Ocean-atmosphere interactions are of paramount importance in both climatic and meteorological contexts. They are known to play important roles from hourly time scales, such as in the intensification of tropical cyclones, to interannual and even longer time scales, such as in El Niño Southern Oscillation mode of variability of the climate system. The focus of this thesis has been on the energy and momentum transfers at the air-sea interface in short time scales processes characterized by extreme conditions. Both the oceanic dynamical response to an extreme atmospheric forcing and the effects of the sea state on the development of a meteorological extreme event are considered. The systems under study are the ocean internal wave field in the wake of a tropical cyclone and the role of the upper ocean thermal state on the development of heavy rainfalls. In particular, the energy exchanges among oceanic internal waves in the wake of an idealized tropical cyclone are studied with a theoretical approach supported by relevant primitve equation numerical simulations. The goal of this analysis is to understand how tropical cyclones might contribute to the internal ocean mixing in locations far from their track. In fact, despite their intermittency in space and time, they are characterized by very intense winds, which are known to excite oceanic internal waves. These are thought to contribute to ocean mixing far from their generation site through their breaking. Since the energy propagation is linked to the spectral features of the waves, a detailed description of the energy partitioning in different vertical modes and frequencies helps to better constrain the extent and the velocity of such energy propagation. A new detailed analytical description of the exchanges leading to the formation of the first superinertial peak is introduced on the basis of the theory developed. Compared to previous works, a realistic oceanic stratification is included and a path for the energy cascade from the large scales of the atmospheric forcing to the small scales of the mixing is highlighted. The second category of extreme events considered are the heavy-rain-producing mesoscale convective systems (MCSs). They are common phenomena along the coasts of the Mediterranean sea and they release large amounts of rain in few hours and over relatively small areas, O(100 km2). It is known that an average warmer sea in the vicinity of their location produces a larger volume of rain, but before this thesis work no information was available on the influence that a spatial pattern of sea surface temperature (SST), with structures on the kilometric scale, might have on the precipitation event. Appropriate atmospheric numerical simulations, run with a non-hydrostatic primitive equation model, shed light on the mechanisms through which submesoscale SST oceanic features can influence the surface wind structure and, in turns, can affect the evolution of the heavy rainfall. It is found that through enhanced vertical momentum mixing in the atmosphere over warmer SST areas, the presence of temperature fronts in the sea can significantly affect the surface wind convergence, which is often the trigger for deep convection in MCSs, over hourly time scales. This might also lead to significant displacement of the rain bands. The possibility of an ocean dynamical feedback related to the vertical temperature profile is then introduced. By means of atmosphere-ocean coupled numerical simulations, it is found that in particular conditions the intense winds in which the MCS is embedded can mix the upper ocean strongly enough to enhance the stability of the atmospheric boundary layer and suppress deep convection. Such conditions, characterized by a shallow mixed layer and strong stratification, are typical of the late summer. This could be the reason why MCSs are generally observed later during the year, when the mixed layer is deeper and this oceanic mitigating effect is absent.

Le interazioni oceano-atmosfera sono di primaria importanza sia in ambito climatico che meteorologico. Sono importanti sia su scale temporali orarie, come nell'intensificazione di cicloni tropicali, che su scale interannuali o interdecadali, come nel modo di variabilità climatica ENSO. Questa tesi si focalizza sui transferimenti di energia e quantità di moto all'interfaccia aria-mare in processi su scale temporali brevi caratterizzati da condizioni estreme. Sono presi in considerazione sia la risposta dinamica dell'oceano ad una forzante atmosferica estrema che l'effetto dello stato del mare sullo sviluppo di un evento meteorologico estremo. I sistemi studiati sono il campo di onde interne oceaniche nella scia di un ciclone tropicale e il ruolo dello stato termico dell'oceano superficiale nello sviluppo di piogge intense. In particolare, gli scambi di energia tra onde interne oceaniche nella scia di un ciclone tropicale idealizzato sono studiati con un approccio teorico supportato da appropriate simulazioni numeriche alle equazioni primitive. Si vuole capire come i cicloni tropicali possano contribuire al mescolamento oceanico interno in luoghi lontani dalla loro scia. Infatti, nonostante siano intermittenti nel tempo e nello spazio, sono caratterizzati da venti molto intensi, che eccitano onde interne oceaniche. Esse contribuiscono al mescolamento lontano dal luogo in cui sono state generate, attraverso la loro rottura. Dato che la propagazione di energia è legata alle loro caratteristiche spettrali, una descrizione dettagliata di come l'energia è divisa tra modi verticali e frequenze aiuta a quantificare l'estensione e la velocità di tale propagazione. Una nuova descrizione analitica degli scambi energetici che portano alla formazione del picco doppio-inerziale viene introdotta sulla base della teoria sviluppata. Rispetto a lavori precedenti, si considera una stratificazione oceanica realistica e viene sottolineata una possibile cascata energetica dalla larga scala della forzante atmosferica alla piccola scala del mescolamento. L'altra categoria di eventi estremi considerata è quella del sistemi convettivi a mesoscala (MCS). Essi sono fenomeni comuni lungo le coste del Mediterraneo e rilasciano abbondanti volumi di pioggia in poche ore e su aree dell'ordine di 100 km2. Si sa che un mare mediamente più caldo in prossimità di un MCS produce più pioggia, ma prima di questa tesi non c'erano informazioni circa l'influenza che un pattern spaziale di temperatura marina superficiale (SST) a scala chilometrica potesse avere sull'evento precipitativo. Opportune simulazioni atmosferiche, eseguite con un modello numerico non-idrostatico alle equazioni primitive, fanno luce sui meccanismi attraverso cui le strutture alla sotto-mesoscala di SST possono influenzare la struttura del vento superficiale e, di conseguenza, possono influenzare l'evoluzione della pioggia intensa. Si trova che, attraverso un maggiore mescolamento verticale di quantità di moto su aree di SST più calda, la presenza di fronti di temperatura nel mare può significativamente influenzare la convergenza superficiale, che è spesso l'elemento scatenante della convezione nei MCS, su scale temporali orarie. Questo potrebbe anche far spostare le linee di pioggia. Viene introdotta, poi, la possibilità di un fenomeno di retroazione oceanico legato al profilo verticale di temperatura. Con simulazioni accoppiate oceano-atmosfera, si trova che, in condizioni particolari, i venti intensi in cui il MCS è inglobato possono mischiare l'oceano superficiale a tal punto che la stabilità atmosferica è aumentata e la convezione è soppressa. Tali condizioni, tipiche della tarda estate, sono caratterizzate da uno strato mescolato sottile e una forte stratificazione. Questo potrebbe essere il motivo per cui i MCS sono generalmente osservati più avanti nell'anno, quando lo strato mescolato è più profondo e tale effetto oceanico di mitigazione è assente.

(2018). Interactions between the ocean and extreme meteorological events. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2018).

Interactions between the ocean and extreme meteorological events

MERONI, AGOSTINO NIYONKURU
2018

Abstract

Ocean-atmosphere interactions are of paramount importance in both climatic and meteorological contexts. They are known to play important roles from hourly time scales, such as in the intensification of tropical cyclones, to interannual and even longer time scales, such as in El Niño Southern Oscillation mode of variability of the climate system. The focus of this thesis has been on the energy and momentum transfers at the air-sea interface in short time scales processes characterized by extreme conditions. Both the oceanic dynamical response to an extreme atmospheric forcing and the effects of the sea state on the development of a meteorological extreme event are considered. The systems under study are the ocean internal wave field in the wake of a tropical cyclone and the role of the upper ocean thermal state on the development of heavy rainfalls. In particular, the energy exchanges among oceanic internal waves in the wake of an idealized tropical cyclone are studied with a theoretical approach supported by relevant primitve equation numerical simulations. The goal of this analysis is to understand how tropical cyclones might contribute to the internal ocean mixing in locations far from their track. In fact, despite their intermittency in space and time, they are characterized by very intense winds, which are known to excite oceanic internal waves. These are thought to contribute to ocean mixing far from their generation site through their breaking. Since the energy propagation is linked to the spectral features of the waves, a detailed description of the energy partitioning in different vertical modes and frequencies helps to better constrain the extent and the velocity of such energy propagation. A new detailed analytical description of the exchanges leading to the formation of the first superinertial peak is introduced on the basis of the theory developed. Compared to previous works, a realistic oceanic stratification is included and a path for the energy cascade from the large scales of the atmospheric forcing to the small scales of the mixing is highlighted. The second category of extreme events considered are the heavy-rain-producing mesoscale convective systems (MCSs). They are common phenomena along the coasts of the Mediterranean sea and they release large amounts of rain in few hours and over relatively small areas, O(100 km2). It is known that an average warmer sea in the vicinity of their location produces a larger volume of rain, but before this thesis work no information was available on the influence that a spatial pattern of sea surface temperature (SST), with structures on the kilometric scale, might have on the precipitation event. Appropriate atmospheric numerical simulations, run with a non-hydrostatic primitive equation model, shed light on the mechanisms through which submesoscale SST oceanic features can influence the surface wind structure and, in turns, can affect the evolution of the heavy rainfall. It is found that through enhanced vertical momentum mixing in the atmosphere over warmer SST areas, the presence of temperature fronts in the sea can significantly affect the surface wind convergence, which is often the trigger for deep convection in MCSs, over hourly time scales. This might also lead to significant displacement of the rain bands. The possibility of an ocean dynamical feedback related to the vertical temperature profile is then introduced. By means of atmosphere-ocean coupled numerical simulations, it is found that in particular conditions the intense winds in which the MCS is embedded can mix the upper ocean strongly enough to enhance the stability of the atmospheric boundary layer and suppress deep convection. Such conditions, characterized by a shallow mixed layer and strong stratification, are typical of the late summer. This could be the reason why MCSs are generally observed later during the year, when the mixed layer is deeper and this oceanic mitigating effect is absent.
PASQUERO, CLAUDIA
air; sea; interactions,; extreme; events
air; sea; interactions,; extreme; events
GEO/12 - OCEANOGRAFIA E FISICA DELL'ATMOSFERA
English
21-mar-2018
SCIENZE CHIMICHE, GEOLOGICHE E AMBIENTALI - 94R
30
2016/2017
open
(2018). Interactions between the ocean and extreme meteorological events. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2018).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/199143
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