Seasonality of Interannual Inter-hemispheric Oscillations over the Past Five Decades

Air mass is inter-hemispherically redistributed, leading to an interesting phenomenon known as the Inter-Hemispheric Oscillation （IHO）. In the present article, the seasonality of the interannual IHO has been examined by employing monthly mean reanalyses from NCEP/NCAR, EAR40, and JRA25 for the period of 1958–2006. It is found that the IHO indices as calculated from different reanalyses are generally consistent with each other. A distinct seesaw structure in all four seasons between the northern and southern hemispheres is observed as the IHO signature in both the surface air pressure anomalies （SAPAs） and the leading EOF component of the anomalous zonal mean quantities. When the SAPAs are positive （negative） in the northern hemisphere, they are negative （positive） in the southern hemisphere. Large values of SAPAs are usually observed in mid- and high-latitude areas in all but the solstice seasons. In boreal summer and winter, relatively stronger perturbations of IHO-related SAPA are found in the Asian monsoon region, which shows a large difference from the status in boreal spring and fall. This suggests that seasonal mean monsoon activity is globally linked via air mass redistribution globally on interannual timescales, showing a very interesting linkage between monsoons and the IHO in the global domain. In all seasons, large values of SAPA always exist over the Antarctic and the surrounding regions, implying a close relation with Antarctic oscillations.

The seasonal cycle of interhemispheric oscillations in mass field of the global atmosphere

Using the daily and monthly data of surface air pressure, meridional wind, radiation and water vapor from NCEP/NCAR reanalysis for the period of 1979―2006, we have examined the seasonal variations of the interhemispheric oscillations (IHO) in mass field of the global atmosphere. Our results have demonstrated that IHO as observed in surface air pressure field shows the distinct seasonal cycle. This seasonal cycle has an interhemispheric seesaw structure with comparable annual ranges of surface air pressure in the Southern and Northern Hemispheres. Mass of water vapor changes out-of-phase between the Southern and Northern Hemispheres, showing clearly a seasonal cycle with its annual range almost equivalent to annual range of the IHO seasonal cycle. Amazingly, the cross-equatorial flow is found to be induced by annual changes in water vapor mass as a response of the atmosphere to seasonal cycle of forcing from hemispheric net surface short- and long-wave radiations. The IHO seasonality exhibits its larger variations in magnitude in mid-latitudes other than in other regions of the globe. Additionally, our results also show that the global air mass is redistributed seasonally not only between the Northern and Southern Hemispheres but also between land and sea. This land-sea air mass redis- tribution induces a zonal pattern of surface air pressure in the Northern Hemisphere but the meridional pattern in the Southern Hemisphere.

Rainfall forecast errors in different landfall stages of Super Typhoon Lekima (2019)

The rainfall forecast performance of the Tropical Cyclone(TC)version Model of Global and Regional Assimilation PrEdiction System(GRAPESTCM)of the China Meteorological Administration for landfalling Super Typhoon Lekima(2019)is studied by using the object-oriented verification method of contiguous rain area(CRA).The major error sources and possible reasons for the rainfall forecast uncertainties in different landfall stages(including near landfall and moving further inland)are compared.Results show that different performance and errors of rainfall forecast exist in the different TC stages.In the near landfall stage the asymmetric rainfall distribution is hard to be simulated,which might be related to the too strong forecasted TC intensity and too weak vertical wind shear accompanied.As Lekima moves further inland,the rain pattern and volume errors gradually increase.The Equitable Threat Score of the 24 h forecasted rainfall over 100 mm declines quickly with the time-length over land.The diagnostic analysis shows that there exists an interaction between the TC and the mid-latitude westerlies,but too weak frontogenesis is simulated.The results of this research indicate that for the current numerical model,the forecast ability of persistent heavy rainfall is very limited,especially when the weakened landing TC moves further inland.