Detecting and interpreting long-term changes in typhoon translation speed in observations remains challenging, contrasting with increased confidence in the poleward migration of typhoons. Here, I ...show a significant relationship between the basin-wide translation speed and the latitudinal position of tropical cyclones in the western North Pacific over 1980–2023. First, because tropical cyclones move faster at higher latitudes, the significant poleward migration (80 km/decade) increases the yearly basin-wide translation speed by 5% over the period. This effect reduces the detectability of a slowing trend. Second, the basin-wide translation speed solely contributed by regional translation speed has slowed by 18%, mostly in the late stage of the cyclone lifecycle. The translation speed slowdown and the poleward migration are likely caused by the same climate drivers through the interconnected large-scale atmospheric circulation between the tropics and subtropics. My findings suggest exacerbated tropical cyclone-related risk in the subtropical regions in a changing climate.
Abstract
The average location of observed western North Pacific (WNP) tropical cyclones (TCs) has shifted north over the last several decades, but the cause remains not fully understood. Here we show ...that, for the annual average, the observed northward migration of WNP TCs is related to changes in TC seasonality, not to a northward migration in all seasons. Normally, peak-season (July–September) TCs form and travel further north than late-season (October–December) TCs. In recent decades, related to less frequent late-season TCs, seasonally higher-latitude TCs contribute relatively more to the annual-average location and seasonally lower-latitude TCs contribute less. We show that the change in TC seasonality is related to the different responses of late-season and peak-season TC occurrence to a stronger Pacific Walker Circulation. Our findings provide a perspective on long-term trends in TC activity, by decomposing the annual-average statistics into seasonal components, which could respond differently to anthropogenic forcing.
Understanding and prediction of tropical cyclone (TC) activity on the medium range remains challenging. Here, we find that the pre-existing westward-moving equatorial waves can inform the risk of TC ...occurrence and intensification, based on a dataset obtained by synchronising objectively identified TCs and equatorial waves in a climate reanalysis. Globally, westward-moving equatorial waves can be precursors to 60-70% of pre-tropical cyclogenesis events, and to >80% of the events with the strongest vorticity, related to the favourable environmental conditions within the pouch of equatorial waves. We further find that when storms are in-phase with westward-moving equatorial waves, the intensification rate of TCs is augmented, whilst in other phases of the waves, storm intensity grows more slowly, or even decays. Coherent wave packets associated with TCs are identifiable up to two weeks ahead. Our findings show that westward-moving equatorial waves can be useful medium-range precursors to TC activity.
The long-term changes of ocean surface waves associated with tropical cyclones (TCs) are poorly observed and understood. Here, we present the global trend analysis of TC waves for 1979-2022 based on ...the ERA5 wave reanalysis. The maximum height and the area of the TC wave footprint in the six h reanalysis have increased globally by about 3%/decade and 6%/decade, respectively. The TC wave energy transferred at the interface from the atmosphere to the ocean has increased globally by about 9%/decade, which is three times larger than that reported for all waves. The global energy changes are mostly driven by the growing area of the wave footprint. Our study shows that the TC-associated wave hazard has increased significantly and these changes are larger than those of the TC maximum wind speed. This suggests that the wave hazard should be a concern in the future.
ECMWF has recently produced its first ocean‐atmosphere coupled twentieth century reanalysis CERA‐20C, with ten ensemble members. CERA‐20C is based on a coupled climate model with data assimilation ...implemented individually for the atmosphere and ocean components. This article focuses on the relationships between 2 m air temperature (T2m) and sea surface temperature (SST) in this new coupled climate reanalysis from the perspective of ensemble statistics. The results show where the atmosphere or the ocean are driving the ensemble variability of temperature at the air–sea interface on varying time‐scales.
In CERA‐20C, the T2m–SST ensemble relationships are changing on diurnal, seasonal and longer time‐scales and also within regions. The T2m ensemble spread is much larger than the SST ensemble spread at high frequencies (3 h), normally with low correlations due to different time‐scales of oceanic and atmospheric dynamics, showing the atmospherically driven variability of T2m. On monthly time‐scales, the T2m spread is mostly slightly lower than the SST spread, with high correlations and strong coupling, reflecting the SST‐forced variability. T2m–SST coupling is strongest where atmospheric and oceanic boundary layers are shallow, e.g. in the summer hemisphere, and weakest where the atmospheric boundary layer becomes deep, e.g. in the ITCZ region. As the twentieth century progresses, ensemble spreads decline, while T2m–SST coupling becomes stronger as both atmosphere and oceans are better constrained to observations. In the Tropics, strong ENSO‐related variability is found in T2m–SST coupling as atmospheric convection centres move interannually to break the SST‐dominated relationships.
Air–sea coupling in the new coupled reanalysis CERA‐20C is examined by focusing on the ensemble variability of temperature at the interface. Surface air temperature is less constrained by SST at high‐frequency time‐scales, but is more forced by SST at long time‐scales. Air–sea coupling is strongest where atmospheric and oceanic boundary layers are shallow, and weakest where the atmospheric boundary layer becomes deep. As more observations become available through the twentieth century, air–sea coupling increases, except in the Tropics.
Sea level has risen significantly in the recent decades and is expected to rise further based on recent climate projections. Ocean reanalyses that synthetize information from observing networks, ...dynamical ocean general circulation models, and atmospheric forcing data offer an attractive way to evaluate sea level trend and variability and partition the causes of such sea level changes at both global and regional scales. Here, we review recent utilization of reanalyses for steric sea level trend investigations. State-of-the-science ocean reanalysis products are then used to further infer steric sea level changes. In particular, we used an ensemble of centennial reanalyses at moderate spatial resolution (between 0.5 × 0.5 and 1 × 1 degree) and an ensemble of eddy-permitting reanalyses to quantify the trends and their uncertainty over the last century and the last two decades, respectively. All the datasets showed good performance in reproducing sea level changes. Centennial reanalyses reveal a 1900–2010 trend of steric sea level equal to 0.47 ± 0.04 mm year−1, in agreement with previous studies, with unprecedented rise since the mid-1990s. During the altimetry era, the latest vintage of reanalyses is shown to outperform the previous ones in terms of skill scores against the independent satellite data. They consistently reproduce global and regional upper ocean steric expansion and the association with climate variability, such as ENSO. However, the mass contribution to the global mean sea level rise is varying with products and its representability needs to be improved, as well as the contribution of deep and abyssal waters to the steric sea level rise. Similarly, high-resolution regional reanalyses for the European seas provide valuable information on sea level trends, their patterns, and their causes.
Abstract
Storm surges are among the deadliest natural hazards, but understanding and prediction of year-to-year variability of storm surges is challenging. Here, we demonstrate that the interannual ...variability of observed storm surge levels can be explained and further predicted, through a process-based study in Hong Kong. We find that El Niño-Southern Oscillation (ENSO) exerts a compound impact on storm surge levels through modulating tropical cyclones (TCs) and other forcing factors. The occurrence frequencies of local and remote TCs are responsible for the remaining variability in storm surge levels after removing the ENSO effect. Finally, we show that a statistical prediction model formed by ENSO and TC indices has good skill for prediction of extreme storm surge levels. The analysis approach can be applied to other coastal regions where tropical storms and the climate variability are main contributors to storm surges. Our study gives new insight into identifying ‘windows of opportunity’ for successful prediction of storm surges on long-range timescales.
The European Centre for Medium‐range Weather Forecasts (ECMWF) has produced the ocean‐atmosphere coupled reanalysis for the twentieth century, CERA‐20C, following on from the similar but ...atmosphere‐only reanalysis ERA‐20C. Here we demonstrate the capability of CERA‐20C in producing more physically consistent ocean and atmosphere boundary conditions, by focusing on sea surface temperature (SST)‐precipitation intraseasonal relationships. CERA‐20C reproduces well the observed SST‐precipitation correlations, while these relationships are poorly represented in ERA‐20C, with the greatest discrepancies in the early 1900s. The improved relationships in CERA‐20C are due to intraseasonal improvements in SST that are not present in the external HadISST2 product. In CERA‐20C, SST‐precipitation relationships are slightly weaker in the 1900s than in the 2000s, mainly due to differences in the assimilated observation density. We also find that the coupled model initialized from CERA‐20C in the 2000s realistically simulates these relationships, while relaxing SST toward HadISST2 tends to damp these relationships. CERA‐20C has improved mean and variance in precipitation over ERA‐20C, but these are mostly due to improvements in the atmospheric model and not due to coupled feedbacks.
Plain Language Summary
Climate reanalyses, reconstructions of past weather and climate conditions, are of importance in the climate research and forecasting community. The European Centre for Medium‐range Weather Forecasts has recently launched its first ocean‐atmosphere coupled reanalysis for the twentieth century, known as CERA‐20C. Many users are expected. One of the main breakthroughs of this state‐of‐the‐art data set is resolving the air‐sea interactions via coupled modeling, while the ocean and atmosphere models are constrained separately by observations. This paper shows that the sea surface temperature‐precipitation relationships are much better reproduced in CERA‐20C than in its predecessors. Such significant improvement in CERA‐20C is due to the ocean surface that more realistically reflects the intraseasonal variations in the atmosphere through the coupled processes. The results highlight the advances of coupled reanalysis in producing physically consistent ocean and atmosphere conditions, indicating that we are moving in the right direction on climate reanalysis. This could have an immediate impact on the plans for producing global climate reanalyses at some institutions. In addition, the metrics of lead‐lag correlations applied here could be easily implemented to other climate data sets, to evaluate the quality of climate data.
Key Points
SST‐precipitation intraseasonal relationships are well captured in CERA‐20C and misrepresented in its predecessor ERA‐20C
Improved relationships in CERA‐20C are due to improved intraseasonal SST through the coupled analysis
The coupled model of CERA‐20C in the 2000s can well simulate SST‐precipitation intraseasonal relationships
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