Jae-Hong Moon

Modeling skin-layer salinity with an extended surface-salinity layer

Due to near-surface salinity stratification, it is problematic to compare satellite-measured surface salinity within the first few centimeters (skin-layer) of the ocean with Argo-measured top-level salinity at about 5 m or with ocean models that do not resolve the skin layer. Although an instrument can be designed to measure the surface salinity, a global scale measurement is currently not available. A regional model can be configured to have a vertical grid in centimeters but it would be computationally prohibited on a global scale due to time step constraints. Here we propose an extended surface-salinity layer (ESSL) within a global ocean circulation model to diagnose skin SSS without increasing the computational cost, while allowing comparable solutions with both satellite and Argo salinity at the respective depths. Using a quarter-degree global ocean model, we show that the ESSL improves near-surface salinity significantly in comparisons with the Aquarius SSS and Argo salinity at 5 and 10 m, respectively. Comparing with data-assimilated HYCOM results reveal that the ESSL provides much stronger seasonal variability of SSS, similar to the Aquarius observations. We also demonstrate that the ESSL solution can be used to constrain the global mean SSS in Aquarius SSS retrieval.

PDO and ENSO modulations intensified decadal sea level variability in the tropical Pacific

According to long-term sea level reconstruction and steric sea level data, regional sea levels in the tropical Pacific have oscillated between east and west on a decadal time scale over the past 60 years, but the oscillation has been intensified significantly in the last three decades. Using conditional composite analysis, we show that the recent intensification in sea level variability is caused by modulation between the Pacific Decadal Oscillation (PDO) and El Nino-Southern Oscillation (ENSO), i.e., an El Nino in a positive PDO or a La Nina in a negative PDO phase. Our analysis of meteorological fields indicates that atmospheric circulation associated with the changes in ENSO-PDO phase relationship plays a positive role in enhancing the decadal sea level oscillation. The intensified sea level oscillation, when superimposed on the global trend of sea level rise, will have profound implications for coastal communities, therefore, the combined effect of PDO and ENSO should be taken into account in the decadal sea level prediction in the tropical Pacific.

Contribution of the Yellow Sea bottom cold water to the abnormal cooling of sea surface temperature in the summer of 2011

Satellite-based sea surface temperature (SST) measurements revealed an abnormal cooling anomaly over the Yellow Sea (YS) in the summer of 2011. Using in situ hydrographic profiles, meteorological fields, and an ocean circulation model with a passive tracer experiment, we identified the cold SST anomaly and its connection with the YS Bottom Cold Water (YSBCW), which occupies the central part of the YS below the thermocline in the summer. The summer SST anomalies in the YS showed three cold peaks in 1993, 2003, and 2011 over the past 20 years, but the reasons for the cooling events were different, as one was due to weakened surface heating and the other was attributed to mixing with the YSBCW. In 1993 and 2003, relatively weak surface heating made the surface water cooler compared with that during the other years, whereas in 2011, a strong vertical mixing of water was induced by a typhoon that passed through the central YS, causing the surface water to cool by ∼8°C and the bottom water to warm up by ∼4°C. A tracer experiment further confirmed that the vertical heat transfers between the warm surface and the cold bottom water masses when the typhoon passed through the YS interior.

Impact of typhoons on the Changjiang plume extension in the Yellow and East China Seas

It is well known that river discharges, winds, ocean currents, and tides are major dynamical factors that determine the distribution and extension of the Changjiang plume (CP) in the Yellow and East China Seas (YECS). Using observations and numerical experiments, this study demonstrates that, in addition to these factors, typhoons in the YECS also play a crucial role in the extension of the CP during the summer season. The hydrographic data observed at the Ieodo Ocean Research Station (IORS) and by a research vessel during the period of Typhoons Ewiniar (0603) and Dianmu (1004) showed that the typhoon-induced strong vertical mixing modified spatial distribution of the CP significantly, resulting in the delay of the CPs extension by as much as up to 20 days. A series of numerical experiments for Typhoon Dianmu also showed that the typhoon plays a blocking role for the extension of CP for up to 17 days through the vertical mixing process and the change of background winds. In particular, it is found that the delay due to Dianmu in 2010 contributed to the avoidance of potential mass mortality of marine life by preventing the low-salinity water from spreading to the aquaculture regions near Jeju Island.

Modeling temperature inversion in southeastern Yellow Sea during winter 2016

A significant temperature inversion with temperature differences larger than 3°C was observed in the southeastern Yellow Sea (YS) during February 2016. By analyzing in situ hydrographic profiles and results from a regional ocean model for the YS, this study examines the spatiotemporal evolution of the temperature inversion and its connection with wind-induced currents in winter. Observations reveal that in winter, when the northwesterly wind prevails over the YS, the temperature inversion occurs largely at the frontal zone southwest of Korea where warm/saline water of a Kuroshio origin meets cold/fresh coastal water. Our model successfully captures the temperature inversion observed in the winter of 2016 and suggests a close relation between northwesterly wind bursts and the occurrence of the large inversion. In this respect, the strong northwesterly wind drove cold coastal water southward in the upper layer via Ekman transport, which pushed the water mass southward and increased the sea level slope in the frontal zone in southeastern YS. The intensified sea level slope propagated northward away from the frontal zone as a shelf wave, causing a northward upwind flow response along the YS trough in the lower layer, thereby resulting in the large temperature inversion. Diagnostic analysis of the momentum balance shows that the westward pressure gradient, which developed with shelf wave propagation along the YS trough, was balanced with the Coriolis force in accordance with the northward upwind current in and around the inversion area.

Uncertainty and sensitivity of wave-induced sea surface roughness parameterisations for a coupled numerical weather prediction model

Abstract The impact of wave-induced sea surface roughness (SSR) parameterisation methods on tropical cyclone (TC) forecasts in a coupled numerical weather prediction (NWP) model is investigated by comparing the results of sensitivity experiments. A wind-dependent SSR parameterisation as a control experiment and five wave-induced SSRs parameterised by wave age, wave steepness, wave-induced stress, and input wave age, which are the most commonly used to estimate the air–sea exchange coefficients, are used in our comparative sensitivity experiments. In this study, we clearly show the magnitude of uncertainties given by the different choice of roughness parameterisations. Our results show that the choice of SSR parameterisation has a considerable influence on the atmospheric boundary layer and wave conditions, leading to a significant difference in both the forecast of TC intensity and the wave fields. The air-sea momentum and enthalpy transports are modified by the differences in exchange coefficients between the various SSR parameterisation methods, which result in differences in frictional convergence, and variation in the amount of energy input to the TC, thus changing the TC intensity. This process is linked to the alteration of the wind fields and then the wave fields. Comparison of observation data with the results obtained via several SSR parameterisation methods indicates that wind speed, and consequently energy transfer to the waves, is reduced by increased SSR, resulting in smaller wave heights. In addition, the increased SSR produces enhanced turbulent heat flux and rainfall rates by inducing an increase in thermal and moisture exchange coefficients.

4DVAR Data Assimilation with the Regional Ocean Modeling System (ROMS): Impact on the Water Mass Distributions in the Yellow Sea

In this study, we evaluate the performance of the recently developed incremental strong constraint 4-dimensional variational (4DVAR) data assimilation applied to the Yellow Sea (YS) using the Regional Ocean Modeling System (ROMS). Two assimilation experiments are compared: assimilating remote-sensed sea surface temperature (SST) and both the SST and in-situ profiles measured by shipboard CTD casts into a regional ocean modeling from January to December of 2011. By comparing the two assimilation experiments against a free-run without data assimilation, we investigate how the assimilation affects the hydrographic structures in the YS. Results indicate that the SST assimilation notably improves the model behavior at the surface when compared to the non-assimilative free-run. The SST assimilation also has an impact on the subsurface water structure in the eastern YS; however, the improvement is seasonally dependent, that is, the correction becomes more effective in winter than in summer. This is due to a strong stratification in summer that prevents the assimilation of SST from affecting the subsurface temperature. A significant improvement to the subsurface temperature is made when the in-situ profiles of temperature and salinity are assimilated, forming a tongue-shaped YS bottom cold water from the YS toward the southwestern seas of Jeju Island.

Correction to: Impact of ocean–wave coupling on typhoon-induced waves and surge levels around the Korean Peninsula: a case study of Typhoon Bolaven

The original version of this article unfortunately contained a mistake.