Tropical Cyclone Prediction on Subseasonal Time-Scales

Here we discuss recent progress in understanding tropical cyclone (TC) subseasonal variability and its prediction. There has been a concerted effort to understand the sources of predictability at subseasonal time-scales, and this effort has continued to make progress in recent years. Besides the Madden-Julian Oscillation (MJO), other modes of variability affect TCs at these time-scales, in particular various equatorial waves. Additionally, TC activity is also modulated by extratropical processes via Rossby wave breaking. There has also been progress in the ability of models to simulate the MJO and its modulation of TC activity. Community efforts have created multi-model ensemble datasets, which have made it possible to evaluate the forecast skill of the MJO and TCs on subseasonal time-scales in multiple forecasting systems. While there is positive skill in some cases, there is strong dependence on the ensemble system considered, the basin examined, and whether the storms have extratropical influences or not. Furthermore, the definition of skill differs among studies. Forecasting centers are currently issuing subseasonal TC forecasts using various techniques (statistical, statistical-dynamical and dynamical). There is also a strong interest in the private sector for forecasts with 3-4 weeks lead time

Global warming hiatus contributed to the increased occurrence of intense tropical cyclones in the coastal regions along East Asia

Abstract The recent global warming hiatus (GWH) was characterized by a La Niña–like cooling in the tropical Eastern Pacific accompanied with the Indian Ocean and the tropical Atlantic Ocean warming. Here we show that the recent GWH contributed significantly to the increased occurrence of intense tropical cyclones in the coastal regions along East Asia since 1998. The GWH associated sea surface temperature anomalies triggered a pair of anomalous cyclonic and anticyclonic circulations and equatorial easterly anomalies over the Northwest Pacific, which favored TC genesis and intensification over the western Northwest Pacific but suppressed TC genesis and intensification over the southeastern Northwest Pacific due to increased vertical wind shear and anticyclonic circulation anomalies. Results from atmospheric general circulation model experiments demonstrate that the Pacific La Niña–like cooling dominated the Indian Ocean and the tropical Atlantic Ocean warming in contributing to the observed GWH-related anomalous atmospheric circulation over the Northwest Pacific

Untangling impacts of global warming and Interdecadal Pacific Oscillation on long-term variability of North Pacific tropical cyclone track density.

How much the observed long-term variability of tropical cyclone (TC) activity is due to anthropogenic global warming (GW) or internal climate variability remains unclear, limiting the confidence in projected future change in TC activity. Here, the relative contributions of GW and the Interdecadal Pacific Oscillation (IPO) to the long-term variability of TC track density (TCTD) over the North Pacific (NP) are quantified on the basis of statistical analyses and climate model simulations. Results show that historical GW mainly reduced (increased) TCTD over the western (eastern) NP, while the positive (negative) IPO corresponds to a NP basin-wide increase (decrease) in TCTD except in some coastal regions. The IPO has a much greater impact on TCTD over the western NP than GW, while the IPO and GW impacts are about equal over the eastern NP during 1960-2019. These findings have important implications for projecting future TC activity over the NP

How Does Tropical Cyclone Genesis Frequency Respond to a Changing Climate?

Abstract Global tropical cyclone (TC) genesis frequency (TCGF) has been documented to decrease or increase linearly in a changing climate. However, our numerical experiments show that the global TCGF exhibits a parabolic relation with spatio‐uniform climate changes in sea surface temperature (SST) from −15 K to 5 K relative to the present climate, with the peak in the 5 K‐cooler climate. The parabolic relation is found in all TC basins except the eastern North Pacific where TCGF keeps increasing with the changing climate. TCGF can be expressed as the product of the frequency of TC seeds and the TC survival rate (SR). Further analysis shows that this parabolic structure in the global TCGF depends on TC seeds rather than the TC SR. The TC SR exhibits an increasing trend with the SST increase, while TC seeds show a consistent change with TCGF, which might be linked to the changes in low‐level relative humidity

Atmospheric modes fiddling the simulated ENSO impact on tropical cyclone genesis over the Northwest Pacific

Abstract The El Niño-Southern Oscillation (ENSO) is crucial to the interannual variability of tropical cyclone (TC) genesis over the western North Pacific (WNP). However, most state-of-the-art climate models exhibit a consistent pattern of uncertainty in the simulated TC genesis frequency (TCGF) over the WNP in ENSO phases. Here, we analyze large ensemble simulations of TC-resolved climate models to identify the source of this uncertainty. Results show that large uncertainty appears in the South China Sea and east of the Philippines, primarily arising from two distinct atmospheric modes: the Matsuno-Gill-mode (MG-mode) and the Pacific-Japan-like pattern (PJ-mode). These two modes are closely associated with anomalous diabatic heating linked to tropical precipitation bias in model simulations. By conditionally constraining either of the modes, we can significantly reduce model uncertainty in simulating the dipole structure of the TCGF anomalies, confirming that it is the atmospheric circulation bias in response to tropical precipitation bias that causes uncertainty in the simulated WNP TCGF

Hemispheric asymmetric response of tropical cyclones to CO<sub>2</sub> emission reduction

Tropical cyclones (TCs) are among the most devastating natural hazards for coastal regions, and their response to human activities has broad socio-economic relevance. So far, how TC responds to climate change mitigation remains unknown, complicating the design of adaptation policies. Using net-zero and negative carbon emission experiments, we reveal a robust hemisphere-asymmetric hysteretic TC response to CO2 reduction. During the decarbonization phase, the Northern Hemisphere TC frequency continues to decrease for several more decades, while the Southern Hemisphere oceans abruptly shifts to a stormier state, with the timescales depending on mitigation details. Such systematic changes are largely attributed to the planetary-scale reorganization of vertical wind shear and midlevel upward motion associated with the hysteretic southward migration of the Intertropical Convergence Zone, underpinned by the Atlantic Meridional Overturning Circulation and El Nino-like mean state changes. The hemispheric contrast in TC response suggests promising benefits for most of the world's population from human action to mitigate greenhouse gas warming, but it may also exacerbate regional socioeconomic disparities, for example by putting more pressure on small open-ocean island states in the Southern Hemisphere to adapt to TC risks.Y