AbstractThe emission of inertia-gravity waves (IGWs) from an exact geostrophic mesoscale eddy in a rotating and stratified fluid is investigated by three-dimensional numerical modeling. An initially balanced geostrophic mesoscale eddy inevitably generates IGWs with spiral patterns in a short transient time period through an instability mechanism. The spiral patterns can be regarded as a universal feature of IGWs which occur in the transient generation process. In the vertical direction, the energy of the IGWs is dominated by mode-1 in the generation and propagation processes, leading to weak dissipation and long-distance propagation. The amplitude of the IGWs increases linearly with the Rossby number in the range 0.04–0.1. Additionally, the IGWs emitted from an anticyclonic vortex are stronger than those radiated from the cyclonic vortex. Anticyclonic and cyclonic geostrophic vortices transfer roughly 0.54% and 0.41% of their kinetic energy to IGWs in this transient generation process, respectively. This transient generation of IGWs can supply an energy pathway from mesoscale eddies to diapycnal mixing processes in the interior of the oceans. After the transient adjustment, the quasi-geostrophic eddy spontaneously generate near-inertial waves (NIWs). NIWs are generated over a long time interval as a forced response to balanced baroclinic mesoscale eddies. For such eddies, NIW generation from balanced flow is an inevitable result as the evolution of eddies. Moreover, the baroclinicity of mesoscale eddies is an essential condition of this NIW generation mechanism. The spontaneously generated NIWs horizontally radiate toward the eddy center and propagate upward in vertical direction. NIW intensity in this spontaneous generation process strengthens with the Rossby and Froude numbers. A solid understanding of the role of the Rossby and Froude numbers is necessary for the parameterization of spontaneous NIW generation from quasi-geostrophic mesoscale eddies in general circulation model.

Ocean mixing influences fluxes of heat, salt, energy and nutrients that are fundamental to climate and ecosystems. Reliable estimation of these fluxes requires high quality turbulence observations.  The increased availability of technologies to do this has rapidly expanded both the mixing research community and the volume of data collected.  This rapid growth, compounded by the absence of standards, is generating challenges in terms of the reliability of mixing estimates. The new ATOMIX (Analysing ocean Turbulence Observations to quantify MIXing) initiative will develop best-practice procedures and quality-control indicators for determining the turbulent dissipation rate -- the critical turbulence quantity for estimating mixing from shear probes and velocity sensors.  These best-practices will support observations from commonly-deployed platforms such as vertical profilers, fixed and moored instruments, and self-propelled gliders.   The three-year effort is supported by SCOR and will enable validation of existing (and future) algorithms in a way that is independent of the programming language being used.  Benchmark datasets with agreed-upon turbulence estimates will be made openly available for a variety of platforms and aquatic environments, along with quality metrics.  The Working Group has members from a range of countries and research agencies. In addition to the turbulence benchmarking, ATOMIX will seek to grow the global mixing community through engagement at key stages of the work.  An open-access wiki and a training workshop geared towards early-career researchers will be rolled-out over the course of the Working Group lifetime.  The initiative seeks to enhance the uptake of high-quality turbulence estimation in order to build a lasting legacy for the ocean science community. 

Historical hydrographic data reveal that low-salinity water can detach from the Yellow River plume in summer. In this study, the mechanism of this detachment is examined using a three-dimensional numerical model that reproduced the observed detachment, including the position and size of the detached low-salinity water patch. Sensitivity experiments showed that tide-induced residual currents and tidal mixing around the Yellow River mouth played key roles in the detachment during spring tide. During the transition from neap tide to spring tide, the intensification of the northward tide-induced residual current and the weakening of the southward density-driven current lead to a net northward residual current (0.03 m/s), transporting high-salinity water to the southern area of low-salinity water. Meanwhile, the gradually strengthened tidal mixing also increased surface salinity, which was apparent in the central area of low-salinity water. With the combination of these two effects, the low-salinity water was separated into two parts during spring tide, i.e., detachment of low-salinity water occurred. The above scenario works with a condition of no wind. With the prevailing southeasterly winds during summer, the northeastward wind-induced surface current promoted detachment by moving the detached low-salinity water northeastward and enlarging its size. In contrast, the northerly wind that prevails in the other seasons drove low-salinity water southward, which then flowed along the coast and hindered detachment. Consequently, the detachment of low-salinity water from the Yellow River plume occurs only in summer.

Tropical cyclone (TC) translation speed is an important parameter. In the context of TC-ocean interaction, faster translation speed can contribute to less TC-induced ocean cooling and thus enables more air-sea enthalpy flux supply to favor TC intensification. In 2018, Kossin published an interesting paper in Nature, reporting a global slow-down of TC translation speed since the 1950s. However, upon close inspection, in the last two decades, TC translation speed actually increased over the western North Pacific (WNP) and neighboring seas. Thus, we are interested to see which sub-region in the WNP and neighboring seas had the most increases during the last two decades, and whether such increases contribute to TC intensification. Our results found statistically significant translation speed increases (~ 0.8 ms^-1 per decade) over the South China Sea. Ruling out other possible factors that may influence TC intensity (i.e., changes in atmospheric vertical wind shear, pre-TC sea surface temperature or subsurface thermal condition), we suggest, in this research, the possible contribution of TC translation speed increases on the observed TC intensity increases over the South China Sea in the last two decades (1998-2017).

El Niño and Southern Oscillation (ENSO) is the most prominent year-to-year climate fluctuation on Earth, alternating between anomalously warm (El Niño) and cold (La Niña) sea surface temperature (SST) conditions in the tropical Pacific. Analyses of Coupled Model Intercomparison Project Phase 3 (CMIP3) and Phase 5 (CMIP5) climate models in a warmer climate show that models disagree considerably on future changes in ENSO properties. In the present study, we suggest decline of sea ice extent (SIE) in the Arctic, which is mainly due to anthropogenic forcing, may influence on ENSO statistics by modulating atmosphere circulation. To test this hypothesis, we conduct idealized experiments in which historical SST for 1951-2016 is restored over the Arctic north of 65N only with different initial condition using coupled global climate model. We found that the North Pacific Oscillation (NPO)-like atmospheric circulation, which is modulated by the reduction of SIE, acts to trigger El Nino-like warming in the tropical Pacific through atmosphere-ocean coupled processes. SIE loss in the Pacific Arctic sector including Chukchi Sea and East Siberian Sea in boreal springtime can induce El Nino-like warming in the following winter. This result implies that connections of tropics-Arctic should be considered to understand the changes in El Nino statistics in a warmer world. 

The El Niño-Southern Oscillation (ENSO) system is the most significant equatorial interannual climate variability pattern and has enormous global climate impacts. The ability to forecast ENSO accurately is crucially important to human livelihoods worldwide. ENSO is characterized by strong winter-peaking sea surface temperature (SST) anomalies in the central-eastern tropical Pacific during the mature phase. However, the bias in simulating ENSO phase-locking behavior persists in climate models. In this study, the ENSO seasonal phase-locking behaviors simulated in 42 Coupled Model Intercomparison Project Phase 6 (CMIP6) models are evaluated by comparing 43 CMIP5 models and observations. Onlya few models (12 CMIP5 and 15 CMIP6) simulated ENSO with a majority proportion of winter-peaking events, which indicates that reasonable ENSO seasonal phase-locking is still a challenge for state-of-the-art climate models. Furthermore, the seasonal cycle of the zonal SST gradient along the equator can explain approximately 24% and 27% of the variance in the ENSO phase-locking for CMIP5 and CMIP6, respectively. In particular, the strengths of the zonal SST gradient in the central-eastern Pacific during boreal spring and autumn are crucial. The biases in simulating the seasonal changes in the zonal SST gradient influence the zonal advective feedback’s strengths that respond to the anomalous SST of ENSO, therefore disturbing the simulation of ENSO phase-locking. Improvement of the simulated ENSO phase-locking should be realized by focusing on the zonal SST gradient’s seasonal cycle.

The characteristics of El-Niño-Southern Oscillation (ENSO) phase-locking in observations and CMIP5 and CMIP6 models are examined in this study. Two metrics based on the peaking month histogram for all El Niño and La Niña events are adopted to delineate the basic features of ENSO phase-locking in terms of the preferred calendar month and strength of this preference. It turns out that most models are poor at simulating the ENSO phase-locking, either showing little peak strengths or peaking at the wrong seasons. By deriving ENSO’s linear dynamics based on the conceptual recharge oscillator (RO) framework through the seasonal linear inverse model (sLIM) approach, various simulated phase-locking behaviors of CMIP models are systematically investigated in comparison with observations. In observations, phase-locking is mainly attributed to the seasonal modulation of ENSO’s SST growth rate. In contrast, in a significant portion of CMIP models, phase-locking is co-determined by the seasonal modulations of both SST growth and phase-transition rates. Further study of the joint effects of SST growth and phase-transition rates suggests that for simulating realistic winter peak ENSO phase-locking with the right dynamics, climate models need to have four key factors in the right combination: (1) correct phase of SST growth rate modulation peaking at the fall; (2) large enough amplitude for the annual cycle in growth rate; (3) amplitude of semi-annual cycle in growth rate needs to be small; and (4) amplitude of seasonal modulation in SST phase-transition rate needs to be small.

El Niño Southern Oscillation (ENSO) is the dominant climate variability that includes coupled air-sea interaction in the equatorial Pacific on interannual time scales. Warm water volume (WWV) variability near the equatorial Pacific serves as the precursor of ENSO events. However, WWV is no longer a good predictor after 1998. Additionally, the Victoria mode (VM), the second dominant Empirical Orthogonal Function (EOF) mode poleward of 20°N of monthly sea surface temperature anomalies (SSTa) in the North Pacific may lead to the development of the ENSO events. Composite analysis confirms that the joint impacts of positive (negative) VM and positive (negative) WWV favor the development of El Niño (La Niña) events. The VM may favor the occurrence of anomalous westerlies in the tropics that drives oceanic Kelvin waves, causing the eastward propagation of WWV to trigger the ENSO events. The main contribution of VM on the WWV change leading to the ENSO development is verified using the Community Earth System Model. When the VM and WWV indices have the same sign, the model tends to develop ENSO events a few months later. An additional sensitivity experiment shows that the correlation between VM and Niño3.4 can be increased if the VM pattern is consistently imposed on the model, suggesting the controlling role of VM on the ENSO events. These results confirm the distinct roles of VM and WWV as the critical predictors of ENSO variability. Finally, although recent studies indicate that WWV is not a good predictor of ENSO events after 2000, the El Niño event in 2015-16 can verify that the joint impacts of VM and WWV variability on ENSO evolution, making the WWV more predictable about the ENSO event in consideration of the VM variability. 

Quasi-decadal climate of the Kuroshio Extension (KE) is pivotal to understanding the North Pacific coupled ocean-atmosphere dynamics and their predictability. Recent observational studies suggest that extratropical-tropical coupling between the KE and the central tropical Pacific El Niño Southern Oscillation (CP-ENSO) leads to the observed preferred decadal time-scale of Pacific climate variability. By combining reanalysis data with numerical simulations from a high-resolution climate model and a linear inverse model (LIM), we confirm that KE and CP-ENSO dynamics are linked through extratropical-tropical teleconnections. Specifically, the atmospheric response to the KE excites Meridional Modes that energize the CP-ENSO (extratropics->tropics), and in turn, CP-ENSO teleconnections energize the extratropical atmospheric forcing of the KE (tropics->extratropics). However, both observations and the model show that the KE/CP-ENSO coupling is non-stationary and has intensified in recent decades after the mid-1980. Given the short length of the observational and climate model record, it is difficult to attribute this shift to anthropogenic forcing. However, using a large-ensemble of the LIM we show that the intensification in the KE/CP-ENSO coupling after the mid-1980 is significant and linked to changes in the KE atmospheric downstream response, which exhibit a stronger imprint on the subtropical winds that excite the Pacific Meridional modes and CP-ENSO.

Whether and how the El Niño-Southern Oscillation (ENSO) exerts an influence on sea surface temperature (SST) variance over the Yellow and East China Seas (YECS) is an ongoing debate. Such debate owes partially to the complexity of multivariate oceanic and atmospheric processes under the evolving climate in both YECS and tropical Pacific. Through estimations on reconstructed datasets for the period 1982−2020 after removing a long-term trend, we found that there was a dramatic shift in the early summer connection between the YECS and the tropical Pacific in the early 2000s. The summer SSTs in the early years seem to be modulated by the western North Pacific Subtropical High (wNPSH) with its marginal coupling to the tropical Pacific. In contrast, an interhemispheric YECS−tropical southeastern Pacific (SEP) coupling appears during recent years. This teleconnection might be attributed to reduced El Niño signature in the tropical Pacific, which likely modulates large-scale atmospheric circulation and rainfall over the YECS, in tandem with simultaneous forming the South Pacific meridional mode (SPMM) in the SEP, highlighting the role of air-sea interaction in the tropical Pacific on shaping the early summer state of the YECS and the SEP. In the presentation, we will discuss whether this abrupt connection is recursive as intrinsic natural variability or an episode under global warming.

Based on a modified mixed layer depth calculation method with reanalysis data SODA 2.2.4, the second leading empirical orthogonal function (EOF) analysis mode (EOF2) of North Pacific winter mixed layer depth (MLD) anomalies is discussed, which has opposite signals centered in Kuroshio Extension (KE) region and central North Pacific. This result is robust among another three reanalysis data with the variance close to the variance of EOF1. The regression pattern of heat flux on the corresponding Principal Components of EOF2 (PC-2) implies that the downward heat flux is positive related with MLD in KE region, and the sensible heat flux is more dominant compared with latent heat flux. Besides the PC-2 is highly positive correlated with Nino3.4 index in interannual time scales which may be explained by Kuroshio transport in KE region that adjusted by ENSO. And on decadal time scales PC2 is highly negative correlated with the Pacific decadal oscillation.

It has been reported that the sea surface temperature (SST) trend of the East China Sea during the 20th century was a couple of times larger than the global mean SST trend. However, the detailed spatial structure of the SST trend in the East China Sea and its mechanism have not been understood. The present study examines the SST trend in the East China Sea from 1901 to 2010 using observational data and a Regional Ocean Modeling System (ROMS) with an eddy-resolving horizontal resolution. A comparison among two observational datasets and the model output reveal that enhanced SST warming occurred along the Kuroshio and along the coast of China over the continental shelf. In both regions, the SST trends were the largest in winter. The heat budget analysis using the model output indicates that the upper layer temperature rises in both regions were induced by the trend of ocean advection, which was balanced to the increasing of surface net heat release. In addition, the rapid SST warming along the Kuroshio was induced by the acceleration of the Kuroshio. Sensitivity experiments revealed that this acceleration was likely caused by the negative wind stress curl anomalies over the North Pacific. In contrast, the enhanced SST warming along the China coast resulted from the ocean circulation change over the continental shelf by local atmospheric forcing.