Basin-scale quasi-geostrophic gyres are common features of large lakes subject to Coriolis force. Cyclonic gyres are often characterized by dome-shaped thermoclines that form due to pelagic upwelling which takes place in their center. At present, dynamics of pelagic upwelling in the Surface Mixed Layer (SML) of oceans and lakes are poorly documented. A unique combination of high-resolution 3D numerical modeling, satellite imagery and field observations allowed confirming for the first time in a lake, the existence of intense pelagic upwelling in the center of cyclonic gyres under strong shallow (summer) and weak deep (winter) stratified conditions/thermocline. Field observations in Lake Geneva revealed that surprisingly intense upwelling from the thermocline to the SML and even to the lake surface occurred as chimney-like structures of cold water within the SML, as confirmed by Advanced Very High-Resolution Radiometer data. Results of a calibrated 3D numerical model suggest that the classical Ekman pumping mechanism cannot explain such pelagic upwelling. Analysis of the contribution of various terms in the vertically-averaged momentum equation showed that the nonlinear (advective) term dominates, resulting in heterogeneous divergent flows within cyclonic gyres. The combination of nonlinear heterogeneous divergent flow and 3D ageostrophic strain caused by gyre distortion is responsible for the chimney-like upwelling in the SML. The potential impact of such pelagic upwelling on long-term observations at a measurement station in the center of Lake Geneva suggests that caution should be exercised when relying on limited (in space and/or time) profile measurements for monitoring and quantifying processes in large lakes.
Numerical wave models have been developed to reproduce the evolution of waves generated in all directions and over a wide range of wavelengths. The amount of wave energy in the different directions and wavelength is the result of a number of physical processes that are not well understood and that may not be represented in parameterizations. Models have generally been tuned to reproduce dominant wave properties: significant wave height, mean direction, dominant wavelengths. A recent update in wave dissipation parameterizations has shown that it can produce realistic energy levels and directional distribution for shorter waves too. Here we show that this new formulation of the wave energy sink can reproduce the variability of measured infrasound power below a frequency of 2 Hz, associated with a large energy level of waves propagating perpendicular to the wind, for waves with frequencies up to at least 1 Hz. The details are sensitive to the balance between the non-linear transfer of energy away from the wind direction, and the influence of dominant and relatively long waves on the dissipation of shorter waves in other directions.
In view of the fact that the practical salinity of the Bohai Sea in January 2007 was significantly higher than the multi-year mean with its horizontal distribution opposite to the latter, the factors that affect the interannual variation of practical salinity in the Bohai Sea are quantitatively analyzed based on the in-situ hydrological measurements, the annual runoff data of the Yellow River into the sea, as well as the precipitation and evaporation reanalysis data of EAR5. The results show that the local freshwater supply not only dominates the magnitude of salinity change in the Bohai Sea, but also causes the salinity in the central Bohai Sea is higher than that in the Bohai Strait in winter in some years which is in inverse to the climatological salinity field. The seesaw distribution characteristics of the freshwater supply of the Bohai, Yellow and East China Seas also strengthen the characteristics that the salinity horizontal distribution opposite to the climatological in the Bohai Sea in some years. Available observations also show that the nutrient and inorganic carbon of the Bohai Sea are much higher than that of the open ocean, which gives a rise of 0.02 ~0.2 g·kg-1 in Absolute Salinity. Therefore, it is necessary to replace the Practical Salinity with the Absolute Salinity for the accurately salinity changes study in the Bohai Sea.
Over the continental slope off Oregon at the US West Coast, at 44.6N, vertical stratification is found to be anomalously weak in July-August of 2014 and 2015 both in a regional ocean circulation model and Conductivity-Temperature-Depth (CTD) profile observations. To understand the responsible mechanism, we focus on the layer between the isopycnal surfaces $\sigma_\theta=26.5$ and 26.25 kg/m3 that is found between depths 100-300 m and represents material properties characteristic of the slope poleward undercurrent and shelf-slope exchange. This layer thickness, about 50 m on average, can be twice as large during the above-mentioned periods. In the 2009-2018 model analysis, this anomaly is revealed over the continental slope only in summers 2014 and 2015 and only off the Oregon and Washington coasts (40-47N). The stratification anomaly is explained as the effect of advection of the seasonal alongslope potential vorticity (PV) gradient by an anomalously strong poleward slope current. In the annual cycle, the zone of strong alongslope PV gradient is found between 40-47N, supported by the local upwelling that results in the injection of the large PV in the bottom boundary layer over the shelf followed by its offshore transport in the slope region. The positive alongslope current anomaly propagates to Oregon with coastally trapped waves as part of the El Niño oceanic response and can be up to 0.1 m/s. Advection by this anomalous poleward current results in transporting the seasonal PV gradient earlier in the season than on average.
The observed retreat and anticipated further decline in Arctic sea ice hold strong climate, environmental, and societal implications. In predicting climate evolution, ensembles of coupled climate models have demonstrated appreciable accuracy in simulating sea ice area and volume trends throughout the historical period. However, individual climate models still show significant differences in simulating the sea ice thickness distribution. To better understand individual model performance in sea ice simulation, nine climate models previously identified to provide plausible sea ice decline and global temperature change were evaluated in comparison with Arctic satellite and reanalysis derived sea ice thickness data, sea ice extent records, and atmospheric reanalysis data of surface wind and air temperature. Assessment found that the simulated spatial distribution of historical sea ice thickness varies greatly between models and that several key limitations persist among models. Primarily, most models do not capture the thickest regimes of multi-year ice present in the Wandel and Lincoln Seas; those that do, often possess erroneous positive bias in other regions such as the Laptev Sea or along the Eurasian Arctic Shelf. From analysis, no model could be identified as performing best overall in simulating historic sea ice, as model bias varies regionally and seasonally. Nonetheless, the bias maps and statistical measures derived from this analysis should enhance understanding of the limitations of each climate model. This research is motivated in-part to inform future usage of coupled climate model projection for regional modeling efforts and enhance climate change preparedness and resilience in the Arctic.
High-resolution simulations by the Regional Ocean Modeling System (ROMS) were used to investigate the dispersal of the San Francisco Bay (SFB) plume over the northern-central California continental shelf during the period of 2011 to 2012. The modeled bulk dynamics of surface currents and state variables showed many similarities to corresponding observations. After entering the Pacific Ocean through the Golden Gate, the SFB plume is dispersed across the shelf via three pathways: (i) along the southern coast towards Monterey Bay, (ii) along the northern coast towards Point Arena, and (iii) an offshore pathway restricted within the shelf break. On the two-year mean timescale, the along-shore zone of impact of the northward-dispersed plume is about 1.5 times longer than that of the southern branch. Due to the opposite surface Ekman transports induced by the northerly or southerly winds, the southern plume branch occupies a broader cross-shore extent, roughly twice as wide as the northern branch which extends roughly two times deeper due to coastal downwelling. Besides these mean characteristics, the SFB plume dispersal also shows considerable temporal variability in response to various forcings, with wind and surface-current forcing most strongly related to the dispersing direction. Applying constituent-oriented age theory, we determine that it can be as long as 50 days since the SFB plume was last in contact with SFB before being flushed away from the Gulf of the Farallones. This study sheds light on the transport and fate of SFB plume and its impact zone with implications for California’s marine ecosystems.
The vertical structure of the Kuroshio Extension (KE) is investigated using velocity measurements from a subsurface mooring array. Mode decomposition based on climatological Temperature/Salinity (T/S) data shows that the barotropic and first baroclinic normal modes dominate the vertical structure of the zonal flow in the KE. This structure is also well described by the leading mode of Empirical Orthogonal Functions (EOFs) that contains the first two vertical normal modes. Further analysis demonstrates that the projection coefficient of the mooring velocity onto the summed vertical mode could be well represented by the surface geostrophic velocity. Therefore, we propose a dynamic method that relates the surface geostrophic flow and the vertical structure of the zonal flow. The applicability of this method is verified with both reanalysis datasets and estimation from hydrographic data. The findings implicate that the KE transport can be well reproduced by surface geostrophic flow and climatological T/S data only.
Polynyas play an important role in climate change with an efficient exchange of heat and matter between the atmosphere and the ocean in polar regions. This study investigated the influence of strong tides and atmospheric forcing on the Amundsen Sea Polynya, especially focusing on large-area polynya events from 2002 to 2020. We found that the geographical locations of the polynyas are closely related to the underwater ridge, where tidal currents are relatively strong. More importantly, strong cross-ridge winds are the “triggers” above the sea surface for the initial formation of the Amundsen Sea Polynya, while strong tides under the sea surface tend to create large-area polynya. Four of the five largest polynya events occurred mainly during spring tides. Only the 2016 event occurred during the normal tide period, which was atmosphere-dominated. Strong tides significantly affect the evolution of polynyas by strengthening the vertical mixing of seawater. Given that ocean in Antarctica might become warmer, tidal mixing might enhance the mixing in the future climate.
Long-term atmospheric water vapor hydrogen (δ2H), oxygen (δ18O), and deuterium excess (d-excess) can provide unique insights into the land-atmosphere coupling processes. The in-situ measurements of atmospheric water vapor δ2H, δ18O, and d-excess were conducted above a reed wetland of Liaodong Bay (2019-2020). We found significant inter-annual variations in atmospheric water vapor isotopes between the two growing (May-September) seasons. The δ2H, δ18O, and d-excess of atmospheric water vapor exhibited different seasonal and diurnal cycles respect to the vertical (i.e., 1 m, 3 m, and 5 m) measurement heights, especially in 2019. The isotopic differences of atmospheric water vapor among vertical measurement heights were more evident in the daytime (8:00-20:00 LST) than at night (20:00-8:00 LST). Rainfall events had a direct impact on the diurnal patterns of water vapor isotopes, and the influences depended on rainfall intensities. However, only week correlations existed between water vapor isotopes and local meteorological factors (R2 = 0.01-0.16, P < 0.001), such as water vapor concentration (w), relative humidity (RH), and surface air temperature (Ta). Based on the back-air trajectory analyses, the spatial-temporal dynamics of atmospheric water vapor isotopes highly synchronized with monsoon activities. The dominant air masses in this region mainly arose from ocean sources, which contributed to 62.1 ± 12.2% (49.4-84.5%) of the total air moisture. High d-excess consistently followed the strong monsoon activities, suggesting predominating impacts of ocean air masses from the East Asian monsoon region. High-resolution measurements of atmospheric water vapor isotopes will improve our understanding of the hydrological cycles in coastal areas.
First-ever measurements of the turbulent kinetic energy (TKE) dissipation rate in the northeastern Strait of Magellan (Segunda Angostura region) taken in March 2019 are reported here. At the time of microstructure measurements, the magnitude of the reversing tidal current ranged between 0.8 and 1.2 ms-1. The probability distribution of the TKE dissipation rate in the water interior above the bottom boundary layer was lognormal with a high median value εmed =1.2x10-6 Wkg-1. Strong vertical shear, (1-2)x10-2 s-1 in the weakly stratified water interior ensued a sub-critical gradient Richardson number, Ri<10-1-10-2. In the bottom boundary layer (BBL), the vertical shear and the TKE dissipation rate both decreased exponentially with the distance from the seafloor ζ, leading to a turbulent regime with the eddy viscosity KM~10-3 m2/s, which varied with the time and location, while being independent of the vertical coordinate in the upper part of BBL (for ζ>~2 meters above the bottom).