Impacts of the Kuroshio Extension Stability on the Storm-track over the North Pacific

The Kuroshio Extension (KE) modulates and influences the weather and climate over the North Pacific and even the entire earth through intense sea-air interactions. In this study, the seasonal responses of the North Pacific storm-track to the KE inter-annual variability are investigated by employing the lag regression approach. The results show that, there are significant seasonal variations in the storm-track responses to the KE stability variability. When the KE is stable in spring, the atmospheric low-level baroclinicity is intensified over the mid-latitudes of the North Pacific, resulting in the enhancement and upstream displacement of the storm-track. Similarly, the summer storm-track is also strengthened as a result of the stronger baroclinicity over the mid-latitudes of the North Pacific. In autumn and winter, the intensified baroclinicity in the upstream region of the storm-track, which is conducive for the development of synoptic eddies to the upstream of its climatological region, causes an upstream displacement of storm-track.


Introduction
In the western boundary current region, the warm ocean currents from the tropics converge with the cold ocean currents from the polar regions, forming a perennial sea surface temperature (SST) front that spans several hundred kilometers meridionally.It is found that the strong SST gradient on the oceanic front can anchor the storm-track by modifying the atmospheric low-level baroclinicity [1] , and the SST anomaly caused by the SST front variation can also modulate the storm-track [2][3][4][5] , which confirms the significant importance of the oceanic front to the atmospheric storm-track.
The SST front mainly affects the storm-track through the atmospheric baroclinicity.After smoothing out the SST front, the simulation results of Small et al. (2014) show that the eddy heat flux and eddy moisture flux of the storm-track over the Atlantic and southern Indian Ocean reduces by 40%, the relative vorticity decreases by 30%, and the intensity of the storm-track is weakened by 20% [6]   .By conducting numerical experiments, Yoshida and Minobe (2017) further found that, the SST front increases the local sea surface pressure deepening rate via strengthening the large-scale condensation latent, thereby strengthening the storm-track [7] .What's more, the responses of the storm-track to the SST front have significant seasonal variation characteristics.In winter (spring), the subpolar front is stronger, and the corresponding near-surface baroclinic instability is enhanced in the middle and east (northwest) part of the storm-track.As a result, more mean available potential energy is converted to the eddy available potential energy, and finally become the eddy kinetic energy.This process is conducive to the storm-track enhancement in its downstream (northwest) [8] .
As one of the strongest western boundary currents in the world, the stability of Kuroshio Extension (KE) is featured by inter-annual fluctuation with a period of 8-10 years [9] .However, the seasonal influence of its stability fluctuation on the North Pacific storm-track and the corresponding physical mechanism are still unclear.Therefore, the scientific problems to be solved in this study are: how the North Pacific storm-track responds to the KE stability in different seasons, and what is the driving mechanism.

Data
The daily-mean sensible and latent heat fluxes with a 1°× 1°resolution are provided by the Woods Hole Oceanographic Institution.The hourly atmospheric re-analysis dataset, which has a horizontal resolution of 0.25°×0.25°and37 levels in the vertical, is downloaded from the fifth major global ReAnalysis data by the European Centre for Medium-Range Weather Forecasts.The sea surface height anomaly (SSHA) from the Archiving, Validation and Interpretation of Satellite Oceanographic (AVISO), available on a 0.25°× 0.25°grid in daily mean, are employed in this study focuses on the period of 1993-2021.The spring, summer, autumn, and winter are defined as March to May, June to August, September to November, and December to following February.To remove the climate trend, all the data are linear detrended before analysis.

Diagnostic methods
The Kuroshio Extension Index (KEI) is used to identify the KE stability, which is calculated as the mean 1-year lowpass filtered SSHA in the region of 31°-36°N, 140°-165°E [10] .The maximum Eady Growth Rate (EGR) is measured as follows: = 0.31 f N ∂U ∂z with f being the Coriolis parameter, U being the horizontal winds, z being the vertical height, N being the Brunt band-Väisälä frequency [11] .The storm-track is valued by the 300-hPa meridional wind variance v'v', in which the prime denotes that a 2.5-6-day pass 31-point filter is employed to extract the signals from synoptic-scale transient eddies.
In the mid-latitudes, the atmospheric responses to the extratropical boundary forcings are generally overlapped with the atmospheric internal variability and remote forcings from the tropic (e.i.El Niño-Southern Oscillation), which makes it difficult to assess the atmospheric responses.To solve this problem, the Equilibrium Feedback Assessment (EFA) proposed by Révelard et al. (2016) [12] is employed in this study.This approach effectively extracts the atmospheric responses to the KE variability from the low-frequency tropical remote forcing and high-frequency atmospheric internal variability.In this study, a 2-month lag is used, because the atmospheric responses in mid-latitudes are most significant when the atmosphere lags the ocean for 2 months.

Results
The anomalous responses of SSHA to KE stability variations exhibit similar distribution patterns in different seasons (figure 1), suggesting that the KE dynamic state is generally steady throughout the year.When KEI is positive, there is a positive (negative) anomaly of SSHA to the south (north) of 35°N, that is, the sea surface height to the south (north) of the KE axis increases (decreases) and thus the current velocity of the KE axis is enhanced, which implies a more stable KE.However, the similar SSHA distributions cause various turbulent heat flux (THF) responses on the sea-air interface in four seasons (figure 2).In spring, the magnitude of the THF anomaly at the sea-air interface is relatively small, and it is not significant at the 90% confidence level, therefore, it can be considered that the THF variation caused by the SST anomaly in spring can be neglected.In summer, the THF in the whole KE region is enhanced approximately by 15 W m -2 .The anomalies in autumn and winter exhibit similar patterns, which are featured by the enhancement of THF (20~30 W m -2 ) in the east part of the KE region.The variability of KE stability changes the baroclinicity of the lower atmosphere by adjusting the THF at the sea-air interactions.Figure 3 depicts the responses of the maximum EGR at the 850-hPa level to the KE stability variability over the North in different seasons, and its corresponding climatological distribution.It can be seen that, in spring, autumn and winter, the large-value area of the EGR extends northeasterly from the Japanese island to the entire North Pacific; in summer, the maximum EGR over the entire North Pacific is weak (<2×10 -6 s -1 ) with a vague structure.On this basis, the responses of the atmospheric EGR to the THF anomaly in the KE region also have obvious seasonal variations.The EGR is strengthened (weakened) in the north (south) of its climatological position in spring.In summer, the EGR respond exhibits a north-south dipole structure over the North Pacific, which corresponds to an increases of EGR over the central North Pacific and a decrease near the Aleutian Islands.In autumn, the response is generally characterized by a reduced (an increased) in the eastern (western) North Pacific, and there is a significant intensification over the Bering Strait.
However, in the cold season, the abnormally enhanced EGR extends eastward from Honshu, Japan to the Northeast Pacific, which is consistent with the location of its climatological region.
The modified EGR further influences the locality and intensity of the North Pacific storm-track.As displayed in figure 4, except that the intensity of the summer storm-track is too weak, the transienteddies-induced momentum transportation of meridional wind shows southwest-northeast zonal distributions in the other three seasons, which extends northeasterly from the Japan to the west coast of North America with a maximum intensity reaching 120 m 2 s −2 .This background state reflects the life cycle of baroclinic waves.The baroclinic waves are mainly generated over the vicinity area of the KE in the western Pacific, and then develop into distinctive barotropic structures farther downstream of the weakened storm-track.The influences of the KE stability on the storm-track show similar patterns in winter and spring.By enhancing the strength of the storm-track in the upper-middle reaches, the storm-track is shifted upstream and enhanced.In summer, the storm-track intensity is increased in its climatological region, which demonstrate an enhancement of the storm-track.In autumn, a positive anomaly (negative anomaly) of the storm-track appears to the north (south) of its climatological region, indicating that the stable KE causes a northward movement of the storm-track.3, but for the storm-track (unit: m 2 s -2 ) at the 300-hPa level.

Summary
KE stability has crucial impacts on the climate of the North Pacific by triggering various atmospheric responses in different seasons.By statistically analysing the seasonal variation of the relationship between KE stability and the storm-track over the North Pacific, the seasonal variation of the relationship between KE stability and responses of sea surface variables and storm-track is obtained in this study.The main conclusions are as follows: (1) The KE stability has strong seasonal consistency, but the associated THF anomaly changes significantly with the seasons.When the KE axis is in a stable state, the THF distributes similar patterns in spring, autumn and winter, corresponding to a decreased (an increased) THF in the western (eastern) part of the KE.In summer, the THF in the entire KE region is enhanced.The opposite patterns hold when the KE axis is unstable.
(2) The responses of storm-track activity to the stability of KE show distinctive seasonal variations.In spring, the atmospheric low-level instability is intensified over the mid-latitudes of the North Pacific, thus leading to an intensified and upstream-shifted storm-track.Similarly, the summer stormtrack is also strengthened as a result of stronger baroclinicity over the mid-latitudes of the North Pacific.The storm-track responses display different spatial patterns in autumn and winter, the enhancement of atmospheric baroclinic instability in the upstream region of the storm-track favors the intensification and northwesterly movement of the storm-track.

Figure 2 .
Figure 2. The estimated surface THF response (shading, unit: W m -2 ) in (a) spring, (b) summer, (c) autumn, and (d) winter onto the KEI 2 months earlier and the corresponding seasonal-mean surface THF (contours, unit: W m -2 ).Stippling indicates the regions where the oceanic responses pass the 90% confidence level.

Figure 4 .
Figure 4.The same as Figure 3, but for the storm-track (unit: m 2 s -2 ) at the 300-hPa level.