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While the fundamental understanding of tropical cyclone (TC) movement is fairly mature, notable advancements are still being made. This paper summarizes new concepts and updates to the existing fundamental theories on TC movement obtained from simplified barotropic models, full-physics models, and data analysis, particularly since 2014. The scope includes recent works on the interaction between a TC and its environment, and the predictability related to TC movement. Although conventional concepts of steering flow, β-gyre, and diabatic heating remain important, a more complete understanding of TC movement governing mechanisms can provide an important basis for further track forecast improvements.

Forecasting tropical cyclone (TC) activities has been a topic of great interest and research. Many studies and existing seasonal forecasting models have examined and predicted the number of TCs (including geneses and landfalls) mainly based on the environmental factors in the peak TC season. However, these predictions can be time-consuming, computationally expensive and uncertain, depending on the efficiency and predictability of the dynamical models. Therefore, here we propose an effective statistical seasonal forecasting model, namely the Sun Yat-sen University (SYSU) Model, for predicting the number of TCs (intensity at tropical storm or above) over the western North Pacific based on the environmental factors in the preseason. The nine categories comprising 103 candidate predictors in 1980–2015 (36 years) are systematically investigated. The best subset selection regression shows that the sea surface temperatures at the tropical North Atlantic and eastern North Pacific in April, the 500 hPa geopotential height difference between April and January at the open ocean southwest of Australia and the 700 hPa geopotential height at the North Pacific in April are the most significant predictors. The correlation coefficient between the modeled results and observations reaches 0.89. The model is successfully validated by leave-one-out, nine-fold cross-validations, and later 5 year (2016–2020) observations. The prediction of the SYSU Model exhibits a 95% hit rate in 1980–2020 (39 out of 41), suggesting an operational potential in the seasonal forecasting of TCs over the western North Pacific.

The typical track of a tropical cyclone (TC) in the Northern Hemisphere is an initial northwestward movement followed by an eventual turning toward the east. Such turning is referred to as recurvature and often explained by the change of the environmental flow that steers the TC. Here we show that even in the absence of background flow, a TC initiated at a high enough latitude can recurve itself. Differential horizontal advection of the planetary vorticity by the TC circulation at different vertical levels leads to the development of vertical wind shear, upper tropospheric anticyclone, and asymmetric distribution of convection. The flow associated with the upper tropospheric anticyclone on the equatorward side of the TC and the diabatic heating associated with the asymmetric convection combine to cause the TC to recurve. Such knowledge, an intrinsic recurvature property of the TC is important in forecasting the TC track when the environmental flow is weak.

This paper presents to date the most complete global climatology of the size of tropical cyclones (TCs) between 1999 and 2009 using the QuikSCAT satellite data. Here, TC size is defined as the azimuthal mean radius of 17 m s−1 surface winds from the TC centre. While the TC size climatology for the Western North Pacific (WNP) and North Atlantic (NA) has been documented in previous studies, those for the Eastern North Pacific (ENP), South Indian Ocean (SI) and South Pacific (SP) have yet to be examined in detail, which is the objective of this study. Among all the basins, TCs over the WNP are the largest and have the largest variance, while those over the ENP are the smallest. In addition, TCs in the Northern Hemisphere (WNP, NA and ENP) have two seasonal size peaks, but those in the Southern Hemisphere (SI and SP) have only one. An important finding is that for all basins, the size of a TC does not necessarily increase with latitude monotonically, but reaches the maximum at some latitudinal region. Such a result agrees well with a recent theoretical study in terms of a balance between the inertial stability associated with the TC circulation and the import of angular momentum into the TC.

This paper is the second part of a comprehensive study on tropical cyclone (TC) size. In Part I, the climatology of TC size and strength over the western North Pacific (WNP) and the North Atlantic was established based on the Quick Scatterometer (QuikSCAT) data. In this second part, the mechanisms that are likely responsible for TC size changes are explored through analyses of angular momentum (AM) transports and synoptic flow patterns associated with the TC. Changes in AM transport in the upper and lower troposphere appear to be important factors that affect TC intensity and size, respectively. The change in TC intensity is positively related to the change in the upper-tropospheric AM export, while the change in TC size is positively proportional to the change in the lower-tropospheric AM import. An examination of the synoptic flow patterns associated with WNP TCs suggests that changes in the synoptic flow near the TC are important in determining the change in TC size, with developments of the lower-tropospheric anticyclonic flows (one to the east and one to the west) bordering the TC being favorable for TC growth and a weakening of the subtropical high to the southeast for TC size reduction. A recurving TC tends to grow if the lower-tropospheric westerlies to its west increase. Moreover, a northward TC movement is related to the change in TC size. For example, a higher northward-moving speed is found for a larger TC, which also agrees well with the AM transport concept.

A comprehensive statistical climatology of the size and strength of the tropical cyclones (TCs) occurring over the western North Pacific (WNP; including the South China Sea) and the North Atlantic (NA; including the Gulf of Mexico and the Caribbean Sea) between 1999 and 2009 is constructed based on Quick Scatterometer (QuikSCAT) data. The size and strength of a TC are defined, respectively, as the azimuthally averaged radius of 17 m s−1 of ocean-surface winds (R17) and the azimuthally averaged tangential wind within 1°–2.5°-latitude radius from the TC center (outer-core wind strength, OCS). The mean TC size and strength are found to be 2.13° latitude and 19.6 m s−1, respectively, in the WNP, and 1.83° latitude and 18.7 m s−1 in the NA. While the correlation between size and strength is strong (r ≈ 0.9), that between intensity and either size or strength is weak. Seasonally, midsummer (July) and late-season (October) TCs are significantly larger in the WNP, while the mean size is largest in September in the NA. The percentage frequency of TCs having large size or high strength is also found to vary spatially and seasonally. In addition, the interannual variation of TC size and strength in the WNP correlate significantly with the TC lifetimes and the effect of El Niño over the WNP. TC lifetime and seasonal subtropical ridge activities are shown to be potential factors that affect TC size and strength.