On the onset of summer monsoon over India in relation to interactions of the monsoon stationary wave with transient baroclinic waves leading to monsoon cyclogenesis

The important problem of the early or late onset of summer monsoon over India is addressed in the present study and found to be related to the structure and behaviour of a monsoon stationary wave that forms over the region due to land-sea thermal contrast and interacts with travelling wave disturbances in the westerlies and the easterlies associated with the subtropical belt over Asia. Depending upon the type of coupling and decoupling that occurs between the interacting waves, monsoon advances towards India either slowly or speedily. Since northward-propagating monsoon depressions are found to accelerate the onset processes. the study carries out a detailed analysis of the interaction processes which give rise to such disturbances and determine their development and movement.

Cyclones of Subtropical Origin in the Southwest Pacific: a Climatology and Aspects of Movement and Development

<p>A comprehensive study on cyclones of subtropical origin (STCs) in the Southwest Pacific is carried out. A brief history of the damage caused by STCs in New Zealand between 1990 and 2005 is given. It shows that approximately 2 to 3 times a year STCs come into the vicinity of New Zealand, mostly affecting the North Island and causing predominantly flood damage. A climatology is compiled with a cyclone track database covering 21 years, providing an overview of the behaviour and characteristics of STCs in this region. Distinct annual and seasonal patterns in frequency, tracks and intensity are revealed. Some of these patterns resemble those of tropical cyclones, in particular those undergoing extratropical transition, while others resemble those of extratropical cyclones in this region. In addition, it is shown that there is a significant increase in the number of summer STCs, which coincides with an increase in sea surface temperatures in the area. The structure and processes involved in the development of STCs are investigated in more detail using data from the United Kingdom Meteorological Office (UKMO) global model spanning 5 years (1999 to 2003). An analysis of the upper-level flow shows that STCs are steered into midlatitudes by upper-level baroclinic waves, m general through interaction with an upper-level trough. Differences in the structure and development of STCs can be attributed to the fact that upper-level baroclinic waves are able to propagate far into the sub tropics in this region. This is also the reason for the existence of three types of STCs, when differentiating by characteristics of their development process. Type 1 STCs are very similar to extratropical cyclones in structure and development. The structure and the development process of Type 3 STCs resemble more those of tropical cyclones. The initial development of Type 2 STCs is similar to that of Type 3, but they then undergo a transition, found to be very similar to that of tropical cyclones undergoing extratropical transition. Interseasonal variations in the upper-level flow over the Southwest Pacific are reflected in the behaviour and characteristics of STCs and subsequently the occurrence of the three types of STCs. During the colder seasons baroclinic waves frequently propagate relatively far into the subtropics in this region. This means STCs not only have a high chance of being picked up by an upper-level trough and undergoing extratropical transition, they are also able to actually form in the vicinity of a trough. Thus, during that time most STCs tend to be either Type 1 or 2. On the other hand, during summer, when baroclinic waves only occasionally propagate into the subtropics, there is a higher frequency of Type 3 STCs. In terms of weather-related threats to New Zealand, the interaction with an upperlevel trough is the cause for STCs coming into the vicinity of New Zealand, while the high rain rates that accompany them, and that are the cause for the extensive, mostly flood-related, damage, are attributed to their place of origin.</p>

Cyclones of Subtropical Origin in the Southwest Pacific: a Climatology and Aspects of Movement and Development

<p>A comprehensive study on cyclones of subtropical origin (STCs) in the Southwest Pacific is carried out. A brief history of the damage caused by STCs in New Zealand between 1990 and 2005 is given. It shows that approximately 2 to 3 times a year STCs come into the vicinity of New Zealand, mostly affecting the North Island and causing predominantly flood damage. A climatology is compiled with a cyclone track database covering 21 years, providing an overview of the behaviour and characteristics of STCs in this region. Distinct annual and seasonal patterns in frequency, tracks and intensity are revealed. Some of these patterns resemble those of tropical cyclones, in particular those undergoing extratropical transition, while others resemble those of extratropical cyclones in this region. In addition, it is shown that there is a significant increase in the number of summer STCs, which coincides with an increase in sea surface temperatures in the area. The structure and processes involved in the development of STCs are investigated in more detail using data from the United Kingdom Meteorological Office (UKMO) global model spanning 5 years (1999 to 2003). An analysis of the upper-level flow shows that STCs are steered into midlatitudes by upper-level baroclinic waves, m general through interaction with an upper-level trough. Differences in the structure and development of STCs can be attributed to the fact that upper-level baroclinic waves are able to propagate far into the sub tropics in this region. This is also the reason for the existence of three types of STCs, when differentiating by characteristics of their development process. Type 1 STCs are very similar to extratropical cyclones in structure and development. The structure and the development process of Type 3 STCs resemble more those of tropical cyclones. The initial development of Type 2 STCs is similar to that of Type 3, but they then undergo a transition, found to be very similar to that of tropical cyclones undergoing extratropical transition. Interseasonal variations in the upper-level flow over the Southwest Pacific are reflected in the behaviour and characteristics of STCs and subsequently the occurrence of the three types of STCs. During the colder seasons baroclinic waves frequently propagate relatively far into the subtropics in this region. This means STCs not only have a high chance of being picked up by an upper-level trough and undergoing extratropical transition, they are also able to actually form in the vicinity of a trough. Thus, during that time most STCs tend to be either Type 1 or 2. On the other hand, during summer, when baroclinic waves only occasionally propagate into the subtropics, there is a higher frequency of Type 3 STCs. In terms of weather-related threats to New Zealand, the interaction with an upperlevel trough is the cause for STCs coming into the vicinity of New Zealand, while the high rain rates that accompany them, and that are the cause for the extensive, mostly flood-related, damage, are attributed to their place of origin.</p>

Planetary-wave induced strengthening of the AMOC forced by poleward intensified Southern Hemisphere westerly winds

The Atlantic meridional overturning circulation (AMOC) plays a key role in determining the distribution of heat and nutrients in the global ocean. Climate models suggest that Southern Ocean winds will strengthen and shift poleward in the future, which could have implications for future AMOC trends. Using a coupled global-ocean sea-ice model at 1/4°horizontal resolution, we study the response of the North Atlantic overturning to two anomalous Southern Ocean wind-forcing (τ+15%), and a poleward intensification(). In both scenarios a strengthening in the North Atlantic overturning develops within a decade, with a much stronger response in the case. In , we find that the primary link between the North Atlantic response and the Southern Ocean forcing is via the propagation of baroclinic waves. In fact, due to the rapid northward propagation of these waves, changes in the AMOC in the case appear to originate in the North Atlantic and propagate southward, whereas in the τ+15% case AMOC anomalies propagate northward from the Southern Ocean. We find the difference to be predominately caused by the sign of the baroclinic waves propagating from the forcing region into the North Atlantic; downwelling in the τ+15% case, versus upwelling in the case. In the case, upwelling waves propagating into the NADW formation regions along shelf-slope topography bringing dense water to the surface. This reduces vertical density gradients leading to deeper wintertime convective overturn of surface waters, and an intensification of the AMOC.

Downstream Suppression of Baroclinic Waves

Baroclinic waves drive both regional variations in weather and large-scale variability in the extratropical general circulation. They generally do not exist in isolation, but rather often form into coherent wave packets that propagate to the east via a mechanism called downstream development. Downstream development has been widely documented and explored. Here we document a novel but also key aspect of baroclinic waves: the downstream suppression of baroclinic activity that occurs in the wake of eastward propagating disturbances. Downstream suppression is apparent not only in the Southern Hemisphere storm track as shown in previous work, but also in the North Pacific and North Atlantic storm tracks. It plays an essential role in driving subseasonal periodicity in extratropical eddy activity in both hemispheres, and gives rise to the observed quiescence of the North Atlantic storm track 1–2 weeks following pronounced eddy activity in the North Pacific sector. It is argued that downstream suppression results from the anomalously low baroclinicity that arises as eastward propagating wave packets convert potential to kinetic energy. In contrast to baroclinic wave packets, which propagate to the east at roughly the group velocity in the upper troposphere, the suppression of baroclinic activity propagates eastward at a slower rate that is comparable to that of the lower to midtropospheric flow. The results have implications for understanding subseasonal variability in the extratropical troposphere of both hemispheres.

The Effect of Quadric Shear Zonal Flows and Beta on the Downstream Development of Unstable Baroclinic Waves

In this paper, the influence of quadric shear basic Zonal flows and β on the downstream development of unstable chaotic baroclinic waves is studied from the two-layer model in wide channel controlled by quasi geostrophic potential vorticity equation. Through the obtained Lorentz equation, we focused on the influence of the quadric shear zonal flow (the second derivative of the basic zonal flow is constant) on the downstream development of baroclinic waves. In the absence of zonal shear flow, chaotic behavior along feature points would occur, and the amplitude would change rapidly from one feature to another, that is, it would change very quickly in space. When zonal shear flow is introduced, it will smooth the solution of the equation and reduce the instability, and with the increase of zonal shear flow, the instability in space will increase gradually. So the quadric shear zonal flow has great influence on the stability in space.

Locating sources of variability in the transition to Structural Vacillation in the baroclinic annulus

&lt;p&gt;The baroclinic rotating annulus is a classic experiment to investigate the transition from regular waves to complex flows. &amp;#160;A well documented transition via Amplitude Vacillation leads to low-dimensional chaos through a sequence of canonical bifurcations. &amp;#160;However, the transition to geostrophic turbulence is usually through a regime of 'Structural Vacillation' (SV) which retains the overall spatial structure of regular waves but includes small-scale variability. &amp;#160;Even though the SV vacillation occurs with a clear time scale, the dynamics of SV cannot usually be described by low-dimensional dynamics. &amp;#160;For example, attractor dimension estimations tend to fail: they may not show any scaling region or converge to an unrealistic values. &amp;#160;Explanations of the origin of SV have variously invoked higher radial modes of the fundamental baroclinic waves, local secondary instabilities in the baroclinic waves caused by high thermal gradients (gravity waves) or velocity shear (barotropic instability), or instabilities within the side-wall (Stewartson) boundary layers.&lt;/p&gt;&lt;p&gt;The aim of this paper is to identify where within the fluid different signals of variability are located at different stages in the transition from a steady wave to pronounced SV. &amp;#160; To this end, a set of experiments in a water-filled rotating annulus with a free surface (inner radius 45 mm, outer radius 120 mm, fluid depth 140 mm) was carried out covering a temperature difference between the heated outer wall and the cooler inner wall of between 6 and 9.5 K, and a range of rotation rates from 0.84 to 2.29 rad/s (&lt;em&gt;Ta&lt;/em&gt;= 4.75 x 10&lt;sup&gt;7&lt;/sup&gt; - 3.53 x 10&lt;sup&gt;8&lt;/sup&gt; and &lt;em&gt;&amp;#920;&lt;/em&gt; = 0.0617 - 0.629). &amp;#160; The flow was observed through an infrared camera capturing the temperatures of the free surface. &amp;#160;Images of the flow were recorded for a period of 15 minutes at a sampling rate of 1 Hz at the lower rotation rates and 2 Hz at the higher rotation rates.&lt;/p&gt;&lt;p&gt;The initial processing of the time series of temperature images involved normalisation of the temperatures followed by rotation of the images to a coordinate system co-rotating with the main baroclinic wave mode. The resulting images were separated into the time-mean wave field and the fluctuation field, resulting in a set of normalised temperature fluctuations at fixed points relative to the main baroclinic wave. &amp;#160; Each of the time series was then used to calculate the power spectrum at that location. &amp;#160;The low-frequency part of the spectra (up until half the tank rotation frequency) was used in a k-means cluster analysis to identify clusters of similar spectral shape and, from this, create a map of which spectral shape was found at which location in the flow field.&lt;/p&gt;&lt;p&gt;The results show isolated locations of a high frequency peak near the inner boundary at the onset of visible fluctuations. &amp;#160;Further into the regime of clear structural vacillations, areas of pronounced variability at lower frequencies become visible at the lee shoulder of the cold jets in the fluid interior, followed by activity where the end of the cold jet interacts with the hot jet emanating from the outer boundary layer.&lt;/p&gt;

Normal Mode Spectra of Idealized Baroclinic Waves

Normal modes are used to investigate the contributions of geostrophic vortices and inertia–gravity waves to the energy spectrum of an idealized baroclinic wave simulation. The geostrophic and ageostrophic modal spectra (GE and AE, respectively) are compared to the rotational and divergent kinetic energy (RKE and DKE, respectively), which are often employed as proxies for vortex and wave energy. In our idealized f-plane framework, the horizontal modes are Fourier, and the vertical modes are found by solving an appropriate eigenvalue problem. For low vertical mode number n, both the GE and AE spectra are steep; however, for higher n, while both spectra are shallow, the AE is shallower than the GE and the spectra cross. The AE spectra are peaked at the Rossby deformation wavenumber knR, which increases with n. Analysis of the horizontal mode equations suggests that, for large wavenumbers k≫knR, the GE is approximated by the RKE, while the AE is approximated by the sum of the DKE and potential energy. These approximations are supported by the simulations. The vertically averaged RKE and DKE spectra are compared to the sum of the GE and AE spectra over all vertical modes; the spectral slopes of the GE and AE are close to those of the RKE and DKE, supporting the use of the Helmholtz decomposition to estimate vortices and waves in the midlatitudes. However, the AE is consistently larger than the DKE because of the contribution from the potential energy. Care must be taken when diagnosing the mesoscale transition from the intersection of the vortex and wave spectra; GE and AE will intersect at a different scale than RKE and DKE, despite their similar slopes.