Spring And Fall: The Neglected Seasons
John Abatzoglou, Desert Research Institute
While a great deal of research has been given to the prominent "peak" seasons of summer and winter, the shoulder seasons remain relatively understudied. Three phenomena with regional focus over western North America will be presented in this talk: (1) the asymmetry of temperature trends between spring and autumn, (2) the influence of the Madden-Julian Oscillation in driving extreme temperature events during the spring, and (3) Santa Ana winds events in southern California.
(1) Observational evidence shows that spring temperatures over western North America have undergone significant warming over the past half century, while autumn temperatures have shown relatively little change. Low- frequency modes of atmospheric variability for spring and autumn are demonstrated to account for a great deal of the seasonal asymmetry, with trends in spring circulation patterns exacerbating regional warming, and trends in autumn circulation patterns counteracting warming. After excluding warming associated with the primary modes of atmospheric variability, temperature trends in spring and autumn over western North America are similar to one another and in broad agreement with seasonal trends from a multimodel ensemble.
(2) Previous studies have examined the influence of the MJO on extreme precipitation events across the midlatitudes; here we examined the role that the MJO has on extreme temperature events over western North America during the spring. The amplification of midlatitude ridge and trough patterns in concert with the eastward propagation of an active MJO signal is shown to provide added predictability of anomalously warm and cold events over the region. As the MJO is a recurrent intraseasonal phenomena, the ability to utilize information on its phase and amplitude may enhance the predictability of weather and extreme events at lead times beyond those of deterministic numerical weather prediction (e.g., week 2 forecasts).
(3) Santa Ana winds are downslope offshore winds that are sporadically observed in southern Californiabetween fall and spring. Due to the complex topography of the region, Santa Ana episodes can sometimes be extremely intense and pose significant environmental hazards, most notably contributing to the incidence and spread of wildfires such as in the wildfires of October 2003 and 2007. An objective methodology is presented to identify the occurrence of Santa Anas using daily mean data from the reanalysis data and Remote Automated Weather Stations. Connections to prominent teleconnection patterns will be discussed.
SECURITY: If you are coming from outside the NOAA campus, please be advised that you will need an on-site sponsor. Please contact that person in advance of the seminar to be put on the list and allow 10 minutes extra on the day of the seminar. Please contact Joe Barsugli (303-497-6042) or Barbara Herrli (303-497-3876) at least a day before the seminar if you have any questions.