The Large-scale empirical forcing function

(1)

Universidade de

Évora

2009

Cristina

Maria Mendes

Andrade

The Large-scale

Empirical Forcing Function

orientador:

Professor Doutor João Alexandre Medina corte-Real

co-orientador:

Professor Doutor João Carlos Andrade santos

1

r'(

J?r.

\2s

(2)

r)

A

todos os que tornaram este trabalho uma realidade quero expressar

o

meu agradeci-mento. Em especial ao meu orientador Professor Doutor Corte-Real e ao meu co-orientador Professor Doutor João Santos pela confiança, disponibilidade amiga e atenção dispensadas.

Ao

colega e amigo Professor

Doutor

João Patrício pela sua sempre

pronta

colaboração nas enumeras peripécias informáticas

e

também como responsável pelo C3

do Instituto

Politécnico de Tomar, sem

o

qual não

teria

sido possível realizar

a

enorme

parte

com-putacional deste

trabalho.

Ao amigo Professor Doutor Joaquim

Pinto

pelas palavras de estímulo e colabora4ão.

A

todos os colegas da área pelo seu apoio.

Aos estimados arguentes e membros do

júri

pelos seus valorosos comentários e sugestões

Agradeço ainda à minha famflia e amigos pelo encorajamento e apoio nas horas de maior desânimo.

(3)

Resumo

A

Equação média temporal

da

vorticidade potencial

do

fluxo atmosférico em coordenadas isobáricas pode

incluir

explicitamente

um termo

(interno)

atmosférico forçador

do

fluxo

perturbado.

De

facto,

desprezando nesta equação de balanço os termos não lineares, este

forçamento pode ser expresso matematicamente

por

uma única função denominada F\rnção

Empírica

Forçadora

(EFF),

correspondente

à

derivada

material

da

vorticidade potencial média

temporal.

Mais, a

EFF

pode ser decomposta na soma de sete componentes, que

in-dividualmente representam mecanismos forçadores de natureza diferente. Estes mecanismos

incluem as componentes diabáticas associadas com

o

forçamento radiativo,

a

libertação de

calor latente e dissipação pelo

atrito,

bem como, componentes relacionadas com os transportes transientes perturbados de entalpia e momento linear. Todos estes factores quantificam o

pa-pel das perturbações transientes no forçamento da circulação atmosférica. Com o objectivo de

compreender a importância da EFF no diagnóstico das anomalias de larga-escala da circulação atmosférica, é analisada a relação entre a EFF e a ocorrência de cristas anticiclónicas intensas

na zona Este do

Atlântico

Norte, consideradas frequentemente precursoras de secas na zona Oeste da Península Ibérica. Para tais eventos, o padrão da EFF apresenta no Atlântico Norte

uma clara estrutura dipolar; forçamento ciclónico (anticiclónico) de vorticidade potencial no sentido ascendente (descendente)

da crista

anormalmente

intensa.

Os resultados apontam igualmente para

uma

maior relevância das componentes que se relacionam com processos

diabáticos. Por último,

estes resultados enfatizam

a

relevância

da

EFF

no diagnóstico de

anomalias de larga-escala, e fornecem igualmente uma melhor compreensão das interacções entre os diferentes mecanismos físicos.

(4)

Abstract

The time.mean potential

vorticity

equation of the atmospheric flow on isobaric surfaces can

explicitly

include an atmospheric (internal) forcing term of the eddy

flow'

In

fact, neglecting

some non-linear terms in this balance equation, this forcing can be mathematically expressed as a single function, ca,lled Empirical Forcing F\rnction (EFF), which is equal to the material

derivative of the time-mean potential

vorticity.

F\rrthermore, the EFF can be decomposed as a sum of seven components, each one representing a forcing mechanism

of

different nature. These mechanisms include diabatic components associated

with

the radiative forcing, latent

heat release and

frictional

dissipation, and components related

to

transient eddy transports

of heat and momentum.

All

these factors quantify the role of the tra^nsient eddies

in

forcing

the atmospheric circulation.

In

order

to

assess the relevance of the

EFF

in

diagnosing large-scale anomalies

in

the

atmospheric circulation,

the

relationship between

the

EFF

and the occurrence

of

strong anticyclonic ridges over the Eastern

North

Atlantic

is analysed, which

are often precursors of severe droughts over Western

lberia.

For such events, the EFF pattern

depicts

a

clear dipolar structure over

the North Atlantic;

cyclonic (anticyclonic) forcing

of

potential

vorticity

is found upstream (downstream) of the anomalously strong ridges. Results also show

that

the most significant components are related

to

the diabatic processes. LastlS these results

highlight

the

relevance

of the EFF in

diagnosing large-scale anomalies, also

(5)

Resumo

da

Tese

portugal _{encontra-se localizado no extremo ocidental da Europa, na Península}

ItÉrica'

Dada

a sua locali zação oregime de precipita4ão é altamente variável e caracteriza-se por uma grande sazonalidade. A precipitação miíxima ocorre durante o Inverno (Dezembro a Fevereiro), sendo embora de destacar as contribuições do final de Outono e início da Primavera. Consequente-mente, os Invernos extremamente secos

ou

extremamente chuvosos tendem

a

ter

um

forte

impacto na disponibilidade de

água.

Deste modo, grandes deficits de precipitaçã,o são

fre-quentemente precursores de secas severas e extremas. Devido a esta vulnerabilidade, com um

grande impacto ambiental, económico e social, é de grande importância entender os

mecan-i.-o,

de l,arga-escala que se encontram ligados à ocorrência destas secas.

E

pois com base

neste objectivo que o estudo descrito neste trabalho se baseia.

O fluxo médio temporal da atmosfera pode ser considerado como um regime forçado que

se encontra condicionado

por

diversos fa,ctores,

tais

como, fontes e sumidouros de entalpia' orografia e contrastes térmicos à superfície.

contudo,

as perturbações do escoamento médio

temloral

induzem fluxos locais transientes de entalpia

e

momento linear que são

elemen-tos importantes nos mecanismos forçadores da circulaçao geral atmosférica. De facto, estas perturLa,ções são essenciais para a compreensão

do

ciclo

da

energética

da

atmosfera, dado

,"r"-

responsáveis pela transforma4ão contínua da energia potencial disponível média zonal em energia potencial disponível das perturbações, que por sua vez, por processos baroclínicos

se transforma em energià cinética perturbada que se converte por processos barotrópicos em energia cinética média zonal do movimento, consequência de processos de dissipação ligados ao

atrito.

A

vorticidade potencial é frequentemente utiliza.da em estudos diagnósticos da circulação atmosférica, sendo comummente escrita na sua formulação original, em superfícies isentrópicas, mas também podendo ser referida

a

um sistema de coordenadas em que a coordenada

ver-tical é u pr".rão

(sistema

p).

A

formulação matemática

da

equação

da

vorticidade

po-tencial em coordenadas isobáricas

é

revisitada no primeiro

capítulo.

Após algumas

aprox-ima4ões matemáticas obtém-se uma versão linearizada daquela equação, onde os vrários termos forçádores são explicitados e representados individualmente. O termo forçador da vorticidade

potlncial

assimétrica

é

expresso como uma única função, designada

por

F\rnção Forçadora

Empírica (EFF). Esta função pode ser decomposta numa soma de sete componentes que rep-resentam diferentes contribuições físicas (associadas a processos diabáticos e de transportes perturbados de entalpia e momento linear) para o termo forçador total da equação de balanço'

Este desenvolvimentà segue

a

formulação original devida

a

Saltzman.

Por

forma a

ilustrar

(6)

x

a aplicabilidade da

EFF

no _{diagnóstico de anomalias atmosféricas de}_larga-escala

,

rrm,case

study'é

apresentado no terceiro capítulo deste trabalho, focando o estabelecimento de cristas de altas pressões _sobre_o

_{Atlântico Norte.}

_A

_influência_das

cristas de altas pressões intensas

no

Atlântico

Norte _{no sistema atmosférico das}_latitudes_{médias, é previamente analisado.}

Os deficits severos _{de precipitação em}_Portugal_estão_{relacionados com padrões anómalos}

da

circulação atmosférica

_de

_larga-escala

_{no Atlântico}

_Norte.

_A

_{intensifica,çao}

das per-turbações de larga-escala levam

a

condições atmosféricas que não _{são favoráveis ao}

estab-elecimento de mecanismos geradores de precipitação em Portugal. _{Com o}_objectivo_{de isolar}

pa'drões _{anómalos de larga-escala sobre o}

_Atlântic;

Norte, em particular cristas anticiclónicas intensas

a

Oeste da Península lbérica, foram considerados _{um conjunto}_de_seis_Invernos

ex-tremamente secos

e

seis _{rnvernos extremamente chuvosos.}_Os_{compósitos destes Invernos}

para

diversas variáveis

e

campos atmosféricos foram analisados a,o longo

da

primeira _e

se-gunda secções do terceiro capítulo e denominam-se

por

'fnvernos Secos (Chuvosos),. As suas.

anomalias foram analisadas por intermédio da diferença entre os Invernos Secos e os Chuvosos.

Na última

secçã,o do terceiro capítulo, os carnpos de seis _{componentes da F\rnção Forçadora}

Í: ":-l"iente

associada a,o

atrito

(quinta)

_"*

e calculada)

"

. p;;;ri" *";,

Forçadora

rorarn analrcadas.

Para os Invernos Secos o _{campo médio do geopotencial apresenta quer}_a,os_b00_quer_{aos 800}

hPa uma crista bem definida sobre o Atlântico Nãrte, enquanto que para os Invernos Chuvosos

o padrã'o é quase

zonal

_{Este resultado é igualmente suportado p"1o}

"u,-po

da anomalia da

vorticidade potencial.

A

existência de um eixo quase

,"iti"ul

_{da anomalia}_miíxima_da_altura

do geopotencial médio zonal para os Invernos Sãcos sugere uma estrutura quase barotrópica equivalente para aquela crista.

A

presença de um

núclÃ

quente intenso na crista a oeste da Península Ibérica _{é claramente desfavorável}_{para a}_{ocorrência de precipitaçã,o em portugal.}

Esta crista _{intensa bloqueia}_{por um}_{lado, a propagação para leste}_dos

sistemas frontais prove-nientes do

Atlântico,

levando a um desvio para Nordestã d,o, storm track, no Atlântico Norte.

Por outro lado, a sua _{estrutura barotrópica}_equivalente,_leva_{a uma clara diferenciação entre}

os jactos subtropical _e

_polar.

_O

_jacto

_subtropical_{sofre nas}_latitudes

médias uma separação

em dois ramos

nà

zona _{Este da}_{América do}

_Norte.

_Esta_separação_{é acompa.nhada}_por

uma intensificação da componente meridional _{da circulação média troposférica, intensificaçã,o}

nor-malmente acompanhada de bloqueio na vizinhançá das Ilhas

Britânicas.

Os transportes de

entalpia _{e de humidade específica apresentarn}

_um

desvio para Nordeste nos Invernos Secos.

Este desvio é igualmente observado nos padrões da taxa dL precipitaçao e água precipitável,

contrastando _com

_o

_{padrão quase}_zonal_{dos Invernos Chuvosos.}

As

regiões de intensidade

miíxima

_{dos transportes de entalpia}_{estão concentradas nas zonas}_preferencia

is

do

,

stonn

track',

_{sugerindo ainda o desvio dos transportes de humidade,}

_o

bloqueio dos sistemas

tran-sientes pela crista _{intensa. Também o padrão dos transportes zonais de momento linear}

está

em concordância com a presença de uma crista _{anticiclónica intensa nos Invernos}_Secos.

A

F\rnção Forçadora Empírica é _{aplicada como ferramenta}_de_diagnóstico_das_{perturbações}

estacioniírias

de

larga-escala associadas

à

ocorrência

de

cristas anticiclónicas intensas no

Atlântico

Norte, _{consideradas como precursor€É da ocorrência de}_secas_severas

_na

zonaoeste

da Península

Ibérica-

Os resultados mostram, que o padrão da

EFF

é dinamicamente

coer-ente com

a

configuração do carnpo do geopotencial. Para os Invernos em estudo, o padrão

(7)

xt

ma,nutenção de vorticidade potencial ciclónica (anticiclónica) na região ascendente

(descen-dente) da crista anticiclónica anormalmente intensa. A anáIise individual das componentes da

EFF

revelou que de entre as seis componentes calculadas, a componente mais importante ao longo da tropàsfera é a primeira. Esta componente está associada ao aquecimento diabático,

excluindo-se

o

calor

latente.

contudo,

a

componente associada ao

calor

latente (segunda componente)

é

igualmente importante

na

baixa troposfera, enquanto os fluxos horizontais transientes àe entalpia (terceira componente) e de momento linear (sexta componente) são

importantes

na alta

troposfera. Estas componentes quando comparadas com as associadas

-"

flu*o.

verticais (qua.ta e setima componentes) são claramente domina'ntes em magnitude'

Deste modo, numa primeira aproximação o padrão da EFF pode ser estimado por intermédio

da primeira componente. Numa segunda aproximação, a estimativa da

EFF

pode ser

mel-ho.adu na

baixa (alta)

troposfera

por

intermédio da inclusão da segunda (terceira e sexta) componente.

Na interpretação dos padrões da

EFF

algumas considerações devem ser tidas em conta'

Embora alguns plocessos internos estejam incluídos

na

definição

da EFF,

os mecanismos forçadores externos

da

circulação atmosférica,

tais

como os efeitos topográficos, os fluxos diabáticos na camada

limite

não são incorporados nesta definição. De facto, estes processos só podem ser incluídos como condições

fronteira.

Tendo em consideração que a formulação

da

EFF

deriva da equação da vorticidade potencial média temporal sobre um período relati-vamente longo de ternpã (ie, uma estação), a análise da

EFF

só permite um diagnóstico das condições dinâmicas, não podendo ser utilizada no estudo da geração (desenvolvimento) das

anomalias. Nesta análise só se pode afirmar que a

EFF

(ou componente da

EFF)

pode ou não contribuir para a manutençã,o de uma perturbação estacionária específica' Mesmo tendo

em consideração as limitações referidas,

a

análise efectuada por intermédio da

EFF

permite compreender as diferentes contribuições dos processos atmosféricos internos que mantêm as anomalias médias temporais axialmente assimétricas do fluxo atmosférico'

(8)

(9)

List

of Figures

List

of

Tables

List

of

Symbols

1 Introduction

2 Data and MethodologY

2.L

Theoretical development of the Empirical Forcing F\rnction

2.L.7

Potential VorticitY

2.1.2

F\rndamental equations and deduction of

the

Empirical Forcing

F\rnc-tion

2.7.3

Empirical Forcing Function components

2.2

Data and methodologY

xv

xxv

xxvll

1 b 5 5 6 11 13

3 Results

3.1 Dynamical mechanisms of the

North Atlantic

atmospheric circulation:

prelim-inary analysis

3.1.1

Precipitation rate and Precipitable water

3.1.2

Sea Ievel Pressure

3.1.3

Mean GeoPotential height

3.1.4

Storm TYack

3.1.5

Potential temPerature

3.1.6

Horizontal transports of enthalpy

3.L.7

Horizontal transports of momentum

3.1.8

Horizontal transports of humidity

3.1.9 Wind

fields and the

jet

stream

Dynamical analysis of the precursors of the

EFF

components

3.2.I

Horizontal and vertical divergences of

enthalpy

' '

'

3.2.2

Horizontal and vertical divergences of

humidity

' '

'

3.2.3

Rates of heat addition

3.2.4 Vorticity

fields

Empirical Forcing Function:

A

Case Study

3.3.1

The

first

and second components of the EFF

3.2 3.3 L9 19 19 22 24 29 30 31 34 37 40 44 44 49 64 56 62 62 xlll

(10)

xlv

_CONTENTS

The

third

and

fourth

components of the EFF

The

sixth

and seventh components of the EFF The Empirical Forcing F\rnction

4 Discussion

and

Conclusions

References

Index

3.3.2 3.3.3 3.3.4 66 69 72 77 81 90

(11)

2.7

2.2

List

of

Figures

Geographical extension of the Euro-Atlantic sector

Map of Portugal and station's Iocations (from Mapping specialists, Lda)

Winter composite of the precipitation rates (shading) in 10-5

mm.day-l

in the

North

Atlantic

sector between 20oN-80oN and 100ow-40oE for (a)

dry

winters,

(b) wet winters, and (c) winter

climatology

20

Difference of the precipitation rates between

dry

and wet winters (contours)

in

iol;

In*.aay-i

and

tie

corresponding p-values of the Student's

t-test

(shad-ing)

in

the

úorth

Atlantic

sector between 20oN-80oN and

100ow-40oE'

2l

winter

composite

of the

precipitable

water

(shading) in.

mm

in

the

North

Atlantic

sector between

zoiN-ab'N

and 100oW-40oE

for (a)

dry

winters, (b)

wet winters, and (c) winter

climatology'

22

Difference

of

the

precipitable water between

dry

in

mm and the correspLnding p-values of the Student's

t-test

(shading)

in

the

North Atlantic

sector betweÃ-ZO'N-gOoN and

100oW-40oE

22

Winter

composite

of the

mean sea level pressure (shading)

in

102

Pa

in

the

North

Atlantic

sector between 20oN-80oN and 100ow-40oE for (a) dry winters,

(b)

wet winters, and (c) winter

climatology'

23

Difference

of the

mearr sea level pressure between

dry

and wet winters

(con-ãrlrii"

f O2 Pa and the corresponding p-values of the Student's t-test (shading)

in

the

North Atlantic

sector between 200N-800N and

1000w-400E.

23

winter

composite of

the

mean geopotential field

in

gpm

at

300 hPa over the Northern Hemisphere between 20oN-85oN for (a) dry winters, and (b) wet

winters'

24

Difference

of

the

mean geopotential

field

between

dry

and wet winters

(con-tours) in gpm at 300 hPa and the corresponding pvalues of the Student's t-test

(rhii"gfã""r

the Northern Hemisphere between 20oN-85'N'

cross-section

of the

zonal-mean (60ow-0oE)

winter

composite

of the

mean geopotential field

in

gpm over the Northern Hemisphere sector between

0'N-85'N for

(a)

dry

winters, and

(b)

wet winters

cross-section of the zonal-mean (60ow-0'E) difference of the mean geopotential field between

dry

in

gpm and the corresponding

P

values of the student's

t-test

(shading) over the Northern Hemisphere between

00N-850N. L4 15 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 25 25

xv

26

(12)

xvl

3.11 cross-section _{of the}_zonal-mean_(60"w-0oE)_difference_{of the}

mean geopoten_

tial

field

and

its

hemispheric mean (shading)

in

gpm and

the

corresponding

winter

mean omega-vertical

velocity

(contours)

in-pa.s-l

orr",

th"-ttio.tt"r,

Hemisphere between OoN-g5oN

for

(a)

dry

winters, and

(b)

wet winters.

3'12

_{Cross-sectionof the zonal-mean}_(60'Ú-0oÊ)_{diffe.e.rce of}

ilre mean geopotential

field

and

its

hemispheric mean (contours)

in

gpm and

the

corresponding

_F

values of the Student's

t-test

_{(shading) over the}_Northern_{Hemisphere between}

00N-950N.

3'13 EOF1

_{of the}

_mean_geopotential

_field

_in

_the

_North

_Atlantic

sector between 20oN-85oN

and

r00ow-40oE

for

.(a)

dry

winters (go.g% explained

,*iur""),

(b)

wet

winters (go.g% _{explained variance),}_and

jc)

_dry-*àt

_winters

_(go.o%

explained variance), and the _{corresponding mean gàopotentiar}_field

_in

_{tti2 gpm}

at

300

hPa.

.

3'14

Chronogram of the normalized

PCI

of the dry-wet winters _{mean geopotential} field

in

102 gpm

in

the

North Atlantic

_{sector between 20oN-g5oN}_and

100oW-400E. .

3'15

Mean geopotential _{field obtained}

_from

_PC1

_in

_the

_{North Atlantic}

sector

be-tween 20oN-85oN _and_{100'w-40oE for the}_{(a) positive composite, and}

(b) neg_

ative composite.

3.16

winter

composite

_{of the storm track}

_(shading)

_in

_gpm

_at

_b00

_hpa,

_and the corresponding mean geopotential

_height

_field

_(contours)

_in

₁₀₂

_gpm

_{i, tt"}

North Atlantic

_{sector between 20oN-85oN and}_t00,w-40oE

_for

_(")

ã;y

winters,

(b) wet winters, (c) winter _{crimatology, and}_(d)_{the difierenc" uàtrre"r,}_dry

and

wet winters.

3'17

Cross-sections of the mean-meridional (60oW-0oE) potential _temperature

_{in K}

in

the Northern _{Hemisphere between}_OoN-gboN

_r",

1ry dry winters, and (b) wet

winters.

3'18

cross-section

of

the

mean-meridional _{(60ow-0oE) difierence}

_of

_the

_potential temperature between dry and wet winters (contours)

in

K

and the

coriespond-ing

p-values

_of

_the

_Student's

_t-test

_{(shading) over}

_{the Northern}

Hemisphere between 0oN-85oN.

3'19 Winter

composite of the

total

horizontal _{transports of enthalpy}

_in

₁₀₂

_X.m.s-i

at

300 hPa

in

the

North

Atlantic

sector between 20oN_g0oN and 100ow _AooL

for

(a)

dry

winters, and

(b)

wet winters.

shading represents

the

intensity

of

thefluxes,andarrowsrepresentthehorizontalfluxes.

3'20

Diflerence of the

total

_{horizontal transports of}_enthalpy

_u"i*"""

_al,

_."a

_*"i

winters (contours)

in

102

K.m.s-l

at 300 _{hPa and the corresponding p-values}

_of

the Student's

t-test

(shading)

in

the

North Atlantic

sector between 20oN-g0oN

and 100"W-40oE.

3'21 Winter

composite of the transient _{horizontal transports of enthalpy}

_in

_K.m.s-l

at

300 hPa

in

the

North

Atlantic

_{sector between iO,N_gO,N}_and_{100ow_40oE}

for

(a)

dry

winters, and

(b)

wet winters.

Shading represents

_the

_intensity

_of

the

_{fluxes, and arrows represent}_{the horizontal fluies.}

3'22

Difference _{of the transient horizontal transports of enthalpy}

between

dry

and wet winters (contours) in

K.m.s-l

at 300 _{hPa and the corresponding p-values}

_of

the Student's

t-test

(shading)

in

the

North Atlantic

and 100oW-40o8. .

LIST

OF FIGURES

32 33 34 26 27 27 28 28 29 30 31 34

(13)

xvll

LIST

OF FIGURES

3.23 Winter composite of the horizontal convergence of the transports of enthalpy in

K.s-l

at 30õ hPa in the

North Atlantic

100oW-400E for (a) dry winters, and (b) wet winters. shading represents the intensity of the fluxes, and arrows represent the horizontal fluxes'

3.24 Winter composite

total

horizontal transports of zonal momentum

in

102 m2's-2

at

300 hPa

in

the

North Atlantic

sector between 20oN-80oN and 100oW-40oE

for

(a) dry

winters, and

(b)

wet

winters.

shading represents

the

intensity

of

the fluxes, and arrows represent the horizontal fluxes'

3.25 Difference of the

total

horizontal transports of momentum between dry and wet

winters (contours)

in

102 m2.s-2 at 300 hPa and the corresponding pvalues

of

the Student,s

t-test

(shading) in the North

Atlantic

and 100oW-40oE. .

3.26

Winter

composite transient horizontal transports

of

zonal momentum

in

102

m2.s-2 at 30b hpa in the North Atlantic sector between 20oN-80oN and 100oW-400E for (a) dry winters, and (b) wet winters. shading represents the intensity

of the fluxes, and arrows represent the horizontal fluxes'

3.27 Difference

of

the

transient horizontal transports

of

momentum between

dry

in

102 m2.s-2

at

300 hPa and the corresponding

pvalues of the student's

t-test

(shading) in the

North Atlantic

sector between 20oN-80oN and 100oW-40oE.

3.28

Winter

composite

of the

mean kinetic energy (contours) and transient mean

kinetic energy (shading)

in

1.01 m2.s-2 at 300 hPa

in

the

North Atlantic

sector

between 20oN-à0oN u.ra 1OO'W-4OoE

for

(a)

dry

winters, and (b) wet winters'

3.29 Difference of

the

transient mean kinetic energy between

dry

and wet winters

(contours)

in

101 m2.s-2

at

300 hPa and

the

corresponding p-values

of

the

student's

t-test

Atlantic

sector between 200N-800N and 1000w-400E

8.30 Winter

_-

composite of the

total

horizontal transports of specific humidity

in

L0-1

g.kg-1.-.r-i

at

300

hpa

in

the

North Atlantic

sector between 0'N-80oN and 1000w-400E

for

(a)

dry

winters, and

(b)

wet

winters.

shading represents the

intensity of the fluxes, and arrows represent the horizontal fluxes.

8.31 Difference of the

total

horizontal transports of specific

humidity

between

dry

in

L0-1

g.kg-l.m.s-l at

300 hPa and

the

corre-sponding pvalues of the student's

t-test

(shading) in the North Atlantic sector between 0oN-80oN and 100'W-40oE.

Winter

composite

of

the

transient-eddy horizontal transports

of

specific

hu-midity

in

10-1

g.kg-l.m.s-l

at

300 hPa

in

the

North Atlantic

sector between 0rN-800N and 1000w-400E

for

(a)

dry

winters, and (b) wet

winters.

shading

represents the intensity of the fluxes, and arrows represent the horizontal fluxes Difierence of the transient-eddy horizontal transports of specific

humidity

be-tween dry and wet winters (contours)

in

Lg-l g.kg-l.m,s

1 at 300 hPa and the

"orr".porrding pvalues of tLe Student's

t-test

(shading)

in

the

North Atlantic

sector between 0oN-80oN and 100oW-40oE'

Winter

composite of

the

specific

humidity

advection

in

10-1

g.kg-l.day-l

,at

300 hPa

in

the

North Atlantic

sector between 200N-600N and 800w-200E for

(a) dry

winters, and

(b)

wet

winters.

shading represents the intensity of the fluxes, and arrows represent the horizontal fluxes'

35 35 35 36 36 37 37 3.32 3.33 3.34 38 38 39 39 40

(14)

xvlll

_LIST

_{OF FIGURES}

3'35

General configuration of the polar and subtropical

jet

streams (from NASA).

3'36 Winter

composite

of

the streamlines and

wind

intensity

i,

m.s-1

at

300 hpa

over the Northern _{Hemisphere between 0oN-g5oN}

_for

_(a)

_dry

_{winters, and}_(b) wet

winters.

Shading represents the

wind intensity

and arrows represent the

streamlines

3'37

Difference of the streamlines and wind intensity between

dry

m.s-l

at 300 hPa and the corresponding pvalues of the Student,s

t-test

(shading) over the Northern _{Hemisphere betwãen 0"N-g5oN.}

3'38

Cross-sections of

the

winter composites of the zonal-mean wind component in

m's-l

at 300 hPa in the North Atiantic sector between 0oN-85oN _{and 60oW-0oE}

for

(a)

dry

winters, and

(b)

wet winters

3'39

Difference of the winter _{composites of the zonal-mean}_wind_{component between}

dry

and wet w-inters (contours)

in m.s-l

at

800 hpa and the cárresponding

p

values of

the

Student's

t-test

(shading)

in

the

North

Atlantic

sectàr between 0oN-85oN and 60oW-0oE.

3'40 Winter

composite

of

the vertically-averaged (500

_-

300

hpa)

mean horizontal divergence of

-enthalpy (shading)

in

10-a m.K.s-2 and the corresponding mean geopotential

field

(contours)

in

102 gpm

in

the

North Atlantic

sector

bãt.",

20oN-65oN and 100ow-40oE for (a) dry _{winters, (b) wet winters, and (c)}_winter

climatology.

3'41

Difference of _{the vertically-averaged (500-300 hPa)}_mean_{horizontal divergence}

of enthalpy between

dry

in

10-a m.K.s-2 u.rã

,h"

corresponding p-values _{of the}_Student's

_t-test

_(shading)

_in

_{the North}

_Atlantic

sector between 20oN-80oN and 100oW_ O1E. .

3'42

Zonal cross-sections _{of the}_{meridional-mean (20oN-60oN)}_of_{the mean}

horizon-tal

divergence

of

entharpy

at

500

_-

a00

hpa

in

10-5

m.K.s-2

in

the

North

Atlantic

between 100ow-20oE

for

(a)

dry

winters, and

(b)

_{wet winters, and at}

1000

_-

_{500 hPa}

_in

_10-a

_{m.K.s-2 for}

_(c)

_dry

_winters,

_and'(d)

wet winters.

3.43 Zonil

_{cross-section of the difference of the}

_{meridional--"u.,}

_{(20rN-60rN) of the}

mean horizontal divergence of enthalpy between dry and wet winters (coítours)

at

(a) 500

_-

300 hpa

in

10-5 m.K.s-2, and

(b)

100ó

_-

500 hpa

in

10-à m.K.s-2

and the corresponding p-values _{of the}_Student's

_t-test

_(shading)

_in

_{the North}

Atlantic

sector between 100oW-20oE

3'44

Winter

composite

of the

vertically-averaged (500

_-

800

hpa)

mean vertical

transports of enthalpy (shading)

in

10-a m.K.s-2 and the corresponding mean

geopotential

field

(contours)

in

102 gpm

in

the

North Atlantic

sector bãtween 20oN-65oN and 100ow-40'E for (a) dry winters, (b) wet winters, and (c) winter

climatology.

3'45

Difference of the vertically-averaged (500

_-

300 hPa) mea,n vertical transports of enthalpy between

dry

and wet winters (contoursj

in

10-a

m.K.s-2

urà tn"

t-test

(shading)

in

the

North

Atlantic

sector between 20oN-80oN and 100oW_ 40"8.

.

. . .

.

3'46

Winter _{composite of the vertically-averaged (500-300 hPa)}_mean_{eddy-horizontal}

divergence of enthalpy (shading)

in

10-6 m.K.s-2 and the

"orr""porràing mean

geopotential

field

(contours)

in

102 gpm

in

the

_{North Atlantic slctor}

_b-et*""r,

20oN-80oN and l00ow-40oE for (a) dry winters, (b) _{wet winters, and (c)}_winter

climatology. 40 41 47 42 42 44 45 45 45 46 46 47

(15)

xlx

LIST

OF

FIGUR^ES 3.47 3.48 3.49 3.50 3.51

Difference

of

the

_-

300

hPa)

mean eddy-horizontal divergence of enthalpy between

a.y

u,t d wet winters (contours)

in

10-6

mx'sa

and

ihe

t-test

(shading)

in

the North

Atlantic

sector between 20oN-80oN and 100'W-40oE'

winter

composite of the vertically-averaged (500-300 hPa) mean eddy-vertical

transports o1enthalpy (shading)

in

10-6 m.K.s-2 and the corresponding mean

geopotential field

(ctntours)

in

102 gpm

in

the

North Atlantic

sector between

ãOr-tt-gOrNf and 100oW-40'É for (a) dry winters, (b) wet winters, and (c) winter

climatology

-

300 hPa) mean eddy-vert-ical

trans-ports

of

enthalpy between

dry

and

wet winters

(contours)

in

10-6 m'K's-2

and the corresponding pvalues of the student's

t-test

(shading)

in

the North

Atlantic

sector between 20oN-80oN a'nd 100oW-40oE'

47 48 48 49 50 50 51

winter

composite of the vertically-averaged (500-300 hPa) mean divergence of

enthalpy (súading)

in

10-5

m.K.s-2

and the corresponding mean geopotential

field

(àániours)

in

L02 gpm

in

the North Atlantic

and 100'W-40oE

fot (ufàry

winters, (b) wet winters, and (c) winter climatology' 48 Difference of

the

-

300 hPa)_ mean divergence of

en-thalpy between

dry

and

*Lt

*irtã.,

(contours)

in

10-5 n

.I't-'

and the corre-,porráirrg p-values of the Student's

t-test

Atlantic

sector

between 20oN-80oN and

100oW-40oE'

49

3.52

Winter

composite of the vertically-averaggd (500

-

300 hPa) mean horizontal divergence of

humidity

(shading) 1n 10-10 m.s-2 and the corresponding mean

geopJtentiat field (contours)

in

L02 gpm

in

the

North Atlantic

sector between 20oN-g0oN and t0ôoW-40,É for (a) dry winters, (b) wet winters, and (c) winter climatologY

3.53 Difference of the vertically-averaged (500-300 hPa) mean horizontal divergence

of humidity

between

dry

i,

t6-to

m's-2

and the

corresponding p-value" áf

tn"

Student's

t-test

(shading)

in

the

North Atlantic

sector between 20oN-80oN and 100oW-40oE'

3.64 Zonil

cross-sections

of the

meridional-mean (20oN-60oN)

of the

mean hori-zontal divergence

of humidity

at

500

-

300 hPa

in

l'0-10

m's-2

in

the North

Atlantic

between 100ow-20oE

for

(a)

dry

winters, and (b) wet winters, and at

1000

_-

500 hPa

in

10-e m.s-2

for

(c)

dry

winters, and (d) wet winters

3.55 Zonat cross-section of the difierence of the meridional-mean (20oN-60'N)

hor-izontal divergence of

humidity

between

dry

at

(a)

500

_-

300 t pa

i,

iO-10

*..-2,

and

at

(b) 1ó00

_-

500 hPa

in

10-e m's-2 and the corresponding pvalues of the student's

t-test

(shading)

in

the

North

Atlantic sector between 100oW-20oE

3.56

Winter

composite

of the

vertically-averaged (5^00

-

300

hPa)

mean vertical

transports of

humidity

(shading)

in

10-10 m.s-2 and the corresponding mean geopotentiat

field

(contours)

in

102 gpm

in

the

North Atlantic

sector between

20oN-g0,N and rOôoW

-

OIE for (a) dry winters, (b) wet winters, and (c) winter

(16)

xx

_LIST

OF FIGURES

3'57 Difference of the _{vertically-averaged (500}

_-

_{300 hPa) mean}_{vertical transports}

of humidity

between

dry

in

10-ro

_-.r-iãl

_tn"

corresponding pvalues of the Student's

t-test

(shading)

in

the

North Atlantic

sector between 20oN-80oN and 100oW_

4OoE.

52

3'58 Winter composite of _{the vertically-averageg (500-800}_hpa)_mean_{eddy-horizontal}

divergence

of humidity

(shading)

in

10-10 m.s-2 and the corr".porrãirg mean geopotential

field

(contours)

in

102 gpm

in

the

North Atlantic

sector

blt*"en

20oN-80oN and 100ow-40oE for (a) dry winters, (b) wet winters, and (c) winter

climatolory.

b2

3'59

Difference

of the

_-

₈₀₀

_hpa)

_mean_{eddy-horizontal} divergence of

humidity

between

dry

in

t0-10 m.s-2 and the corresponding _{pvalues of the Student's}

_t-test

_(shading)

_in

_the

_North

Atlantic

sector between 20oN-g0'N and 100ow- 4oo1.

.

₅₈

3'60 Winter composite of the vertically-averaqgd _{(500-400 hpa)}_{mean eddy-vertical}

transports of

^humidity (shading)

in

10-10

D.s-2,

and the corr"spo.rding mean geopotential

field

(contours)

in

102 gpm

in

the

North Atlantic

sector

bãt*"",

20oN-80oN and 100ow-40oE for (a) dry winters, _{(b) wet winters, and (c)}_winter

climatology.

bB

3'61

_-

300 _{hPa) mean eddy-vertical}

trans-ports of

humidity

between

dry

in

t0-10

_-..-i

urd

the corresponding p-values _of

_the

_Student's

_t-test

_(shading)

_in

_the

North

At-lantic sector between 20oN-g0oN _{a,nd 100rW_}

_AO,E.

bB

3'62

Winter _{composite of the vertically-^averaged}_{(500-300 hpa)}

mean divergence

of

humidity

_{(shading) in- 10-10}

_D.s-2,

_and

_the

_{corresponding mean geopãtential}

field

(contours)

in

102 gpm

in

the

North Atlantic

_{sector between 20oN-g0rN} and 100oW-40oE

for

(a)

dry

winters, (b) wet winters, and (c) winter climatology. 54

3'63

Difference

of the

_{vertically-averaged (500}

_-

₃₀₀

_hpa)

_{mean divergence}

_of

hu-midity

between

dry

in

10-10 m.s-2

"rrã-

trr"

"àr.o

sponding pvalues _{of the Student's}

_t-test

_{(shading) in the}_{North Atlantic}

sector

3'64

Winter

comPosite.of the vertical\r-averaged (500

_-

800 hpa)

total

rate of heat

addition

(shading)

in

10-2

J.r-1,

and

the

_{corresponding mean geopotential}

field

(contours)

_in

102 gpm

in

the

North Atlantic

and 100oW-40oE

for

(a)

dry

winters, (b) _{wet winters, and}_{(c) winter climatology.}₅₅

3'65

_-

₃₀₀_hpa)

_total

_{rate of heat}_addition

between

dry

in

tO-z

Js-r

and the corresponding p-values of

the

Student's

t-test

(shading)

in

the North

Atlantic

sector Letween 20oN-80oN and

100oW-4OoE.

5b

3'66

Winter

composite of

the

_{vertically-averaged (500}

_-

₈₀₀

_hpa)

_{rate of heat}

ad-dition,

per

unit

mass, due

to

_{condensatiror, 1àrrrairg)}

_i,

_tola

_J.-1,

""ã

tr,"

corresponding mean geopotential field (contours)

in

t02

gp-

in

the

úo.tt

at_ lantic _{sector between 20oN-g0oN}_{and tô0rw-40oE for}

_(rfa.y

*i.rt"r.,

_fúj*"t

-

__

yilt".s,

and (c) winter

climatology.

56

3.67 Difference of the verticalry-averaged (500

_-

300 hpa) rate of heat addition, per

unit

mass, due to condensation between

dry

iô-3

J.s-l

and

the

corresponding p-values _of

_the

_student,s

_t-test

_(shading)

_in

_the

North Atlantic

_{sector between 20oN-g0oN}_and_100ow-

_4ooL.

(17)

LIST

OF

FIGUR"ES

8.68

Winter

composite of the vertical component of the relative

vorticity

(shading)

in

10-6

s-l

at

300 hPa

in

the North

Atlantic

100oW-40oE

for

(a)

dry

winters, (b) wet winters, and (c) winter climatology. .

8.6g Difference of the vertical component of the relative

vorticity

between

dry

in

10-6 s-1

at

300 hPa and the corresponding p-values

of the Student's

t-test

Atlantic

and 100oW-40oE.

3.20

Winter

composite

of

the

mean meridional gradient

of the

absolute

vorticity

(shading)

in

L0-11

m-1.s-1

at

300 hPa

in

the

North

Atlantic

sector between 00N-800N and 100rw-400E

for

(a)

dry

winters, and (b) wet winters

B.Z1 Difference of the mean meridional gradient of the absolute

vorticity

between

dry

in

10-11

m-l.s-1

at 300 hPa and the corresponding

pvalues of the Student's

t-test

(shading) in the

North

Atlantic sector between 0'N-80oN and 100oW-40oE.

3.72

Winter

composite

of the

absolute

vorticity

(shading)

in

1'0-5

s-I

at

300 hPa

and

the

corresponding mean geopotential

field

(contours)

in

102 gpm

in

the

North Atlantic

sector between 0'N-80',N and 100ow-40oE

for

(a)

dry

winters,

(b) wet winters, and (c) difference between

dry

and wet winters

B.Z3 Difference of the absolute

vorticity

between

dry

and wet winters (contours) in 10-5

s-1 at

300

hPa

and

the

corresponding p-values

of the

Student's t-test (shading) in the

North Atlantic

sector between 0oN-80oN and 100'w-400E,. 3.74

Winter

composite of

the

potential

vorticity

(shading)

in

10-a

s-l

at

300 hPa

and

the

corresponding mea,n geopotential

field

(contours)

in

102 gpm

in

the

North

Atlantic

sector between 20oN-80oN and l.00oW-4008 for (a) dry winters,

(b) wet winters, and (c) difference between

dry

and wet winters

B.Z5 Difference

of the potential

vorticity

between

dry

in

10-a

s-l

at

300 hPa and the corresponding p-values of the Student's t-test

(shading)

in

the

North Atlantic

sector between 20oN-80oN and 100ow-40oE. .

3.76 Winter composite of the potential

vorticity

advection (shading)

in

10-10

:-t tt

300 hPa and the corresponding mean geopotential field (contours)

in

102 gpm

in

the

North Atlantic

for

(a)

dry

winters,

and

(b) wet winters

3.77 Difference

of the

potential

vorticity

advection between

dry

and

wet

winters

(contours)

in

10-10

s-l

at

300

hPa and

the

corresponding

pvalues

of

the Student's

t-test

Atlantic

sector between 20oN-80oN and 1000w-400E

B.Z8 Zonal cross-sections

of

the

difference

of the

meridional-mean (35oN-50oN)

of

the first EFF

component pattern between

dry

and wet winters

for the North

Atlantic

between

á0rW-20'E

at (a)

500

_-

300 hPa

in

10-10

s-2,

and

at

(b)

1000

_-

500 hPa

in

10-e s-2.

3.79 Vertically-averaged (500

_-

300 hPa)

first EFF

component (shading)in l'0-10

s-2

and

th"

"orroponding

mean geopotential

field

(contours)

in

102 gpm in

the

North Atlantic

between 30oN-60oN and 80oW-20oE for (a)

dry

winters (b)

wet

winters

(c) winter

climatology,

and

(d)

difference between

dry

and wet

xxl

57 57 58 58 59 59 59 60 60 61 63 winters 63

(18)

xxll

_LIST

_{OF FIGURES}

3.80 3.81 3.82 3.83 3.84

_-

500 hPa)

first

EFF

component (shading)

in

10-e

s-2 and _{the corresponding mean geopotential fierd}_(contor.r)

_in

_ro2

_ãó-

_i1

_irr"

North Atlantic

between 30,N-G0'N and 80ow-20oE

for

(a)

dry

winters (b) wet

winters (c) winter climatology, and

(d)

difference between

dry

and wet

winters.

64

Difference of the vertically-averaged _{first EFF}_{component between}_dry_{and wet}

winters (co:rtours)

at

(a)

500-800

hpain

10-10 s-2, and

at (b)

1000-500 hpa

in

10-e s-2 and _{the corresponding p-values of the student,s}

_t-test

_(shading)_in

the

North Atlantic

between B0oN-60oN a,nd

80oW-20oE.

₆₄

zonal

cross-sections

of the

difference

of the

meridional-mean (B5oN-50oN)

_of

the second EFF component pattern between

dry

and wet winters

in

the North

Atlantic

between 80ow-20oF,

at

^(a) 500

-

300 hpa

in

10-11

s-2,

and

at

(b) 1000

_-

500 hPa layer

in

16-10

.-2.

₆₅ Vertically-averaged (500-300 hPa) second _{EFF component (shading)}_in_19-11

_.-2

and

the

field

(contours)

in

102 gpm

in

the

North Atlantic

between 30"N-60oN and 80ow-20oE

for

(a) dry winters (b) wet

winters (c) winter climatology, and (d) difference between

dry

and wet

winters.

65

_-

800

hpa)

second

EFF

component between

dry

i., 10-tt

s-2,

and

the

t-test

(shading)

in

the North Atla,ntic between

BgoN-60oN and 80'W-20oE.

3.85

Zonal cross-sections

of the

difference

of the

meridional-mean (35oN-50oN) of

the

third

EFF

component

pattern

between

dry

and wet winters

in the

North

Atlantic

between

80'w-20oE

at

(a)

500

_-

800

hpa

in

10-11

s-2,

and

at

(b)

1000

_-

500 hPa

i,

19-11

r-2.

3.86 ve-rtically-averaged (500

_-

900

hpa)

third

EFF

component (shading)

in

10-1r

s-2 and the corresponding mean geopotential field

(cãntor..ji,

102"gpm

inthe

North Atlantic

between B0oN-60oN and 80oW-20oE

for

(a) dry winters (b) wet

dry

and wet

winters.

6Z

3.87 _{Vertically-averaged (1000-500 hPa)}

_third

_{EFF component (shading)}

_i,

_19-11

_.-2

and

the

field

(contours)

in

102 gpm

in

the

North Atlantic

between B0oN-60oN and g0ow-20oE

for

(a)

dry

*inús

(b) wet

winters (c) winter climatology, and

(d)

difference between

dry

and wet

winters.

68 66 66

69

3.88

3.89

Difference of the vertically-averaged

third

EFF component between dry and wet winters-(contours)

at

(a)

500-800

hpain

10-11 s-2, and

at (b)

1000-500 hpa

in

10-11

s-2

and the corresponding p-varues _{of the studentis}

_t-test

_(shading)

in

the

North

Atlantic

between B0oN-60rN and g0oW-20"E. .

Zond, cross-sections

of the

difference

of the

meridional-mean (35rN-50oN) of the

fourth

EFF component

pattern

between

dry

and wet winters

in

the North

Atlantic

between 80ow-20oE

at

(a)

500

_-

300 hpa

in

10-12

s-2,

and

at

(b)

1000

_-

500 hPa

i,

19-11

.-2.

68

3.90 Vertically-averaged (500-300 hPa) fourth EFF component (shading) in 19-11

,-2

and

the

field

(contours)

in

102 gpm

in

the

North Atlantic

between B0oN-60oN and 80ow-20oE for (a)

dry

winters (b) wet

(19)

LIST

OF FIGURES

xxlll

3.91 3.92 3.93 3.94 3.95

Zonal cross-sections

of the

difference

of the

meridional-mean (35oN-50oN) of the

sixth EFF

component

pattern

between

dry

and wet winters

in

the North

Atlantic

between 80oW-20oE

at

(a)

500

_-

300 hPa

in

10-11

s-2,

and

at

(b)

1000

_-

500 hPa

ir,

19-11

.-2.

Vertically-averaged (500

_-

300 hPa) sixth

EFF

component (shading)

in

10-11

s-2 and the corresponding mean geopotential field (contours)

in

102 gpm in the

North Atlantic

between 30oN-60oN and 80oW-20oE for (a) dry winters (b) wet

dry

and wet winters Zond, cross-sections

of the

difference

in

the

meridional-mean (35oN-50oN) of

the seventh EFF component pattern between dry and wet winters in the North

Atlantic

between 80oW-20oE

at

(a)

500

_-

300 hPa

in

10-12

s-2,

and

at

(b)

1000

_-

500 hPa

i,

19-12

r-2.

Vertically-averaged (500-300 hPa) seventh EFF component (shading) in L0-12

s-2 and the corresponding mean geopotential field (contours)

in

102 gpm in the

North Atlantic

between 30oN-60oN and 80oW-20oE for (a)

dry

winters (b) wet

dry

and wet winters Vertically-averaged (500

_-

300 hPa) sum of second,

third

and

sixth

EFF

com-ponent (shading)

in

10-11

s-2

and the corresponding mean geopotential field (contours)

in

102 gpm

in

the North

Atlantic

between 30oN-60oN and

80oW-20oE

for

(a)

dry

winters (b) wet winters (c) winter climatology, and (d)

differ-ence between

dry

and wet winters.

3.96 Vertically-averaged (1000

_-

500 hPa) sum of second,

third

and sixth EFF

com-ponent (shading)

in

10-11

s-2

and the corresponding mean geopotential field (contours)

in

102 gpm

in

the North

Atla^ntic between 30oN-60oN and

80'W-20oE for (a)

dry

winters (b) wet winters (c) winter climatology, and (d) differ-ence between

dry

and wet winters.

3.97 Zonal cross-sections

of the

difference

of the

meridional-mean (35'N-50oN) of

the EFF pattern

between

dry

and wet winters

in

the

North Atlantic

between

80oW-20oE

at

(a)

500

_-

300 hPa

in

10-10

s-2,

and

at (b)

1000

_-

500 hPa in

10-e s-2.

3.98 Vertically-averaged

(500-300

hPa)

EFF

(shading)

in

10-10 s-2 and the corre-sponding mean geopotential field (contours)

in

102 gpm in the

North

Atlantic

between 30oN-60oN and 80oW-20oE

for (a) dry

winters

(b)

wet winters

(c)

winter climatology, and

(d)

difference between

dry

and wet winters

3.99 Vertically-averaged

(1000-500

hPa)

EFF

(shading)

in

10-e s-2 and the corre-sponding mearr geopotential field (contours)

in

102 gpm in the

North

Atlantic

between 30oN-60oN and 80oW-20oE

for (a) dry

winters

(b)

wet winters

(c)

winter climatology, and

(d)

difference between

dry

and wet winters

3.l00Difference of

the

vertically-averaged

EFF

between

dry

and wet winters

(con-tours) at (a)

500-300

hPa

in

L0-10 s-2, and

at

(b) 1000-500 hPa in L0-e s-2, and the corresponding pvalues

of

the Student's

t-test

(shading)

in

the North

Atlantic

between 30oN-60oN and 80oW-20oE.

70 70 77

7t

72 74 74 75 73 75

(20)

(21)

2,L

2.2 2.3

List

of

Tables

List

of variables and respective units

List

of

the

geographical locations of

the

four Portuguese stations used

in

this study

List

of the extremely

dry

and extremely wet winters

15 15

t4

(22)

(23)

List

of

Symbols

The following notation was used

in

the text.

:6.377 x

106 m

:7.292

x

10-5 rad s-1

:

9.80 ms-2

:

AcOSg# da :AÉ

v:

ui+uj

d,o

u:

_ã,

Q:

gz

T

%:7004

J

kg-t

6-r

R:287

J

kg-t

6-t

FÀ Fe q 4a

qt

r

o

ÍÍ"

Longitude Latitude Pressure Time

Mean radius of the earth

Angular velocity of the Earth Apparent

gravity

acceleration Zonal wind component

Meridional wind component

Horizontal wind vector

Omega vertical wind comPonent Geopotential

Air

temperature

Specific heat

at

constant pressure

Gas constant for

dry

air

Specific zonal

frictional

force Specific meridional frictional force Specific

humidity

Specific diabatic heating rate due to conduction and friction

Specific diabatic heating rate due

to

radiation

Specific diabatic heating rate due to latent heat release

Static

stability

parameter

Empirical forcing function

n-th

component of the empirical forcing function Total rate of heat addition

Potential

vorticity

Absolute

vorticity

Relative

vorticity

À

I

p

t

a

o

I

u a qe

a

il

rl C

Í

:q:qrqaqL

xKvll

(24)

xxvlll

_LIST

_{OF SYMBOLS}

P

E

Rs

W

0

o

o'

t1 (

_).

o

Precipitation rate Evaporation rate Surface runoff Precipitable water Potential temperature Time-mean

Departure from time-mean Zonal-mean

Departure from zonal-mean Areolar-mean

(25)

Ghapter

1 !ntroduction

Portugal is located in the western portion of Europe in the Iberian Peninsula between 37o-42oN

and 6.5o-9.5oW. Due to its location the precipitation regime is highly unbalanced and is char-acterized by a strong seasonal behaviour. The maximum rainfall occurs

in

winter (December

to February; hereafter

DJF),

despite the contributions of late autumn and early spring. As a

result, extremely wet or extremely dry winters tend to have strong impacts on water

availabil-ity

_{and management practices [García-Herrera}

et al.,

2007]. Severe precipitation deficits in

Portugal are largely related to anomaly patterns in the North Atlantic large-scale atmospheric

flow.

The dependence of winter precipitation on the

North Atlantic

Oscillation

(NAO),

and baroclinic wave

activity

associated

to

circulation weather types or weather regimes has been

the subject

of

several studies _[Corte-Real

et

al.,

1995; Fernandez

et

al., 2003; Goodess and Jones, 2002; Santos et

al.,

2005,2007a;

Tligo

and Da Camara, 2000; Ulbrich and Christoph,

1999a;

W*g,

2002; Wiin-Nielsen, 2003].

Severe precipitation deficits

in

Portugal result.from a enhancement of the stationary wave

pattern of

the

atmospheric large-scale circulation over

the North

Atlantic.

This

enhance-ment

of

the

large'scale stationary eddies leads

to

atmospheric conditions

that

are clearly unfavourable

to

the

establishment

of

rain-generating mechanisms over

Portugal.

Therefore

severe precipitation deficits are often precursors of severe or extreme drought episodes. Due

to

this vulnerability,

with

great environmental, economical and social significance,

it

is of great importance to better understand the large-scale mechanisms

that

are linked to the occurrence

of such droughts.

The

temporal mean flow

of the

atmosphere can be regarded as

a

forced regime

that

is

conditioned by several factors, such as, heat release, topography and surface thermal contrasts [Tbewartha and Horn, 1980; Watlace and Hobbs, 2006]. However, eddies embedded in the

time

mean flow induce local transient heat and momentum fluxes

that

are key forcing mechanisms

of

the general atmospheric

circulation.

In

fact,

such eddies are essential

for

explaining the energy cycle of the atmosphere, as they continuously transform zonal-mean available potential

energy

into

eddy

and

zonal-mean

kinetic

energy

of the

flow

that is

ultimately

dissipated

through cascading

frictional

dissipation _{processes [e.g.,}Peixoto and

Oort,

1992]. The

key

role played

by

the

eddy transports concerning

the

maintena,nce

of

large.scale atmospheric anomalies through

their

interaction

with

the zonal-mean flow has been already demonstrated

in

many previous studies

_[e.g.,

Andrade

et

al.,

2008c,

d;

Black, 1998; Edmon

et aI.,

1980;

(26)

2

Introduction

Holopainen et al., 1982; Hoskins and Valdes, 1990; Lau and Nath, 1991; Rodríguez-Puebla et

al., 2001; Santos and Corte-Real, 2006; Santos and Leite, 2009a].

Vorticity

is a widely used property when describing the dynamical characteristics of the

flow, and

its

equation is commonly used for diagnosis and prediction of fluid _{motions [Holton,}

2004; Hoskins

et

al.,

1985]. Potential

vorticity is a

quantity

that

combines

vorticity

and

static

stability

and verifies an

invertibility

principle

that

can be stated as follows _[Bluestein,

1993]:

if

the distribution

of

potential

vorticity

is known, then

vorticity, static stability

and

wind field can also be determined, providing the required boundary conditions and a balance

condition.

The

concept

of

potential

vorticity

is often used

in

the

study

of the

atmospheric

circulation, particularly

when considering

an

adiabatic frictionless

flow

_[Satbyand Roger, 1996];

in

this

case

it

is materially

conserved (local

rate of

change

is

entirely

bala,nced by

advection).

Therefore,

potential

vorticity is

commonly

written

in

its

original

formulation

on

isentropic surfaces

_[Ertel,

1962]. However, many subsequent studies have also used the

potential

vorticity

defined on isobaric surfaces _[e.g.,Hartmann,1977; Lau a,nd Wallace, 1979].

In

fact,

potential

vorticity

is

also

materially

conserved when using

the

quasi-geostrophic

formulation of the potential

vorticity

under the quasi-geostrophic assumptions and following

the geostrophic wind on an isobaric surface _[Holton,2004].

The mathematical formulation of the potential

vorticity

equation

in

isobaric coordinates

is

revisited

in this study.

Some mathematical approximations and rearrangements are un-dertaken so as

to

obtain

a linearised version

of the

balance equation, where various forcing

terms are

explicitly

and independently represented. The forcing term of the stationary-eddy

(asymmetric) potential

vorticity

balance is expressed as a single function, called the

Empir-ical

Forcing F\rnction (hereafter

EFF).

This

function can be

in turn

decomposed as a sum

of seven components

that

represent different physical contributions (associated

with

diabatic

processes and transient eddy enthalpy and momentum transports) to the

total

forcing term

of

the

balance equation. This development closely follows the original formulation undertaken

by

_{Saltzman [1962].}

In

order

to

illustrate the

applicability of

the EFF

in

diagnosing large-scale atmospheric anomalies,

a

case

study

focusing on the establishment

of

ridges over the Eastern

North Atlantic

is presented. As already mentioned, previous studies have shown

that

the

frequency and persistence of such events are ofben associated

with

droughts over western

Iberia

_{[García-Herrera}

et

al., 2007; Hanson

et

al.,

2007; Santos and Leite, 2009a; Santos et

ú.,2007a).

The influence of those enhanced ridges over the Eastern North

Atlantic

in the mid-latitude weather systems

is

also tested

here.

In

fact,

these systems have

their

origin

in

processes

predicted under the theory of baroclinic

instability

and their development is closely associated

with

the

above referred forcing mechanisms. Such extra-tropical migratory systems follow a

path

in

the belt

of the prevailing westerly winds and derive

their

existence and growth from

the

conversion

of

available

potential

energy

to

kinetic

energy

by

reducing

the

temperature

gradients,

lifüing mid-latitude warm

air

and

sinking cold

polar

air

_[Charney,Lg47;

Edy,

1949].

They

also play a central role

in

the angular momentum budget of the atmosphere and are

primarily

responsible for maintaining the westerlies against surface

friction

_{[Peixoto and}

Oort,

1992]. One common measiure of synoptic

activity

is the 'storm

track',

which Blackmon

[1976] defined as the standard deviation of the bandpass-filtered (2-6 days)

variability at

500

(27)

3

as the tropospheric counterparts of surface cyclones and high pressure _{systems [Blackmon et}

al., 1984a, b; Wallace and Gutzler, 1981,; Wallace et al., 1988]. This variable has been widely

used

to

quantify synoptic

activity, both for

observational and model data _{sets [e.g., Chang}

and Fb, 2002; Chang, 2009; Hoskins and Valdes, 1990; Stephenson and Held, 1993; Ulbrich et al., 2008;

Yin,

2005].

In

this study, the objectives are two.fold.

First,

it

is intended to present the mathematical development of the EFF, and secondly to demonstrate its relevance in diagnosing large-scale anomaly patterns by considering a pertinent case study.

The structure of

this

thesis is as follows:

in

Section 2

it

is presented

the theoretical and mathematical development of the EFF and

its

components,

as well as, the data and methodologies;

in

Section 3 results obtained for the selected case study are presented,

in

the following

sequence:

first,

the dynamical mechanisms of the

North Atlantic

circulation are discussed,

second, the dynamical analysis of the precursors fields of the

EFF

components, is

carried

out

and

third,

the Empirical Forcing Function is analysed

(28)