1:45 PM - 2:00 PM
[AOS21-01] Marine heatwaves around Japan over the past four decades: Links to low-frequency variability in sea surface temperature
Keywords:marine heatwaves, climate variability, global warming, MGD-SST
Marine heatwaves (MHWs) are characterized by extremely high temperatures persisting for at least several consecutive days. Around Japan, the MHWs are suggested to impact human activities, particularly fisheries (Miyama et al. 2021) and air temperatures above Japan (Sato et al. 2024). Recently, the MHWs have been observed more frequently in the global ocean, and this increase is partly attributed to climate variability including global warming (Oliver et al. 2018; Lee et al. 2023), as the MHWs are affected by climate variability (Joh and Di Lorenzo 2017; Kawakami et al. 2024). However, the extent to which longer time-scale variability affects MHWs around Japan is poorly understood.
In this study, these impacts are quantified using a satellite-based sea surface temperature (SST) dataset, MGDSST (Kurihara et al. 2006). To detect the large-scale MHW characteristics around Japan, 10 regions, each spanning 10° in longitude and 5° in latitude, are analyzed. The analysis of four decades of SST data shows that the annual MHW days averaged over the past 40 years are approximately 30 days, with the longest duration being 109 days. To evaluate the effect of the warming trend on the MHW detection, MHWs are identified using the detrended SST data. The results indicate that annual MHW days from the 2010s are reduced by 65% due to detrending. Additionally, the MHWs are identified using the detrended SST data without low-frequency variability. The annual MHW days averaged for 10 regions are reduced by 56% when applying a high-pass filter with a 1-year cutoff and by 20% with an 8-year cutoff.
These findings indicate that longer time-scale variability creates favorable conditions for the MHWs to occur. Around Japan, this longer time-scale variability is associated with variability in the North Pacific Ocean and is well correlated with the second and third EOF modes of North Pacific SST. This implies that improving the projection accuracy of North Pacific SST variability could enhance the accuracy of MHW projections around Japan.
In this study, these impacts are quantified using a satellite-based sea surface temperature (SST) dataset, MGDSST (Kurihara et al. 2006). To detect the large-scale MHW characteristics around Japan, 10 regions, each spanning 10° in longitude and 5° in latitude, are analyzed. The analysis of four decades of SST data shows that the annual MHW days averaged over the past 40 years are approximately 30 days, with the longest duration being 109 days. To evaluate the effect of the warming trend on the MHW detection, MHWs are identified using the detrended SST data. The results indicate that annual MHW days from the 2010s are reduced by 65% due to detrending. Additionally, the MHWs are identified using the detrended SST data without low-frequency variability. The annual MHW days averaged for 10 regions are reduced by 56% when applying a high-pass filter with a 1-year cutoff and by 20% with an 8-year cutoff.
These findings indicate that longer time-scale variability creates favorable conditions for the MHWs to occur. Around Japan, this longer time-scale variability is associated with variability in the North Pacific Ocean and is well correlated with the second and third EOF modes of North Pacific SST. This implies that improving the projection accuracy of North Pacific SST variability could enhance the accuracy of MHW projections around Japan.