Abstract

A weakly nonlinear theory is presented that may explain the persistence of the two main types of low-frequency anomalies observed in the midlatitude jet stream by Dole and Gordon (1983). The theory describes how nonlinearity can balance dispersion effects for both split jet stream anomalies (which resemble blocking flows) and for jet intensification anomalies. It is shown that the variation of the potential or refractive-index function ≡ dq/dψ across the jet stream is crucial for determining which types of anomaly will tend to persist. Although the theory is only weakly nonlinear it is argued that the same dynamical mechanisms will remain important in the high-amplitude regime particularly for the intense-jet anomalies. In the split anomalies the potential vorticity contours can easily become closed at high amplitude hence trapping air parcels (this is the origin of the strongly nonlinear modon solutions). However, even for very strong intense-jet anomalies the potential vorticity contours may remain open and then no air trapping occurs, thus, the variations in the cross jet stream potential function remain important. Initial value numerical experiments are presented to demonstrate that both types of anomaly are close to persistent states of the full barotropic vorticity equation, even at amplitudes that are beyond the strict range of validity of the weakly nonlinear theory. Some discussion and investigation of the possible role of critical lines in preventing dispersion into equatorial latitudes is also presented. Finally, the possibility of testing this theory by making appropriate diagnostic measurements is considered.

Abstract

A diagnostic study of persistent intense jet stream events in the Pacific has been carried out with a 15 winter NMC dataset to assess the relevance of the weakly nonlinear model recently proposed by Haines and Malanotte-Rizzoli. Composited data from 14 episodes of persistent intensification anomalies in the central and eastern Pacific have been analyzed with scatter diagrams of potential vorticity q plotted against geopotential Φ on the 300-mb surface. The slope of the functional relationship gives a measure of the wavelength independent component of the refractive index (n ² = −Λ₀ = −dq ₀/dψ ₀). The theoretical model suggests that if dq ₀/dψ ₀ is more negative on the northern and southern flanks, a local intense region within the jet stream may be abnormally persistent. The composited dataset shows that this condition is satisfied during the postonset period as defined by Dole. In contrast, the climatology and the mean flow before onset does not show much variation in Λ₀ across the jet. Results are encouraging, but higher-resolution data is needed to draw firm conclusions.

Abstract

We examine the nonlinear evolution of barotropic β-plane jets on a periodic domain with a pseudospectral. A calculation of the linear growth rate yields an infected U-shaped curve on the β versus k ₀ plane which separates regions of stability and instability. This curve aids in clarifying the morphology of the nonlinear structures which evolve from monochromatic small-amplitude perturbations of wavenumber k ₀. At very small or zero β, we recover and further quantify previously obtained results, including formation of: dipolar vortex structures or bound pools of opposite-signed vortex regions at small k ₀; staggered streets isolated vortex pools at intermediate k ₀; and “cat-eyes” or staggered connected pools of vorticity at large but still unstable k ₀.

As β is increased, the jet exhibits quite different evolutionary patterns. At low k ₀, where the laminar jet may be stable, we find a multistage instability. First, neutrally stable long-wavelength modes of small amplitude interact nonlinearly to produce harmonics in the linear unstable band. These grow at an exponential rate until a near-steady wake appears. However, the wake is unstable to the initial long wavelength modes and a rapid merger (i.e., backward energy cascade) occurs.

At an intermediate k ₀, the presence of β causes a “reversal” of vortex pools in the meridional direction of the near-steady vortex street. That is for a west-to-east flowing jet the Positive pools of vorticity are south of the negative pools causing a decrease in the near-steady velocity of the jet. Retrograde Rossby radiation is observed and weak “shingles” or cast-away vortex pools are observed. The meander amplitude pulsates, pumping Rossby radiation into the far field. The merger and binding processes also occur on a jet excited by many harmonics, with an ensuing chaos that is very sensitive to initial conditions.

Abstract

Based on a high-resolution (0.1° × 0.1°) regional ocean model covering the entire northern Pacific, this study investigated the seasonal and interannual variability of the Indonesian Throughflow (ITF) and the South China Sea Throughflow (SCSTF) as well as their interactions in the Sulawesi Sea. The model efficiency in simulating the general circulations of the western Pacific boundary currents and the ITF/SCSTF through the major Indonesian seas/straits was first validated against the International Nusantara Stratification and Transport (INSTANT) data, the OFES reanalysis, and results from previous studies. The model simulations of 2004–12 were then analyzed, corresponding to the period of the INSTANT program. The results showed that, derived from the North Equatorial Current (NEC)–Mindanao Current (MC)–Kuroshio variability, the Luzon–Mindoro–Sibutu flow and the Mindanao–Sulawesi flow demonstrate opposite variability before flowing into the Sulawesi Sea. Although the total transport of the Mindanao–Sulawesi flow is much larger than that of the Luzon–Mindoro–Sibutu flow, their variability amplitudes are comparable but out of phase and therefore counteract each other in the Sulawesi Sea. Budget analysis of the two major inflows revealed that the Luzon–Mindoro–Sibutu flow is enhanced southward during winter months and El Niño years, when more Kuroshio water intrudes into the SCS. This flow brings more buoyant SCS water into the western Sulawesi Sea through the Sibutu Strait, building up a west-to-east pressure head anomaly against the Mindanao–Sulawesi inflow and therefore resulting in a reduced outflow into the Makassar Strait. The situation is reversed in the summer months and La Niña years, and this process is shown to be more crucially important to modulate the Makassar ITF’s interannual variability than the Luzon–Karimata flow that is primarily driven by seasonal monsoons.