Impacts of North Pacific Subtropical and Subarctic Oceanic Frontal Zones on the Wintertime Atmospheric Large-Scale Circulations

Abstract

An oceanic frontal zone is a confluent region of warm and cool ocean currents, characterized by a strong meridional gradient of sea surface temperature (SST). High-resolution SST observations show that the wintertime North Pacific exhibits a unique double-oceanic-front structure, with a subtropical frontal zone (STFZ) and a subarctic frontal zone (SAFZ), whose impacts on the weather and climate over the East Asia–North Pacific–North American region need further investigation. In this study, we conduct groups of multiyear and ensemble simulations using a WRF high-resolution regional climate model, through which the different impacts of the STFZ and SAFZ on the wintertime atmospheric circulations are identified and compared. Our multiyear simulations show that the STFZ, although with weaker intensity, exerts evident and consistent impacts on the storm track and westerly jet in the North Pacific by enhancing and elongating the eddy activity, zonal wind, and Aleutian low. The SAFZ exhibits coherent impacts on the low-level atmospheric baroclinicity and storm track; however, its impacts on the upper-level storm track and atmospheric circulations are divergent, exhibiting strong year-by-year difference. Our study suggests that the SAFZ’s impacts on the atmospheric circulations strongly depend on the background mean state, which contributes to the divergent impacts of the SAFZ. Furthermore, our results highlight the role of diabatic heating for the above different impacts of the STFZ and SAFZ on the atmosphere. We argue that the much deeper diabatic heating induced by the STFZ, via affecting the baroclinicity through the whole troposphere, can exert consistent influence on eddy activities and atmospheric circulations.

Climatic Effects of Spring Mesoscale Oceanic Eddies in the North Pacific: A Regional Modeling Study

“…This result is similar to that of Huang et al [54], which showed that the climatic effects of the wintertime doubleoceanic-front structure in the NPac are characterized by weaker atmospheric responses to the northern ocean fronts. As noted previously [8][9][10][11][12]14,54], the turbulent heat fluxes may also play crucial roles in mesoscale air-sea interactions. Thus, we further analyzed the composite differences of spring turbulent heat fluxes between EddySSTA + N25N and SmthSST, and EddySSTA + S25N and SmthSST runs.…”

“…We found that the LHF averaged over the KE ( 130 3b,d). Such overestimated surface turbulent heat fluxes (SHF and LHF) have also been reported in many previous wintertime model simulations [53,54]. Overall, the WRF model can roughly and reasonably reproduce the spatial patterns of SHF and LHF.…”

“…These amplification effects of OEs on the main body of the lower-level storm track and the downstream west coast of North America in spring are quite similar to the effects of wintertime mesoscale SSTAs in the whole NPac [29,30]. However, such effects of OEs are relatively weaker than ocean fronts, which is likely due to their spatial randomness [15,16,53,54]. In the upper troposphere, OEs induce significant negative EKE anomalies over the central NPac (Figure 5e) and thus weaken the upper-level storm track, which is consistent with Foussard et al [31], although their model domain is close to the Antarctic Circumpolar Current.…”

“…Generally, the lower-level storm track largely depends on the meridional gradient of the lower-level atmospheric temperature or SST, and thus the atmospheric baroclinicity [15,53,54]. However, because of their spatial scales, much smaller than the atmospheric Rossby deformation radius, mesoscale OEs may exert limited impacts on the lower-level atmospheric baroclinicity.…”

Quantifying the dynamics of the interannual variabilities of the wintertime East Asian Jet Core

“…This result shows that during winter, the Pacific Subtropical oceanfront is stronger when the East Asian jet is intensified. The stronger oceanfront may feedback and help to maintain the downstream intensification of the upper-tropospheric jet through eddy activities and thermal wind balance, as suggested by a recent regional model study (Huang et al 2020). In addition, significant but weaker SST anomalies are also found in tropical Indian Ocean and equatorial central Pacific, in general similar to that with the variability of East Asian winter monsoon in Wang and Chen (2014), suggesting the connection between the variations of the two (Wang and Lu 2017;Chowdary et al 2019).…”