2010.01.07
Analysis of Fall Transition Season (Sept-Early Dec) 2009
– Why has the weather been so violent?
As can be seen by the following forecast map, the Fall Transition and early Winter Season of 2009 has been particularly heavy in the North Pacific, with several very deep low pressure systems, long fetches of over 1,000nm with very strong winds, and extremely wide areas of over 7m seas/swells to south of 30N.
Reasons for such violent weather are related to several features, including the El Niño phenomenon, sea surface temperatures, global circulation patterns and slightly higher than average tropical cyclone activity in the Northwest Pacific, many of which recurved into the subtropics, bringing warm, moist air which helps fuel explosive development of low pressure systems.
This chart shows the current mid-winter situation. There are larger and more areas of heavy weather evident. Under such severe conditions, the Master should not be left to his own devices for making important decisions.
Introduction
The period September through early December has boasted ten low pressure systems deeper than 970mb, four of which showed central pressures less than 955mb (Figure 1).
Figure 1
Ninety-day average wave heights (m) and storm tracks for the period 11 September – 09 December. Courtesy Climate Prediction Center
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/stormtracks/strack_alaska.shtml
The presence of a moderate El Niño over the past several months (Figure 2), and its effect on Tropical Storm development, sea surface temperatures and global circulation patterns can be traced as the reason for this particularly heavy weather.
Figure 2
Time series of area averaged sea surface temperature anomalies in the Niño regions [Niño-1+2 (0-10S, 90W-80W), Niño-3 (5N-5S, 150W-90W), Niño-3.4 (5N-5S, 170W-120W), Niño-4 (5N-5S, 150W-160E)]. SST anomalies are departures from the 1971-2000 base period weekly means. (Xue et. al., 2003, J. Climate, 16, 1601-1612). Graphics courtesy Climate Prediction Center.
El Niño
During El Niño events, the atmospheric pattern is characterized by an eastward extension of the East Asian jest stream from the International Date Line to the southwestern United States, and shifting of that jet stream further south through the mid-latitudes. This results in an amplified main low track and enhanced Aleutian low, meaning that lows within the main low track tend to be stronger, and track further south through the basin.
Tropical Cyclones and Sea Surface Temperatures
The seven recurving tropical systems (as well as several tropical depressions that also recurved) during this period brought rich tropical moisture in the mid-latitudes, which helped prime the atmosphere for more violent weather during this transition season in two ways;
The seven recurving tropical systems (as well as several tropical depressions that also recurved) during this period brought rich tropical moisture in the mid-latitudes, which helped prime the atmosphere for more violent weather during this transition season in two ways;
- Recurving tropical systems bring warm, humid air to the mid latitudes.
- Strong recurving tropical systems typically have strong southerly winds which along the eastern side of the system
(These strong winds can serve to ‘push’ warm surface waters northward from the tropics into the mid-latitudes)
The PNA and El Niño
Eight of the ten storms that were observed to have central pressures of less than 970mb occurred when the PNA Index*was in a positive phase (Figure 9), meaning that the Aleutian Low, and the main low track was enhanced.
Conclusion
The presence of several recurving tropical systems over the western Pacific affected the weather pattern during the transition season in two ways. Firstly, warm, humid tropical air was carried into the mid-latitudes as these systems tracked northeastward and merged with the main low track.
Secondly, several strong recurving typhoons served to ‘push’ warmer tropical waters northward into the mid-latitudes, causing a warm SST anomaly to develop across the western Pacific. This allowed low pressure systems that developed off the coast of Japan to remain over warmer waters for longer; when those lows then interacted with cold air at higher latitudes, the resulting dynamics allowed lows to deepen further, and often more rapidly, than typically is expected.
Similarly, the effects of El Niño on the general weather pattern of the North Pacific, and El Niño’s interaction with the Pacific/North American pattern resulted in an enhanced Aleutian low pressure system across the central Pacific and Gulf of Alaska, resulting in a more active main low track across the basin.
The enhanced main low track served to allow stronger low pressure systems to develop (in concert with the observed SST anomaly), which caused the several violent storms that were observed over the course of this transition season.
* The Pacific North American (PNA) Index is a climate index that describes one of the most prominent modes of low-frequency variability in the Northern Hemisphere extratropics.
