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>

Acceleration of tropical cyclones as a proxy for extratropical interactions: synoptic-scale patterns and long-term trends

It is well known that rapid changes in tropical-cyclone motion occur during interaction with extratropical waves. While the translation speed has received much attention in the published literature, acceleration has not. Using a large data sample of Atlantic tropical cyclones, we formally examine the composite synoptic-scale patterns associated with tangential and curvature components of their acceleration. During periods of rapid tangential acceleration, the composite tropical cyclone moves poleward between an upstream trough and downstream ridge of a developing extratropical wave packet. The two systems subsequently merge in a manner that is consistent with extratropical transition. During rapid curvature acceleration, a prominent downstream ridge promotes recurvature of the tropical cyclone. In contrast, during rapid tangential deceleration or near-zero curvature acceleration, a ridge is located directly poleward of the tropical cyclone. Locally, this arrangement takes the form of a cyclone–anticyclone vortex pair. On average, the tangential acceleration peaks 18 h prior to extratropical transition, while the curvature acceleration peaks at recurvature. These findings confirm that rapid acceleration of tropical cyclones is mediated by interaction with extratropical baroclinic waves. Furthermore, the tails of the distribution of acceleration and translation speed show a robust reduction over the past 5 decades. We speculate that these trends may reflect the poleward shift and weakening of extratropical Rossby waves.

The Response of Extratropical Transition of Tropical Cyclones to Climate Change: Quasi-Idealized Numerical Experiments

This study uses small ensembles of convection-allowing, quasi-idealized simulations to examine the response of North Atlantic tropical cyclones (TCs) undergoing extratropical transition (ET) to climate change. Using HURDAT2 and ERA5 data over a 40-yr period from 1979 to 2018, we developed storm-relative composite fields for past North Atlantic recurving, oceanic ET events. The quasi-idealized present-day simulations are initialized from these composites and run in an aquaplanet domain. A pseudo–global warming approach is used for future simulations: Thermodynamic changes between late twenty-first century and twentieth century, derived from an ensemble of 20 CMIP5 GCMs under the RCP8.5 scenario, are added to the present-day initial and lateral boundary conditions. The composite-initialized present-day simulations exhibit realistic ET characteristics. Future simulations show greater intensity, heavier precipitation, and stronger downstream midlatitude wave train development relative to the present-day case. Specifically, the future ET event is substantially stronger before ET completion, though the system undergoes less reintensification after ET completion. Reductions in lower-tropospheric baroclinicity associated with Arctic amplification could contribute to this result. The future simulation exhibits 3-hourly ensemble-mean precipitation rate increases ranging from ~23% to ~50%, depending on ET phase and averaging radius. In addition, larger eddy kinetic energy accompanies the future storm, partly created by increased baroclinic conversion, resulting in stronger amplification of downstream energy maxima via intensified ageostrophic geopotential flux convergence and divergence. These results suggest that future TCs undergoing ET could have greater potential to cause high-impact weather in western Europe through both direct and remote processes.

Ciclone Michael: gênese e transição extratropical

Os ciclones de escala sinótica são responsáveis por grandes mudanças no tempo das regiões onde atuam. Nos últimos anos tem aumentado o número de sistemas tropicais severos. Por exemplo, o ciclone Michael, ocorrido em outubro de 2018, causou muitos danos nos Estados Unidos (U$25 bilhões em prejuízo), inclusive 16 óbitos. Diante desse contexto, o objetivo do presente estudo é a análise sinótica da gênese e transição extratropical do ciclone Michael. Michael teve gênese no mar do Caribe, no dia 6 de outubro de 2018, associada a uma perturbação ciclônica em baixos níveis da atmosfera. O sistema chegou a categoria 5 na escala de Saffir-Simpson no dia 10 de outubro; já no dia 11 de outubro transicionou para ciclone extratropical e decaiu no dia 18 de outubro. A análise sinótica mostra que transição extratropical ocorre à medida que o sistema interage com uma região de intenso gradiente horizontal de temperatura do ar. Cyclone Michael: genesis and extratropical transition A B S T R A C T Synoptic-scale cyclones are responsible for major changes in the weather in the regions where they act. In recent years the number of severe tropical systems has increased. For example, cyclone Michael, which occurred in October 2018, caused a lot of damage in the United States ($ 25 billion in damage) including 16 deaths. Given this context, the objective of the present study is the synoptic analysis of the genesis and extratropical transition of cyclone Michael. Michael had genesis in the Caribbean Sea on October 6, 2018, associated with a cyclonic disturbance in low levels of the atmosphere. The system reached category 5 on the Saffir-Simpson scale on 10 October; on the 11 October it transitioned to an extratropical cyclone and decayed on the 18 October. The synoptic analysis shows that extratropical transition occurs as the system interacts with a region of intense horizontal air temperature gradient. Keywords: cyclone, synoptic analysis, extratropical transition.

Storm-Scale Dynamical Changes of Extratropical Transition Events in Present-Day and Future High-Resolution Global Simulations

Tropical cyclones (TCs) propagating into baroclinic midlatitude environments can transform into extratropical cyclones, in some cases resulting in high-impact weather conditions far from the tropics. This study extends analysis of extratropical transition (ET) changes in multi-seasonal global simulations using the Model for Prediction Across Scales-Atmosphere (MPAS-A) under present-day and projected future conditions. High-resolution (15 km) covers the Northern Hemisphere; TCs and ET events are tracked based on sea-level pressure minima accompanied by a warm core and use of a cyclone phase space method. Previous analysis of these simulations showed large changes in ET over the North Atlantic (NATL) basin, with ET events exhibiting a 4–5° northward latitudinal shift and a ~6 hPa strengthening of the post-transition extratropical cyclone. Storm-relative composites, primarily representing post-transformation cold-core events, indicate that this increase in post-transition storm intensity is associated with an intensification of the neighboring upper-level trough and downstream ridge, and a poleward shift in the storm center, conducive to enhanced trough-TC interactions after ET completion. Additionally, the future composite ET event is located in the right-jet entrance of an outflow jet that is strengthened relative to its present-day counterpart. Localized impacts associated with ET events, such as heavy precipitation and strong near-surface winds, are significantly enhanced in the future-climate simulations; 6-hourly precipitation for NATL events increases at a super-Clausius-Clapeyron rate with area-average precipitation increasing over 30%. Furthermore, intensified precipitation contributes to enhanced lower-tropospheric potential vorticity and stronger upper-tropospheric outflow, implying the potential for more extreme downstream impacts under the future climate scenario.

Acceleration of Tropical Cyclones As a Proxy For Extratropical Interactions: Synoptic-Scale Patterns and Long-Term Trends

It is well known that rapid changes in tropical cyclone motion occur during interaction with extratropical waves. While the translation speed has received much attention in the published literature, acceleration has not. Using a large data sample of Atlantic tropical cyclones, we formally examine the composite synoptic-scale patterns associated with tangential and curvature components of their acceleration. During periods of rapid tangential acceleration, the composite tropical cyclone moves poleward between an upstream trough and downstream ridge of a developing extratropical wavepacket. The two systems subsequently merge in a manner that is consistent with extratropical transition. During rapid curvature acceleration, a prominent downstream ridge promotes recurvature of the tropical cyclone. In contrast, during rapid tangential or curvature deceleration, a ridge is located directly poleward of the tropical cyclone. Locally, this arrangement takes the form of a cyclone-anticyclone vortex pair somewhat akin to a dipole block. On average, the tangential acceleration peaks 18 hours prior to extratropical transition while the curvature acceleration peaks at recurvature. These findings confirm that rapid acceleration of tropical cyclones is mediated by interaction with extratropical baroclinic waves. Furthermore, The tails of the distribution of acceleration and translation speed show a robust reduction over the past 5 decades. We speculate that these trends may reflect the poleward shift and weakening of extratropical Rossby waves.

Large-Scale Conditions for Reintensification after the Extratropical Transition of Tropical Cyclones in the Western North Pacific Ocean

Tropical cyclones that complete extratropical transition (ETTCs) in the western North Pacific are statistically analyzed to clarify the large-scale conditions for their reintensification. A dataset of ETTCs is grouped into intensifying, dissipating, and neutral classes based on the best track data documented by the Japan Meteorological Agency during the period 1979–2018. Intensifying ETTCs are most frequent in September–October, whereas dissipating ETTCs are most frequent in the later season, October–November. Intensifying ETTCs occur at higher latitudes than dissipating ETTCs, where the upper levels are characterized by high potential vorticity (PV) and a steep horizontal gradient of PV. The composite analysis demonstrates that intensifying ETTCs are associated with deep upper-level troughs to their northwest, intense ridge building to their northeast, and strong updrafts to their north associated with vorticity advection and warm-air advection. These results statistically support the findings of previous studies. Furthermore, an analysis using a time filter demonstrates the relationship between planetary-scale environments and synoptic-scale dynamics in the upper levels. The high PV to the northwest of ETTCs is attributed not only to eastward-moving troughs, but also to the environmental PV. The low PV to the northeast of ETTCs results from the negative PV formation associated with ridge building, which almost cancels the environmental PV. Thus, the environmental PV at relatively high latitudes enhances the intensity of positive PV to the northwest of ETTCs, and increases the upper limit of the magnitude of ridge building to the northeast.

On the accumulation of enhanced vertical shear of the horizontal wind in the upper troposphere / lower stratosphere

&lt;p&gt;&lt;span&gt;The extratropical transition or mixing layer indicates the chemical transition from well mixed troposphere to the stably stratified stratosphere . It is located around the classical defitinition of the tropopause and is defined by a set of unique tracer-tracer correlations. Physically, it is the result of cross-tropopause transport, however, many processes associated with the formation and maintenance of the extratropical transition layer with its very distinct features as well as the importance of its overlap with the tropopause inversion layer (TIL) are still a subject of research. In particular, turbulent motions in the UTLS and their relative importance for the ExTL are still unknown.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;span&gt;We analyse the top end of the spectrum of vertical shear of the horizontal wind, S&lt;sup&gt;2&lt;/sup&gt;, in the troposphere and stratosphere as a proxy for turbulent motions. For this we use 10 years of ECMWF (European Centre for Medium-Range Weather Forecasts) ERA5 reanalysis data. We focus our analysis on the Northern Hemisphere extratropical UTLS, and more specifically on the Northern Pacific and Atlantic sectors. We find strong signatures of high S&amp;#178; just above the tropopause in both region. However, differences between the two regions are evident due to difference in the jet stream characteristics in these two regions . The areas of strong vertical wind gradients appear as regions of reduced Richardson numbers in the elsewhere highly dynamically stable lowermost stratosphere. We compare features of these regions in the model output with known characteristics of the extratropical transition layer to see if they are linked.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;span&gt;&lt;em&gt;This work was supported by DFG grant no. KU 3524/1-1.&lt;/em&gt;&lt;/span&gt;&lt;/p&gt;