Dynamic Conditions of the Atmosphere Accounting for the Opposite Trends in Tropical Cyclone Count in the Pacific

Tropical cyclones<!–[if supportFields]> XE “cyclones” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> are considered to be one of the more devastating weather phenomena in terms of their effect on human life and global economy.  How global warming will affect these events is widely debated amongst scientists. It is understood that tropical cyclone genesis is dependent on sea surface temperature (SST)<!–[if supportFields]> XE “sea surface temperature (SST)” <![endif]–><!–[if supportFields]><![endif]–>, since a higher SST provides a cyclone with higher ocean thermal energy.  Tropical cyclones typically develop from a tropical disturbance, and only a small percentage of cyclones result from such disturbances. It is implied that with higher SST levels and a temperature increase there will be a similar increase in tropical storms, however, there are opposite trends of cyclone frequency in the western and central Pacific.  Li et al. (2010) created and examined a model that would explain the variations in the tropical cyclone genesis in different areas of the Pacific. Their results suggest that the major factor that accounts for the distinctly opposite tropical cyclone trends is the constantly changing condition of the atmosphere. They conclude that the projected shift in tropical cyclone activity might pose a great threat to Hawaii and the central Pacific islands. —Brian Nadler
 Li, T., Kwon, M., Zhao, M., Kug, J., Luo,J., and Yu, W. 2010.  Global warming shifts Pacific tropical cyclone<!–[if supportFields]>XE “cyclone”<![endif]–><!–[if supportFields]><![endif]–> location. Geophysical Research Letters 37, 1–5.

Li et al. examined data demonstrating opposite effects of global warming in different areas of the Pacific.  They created a high-resolution global model that can more accurately predict the results shown in previous studies. The model was then tested to ensure that it is applicable to a wider range of results than just Pacific tropical cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> increases and decreases.  The authors used data from the International Pacific Research Center in Hawaii, Korea Ocean Research and Development Institute, First Institute of Oceanography in China<!–[if supportFields]> XE “China” <![endif]–><!–[if supportFields]><![endif]–>, and the GFDL, NOAA in Princeton New Jersey to compile a suitable model to predict tropical cyclone genesis.  The results suggested an increase in variations in atmospheric conditions in the north central Pacific region (termed synoptic-scale disturbances), and a decrease in variations in the northwestern Pacific region. These changes in conditions create wind shears that have significant effects on the increases and decreases in tropical cyclone activity.
Li et al. discovered that global warming weakens the trade winds in the Pacific, and that the Walker circulation is weakened, causing a weakening of the North Pacific monsoon<!–[if supportFields]>XE “monsoon” <![endif]–><!–[if supportFields]><![endif]–> season. The weakening of heating caused by the monsoon season leads to a reduction of tropical cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> frequency. In contrast, in the central Pacific, the SST<!–[if supportFields]>XE “sea surface temperature (SST)”<![endif]–><!–[if supportFields]><![endif]–> gradient is reduced and there is a higher localized SST, which results in an increase in tropical cyclone frequency. The model tends to overestimate tropical cyclone genesis, so the authors suggest caution when interpreting the results—however, the shift of cyclone activity in the Pacific is significant to the millions of people living in Hawaii and central Pacific islands. 
Even with the overestimation, developing a model that can accurately predict tropical cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> activity and take into account so many variables is particularly important. There are still uncertainties with regards to physics in the models uses and SST<!–[if supportFields]> XE “sea surface temperature (SST)” <![endif]–><!–[if supportFields]><![endif]–> warming patterns, but being able to take these into account may have implications for determining the effects of tropical cyclone activity on a global scale, rather than limiting it to the Pacific.

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