Seasonal-to-Decadal Variability and Prediction of the Kuroshio Extension in the GFDL Coupled Ensemble Reanalysis and Forecasting System

Abstract

The Kuroshio Extension (KE), an eastward-flowing jet located in the Pacific western boundary current system, exhibits prominent seasonal-to-decadal variability, which is crucial for understanding climate variations in northern midlatitudes. We explore the representation and prediction skill for the KE in the GFDL SPEAR (Seamless System for Prediction and EArth System Research) coupled model. Two different approaches are used to generate coupled reanalyses and forecasts: (1) restoring the coupled model’s SST and atmospheric variables toward existing reanalyses, or (2) assimilating SST and subsurface observations into the coupled model without atmospheric assimilation. Both systems use an ocean model with 1° resolution and capture the largest sea surface height (SSH) variability over the KE region. Assimilating subsurface observations appears to be critical to reproduce the narrow front and related oceanic variability of the KE jet in the coupled reanalysis. We demonstrate skillful retrospective predictions of KE SSH variability in monthly (up to 1 year) and annual-mean (up to 5 years) KE forecasts in the seasonal and decadal prediction systems, respectively. The prediction skill varies seasonally, peaking for forecasts initialized in January and verifying in September due to the winter intensification of North Pacific atmospheric forcing. We show that strong large-scale atmospheric anomalies generate deterministic oceanic forcing (i.e., Rossby waves), leading to skillful long-lead KE forecasts. These atmospheric anomalies also drive Ekman convergence/divergence that forms ocean memory, by sequestering thermal anomalies deep into the winter mixed layer that re-emerge in the subsequent autumn. The SPEAR forecasts capture the recent negative-to-positive transition of the KE phase in 2017, projecting a continued positive phase through 2022.

Exceptional multi-year prediction skill of the Kuroshio Extension in a high-resolution decadal prediction system

“…However, it has not been examined whether the decadal KE variability can be predicted on multi-year timescales in more comprehensive, fully coupled prediction systems until very recently. Specifically, a study based on a newly developed decadal prediction system (GFDL-SPEAR 23 ) shows skillful multi-year prediction of KE 24 , in agreement with the results from previous studies based on simpler models.…”

“…ACC from HRDP is always above the upper limit of the subsampled DPLE range, except at LY 1 against the observations, suggesting that the higher skill of HRDP is unlikely to arise by chance. While the skill of HRDP is higher than that recently reported using GFDL-SPEAR 24 , the skill of DPLE (with resolution comparable to that of GFDL-SPEAR) is noticeably lower. The HRDP skill is also higher than that obtained using a linear reduced gravity model 17 where forcing is only wind-driven Ekman pumping (thus less contamination of the predictable signal by other forcings and processes).…”

“…Previous studies emphasize the westward Rossby wave propagation induced by WSC forcing in the central to eastern NP as the primary source of predictability of KE 17,18,20,22,24 . In FOSI-H, lead-lag correlation maps of annual-mean SSH onto its KEI (Fig.…”

On the seasonal prediction and predictability of winter surface Temperature Swing Index over North America

“…The focus of this study is on the December-January-February (DJF) season from SRF initialized on 1 st December. SPEAR's SRF has shown significant seasonal forecast skill in predicting a wide range of essential indicators of climate variability, including but not limited to the SST, SAT over land, mid-latitude baroclinic waves, Antarctic/Arctic sea ice, Kuroshio extension, North American summertime heat extremes, and atmospheric rivers over Western North America (Lu et al, 2020;Bushuk et al, 2021Bushuk et al, , 2022Tseng et al, 2021;Zhang et al, 2021;Jia et al, 2022;Joh et al, 2022).…”

Skillful Multiyear Prediction of the Kuroshio and Gulf Stream Jets and Eddy Activity

“…While findings from these studies have provided important insights into the potential predictability of WBCs, forecast skill for spatiotemporal variations in frontal‐scale oceanic features in the WBC regions has not been rigorously assessed in an operational sense. Several recent studies have demonstrated operational dynamical forecasting systems that can make skillful muti‐year predictions of large‐scale features of the Kuroshio Extension (KE) (Joh et al., 2022; Kim et al., 2023; Siqueira et al., 2021). However, the horizontal resolutions of ocean models and/or initial conditions adopted in these studies may not be sufficient to explicitly resolve the frontal structure of WBC jets and vigorous mesoscale eddies.…”