Hotspots and drivers of compound marine heatwaves and low net primary production extremes

Abstract

Abstract. Extreme events can severely impact marine organisms and ecosystems. Of particular concern are multivariate compound events, namely when conditions are simultaneously extreme for multiple ocean ecosystem stressors. In 2013–2015 for example, an extensive marine heatwave (MHW), known as the Blob, co-occurred locally with extremely low net primary productivity (NPPX) and negatively impacted marine life in the northeast Pacific. Yet, little is known about the characteristics and drivers of such multivariate compound MHW–NPPX events. Using five different satellite-derived net primary productivity (NPP) estimates and large-ensemble-simulation output of two widely used and comprehensive Earth system models, the Geophysical Fluid Dynamics Laboratory (GFDL) ESM2M-LE and Community Earth System Model version 2 (CESM2-LE), we assess the present-day distribution of compound MHW–NPPX events and investigate their potential drivers on the global scale. The satellite-based estimates and both models reveal hotspots of frequent compound events in the center of the equatorial Pacific and in the subtropical Indian Ocean, where their occurrence is at least 3 times higher (more than 10 d yr−1) than if MHWs (temperature above the seasonally varying 90th-percentile threshold) and NPPX events (NPP below the seasonally varying 10th-percentile threshold) were to occur independently. However, the models show disparities in the northern high latitudes, where compound events are rare in the satellite-based estimates and GFDL ESM2M-LE (less than 3 d yr−1) but relatively frequent in CESM2-LE. In the Southern Ocean south of 60∘ S, low agreement between the observation-based estimates makes it difficult to determine which of the two models better simulates MHW–NPPX events. The frequency patterns can be explained by the drivers of compound events, which vary among the two models and phytoplankton types. In the low latitudes, MHWs are associated with enhanced nutrient limitation on phytoplankton growth, which results in frequent compound MHW–NPPX events in both models. In the high latitudes, NPPX events in GFDL ESM2M-LE are driven by enhanced light limitation, which rarely co-occurs with MHWs, resulting in rare compound events. In contrast, in CESM2-LE, NPPX events in the high latitudes are driven by reduced nutrient supply that often co-occurs with MHWs, moderates phytoplankton growth, and causes biomass to decrease. Compound MHW–NPPX events are associated with a relative shift towards larger phytoplankton in most regions, except in the eastern equatorial Pacific in both models, as well as in the northern high latitudes and between 35 and 50∘ S in CESM2-LE, where the models suggest a shift towards smaller phytoplankton, with potential repercussions on marine ecosystems. Overall, our analysis reveals that the likelihood of compound MHW–NPPX events is contingent on model representation of the factors limiting phytoplankton production. This identifies an important need for improved process understanding in Earth system models used for predicting and projecting compound MHW–NPPX events and their impacts.

Drivers of Surface Ocean Acidity Extremes in an Earth System Model

“…In the tropical regions, increased nutrient concentrations are simulated during OAX events (not shown), which may cause increased phytoplankton growth there. In the mid‐to‐high latitudes, low biological production may be connected to nutrient limitation and/or low phytoplankton biomass due to enhanced zooplankton grazing under the elevated temperatures (Le Grix et al., 2022).…”

Potential Predictability of the Spring Bloom in the Southern Ocean Sea Ice Zone

“…It is important to note that the real-world applicability of the timescales and mechanisms of predictability found in this study depends inherently on the fidelity of the model's representation of primary production in the Southern Ocean sea ice zone. In Section 2 we noted that the GFDL-ESM2M model captures large-scale biogeochemical processes reasonably well (Dunne et al, 2012(Dunne et al, , 2013Frölicher, Aschwanden, et al, 2020;Le Grix et al, 2022), but exhibits a notably low bias in interannual variability of NPP in the sea-ice zone (Figure S1 in Supporting Information S1). It is likely that certain processes, such as the dynamic link between NPP and sea-ice cover (a key component of the predictability in the model), are more complex in reality.…”

“…Representation of NPP is challenging to assess in this part of the ocean, considering the paucity of comprehensive observations. Comparison to satellite ocean color (Dunne et al, 2013, see their Figure 3) and observation-based estimates of NPP (Le Grix et al, 2022, see their Figure B1) shows that the model captures the right order of magnitude of biological activity close to the Antarctic continent, but generally has a positive bias in open ocean regions such as the eastern Weddell Gyre. Figure S1 in Supporting Information S1 shows that NPP in a historically forced simulation of GFDL-ESM2M has low variance on inter-annual timescales in comparison to observation-based products, particularly in open-ocean regions (note that this figure is a deseasonalized version of Figure B1 from Le Grix et al, 2022).…”

Extreme and compound ocean events are key drivers of projected low pelagic fish biomass

“…Dependencies between ocean variables are reflected in the logistic regression coefficients and may impact our results. In particular, the negative correlation between net primary production and temperature in GFDL ESM2M is overestimated in the tropics and strongly underestimated around Antarctica when compared to observation-based data (Le Grix et al, 2022).…”

“…Extreme events, such as marine heatwaves (Hobday et al, 2016) or low net primary production (NPP) events (Le Grix et al, 2021, 2022), have been linked to a range of negative impacts on marine organisms and ecosystems (Cavole et al, 2016; Smale et al, 2019; Smith et al, 2023; Wernberg et al, 2013, 2016), including the collapse of entire ecosystems (e.g., Wernberg, 2021). Of particular concern are compound events, which occur when conditions are anomalous for multiple ocean ecosystem drivers (Burger et al, 2022; Gruber et al, 2021; Le Grix et al, 2021, 2022; Zscheischler et al, 2018). The “Blob,” a prolonged and extensive marine heatwave that occurred from 2013 to 2015 in the Northeast Pacific, illustrates the potential threat posed by marine compound events on ecosystems.…”