etd @ Mason (Electronic Theses and Dissertations) >
College of Science >
Please use this identifier to cite or link to this item:
|Title: ||The Impact of ENSO on the Extratropics|
|Author(s): ||Jin, Daeho|
|Issue Date: ||11-Jul-2008|
|Abstract: ||The impact of tropical remote forcing on the extratropics is examined with ideal
El Niño-Southern Oscillation (ENSO) forcing in the tropical Pacific. A set of numerical
experiments are described in which perfectly periodic ENSO is prescribed in the tropical
Pacific as a lower boundary condition, and a slab mixed layer ocean model is coupled to
an Atmospheric General Circulation Model (AGCM) in all other ocean basins.
First, the role of subtropical air-sea coupling is investigated by changing the
tropical Pacific forcing region, i.e., narrow (10°S~10°N) vs. broad (20°S~20°N) forcing
region. When the tropical Pacific is prescribed to be only the climatological annual cycle,
different SST in the subtropics results. This, in turn, leads to different atmospheric
motions, and consequently affects the adjacent extratropical atmosphere. The effect is
limited to the Pacific basin only. When the tropical Pacific includes ENSO, meridionally
broad structure of SST forcing intensifies the meridional atmospheric circulation in the
North Pacific basin, and, hence, the extratropical response to ENSO increases.
Secondly, the relationship between remote ENSO forcing and seasonality is
examined. Here we compare the response to perfectly periodic ENSO forcing that peaks
in boreal summer versus boreal winter. The results indicate that the maximum
extratropical response to the ENSO is determined by the local seasonality rather than the
temporal phase of ENSO. When the peak of ENSO is in boreal summer, the surface heat
flux in the North Pacific is maximized in the boreal winter, six months earlier than the
peak of ENSO. At the same time, the evolution of SST in the South Pacific is very
similar to that of the observed North Pacific. The tropical atmosphere linearly responds to
the prescribed SST forcing, but the atmospheric bridge connecting the deep tropics to the
extratropics occurs in specific seasons.
Lastly, we examined how the extratropical response to ENSO varies depending on
the period of ENSO. Ideally periodic two, four, and six year ENSO period experiments
were performed. When the ENSO responses mature in the North Pacific, the composite
patterns are similar among experiments, but the variance is sensitive to the ENSO
periodicity. The extratropical response to ENSO is damped by local air-sea interaction.
This local damping has a time-scale that is considerable longer than one year. Hence, the
high frequency ENSO, in which the time-scale between successive ENSO events is
shorter than the local damping process, results in increased variances of the ENSO forced
pattern. In addition, the La Niña forced pattern persists longer than the El Niño forced
|Appears in Collections:||College of Science|
Items in MARS are protected by copyright, with all rights reserved, unless otherwise indicated.