Inner-core-scale Trend Analysis Of Tropical Cyclones In The Northwest Pacific And Its Mechanism

Studies on tropical cyclone(TC)inner-core size have become increasingly active in recent years.However,few studies have investigated the trend of TC inner-core size.In this study,we use the TC datasets provided from the China Meteorological Administration(CMA)and the Japan Meteorological Agency(JMA)to calculate TC inner-core size trend from 1981 to 2016 with statistical analysis methods such as correlation analysis,regression analysis,and synthetic analysis.Here,we introduce a new index to measure TC inner-core size and calculate the observed trend.This index can greatly reduce the influence of data heterogeneity and uncertainty.It also considers public concern because the new index is mainly determined by the inner-core size of strong TCs,which attract more public attention than weak TCs.In addition,numerical experiments are used to study the physical mechanism of the TC inner-core size downward trend.Here are the main conclusions:1)In the past decades,the TC inner-core size trend over the western north pacific shows a significant downtrend that is significant above the 99% confidence level,when the new index is used.The new index shows three main advantages through our quantitative analyses.Fisrt,it is not sensitive to the relatively inaccurate data during the TC lifetime.Secondly,the new index is not sensitive to weak TCs.At last,more consistent results of the new index can be obtained using datasets from different agencies.2)The downward trend of the TC inner-core size is related to the TC intensity.Here,we find that there is a significant anti-correlation between the intensity of TC and the inner-core size of TC.We also find that the power dissipation index(PDI)of strong TCs over the Western North Pacific from 1981 to 2016 shows an increasing trend.Based on the significant anticorrelation between the TC intensity and the inner-core size,the trend of TC inner-core size would decrease significantly.The convective activity and inertial stability contribute to the anticorrelation between TC intensity and TC inner-core size.TCs with large inner-core size need more convective heating during intensification and have better inertial stability,which inhibits inflow from transporting higher angular momentum air inward within the boundary layer;this causes TCs with large inner-core size have more difficulties to reach their potential maximum intensity and are more likely to be weaker than TCs with smaller inner-core size.3)The downward trend of TC inner-core size is related to sea surface temperature(SST)and TC relative SST.We find that there is a significant positive correlation coefficient between SST in the TC main active area and TC intensity,and a significant anti-correlation coefficient between SST and TC inner-core size.Their correlation is mainly attributed to the air-sea interface and convective activities.The air-sea interface plays an important role on TC intensity change caused by relative SST.As the inflow air gets closer to the eyewall,the colder remote SST from the TC center(which can lead to the result that the TC would have warmer relative SST)can gradually decrease the surface air temperature and moisture near the TC center,and thus increase air-sea temperature differences and moisture differences between the TC center and the area outside it,which will lead to more energy fluxes entering the eyewall with TC inner-core size shrinking.Moreover,with the increase of SST,the pressure gradient force in the TC inner-core region would increase,which intensifies the tangential wind in the TC inner region,causing the field of tangential wind shrink inward.As a result,TC inner-core size would get decreased.When TCs get their mature stage,the convective activities in the TC inner-core region become more active,resulting in the enhancement of TC intensity and the decrease of TC inner-core size.4)There is a significant,abrupt decadal change of TC inner-core size around 1999.From1981 to 1999,TC inner-core size over the western north pacific shows an upward trend.After1999,TC inner-core size over the western north pacific shows a downward trend.The abrupt decadal change is related to the change of TC track and the environmental differences between the interdecadal pacific oscilation(IPO)positive phase and negative phase.TCs have a poleward movement from 1981 to 1999,and then they turn to the equatorial region from 1999 to 2016.As a result,TC relative SST would decrease firstly,and then increase.Therefore,from1981 to 2016,the TC inner-core size also has an upward trend firstly,and then has a downward trend.The other possible mechanism may be related to IPO.IPO index and TC inner-core size have a significant positive correlation.Based on the analysis of environmental differences between IPO positive and negative phase,the east wind is stronger in IPO negeative phase than positive phase,thus more energy would be exchanged between the sea surface and the air,which attributes to the enhancement of TC intensity.Based on the significant anti-correlation coefficient between the TC intensity and the TC inner-core size,since TC intensity trend has a reversal around 1999,TC inner-core size would also have an abrupt change.