Shijian Hu

Pacific western boundary currents and their roles in climate

Pacific Ocean western boundary currents and the interlinked equatorial Pacific circulation system were among the first currents of these types to be explored by pioneering oceanographers. The widely accepted but poorly quantified importance of these currents—in processes such as the El Niño/Southern Oscillation, the Pacific Decadal Oscillation and the Indonesian Throughflow—has triggered renewed interest. Ongoing efforts are seeking to understand the heat and mass balances of the equatorial Pacific, and possible changes associated with greenhouse-gas-induced climate change. Only a concerted international effort will close the observational, theoretical and technical gaps currently limiting a robust answer to these elusive questions.

Direct Measurements of the Luzon Undercurrent

AbstractThe Luzon Undercurrent (LUC) was discovered about 20 years ago by geostrophic calculation from conductivity–temperature–depth (CTD) data. But it was not directly measured until 2010. From November 2010 to July 2011, the LUC was first directly measured by acoustic Doppler current profiler (ADCP) from a subsurface mooring at 18.0°N, 122.7°E to the east of Luzon Island. A number of new features of the LUC were identified from the measurements of the current. Its depth covers a range from 400 m to deeper than 700 m. The observed maximum velocity of the LUC, centered at about 650 m, could exceed 27.5 cm s−1, four times stronger than the one derived from previous geostrophic calculation with hydrographic data. According to the time series available, the seasonality of the LUC strength is in winter > summer > spring. Significant intraseasonal variability (ISV; 70–80 days) of the LUC is exposed. Evidence exists to suggest that a large portion of the intraseasonal variability in the LUC is related to the w...

Interannual variability of the Indonesian Throughflow: The salinity effect

The Indonesian Throughflow (ITF) region possesses strong mixing and experiences significant freshwater input, but the role of salinity variability in the Indonesian Seas remains unclear. The goal of this study is to understand how salinity variability influences the ITF transport on interannual time scales. The ITF transport is calculated using observations and assimilation data sets and verified using direct ITF transport estimates. We find that the halosteric component of the ITF transport contributes (36 +/- 7)% of the total ITF variability, in contrast to (63 +/- 6)% by the thermosteric component. Thus, while not dominant, this result nonetheless implies that the salinity variability in the Indonesian Seas is of remarkable importance in determining the interannual variability of ITF transport. Correlation analysis indicates that the interannual variability of the total ITF transport is mainly influenced by the El Nino-Southern Oscillation (ENSO) rather than the Indian Ocean Dipole. Under the ENSO cycle, the Walker Circulation shifts longitudinally resulting in fluctuations in precipitation over the Indonesian Seas that modulates salinity and subsequently influences the interannual variability of ITF transport. This result signals the importance of precipitation and the subsequent salinity effect in determining the interannual variability of the ITF transport. The role of wind forcing and oceanic planetary waves is also revisited using this newly calculated ITF transport series. ENSO-related wind forcing is found to modulate the ITF transport via Rossby waves through the wave guide in the Indonesian Seas, which is in agreement with previous studies.

Observed strengthening of interbasin exchange via the Indonesian seas due to rainfall intensification

Abstract A proxy of the Indonesian Throughflow (ITF) transport, developed using in situ hydrographic measurements along with assimilations, shows a significant strengthening trend during the past decade. This trend is due to a freshening and subsequent increase in the halosteric component of the ITF transport associated with enhanced rainfall over the Maritime Continent over the same period. The strengthening of the ITF transport leads to a significant change in heat and freshwater exchange between the Pacific and Indian Oceans and contributes to the warming and freshening of the eastern Indian Ocean. The combined effect of the ITF transport of mass and freshwater along with tropical rainfall plays a very important role in the climate system.

Tropical Meridional Overturning Circulation Observed by Subsurface Moorings in the Western Pacific

Meridional ocean current in the northwestern Pacific was documented by seven subsurface moorings deployed at 142°E during August 2014-October 2015. A sandwich structure of the tropical meridional overturning circulation (TMOC) was revealed between 0–6°N that consists of a surface northward flow (0–80 m), a thermocline southward flow (80–260 m; 22.6–26.5 σθ), and a subthermocline northward flow (260–500 m; 26.5–26.9 σθ). Based on mooring data, along with satellite and reanalysis data, prominent seasonal-to-interannual variations were observed in all three layers, and the equatorial zonal winds were found to be a dominant cause of the variations. The TMOC is generally stronger in boreal winter and weaker in summer. During 2014–2015, the TMOC was greatly weakened by westerly wind anomalies associated with the El Niño condition. Further analysis suggests that the TMOC can affect equatorial surface temperature in the western Pacific through anomalous upwelling/downwelling and likely plays a vital role in the El Niño-Southern Oscillation (ENSO).

Structure and Variability of the North Equatorial Current/Undercurrent from Mooring Measurements at 130°E in the Western Pacific

The mean structure and variability of the North Equatorial Current/Undercurrent (NEC/NEUC) are investigated with one-year Acoustic Doppler Current Profilers measurements from 4 subsurface moorings deployed at 10.5°N, 13°N, 15.5°N, and 18°N along 130°E in the western Pacific. The strong westward flowing NEC ranges from the sea surface down to 400 m, and the mean zonal velocity of the NEC at 10.5°N is around −30 cm/s at the depth of 70 m. Eastward flowing NEUC jets are detected below the NEC at 10.5°N and 13°N, and the depth of the NEUC could reach at least 900 m. The mean velocity of the NEUC is around 4.2 cm/s at 800 m. No eastward undercurrents are observed at 15°N and 18°N. The mooring measurements also reveal a strong intraseasonal variability of the currents at all 4 mooring sites, and the period is around 70–120 days. The vertical structure of this intraseasonal variability varies at different latitudes. The variability of the NEUC jets at 10.5°N and 13°N appears to be dominated by subthermocline signals, while the variability of the currents at 15.5°N and 18°N is dominated by surface-intensified signals.

Niño4 as a Key Region for the Interannual Variability of the Western Pacific Warm Pool

The Western Pacific Warm Pool (WPWP) plays an important role in the global climate through modulating deep convections, ENSO, monsoon onsets, etc. Due to the vast spatial range and huge heat storage of the WPWP, near-real-time monitoring of its three-dimensional variations remains challenging. Based on Argo observations and three reanalysis data sets, we find that the Nino4 sea surface temperature (SST) index captures the interannual variability of the WPWP well. The Nino4 SST can explain approximately half of the variance of the WPWP heat content and almost all the variance of the east-west migration of the WPWP. An assessment of 31 CMIP5 models also reveals that models with larger interannual spectral powers and amplitudes of the Nino4 SST tend to simulate larger variations in the heat content and east-west migration of the WPWP. A surface heat budget analysis further shows that the Nino4 SST and WPWP are physically connected through basin-scale horizontal advections of mean temperatures by anomalous horizontal currents, which dominate the interannual variations of both the Nino4 SST and WPWP. Our results indicate that the Nino4 SST can efficiently estimate the interannual WPWP changes and a reliable predictor of the onset time of the South China Sea summer monsoon and Bay of Bengal summer monsoon, without the need to calculate the eastern boundary location and heat content of the WPWP. Moreover, a better simulation of the SST and horizontal currents in the Nino4 region can help to reduce model bias when reproducing the WPWPs interannual variabilities.