Wintertime sea surface temperature variability modulated by Arctic Oscillation in the northwestern part of the East/Japan Sea and its relationship with marine heatwaves

Abstract

The northwestern part of the East/Japan Sea (EJS) is a region with large sea surface temperature (SST) variability and is known as a hotspot of marine heatwaves (MHW) stress for marine environments that peaked in boreal winter (January-February-March). This could have profound impacts on the marine ecosystems over the EJS. Here, we used a set of high-resolution satellite and reanalysis products to systematically analyze the spatiotemporal SST variations and examine their linkage to a large-scale mode of climate variability, such as the Arctic Oscillation (AO). The results show that AO-related wind forcing modulates the SST variability over the EJS via the oceanic dynamic adjustment processes. In particular, the abnormally warm SSTs in the northwestern part of the EJS are driven by the anomalous anticyclonic eddy-like circulation and Ekman downwelling during a positive AO phase. This physical linkage between a positive AO and the abnormally warm SST could be conducive to MHW occurrences in the EJS as in the extremely positive AO event during the winter of 2020. These results have implications that the MHW occurrences in the EJS could be amplified by natural climate variability along with long-term SST warming.

Intrinsic Mode-Based Network Approach to Examining Multiscale Characteristics of Sea Surface Temperature Variability

“…Furthermore, a dipole structure between the south of Vladivostok and the EKB during winter, as seen in the IE spatial pattern of IMF-4 in Figure 7, is consistent with the climatological surface wind stress curl pattern during JFM [43]; the temporal scale of this wind stress curl could be about 60 days. Appl.…”

“…2024, 14, x FOR PEER REVIEW 9 of 22 small portion of energy to the SSTA variability, most seasonality-related features could be associated with these small-scaled modes or any related forcing. Furthermore, a dipole structure between the south of Vladivostok and the EKB during winter, as seen in the IE spatial pattern of IMF-4 in Figure 7, is consistent with the climatological surface wind stress curl pattern during JFM [43]; the temporal scale of this wind stress curl could be about 60 days. Similar to the IE spatial patterns of high-mode IMFs, as shown in Figure 3, the IE patterns of IMF-6 through IMF-10 are nearly persistent, irrespective of seasons (Figures 4-7).…”

“…As shown in Figure 3, the IE spatial patterns of all-mode IMFs for full duration (from September 1981 to April 2023) show distinctly scale-dependent features; for the low-mode IMFs (1 through 4), the region with relatively high energy density extends from the central part toward the northern part of the EJS, while the energy of IMFs with high mode (6 through 9) is mainly focused on a varying tongue-shaped region (from the EKB toward the SPF). The tongue-shape region located at mid-latitude (~40 • N) is consistent with the region, which is characterized by a large standard deviation of SSTA [43] and exhibits a distinct seasonality of marine heatwave intensity [44]. The largest interannual SST variability observed in the EKB [13], which could be associated with the surface wind variability during winter time, is also confirmed by the IE spatial pattern of IMF-8 (Figure 3).…”

“…A most noticeable difference is the pattern between summer and winter for low-mode IMFs (IMF-1 through IMF-4) with timescales shorter than a seasonal scale (~60 days); those modes, in summer seasons, show a basin-wide IE spatial pattern from the southern EJS extending towards the central EJS (Figure 5), along with the highest IE level at the northwestern part of the EJS, including the ESIW formation site, while in winter seasons, the same modes make an elongated belt-shape from the western end of the SPF extending towards the west coast of northern Japan (Figure 7). This distinct IE spatial pattern by lowmode IMFs (IMF-1 through IMF-4) in winter seasons could be closely related to the Arctic Oscillation (AO) forcing because significant SSTA variability was observed in the negative AO phase along the Japan coast [43]. Although these small-scaled modes contribute a Next, we examine the seasonality of the IE spatial patterns of all IMFs in Figures 4-7.…”

“…The tongue-shape region from the EKB extending towards the SPF has been reported to be greatly affected by the surface current circulations; in the EKB, a cyclonic eddy is often formed and induces anomalous downwelling (upwelling) during a positive (negative) AO phase [43,45], and the EKWC affected by the AO is a significant factor in the SSTA variations [46,47]. Note that the EKWC strength index has a decadal timescale [43]. Thus, Similar to the IE spatial patterns of high-mode IMFs, as shown in Figure 3, the IE patterns of IMF-6 through IMF-10 are nearly persistent, irrespective of seasons (Figures 4-7).…”