Intense extratropical cyclones are one of the major natural threats in mid-latitudes and are often responsible for large socioeconomic impacts (Munich Re, 2015). Their impacts include strong winds, heavy precipitation, and in some cases storm surges (e.g. Lamb, 1991) In particular, cases associated with explosive cyclogenesis (Sanders and Gyakum, 1980; pressure decrease larger than 24 hPa in 24 h at 60∘ N, or equivalent) are associated with particularly large impacts and often with low predictability (e.g. Wernli et al., 2002; Fink et al., 2009). Such systems are often referred to in the literature as “bombs”. According to Shapiro et al. (1998), explosive cyclogenesis result from different mechanisms that include upper-level cyclonic vorticity advection, low-level warm air advection, and latent heat release. This may be supported by upper-tropospheric Rossby wave breaking, which constrains and intensifies the upper-level jet stream and thus contributes to intense cyclone developments (e.g. Hanley and Caballero, 2012; Gómara et al., 2014). According to Aubert (1957), the latent heating influence is significant not only in the pressure distribution but also in the vertical motion field. In particular, Aubert (1957) states that this mechanism lowers the heights of isobaric surfaces in the lower troposphere, and raises them in the upper. In agreement, Tsou et al. (1987) found that even for cases with strong vorticity advection and the differential thermal advection, latent heat release is still primary cause of the pressure falling below 900 hPa during the period of most abundant precipitation. The release of latent heat contributes to increase the storm's available potential energy, deepening the cyclone while decreasing the horizontal scale of the region of ascent particularly in maritime cyclones (e.g. Emanuel, 1987; Snyder and Lindzen, 1991; Davis, 1992).

Several studies have confirmed the occurrence of a maximum of latent and sensible heat availability in the lower troposphere near the warm sector of the cyclone and documented the contribution of moist diabatic processes such as latent heat release by cloud condensation processes to the intensification of extratropical cyclones (Pinto et al., 2009; Liberato et al., 2012; Ludwig et al., 2014). However, the relative contribution of diabatic processes to the cyclone deepening may differ considerably from case to case. For example, Fink et al. (2012) showed for selected explosive cyclogenesis cases that while diabatic processes played a key role for storms such as Xynthia and Klaus, other storms are largely baroclinic driven (e.g. Kyrill and Martin). These results have recently been extended by Pirret et al. (2017), who provided evidence that baroclinic processes generally dominate the majority of storms. On the other hand, the contributions from diabatic processes varies strongly from cases to case, and are only dominant for 10 out of 58 cases. The role of the diabatic contribution is strongly related to the period of time that a storm remains equatorward side of the jet, where warm, moist air is present.

The higher moisture availability in the North Pacific and North Atlantic basins are controlled by so-called atmospheric rivers (ARs; e.g. Newell and Zhu, 1994; Zhu and Newell, 1998; Bao et al., 2006; Ralph and Dettinger, 2011; Gimeno et al., 2016). ARs are relatively narrow (on average 500 km) corridors of enhanced water vapour (WV) transport in the lower troposphere that can extend for thousands of kilometres. According to the American Meteorological Society glossary ARs are “a long, narrow, and transient corridor of strong horizontal water vapour transport that is typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone”. The definition also states that the water vapour in ARs is supplied by sourced of tropical and/or extratropical origin (e.g. Ramos et al.2016a; Eiras et al., 2017) and that ARs can lead to heavy precipitation whenever these systems are forced upward – either by mountains or by ascent in the warm conveyor belt. Horizontal water vapour transport in the midlatitudes occurs primarily in atmospheric rivers and is focused in the lower troposphere. The importance of ARs in extreme precipitation events and floods has been analysed in detail for the west coast of the USA (particularly for California) over the last decade (e.g. Ralph et al., 2004; Neiman et al., 2008; Dettinger et al., 2011). Similar conclusions have been reached for Europe (e.g. Malguzzi et al., 2006; Lavers et al., 2012; Liberato et al., 2012; Ramos et al., 2015; Eiras-Barca et al., 2016; Brands et al., 2017) and other regions of the world (e.g. Viale and Nuñez, 2011; Mahoney et al., 2016; Blamey et al., 2018).

Given the role of latent heat release in the development of explosive cyclogenesis, this suggests that explosive cyclogenesis in the mid-latitudes may be influenced by the presence of an AR. Additionally, the release of sensible heat in the vicinity of the cyclone will enhance the convective instability of the AR. Previous studies showed for selected case studies that explosive development can indeed be driven by the presence of an AR (Zhu and Newell, 1994; Ferreira et al., 2016). For example, Ferreira et al. (2016) have provided evidence on the role of ARs over the western and central (sub)tropical Atlantic towards cyclone development, which converged into the cyclogenesis region and then moved along with the storm towards Europe. However, to the best of our knowledge, no ample assessments analysing the role of ARs in the explosive deepening have been performed from the climatological perspective. The main objective of this study is to provide a comprehensive evaluation on the role of the ARs in the explosive deepening of North Atlantic and North Pacific extratropical cyclones between 1979 and 2011 for the extended winter months (ONDJFM), focusing on the spatial concurrence and the timing of both features.

The manuscript is organized as follows: Sect. 2 describes the data and methods, while the results are presented in Sect. 3; finally, the conclusions are given and discussed in Sect. 4.

We use ECMWF ERA-Interim Reanalysis (Dee et al., 2011) between 1979 and 2011 for our study. For the cyclone detecting and tracking methodology (see Sect. 2.1), 6-hourly instantaneous 1000 hPa geopotential height fields at a resolution of 0.75∘×0.75∘ are considered. For the detecting and tracking of the ARs' structures (see Sect. 2.2), we used moisture and wind values at multiple vertical pressure levels to compute the integrated water vapour column (IWV) and the vertically integrated horizontal water vapour transport (IVT) at the same resolution.

Both AR detection methodologies were applied to the entire cyclone database. The obtained information was used to estimate the relevance of the ARs in the occurrence of explosive cyclogenesis and compare it with the corresponding NEC results. First, we analyse the samples of cyclones in each sub-set in terms of the evolution of core pressure over time. Supplement Fig. S1 depicts the distributions of core pressure values from -36 h until +36 h from MDP, for both the North Atlantic and North Pacific basins and for EC and NEC. Please note that the number of cyclones included in the statistics changes over time (Supplement Table S1), as not all systems have the same lifetime. EC systems typically deepen around 30 hPa during their lifetime and attain a minimum core pressure around MDP+6 or +12 h. Afterwards, occlusion advances and the systems fill in and consequently core pressure slowly increases with time. On the other hand, the pressure changes for NEC is typically much smaller (around 10 hPa) and the minimum peak intensity is difficult to identify since the core pressure more or less is stable after MDP. The increase in core pressure over time after MDP+6 h is thus not identifiable for NEC systems.

Regarding the concurrence of these events with ARs, Fig. 3 shows the ratios of observed coincidence between the EC and the presence of an AR within a 1500 km radius of the cyclone for the North Atlantic (Fig. 3a) and for the North Pacific (Fig. 3b), including the variance as the error bars for each MDP. The first prominent result is high ratios of coincidence between ARs for EC, peaking between 70 and 80 % for time lags MDP-6 h to MDP+6. When focusing on the North Atlantic region (Fig. 3a), the maximum (∼78 %) is found for the MDP+6 h using the GUAN2016, while using the EIRAS2016 the maximum (∼74 %) is at the MDP timing. Likewise, for the Pacific Basin (Fig. 3b) the ratios of coincidence with the ARs reach a maximum of 78 % when using the GUAN2016 on the MDP-6, while when using the EIRAS2016 its maximum (∼72 %) is found at the MDP. The results for the EC are in line with those found by Zhu and Newell (1994) and Ferreira et al. (2016) for a few selected case studies, where ARs were identified near the cyclones during an explosive cyclogenesis. In addition, we show here that the temporal coincidence between the ARs and explosive deepening of the cyclone takes place primarily between -6 h and +6 h around the MDP. Our results support the findings by Pirret et al. (2017), as the presence of the ARs will enhance the warm advection ahead of the cyclone core during the development phase.

For NEC, the concurrence of ARs during the development phase is considerably smaller. For the North Atlantic Region (Fig. 3a), the values for NEC range from about 55 % with the GUAN2015 and close to 45 % with the EIRAS2016 method. For the North Pacific Basin (Fig. 3b), results for the NEC are similar (but lower ratios) to those found for the North Atlantic Basin, ranging from approximately 46 % in the GUAN2015 to nearly 42 % when using EIRAS2016. In addition, for the NEC, there is apparently an increase in the ratio of coincidence between the position of the NEC and the presence of the ARs from -36 h to +36 h. This can be associated with the convergence of moisture along the frontal system of the cyclones (Dacre et al., 2015) along with tropical moisture export episodes (Knippertz and Wernli (2010) which can potentiate the formation of an AR in the latter stages of the NEC.

The conclusions attained with both methodologies are very similar. While a clear peak is identified close to the MDP for EC, for NEC a stable relationship with the ARs is identified in both methodologies with almost no changes in the ratio of coincidence when analysing the different 6 h time frames. However, the ratio of coincidence is always higher when using the GUAN2015.

Regarding the spatial distribution of the EC-AR coincidences, no conclusions can be achieved based on our results. Supplement Fig. S2 shows the position of the EC during the MDP if the AR coincidence was detected (red dots) and if it was not (black crosses). At first sight, the coincidences and non-coincidences of explosive cyclogenesis with ARs seem to be roughly equally distributed throughout the Atlantic and Pacific domains. However, on the downstream end of the storm tracks, cases with concurrent AR seem to dominate (e.g. over the British Isles). Still, no general conclusions can be made on a possible relation on the location of explosive cyclogenesis and concurrence (or non-concurrence) with anomalous moisture flux near the cyclones.

In order to analyse the flux of moisture near the cyclones within ±36 h of the MDP, spatial composites of the IVT within radius of 1500 km from the cyclone core were computed for each time step, for the EC and for the NEC and for both domains. Figure 4 shows the composites of the IVT in the surroundings of EC between -36 h and +36 h from the MDP at 6 h time steps for the North Atlantic, while Fig. 5 shows the same fields but for NEC. One key difference between both figures is noticeable differences in the IVT fields, which implies the presence of intense IVT values akin to AR-like structures in explosive cyclogenesis when compared to the NEC events.

Since ARs are commonly spatially associated with the warm sector of the cyclone, the evolution of the IVT fields throughout the 36 h time frame adjacent to the MDP frame depicts the general frontal evolution over the cyclogenesis event. Note that the AR is already quite prominent at MDP-36 h for EC events, and its position slowly rotates around the cyclone core MDP with slightly increasing intensities. Also noteworthy is the fact that, after the MDP point, not only does the AR strongly weaken but its central axis also tends to be detached from the cyclone with increasing time, corresponding to the detachment from the warm sector from the cyclone core at later stages of cyclone development (occlusion is initiated). This conclusion is in line with that obtained by Zhu and Newell (1994) for a much smaller number of cases. While the EC samples are dominated by systems associated with an AR (Fig. 3), this is not always the case. In order to evaluate this in more detail, we analysed additional composites by separating the EC cyclones in two categories, namely EC with AR (EC-AR) and without AR (EC-nonAR; Supplement Fig. S3). As expected, results show that there is a considerable difference in the composites between EC-AR and EC-nonAR, with high values of IVT identified for EC-AR and considerable lower values for EC-nonAR. The figures for EC-nonAR are more similar to NEC systems (not shown).

Supplement Figs. S4 and S5 show the composites for EC and NEC events but for the North Pacific domain. No meaningful differences can be observed between the North Atlantic and the North Pacific basins, and thus the conclusions are the same for the North Atlantic. The results suggest the importance of latent heat released when the cyclones encounter the ARs, leading to intense condensation process, thus providing an important source of energy when the cyclone is in its deepening phase (e.g. Danard, 1964; Bullock and Jonhson, 1971; Whitaker and Davis, 1994).

The results presented in this section have revealed two main new insights. First, the highest ratio of the present of the ARs in the vicinity of EC is found within ±6 h of the MDP. Second, it is apparent for EC events that the AR is located very close to the cyclone centre prior to MDP, while the AR becomes detached from the EC core once the cyclone stops deepening. As a result, we confirm the hypothesis that the presence of an AR raises the odds of an explosive cyclogenesis occurrence and is thus not only merely a statistical relationship as suggested by Ferreira et al. (2016) for three modelled EC case studies.

We investigated the importance of ARs in the development of explosive cyclogenesis on both North Atlantic and North Pacific basins using two different algorithms for AR identification. With this aim, the concurrence of the presence of AR in the vicinity of developing cyclones was quantified over different time lags. The main results are summarized in the following:

ARs are present very frequently within the vicinity of cyclones undergoing EC, reaching maximum values close to 80 % near the MDP (±6 h) for both domains. The concurrence of ARs with NEC is 42–46 % for NEC.

Regarding future climate projections, Ramos et al. (2016b) showed that most models from CMIP5 project a coherent increase in IVT values over the North Atlantic Basin and an increase in the number of ARs that hit western Europe by the end of the 21st century, although this is more evident with emissions scenario RCP8.5 than with scenario RCP4.5. Taken together with the new insights of the current paper, such climate change scenarios imply that the probability of intense extratropical explosive cyclones will increase in future decades.