Normal-mode analysis of the role of tropics in global atmospheric predictability

Kosovelj, Katarina (author), Žagar, Nedjeljka (mentor)

Tropical circulation features associated with convective heating affect the extratropical variability trough various teleconnections. Some properties of this tropical-extratropical coupling are addressed in this thesis. The novelty of the applied methodology is the modal decomposition technique that provides scale-dependent diagnostics in terms of the Rossby and inertio-gravity (IG) waves. These waves are eigensolutions of the linearized primitive equations, termed the normal modes and their horizontal structure is described by the Hough harmonics. Known for a long time, the normal-mode functions (NMFs) have recently been applied also for the validation of weather and climate model variability. The central question addressed in the thesis is the quantification of the global Rossby and IG wave response to tropical heating perturbations resembling different phases of Madden-Julian Oscillation (MJO). This question is addressed by numerical simulations using the ICTP AGCM (SPEEDY) model. Although simplified with respect to the full scale state-of-the-art climate and weather prediction models, SPEEDY proves capable to reproduce large-scale patterns of both transient (short-term) and average (climatological) circulation. The model simulates the partition of the spatial and temporal variability between the zonal and meridional modes, zonal mean state and waves, the Rossby and IG part of circulation reasonably well in comparison to the ERA-20C reanalyses. It is shown that initial uncertainties is SPEEDY grow in a similar fashion as in the numerical weather prediction systems. The model is able to fairly reproduce forecast uncertainties associated with the Rossby and IG modes with temporal and spatial characteristics found in an operational ensemble prediction systems. This justifies using SPEEDY for studying the transient properties of the global response to tropical heating perturbations. Four ensembles of simulations with the MJO-like heating perturbations in 100 winters with different background conditions are performed. Two ensembles are forced with dipole and another two with monopole heating and cooling perturbations, mimicking various phases of the MJO life cycle. The perturbations are applied on temperature tendencies and include feedbacks from the model convection and large-scale condensation schemes. The results show that the realistic formulation of heating perturbations involving moist feedbacks does not produce significantly different results from dry model with imposed temperature perturbations used in previous studies. The response is mostly defined by a positive heating anomaly, and the interaction of the two poles of dipole acts to localize the tropical response and to affect the shape and orientation of the Rossby wave train emanating from the subtropics. The short-term response is projecting predominantly on the IG modes, especially on the Kelvin wave. In the medium time range, the Rossby modes dominate the response. Although the n=1 Rossby mode is the main contributor to the Rossby wave variance at all time scales, it is shown that the modes with the meridional index n>1 contribute over 50 % of the medium-range Rossby wave response. Finally, numerical experiments without large-scale orography components in the northern hemisphere (NH) are performed to study the orography impact on the extratropical propagation of the Rossby wave developed in response to MJO heating perturbation. A relatively small statistically significant impact is found.

• Language: English
• Keywords: Madden-Julian Oscillation, normal modes, ICTP AGCM (SPEEDY), general circulation, predictability
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2019
• PID: 20.500.12556/RUL-111890
• COBISS.SI-ID: 385193
• Publication date in RUL: 17.10.2019

Secondary language

• Language: Slovenian
• Title: Analiza vloge tropskih predelov v globalni napovedljivosti ozračja s pomočjo normalnih načinov

Abstract

Lastnosti tropske cirkulacije, povezane s konvekcijo, vplivajo na variabilnost cirkulacije izven tropov preko različnih povezav na daljavo. V tej disertaciji se ukvarjam z značilnostmi nekaterih takšnih povezav. Novost uporabljene metodologije je, da omogoča analizo Rossbyjevih in inercijsko težnostnih valov v modalnem prostoru, v odvisnosti od prostorske skale. Omenjeni valovi so lastne rešitve lineariziranih osnovnih gibalnih enačb, imenujemo se normalni načini gibanj. Njihove horizontalne strukture opisujejo Houghovi harmoniki. Normalni načini gibanj (NMF) so poznani že dolgo, pred kratkim pa so bili uporabiljeni tudi za validacijo variabilnosti klimatskih modelov in modelov za numerično napovedovanje vremena. Glavni poudarek te disertacije je na kvantifikaciji globalnega odziva Rossbyjevih in IG valov na anomalije v tropske diabatnem gretju, podobne različnim fazam Madden-Julianove oscilacije (MJO). Numerične simulacije v tej disertaciji so narejene z modelom ICTP AGCM (SPEEDY). V primerjavi z modernimi klimatskimi modeli in modeli za napovedovanje vremena na visoki ločljivosti je SPEEDY poenostavljen, a še vedno je sposoben reproducirati vzorce kratkoročne in povprečne klimatološke cirkulacije na velikih prostorskih skalah. V primerjavi z reanalizo ERA-20C, je SPEEDY sposoben simulirati porazdelitev energije in časovne variabilnosti med zonalne in meridionalne načine, med zonalno povprečje in valove ter med Rossbyjevo in IG komponento cirkulacije. Na vremenskih časovnih skalah nezanesljivosti v SPEEDYju rastejo na podobnih časovnih skalah kot v modelih za numerično napovedovanje vremena. SPEEDY je tudi sposoben generirati modelske nezanesljivosti v Rossbyjevih in IG načinih, na primernih geografskih območjih in na primernih časovnih skalah. Zaradi tega je SPEEDY primeren za študije globalnega odziva na anomalije tropskega diabatna gretja. Narejeni so bili štirje ansambli simulacij z anomalijami podobnimi MJO, v 100 zimah z različnimi vremenskimi pogoji. Dva ansambla sta imela dodano anomalijo v obliki dipola (gretje in hlajenje) in dva v obliki monopola (gretje ali hlajenje). Takšne perturbacije simulirajo različne faze MJO. Anomalije so bile dodane na tendence temperature, vsebujejo pa povratne zanke iz modelskih shem za konvekcijo in kondenzacijo na velikih prostorskih skalah. Rezultati kažejo, da realistična formulacija motenj v diabatnem gretju, z upoštevanimi povratnimi zankami zaradi vlažnih procesov, ne daje bistveno drugačnih rezultatov kot jih dajo modeli brez vlage z dodanimi perturbacijami v temperaturi, kakršni so bili pogosto uporabljeni v podobnih študijah. Odziv cirkulacije je večinoma pogojen s pozitivno anomalijo v gretju, vendar meddelovanje obeh polov (pozitivne in negativne anomalije) v dipolu lokalizira odziv v tropih in vpliva na obliko in orientacijo Rossbyjevih valov, ki se širijo iz subtropskih območij. Kratkoročni odziv se projicira predvsem na IG načine, še posebej na Kelvinov val. V srednjeročnem odzivu dominirajo Rossbyjevi načini, Rossbyjevi načini z meridionalnim indeksom večjim od 1 (Rossbyjevi načini n>1) prispevajo več kot 50 % variance k Rossbyjevemu delu odziva. Največji prispevek k varianci Rossbyjevih valov prispeva Rossbyjev način n=1, ne glede na časovno skalo odziva. Na koncu so bili dodatno narejeni se poskusi z odstranitvijo visokih gorovij severne poloble, za študijo vpliva orografije na razširjanje odziva na severni polobli izven tropov. Statistično značilen vpliv je majhen.

• Keywords: Madden-Julianova oscilacija, normalni načini gibanj, ICTP AGCM (SPEEDY), splošna cirkulacija ozračja, napovedljivost