Eastward Eddy Shedding of the Asian Summer Monsoon Anticyclone

<p>Eastward eddy shedding of the Asian summer monsoon (ASM) anticyclone has a large impact on the chemical composition of the upper troposphere and lower stratosphere (UTLS) over the western Pacific. Here we investigate the dynamical mechanism of eastward eddy shedding in July and August using 41 years of the ERA5 6-hourly reanalysis data. We perform composite analyses of meteorological variables focusing on the eastward eddy shedding events with the presence of anticyclonic centers falling between 135<sup>•</sup>-140<sup>•</sup>E. The composited outgoing longwave radiation anomalies suggest enhanced convection near the Philippines Sea and the East China Sea one week beforehand. In the tropopause level, we see evident eastward propagating geopotential and meridional wind anomalies from the North Atlantic jet exit toward the western Pacific embedded along the extratropical westerly jet during day -10 to day 0. In the lower troposphere, we find that the geopotential anomalies aligned meridionally from the east Asian coast to the North Pacific to the northern North America during day -7 to day 0. The wave-activity flux is evaluated to identify the origin and propagation of the energy of the Rossby wave–like perturbation. In the UTLS we find a strong southeastward-pointing flux along 40<sup>•</sup>-50<sup>•</sup>N, resembling the Silk Road pattern. While in the lower troposphere, we also see a northeastward-pointing flux originating from tropical Philippine Sea across Japan to North America, resembling the Pacific-Japan pattern. Additional analysis is needed to study the relationship between the Silk Road pattern and the Pacific-Japan pattern.</p>

Zonal scale and temporal variability of the Asian monsoon anticyclone in an idealised numerical model

The upper-level monsoon anticyclone is studied in a 3-D dry dynamical model as the response of a background circulation without any imposed zonal structure to a steady imposed zonally confined heat source. The characteristics of the background circulation are determined by thermal relaxation towards a simple meridionally varying state, which gives rise to baroclinic instability if meridional gradients are sufficiently large. This model configuration allows study of the dependence of the monsoon anticyclone response on characteristics of both the imposed heating and the background state, in particular including interactions between the anticyclone and the active dynamics on its poleward side in the form of the jet and baroclinic eddies. As characteristics of forcing and background state are varied a range of different behaviours emerges, many of which strongly resemble phenomena and features associated with the monsoon anticyclone as observed in re-analysis data. For a resting background state the time-mean anticyclone is highly extended in longitude to the west of the forcing region. When the active mid-latitude dynamics is included the zonal extent of the time-mean anticyclone is limited, without any need for the explicit upper-level momentum dissipation which is often included in simple theoretical models, but difficult to justify physically. We further describe in detail the spontaneous emergence of temporal variability in the form of westward eddy shedding from the monsoon anticyclone for varying strength of the imposed heating. By varying the strength of the background mid-latitude dynamics we observe a transition of the system from a state with periodic westward eddy shedding to a state dominated by eastward shedding. The details of the time-mean structure and temporal evolution depend on the structure of the background flow and for certain flows the monsoon anticyclone shows signs of both westward and eastward shedding.

Instabilities and Multiscale Interactions Underlying the Loop Current Eddy Shedding in the Gulf of Mexico

A recently developed tool, the multiscale window transform, along with the theory of canonical energy transfer is used to investigate the roles of multiscale interactions and instabilities in the Gulf of Mexico Loop Current (LC) eddy shedding. A three-scale energetics framework is employed, in which the LC system is reconstructed onto a background flow window, a mesoscale eddy window, and a high-frequency eddy window. The canonical energy transfer between the background flow and the mesoscale windows plays an important role in LC eddy shedding. Barotropic instability contributes to the generation/intensification of the mesoscale eddies over the eastern continental slope of the Campeche Bank. Baroclinic instability favors the growth of the mesoscale eddies that propagate downstream to the northeastern portion of the well-extended LC, eventually causing the shedding by cutting through the neck of the LC. These upper-layer mesoscale eddies lose their kinetic energy back to the background LC through inverse cascade processes in the neck region. The deep eddies obtain energy primarily from the upper layer through vertical pressure work and secondarily from baroclinic instability in the deep layer. In contrast, the canonical energy transfer between the mesoscale and the high-frequency frontal eddy windows accounts for only a small fraction in the mesoscale eddy energy balance, and this generally acts as a damping mechanism for the mesoscale eddies. A budget analysis reveals that the mesoscale eddy energy gained through the instabilities is balanced by horizontal advection, pressure work, and dissipation.

Influence of an Island on Hysteresis of a Western Boundary Current Flowing across a Gap

We investigate impact of an island on hysteresis of a western boundary current (WBC) flowing across a gap using a nonlinear 1.5-layer ocean model. The results of hysteresis curves show the island in the middle of the gap facilitates the WBC intrusion. The inserted (removed) island in the middle of the gap promotes the WBC to shed eddy (leap across the gap) when the WBC path transits from the periodic penetrating (leaping) to the leaping (periodic penetrating) regime without (with) an island. Vorticity balance analysis reveals that the WBC transition from the eddy-shedding (leaping) to the leaping (eddy-shedding) regime is induced by increased (decreased) meridional advection. Moreover, the critical Reynolds number of the WBC at the Hopf bifurcation is not sensitive to the size and location of the island when the total gap width is fixed. The critical Reynolds number of the WBC translating from the eddy shedding to the leaping regime increases when either the total gap width increases or the island’s meridional length increases; however, the critical Reynolds number is inversely proportional to the width of the southern gap with fixed total gap width and enlarged island length. The island promotes the WBC to shed eddy except when the island is near the northern barrier. The influence of an eastward-shifted island on the WBC transition from the eddy-shedding to the leaping regime is gradually reduced when the island is east of the Munk layer.

Transport of trace gases via eddy shedding from the Asian summer monsoon anticyclone and associated impacts on ozone heating rates

The highly vibrant Asian summer monsoon (ASM) anticyclone plays an important role in efficient transport of Asian tropospheric air masses to the extratropical upper troposphere and lower stratosphere (UTLS). In this paper, we demonstrate long-range transport of Asian trace gases via eddy-shedding events using MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) satellite observations, ERA-Interim reanalysis data and the ECHAM5–HAMMOZ global chemistry-climate model. Model simulations and observations consistently show that Asian boundary layer trace gases are lifted to UTLS altitudes in the monsoon anticyclone and are further transported horizontally eastward and westward by eddies detached from the anticyclone. We present an event of eddy shedding during 1–8 July 2003 and discuss a 1995–2016 climatology of eddy-shedding events. Our analysis indicates that eddies detached from the anticyclone contribute to the transport of Asian trace gases away from the Asian region to the western Pacific (20–30∘ N, 120–150∘ E) and western Africa (20–30∘ N, 0–30∘ E). Over the last two decades, the estimated frequency of occurrence of eddy-shedding events is ∼68 % towards western Africa and ∼25 % towards the western Pacific. Model sensitivity experiments considering a 10 % reduction in Asian emissions of non-methane volatile organic compounds (NMVOCs) and nitrogen oxides (NOx) were performed with ECHAM5–HAMMOZ to understand the impact of Asian emissions on the UTLS. The model simulations show that transport of Asian emissions due to eddy shedding significantly affects the chemical composition of the upper troposphere (∼100–400 hPa) and lower stratosphere (∼100–80 hPa) over western Africa and the western Pacific. The 10 % reduction of NMVOCs and NOx Asian emissions leads to decreases in peroxyacetyl nitrate (PAN) (2 %–10 % near 200–80 hPa), ozone (1 %–4.5 % near ∼150 hPa) and ozone heating rates (0.001–0.004 K day−1 near 300–150 hPa) in the upper troposphere over western Africa and the western Pacific.

Strouhal Number for VIV Excitation of Long Slender Structures

The prediction of Vortex-Induced Vibration (VIV) of cylinders under fluid flow conditions depends upon the eddy shedding frequency, conventionally described by the Strouhal Number. The most commonly cited relationship between Strouhal Number and Reynolds Number for circular cylinders was developed by Lienhard [1], whereby the Strouhal Number exhibits a consistent narrow band of about 0.2 (conventional across the sub-critical Re range), with a pronounced hump peaking at about 0.5 within the critical flow regime. The source data underlying this relationship is re-examined, wherein it was found to be predominantly associated with eddy shedding frequency about fixed or stationary cylinders. The pronounced hump appears to be an artefact of the measurement techniques employed by various investigators to detect eddy-shedding frequency in the wake of the cylinder. A variety of contemporary test data for elastically mounted cylinders, with freedom to oscillate under one degree of freedom (i.e. cross flow) and two degrees of freedom (i.e. cross flow and in-line) were evaluated and compared against the conventional Strouhal Number relationship. It is well established for VIV that the eddy shedding frequency will synchronise with the near resonant motions of a dynamically oscillating cylinder, such that the resultant bandwidth of lock-in exhibits a wider range of effective Strouhal Numbers than that reflected in the narrow-banded relationship about a mean of 0.2. However, whilst cylinders oscillating under one degree of freedom exhibit a mean Strouhal Number of 0.2 consistent with fixed/stationary cylinders, cylinders with two degrees of freedom exhibit a much lower mean Strouhal Number of around 0.14–0.15. Data supports the relationship that Strouhal Number does slightly diminish with increasing Reynolds Number. For oscillating cylinders, the bandwidth about the mean Strouhal Number value appears to remain largely consistent. For many practical structures in the marine environment subject to VIV excitation, such as long span, slender risers, mooring lines, pipeline spans, towed array sonar strings, and alike, the long flexible cylinders will respond in two degrees of freedom, where the identified difference in Strouhal Number is a significant aspect to be accounted for in the modelling of its dynamic behaviour.