Drivers of upper ocean heat content extremes around New Zealand revealed by Adjoint Sensitivity Analysis

Abstract

Marine heatwaves can have devastating ecological and economic impacts and understanding what drives their onset is crucial to achieving improved prediction. A key knowledge gap exists around the subsurface structure and temporal evolution of MHW events in continental shelf regions, where impacts are most significant. Here, we use a realistic, high-resolution ocean model to identify marine heatwaves using upper ocean heat content (UOHC) as a diagnostic metric. We show that, embedded in the inter-annual variability of UOHC across the Tasman Sea, regional UOHC around New Zealand varies at short temporal and spatial scales associated with local circulation which drives the onset of extreme events with median duration of 5–20 days. Then, using a novel application of Adjoint Sensitivity Analysis, we diagnose the regional drivers of extreme UOHC events and their 3-dimensional structure. We compute the sensitivity of UOHC to changes in the ocean state and atmospheric forcing over the onset of MHW events using ensembles of between 34 and 64 MHW events across 4 contrasting regions over a 25-year period. The results reveal that changes in regional UOHC on short (5-day) timescales are largely driven by local ocean circulation rather than surface heat fluxes. Where the circulation is dominated by boundary currents, advection of temperature in the mixed layer dominates the onset of extreme UOHC events. Higher magnitude MHW events are typically associated with shallower mixed layer and thermocline depths, with higher sensitivity to temperature changes in the upper 50–80 m. On the west coast, where boundary currents are weak, UOHC extremes are sensitive to density changes in the upper 1,000 m and likely caused by downwelling winds. Our results highlight the importance of understanding the different temporal and spatial scales of UOHC variability. Understanding the local circulation associated with heat content extremes is an important step toward accurate MHW predictability in economically significant shelf seas.

Comparison of 4-dimensional variational and ensemble optimal interpolation data assimilation systems using a Regional Ocean Modeling System (v3.4) configuration of the eddy-dominated East Australian Current system

“…With a 4D-Var system, the use of the adjoint model can provide useful insight into the sensitivity of the ocean state to prior changes in state variables or forcings (e.g. Powell et al, 2013;Kerry et al, 2022) and the direct quantification of observation impacts (e.g. Powell, 2017;Kerry et al, 2018).…”

Characterizing the Variability of Boundary Currents and Ocean Heat Content Around New Zealand Using a Multi‐Decadal High‐Resolution Regional Ocean Model

“…UOHC is a vital metric to understand heat transfer in and out of the upper ocean and is of key interest for understanding ocean warming (e.g., J. Li et al, 2022b, and others) and the subsurface effects of marine heatwaves (Behrens et al, 2019;Kerry et al, 2022). UOHC quantifies the heat in the upper ocean, and is given by…”

“…UOHC is a vital metric to understand heat transfer in and out of the upper ocean and is of key interest for understanding ocean warming (e.g., J. Li et al., 2022b, and others) and the subsurface effects of marine heatwaves (Behrens et al., 2019; Kerry et al., 2022). UOHC quantifies the heat in the upper ocean, and is given by $$$\int \nolimits_{-{z}_{T}}^{0}\rho (z){C}_{p}T(z)dz$$$ where − z T is the depth of the upper layer, ρ ( z ) is the density at depth z , C p is the specific heat of sea water, and T ( z ) is the water temperature at depth z .…”

“…This characterization represents a crucial first step toward understanding how NZ's ocean might respond to global warming and toward predicting the likelihood, severity, and timing of ocean temperature extremes. The understanding gained by this study, combined with advanced modeling tools, has been used to study the dominant drivers of extended periods of anomalously warm ocean temperatures (marine heatwaves, Hobday et al., 2016) in NZ's shelf seas (Kerry et al., 2022). We focus on characterizing the mean and variability of boundary currents and UOHC around NZ at intra‐annual, seasonal, and intra‐annual timescales; future studies could use the hindcast to look at long‐term trends and to diagnose the dynamical drivers of boundary current and UOHC variability across the region.…”

Comparison of 4-Dimensional Variational and Ensemble Optimal Interpolation data assimilation systems using a Regional Ocean Modelling System (v3.4) configuration of the eddy-dominated East Australian Current System

“…With a 4D-Var system, the use of the adjoint model can provide useful insight into the sensitivity of the ocean state to prior changes in state variables or forcings (e.g. Powell et al, 2013;Kerry et al, 2022) and the direct quantificiation of observation impacts (e.g. Powell, 2017;Kerry et al, 2018).…”