Intensity fluctuations in Hurricane Irma (2017) during a period of rapid intensification

Intensity fluctuations observed during a period of rapid intensification of Hurricane Irma (2017) between 04 September and 06 September were investigated in a detailed modelling study using an ensemble of Met Office Unified Model (MetUM) convection permitting forecasts. These intensity fluctuations consisted of alternating weakening and strengthening phases. During weakening phases the tropical cyclone temporarily paused its intensification. It was found that weakening phases were associated with a change in the potential vorticity structure, with a tendency for it to become more monopolar. Convection during strengthening phases was associated with isolated local regions of high relative vorticity and vertical velocity in the eyewall, while during weakening phases the storm became more azimuthally symmetric with weaker convection spread more evenly. The boundary layer was found to play an important role in the cause of the intensity fluctuations with an increase in the agradient wind within the boundary layer causing a spin--down just above the boundary layer during the weakening phases whereas during the strengthening phases the agradient wind reduces. This study offers new explanations for why these fluctuations occur and what causes them.

The role of tropopause polar vortices in the intensification of summer Arctic cyclones

Human activity in the Arctic is increasing as new regions become accessible, with a consequent need for improved understanding of hazardous weather there. Arctic cyclones are the major weather systems affecting the Arctic environment during summer, including the sea ice distribution. Mesoscale to synoptic-scale tropopause polar vortices (TPVs) frequently occur in polar regions and are a proposed mechanism for Arctic cyclone genesis and intensification. However, while the importance of pre-existing tropopause-level features for cyclone development, as well as being an integral part of the three-dimensional mature cyclone structure, is well established in the mid-latitudes, evidence of the importance of pre-existing TPVs for Arctic cyclone development is mainly limited to a few case studies. Here we examine the extent to which Arctic cyclone growth is coupled to TPVs by analysing a climatology of summer Arctic cyclones and TPVs produced by tracking both features in the latest ECMWF reanalysis (ERA5). The annual counts of Arctic cyclones and TPVs are significantly correlated for features with genesis either within or outside the Arctic, implying that TPVs have a role in the development of Arctic cyclones. However, only about one-third of Arctic cyclones have their genesis or intensify while a TPV of Arctic origin is (instantaneously) within about twice the Rossby radius of the cyclone centre. Consistent with the different track densities of the full sets of Arctic cyclones and TPVs, cyclones with TPVs within range throughout their intensification phase (matched cyclones) track preferentially over the Arctic Ocean along the North American coastline and Canadian Arctic Archipelago. In contrast, cyclones intensifying distant from any TPV (unmatched cyclones) track preferentially along the northern coast of Eurasia. Composite analysis reveals the presence of a distinct relative vorticity maximum at and above the tropopause level associated with the TPV throughout the intensification period for matched cyclones and that these cyclones have a reduced upstream tilt compared to unmatched cyclones. Interaction of cyclones with TPVs has implications for the predictability of Arctic weather, given the long lifetime but relatively small spatial scale of TPVs compared with the density of the polar observation network.

Nonseasonal Variations in Near-Inertial Kinetic Energy Observed Far Below the Surface Mixed Layer in the Southwestern East Sea (Japan Sea)

Near-inertial internal waves (NIWs) generated by surface wind forcing are intermittently enhanced below and within the surface mixed layer. The NIW kinetic energy below the surface mixed layer varies over intraseasonal, interannual, and decadal timescales; however, these variations remain unexplored, due to a lack of long-term, in situ observations. We present statistical results on the nonseasonal variability of the NIW kinetic energy 400 m below the surface mixed layer in the southwestern East Sea, using moored current measurements from 21 years. We used long time series of the near-inertial band (0.85–1.15 f) kinetic energy to define nine periods of relatively high (period high) and seven periods of relatively low (period low) NIW kinetic energy. The NIW kinetic energy average at period high was about 24 times higher than that at period low and those in specific years (2003, 2012–2013, 2016, and 2020) and decade (2010s) were significantly higher than those in other years and decade (2000s). Composite analysis revealed that negative relative vorticity and strong total strain significantly enhance NIW kinetic energy at 400 m. The relative vorticity was negative (total strain was positively enhanced) during seven (six) out of nine events of period high. NIW trapping in a region of negative relative vorticity and the wave capture process induce nonseasonal variations in NIW kinetic energy below the surface mixed layer. Our study reveals that, over intraseasonal, interannual, and decadal timescales, mesoscale flow fields significantly influence NIWs.

Possible linkage between asymmetry of atmospheric meridional circulation and tropical cyclones in the Central Pacific during El Niño years

The El Niño–Southern Oscillation is one of the most important drivers of climate change on Earth, and is characterised by warmer (El Niño) or colder (La Niña) ocean surface temperatures in the equatorial Pacific. Tropical cyclones (TCs) and meridional circulation are the most influential weather events and climate phenomena, respectively. However, the link between TCs and meridional circulation anomalies (MCA) during El Niño years is unclear. Therefore, we calculated the accumulated cyclone energy index of TCs and the mass stream function of MCA from 1980 to 2018. Our results showed that TCs were closely related to the asymmetry of the MCA in the Central Pacific during El Niño years. An updraft anomaly in the North Pacific was found, which affected the response of MCA to El Niño from May to October during El Niño years. Therefore, the MCA intensity difference between the North and South Pacific increased, and the asymmetry was strengthened. This phenomenon may be strengthened by the combined effects of the equatorial westerly wind, relative vorticity, and warm ocean surfaces, which are controlled by El Niño. The equatorial westerly wind produces positive shear north of the equator, which increases the relative vorticity. The increase in relative vorticity is accompanied by a monsoon trough, leading to increased precipitation and updrafts. The background of the relative vorticity, updraft, and monsoon trough may be conducive to the generation and development of TCs. Our results prove that the possible link between TCs and the asymmetry of the MCA during El Niño years is derived from the combined effect of the equatorial westerly wind, relative vorticity, and warm ocean surfaces, thus providing a partial explanation for the link between TCs and the MCA.

Impact of Extratropical Cyclone Intensity and Speed on the Extreme Wave Trends in the Atlantic Ocean

This work analyzes the extratropical cyclone-related extreme waves in the ocean surface and their trends in the North and South Atlantic Oceans. Atmospheric and ocean wave products are obtained from ERA5, from 1979 to 2020 with 1-hourly outputs, covering 42 years with the present climate changes evaluated by the difference between the two 21-years time slices. The cyclones are tracked through the relative vorticity at 850 hPa and then associated with extreme wave events using an automated scheme that searches for an extreme wave region 1500 km from the centre of the cyclone, following criteria that exclude possible swell dominated events. The hot-spot regions of cyclone-related waves occurrence found by the method are in agreement with previous studies and are related to the cyclogenesis region and storm track orientation. Most cyclones associated with extreme wave events are generated in the western boundary of the domains. The east-poleward side of the ocean basins presents the highest density of occurrences related to the higher density of cyclone track and the dominance of more mature stage cyclones while in the west side prevail systems on developing stages, with notable propagating fronts and consequently, lower wind persistence. Changes in occurrence cannot be explained just by the storm track variation during the period due to the lack of statistical confidence. However, the wave occurrence responds to changes in the cyclone intensity, modulated by cyclone displacement speed. Regions with an increase of extreme waves are related to the effect of more intense cyclones or cyclones with slower propagation, being the last associated with a longer interaction of winds with the ocean surface.

A global climatology of polar lows investigated for local differences and wind-shear environments

Polar lows are intense mesoscale cyclones developing in marine polar air masses. This study presents a new global climatology of polar lows based on the ERA-5 reanalysis for the years 1979–2020. Criteria for the detection of polar lows are derived based on a comparison of six polar-low archives with cyclones derived by a mesoscale tracking algorithm. The characteristics associated with polar lows are considered by the criteria: (i) intense cyclone: large relative vorticity, (ii) mesoscale: small vortex diameter, and (iii) development in the marine polar air masses: combination of low dry-static stability and low potential temperature at the tropopause. Polar lows develop in all marine areas adjacent to sea ice or cold landmasses, mainly in the winter half-year. The length and intensity of the season are regionally dependent. The highest density appears in the Nordic Seas. For all ocean sub-basins, forward-shear polar lows are the most common, whereas weak shear and those propagating towards warmer environments are second and third most frequent, depending on the area. Reverse-shear polar lows and those propagating towards colder environments are rather seldom, especially in the Southern Ocean. Generally, PLs share many characteristics across ocean basins and wind-shear categories. The most remarkable difference is that forward-shear polar lows are often occurring in stronger vertical wind shear, whereas reverse-shear polar lows feature lower static stability. Hence, the contribution to a fast baroclinic growth rate is slightly different for the shear categories.

Slowdown in the Decay of Western North Pacific Tropical Cyclones Making Landfall on the Asian Continent

This study finds an increasing trend in the decay timescale (τ) of western North Pacific (WNP) tropical cyclone (TCs) making landfall on the Asian continent from 1966–2018. Statistical analysis of individual landfalling TCs shows that τ is significantly positively linked to soil wetness, 850-hPa relative vorticity and 200-hPa divergence, whereas it is weakly correlated with 700–500-hPa relative humidity and 850–200-hPa vertical wind shear. For TCs hitting southeastern China, the observed increasing τ is likely caused by enhanced 850-hPa vorticity and 200-hPa divergence. For TCs hitting southern China, increasing τ is likely driven by increased 850-hPa vorticity. By comparison, there are no significant trends in environmental variables over the eastern Indo-China Peninsula, and τ has not significantly changed in this region. Our results imply that the increasing τ of WNP landfalling TCs on the Asian continent are more likely caused by changes in dynamic variables than changes in thermodynamic variables.

Variability of Tropical Cyclone Landfalls in China

The reported decreasing trend of the annual tropical cyclone (TC) landfalls in Southern China and increasing trend in Southeastern China in recent decades are confirmed to be an abrupt shift occurred at the end of the 20th century, based on a statistical analysis. The opposite trends in the two adjacent regions are often considered as a result of tropical cyclone landfalls in southern China being deflected northward. However, it is demonstrated in this study that they are phenomenally independent. In fact, the abrupt decrease of TC landfalls in Southern China occurs due to an abrupt decrease of the westward events in the post-peak season (October-December), as the consequence of a significant decrease of the TC genesis frequency in the southeastern part of the western North Pacific (WNP) ocean basin. On the other hand, the abrupt increase of TC landfalls in Southeastern China occurs due to an abrupt increase of the northwest events in the peak season (July-September), as the consequence of a statistically westward shift of the TC genesis. The relevant variations of the TC genesis are shown to be mainly caused by the decreased relative vorticity and the increased vertical wind shear, which, however, are intrinsically related to the accelerated zonal atmospheric circulation driven by a La Niña-like sea surface warming pattern over WNP developed after the end of 20th century.

What Caused The Increase of Tropical Cyclones In The Western North Pacific During The Period of 2011-2020?

Based on satellite era data after 1979, we find that the tropical cyclone (TC) variations in the Western North Pacific (WNP) can be divided into three-periods: a high-frequency period from 1979-1997 (P1), a low-frequency period from 1998-2010 (P2), and a high-frequency period from 2011-2020 (P3). Previous studies have focused on WNP TC activity during P1 and P2. Here we use observational data to study the WNP TC variation and its possible mechanisms during P3. Compared with P2, more TCs during P3 are due to the large-scale atmospheric environmental conditions of positive relative vorticity, negative vertical velocity and weak vertical wind shear. Warmer SST is found during P3, which is favorable for TC genesis. The correlation between the WNP TC frequency and SST shows a significant positive correlation around the equator and a significant negative correlation around 36°N, which is similar to the warm phase of the Pacific Decadal Oscillation (PDO). The correlation coefficient between the PDO and TC frequency is 0.71, significant at 99% confidence level. The results indicate that the increase of the WNP TC frequency during 2011-2020 is associated with the phase transition of the PDO and warmer SST. Therefore, more attention should be given to the warmer SST and PDO phase when predicting WNP TC activity.

Observed Near-Inertial Waves in the Northern South China Sea

Characteristics of near-inertial waves (NIWs) induced by the tropical storm Noul in the South China Sea are analyzed based on in situ observations, remote sensing, and analysis data. Remote sensing sea level anomaly data suggests that the NIWs were influenced by a southwestward moving anticyclonic eddy. The NIWs had comparable spectral density with internal tides, with a horizontal velocity of 0.14–0.21 m/s. The near-inertial kinetic energy had a maximum value of 7.5 J/m3 and propagated downward with vertical group speed of 10 m/day. Downward propagation of near-inertial energy concentrated in smaller wavenumber bands overwhelmed upward propagation energy. The e-folding time of NIWs ranged from 4 to 11 days, and the larger e-folding time resulted from the mesoscale eddies with negative vorticity. Modified by background relative vorticity, the observed NIWs had both red-shifted and blue-shifted frequencies. The upward propagating NIWs had larger vertical phase speeds and wavelengths than downward propagating NIWs. There was energy transfer from the mesoscale field to NIWs with a maximum value of 8.5 × 10−9 m2 s−3 when total shear and relative vorticity of geostrophic currents were commensurate. Our results suggest that mesoscale eddies are a significant factor influencing the generation and propagation of NIWs in the South China Sea.