Tropical cyclone frequency in the north Indian Ocean in relation to southern oscillation phenomenon

The present paper deals with the influence of Southern Oscillation (SO) on the frequency of tropical cyclones in the north Indian Ocean. The results show that during the negative phase of SO the frequency of tropical cyclones and depressions over the Bay of Bengal and the Arabian Sea diminishes in May which is most important pre-monsoon cyclone month. The correlation coefficient between the frequency of cyclones and depressions and the Southern Oscillation Index (SOI) is +0.3 which is significant at 99% level. Post-monsoon cyclone frequency in the Bay of Bengal during November shows a significant positive correlation with SOl implying that it also decreases during the negative phase of SO. Thus there is a reduction in the tropical cyclone frequency over the Bay of Bengal during both intense cyclone months May and November in EI-Nino/Southern Oscillation (ENSO) epochs. Therefore it would not be correct to say that ENSO has no impact on the cyclogenesis in the north Indian Ocean. It is true that ENSO has no significant impact on the frequency of cyclones in the Arabian Sea. ENSO also seems to affect the rate of intensification of depressions to cyclone stage. The rate of intensification increases in May and diminishes in November in the north Indian Ocean during ENSO. The results are based on the analysis of monthly frequencies of tropical cyclones and depressions and SOI for the 100 year period from 1891-1990.

Trends and fluctuations of the cyclonic systems over North Indian Ocean

The data of the monthly cyclone frequencies over North Indian Ocean for the period 1877-1998 has been analysed to study the long-term trends and fluctuations. Analysis has been made separately for depressions and cyclones for the pre-monsoon, monsoon and post-monsoon seasons along with the annual frequencies. The data was subjected to 11-year moving averages and the epochs of increasing and decreasing trends have been identified. A consistent decreasing trend after 1950s is a notable feature. The time series of the monthly cyclone frequency were passed through a high-pass filter to eliminate periods greater than 21 years and then subjected to spectrum analysis using Maximum Entropy Method to obtain dominant periodicities. Three significant periodicities at 2.2-2.8; 3.5-6.5 and 10-15 years have been identified which could be attributed to QBO, ENSO and decadal frequencies.

Characterizing the highest tropical cyclone frequency in the Western North Pacific since 1984

The 2018 boreal summer in the Western North Pacific (WNP) is highlighted by 17 tropical cyclones (TC)—the highest record during the reported reliable years of TC observations. We contribute to the existing knowledge pool on this extreme TC frequency record by showing that the simultaneous highest recorded intensity of the WNP summer monsoon prompted the eastward extension of the monsoon trough and enhancement of tropical convective activities, which are both favorable for TC development. Such changes in the WNP summer monsoon environment led to the extreme TC frequency record during the 2018 boreal summer. Meanwhile, the highest record in TC frequency and the intensity of the WNP summer monsoon are both attributed with the combined increase in the anomalous westerlies originating from the cold tropical Indian Ocean sea surface temperature (SST) anomalies drawn towards the convective heat source that is associated with the warm central Pacific SST anomalies. Our results provide additional insights in characterizing above normal tropical cyclone and summer monsoon activities in the WNP in understanding seasonal predictable horizons in the WNP, and in support of disaster risk and impact reduction.

Generating reliable estimates of tropical cyclone induced coastal hazards along the Bay of Bengal for current and future climates using synthetic tracks

Deriving reliable estimates of design water levels and wave conditions resulting from tropical cyclones is a challenging problem of high relevance for, among others, coastal and offshore engineering projects and risk assessment studies. Tropical cyclone geometry and wind speeds have been recorded for the past few decades only, therefore resulting in poorly reliable estimates of the extremes, especially at regions characterized by a low number of past tropical cyclone events. In this paper, this challenge is overcome by using synthetic tropical cyclone tracks and wind fields generated by the open source tool TCWiSE (Tropical Cyclone Wind Statistical Estimation), to create thousands of realizations representative for 1,000 years of tropical cyclone activity for the Bay of Bengal. Each of these realizations is used to force coupled storm surge and wave simulations by means of the processed-based Delft3D Flexible Mesh Suite. It is shown that the use of synthetic tracks provides reliable estimates of the statistics of the first-order hazard (i.e. wind speed) compared to the statistics derived for historical tropical cyclones. Based on estimated wind fields, second-order hazards (i.e. storm surge and waves) are computed. The estimates of the extreme values derived for wind speed, wave height and storm surge are shown to converge within the 1,000 years of simulated cyclone tracks. Comparing second-order hazard estimates based on historical and synthetic tracks show that, for this case study, the use of historical tracks (a deterministic approach) leads to an underestimation of the mean computed storm surge up to −30 %. Differences between the use of synthetic versus historical tracks are characterized by a large spatial variability along the Bay of Bengal, where regions with a lower probability of occurrence of tropical cyclones show the largest difference in predicted storm surge and wave heights. In addition, the use of historical tracks leads to much larger uncertainty bands in the estimation of both storm surges and wave heights, with confidence intervals being +80 % larger compared to those estimated by using synthetic tracks (probabilistic approach). Based on the same tropical cyclone realizations, the effect that changes in tropical cyclone frequency and intensity, possibly resulting from climate change, may have on modelled storm surge and wave heights were computed. An increase in tropical cyclone frequency of +25.6 % and wind intensity of +1.6 %, based on literature values, could result in an increase of storm surge and wave heights of +11 % and +9 % respectively. This suggest that climate change could increase tropical cyclone induced coastal hazards more than just the actual increase in maximum wind speeds.

Pacific Decadal Oscillation Causes Fewer Near-Equatorial Cyclones in North Indian Ocean

Tropical cyclones do not form easily near the equator but can intensify rapidly, leaving little time for preparation. We investigated the number of near-equatorial (originating between 5°N and 11°N) tropical cyclones over the north Indian Ocean during post-monsoon seasons (October to December) over the past 60 years. A marked 43% decline in the number of such cyclones was detected in recent decades (1981-2010) compared to earlier (1951-1980). This decline in tropical cyclone frequency is primarily due to the weakened low-level vorticity modulated by the Pacific Decadal Oscillation (PDO). In the presence of basin-wide warming at low latitudes, and a favorable phase of the PDO, both the intensity and frequency of such cyclones is expected to increase. Such dramatic and unique changes in tropical cyclonic activity due to the interplay between natural variability and climate change call for appropriate planning and mitigation strategies.

Sensitivity of Northern Hemisphere Cyclone Detection and Tracking Results to Fine Spatial and Temporal Resolution using ERA5

Lagrangian detection and tracking algorithms are frequently used to study the development, distribution, and trends of extratropical cyclones. Past research shows that results from these algorithms are sensitive to both spatial and temporal resolution of the gridded input fields, with coarser resolutions typically underestimating cyclone frequency by failing to capture weak, small, and short-lived systems. The fifth-generation atmospheric reanalysis from the European Centre for Medium-Range Weather Forecasts (ERA5) offers finer resolution, and therefore more precise information regarding storm locations and development than previous global reanalyses. However, our sensitivity tests show that using ERA5 sea-level pressure fields at their finest possible resolution does not necessarily lead to better cyclone detection and tracking. If a common number of nearest neighbors is used when detecting minima in sea-level pressure (like past studies), finer spatial resolution leads to noisier fields that unrealistically break up multi-center cyclones. Using a common search distance instead (with more neighbors at finer resolution) resolves the issue without smoothing inputs. Doing this also makes cyclone frequency, lifespan, and average depth insensitive to refining spatial resolution beyond 100 km. Results using 6-h and 3-h temporal resolutions have only minor differences, but using 1-h temporal resolution with a maximum allowed propagation speed of 150 km h-1 leads to unrealistic track splitting. This can be counteracted by increasing the maximum propagation speed, but modest sensitivity to temporal resolution persists for several cyclone characteristics. Therefore, we recommend caution if applying existing algorithms to temporal resolutions finer than 3-h and careful evaluation of algorithm settings.

Intense east coast lows and associated rainfall in eastern Australia

East coast lows (ECLs) are low pressure systems that occur near the east coast of Australia. But not all lows cause the same level of impact, and a small proportion of ECLs are responsible for more than half of all days with widespread rainfall above 50mm in this region. In this study, we combine analyses of cyclones at both the surface and 500hPa levels to assess the locations of cyclones responsible for widespread heavy rainfall on the east coast. We found that the majority of days with widespread totals above 100mm on the east coast occur when a low at 500hPa over inland southeast Australia coincides with a surface low located more directly over the east coast. Such events occur on about 15 days per year but are responsible for more than 50% of days with widespread heavy rainfall on the eastern seaboard of Australia. We also found that extreme rainfall was most likely when both the surface and upper cyclones were very strong, when measured using the maximum Laplacian of pressure/height. The seasonal frequency of cyclones at the surface and 500hPa were found to be only weakly correlated with each other and often had opposing relationships (albeit weak in magnitude) with both global climate drivers and indices of local circulation variability. Trends in cyclone frequency were weak over the period 1979–2019, but there was a small decline in the frequency of deep cyclone days, which was statistically significant in some parts of the southeast. Understanding which ECLs are associated with heavy rainfall will help us to better identify how future climate change will influence ECL impacts.