Why are rings regularly shed in the western equatorial Atlantic but not in the western Pacific?     

Doron Nof 《Progress in Oceanography》1996,38(4):417-451

The western equatorial Atlantic is characterized by the formation and shedding of 3–4 large anticyclonic rings per year. These rings originate from the North Brazil Current which, in response to the vanishing wind stress curl (over the ocean interior), retroflects and turns eastward at around 4°N. After their formation and shedding the rings propagate toward the northwest along the South American coast carrying an annual average of about 4Sv. As such, the rings constitute an important part of the meridional heat flux in the Atlantic.The same cannot be said, however, of the western equatorial Pacific. Here, the situation is entirely different even though the South Equatorial Current retroflects at roughly the same latitude as its Atlantic counterpart, the North Brazil Current. Although the South Equatorial Current retroflection is flanked by two quasi-permanent eddies (the so-called Halmahera and the Mindanao eddies), these eddies are an integral part of the current itself and are not shed. Consequently, they are not associated with any meridional heat flux. An important question is, then, why the two oceans behave in such a fundamentally different way even though the source of the rings, the retroflected currents, are very similar in the two oceans.To answer this question, the two oceans are compared using recently developed analytical and numerical models for the western equatorial oceans. It is first pointed out that, according to recent developments in the modelling of the western equatorial Atlantic, the North Brazil Current retroflection rings are formed, shed and drift to the west because, in the Atlantic, this is the only way by which the momentum flux of the approaching and retroflecting current can be balanced. In this scenario, the northwestward flow force exerted by the approaching and retroflecting North Brazil Current (analogous to the force created by a rocket) is balanced by the southwestward force exerted by the rings as they are formed (analogous in some sense to the kickback associated with a firing gun).On the other hand, in the western equatorial Pacific, the formation and shedding of rings is unnecessary because the southward flowing Mindanao Current provides an alternative mechanism for balancing the northward momentum flux of the South Equatorial Current. This implies that it is the absence of a counter current (such as the Mindanao) in the western Atlantic that causes the formation and shedding of North Brazil Current rings. A remaining difficulty with the above scenario is that most colliding and retroflecting currents (i.e. the Mindanao and South Equatorial currents) are not “balanced” in the sense that they cannot be stationary but rather must drift along the coast. It is shown that, in the case of the western Pacific, the long-shore migration is arrested by the Indonesian Throughflow which allows the “unbalanced” fraction of the approaching currents to leak out into the Indian Ocean. This resolves the above difficulty and allows the retroflection to be approximately steady.  相似文献   

Degradation of Adsorbed Sarin on Activated Carbons: A 31P‐MAS‐NMR Study     

Doron Kaplan  Liora Shmueli  Ido Nir  Daniel Waysbort  Ishay Columbus 《洁净——土壤、空气、水》2007,35(2):172-177

The fate of the OP nerve agent isopropyl methylphosphonofluoridate (sarin) on granular activated and metal‐impregnated activated carbons that are used in gas‐mask filters was investigated by means of ³¹P magic‐angle‐spinning (MAS) NMR spectroscopy. The results show that most of the adsorbed sarin on extensively dried carbons decomposes with a half‐life of 5–12 days. A MAS‐NMR signal of the degradation product isopropyl methylphosphonic acid (IMPA) appears in the spectra of sarin on non‐impregnated carbons. The IMPA signal is not visible in the spectra from metal‐impregnated carbons, probably due to strong binding of the acid molecule to paramagnetic Cr³⁺ and Cu²⁺ ions. Exposure of BPL and ASC carbons to air of 53% relative humidity shortens the degradation time by approximately an order of magnitude. Wetting shortens the half‐life of sarin on BPL carbon to approximately 2 hours.  相似文献   

Modons and monopoles on a γ-plane     

Doron Nof 《地球物理与天体物理流体动力学》2013,107(1-3):71-87

Abstract

The gamma plane approximation introduced in this study corresponds to a nonlinear horizontal shallow flow in a plane where, in addition to the familiar linear variation of f (i.e., β), there is a quadratic variation with latitude. Such a plane may have some application to the mesoscale oceanic flow in the immediate vicinity of the North Pole because at the pole the linear gradient (β) vanishes so that the quadratic variation (γ) is the dominant gradient. It is also applicable to the flow near the center of a rotating (laboratory) tank.

Exact analytical solutions analogous to the stationary barotropic mid-latitude modons (Stern, 1975) are constructed. First, it is shown that, for a modon situated slightly off the pole (i.e., both β and γ are present) the condition of stationarity (in a resting ocean) takes the form β ∫∫ Ψ dxdy — 2γ ∫∫ yΨ dxdy = 0, where Ψ is the streamfunction and x and y are Cartesian coordinates pointing eastward and northward, respectively. Secondly, it is shown that due to the presence of γ, the cyclonic cell situated to the north increases in size and engulfs the southern anticyclone which decreases in size. Namely, as the pole is approached the engulfing cyclone grows whereas the anticyclone shrinks. Ultimately, when the center of the modon (whose diameter is R) reaches a critical distance from the pole (0.1227 R) the anticyclone diminishes to merely a point. Modons that are closer than this critical distance to the pole cannot contain an anticyclone. Far away from the pole our solution reduces to the familiar mid-latitude β-plane modon as should be the case.

In contrast to these dramatic effects of γ on modons, the migration of monopoles (i.e., isolated cyclones or anticyclones) is almost unaffected by γ even though γ is of the same order of (or larger than) β. This results from the fact that the γ-induced perturbations are symmetrical (with respect to north and south) whereas those due to β are asymmetrical. It is shown that, as in other eddies, self-propulsion is primarily caused by asymmetrical perturbations so that disturbances due to γ have almost no influence on the migration.  相似文献