Effects of an along-shelf current on the generation of internal tides near the critical latitude

The effects of along-shelf barotropic geostrophic currents on internal wave generation by the $K_1$ tide interacting with a shelf at near-critical latitudes are investigated. The horizontal shear of the background current results in a spatially varying effective Coriolis frequency which modifies the slope criticality and potentially creates blocking regions where freely propagating internal tides cannot exist. This paper is focused on the barotropic to baroclinic energy conversion rate, which is affected by a combination of three factors: slope criticality, size and location of the blocking region where the conversion rate is extremely small and the internal tide (IT) beam patterns. All of these are sensitive to the current parameters. In our parameter space, the current can increase the conversion rate up to 10 times.

A diagnostic study on the energetic aspects of weak/strong spell of north east monsoon

An attempt has been made to study dynamics of consecutive weak/strong spell of north east monsoon for the years, 2009 and 2010 from an energetics aspect. For that different energy terms, their generation and conversion among different energy terms have been computed for consecutive weak and strong phases during Oct to Dec of the above two years over a limited region between 70 °E to 85 °E, 5 °N to 20 °N. These computations are based on daily NCEP 2.5° × 2.5° data for the same period. The transition from weak phase to strong phase of north east monsoon (NEM) observed to be associated with an enhancement in conversion of zonal available potential energy (Az) to zonal kinetic energy (Kz), implying a strengthening of Hadley circulation, favouring the above transition. It is also observed that the transition from weak phase to strong phase is associated with enhanced Baroclinic energy conversion

Energetics of transient eddies related to the midwinter minimum of the North Pacific storm-track activity

Storm-track activity over the North Pacific climatologically exhibits a clear minimum in midwinter, when the westerly jet speed sharply maximizes. This counterintuitive phenomenon, referred to as the “midwinter minimum (MWM)”, has been investigated from various perspectives, but the mechanisms are still to be unrevealed. Toward better understanding of this phenomenon, the present study delineates the detailed seasonal evolution of climatological-mean Eulerian statistics and energetics of migratory eddies along the NP storm-track over 60 years. As a comprehensive investigation of the mechanisms for the MWM, this study has revealed that the net eddy conversion/generation rate normalized by the eddy total energy, which is independent of eddy amplitude, is indeed reduced in midwinter. The reduction from early winter occurs mainly due to the decreased effectiveness of the baroclinic energy conversion through seasonally weakened temperature fluctuations and the resultant poleward eddy heat flux. The reduced net normalized conversion/generation rate in midwinter is also found to arise in part from the seasonally enhanced kinetic energy conversion from eddies into the strongly diffluent Pacific jet around its exit. The seasonality of the net energy influx also contributes especially to the spring recovery of the net normalized conversion/generation rate. The midwinter reduction in the normalized rates of both the net energy conversion/generation and baroclinic energy conversion was more pronounced in the period before the late 1980s, during which the MWM of the storm-track activity was climatologically more prominent.

Energetics of Interactions between African Easterly Waves and Convectively Coupled Kelvin Waves

Perturbation kinetic and available energy budgets are used to explore how convectively coupled equatorial Kelvin waves (KWs) impact African easterly wave (AEW) activity. The convective phase of the Kelvin wave increases the African easterly jet’s meridional shear, thus enhancing the barotropic energy conversions, leading to intensification of southern track AEWs perturbation kinetic energy. In contrast, the barotropic energy conversion is reduced in the suppressed phase of KW. Baroclinic energy conversion of the southern track AEWs is not significantly different between Kelvin waves’ convective and suppressed phases. AEWs in the convective phase of a Kelvin wave have stronger perturbation available potential energy generation by diabatic heating and stronger baroclinic overturning circulations than in the suppressed phase of a Kelvin wave. These differences suggest that southern track AEWs within the convective phase of Kelvin waves have more vigorous convection than in the suppressed phase of Kelvin waves. Barotropic energy conversion of the northern track AEWs is not significantly different between Kelvin waves’ convective and suppressed phases. The convective phase of the Kelvin wave increases the lower-tropospheric meridional temperature gradient north of the African easterly jet, thus enhancing the baroclinic energy conversion, leading to intensification of northern track AEWs perturbation kinetic energy. In contrast, the baroclinic energy conversion is reduced in the suppressed phase of KW. These results provide a physical basis for the modulation of AEWs by Kelvin waves arriving from upstream.

Transient eddy kinetic energetics on Mars in three reanalysis datasets

The ability of Martian reanalysis datasets to represent the growth and decay of short-period (1.5 < P < 8 sol) transient eddies is compared across the Mars Analysis Correction Data Assimilation (MACDA), Open access to Mars Assimilated Remote Soundings (OpenMARS), and Ensemble Mars Reanalysis System (EMARS). Short-period eddies are predominantly surface-based, have the largest amplitudes in the northern hemisphere, and are found, in order of decreasing eddy kinetic energy amplitude, in Utopia, Acidalia, and Arcadia Planitae in the northern hemisphere, and south of the Tharsis Plateau and between Argyre and Hellas Basins in the southern hemisphere. Short-period eddies grow on the upstream (western) sides of basins via baroclinic energy conversion and by extracting energy from the mean flow and long-period (P > 8 sol) eddies when interacting with high relief. Overall, the combined impact of barotropic energy conversion is a net loss of eddy kinetic energy, which rectifies previous conflicting results. When Thermal Emission Spectrometer observations are assimilated (Mars years 24–27), all three reanalyses agree on eddy amplitude and timing, but during the Mars Climate Sounder (MCS) observational era (Mars years 28–33), eddies are less constrained. The EMARS ensemble member has considerably higher eddy generation than the ensemble mean, and bulk eddy amplitudes in the deterministic OpenMARS reanalysis agree with the EMARS ensemble rather than the EMARS member. Thus, analysis of individual eddies during the MCS era should only be performed when eddy amplitudes are large and when there is agreement across reanalyses.

The influence of a geostrophic current on the internal tide generation

&lt;p&gt;We investigate the influence of a barotropic geostrophic current on&lt;br&gt;internal tide (IT) generation over a shelf slope.&lt;br&gt;The current $V_g(x)$ is modeled as an idealized Gaussian function centered at&lt;br&gt;$x_0$ with width $x_r$ and maximum velocity $V_{max}$.&lt;br&gt;The bathymetry is modelled as a linear slope with smoothed corners.&lt;br&gt;We calculate the total barotropic-to-baroclinic energy conversion $C =&lt;br&gt;\int \overbar{C} \,dx = \int \int \rho' g W \,dx\, dz$.&amp;#160;&lt;br&gt;$\overbar{C}(x,t)$ can be either positive or negative. Positive (negative) conversion means energy is&lt;br&gt;converted from barotropic to baroclinic (baroclinic to barotropic)&lt;br&gt;waves.&amp;#160;&lt;br&gt;The main conclusions are: 1) $V_g(x)$ changes the effective&lt;br&gt;frequency $f_{eff}$. This has a direct impact on the slope of the IT&lt;br&gt;characteristics and the slope criticality, which affects the total&lt;br&gt;conversion rate;&lt;br&gt;2) Since $(V_g)_x$ is not a constant value, $f_{eff}$ varies along the&lt;br&gt;slope. This has a significant effect on the IT beam generation&lt;br&gt;location and its propagation path. If the current is strong enough so&lt;br&gt;that $f_{eff}$ is greater than the barotropic tidal frequency $\sigma_T$, a blocking&lt;br&gt;region is formed where the conversion vanishes and IT propagation is blocked;&lt;br&gt;3) Changes of sign in $\bar{C}(x,t)$ correspond to the locations where&lt;br&gt;IT beams reflect from the boundaries. As a result, the total conversion rate $C$ is&lt;br&gt;also strongly affected by the IT beam pattern.&lt;br&gt;In conclusion, the total conversion rate $C$ is affected by a&lt;br&gt;combination of three factors: slope criticality, size and location of the blocking&lt;br&gt;region and the IT beam patterm, all of which can be varied by changing&lt;br&gt;the strength, width and location of the geostrophic current $V_g(x)$.&lt;/p&gt;

A teleconnection pattern of 10-30-day atmospheric oscilations over North Pacific during summer: Chracteristics and maintainance mechanism

&lt;p&gt;The present study investigates the intraseasonal variations of meridional winds over North Pacific during summer based on reanalysis datasets. It is shown that the band of 10-30 days is the main component of total intraseasonal varaitions. We identified a teleconnection pattern over North Pacific at this band . This teleconnection pattern is characterized by a zonally-oriented wave-like structure with a zonal wavenumber 5, and does not show a phase-locking feature. In addition, the anomalies associated with the teleconnection pattern exhibit a roughly baratropic structure. Further analyses suggest that the teleconnection pattern can gain energy from the basic flow through the baroclinic energy conversion, while the barotropic energy conversion plays a trivial role.&lt;/p&gt;

Maintenance Mechanism for the Teleconnection Pattern over the High Latitudes of the Eurasian Continent in Summer

There is a zonally oriented teleconnection pattern over the high-latitude Eurasian continent, which is maintained through baroclinic energy conversion. In this study, we investigate the unique features of the maintenance mechanism of this teleconnection. It is found that the baroclinic energy conversion is most efficient in both the midtroposphere and the lower troposphere, and that the baroclinic energy conversion in the lower troposphere is comparable to that in the midtroposphere. Further results indicate that the basic state plays a crucial role in the baroclinic energy conversion. For both the middle and lower troposphere, the atmospheric stability is low and the Coriolis parameter is large over high-latitude Eurasia, favoring strong baroclinic energy conversion. Particularly, in the lower troposphere, the atmospheric stability exhibits a clear land–sea contrast, favoring baroclinic energy conversion over the continents rather than the oceans. Furthermore, in the lower troposphere, the in-phase configuration of the meridional wind and temperature anomalies, which results from the strong meridional gradient of mean temperature around the north edge of the Eurasian continent, also significantly contributes to baroclinic energy conversion. This study highlights the role of the basic state of temperature rather than zonal wind in maintaining the high-latitude teleconnection through baroclinic energy conversion.

The Intraseasonal Variability of African Easterly Wave Energetics

African easterly waves (AEWs) and associated perturbation kinetic energy (PKE) exhibit significant intraseasonal variability in tropical North Africa during boreal summer. Consistent with East Africa (e.g., east of Lake Chad) being an initiation region for AEWs, previous studies have shown that increased East African PKE precedes and leads to increased West African AEW activity on intraseasonal time scales. In this study, reanalysis budgets of PKE and perturbation available potential energy (PAPE) are used to understand this behavior. The variability of PKE and PAPE sources is analyzed as a function of Madden–Julian oscillation (MJO) phase and a local 30–90-day West African PKE index to diagnose when and where eddy energy conversions terms are important to periods of increased or decreased intraseasonal AEW activity. In East Africa, an increased meridional temperature gradient locally enhances baroclinic energy conversion anomalies to initiate periods of increased intraseasonal AEW activity. Downstream barotropic and baroclinic energy conversions associated with strong AEWs are important for the maintenance of intraseasonal AEW activity in West Africa. Barotropic energy conversions dominate south of the African easterly jet (AEJ), while baroclinic energy conversions are most important north of the AEJ. In both East and West Africa, diabatic heating does not appear to aid intraseasonal PKE creation. Instead, negative PAPE tendency anomalies due to the diabatic heating–temperature covariance act as a negative feedback to increased baroclinic energy conversion downstream in the AEJ.