {\displaystyle \begin{array}{c}T={28.88}^{{}^{\circ}}C\ for\ 0<z<-30m,\\ {}T=28.88+\left[\left(28.88-8.5\right)\tanh \left[\frac{z+30}{175.9}\right]\right]\ for-30m<z<-500m,\\ {}T=-3552.5{z}^{-0.967}\ for-500m>z.\end{array}} $$

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Nitrate is modeled by the set of equations:

$$ {\displaystyle \begin{array}{c}\mathrm{NO}3=0.05\mathrm{forT}\ge {26.8}^{{}^{\circ}}\mathrm{C},\\ {}\mathrm{NO}3=-1.3458\mathrm{T}+\mathrm{36.094.}\end{array}} $$

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Nitrate is a function of temperature, also fitted using WOA data (Huang and Oey 2015). The initial phytoplankton profile is a shifted Gaussian function (Platt and Sathyendranath 1988; Sathyendranath et al. 1995; Huang and Oey 2015), such that phytoplankton concentrations are zero at approximately 200 m depth, with a value of 0.12 mmol N m⁻³ at the surface and a maximum of 0.37 mmol N m⁻³ at approximately 60 m depth.