Abstract
To investigate influences of oceanic intrinsic/internal variability and its interannual-to-decadal modulations on the Kuroshio Extension (KE) jet speed and associated eddy activity, a ten-member ensemble integration of an eddy-resolving ocean general circulation model forced by 1965-2016 atmospheric reanalysis is conducted. We found a distinct time-scale dependence in the ratio of forced and intrinsic variability of the KE jet speed. On decadal time scale, the ratio of the magnitude of intrinsic variability to that of atmospheric-driven variability is 0.69, suggesting it is largely atmospheric-driven. In contrast, on interannual time scales, the KE jet speed has large ensemble spread, indicating that it is strongly affected by intrinsic variability and has substantial uncertainty. For eddy activity, ratios of variability in ensemble average and spread also depend on region. In the downstream KE [32°-38°N, 153°-165°E], variability in the ensemble average eddy activity dominates (1.5 times) over rms of its ensemble spread on decadal time scale, and is positively correlated with current speed. Ensemble mean eddy activity in the downstream KE region is correlated (r=0.59) with ensemble mean current speed variability in the central North Pacific 4 years earlier, consistent with westward propagation of wind-driven jet speed anomalies. This linkage is robust even for each ensemble member with the significant lagged correlation also found in seven out of ten ensemble members, suggesting possibility of prediction of the eddy activity. In contrast, eddy activity in the upstream KE [32°-38°N, 141°-153°E] shows very large intrinsic and limited atmospheric-driven variability with a ratio of the former to the latter of 1.96. These results suggest intrinsic variability needs to be considered in interannual variability of strong ocean jet. The dependence of these findings to the model specificities need to be further explored.
Highlights
The Kuroshio is the western boundary current of the subtropical gyre in the North Pacific
On the interannual time scales, the Kuroshio Extension (KE) jet speed has a large ensemble spread, indicating that it is strongly affected by intrinsic variability and has substantial uncertainty
While the modeled sea surface temperature (SST) field tends to be restored to observed surface air temperature field through the estimation of surface heat fluxes based on the bulk formulae, SSTs are strongly influenced by oceanic dynamics in the strong current regions and can have the ensemble spreads associated with the spreads of the current field
Summary
The Kuroshio is the western boundary current of the subtropical gyre in the North Pacific. The Kuroshio and KE system transports heat from the tropics to the mid-latitude, and release huge amounts of heat to the atmosphere (about 1.7 × 108 W in the annual mean net surface heat flux from the KE region [30◦–40◦N, 141◦–165◦E], compared to 7.9 × 108 W in the whole Northern Hemisphere oceans to the north of 30◦N based on the third version of the Japanese Ocean Flux Data Sets with the Use of Remote Sensing Observations (J-OFURO3; Tomita et al, 2018) to impact the atmosphere aloft Through such processes, the strong warm currents and associated sea surface temperature (SST) frontal structure affect the development of cyclones (Kuwano-Yoshida and Minobe, 2017; Hirata et al, 2018) and the large-scale atmospheric circulations (Minobe et al, 2008; Frankignoul et al, 2011; O’Reilly and Czaja, 2015), inducing basin–scale air–sea interactions (Qiu et al, 2014) extending to the tropical Pacific (Joh and Di Lorenzo, 2019). Because of these possible influences on the atmosphere and ocean, including geochemical variables, it is important to improve our understandings of variability in the KE and associated eddies
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