|
By Rob Mann and Jason Moreland As of May 24, 2010 ...Signs point to one of the most active on record... This is our eighth Atlantic basin hurricane season forecast, and the first issued as a part of 28storms.com. Last year's forecast of total tropical cyclone activity proved to be accurate: the 2009 Atlantic basin hurricane season finished below the long-term average with 9 named storms (8-11 predicted), 3 hurricanes (3-5 predicted), and 2 major hurricanes (1-2 predicted). Due to last year's success, the methodology of this aspect of the forecast remains unchanged. However, the predicted synoptic pattern and resulted hurricane tracks during the peak of the season did not fully materialize. A thorough post-review of what went wrong was conducted this winter. Since then, a new and more promising methodology with emphasis on meteorological causation has been developed. Please keep in mind that this forecast is unofficial and experimental. El Niño Southern Oscillation (ENSO) ENSO is a coupled ocean-atmosphere phenomenon that is most notably characterized by sea-surface temperature anomaly (SSTA) changes in the equatorial Pacific. These alterations impact the global atmospheric circulations and, thus, have a drastic impact on Atlantic basin tropical cyclone activity. Recent Trends The strong El Niño observed this past winter has dissipated. SSTAs in the Niño 3.4 region (120-170°W) of the equatorial Pacific have dropped from 1.2°C in mid-March to -0.1°C in the most recent weekly update, well below the accepted El Niño threshold of 0.5°C. Correspondingly, cloudiness has decreased and trade winds have strengthened to near-normal levels along the equatorial Pacific. Meanwhile, temperature anomalies below the surface of the equatorial Pacific have cooled dramatically over the past two months, associated with a steepening of the thermocline. An anomalous cool pool currently stretches from 150°E to the Peruvian coast, at a depth from 200m in the west and rising to the surface in the east.  The most dramatic ENSO-related reversal over the past two months has been in the Southern Oscillation Index (SOI), a measure of sea level pressure (SLP) difference between Tahiti and Darwin, Australia. The monthly SOI materialized between -10.0 and -15.0 SD from January to March, but leaped to 15.2 SD in April and will likely finish above 5.0 for May. Such a positive SOI is associated with lower SLP over the Maritime Continent and stronger trade winds across the equatorial Pacific. This and the above trends indicate that not only has the El Niño dissipated, but that a La Niña may soon take hold.  Analysis and Forecast One factor to consider regarding ENSO evolution is global atmospheric angular momentum (GLAAM). GLAAM simply measures how fast the Earth's atmosphere is rotating. A La Niña favors negative GLAAM since it is usually associated with slower mid-latitude westerly winds and faster trade winds. However, GLAAM can also impact ENSO. When Earth's atmospheric angular momentum is negative, the angular momentum of the solid Earth has to be proportionally positive to compensate because angular momentum cannot be created nor destroyed. Hence, trade winds that reflect from the Andes Mountains are thrust westward at quicker rate during a negative GLAAM, which cools SSTAs in the equatorial Pacific off the Peruvian coast. GLAAM exhibits interdecadal variability, and since 1998 has shown a negative tendency. This elevates the chance of a La Niña forming this year. It is also worth examining the state of the 11-year sunspot cycle. Low levels of sunspot activity are accompanied by a decrease in solar wind. Solar wind helps deflect galactic cosmic rays (GCRs), which are high-energy charged particles that bombard the solar system from outside. It has been theorized that GCRs aid cloud growth, especially over the Maritime Continent since that is where cloud tops are the highest. Increased GCRs may then help lower the SLP and favor a cooler ENSO. Since 1949, the only La Niña events that formed during rising sunspot activity were in 1988 and 1998. These were on the heels of a strong two-year El Niño and an extremely strong El Niño, respectively, so the balancing influence of stronger downwelling Rossby waves overcame the solar impact. Every other La Niña (1954, 1961, 1964, 1970, 1973, 1984, 1995, and 2007) formed during a period of minimum or falling sunspot activity. On the other hand, the El Niño events in the first half of 1958, 1966, 1969, 1977, 1978, 1983, 1987, 1992, and 2003 decayed or persisted during a period of rising sunspot activity. Incidentally, none of these events gave way to a La Niña before winter. Although sunspot activity increased this past winter, it has since fallen and remains at very low levels. This favors La Niña development. Based on the above data, it is no surprise that a La Niña appears to be developing. The La Niña is expected to be fully established by July and strengthen into a moderate event as the season progresses. A La Niña supports lower SLPAs and less wind shear across the tropical Atlantic. This will have an enhancing effect on tropical cyclone activity in the Atlantic basin. Atlantic Multidecadal Oscillation (AMO) The AMO is a long-term pattern of SSTA variability in the North Atlantic. This phenomenon exerts a strong influence on Atlantic basin tropical cyclone activity via modifying the SSTAs over the tropical Atlantic. Recent Trends The AMO is currently warmer than it has been in any other spring since 1856. The latest monthly AMO value was 0.478°C, the fourth warmest in any month on record. More pertinent to tropical cyclone activity, however, are SSTAs in the tropical Atlantic, rather than the entire North Atlantic. SSTAs in this specific region are best measured by the Tropical North Atlantic (TNA) index. While the TNA dataset only extends back to 1950, both the March and April values set new warm records at 1.32°C and 1.38°C, respectively. SSTAs in the region have only increased further this month, so the record is likely to be broken for the third straight month in a row.  It is no accident that SSTAs in the tropical Atlantic are so warm. The past winter and spring featured an abnormally weak subtropical ridge due to the extremely high pressure anomalies across the arctic and northern Atlantic. This reduced northerly flow along the West African coast and trade winds in the tropics. In turn, the Canary current slowed and the SSTs were allowed to warm much more dramatically than normal. Moreover, the strong El Niño shifted the Walker circulation so that upper-tropospheric air began sinking over the tropical Atlantic. This increased the SLPAs over the region. Since the tropical Atlantic has an inherently lower SLP than the subtropical Atlantic, this reduced the SLP gradient between the tropics and the subtropics. Over time, this weakened the trade winds across the tropical Atlantic and helped warm the SSTAs.  Analysis and Forecast The aforementioned ENSO-TNA correlation helps explain the current warm TNA, but it also suggests that the SSTAs will moderate over the next several months. The decay of the El Niño will give way to increased low-level convergence, and consequently lower SLPAs, over northern South America and the tropical Atlantic. This will have a net cooling effect on SSTAs as the weakened meridional SLP gradient increases the trade winds. However, the synoptic pattern through July is expected to favor a continuance of warm tropical Atlantic SSTAs, as will be discussed in the next section. Furthermore, there is a statistically significant correlation coefficient of approximately 0.7 between fall SSTs in the tropical Atlantic and global mean temperature. Given the relatively warm global mean temperature so far this year and over the past ten years, significant cooling of the SSTAs in the tropical Atlantic through the fall is unlikely. Finally, it is important to note that ocean currents over the tropical Atlantic are easterly, due to the prevailing trade winds along the southern periphery of the subtropical ridge. Therefore, warm SSTAs across the tropical Atlantic should continue to spread westward into the Caribbean Sea and Gulf of Mexico. Since tropical Atlantic SSTAs are currently the warmest on record by 0.4-0.5°C, it would take significant cooling, more than 0.8°C in five months, to fall below record monthly levels. This is unlikely to occur, as the above data suggests there will only be a gradual moderation of SSTAs in the area through the season. SSTAs in the Caribbean Sea and Gulf of Mexico will probably continue warming to well-above average levels over the next several months. Such warm SSTAs across the entire tropical basin are extremely conducive for tropical cyclone development, as they favor lower SLPAs and provide increased fuel for convection. June and July Synoptic Pattern During the Summer Solstice, the Earth's axis tilts the most toward the Sun. Since the higher latitudes of the northern hemisphere observe the greatest shift in sunlight, they warm more dramatically than the tropics. This weakens the meridional temperature gradient across the northern hemisphere, and as a result, the synoptic pattern becomes far more stable than in the cold season. Planetary wavelengths shorten, which reduces the impact of ENSO forcing on the Atlantic basin pattern. This dynamical shift in the summer allows spring SSTAs and soil moisture anomalies to have an influence on the synoptic pattern through positive feedback. Warm SSTAs causes surface air to rise quicker and then diverge in the upper troposphere, forming anomalous upper-level ridging. Similarly, anomalously dry soil reduces evaporation. This heats the surface air and likewise favors more upper-level ridging. Cool SSTAs and anomalously wet soil have the opposite effects. Cool SSTAs south of the Gulf of Alaska and more neutral SSTAs along the western United States coast over the past two months should correspond to anomalous troughing and anomalous ridging, respectively. Unusually dry soil over the western United States supports such a setup. Additionally, abnormally wet soil has persisted over much of the Great Plains, which should correspond to an anomalous trough. Soil in the eastern states, while wetter than normal in places, has been drier than in the Central Plains. Crucially, Atlantic SSTAs close to the United States east coast and offshore the Canadian Maritimes have been above average. This favors a downstream anomalous ridge centered roughly over the Canadian Maritimes. Cool SSTAs in the subtropical Atlantic and warm SSTAs offshore West Africa and in the tropics support anomalous troughing and ridging in those respective areas. Tropical Cyclones Low SSTs normally limit tropical cyclone activity before August. However, the record warm SSTAs in the tropical Atlantic basin combined with a developing La Niña portends a significant amount of tropical cyclone activity in June and July. The only other years since 1950 that had spring tropical Atlantic SSTAs even close to present levels were 1958, 1969, and 2005. The former two years observed a continuance of El Niño conditions through the summer, which overcame the warm SSTAs. 2005, however, had no El Niño during the summer and produced seven named storms in the tropics before August. Since SSTAs in the tropical Atlantic and Caribbean Sea this month have been even warmer than in May 2005, there is no obvious reason to expect less early tropical cyclone activity. Therefore, 7-8 named storms, 3-4 hurricanes, and 1-2 major hurricanes are forecast to develop before August this year. The forecast of abnormal low-latitude ridging, due to the SSTAs, suggests that tropical cyclones that form east of the Caribbean Sea will probably track toward land. This increases the risk of a hurricane landfall in the Antilles and the Bahamas. Beyond these islands, the anticipated synoptic pattern of a mean trough in the Central Plains and a downstream anomalous ridge near the Canadian Maritimes suggests a heightened hurricane landfall threat for the Carolinas, Florida peninsula, and central Gulf coast. These regions will also be at risk from any hurricane that forms in the Caribbean Sea, Gulf of Mexico, or western Atlantic. Given the Central Plains trough, however, it is unlikely that Texas, Mexico, or Central America will face a hurricane landfall before August.  August and September Synoptic Pattern As the Earth's axis tilts further away from the Sun, less sunlight reaches the high latitudes of the northern hemisphere and so the meridional temperature gradient increases. This destabilizes the synoptic pattern and increases planetary wavelengths. Hence, soil moisture and SSTAs in the North Pacific and Atlantic lose influence. On the other hand, forcing from ENSO becomes more important. When rising air at the equator reaches the tropopause, a large portion of it diverges poleward to transfer the latent heat. Due to the Coriolis effect and conservation of angular momentum, air is forced eastward in the mid-latitudes, marking the poleward extent of the Hadley cell. SSTA variations in the equatorial Pacific influence how quickly air is transferred poleward. When planetary wavelengths lengthen, ENSO is thus able to impact the entire northern hemisphere synoptic pattern. An intensifying La Niña will likely be in place by August. This will result in less rising air in the equatorial Pacific and a slower poleward transfer of heat. The Coriolis effect will take longer to turn the air completely eastward and, therefore, the Hadley cell will expand relative to average. As fall approaches, this should increasingly result in anomalous ridging across most of the subtropical North Pacific. Downstream anomalous troughing will likely develop near the United States west coast, which, in turn, should force an anomalous ridge over the central and eastern United States. The Atlantic basin synoptic pattern, while more variable, will likely be characterized by an anomalous trough offshore the Canadian Maritimes and an anomalous ridge near the Azores. The Quasi-Biennial Oscillation (QBO), a periodic stratospheric oscillation of zonal winds in the equatorial stratosphere, also has a greater role in August and September. During an easterly QBO, equatorial zonal winds descend upon the tropopause in the same direction as the tropospheric flow and raise the tropopause. This sparks a compensating poleward ripple effect on the tropopause height, which decreases in the subtropics and increases in the mid-latitudes relative to average. The result is increased upper-level ridging across the mid-latitudes and negative GLAAM. Since the QBO will probably rise to near-neutral levels by September, a more amplified synoptic pattern is expected outside the North Pacific. Thus, the aforementioned anomalous mean troughs should be relatively deep. Tropical Cyclones Since SSTs are the warmest in August and September, and wind shear is low, most tropical cyclone activity usually occurs in these two months. This August and September, SSTAs across the tropical Atlantic, Caribbean Sea, and Gulf of Mexico should remain extremely warm. The La Niña will likely be intensifying into a moderate event. This is a very favorable amalgam for tropical cyclone development during the peak of the season. The years with the warmest July-August tropical basin SSTAs were 1958, 1995, 1998, 2004, and 2005. All had above-average tropical cyclone activity in August and September. In 2010, the amplified synoptic pattern will be particularly conducive for tropical cyclone development in the tropical Atlantic, as it was in 1995, 1998 and 2004. The anomalous Canadian Maritimes trough will also allow cutoff lows to retrograde under the anomalous central-eastern United States ridge. Given the warm SSTAs, several such lows will probably aid in tropical cyclone formation. All considered, 11-12 named storms, 8-9 hurricanes, and 4-5 major hurricanes are forecast to develop in August and September. Based on the Canadian Maritimes weakness, most, if not all, of the tropical cyclones that form east of the Caribbean Sea will recurve away from land. Moreover, the moderate La Niña and extremely warm SSTAs will shift the intertropical convergence zone (ITCZ) much further north than normal. Because tropical waves move along the axis of the ITCZ, there is a diminished chance of a hurricane tracking through the Caribbean Sea and hitting Central America or the Yucatán peninsula. However, several hurricanes should form in the western Atlantic and Gulf of Mexico. The anomalous ridge over the central and eastern United States suggests that these will pose a threat to the Bahamas, eastern Florida peninsula, central Gulf coast, and western Gulf coast. The expected mean position of the ridge indicates a low hurricane risk to the United States east coast north of Florida, with the possible exception of the Outer Banks. 
October and November Synoptic Pattern The Earth's axis continues to tilt away from the Sun as fall progresses. Less sunlight reaches the high latitudes of the northern hemisphere, so the meridional temperature gradient further increases. The synoptic pattern continues to destabilize as planetary wavelengths increase. Forcing from ENSO becomes even more important, but wavelengths increase enough that progression of the Madden-Julian Oscillation (MJO) over the Indian Ocean more notably impacts the North American and Atlantic synoptic pattern. The MJO is characterized by patterns of anomalous wind flow and rainfall over the tropics. Unfortunately, it is difficult to predict the progression of the MJO many months in advance. As the La Niña matures, the inclination of the thermocline will flatten, which will reduce the amount of upwelling in the eastern equatorial Pacific. Since trade winds carry the cool SSTAs westward, it is expected that the difference in SSTAs between the western and eastern equatorial Pacific will increase through the fall. This favors a westward shift in the strongest anomalous ridging over the North Pacific. However, due to the longer planetary wavelengths, this should not influence the mean position of the anomalous downstream trough. Thus, an anomalous ridge will likely persist over the central and eastern United States through most of October and perhaps into November. Such a synoptic anomaly will likely maintain an abnormal trough near the Canadian Maritimes. However, the synoptic pattern is expected to be more variable than in August and September, given the heightened relevance of the MJO. Tropical Cyclones As SSTs cool and wind shear increases, tropical cyclone activity typically decreases in October and, especially, November. Although tropical Atlantic SSTAs will not be as extreme by October, the current levels and anticipated rate of decline still suggests warm fall levels incomparable to any other year except 2005. Such SSTAs and the La Niña, as in the first four months, favors well above-average tropical cyclone activity after September. Six named storms formed in the tropical latitudes during October and November of 2005. An additional five named storms, including one in December, formed in the subtropical latitudes. However, such cyclone development is a more function of the synoptic pattern, rather than SSTAs. While the synoptic pattern during the last two months of the season could allow for a few named storms in the subtropical latitudes, there is not enough certainty to predict as many as occurred in 2005. Therefore, 6-7 named storms, 3-4 hurricanes, and 1-2 major hurricanes are forecast to develop after September. The anticipated mean synoptic pattern during October and November is actually quintessential of a cold Pacific Decadal Oscillation (PDO), an oscillation of climate variability in the North Pacific. This is reasonable since the PDO, as with the GLAAM, has been in a multidecadal cold cycle since 1998. In such a synoptic pattern, the western Florida peninsula has, historically, more frequently been hit. Given that multiple hurricanes are possible in the Caribbean Sea, this area will be at risk for a hurricane landfall in October. This also heightens the hurricane threat for the Bahamas, Cuba, Central America, and the Yucatán peninsula.  Conclusion The record warm tropical Atlantic SSTAs in tandem with a developing La Niña is an unprecedented combination. Accordingly, this is an alarming forecast. 28storms.com 2010 Atlantic Basin Hurricane Season Forecast If you have any questions or comments about this forecast, please email us. |