scholarly journals An Intraseasonal Genesis Potential Index for Tropical Cyclones during Northern Hemisphere Summer

2018 ◽  
Vol 31 (22) ◽  
pp. 9055-9071 ◽  
Author(s):  
Ja-Yeon Moon ◽  
Bin Wang ◽  
Sun-Seon Lee ◽  
Kyung-Ja Ha
Keyword(s):  
Northern Hemisphere ◽  
North Pacific ◽  
Large Scale ◽  
North Indian Ocean ◽  
Vertical Shear ◽  
Boreal Summer ◽  
Tropical Cyclone Genesis ◽  
Coriolis Parameter ◽  
Genesis Potential Index ◽  
Potential Index

Abstract An intraseasonal genesis potential index (ISGPI) for Northern Hemisphere (NH) summer is proposed to quantify the anomalous tropical cyclone genesis (TCG) frequency induced by boreal summer intraseasonal oscillation (BSISO). The most important factor controlling NH summer TCG is found as 500-hPa vertical motion (ω500) caused by the prominent northward shift of large-scale circulation anomalies during BSISO evolution. The ω500 with two secondary factors (850-hPa relative vorticity weighted by the Coriolis parameter and vertical shear of zonal winds) played an effective role globally and for each individual basin in the northern oceans. The relative contributions of these factors to TCG have minor differences by basins except for the western North Atlantic (NAT), where low-level vorticity becomes the most significant contributor. In the eastern NAT, the BSISO has little control of TCG because weak convective BSISO and dominant 10–30-day circulation signal did not match the overall BSISO life cycle. The ISGPI is shown to reproduce realistic intraseasonal variability of TCG, but the performance is phase-dependent. The ISGPI shows the highest fidelity when BSISO convective anomalies have the largest amplitude in the western North Pacific and the lowest when they are located over the north Indian Ocean and eastern North Pacific. Along the NH major TCG zone, the TCG probability changes from a dry to a wet phase by a large factor ranging from 3 to 12 depending on the basins. The new ISGPI for NH summer can simulate more realistic impact of BSISO on TC genesis compared to canonical GPI derived by climatology.

scholarly journals An Anomalous Genesis Potential Index for MJO Modulation of Tropical Cyclones

2017 ◽  
Vol 30 (11) ◽  
pp. 4021-4035 ◽  
Author(s):  
Bin Wang ◽  
Ja-Yeon Moon
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Vertical Wind Shear ◽  
Vertical Motion ◽  
General Circulation Models ◽  
Intraseasonal Variation ◽  
Coriolis Parameter ◽  
Genesis Potential Index ◽  
Low Level ◽  
Potential Index

Abstract Modulation of tropical cyclone (TC) genesis by the Madden–Julian oscillation (MJO) has been quantitatively diagnosed by using a climatological genesis potential index (GPI). Analysis of TC genesis during November–April of 1979–2014 indicates the most effective factors controlling intraseasonal TC genesis are 850-hPa relative vorticity weighted by the Coriolis parameter fζr850 and 500-hPa vertical motion ω500. The total vertical wind shear and maximum potential intensity are unimportant, and the role of 600-hPa relative humidity is greatly represented by ω500. The MJO modulates TC genesis primarily through changing low-level vorticity induced by its Rossby wave gyres and meridional shears of equatorial zonal winds. A new intraseasonal GPI (ISGPI) is proposed to quantify the MJO’s modulation of TC genesis. The ISGPI significantly improves representation of intraseasonal variation of TC genesis in the tropics and in each subregion of the southern Indian Ocean, Australian monsoon, and South Pacific. In the hot spots of the Southern Hemisphere TC genesis zone, the probability of TC genesis can differ by a factor of 5–19 as a result of MJO modulation. The results suggest that the large-scale factors controlling TC genesis may vary with different time scales, and the climatological GPI may not be quite applicable for diagnoses of climate variability and future change of TC genesis potential. To simulate realistic impacts of the MJO on TC genesis, general circulation models must reproduce not only realistic eastward propagation but also the MJO low-level circulation structure. Application of the new ISGPI may have a large potential to improve dynamical subseasonal prediction of TC genesis.

Investigating the Use of a Genesis Potential Index for Tropical Cyclones in the North Atlantic Basin

2012 ◽  
Vol 25 (24) ◽  
pp. 8611-8626 ◽  
Author(s):  
Cindy L. Bruyère ◽  
Greg J. Holland ◽  
Erin Towler
Keyword(s):  
Tropical Cyclones ◽  
North Atlantic ◽  
Large Scale ◽  
Vertical Shear ◽  
Genesis Potential Index ◽  
The North ◽  
Potential Index ◽  
Cyclone Genesis ◽  
Future Work ◽  
Cyclone Frequency

Abstract Large-scale environmental variables known to be linked to the formation of tropical cyclones have previously been used to develop empirical indices as proxies for assessing cyclone frequency from large-scale analyses or model simulations. Here the authors examine the ability of two recent indices, the genesis potential (GP) and the genesis potential index, to reproduce observed North Atlantic cyclone annual frequency variations and trends. These skillfully estimate the mean seasonal variation of observed cyclones, but they struggle with reproducing interannual frequency variability and change. Examination of the independent contributions by the four terms that make up the indices finds that potential intensity and shear have significant skill, while moisture and vorticity either do not contribute to or degrade the indices’ capacity to reproduce observed interannual variability. It is also found that for assessing basinwide cyclone frequency, averaging indices over the whole basin is less skillful than its application to the general area off the coast of Africa broadly covering the main development region (MDR). These results point to a revised index, the cyclone genesis index (CGI), which comprises only potential intensity and vertical shear. Application of the CGI averaged over the MDR demonstrates high and significant skill at reproducing interannual variations and trends in all-basin cyclones across both reanalyses. The CGI also provides a more accurate reproduction of seasonal variations than the original GP. Future work applying the CGI to other tropical cyclone basins and to the downscaling of relatively course climate simulations is briefly addressed.

scholarly journals Absence of Tropical Cyclone Genesis Over the Western North Pacific in July 2020 and Its Prediction by CFSv2

2021 ◽  
Vol 9 ◽  
Author(s):  
Baoqiang Tian ◽  
Ke Fan
Keyword(s):  
North Atlantic ◽  
Environmental Conditions ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
Walker Circulation ◽  
Feedback Mechanism ◽  
North Indian Ocean ◽  
Tropical Cyclone Genesis ◽  
The Impact

There were no TCs generated in July 2020 over the western North Pacific (WNP), which was the first time this had happened during since 1980. This study attempts to understand the cause of there having been no TCs generated in July 2020, and evaluates the prediction skill for the large-scale environmental conditions associated with the TC genesis number (TCGN). Results show that the main causes were the abnormal warming of sea surface temperature (SST) in the North Indian Ocean (NIO) and North Atlantic in July and the abnormal decrease in SST from April and May in the Niño4 region. The NIO SST can affect the large-scale environmental conditions via the SST–precipitation–wind feedback mechanism. Through the interaction between the tropical North Atlantic and the NIO, the abnormally warm North Atlantic SST further strengthened the impact of the NIO SST on the environmental conditions. The monthly difference (MD) of the Niño4 index from April to May is significantly correlated with the TCGN in July. Not only can the Niño4 MD in May affect the environmental conditions by affecting the WNP anticyclone, but it can also affect the NIO SST and precipitation anomalies through a shift in the position of the Walker circulation. Besides, the activity of the MJO also had a certain impact on the absence of TC genesis in July 2020. Although CFSv2 can successfully predict the local feedback affecting the July TCGN, it fails to forecast the large-scale environmental conditions associated with the absence of TC genesis over the WNP in July 2020.

Developing versus Nondeveloping Disturbances for Tropical Cyclone Formation. Part II: Western North Pacific

2012 ◽  
Vol 140 (4) ◽  
pp. 1067-1080 ◽  
Author(s):  
Bing Fu ◽  
Melinda S. Peng ◽  
Tim Li ◽  
Duane E. Stevens
Keyword(s):  
Tropical Cyclone ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
Relative Vorticity ◽  
Tropical Cyclone Genesis ◽  
Horizontal Shear ◽  
The North ◽  
Thermodynamic Variables ◽  
Cyclone Genesis

Global daily reanalysis fields from the Navy Operational Global Atmospheric Prediction System (NOGAPS) are used to analyze Northern Hemisphere summertime (June–September) developing and nondeveloping disturbances for tropical cyclone (TC) formation from 2003 to 2008. This is Part II of the study focusing on the western North Pacific (WNP), following Part I for the North Atlantic (NATL) basin. Tropical cyclone genesis in the WNP shows different characteristics from that in the NATL in both large-scale environmental conditions and prestorm disturbances. A box difference index (BDI) is used to identify parameters in differentiating between the developing and nondeveloping disturbances. In order of importance, they are 1) 800-hPa maximum relative vorticity, 2) rain rate, 3) vertically averaged horizontal shear, 4) vertically averaged divergence, 5) 925–400-hPa water vapor content, 6) SST, and 7) translational speed. The study indicates that dynamic variables are more important in TC genesis in the WNP, while in Part I of the study the thermodynamic variables are identified as more important in the NATL. The characteristic differences between the WNP and the NATL are compared.

Future Change of Northern Hemisphere Summer Tropical–Extratropical Teleconnection in CMIP5 Models*

2014 ◽  
Vol 27 (10) ◽  
pp. 3643-3664 ◽  
Author(s):  
June-Yi Lee ◽  
Bin Wang ◽  
Kyong-Hwan Seo ◽  
Jong-Seong Kug ◽  
Yong-Sang Choi ◽  
...  
Keyword(s):  
North America ◽  
Northern Hemisphere ◽  
Summer Monsoon ◽  
North Pacific ◽  
Western North Pacific ◽  
Cmip5 Models ◽  
Boreal Summer ◽  
Moderate Increase ◽  
Coupled Models ◽  
Teleconnection Patterns

Abstract Two dominant global-scale teleconnections in the Northern Hemisphere (NH) extratropics during boreal summer season (June–August) have been identified: the western North Pacific–North America (WPNA) and circumglobal teleconnection (CGT) patterns. These teleconnection patterns are of critical importance for the NH summer seasonal climate prediction. Here, how these teleconnections will change under anthropogenic global warming is investigated using representative concentration pathway 4.5 (RCP4.5) experiments by 20 coupled models that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5). The six best models are selected based on their performance in simulation of the two teleconnection patterns and climatological means and variances of atmospheric circulation, precipitation, and sea surface temperature. The selected models capture the CGT and its relationship with the Indian summer monsoon (ISM) reasonably well. The models can also capture the WPNA circulation pattern but with striking deficiencies in reproducing its associated rainfall anomalies due to poor simulation of the western North Pacific summer monsoon rainfall. The following changes are anticipated in the latter half of twenty-first century under the RCP4.5 scenario: 1) significant weakening of year-to-year variability of the upper-level circulation due to increased atmospheric stability, although the moderate increase in convective heating over the tropics may act to strengthen the variability; 2) intensification of the WPNA pattern and major spectral peaks, particularly over the eastern Pacific–North America and North Atlantic–Europe sectors, which is attributed to the strengthening of its relationship with the preceding mature phase of El Niño–Southern Oscillation (ENSO); and 3) weakening of the CGT due to atmospheric stabilization and decreasing relationship with ISM as well as weakening of the ISM–ENSO relationship.

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