Intraseasonal variability of North Pacific Intermediate Water induced by mesoscale eddies

Abstract. The North Pacific Intermediate Water (NPIW) is one of the most crucial water masses in the global ocean, significantly impacting physical, biological, chemical, and ecological processes. The challenges inherent in direct continuous observation of NPIW have been limiting the understanding of its short-term variability. Utilizing 14 months of data from three moorings (146° E, 25° N, M1; 122.6° E, 22.3° N, M2; 126° E, 18° N, M3), this study reveals the characteristics of the NPIW and its consistent intraseasonal variability from 60 to days across a range of latitudes and spatiotemporal scales. Direct measurement show depth variations at 700 m, 600 m, and 550 m for M1, M2, and M3, respectively. The analysis reveals a significant association between NPIW variation and mesoscale eddies, evidenced by lead-lag coefficients of 0.6, 0.5, and 0.55 for SLA and salinity at M1, M2, and M3. During anticyclonic (cyclonic) eddies, a positive (negative) SLA corresponds to relatively warm (cooler) and saline (fresh) characteristics of NPIW. Further analysis has shown that due to the inverse S-shaped structure of salinity in the North Pacific region, the vertical movement of water masses within mesoscale eddies leads to inverse phase changes between the NPIW and deeper water. Also the circulation and water masses near the western boundary are relatively complex, mesoscale eddies also induce mixing of the surrounding water masses and thus modify the NPIW properties. The result found that under the influence of the eddy, the change in salinity in the intermediate layer can reach to 0.3 psu, and the depth of the low-salt core can vary by hundreds of meters. Therefor studying the variability of NPIW is crucial for accurately predicting mesoscale eddy transport of heat and energy to ocean's intermediate layer, and understanding its response to climate change, its role in the global carbon cycle, and its impact on marine ecosystems.