Abstract

The nonlinear collision of two western boundary currents (WBCs) of Munk thickness L _M colliding near a gap of width 2a is studied using a 1.5-layer, reduced-gravity, quasigeostrophic ocean model. The work is a continuation of our recent study on nonlinear collision of two equal-strength WBCs at a wide gap. It is found that, for narrow gaps, a < 5.7L _M , and both of the WBCs fail to penetrate into the western basin due to the restriction of friction; for intermediate size gaps, 5.7L _M ≤ a < 9.6L _M , and multiple equilibrium states exist for the colliding WBCs: the penetrating state, the choking state, and the eddy-shedding state. The current system transits between them through a hysteresis procedure, with transitions at different Reynolds numbers from those in the equal-transport case. The stronger WBC tends to intrude more deeply into the western basin than the weaker WBC; for wide gaps, a > 9.6L _M , and only penetrating and eddy-shedding states exist. No choking state is identified for either WBC. It is found that the critical gap width for the disappearance of the choking state decreases with the asymmetry of the WBC system. The theory is used to explain some of the circulation features at the entrance of the Indonesian Throughflow in the western Pacific Ocean recently observed with satellite-tracked surface drifters.

Abstract

The nonlinear collision of two western boundary currents (WBCs) of equal transport at a gap of the western boundary is studied using a 1.5-layer reduced-gravity quasigeostrophic ocean model. It is found that, when the gap (of width 2a) is narrow, a ≤ 7.3L_M (L_M the Munk thickness), neither of the WBCs can penetrate into the western basin because of the restriction of the viscous force. When 7.3L_M < a < 9.0L_M , both WBCs penetrate into the western basin for small transport and choke for large transport. When 9.0L_M ≤ a ≤ 9.6L_M , the two WBCs penetrate for small transport, choke for intermediate transport, and shed eddies periodically for large transport. When a > 9.6L_M , no steady choking state is found. Instead, the WBCs have only two equilibrium states: the penetrating and the periodic eddy shedding states. A Hopf bifurcation is found for a > 9.0L_M . The Reynolds number (Re) of the Hopf bifurcation is sensitive to the magnitude of γ(a/L_M ) and the baroclinic deformation radius, being small for larger γ or deformation radius. In addition, a reverse Hopf bifurcations is identified in the decreased Re experiments, occurring at a smaller Re than that of the Hopf bifurcation. The Re of the reverse Hopf bifurcation is not sensitive to the magnitude of the baroclinic deformation radius.

Hysteresis behavior of the WBCs is found for a > 9.0L_M , because of the existence of the Hopf and reverse Hopf bifurcations. In between them, steady penetrating or choking states coexist with eddy-shedding states. The steady states are found to be sensitive to perturbations of relative vorticity and can transit to periodic eddy-shedding states at the forcing of a mesoscale eddy.

Abstract

Hysteresis of a western boundary current (WBC) flowing by a wide gap of a western boundary and the dynamics of the WBC variations associated with the impingement of mesoscale eddies from the eastern side of the gap are studied using a 1.5-layer reduced-gravity quasigeostrophic ocean model. The study focuses on two issues not covered by existing studies: the effects of finite baroclinic deformation radii and time dependence perturbed by mesoscale eddies. The results of the study show that the hysteresis of the WBC of finite baroclinic deformation radii is not controlled by multiple steady-state balances of the quasigeostrophic vorticity equation. Instead, the hysteresis is controlled by the periodic penetrating and the leaping regimes of the vorticity balance. The regime of the vorticity balance inside the gap is dependent on the history of the WBC evolution, which gives rise to the hysteresis of the WBC path. Numerical experiments have shown that the parameter domain of the hysteresis is not sensitive to the baroclinic deformation radius. However, the domain of the periodic solution, which is determined by the lower Hopf bifurcation of the nonlinear system, is found to be sensitive to the magnitude of the baroclinic deformation radius. The lower Hopf bifurcation from steady penetration to periodic penetration is found to occur at lower Reynolds numbers for larger deformation radii. In general, the lower Hopf bifurcation stays outside the hysteresis domain of the Reynolds number. However, for very small deformation radii, the lower Hopf bifurcation falls inside the hysteresis domain, which results in the transition from the leaping to the penetrating regimes of the WBC to skip the periodic regime and hence the disappearance of the upper Hopf bifurcation.

Mesoscale eddies approaching the gap from the eastern basin are found to have significant impact on the WBC path inside the gap when the WBC is at a critical state along the hysteresis loop. Cyclonic (anticyclonic) eddies play the role of reducing (enhancing) the inertial advection of vorticity in the vicinity of the gap so that transitions of the WBC path from the leaping (periodic penetrating) to the periodic penetrating (leaping) regimes are induced. In addition, cyclonic eddies are able to induce transitions of the WBC from the periodic penetrating to the leaping regimes through enhancing the meridional advection by its right fling. The transitions are irreversible because of the nonlinear hysteresis and are found to be sensitive to the strength, size, and approaching path of the eddy.

Abstract

Based on first principles, a theoretical model for El Niño-Southern Oscillation (ENSO) is derived that consists of prognostic equations for sea surface temperature (SST) and for thermocline variation. Considering only the large-scale, equatorially symmetric, standing basin mode yields a minimum dynamic system that highlights the cyclic, chaotic, and season-dependent evolution of ENSO.

For a steady annual mean basic state, the dynamic system exhibits a unique limit cycle solution for a fairly restricted range of air-sea coupling. The limit cycle is a stable attractor and represents an intrinsic interannual oscillation of the coupled system. The deepening (rising) of the thermocline in the eastern (western) Pacific leads eastern Pacific warming by a small fraction of the cycle, which agrees well with observation and plays a critical role in sustaining the oscillation. When the nonlinear growth of SST anomalies reaches a critical amplitude, the delayed response of thermocline adjustment provides a negative feedback, turning over warming to cooling or vice versa.

When the basic state varies annually, the limit cycle develops a strange attractor and the interannual oscillation displays inherent deterministic chaos. On the other hand, the transition phase of the oscillation tends to frequently occur in boreal spring when the basic state is most unstable. The strongest boreal spring instability is due to the weakest mean upwelling and largest vertical temperature difference across the mixed layer base. The former minimizes the negative feedback of mean upwelling, whereas the latter maximizes the positive feedback of anomalous upwelling effects on SST; both favor spring instability. It is argued that the season-dependent coupled instability may be responsible for the tendencies of ENSO phase locking with season and period-locking to integer multiples of the annual period, which, in turn, create irregularities in oscillation period and amplitude.

Abstract

The South China Sea (SCS) is affected by two intraseasonal components in summer: the Madden–Julian oscillation (MJO) and the quasi-biweekly oscillation (QBWO). In the present study, the impacts of the MJO and QBWO on tropical cyclones (TCs) locally formed in the SCS (local TCs) in summer are investigated. The results show that both the MJO and QBWO can affect the genesis frequency, location, and motion of the local TCs. More TCs form in the convectively active phases of the MJO and QBWO in the northern SCS. With the northward propagation of the MJO and QBWO convective signals, the major TC genesis location also shifts northward. Since the western Pacific subtropical high shifts eastward (westward) when convection associated with the MJO and QBWO in the northern SCS is enhanced (suppressed), the steering flow in the major TC genesis region is favorable for the eastward (westward) movement of TCs. Results from the composite analysis of the steering flow indicate that both the MJO and QBWO play an important role in controlling the motion of the eastward-moving TCs.

Abstract

Triple collocation (TC) is a popular technique for determining the data quality of three products that estimate the same geophysical variable using mutually independent methods. When TC is applied to a triplet of one point-scale in situ and two coarse-scale datasets that have the similar spatial resolution, the TC-derived performance metric for the point-scale dataset can be used to assess its spatial representativeness. In this study, the spatial representativeness of in situ snow depth measurements from the meteorological stations in northeast China was assessed using an unbiased correlation metric ${\rho }_{t,X_1}^2$ estimated with TC. Stations are considered representative if ${\rho }_{t,X_1}^2\ge 0.5$ ; that is, in situ measurements explain no less than 50% of the variations in the “ground truth” of the snow depth averaged at the coarse scale (0.25°). The results confirmed that TC can be used to reliably exploit existing sparse snow depth networks. The main findings are as follows. 1) Among all the 98 stations in the study region, 86 stations have valid ${\rho }_{t,X_1}^2$ values, of which 57 stations are representative for the entire snow season (October–December, January–April). 2) Seasonal variations in ${\rho }_{t,X_1}^2$ are large: 63 stations are representative during the snow accumulation period (December–February), whereas only 25 stations are representative during the snow ablation period (October–November, March–April). 3) The ${\rho }_{t,X_1}^2$ is positively correlated with mean snow depth, which largely determines the global decreasing trend in ${\rho }_{t,X_1}^2$ from north to south. After removing this trend, residuals in ${\rho }_{t,X_1}^2$ can be explained by heterogeneity features concerning elevation and conditional probability of snow presence near the stations.

Abstract

One central problem in the study of wind-generated gravity waves is the energy balancing process in the equilibrium spectral subrange. In considering the predicted equilibrium spectral forms from physical models proposed by Kitaigorodskii, other investigators accepted that the statistical equilibrium state is effectively characterized by the wave action conservation law: δE/δt+C⃗ _g ·∇E = 0, where E is the wave energy spectrum and C⃗ _g = ∇_kω(k) is the group velocity. Here the continuous wavelet transform is used to analyze typical sets of wind-generated gravity wave data obtained both in the ocean and in a wind-wave channel. This “space scale” analysis is shown to provide the first visual evidence that when the fetch is not very short, the wave action conservation law mentioned above is not convenient to describe the dynamics of the wave components in the equilibrium range estimated from its energy spectrum.

Abstract

Incident photosynthetically active radiation (PAR) is an important parameter for terrestrial ecosystem models. Because of its high temporal resolution, the Geostationary Operational Environmental Satellite (GOES) observations are very suited to catch the diurnal variation of PAR. In this paper, a new method is developed to derive PAR using GOES data. What makes this new method distinct from the existing method is that it does not need external knowledge of atmospheric conditions. The new method retrieves both atmospheric and surface conditions using only at-sensor radiance through interpolation of time series of observations. Validations against ground measurement are carried out at four “FLUXNET” sites. The values of RMSE of estimated and ground-measured instantaneous PAR at the four sites are 130.71, 131.44, 141.16, and 190.22 μmol m⁻² s⁻¹, respectively. At the four validation sites, the RMSE as the percentage of estimated mean PAR value are 9.52%, 13.01%, 13.92%, and 24.09%, respectively; the biases are −101.54, 16.56, 11.09, and 53.64 μmol m⁻² s⁻¹, respectively. The independence of external atmospheric information enables this method to be applicable to many situations in which external atmospheric information is not available. In addition, topographic impacts on surface PAR are examined at the 1-km resolution at which PAR is retrieved using the GOES visible band data.

Abstract

The interdecadal variability of cross-equatorial meridional winds in the eastern Pacific (V _EP), which has been shown to correlate with the amplitude of El Niño–Southern Oscillation (ENSO) on the interdecadal time scale, is investigated using long-term observations and 22 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6). Both observations and models exhibit a tight negative correlation between interannual variations of V _EP and ENSO, and this relationship is synchronized with the interdecadal variability of V _EP. In long-term observations, ENSO amplitude modulation is still seen to be out-of-phase with interdecadal variability of V _EP. This relationship, however, is substantially underestimated among CMIP6 models, particularly in the historical simulations. The interdecadal variability of V _EP is associated with both the interdecadal Pacific oscillation (IPO) and the Atlantic multidecadal oscillation (AMO) in observations. However, most CMIP6 preindustrial control (PI-control) experiments have no link between the V _EP and AMO. In contrast, in historical simulations, the multimodel mean of interdecadal V _EP shows a significant fluctuation with AMO around the 1980s, which might be caused by the anthropogenic aerosol forcing. Consequently, the interdecadal variation of the AMO–V _EP relationship is likely a response to external forcing while the IPO–V _EP relationship is mainly modulated by internal climate variability. Another plausible factor causing the weak AMO–V _EP relationship in PI-control runs is the unrealistic relationship between modeled SSTs in the tropical Pacific and North Atlantic. Furthermore, model biases in the tropical Pacific may account for the weak relationship between interdecadal V _EP and ENSO amplitude in CMIP6 models.

Abstract

The present study investigates the impact of the Madden–Julian oscillation (MJO) on the South China Sea (SCS) in summer with three types of models: a theoretical Sverdrup model, a 1.5-layer reduced gravity model, and a regional ocean model [Regional Ocean Modeling System (ROMS)]. Results show that the ocean circulation in the SCS has an intraseasonal oscillation responding to the MJO. During its westerly phase, the MJO produces positive (negative) wind stress curl over the northern (southern) SCS and thus induces an enhanced cyclonic (anticyclonic) circulation in the northern (southern) SCS. This not only cools sea surface temperature (SST) but also decreases (increases) subsurface temperature in the northern (southern) SCS. During its easterly phase, the MJO basically produces a reversed but weaker influence on SCS ocean circulation and temperature. Thus, the MJO can have an imprint on the summer climatology of SCS circulation and temperature. The authors' analysis further indicates that the MJO's dynamic effect associated with wind is generally more important than its thermodynamic effect in modulating the regional ocean circulation and temperature. The present study suggests that the MJO is important for summer ocean circulation and temperature in the SCS.