A Survey of Statistical Relationships between Tropical Cyclone Genesis and Convectively Coupled Equatorial Rossby Waves

Convectively coupled equatorial Rossby waves (ERW) modulate tropical cyclone activities over tropical oceans. This study presents a survey of the statistical relationship between intraseasonal ERWs and tropical cyclone genesis (TCG) over major global TC basins using four-decade-long outgoing longwave radiation (OLR) and TC best-track datasets. Intraseasonal ERWs are identified from the OLR anomalies using an empirical orthogonal function (EOF) analysis method without imposing equatorial symmetry. We find that westward-propagating ERWs are most significant in four tropical ocean basins over the summer hemisphere and that ERWs exhibit similar northeast-southwest (southeast-northwest) tilted phase lines in the northern (southern) hemisphere, with an appreciable poleward advance of wave energy in most TC basins. The EOF-based ERW indices quantitatively show that ERWs significantly modulate TC genesis. The convectively active (suppressed) phases of ERWs coincide with increased (reduced) TCG occurrences. The TCG modulation by ERWs achieves the maximum where the ERWs propagate through the climatological TCG hotspots. As a result, the total number of TCG occurrences in the TC basins varies significantly according to the ERW phase. The ERW-TCG relationship is significant over the northwestern Pacific Ocean, northeastern Pacific Ocean, and the northern Indian Ocean during the northern summer seasons. In the southern summer season, the ERW-TCG relationship is significant over the southern Indian Ocean, Indonesian-Australia basin, and the southwestern Pacific Ocean. However, ERW activities are weak in the main TC development region of the Atlantic Ocean; and the impact on Atlantic TCG appears to be insignificant.

Tropical cyclone Genesis Potential Parameter (GPP) and it’s application over the north Indian Sea

bl 'kks/k i= esa mRrjh fgUn egklkxj esa m".kdfVca/kh; pØokr ds cuus ds foHko izkpy ¼th- ih- ih-½ dk fo’ys"k.k fd;k x;k gSA dksVy }kjk fodflr ¼2009½ pØokr cuus ds foHko izkpy dk vkdyu pkj ifjofrZrkvksa ds vk/kkj ij fd;k x;k gS tks bl izdkj gS % 850 gSDVkikLdy ij Hkzfeyrk] e/; {kksHkeaMyh; lkisf{kd vknzZrk] e/; {kksHkeaMyh; vfLFkjrk vkSj ml LFkku ds lHkh fxzM IokbaVksa ij m/okZ/kj iou vi:i.kA bu fLFkfr;ksa esa fxzM IokbaV ij th-ih-ih- ij ;g fopkj fd;k x;k fd lHkh ifjorhZ Hkzfeyrk] e/; {kksHkeaMyh; lkisf{kd vknzZrk] e/; {kksHkeaMyh; fLFkjrk vkSj m/okZ/kj iou vi:i.k 'kwU; ls cM+k gS vkSj ;g ekuk x;k gS fd tc buesa ls dksbZ Hkh ifjorhZ 'kwU; ls de ;k cjkcj gks rks og 'kwU; gh ekuk tk,xkA ;wjksih; e/;kof/k ekSle iwokZuqeku dsUnz ¼b-lh-,e-MCY;w-,Q-½ fun’kZ vk¡dM+ksa dk mi;ksx djrs gq, bu ifjofrZrkvksa dk vkdyu fd;k x;k gSaA b- lh- ,e- MCY;w- ,Q- fun’kZ dh lwpukvksa ¼http://www.imd.gov.in/section/nhac/dynamic/analysis.htm ij miyC/k½ dk okLrfod le; dk mi;ksx djrs gq, lkr fnuksa rd ds fy, tsusfll izkpy ds iwokZuqeku Hkh rS;kj fd, x,A ml {ks= esa th-ih-ih- ds mPprj ekuksa ls ml LFkku ds tsfufll ds mPprj foHko dk irk pyk gSA ml LFkku ij th-ih-ih- ds eku 30 ds cjkcj vFkok vf/kd gksus dh fLFkfr esa pØokr mRifRr ds fy, mPp foHko {ks= ik;k x;k gSA izkpy ds fo’ys"k.k vkSj 2010 esa pØokrh fo{kksHkksa ds nkSjku budh izHkko’khyrk ls mRrjh fgUn egklkxj esa pØokr mRifÙk ds fy, iwokZuqeku lwpd flaxuy ¼4&5 fnu igys½ ds :i esa vkSj fodkl dh vkjafHkd voLFkkvksa esa fodflr vkSj xSj&fodflr iz.kkfy;ksa ds rzhohdj.k ds fy, foHko dk fu/kkZj.k gsrq budh mi;ksfxrk dh iqf"V gqbZ gSA An analysis of tropical cyclone genesis potential parameter (GPP) for the North Indian Sea is carried out. The genesis potential parameter developed by Kotal et al. (2009) is computed based on the product of four variables, namely: vorticity at 850 hPa, middle tropospheric relative humidity, middle tropospheric instability and the inverse of vertical wind shear at all grid points over the area. The GPP at a grid point is considered under the conditions that all the variables vorticity, middle tropospheric relative humidity, middle tropospheric instability and the vertical wind shear are greater than zero and it is taken as zero when any one of these variables is less or equal to zero. The variables are computed using the European Centre for Medium Range Weather Forecast (ECMWF) model data. Forecast of the genesis parameter up to seven days is also generated on real time using the ECMWF model output (available at http://www.imd.gov.in/section/nhac/dynamic/Analysis.htm). Higher value of the GPP over a region indicates higher potential of genesis over the region. Region with GPP value equal or greater than 30 is found to be high potential zone for cyclogenesis. The analysis of the parameter and its effectiveness during cyclonic disturbances in 2010 affirm its usefulness as a predictive signal (4-5 days in advance) for cyclogenesis over the North Indian Sea and for determining potential for intensification of developing and non-developing systems at the early stages of development.

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.

Intraseasonal variability of ocean surface wind waves in the western South Atlantic: the role of cyclones and the Pacific South American pattern

Extratropical cyclones are known to generate extreme significant wave height (swh) values at the ocean surface in the western South Atlantic (wSA), which are highly influenced by intraseasonal scales. This work aims to investigate the importance of intraseasonal timescales (30–180 d) in the regional climatology of waves and its atmospheric forcing. The variability is explained by analyzing the storm track modulation due to westerly winds. These winds present timescales and spatial patterns compatible with the intraseasonal component of the Pacific South American (PSA) patterns. The analyses are made using ECMWF’s ERA5 from 1979 to 2019 and a database of extratropical cyclones based on the same reanalysis. Empirical orthogonal function (EOF) analyses of the 10 m zonal wind and swh are used to assess the regime of westerlies and waves in the wSA. The EOF1 of the 10 m zonal wind (u10) presented a core centered at 45∘ W and 40∘ S, while the EOF2 is represented by two cores organized into a seesaw pattern with a center between 30–40∘ S and another to the south of 40∘ S. Composites of cyclone genesis and track densities as well as swh fields were calculated based on the phases of both EOFs. In short, EOF phases presenting cores with a positive (negative) u10 anomaly provide a favorable (unfavorable) environment for cyclone genesis and track densities and, therefore, positive (negative) swh anomalies. The modulation of the cyclone tracks is significant for extreme values of the swh. The spatial patterns of the EOFs of u10 are physically and statistically consistent with 200 and 850 hPa geopotential height signals from the Pacific, indicating the importance of the remote influence of the PSA patterns over the wSA.

Nonlinear Analysis of Tropical Waves and Cyclogenesis Excited by Pressure Disturbance in Atmosphere

The horizontal equations of motion for an inviscid homogeneous fluid under the influence of pressure disturbance and waves are applied to investigate the nonlinear process of solitary waves and cyclone genesis forced by a moving pressure disturbance in atmosphere. Based on the reductive perturbation analysis, it is shown that the nonlinear evolution equation for the wave amplitude satisfies the Korteweg–de Vries equation with a forcing term (fKdV equation for short), which describes the physics of a shallow layer of fluid subject to external pressure forcing. Then, with the help of Hirota’s direct method, the analytic solutions of the fKdV equation are studied and some exact vortex solutions are given as examples, from which one can see that the solitary waves and vortex multi-pole structures can be excited by external pressure forcing in atmosphere, such as pressure perturbation and waves. It is worthy to point out that cyclone and waves can be excited by different type of moving atmospheric pressure forcing source.

Mesoscale interactions during the genesis and intensification of October 1999 Orissa super cyclone

lkj & ;g loZfofnr rF; gS fd iw.kZ fodflr m".kdfVca/kh; pØokr esa izk;% v{klekuqikfrd lajpuk ikbZ tkrh gS tcfd pØokr ds cuus dh voLFkk esa vR;kf/kd vlaxfr fn[kkbZ nsrh gSA iz’kkar egklkxj esa gky gh esa fd, x, v/;;uksa vkSj izs{k.kksa ls ;g irk pyk gS fd m".kdfVca/kh; pØokrksa dh mRifRr dk irk yxkkus esa eslksLdsy dh ijLij fØ;k,sa egRoiw.kZ Hkwfedk fuHkk ldrh gSaA m".kdfVca/kh; pØokr dh mRifRr ds vk/kqfud fl)kar Hkh mi;qZDr iwoZdfFkr rF; ij vk/kkfjr gSaA bl 'kks/k&Ik= esa vkbZ- vkj- mixzg ls izkIr foEckoyh vkSj cM+s iSekus ij Hkzfeyrk ds {ks=ksa dk fo’ys"k.k izLrqr fd;k x;k gSA ftuesa ;g ns[kk x;k gS fd 1999 esa mM+hlk esa vk, egkpØokr dh Hkh izkjfEHkd voLFkkvksa esa eslksLdsy ls pØokr ds coaMj dh ijLij fØ;kvksa dk irk yxk gSA It is well known that a mature tropical cyclone is known to have a nearly axisymmetric structure but that the formation stage exhibits considerable asymmetry. Recent studies and observations in the Pacific indicate that mesoscale interactions could play an important role in the genesis of tropical cyclones. Modern theories of tropical cyclone genesis are also based on this premise. In this paper, an analysis of the IR satellite imagery and large scale vorticity fields is presented, which shows that mesoscale vortex interactions occur in the early stages of the 1999 Orissa super cyclone also.

Understanding differences in tropical cyclone activity over the Arabian Sea and Bay of Bengal

Within the North Indian Ocean basin, tropical cyclone (TC) activity over the Bay of Bengal (BoB) is substantially greater than over the Arabian Sea (AS). The authors attempt to quantify the roles of specific environmental factors in order to understand the reasons for this difference between the two basins. Environmental variables are considered in the basin as a whole and in the immediate times and places at which cyclogenesis and storm intensification occur.The results for the two sub-basins are compared to determine which environmental variablessignificantly between the sub-basins. A tropical cyclone genesis index (TCGI) is also examined to determine whether the AS- differedBased on that partial success, climatologies of the individual factors that comprise the index are examined to determine which ones are most important in the difference