Analysis of the Polar Vortex Trend in the Northern Hemisphere under Climate Change Conditions

1. Today, climate change problem is considered as one of the most common scientific
subject, even as social and political issues, in fact it is not novelty issue. Principally,
because the change and movement are one of the component in macro scale climatic
systems, discovering the legitimacy of these systems, which mainly is cyclic, it is can be
consider as a tool which can help to predict future behavior of the system. The polar
vortex (PV) is defined as a large-scale upper-level cyclonic circulation in the middle
and upper troposphere which is centered on the polar region. Measurement undertaken,
observational investigation and modeling indicate the oscillation cycles of polar vortex.
There are two effective mechanisms for oscillation cycles of polar vortex that could be
divided into two categories: internal and external. The internal mechanism is included
investigation of oscillation cycles of polar vortex by applying thermal and wavy forcing,
which is independent of time and leads to identify the effect of the stratosphere on the
troposphere. External mechanism is included investigation of oscillation cycles of polar
vortex by applying thermal and wavy forcing, which is depends on time and leads to
identify the effect of the troposphere on the stratosphere. Trend changes on the macro
scale climatic system, such as polar vortex has a significant role on climate change of
the surface. Variability in the atmospheric circulation at the hemispheric-scale can have
a direct impact on variations in the surface environmental parameters, such as
temperature, precipitation, and pollutant transport. One indicator of the behavior of the
hemispheric-scale circulation is the polar vortex. The polar vortex can be used for study
the holistic hemispheric-scale sub-tropical circulation because it represents the
characteristics of long-wave, upper-level ridge-trough configuration around the entire
(northern or southern) hemisphere at a given point in time. The PV is defined as the
large-scale upper-level cyclonic circulation in the middle and upper troposphere
centered on the polar region. The most fundamental question addressed in this research
is whether the northern hemisphere polar vortex (NHPV) exhibits long-term trends in
area over the 1948-2007 periods. Therefore, the first part of this study answers the
question: How can we characterize the long-term trend of the area of the NHPV? It is
hypothesized that the global warming signal would be associated with a reduction in
area in the NHPV over time as the cold pool of air over the poles shrinks.
2. Material and In this research, daily mean data of 500 hPa geopotential height were used from the
National Center for Environmental Prediction & National Center of Atmospheric
Research (NCEP/NCAR) during the period of 1948 to 2007. Their spatial resolution is
2/5 * 2/5 degree with 36 * 144 pixels. The 500 hPa geopotential height (i.e., constant
pressure surface) was selected for analysis for several reasons. First, since the 500 hPa
level is in the middle of the atmosphere, it represents characteristics of both the lower
and upper atmosphere. Also, most steering of mid-latitude systems occurs at the 500
hPa level and geostrophic flow occurs at this level as well. A specific isohypse(contour)
is selected to represent the NHPV for each month in 500 hpa level (table 1).
Monthly polar vortex area is calculated with Grads software. This research utilizes a
statistical approach to determine the trend in the northern hemisphere (NH) polar
vortex. For comparison of area means, used from Paired-Samples T Test and for the
detection of polar vortex area trend used from Kendall's tau trend test in SPSS software.
Paired-Samples T Test is one of the statistical parametric tests to compare the mean and
variance of the two dependent groups in a region.
3. By doing the Kendall tau test in SPSS, it was determined a reduction in the polar vortex
area in all months. Polar vortex area has a decreasing trend during May, June, July,
August, September and March which is significant at 0.01 level.
The biggest reduction trend is related to August and lowest to December. The show that the polar vortex during the whole period is declining, but this decline was
higher during summer months. The most of decline in the polar vortex area is observed
in August that the rate of about 93,000 square kilometers per year. The decline rate has
accelerated in the second half of period. Generally, the decline of polar vortex area
caused by macro-scale climate change and planetary scale, That certainly global
warming and consequently advancement of subtropical high pressure will be one of the
components. The results of Paired-Samples Test showed that the mean difference of
polar vortex area is negative for all months and this amount represents a reduction in
polar vortex area during thirty years of second period compared to the first period.
4. The change in configuration of polar vortex (trough and ridge) and transformation from
zonal to meridional as a result of reducing of meridional pressure gradient caused
regional climatic anomalies such as the change in the type and amount of precipitation
and temperature. Generally, the decline of polar vortex area caused by macro-scale climate change and planetary Scale, that certainly global warming and consequently
advancement of subtropical high pressure will be one of the effective components.