Multi-decadal trends in the tropical Pacific western boundary currents retrieved from historical hydrological observations

Shijian Hu,Fan Wang,Cong Guan,Linchao Xin,Jie Ma,Dunxin Hu,Shihan Li,Xi Lu

doi:10.1007/s11430-020-9703-4

Abstract

As large-scale ocean circulation is a key regulator in the redistribution of oceanic energy, evaluating the multi-decadal trends in the western Pacific Ocean circulation under global warming is essential for not only understanding the basic physical processes but also predicting future climate change in the western Pacific. Employing the hydrological observations of World Ocean Atlas 2018 (WOA18) from 1955 to 2017, this study calculated the geostrophic currents, volume transport and multi-decadal trends for the North Equatorial Current (NEC), the North Equatorial Countercurrent (NECC), the Mindanao Current (MC), the Kuroshio Current (KC) in the origin and the New Guinea Coastal Undercurrent (NGCUC) within tropical western Pacific Ocean over multi-decades. Furthermore, this study examined the contributions of temperature and salinity variations. The results showed significant strengthening trends in NEC, MC and NGCUC over the past six decades, which is mainly contributed by temperature variations and consistent with the tendency in the dynamic height pattern. Zonal wind stress averaged over the western Pacific Ocean in the same latitude of each current represents the decadal variation and multi-decadal trends in corresponding ocean currents, indicating that the trade wind forcing plays an important role in the decadal trend in the tropical western Pacific circulation. Uncertainties in the observed hydrological data and trends in the currents over the tropical western Pacific are also discussed. Given that the WOA18 dataset covers most of the historical hydrological sampling data for the tropical western Pacific, this paper provides important observational information on the multi-decadal trend of the large-scale ocean circulation in the western Pacific.

Highlights

The ocean is the largest heat storage in the earth’s climate system (Chen and Tung, 2014; Cheng et al, 2017)
3.1 Multi-decadal mean Western Boundary Current (WBC) in the tropical Pacific Figure 2 displays the mean currents retrieved from World Ocean Atlas 2018 (WOA18) hydrological data for each WBC section (Figure 2a, 2b, 2d, 2e) and the mean horizontal field of absolute velocity
The distribution of boundary currents in the tropical Pacific is prominent in Figure 2c, with the North Equatorial Current (NEC) bifurcating as the Kuroshio Current (KC) and the Mindanao Current (MC) to the east of east Philippines and converging with the New Guinea Coastal Undercurrent (NGCUC), eventually entering the North Equatorial Countercurrent (NECC)

Summary

Introduction

The ocean is the largest heat storage in the earth’s climate system (Chen and Tung, 2014; Cheng et al, 2017). Redistributing the heat into the ocean which the ocean dynamic process formulates the basic pattern of the ocean environment, mainly resulting from the large-scale ocean circulation that is a basic physical process of ocean energy redistribution (Ganachaud and Wunsch, 2000; Wu et al, 2012; Forget and Ferreira, 2019; Wu, 2020). No. varies over the past decades, i.e., a long-term trend, for understanding and predicting future climate variations. Given long-term climate warming, the global mean sea surface wind speed accelerates (Reguero et al, 2019; Young and Ribal, 2019), resulting in an accelerating trend in the global mean ocean circulation over the last 20 years (Hu et al, 2020). The response of ocean circulation to global climate variation is not evenly distributed, i.e., considerably varying with regional characteristics (Hu et al, 2015)

Methods

Results

Conclusion