Recent Tropical Expansion: Natural Variability or Forced Response?

Previous studies have documented a poleward shift in the subsiding branches of Earth’s Hadley circulation since 1979 but have disagreed on the causes of these observed changes and the ability of global climate models to capture them. This synthesis paper reexamines a number of contradictory claims in the past literature and finds that the tropical expansion indicated by modern reanalyses is within the bounds of models’ historical simulations for the period 1979–2005. Earlier conclusions that models were underestimating the observed trends relied on defining the Hadley circulation using the mass streamfunction from older reanalyses. The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but models suggest that the factors driving the expansion differ between the hemispheres. In the SH, increasing greenhouse gases (GHGs) and stratospheric ozone depletion contributed to tropical expansion over the late twentieth century, and if GHGs continue increasing, the SH tropical edge is projected to shift further poleward over the twenty-first century, even as stratospheric ozone concentrations recover. In the NH, the contribution of GHGs to tropical expansion is much smaller and will remain difficult to detect in a background of large natural variability, even by the end of the twenty-first century. To explain similar recent tropical expansion rates in the two hemispheres, natural variability must be taken into account. Recent coupled atmosphere–ocean variability, including the Pacific decadal oscillation, has contributed to tropical expansion. However, in models forced with observed sea surface temperatures, tropical expansion rates still vary widely because of internal atmospheric variability

Atmospheric Response to Orbital Forcing and 20th Century Sea Surface Temperatures

This study investigates modes of atmospheric variability in response to changes in Earth\u27s orbit and changes in 20th century sea surface temperatures (SST). The orbital forcing is manifested by a change in obliquity and precession, and changes the distribution of the top-of-atmosphere insolation. A smaller obliquity reduces the the annual insolation that the poles receive and increases the annual insolation in the tropics. As the meridional insolation gradient increases, the zonal mean atmospheric-ocean circulation increases. The resulting climate also has a reduced global mean temperature due to the effect of climate feedbacks. This cooling can be attributed to a reduced lapse rate, increased cloud fraction. reduced water vapor in the atmosphere, and an increase in the surface albedo. A change in the precession, as the perihelion shifts from the winter to the summer solstice, causes a strengthening as well as an expansion of the N. Pacific summer subtropical anticyclone. This anticyclonic anomaly can be attributed to the weakening of the baroclinic activity, but also represents the circulation response to remote and local diabatic heating. The remote diabatic heating is associated with monsoonal activity in the SE Asia and North Africa. Regarding the 20th century SST forcing, it is represented by a multidecadal variability in the inter-hemispheric SST difference. This change in the SST causes a latitudinal shift in the ascending branch of the Hadley cell and precipitation in the tropics, as well as an increase in the atmospheric meridional heat transport from the warmer to the colder hemisphere

Simulated variability in the mean atmospheric meridional circulation over the 20th century

The variability in the zonally averaged atmospheric meridional circulation over the last 150 years is investigated using two atmospheric general circulation models (GCMs). A multi‐decadal signal, with an approximately 70 year timescale, is identified in the cross‐equatorial circulation. This signal is associated with a latitudinal shift in the ascending branch of the Hadley cell and precipitation in the tropics, as well as a change in the atmospheric meridional heat transport. These changes are well‐correlated with the inter‐hemispheric SST difference: When Northern Hemisphere sea surface temperatures (SSTs) are colder than the Southern Hemisphere, the ITCZ and precipitation shift to the south in a zonal mean sense, and the northward atmospheric energy transport increases. Previous studies with idealized climate forcings have shown similar results, but the findings presented here highlight the potential relevance of the inter‐hemispheric SST contrast for understanding 20th century climate changes

Arithmetic Functions & The Riemann's zeta function

Η παρούσα πτυχιακή εργασία αποτελεί μια εισαγωγή σε βασικές έννοιες της Θεωρίας Αριθμών. Πιο συγκεκριμένα μελετάμε τις αριθμητικές συναρτήσεις και την συνάρτηση ζ του Riemann, και τέλος προσπαθήσαμε να συνδέσουμε τις δύο αυτές έννοιες. Στο πρώτο κεφάλαιο παραθέτουμε τους βασικούς ορισμούς της Θεωρίας Αριθμών, όπως αυτούς των πρώτων αριθμών, των αριθμητικών συναρτήσεων και κάποια σημαντικά θεωρήματα όπως το Μικρό Θεώρημα του Fermat, το Θεώρημα Auler- Fermat και αυτό του Legendre. Στο δεύτερο κεφάλαιο εξετάζουμε κάποιες πολύ γνωστές αριθμητικές συναρτήσεις, όπως αυτη του Euler, του Mobius και τις συναρτήσεις των διαιρετών, όπως είναι η τ(n) και η σ(n). Στη συνέχεια αποδεικνύουμε βασικές ιδιότητες τους και εξετάζουμε αν είναι πολλαπλασιαστικές. Το τρίτο κεφάλαιο εξετάζει τις συναρτήσεις π(x) και li(x), που αν και δεν είναι αριθμητικές συναρτήσεις παρουσιάζουν ενδιαφέρον στην μελέτη των πρώτων αριθμών. Επίσης δίνουμε ένα αριθμητικό φράγμα για την π(x) μέσω μιας ανισότητας και το αποδεικνύουμε. Με το τρίτο κεφάλαιο ολοκληρώνεται το μέρος της Θεωρίας Αριθμών και προχωράμε στην Αναλυτική Θεωρία Αριθμων μελετώντας την συνάρτηση ζ και την εικασία του Riemann. Αναλυτικότερα, στο τέταρτο κεφάλαιο βλέπουμε τα βασικά σημείας της εργασίας του Riemann με τίτλο “On the Number of Primes less than a Given Magnitude” και πως καταλήγει στην καλύτερη προσέγγιση της συνάρτησης π(x). Υπολογίζουμε αριθμητικές τιμές της ζ για το 2 και για κάθε άρτιο, και τέλος βλέπουμε μία εφαρμογη της. Στη συνέχεια, το πέμπτο κεφάλαιο αναφέρεται στην Εικασία του Riemann με την βοήθεια της συνάρτησης ξ(x). Εν συνεχεία αποδεικνύουμε μέσω εργαλείων ασυμπτωτικής ανάλυσης μια αναγκάια συνθήκη της Εικασίας. Τέλος, στο έκτο κεφάλαιο βλέπουμε την συνάρτηση ζ σαν γεννήτρια συνάρτηση και τη σχέση της με τις αριθμητικές συναρτήσεις. Με αυτό τον τρόπο συνδέονται τα δύο μέρη της διπλωματικής εργασίας και κλείνει η εργασία.The primary objective in my diploma thesis is to make an introduction to Arithmetic Functions and to Riemann’s Zeta Function. The first chapter is introductory and contains the basic definitions and theorems in Number Theory. The second chapter is about the basic arithmetic functions, such as the function of Euler and Mobius. The third chapter focuses on functions π(x) and li(x), so we can study the distribution of prime numbers on the set of real numbers. However, the first three chapters concern Number Theory, unlike the last three chapters, where we deal with Analytic Number Theory, by introducing the Riemann’s Zeta function and the Riemann Hypothesis. Specifically, the forth chapter focuses on the basic points of Riemann’s paper “On the Number of Primes Less than a Given Magnitude” and we compute some arithmetic values for the Riemann Zeta Function. Moreover, the objective of the fifth chapter is the Riemann Hypothesis and the proof of a sufficient condition of the Hypothesis. In conclusion, in the sixth chapter, we study the Zeta Function as a generating function and we connect it to the arithmetic functions.Κοσμάς Ν. Μάντση

Bank Evaluation with the CAMELS methodology and Stress Testing: The case of Greek National Bank

40 σ.Εθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Μαθηματική Προτυποποίηση σε Σύγχρονες Τεχνολογίες στην Οικονομία”Καθώς η παγκόσμια οικονομία καλείται να αντιμετωπίσει τις συνέπειες της κρίσης, που ξέσπασε στις Ηνωμένες Πολιτείες Αμερικής (Η.Π.Α.) και εξαπλώθηκε παγκοσμίως και στον Ευρωπαϊκό χώρο, γίνεται ολοένα και πιο σαφές πως ο παγκόσμιος τραπεζικός τομέας έχει τρωτά σημεία και αδυναμίες, που μπορούν να έχουν καταστροφικές συνέπειες. Στον απόηχο της κρίσης φάνηκε η αδυναμία των ελεγκτικών μηχανισμών να προβλέψουν και να αντιμετωπίσουν ένα οικονομικό «κραχ». Έτσι, μεγάλο είναι το ενδιαφέρον στην αναβάθμιση των υπαρχόντων ρυθμιστικών πλαισίων αλλά και στην ανάπτυξη νέων εργαλείων αξιολόγησης των χρηματοπιστωτικών ιδρυμάτων καθώς και πρόβλεψης της συμπεριφοράς τους σε περιόδους ακραίων οικονομικών μεταβολών. Ένα σύγχρονο εργαλείο στα χέρια των ελεγκτικών μηχανισμών, καθώς και των τραπεζιτών, είναι τα λεγόμενα stress tests. Στην παρούσα πτυχιακή εργασία σκοπός μας είναι να μελετήσουμε τα βασικά stress test ως εργαλεία αξιολόγησης και πρόβλεψης για την λειτουργία, την πορεία και την φερεγγυότητα μιας τράπεζας. Στο πρώτο κεφάλαιο παρουσιάζεται η εξέλιξη της κρίσης από τις ΗΠΑ στην Ευρώπη και στην συνέχεια αναλύεται η περίπτωση της Ελλάδα. Στο δεύτερο κεφάλαιο μελετάται το ελληνικό τραπεζικό σύστημα και πως αυτό επηρεάστηκε από την κρίση χρέους. Στο τρίτο κεφάλαιο μελετάται και αξιολογούμε την Εθνική Τράπεζα της Ελλάδος (E.T.E.) με την βοήθεια τους συστήματος CAMELS. Στο τέταρτο κεφάλαιο παρουσιάζεται και αξιολογείται η συστημική τράπεζα Ε.Τ.Ε (Εθνική Τράπεζα Ελλάδος) με την βοήθεια των stress test. Στο πέμπτο κεφάλαιο σχολιάζονται τα αποτελέσματα και προτείνονται τρόποι αντιμετώπισης των κινδύνων. +As the global financial system tried to deal with the consequences of the financial crisis, which started in the USA and spread to the rest of the world, it became more than clear that the bank sector suffered from systemic weaknesses and vulnerabilities. In the wake of the crisis everyone understood the weakness of supervisory authorities to predict and cope with a financial "crash". So there is an interesting growing in upgrading existing regulatory frameworks and develop new, in order to predict the behavior of financial institutions in times of extreme macroeconomic changes. A modern tool in the hands of authorities and banks are the so-called stress tests. In this paper we try to combine traditional tools, as the CAMELS methodology, with more modern, as stress testing, to evaluate the credit and liquidity risk of a systemic Greek bank, National Bank of Greece (NBG). The first two chapters we analyze the macroeconomic environment and the bank system. In the third chapter we present the CAMELS evaluating system and its application on NBG. In the fourth chapter we study basic stress testing methodologies and we apply them in NBG. In the final chapter we give a conclude evaluation of the bank and present the challenges that the bank has to deal with in the future.Κοσμάς Ν. Μάντσή

Climate Feedbacks in Response to Changes in Obliquity

Abstract The feedbacks involved in the response of climate to a reduction of Earth’s obliquity are investigated in the GFDL Climate Model version 2.1 (CM2.1). A reduction in obliquity increases the meridional gradient of the annual mean insolation, causing a strengthening of the atmospheric and ocean circulation that transports more heat poleward. The heat transport does not balance the direct obliquity forcing completely, and additional local radiative fluxes are required to explain the change in the equilibrium energy budget. The surface temperature generally increases at low latitudes and decreases at high latitudes following the change in the insolation. However, in some areas, the sign of the temperature change is opposite of the forcing, indicating the strong influence of feedbacks. These feedbacks are also responsible for a decrease in the global mean temperature despite that the change in the global mean insolation is close to zero. The processes responsible for these changes are increases in the ice fraction at high latitudes and the global cloud fraction—both of which reduce the absorbed solar radiation. A reduction in the global greenhouse trapping, due to changes in the distribution of the water vapor content of the atmosphere as well as a change in the lapse rate, has an additional cooling effect. Among these feedbacks, clouds and the lapse rate have the larger contribution, with water vapor and surface albedo having a smaller effect. The implications of the findings presented here for interpretation of obliquity cycles in the paleoclimate record are discussed

Mechanisms of Mid-Holocene Precipitation Change in the South Pacific Convergence Zone

International audienc

Precessional Cycles and Their Influence on the North Pacific and North Atlantic Summer Anticyclones

Abstract The response of the Northern Hemisphere summer anticyclones to a change in the timing of perihelion is investigated using the GFDL Climate Model version 2.1 (CM2.1). The orbital forcing consists of changes in the seasonal cycle of the top-of-atmosphere insolation as the perihelion shifts from the Northern Hemisphere winter to the Northern Hemisphere summer solstice. The North Pacific summer anticyclone experiences a large strengthening as well as a northward and westward expansion. The North Atlantic subtropical high experiences a smaller change that consists of a slight westward expansion but little change in strength. Experiments with a primitive equation atmospheric model show that these changes represent the circulation response to changes in the diabatic heating, both local and remotely. The remote diabatic forcing is associated with changes in the Southeast Asian and African summer monsoons, and the local forcing is dominated by a combined effect of a change in low clouds and local precipitation