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Is the poleward migration of tropical cyclone maximum intensity associated with a poleward migration of tropical cyclone genesis?

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A recent study showed that the global average latitude where tropical cyclones achieve their lifetime-maximum intensity has been migrating poleward at a rate of about one-half degree of latitude per decade over the last 30 years in each hemisphere. However, it does not answer a critical question: is the poleward migration of tropical cyclone lifetime-maximum intensity associated with a poleward migration of tropical cyclone genesis? In this study we will examine this question. First we analyze changes in the environmental variables associated with tropical cyclone genesis, namely entropy deficit, potential intensity, vertical wind shear, vorticity, skin temperature and specific humidity at 500 hPa in reanalysis datasets between 1980 and 2013. Then, a selection of these variables is combined into two tropical cyclone genesis indices that empirically relate tropical cyclone genesis to large-scale variables. We find a shift toward greater (smaller) average potential number of genesis at higher (lower) latitudes over most regions of the Pacific Ocean, which is consistent with a migration of tropical cyclone genesis towards higher latitudes. We then examine the global best track archive and find coherent and significant poleward shifts in mean genesis position over the Pacific Ocean basins.
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Vol.:(0123456789)
1 3
Clim Dyn (2018) 50:705–715
DOI 10.1007/s00382-017-3636-7
Is thepoleward migration oftropical cyclone maximum intensity
associated withapoleward migration oftropical cyclone genesis?
AnneSophieDaloz1 · SuzanaJ.Camargo2
Received: 29 November 2016 / Accepted: 16 March 2017 / Published online: 3 April 2017
© Springer-Verlag Berlin Heidelberg 2017
1 Introduction
Kossin et al. (2014) showed that in the past few dec-
ades (1982–2009) the large-scale environment of tropi-
cal cyclones has evolved over the tropics and subtropics.
Indeed, favorable conditions for the development of tropical
cyclones have migrated towards higher latitudes (vertical
wind shear and potential intensity), moving from the trop-
ics closer to the subtropics. With a globally homogenized
record of intensity (Kossin etal. 2013) and a global best-
track archive (Knapp et al. 2010), they also demonstrated
that the location where observed tropical cyclones reach
their maximum intensity has been migrating towards higher
latitudes. More recently, Kossin etal. (2016) used observa-
tions and simulations to examine the changes in lifetime-
maximum intensity and tropical cyclone exposure for the
present and future climates over the western North Pacific
Ocean. The projections of tropical cyclones were simu-
lated by, and downscaled from, an ensemble of numerical
Coupled Model Intercomparison Project Phase 5 (CMIP5)
models (Taylor et al. 2012). They showed a poleward
migration of lifetime-maximum intensity (LMI) latitude in
the present century and continuing into the future using one
of the representative concentration pathways (RCP8.5). A
possible mechanism responsible for these global and local
changes is the expansion of the tropics (Lucas etal. 2014),
however this link has not been proved yet.
The current study expands on the findings by Kossin
etal. (2014, 2016) by analyzing the possible origin of the
poleward migration of the LMI latitude. More precisely, we
would like to answer the following question: Is the pole-
ward migration of tropical cyclones’ LMI location related
to a poleward migration in tropical cyclone genesis loca-
tion? Tropical cyclones are very sensitive to the large-scale
environment both during their genesis and development
Abstract A recent study showed that the global average
latitude where tropical cyclones achieve their lifetime-
maximum intensity has been migrating poleward at a rate
of about one-half degree of latitude per decade over the last
30years in each hemisphere. However, it does not answer
a critical question: is the poleward migration of tropical
cyclone lifetime-maximum intensity associated with a pole-
ward migration of tropical cyclone genesis? In this study
we will examine this question. First we analyze changes
in the environmental variables associated with tropical
cyclone genesis, namely entropy deficit, potential intensity,
vertical wind shear, vorticity, skin temperature and specific
humidity at 500 hPa in reanalysis datasets between 1980
and 2013. Then, a selection of these variables is combined
into two tropical cyclone genesis indices that empirically
relate tropical cyclone genesis to large-scale variables. We
find a shift toward greater (smaller) average potential num-
ber of genesis at higher (lower) latitudes over most regions
of the Pacific Ocean, which is consistent with a migration
of tropical cyclone genesis towards higher latitudes. We
then examine the global best track archive and find coher-
ent and significant poleward shifts in mean genesis position
over the Pacific Ocean basins.
Keywords Tropical cyclone genesis· Poleward
migration· Tropical cyclone genesis index· Observations
* Anne Sophie Daloz
adaloz@wisc.edu
1 Space andScience Engineering Center, University
ofWisconsin-Madison, 1225 West Dayton Street, Madison,
WI53706, USA
2 Lamont-Doherty Earth Observatory, Columbia University,
Palisades, NY, USA
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... Prior research has documented observed trends in TCs worldwide including the NATL using various metrics [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] . Those metrics measure the TC formation, best-track and intensity, the latitude at which TCs reach their peak intensity, as well as the damage associated with TCs 3,14,21,22 . ...
... Figure 1a shows a linear trend of the JJASON-mean latitude and longitude of all TCs with the maximum wind speed exceeding 34 kts. The linear trend of latitude and longitude of all TCs is insignificant (Fig. 1a, b), which is in line with previous study 20 . However, when we only consider the TCs that later reach the hurricane intensity (TY), there is a significant southward shift in the genesis location of hurricane, exhibiting a linear trend of 0.114°latitude/year since 1979 ( Fig. 1c and Supplementary Fig. 1b). ...
... Previous studies have found a tendency of a southward movement of TC genesis and TC LMI over the NATL, but the trend is insignificant 11,20 . Present study reveals that the trend is dependent on the TC intensity. ...
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... Hemisphere (NH, SH) respectively ). Daloz and Camargo (2017) found LMI migration corresponds to poleward migration of Pacific TC genesis. ...
... 0.45 latitude decade -1 ) and an equatorward shift in seasonal-mean eastern North Pacific lysis by 0.33latitude decade -1 . These results generally agree with estimates derived in other analyses (e.g.,Kossin et al. 2014;Wang et al. 2016;Daloz and Camargo 2017). ...
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... Contrasting with no consensus in the long-term trends of TC translation speed in the WNP, there is a better agreement in the trends of TC latitudinal position 12 . The annual average location of WNP TCs (i.e. the basin-wide position) was found to migrate poleward over the satellite era [25][26][27] . A poleward trend was also simulated in general circulation models with anthropogenic forcing 26,28 . ...
... To exclude the uncertainty in weak storms in the Best Track 42 , this study only considers TCs that reach severe tropical storm intensity and above during the lifetime, i.e. the maximum sustained wind speed ≥24.5 m/s. TC genesis location is defined by the first track point where TC intensity reaches 17.5 m/s 27,29 . Note that both JMA and HKO data use 10-min mean for measuring maximum sustained wind speed, and CMA and JTWC use 2-and 1-min means, respectively. ...
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... higher latitude regions of East Asia (Chan, 2005;Kossin et al., 2016;Peduzzi et al., 2012). The significant contribution of the poleward shift of TC genesis location to the poleward migration of TC lifetime maximum intensity location has been highlighted in prior studies (Daloz & Camargo, 2017;Gulev & Studholme, 2018;R. Wang & Wu, 2019). ...
... Wang & Wu, 2019). Studies have pointed out that natural variability such as ENSO and the Pacific decadal oscillation (PDO) cannot fully explain the poleward shift in TC genesis location and have highlighted that other modes of natural variability may also be driving this poleward shift (Chan & Liu, 2013;Daloz & Camargo, 2017;Kossin et al., 2014Kossin et al., , 2016. ...
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Plain Language Summary The observed poleward shift in tropical cyclone (TC) genesis over the western North Pacific has increased the threat from these storms to East Asia. Until now, it remains unclear whether TC genesis will continue to move poleward in the future, due to both a relatively short reliable observational TC record as well as model inconsistencies. This study provides evidence that the poleward migration of TC genesis will likely continue due to anthropogenic warming. We find that there is a strong relationship between TC genesis latitude and a mode of Pacific sea surface temperature (SST) variability termed mega‐ENSO. We find using a suite of 30 climate models comprising the Coupled Modeled Intercomparison Project Phase 6 (CMIP6) that future poleward trends in TC genesis latitude are likely to continue. This poleward shift is observed whether using projected trends in mega‐ENSO or an index that measures the conduciveness of the environment for TC formation. Modeled tropical cyclones in high resolution simulations from CMIP6 also show a continued projected poleward shift in TC genesis. This poleward shift in TC genesis appears largely related to changes in extra‐tropical SST warming associated with mega‐ENSO.
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The authors examine the change in tropical cyclone (TC) tracks that results from projected changes in the large-scale steering flow and genesis location from increasing greenhouse gases. Tracks are first simulated using a Beta and Advection Model (BAM) and NCEP-NCAR reanalysis winds for all TCs that formed in the North Atlantic Ocean's Main Development Region (MDR) for the period 1950-2010. Changes in genesis location and large-scale steering flow are then estimated from an ensemble mean of 17 models from phase 3 of the Coupled Model Intercomparison Project (CMIP3) for the A1b emissions scenario. The BAM simulations are then repeated with these changes to estimate how the TC tracks would respond to increased greenhouse gases. As the climate warms, the models project a weakening of the subtropical easterlies as well as an eastward shift in genesis location. This results in a statistically significant decrease in straight-moving (westward) storm tracks of similar to 5.5% and an increase in recurving (open ocean) tracks of similar to 5.5%. These track changes decrease TC counts over the southern Gulf of Mexico and Caribbean by 1-1.5 decade(-1) and increase counts over the central Atlantic by 1-1.5 decade(-1). Changes in the large-scale steering flow account for a vast majority of the projected changes in TC trajectories.
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A Poisson regression between the observed climatology of tropical cyclogenesis (TCG) and large-scale climate variables is used to construct a TCG index. The regression methodology is objective and provides a framework for the selection of the climate variables in the index. Broadly following earlier work, four climate variables appear in the index: low-level absolute vorticity, relative humidity, relative sea surface temperature (SST), and vertical shear. Several variants in the choice of predictors are explored, including relative SST versus potential intensity and satellite-based column-integrated relative humidity versus reanalysis relative humidity at a single level; these choices lead to modest differences in the performance of the index. The feature of the new index that leads to the greatest improvement is a functional dependence on low-level absolute vorticity that causes the index response to absolute vorticity to saturate when absolute vorticity exceeds a threshold. This feature reduces some biases of the index and improves the fidelity of its spatial distribution. Physically, this result suggests that once low-level environmental vorticity reaches a sufficiently large value, other factors become rate limiting so that further increases in vorticity (at least on a monthly mean basis) do not increase the probability of genesis. Although the index is fit to climatological data, it reproduces some aspects of interannual variability when applied to interannually varying data. Overall, the new index compares positively to the genesis potential index (GPI), whose derivation, computation, and analysis is more complex in part because of its dependence on potential intensity.