The Interacting Dynamics of Tropical and Extratropical Climate: Insights from Observations, and Low-order and General Circulation Models

Using methods from dynamical systems theory in observations, low-order and general circulation models (GCMs), this dissertation explores (a) the response of midlatitude jet and eddy energy to climate change and variability, and (b) variability in predictability of the first kind of the El Niño/Southern Oscillation (ENSO) phenomenon. First, an analysis framework inspired by the Lorenz-1984 model is developed to study the relationship of the probability structure of the North Atlantic jet stream and storm track (location and strength) with (a) hemispheric surface temperature gradients (equator-to-pole gradient and ocean-land contrast), and (b) ENSO. Both the equator-to-pole gradient and the ocean-land contrast are projected to decrease in response to greenhouse gas forcing. The shifts in the probability structure of jet and eddy energy in relation to decreasing surface temperature gradients are in the opposite direction than the shifts for El Niño forcing. However, in climate change projections, the El Niño-like tropical pacific warming dominates the response of the jet/eddy energy probability, resulting in a strengthening and equatorward shift of the subtropical jet. The response of the subpolar jet is separate (poleward shift and strengthening), indicating that the combined effect of the tropical and extratropical SST changes under strong greenhouse gas forcing may set up conditions for a separation of the jet stream in the North Atlantic. Then, ENSO predictability of the first kind is examined in observations and in pre-industrial model simulations, using local lyapunov exponents. Multidecadal variations in ENSO predictability are shown in a 2000-yr long simulation from the Geophysical Fluid Dynamics Laboratory (GFDL) CM2.1 model. The GCM is found to be less predictable than nature and than an intermediate model of the tropical Pacific (Zebiak-Cane model). Finally, it is shown that increased predictability is associated with a deeper thermocline in the west Pacific up to five years prior to the peak of the event, along with an earlier deepening of the thermocline in the east Pacific in the months preceding the peak. This dissertation therefore illustrates that the analysis of key features of tropical and extratropical climate in a physically meaningful "reduced space" can provide a focused interpretation of GCM projections for climate change and variability

Projections of climate extremes under potential climate change as represented by changing equator to pole temperature gradient and land ocean temperature contrast.

Under climate variability and anthropogenic forcing, the Equator-to-Pole Temperature Gradient (EPG) and the Ocean-Land Temperature Contrast (OLC) undergo systematic changes, which can be associated with the equatorial pacific circulation patterns via teleconnections, and with the Atlantic Meridional Overturning Circulation (AMOC) via ocean-atmosphere coupling. We couple the Lorenz ’84 atmospheric model, a Box AMOC model (after Roebber 1994), and an ENSO coupled ocean-atmosphere model (Tziperman et al, 1994) to explore the sensitivity of the strength, position and other statistics of the mid-latitude wind components to changes in the aforementioned systems and components. Sea ice and water balances are not explicitly modeled. We then develop and discuss projections of the changes in persistence, low frequency variability, and frequency of extremes in key climatic parameters, as specific climate changes, anticipated under anthropogenic forcing in the 21st century, are postulated

Surface Temperature Gradients as Diagnostic Indicators of Mid-latitude Circulation Dynamics.

Zonal and meridional surface temperature gradients are considered to be determinants of large-scale atmospheric circulation patterns. However, there has been limited investigation of these gradients as diagnostic aids. Here, the 20th century variability in the Northern Hemisphere Equator-to-Pole temperature Gradient (EPG) and the Ocean-Land temperature Contrast (OLC) is explored. A secular trend in decreasing EPG and OLC is noted. Decadal and interannual (ENSO-related) variations in the joint distribution of EPG and OLC are identified, hinting at multistable climate states that may be indigenous to the climate or due to changing boundary forcings. The NH circulation patterns for cases in the tails of the joint distribution of EPG and OLC are also seen to be different. Given this context, this paper extends past efforts to develop insights into jet stream dynamics using the Lorenz-1984 model, which is forced directly and only by EPG and OLC. The joint probability distribution of jet stream and eddy energy, conditional on EPG and OLC scenarios, is investigated. The scenarios correspond to (a) warmer vs. colder climate conditions, and (b) polarized ENSO phases. The latter scenario involves the use of a heuristic ENSO model to drive the Lorenz-1984 model, via a modulation of the EPG or the OLC. As with GCMs, the low-order model reveals that the response to El Niño forcing is not similar to an anthropogenic warming signature. The potential use of EPG and OLC as macro-level indicators of climate change and variability and for comparing results across GCMs and observations is indicated

Space-time structure of extreme precipitation in Europe over the last century: a climate perspective

Historical observations show a significant change of globe temperature distribution as a consequence of global warming. In the midlatitude , and specifically in Europe, annual and seasonal changes of the midlatitude climate driving variables as EPG ( equator pole gradient) and OLC ( ocean land contrast ) were recorded, show significant trends, as shown in fig.1 and 2 . As a consequence of these changes a spatio-temporal trends in extreme precipitation in Europe is expected. We analyze over a century of continuous rainfall data available from the ECA&D archive for spatio-temporal trends in extreme precipitation. The data base includes 515 stations with records longer than 100 years. For each station, we identify daily rainfall events in the winter 6 months (Oct-Mar) that exceed the 99th percentile of daily rainfall. An annual time series of the frequency of such events is created, as well as an annual time series of the average daily rainfall in these events. Space and time analyses of the variation of the frequency and intensity time series are then pursued using multivariate time and frequency domain (multi-taper method) methods. The key trends and organized spectral modes identified can be related to potential anthropogenic change and to well established climate indices (e.g., NAO, EAWR and SL). The simultaneous analysis of monotonic trends over the secular period and quasi -oscillatory phenomena is informative as to the attribution of changes in extreme precipitation over the region

Ch. 21 ENSO in a Changing Climate: Challenges, Paleo-Perspectives, and Outlook

The El Niño Southern Oscillation (ENSO) phenomenon is a dominant force driving year‐to‐year climate variability with ecological and socioeconomic impacts that reverberate around the globe. The complex processes that govern ENSO and its impacts have generated intense research over the past decades, reviewed in previous chapters: a better understanding of how ENSO responds to anthropogenic climate change requires effort in resolving how ENSO responds to and interacts with a multitude of factors such as weather‐scale phenomena, volcanic eruptions, orbital forcing, etc. This chapter highlights some key unresolved issues in ENSO, supplemented by analysis of paleoclimate data and past and future state‐of‐the‐art climate model simulations. First, paleo‐ENSO reconstructions indicate a weakening of ENSO variability accompanying a weaker seasonal cycle, albeit lacking a clear orbital signal. This apparent positive correlation between changes in the magnitude of the seasonal cycle and ENSO amplitude seems to hold in future greenhouse‐gas forcing scenarios. Yet the mechanisms behind this relationship remain unclear, as accelerated paleoclimate model simulations under orbital forcing show the opposite relationship, in accordance with the idea of frequency entrainment in nonlinear oscillatory systems. These results underscore another prominent unresolved question: is ENSO a nonlinear system exhibiting regime‐like behavior (internally generated or in response to external forcing), or is ENSO a stochastically forced linear system whose behavior is modulated by noise? The community’s efforts to answer this question face the limitations imposed by the short instrumental record. Paleoclimate reconstructions provide extensions of this record although there persist sampling issues and uncertainties surrounding the manifestation of the ENSO signal in hydroclimate records which are influenced by local and regional processes. In its conclusion, this chapter highlights recent research directions and underscores the need for sustained and improved observations, paleo‐proxy reconstructions, hierarchical climate modeling, theories, and collaboration across disciplines toward addressing the open ENSO questions.TRU

Climate impacts of the El Niño-Southern Oscillation on South America

International audienceThe climate of South America (SA) has long held an intimate connection with El Niño, historically describing anomalously warm sea-surface temperatures off the coastline of Peru. Indeed, throughout SA, precipitation and temperature exhibit a substantial, yet regionally diverse, relationship with the El Niño-Southern Oscillation (ENSO). For example, El Niño is typically accompanied by drought in the Amazon and north-eastern SA, but flooding in the tropical west coast and south-eastern SA, with marked socio-economic effects. In this Review, we synthesize the understanding of ENSO teleconnections to SA. Recent efforts have sought improved understanding of ocean-atmosphere processes that govern the impact, inter-event and decadal variability, and responses to anthropogenic warming. ENSO's impacts have been found to vary markedly, affected not only by ENSO diversity, but also by modes of variability within and outside of the Pacific. However, while the understanding of ENSO-SA relationships has improved, with implications for prediction and projection, uncertainty remains in regards to the robustness of the impacts, inter-basin climate interactions and interplay with greenhouse warming. A coordinated international effort is, therefore, needed to close the observational, theoretical and modelling gaps currently limiting progress, with specific efforts in extending palaeoclimate proxies further back in time, reducing systematic model errors and improving simulations of ENSO diversity and teleconnections

Changing El Niño–Southern Oscillation in a warming climate

Originating in the equatorial Pacific, the El Niño–Southern Oscillation (ENSO) has highly consequential global impacts, motivating the need to understand its responses to anthropogenic warming. In this Review, we synthesize advances in observed and projected changes of multiple aspects of ENSO, including the processes behind such changes. As in previous syntheses, there is an inter-model consensus of an increase in future ENSO rainfall variability. Now, however, it is apparent that models that best capture key ENSO dynamics also tend to project an increase in future ENSO sea surface temperature variability and, thereby, ENSO magnitude under greenhouse warming, as well as an eastward shift and intensification of ENSO-related atmospheric teleconnections — the Pacific–North American and Pacific–South American patterns. Such projected changes are consistent with palaeoclimate evidence of stronger ENSO variability since the 1950s compared with past centuries. The increase in ENSO variability, though underpinned by increased equatorial Pacific upper-ocean stratification, is strongly influenced by internal variability, raising issues about its quantifiability and detectability. Yet, ongoing coordinated community efforts and computational advances are enabling long-simulation, large-ensemble experiments and high-resolution modelling, offering encouraging prospects for alleviating model biases, incorporating fundamental dynamical processes and reducing uncertainties in projections. Key points Under anthropogenic warming, the majority of climate models project faster background warming in the eastern equatorial Pacific compared with the west. The observed equatorial Pacific surface warming pattern since 1980, though opposite to the projected faster warming in the equatorial eastern Pacific, is within the inter-model range in terms of sea surface temperature (SST) gradients and is subject to influence from internal variability. El Niño–Southern Oscillation (ENSO) rainfall responses in the equatorial Pacific are projected to intensify and shift eastward, leading to an eastward intensification of extratropical teleconnections. ENSO SST variability and extreme ENSO events are projected to increase under greenhouse warming, with a stronger inter-model consensus in CMIP6 compared with CMIP5. However, the time of emergence for ENSO SST variability is later than that for ENSO rainfall variability, opposite to that for mean SST versus mean rainfall. Future ENSO change is likely influenced by past variability, such that quantification of future ENSO in the only realization of the real world is challenging. Although there is no definitive relationship of ENSO variability with the mean zonal SST gradient or seasonal cycle, palaeoclimate records suggest a causal connection between vertical temperature stratification and ENSO strength, and a greater ENSO strength since the 1950s than in past centuries, supporting an emerging increase in ENSO variability under greenhouse warming