Nonlocality of Atlantic tropical cyclone intensities
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The assumption that tropical cyclones respond primarily to sea surface temperatures (SSTs) local to their main development regions underlies much of the concern regarding the possible impacts of anthropogenic greenhouse warming on tropical cyclone statistics. Here the observed relationship between changes in sea surface temperature and tropical cyclone intensities in the Atlantic basin is explored. Atlantic tropical cyclone intensity fluctuations and storm numbers are shown to depend not only upon SST anomalies local to the Atlantic main development region, but also in a negative sense upon the tropical mean SST. This behavior is shown in part to be consistent with changes in the tropical cyclone potential intensity that provides an upper bound on storm intensity. However, Atlantic tropical cyclone intensity fluctuations are more nonlocal than the potential intensity itself and specifically vary along with Atlantic main development region SST anomalies relative to the tropical mean SST. This suggests that there is no straightforward link between warmer SSTs in the main development region and more intense tropical cyclones.Keywords:
Atlantic hurricane
Tropical cyclogenesis
Tropical cyclone scales
African easterly jet
Tropical Atlantic
Abstract The automatic tracking technique used by Thorncroft and Hodges has been used to identify coherent vorticity structures at 850 hPa over West Africa and the tropical Atlantic in the 40-yr ECMWF Re-Analysis. The presence of two dominant source regions, north and south of 15°N over West Africa, for storm tracks over the Atlantic was confirmed. Results show that the southern storm track provides most of the storms that reach the main development region where most tropical cyclones develop. There exists marked seasonal variability in location and intensity of the storms leaving the West African coast, which may influence the likelihood of downstream intensification and longevity. There exists considerable year-to-year variability in the number of West African storm tracks, both in numbers over the land and continuing out over the tropical Atlantic Ocean. While the low-frequency variability is well correlated with Atlantic tropical cyclone activity, West African rainfall, and SSTs, the interannual variability is found to be uncorrelated with these. In contrast, variance of the 2–6-day-filtered meridional wind, which provides a synoptic-scale measure of African easterly wave activity, shows a significant, positive correlation with tropical cyclone activity at interannual time scales.
African easterly jet
Atlantic hurricane
Extratropical cyclone
Tropical Atlantic
Cyclogenesis
Tropical cyclone scales
Storm track
Tropical cyclogenesis
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The tropical cyclone rainfall climatological study performed for the North Pacific was extended to the North Atlantic. Similar to the North Pacific tropical cyclone study, mean monthly rainfall within 444 km of the center of the North Atlantic tropical cyclones (i.e., that reached storm stage and greater) was estimated from passive microwave satellite observations during an 11-yr period. These satellite-observed rainfall estimates were used to assess the impact of tropical cyclone rainfall in altering the geographical, seasonal, and interannual distribution of the North Atlantic total rainfall during June–November when tropical cyclones were most abundant. The main results from this study indicate 1) that tropical cyclones contribute, respectively, 4%, 3%, and 4% to the western, eastern, and entire North Atlantic; 2) similar to that observed in the North Pacific, the maximum in North Atlantic tropical cyclone rainfall is approximately 5°–10° poleward (depending on longitude) of the maximum nontropical cyclone rainfall; 3) tropical cyclones contribute regionally a maximum of 30% of the total rainfall northeast of Puerto Rico, within a region near 15°N, 55°W, and off the west coast of Africa; 4) there is no lag between the months with maximum tropical cyclone rainfall and nontropical cyclone rainfall in the western North Atlantic, whereas in the eastern North Atlantic, maximum tropical cyclone rainfall precedes maximum nontropical cyclone rainfall; 5) like the North Pacific, North Atlantic tropical cyclones of hurricane intensity generate the greatest amount of rainfall in the higher latitudes; and 6) warm El Niño–Southern Oscillation events inhibit tropical cyclone rainfall.
African easterly jet
Tropical Atlantic
Tropical cyclone scales
Atlantic hurricane
Longitude
Tropical cyclogenesis
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The 2013 season was forecast by the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (and many other seasonal forecast centers) to be much more active in terms of tropical cyclone activity than it turned out to be. The season was characterized by tropical cyclone activity that was well below normal and produced significantly fewer named storms than expected. This study investigates the reasons behind the poor forecast by analyzing the differences in the 2013 season compared to climatology from the previous 18 years (1995-2012), a very active Atlantic hurricane period associated with a multidecadal oscillation in the thermohaline circulation (Klotzbach & Gray, 2008).
Specifically, this study focuses in large part on the analysis of accumulated cyclone energy (ACE), which is used by NOAA to determine how “active” an individual tropical cyclone is throughout its life cycle, and how “active” a season is as a whole. ACE is calculated by using the formula ACE = 10-4Sv2 where v is the estimated sustained maximum wind speed measured in knots. This is calculated every six hours, typically at 0000, 0600,1200, and 1800 UTC. For this study, the 2013 tropical cyclone tracks were mapped using ArcGIS software and the ACE for all 2013 was calculated using data collected from NOAA. The Atlantic basin is then subdivided into three regions where tropical cyclones typically form throughout the season: East Atlantic (15°W-45°W), Mid-Atlantic (45°W-75°W) and Gulf of Mexico/Immediate Eastern U.S. Seaboard (75°W - 105°W). For each region, the total ACE for the 2013 season as well as the number of hurricane days is calculated. Then, the ACE values are calculated for each individual month (June-December). These values are compared with “typical” averages and analyzed. Additionally, the physical tracks of 2013 Atlantic tropical cyclones are analyzed and compared to those of a “typical” season using ArcGIS. This information quantifies the extent to which tropical cyclones in 2013 formed in anomalous locations or took anomalous paths compared to the 1995-2012 average.
It can be concluded from this study that the total ACE for 2013 was well below the average ACE value of a “typical” season. When analyzed by month, ACE for the Atlantic basin generally followed the expected climatology trend with a peak in September, but values were much smaller than climatology. It was found that 2013 ACE for the East Atlantic was about average and as expected, however ACE values from the Mid-Atlantic and Gulf of Mexico/Immediate Eastern U.S. Seaboard were well below average. Climatology suggests that most tropical cyclone activity should be in the Mid-Atlantic region, but this was not the case for 2013 when most activity was located in the East Atlantic. Additionally, it was found that the number of hurricane days in 2013 was well below average, with the Mid-Atlantic region being particularly anomalous with no hurricane days in 2013.
This study will be a useful resource for meteorologists and climatologists to continue analyzing the 2013 season, and will serve as a basis for determining possible causes of the anomalous geographic distribution of tropical cyclone activity in 2013. The information herein will also be beneficial in observing long-term trends and improving seasonal outlooks in the future.
Atlantic hurricane
Tropical cyclone scales
Tropical cyclogenesis
African easterly jet
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Tropical cyclogenesis
African easterly jet
Cyclogenesis
Predictability
Tropical cyclone scales
Rainband
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A total of 69 tropical waves (also known as African and easterly waves) were counted in the Atlantic basin during the 1992 hurricane season. As was the case in 1991, the waves were, in general, relatively weak. These waves led to the formation of only four tropical depressions in the Atlantic hurricane basin, of which one intensified into a tropical storm and another intensified into Hurricane Andrew. Andrew was the only 1992 Atlantic hurricane to originate from a tropical wave. There were five additional tropical depressions that were primarily initiated by systems of nontropical origin. These produced three hurricanes and one tropical storm. It appears that tropical waves led to the formation of practically all of the eastern Pacific tropical cyclones in 1992.
Atlantic hurricane
African easterly jet
Tropical Atlantic
Tropical cyclogenesis
Tropical cyclone scales
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A total of 70 tropical waves (also known as African or easterly waves) were counted in the Atlantic basin during the 1993 hurricane season. These waves led to the formation of 9 of the 10 total number of tropical cyclones in the Atlantic hurricane basin. It appears that tropical waves led to the formation of practically all of the eastern Pacific tropical cyclones in 1993.
Atlantic hurricane
African easterly jet
Tropical Atlantic
Tropical cyclone scales
Tropical cyclogenesis
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Tropical cyclogenesis
Atlantic hurricane
Madden–Julian oscillation
Tropical cyclone scales
Tropical Atlantic
African easterly jet
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The interannual variability of outgoing longwave radiation (OLR) over Africa from the Advanced Very High Resolution Radiometer (AVHRR) and zonal wind speed in the African easterly jet (AEJ) is analyzed and discussed in the context of Atlantic tropical cyclone activity. It is found that hurricane and tropical storm totals in the Atlantic basin are closely related to the African meridional OLR contrast (AMOC). It is suggested that the AMOC provides a simple yet novel way to simultaneously characterize the meridional temperature gradient and ITCZ activity, both of which play integral roles in generating African easterly waves. Complimentary to observed relationships between Sahel rainfall and Atlantic tropical cyclone activity, the potential for the AMOC to augment existing techniques used in preparing Atlantic hurricane season outlooks is also discussed.
African easterly jet
Atlantic hurricane
Intertropical Convergence Zone
Tropical cyclogenesis
Tropical Atlantic
Tropical cyclone scales
Predictability
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The assumption that tropical cyclones respond primarily to sea surface temperatures (SSTs) local to their main development regions underlies much of the concern regarding the possible impacts of anthropogenic greenhouse warming on tropical cyclone statistics. Here the observed relationship between changes in sea surface temperature and tropical cyclone intensities in the Atlantic basin is explored. Atlantic tropical cyclone intensity fluctuations and storm numbers are shown to depend not only upon SST anomalies local to the Atlantic main development region, but also in a negative sense upon the tropical mean SST. This behavior is shown in part to be consistent with changes in the tropical cyclone potential intensity that provides an upper bound on storm intensity. However, Atlantic tropical cyclone intensity fluctuations are more nonlocal than the potential intensity itself and specifically vary along with Atlantic main development region SST anomalies relative to the tropical mean SST. This suggests that there is no straightforward link between warmer SSTs in the main development region and more intense tropical cyclones.
Atlantic hurricane
Tropical cyclogenesis
Tropical cyclone scales
African easterly jet
Tropical Atlantic
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Increased occurrence of more intense tropical storms intruding further poleward has been foreshadowed as one of the potential consequences of global warming. This scenario is based almost entirely on the general circulation model predictions of warmer sea surface temperature (SST) with increasing levels of atmospheric C02 and some theories of tropical cyclone intensification that support the notion of more intense systems with warmer SST. Whether storms are able to achieve this theoretically determined more intense state depends on whether the temperature of the underlying water is the dominant factor in tropical cyclone intensification. An examination of the historical data record in a number of ocean basins is used to identify the relative importance of SST in the tropical cyclone intensification process. The results reveal that SST alone is an inadequate predictor of tropical cyclone intensity. Other factors known to affect tropical cyclone frequency and intensity are discussed.
Tropical cyclogenesis
Tropical cyclone scales
African easterly jet
Atlantic hurricane
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Citations (120)