ArticlePublisher preview available

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

January 2018
Climate Dynamics 50(4):1-11

DOI:10.1007/s00382-017-3636-7

Authors:

Anne Sophie Daloz

CICERO

Suzana J. Camargo

Columbia University

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.

Seasonal mean differences between 1997 and 2013 and 1980–1996 for: a entropy deficit, b PI, c shear, d vorticity, e skin temperature and f specific humidity at 500 hPa for MERRA reanalysis, calculated in August–September–October (ASO) for the northern hemisphere and January–February–March (JFM) in the southern hemisphere

…

Seasonal climatological number of tropical cyclone genesis for the period 1980–2013 derived from the GPI, for a MERRA and b ERA-Interim, from the TCGI for c ERA-Interim, d observations (IBTrACS), show for ASO in the northern hemisphere and JFM in the southern hemisphere. In units of number of tropical cyclones per 20 years by 5° gridbox

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Seasonal mean differences of GPI (a MERRA and b ERA-Interim) and TCGI (c ERA-Interim) between 1997 and 2013 and 1980–1996. Black lines indicate the 95% confidence level using a t test

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Global map of tropical centers and their regions of responsibility as defined by the World Meteorological Organization (WMO) with their corresponding regional numbers included in Table 1

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Time series of annual-mean latitude of tropical cyclone genesis calculated from the best-track archive IBTrACS from 1980 to 2013 for: a West Pacific basin (JTWC), b East Pacific basin (HURDAT), c South Pacific basin (Wellington) and d North Atlantic basin (HURDAT). Linear trend lines are presented with their 95% two-sided confidence intervals

…

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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

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.

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Article

May 2011

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.

The poleward migration of the location of tropical cyclone maximum intensity

Article

May 2014
NATURE

Temporally inconsistent and potentially unreliable global historical data hinder the detection of trends in tropical cyclone activity. This limits our confidence in evaluating proposed linkages between observed trends in tropical cyclones and in the environment. Here we mitigate this difficulty by focusing on a metric that is comparatively insensitive to past data uncertainty, and identify a pronounced poleward migration in the average latitude at which tropical cyclones have achieved their lifetime-maximum intensity over the past 30 years. The poleward trends are evident in the global historical data in both the Northern and the Southern hemispheres, with rates of 53 and 62 kilometres per decade, respectively, and are statistically significant. When considered together, the trends in each hemisphere depict a global-average migration of tropical cyclone activity away from the tropics at a rate of about one degree of latitude per decade, which lies within the range of estimates of the observed expansion of the tropics over the same period. The global migration remains evident and statistically significant under a formal data homogenization procedure, and is unlikely to be a data artefact. The migration away from the tropics is apparently linked to marked changes in the mean meridional structure of environmental vertical wind shear and potential intensity, and can plausibly be linked to tropical expansion, which is thought to have anthropogenic contributions.

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

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