Long-term variations of physical properties of seawater in the East Sea (Japan Sea) revealed by heat content and water mass analysis

Abstract

The global ocean has undergone dramatic warming since the 1950s. As a consequence, the ocean heat content which plays a major role in the variability of Earths heat balance significantly increases most parts of the global ocean. Moreover, it has been reported that the global meridional overturning circulation (MOC), whose stability and sensitivity to the hydrological cycle and atmospheric forcing are keys to solving uncertainties regarding future ocean changes, would be continuously weakened associated with the global warming. Although the East Sea (ES) is a small marginal sea in the northwestern Pacific, it is often called as a miniature ocean because of a remarkable dynamic similarity with the global ocean. Addressing the response of this marginal sea to the global climate change would be helpful for better understanding present and future environmental changes in the global ocean. In this study, therefore, how the upper ocean heat content (OHC) and ES ventilation system changed under the continued global warming is investigated. Long-term variability of non-seasonal OHC in the upper 500 m in the ES exhibits a distinct east-west contrast during the recent 30 years. The contrasting OHC variations are revisited and investigated more in detail by analyzing two observational datasets, the gridded data from 1976 to 2007 and in situ data from 1976 to 2011 in the southwestern ES that covers the zone of western boundary current. The OHC variability shows in-phase and predominant decadal variation in both east and west regions before 1995, but uncorrelated and predominant interannual variations after 1995. Heaving effects due to major branches of warm currents in the ES, the East Korea Warm Current (EKWC) in the western part and other two branches in the eastern part, mainly contribute to the OHC variations. The heaving effect in the southwestern ES is shown to be associated with changes in winter wind stress curl field in the northern ES. Weakening of the subpolar gyre due to weakening of positive wind stress curl in the Japan Basin related with wintertime Western Pacific teleconnection pattern and Siberian High appears to enhance the northward penetration of the EKWC resulting in an increase of OHC in the southwestern ES. The heaving effect in the southeastern ES is significantly correlated with the Siberian High, but the causative mechanism is inconclusive. This study also demonstrates the importance of using highly-resolved datasets for areas affected by strong and narrow boundary currents in computing and understanding the OHC variability. Also presented in this study are evidences suggesting vigorous changes in the ventilation system of the ES during the latest decade from structure and volume of deep (> 500 m) water masses — Central Water (CW), Deep Water (DW), and Bottom Water (BW). A shift in the seas ventilation system from BW to CW under a warming climate was previously suggested from the deepening of CW-DW and DW-BW boundaries, providing a clue to future changes in global MOC. Here, a slowdown of the deepening since 2000s is newly found associated with re-initiation of the BW formation. A simple one-dimensional (1-D) advection-diffusion model supports a halt of CW formation and continual oxygen supply to DW since 2000s with more chances of the BW formation. A new projection, associated with open-ocean and slope convections in the northern ES, is also provided here allowing the three-layered deep-water structure without loss of BW, at least by 2040. As a conclusion, the OHC variations and the ventilation system in the ES show significant change in the recent decade and it is suggested that the recent changes of physical properties of seawater in the ES are primarily affected by the atmospheric and oceanic advective or convective processes occurring within and outside the ES rather than an increase of the water temperature due to the global warming. In short, the OHC variations and deep ventilation system in the ES support a previous assertion on the East Sea as a miniature ocean.