The Madden-Julian oscillation (MJO), also known as the 30-60day oscillation, is the strongest of the intraseasonal climate oscillations in the tropics and has significant derivative effects on extratropical circulation and intraseasonal climate. It has recently been shown that the stratospheric quasi-biennial oscillation (QBO) modulates the amplitude of the boreal winter MJO such that MJO amplitudes are larger on average during the easterly phase (QBOE) than during the westerly phase (QBOW). A major possible mechanism is the decrease in static stability in the lowermost stratosphere under QBOE conditions resulting from relative upwelling associated with the QBO-induced meridional circulation. Here evidence is presented that tropical upwelling changes related to the 11year solar cycle also modulate the boreal winter MJO. Based on 37.3years of MJO amplitude data, the largest amplitudes and occurrence rates, and the weakest static stabilities in the tropical lower stratosphere, occur during the QBOE phase under solar minimum (SMIN) conditions while the smallest amplitudes and strongest static stabilities occur during the QBOW phase under solar maximum (SMAX) conditions. Conversely, when the QBO and solar forcings are opposed (QBOW/SMIN and QBOE/SMAX), the difference in occurrence rates becomes statistically insignificant. During the coming solar minimum, at least one additional winter in the QBOE/SMIN category should occur (possibly as early as 2017/2018) during which especially large MJO amplitudes are expected and an initial test of these results will be possible. Plain Language Summary An ongoing issue in climate science is the extent to which upper atmospheric processes, including solar forcing, can influence tropospheric climate. It has recently been shown that an internal oscillation of the stratosphere, the quasi-biennial oscillation, can modulate the amplitude and occurrence rate of intraseasonal climate oscillations in the tropical Pacific during northern winter. These intraseasonal oscillations, the most important of which is the 30-60day Madden-Julian oscillation, have significant derivative effects on climate outside of the tropics, including impacts on rainfall events over the continental United States. Here evidence is presented that the amplitude of the Madden-Julian oscillation during northern winter is also modulated by the 11year solar cycle. The modulation is such that amplitudes and occurrence rates are largest under solar minimum conditions when the quasi-biennial oscillation is in its easterly phase and smallest under solar maximum conditions when the quasi-biennial oscillation is in its westerly phase. However, the available time record (37.3years of satellite measurements) is limited. During the coming solar minimum, at least one additional winter in the solar minimum/easterly category should occur (possibly as early as 2017/2018) during which larger-than-average amplitudes are expected and an initial test of the proposed relationship will be possible.

The Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) is a truck-mounted C-band, Doppler radar that was deployed during the Dynamics of the Madden-Julian Oscillation (DYNAMO) / Cooperative Indian Ocean Experiment on interseasonal variability in the year 2011 (CINDY2011) campaign on Addu Atoll, Maldives. One of SMART-R’s objectives was to provide continuous volume scans of precipitating clouds during all phases of the Madden-Julian Oscillation (MJO) for the full duration of the campaign. Data from SMART-R is available for 2 October 2011 through 9 February 2012. Every 10 minutes a full volume scan was produced, which was subsequently run through quality control algorithms that, among other filters, performed a calibration correction, noise filtering, and an attenuation correction. It was observed that data from SMART-R appeared to be slanted towards the WNW, and after analysis, a 0.75◦ tilt correction was applied towards azimuth 285◦. The data was then converted into Cartesian coordinates and an additional noise filter was applied. NETCDF files with radial velocities and corrected reflectivity were produced. From the reflectivity observations, a suite of products including rain maps, echo- top heights and convective/stratiform separations were produced. A modified version of the convective/stratiform separation was developed in an attempt to classify shallow and weak convection more correctly. The modified algorithm utilizes an isolation parameter set to 10 km to the north, south, east, and west, a 10-dBz echo-top height threshold set to 9 km, and a 16-dBz reflectivity threshold at 3 km to ensure only isolated, shallow, and weak rain originally classified as stratiform, is reclassified as convection. Analyses of these products clearly suggest two MJO events occurring in October and November as indicated by the Wheeler and Hendon Multivariate MJO index. While stratiform rain almost always encompassed a larger area of the radar domain, convective rain was the larger producer of rain with the exception of active MJO periods. In addition, echo-top height counts are observed to increase in both vertical structure and frequency as the MJO initiates and becomes active over the radar domain. Possible connections are also made between echo-top height data and humidity retrievals from soundings launched on Addu Atoll. It appears that during MJO initiation, convective echo tops lead the moistening of the mid troposphere, while during suppressed phases, the convective echo tops lag behind the moistening of the mid troposphere. Wind shear also appears to be weaker during an active MJO event, and increase as the active MJO exits the region. From these observations, as well as other rain statistics including the diurnal cycle, indicators for a localized MJO index are proposed that are based on local radar and sounding data, rather than satellite and reanalysis observations of wind and outgoing long-wave radiation.

A census of 24 coupled (CMIP) and 13 uncoupled (AMIP) models from the Intergovernmental Panel on Climate Change (IPCC) fourth assessment report (AR4) were compared with observations and reanalysis to show varied ability of the models to simulate Caribbean precipitation and mechanisms related to precipitation in the region. Not only were errors seen in the annual mean, with CMIP models underestimating both rainfall and sea surface temperature (SST) and AMIP models overestimating rainfall, the annual cycle was also incorrect. Large overestimates of precipitation at all SSTs (and particularly above 28 degrees C) and at vertical circulations less than -10 hPa/day (the deep convective regime) were inherent in the atmospheric models with models using spectral type convective parameterizations performing best. In coupled models, however, errors in the frequency of occurrence of SSTs (the distribution is cold biased) and deep convective vertical circulations (reduced frequency) lead to an underestimation of Caribbean mean precipitation. On daily timescales, the models were shown to produce too frequent light rainfall amounts (especially less than 1 mm/day) and dry extremes and too few heavy rainfall amounts and wet extremes. The simulation of the mid-summer drought (MSD) proved a challenge for the models, despite their ability to produce a Caribbean low-level jet (CLLJ) in the correct location. Errors in the CLLJ, such as too strong magnitude and weak semi-annual cycle, were worse in the CMIP models and were attributed to problems with the location and seasonal evolution of the North Atlantic subtropical high (NASH) in both CMIP and AMIP models. Despite these discrepancies between models and observations, the ability of the models to simulate the correlation between the CLLJ and precipitation varied based on season and region, with the connection with United States precipitation particularly problematic in the AMIP simulations. An observational study of intraseasonal precipitation in the Caribbean showed an explicit connection between the Madden-Julian oscillation (MJO) and Caribbean precipitation for the first time. Precipitation anomalies up to 50 percent above (below) the annual mean are observed in phases 1 and 2 (5 and 6) of the MJO and are related to changes in the CLLJ, that is also modulated by the MJO. Considerable progress has been made on identifying both problems and successes in the simulation of Caribbean climate in general circulation models, but many areas still require investigation.

The intraseasonal Oscillation (ISO) is a very strong and coherent mode of variability observed in the Earths climate. Rainfall variability in the intraseasonal timescale is particularly strong in the Tropics and it directly interacts with the South Asian monsoon during boreal summer and with the Australian monsoon during winter. A detailed analysis of the horizontal and vertical structure of the ISO during both summer and winter is presented in this work considering the coupled ocean-atmosphere system. In addition, the role of the intraseasonal variability of the Southeast Asian monsoon is studied in detail. From the applications point of view, the intraseasonal time scale is arguably the most important period of variability. However, extended forecasting of intraseasonal activity has proven to be a difficult task for the state of the art numerical models. In order to improve the forecasts of the ISO activity over the Southeast Asian monsoon region, a physically based empirical scheme was designed. The scheme uses wavelet banding to separate the predictand and predictors into physically significant bands where linear regression followed by recombination of the bands is used to generate the forecast. Results of the empirical scheme suggest that isolating the evolution of the intraseasonal signal from higher frequency variability and noise improve the skill of the prediction. The hypothesis is that a similar phenomenon occurs in numerical models: The strong intraseasonal signal is eroded by high frequency errors due to the model parameterizations, especially in convection. To evaluate the hypothesis, a coupled ocean-atmosphere model was run in ensemble mode for 30 day periods initialized daily for 20 days before to 20 days after major intraseasonal oscillations, allowing the examination of the skill of the model relative to the phase of the oscillation. The results, which confirm the previous hypothesis, represent well the observations for about 7 days after which the magnitude of the errors is greater than the signal itself. An integration scheme was developed for the coupled ocean-atmosphere general circulation model in order to mimic the philosophy of the empirical scheme and use for 30-day forecasts. The propagation features associated to ISO activity are improved.

Intraseasonal oscillations (ISOs) are important elements of the tropical climate with time-scales of 20-80 day. The ISO is poorly simulated and predicted by numerical models. This work presents a joint diagnostic and modeling study of the ISO that examines the hypothesis that local coupling between the ocean and the atmosphere is essential to the existence and evolution of the ISO in the Indo-Pacific warm pool region. Low-level moistening during the transition phase preconditions the atmosphere for deep convection. The vertical structure of ISO from the ECMWF coupled model during different phases of the oscillation as well as the skill of the model in simulating the processes that occur during the transition phase were studied. The forecast skill of the vertical structure associated with the ISO is greater for winter than for summer events. Predictability of the convective period is poor when initialized before the transitional phase. When initialized within the transition period including lower tropospheric moistening, predictability increases substantially, although the model parameterizations appears to trigger convection quickly without allowing an adequate buildup of CAPE during the transition. The model tends to simulate a more stable atmosphere compared to data, limiting the production of deep convective events. Two different one-dimensional coupled models are used to analyze the role of local ocean-atmosphere coupling in generating ISO. The ocean component is a one-dimensional mixed layer model. In the first model the atmospheric component corresponds to the SCCM. Results suggest that convection in the model tends to be "overactive," inhibiting development of lower frequency oscillations in the atmosphere. In the second case, the atmospheric component is a semi-empirical model that allows reproducing the coupled ISO over long integration periods including only local mechanisms. In the semi-empirical scheme the rate of change of atmospheric variables is statistically related to changes in SST. The stable state of this model is a quasi-periodic oscillation with a time scale between 25 and 80 days that matches well the observed ISO. Results suggest that the period of the oscillation depends on the characteristics of the ocean mixed layer, with a higher frequency oscillation for a shallow mixed layer.

The Madden-Julian Oscillation (MJO) and the Convectively Coupled Equatorial Waves (CCEW) are coherent structures of convection and various large-scale fields. These phenomena are not well understood, despite their importance to the tropical climate. A toy model of the CCEW consisting of a pair of shallow water wave modes coupled by a simple convective parameterization is considered. The linear behavior of the system is analyzed, showing a growth spectrum that is similar to the spectrum that is observed. To explore the processes involved in propagation and maintenance of the MJO disturbance, we analyze the MSE budget of the disturbance within a numerical model. In an idealized experiment, the column-integrated long-wave heating is the only significant source of column-integrated MSE acting to maintain the MJO-like anomaly balanced against the combination of column-integrated horizontal and vertical advection of MSE and Latent Heat Flux. Eastward propagation of the MJO-like disturbance is associated with MSE generated by both column-integrated horizontal and vertical advection of MSE, with the column long-wave heating generating MSE that retards the propagation. The contribution to the eastward propagation by the column-integrated horizontal advection term is dominated by meridional advection of MSE by anomalous synoptic eddies caused by the suppression of eddy activity ahead of the MJO convection. This suppression is linked to the barotropic conversion mechanism; with the gradients of the low frequency wind experienced by the synoptic eddies within the MJO envelope acting to modulate the Eddy Kinetic Energy. The meridional eddy advection’s contribution to poleward propagation is dominated by the mean state’s (meridionally varying) eddy activity acting on the anomalous MSE gradients associated with the MJO. In a follow-up experiment, the variations in the propagation speed of MJO with variations in the imposed SST distribution are seen to be driven by the variations in meridional advection of the mean MSE profile by the MJO-related winds, which are in turn dominated by the variations in the mean MSE profile due to the variations of the SST. A brief investigation of the MSE budget for a more realistic case shows an increase in the MSE sink due to meridional advection as the MJO progresses from genesis over the Indian Ocean to decay in the central Pacific. The increase in this sink appears to be the cause of MJO’s demise. / Physics

This thesis considers atmospheric superrotation, a state of westerly equatorial winds which must be maintained by up-gradient eddy momentum fluxes. Superrotation has appeared in simulations of warm climates that generate enhanced Madden-Julian Oscillation (MJO)-like variability. This led to hypotheses that the warmer atmospheres of the early Pliocene and Eocene may have been superrotating, and that the phenomenon may be relevant to future climate projections. / Earth and Planetary Sciences

Over the 1951-2009 time period, 47% of all tropical systems in the Atlantic Basin transitioned to post-tropical storms. These storms are capable of producing hurricaneforce winds, torrential, flooding rains and storm surge that floods coastal areas. This study adds to previous climatological work by completing a case-study of Hurricane Ike (2008) and examining how teleconnections such as the El Niño Southern Oscillation (ENSO), the Madden-Julian Oscillation (MJO), the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO) contribute to the strength of a transitioning post-tropical storm. T-tests performed show strong statistical relationships between an increase (decrease) in post-tropical storm frequency and warm PDO – La Niña (cold PDO – La Niña), cold PDO – ENSO neutral (warm PDO – ENSO neutral), and warm (cold) AMO conditions. Moreover, nearly significant results were found for the same increase (decrease) and La Niña seasons since (pre) 1980 and for cold (warm) PDO conditions. Modeling the MJO suggests that increased (decreased) relative humidity associated with the wet (dry) phase could increase (decrease) precipitation output from the storm and decrease (increase) forward speed of the storm, decreasing (increasing) wind speeds observed at the surface.

L'Oscillation de Madden-Julian (MJO) est le principal mode de variabilité intrasaisonnière (20-90 jours) dans les tropiques. La MJO se caractérise par une perturbation de la dynamique de grande échelle et des précipitations se propageant principalement vers l'Est à une vitesse 5 m/s à l'équateur, et aussi vers le Nord en été boréal. Malgré son importance, cette perturbation est mal reproduite dans les modèles de circulation générale (MCG). Ce défaut des MCGs a été attribué en partie à un déclenchement trop fréquent du schéma de convection profonde du modèle qui préserverait trop la stabilité statique dans les tropiques et empêcherait la formation de fortes perturbations organisées de la convection et le développement de la réponse dynamique de grande échelle qui en résulte. Cette thèse a donc pour objectif d'étudier l'impact de l'inhibition de la convection sur l'état moyen et la variabilité tropicale dans le MCG LMDZ. Pour cela, deux paramètres - la fermeture et l'entraînement - d'un même schéma convectif, qui étaient initialement basés sur le profil vertical de la convergence d'humidité, ont été modifiés. Le taux entraînement du schéma modifié est proportionnel à l'humidité relative de l'environnement. Des études de sensibilité dans un modèle unidimensionnel montrent que, par rapport au schéma initial, cette modification a pour effet d'inhiber la convection dans une atmosphère sèche et de la favoriser en atmosphère humide, les précipitations deviennent donc plus rares et plus intenses. Avec ce nouvel entraînement, les simulations globales du modèle atmosphérique LMDZ présentent effectivement une plus forte variabilité tropicale de la dynamique et des précipitations à toutes les échelles de temps. Le cycle saisonnier des précipitations est aussi mieux reproduit ainsi que la propagation vers l'Est et vers le Nord des perturbations intrasaisonnières. L'ajout d'une fermeture en CAPE modifie peu ce comportement, mais accélère légèrement la propagation vers le Nord et vers l'Est de la MJO, et donne un état moyen et une variabilité un peu plus réaliste dans les tropiques. / The Madden-Julian oscillation (MJO) is the dominant mode of the Tropical intraseasonal variability. The MJO is characterized by a wide region of rain perturbation propagating eastward along the equator at a speed of about 5 m/s, from the Indian Ocean to the Central Pacific with a period of 20-90 days. Its boreal summer counterpart, referred as the Boreal Summer Intraseasonal Oscillation (BSISO), propagates both eastward and northward. The LMDZ atmospheric global circulation model (GCM), like most climate models, still has difficulties to represent correctly these two modes of tropical intraseasonal variability. One of the causes of this deficiency has been attributed to the lack of intense rain events organized at large scale. If the convection is triggered too easily in a GCM, the convective instability cannot accumulate in a column and trigger an organized convection at a basin scale. This thesis consisted in testing the role of several ways of inhibiting the convection in the simulation of the MJO in the LMDZ GCM. For this purpose, we have first evaluated the impact of entrainment and closure assumptions of the convective scheme on the structure of the atmospheric column using a 1D model and on the tropical variability using the LMDZ GCM. An entrainment that is dependant on the relative humidity of the environment has been implemented in the Tiedtke (1989) convective scheme. The new entrainment inhibits the convection in a dry environment and promotes the convection in a humid environment. This favours strong ascents and subsidence and consequently increases the variance of precipitation at all time scales. The new closure, based on the convective instability (CAPE) instead of the moisture convergence, decreases the occurrence of strong rain events and gives a more realistic mean state. The east-west ratio is increased at intraseasonal scale but it is not sufficient to organise correctly the eastward propagation related to the MJO. The seasonal cycle of the tropical precipitation over India is however improved. The LMDZ is able to reproduce correctly the signature of the northward propagation of the BSISO with a period of 40 days in Boreal Summer. We have shown, with A Local Mode Analysis (LMA), that the new entrainment produces intraseasonal perturbations related to the northward propagation of the BSISO which are more reproducible and more organized on the northern Indo-Pacific region.

Humans’ ability to combat heat stress through sweat-based evaporative cooling is modulated by air temperature and humidity, rendering human health highly sensitive to humid heat extremes. While the field of climate science has studied extreme dry bulb temperatures for decades, exploration of the physical drivers producing extreme humid heat is nascent in comparison. Two major areas of development for the evolving field are: 1) improving understanding of the local drivers of extreme humid heat, and 2) collating a set of universal physical mechanisms which generate humid heat extremes across the planet. The four chapters of this dissertation together advance each of these goals. Chapter 1 relates the occurrence of extreme humid heat in the Persian Gulf and South Asia to two related modes of intraseasonal climate variability, namely the Madden–Julian oscillation (MJO) and the boreal summer intraseasonal oscillation (BSISO). Wet bulb temperatures (Tw) sufficiently high to impact human health are found to be almost twice as likely during certain oscillation phases than in others. Humid heat anomalies in each region are driven by distinct local circulation patterns and variations in moisture. Chapter 2 evaluates the influence of monsoon onset and subseasonal precipitation variability on the occurrence of extreme Tw across South Asia. Extreme Tw events often occur on rainy days during the monsoon season. However, the influence of precipitation on Tw varies with the background specific humidity climatology. In climatologically drier areas, positive Tw anomalies tend to occur when precipitation increases due to either early onset or wet spells during the monsoon. In contrast, in climatologically humid areas, positive Tw anomalies occur during periods of suppressed precipitation, including delayed onset and dry spells during the monsoon. Chapter 3 analyzes the dynamics of the record-breaking extreme heat event in Rio de Janeiro in November 2023. The heatwave was associated with persistent atmospheric blocking, potentially linked to developing El Niño conditions. Temperatures were intensified by declining soil moisture and elevated local sea surface temperatures, and the event was finally terminated due to the onset of precipitation. This chapter also evaluates the influence of climate change on the frequency of heat extremes, finding a significant increase in the frequency of high heat days throughout Brazil over the past four decades. Further, the frequency of spring heat extremes is expected to increase in the future, though highly dependent upon our future emissions pathway. Chapter 4 explores the combinations of temperature and humidity contributing to humid heat experienced across the globe. In addition to using traditional metrics, this chapter derives a novel variable named “stickiness,” which quantifies the relative contributions of temperature and specific humidity to a given Tw. Consistent across metrics, high magnitudes of Tw tend to occur in the presence of anomalously high moisture, with temperature anomalies of secondary importance. Nonetheless, there is a broad range of stickiness observed for a given Tw across moderate-to-high Tw thresholds relevant to socioeconomic impacts.