This project studies the dynamics of the recurrent patterns of variability of the large-scale atmospheric cicrulation, including the Pacific/North American Pattern, the North Atlantic Oscillation, and the Northern and Southern Hemisphere Annular Modes. These patterns, sometimes referred to as teleconnection patterns, have a large influence on weather and climate at the earth's surface and thus have a variety of important impacts. The work has three goals: (1) to investigate the basic dynamical processes associated with teleconnection patterns; (2) to to understand the implications of teleconnection patterns on the predictability of atmospheric flow on synoptic to intraseasonal timescales; and 3) to understand the implications of teleconnection patterns for interannual to decadal atmospheric variability. The method of Self Organizing Maps is used to identify the teleconnection patterns, and a combination of statistical techniques and idealized numerical models will be used to understand the underlying dynamics of the patterns.
The work has relevance to society because the weather and climate fluctuations associated with teleconnection patterns are important for human activities and the natural world. The results of the work have the potential to improve medium- to long-range forecasting, and preliminary results are already in use at the National Oceanic and Atmospheric Administration's Climate Prediction Center. In addition, the work supports a graduate student, thereby providing support and training to the next generation of scientists.
Intellectual Merit of Research Atmospheric teleconnections are wave-like patterns that connect the weather from distant locations across the globe, with a typical horizontal scale of 3000-6000 km. Teleconnections vary over a broad range of time scales, from day-to-day weather to the much longer inter-decadal time scale (longer than 10 years). The aim of this grant proposal was to understand the physical processes that drive the day-to-day variation of teleconnections, and their inter-decadal variability. This research resulted in the publication of 17 peer-reviewed papers. Here, we provide some of the key findings reported in these papers on the linkage between convection (intense widespread thunderstorm activity) in the tropics and extratropical teleconnections, hence the midlatitude weather. The most prominent weather phenomenon in the tropics is known as the Madden-Julian Oscillation (MJO). This 30-60 day oscillation is characterized by increased convection over the Indian Ocean and decreased convection over the western Pacific Ocean, and vice versa. On time scales greater than one year, El Niño/Southern Oscillation (ENSO) tropical variability. ENSO is characterized by increased convection near Indonesia and reduced convection in the central tropical Pacific (the La Niña phase), and vice versa (the El Niño phase). Our findings show that there has been an interdecadal trend over that past 30 years toward more MJO events with intense convection over Indonesia, and a trend toward more La Niña-like convection. The impact of this MJO and La Niña trend has been to excite teleconnections that warm the Arctic . We refer to this tropical driving of a warmer Arctic as the Tropically Excited Arctic warMing mechanism (TEAM) mechanism (Fig. 1). Our findings raise the possibility that the majority of the Arctic Amplification, i.e., the Arctic being the region of most rapid warming, could be driven by tropical convection. Detailed analysis reveals that the Arctic warming during the cold season is realized through the teleconnection patterns transporting additional heat and water vapor into the Arctic, with the latter being more important. An important impact of adding water vapor into the Arctic is the warming of the surface through the formation of clouds which are effective at emitting infrared radiation downward. This indicates that the Arctic temperature can be better predicted by further improving our understanding of the mechanism that excites the teleconnections. We also investigated the impact of increased tropical convection (a signal of global warming) and declining stratospheric ozone on the inter-decadal poleward shift of the atmospheric jets in the Southern Hemisphere. Understanding the underlying mechanism is important because the poleward jet shift influences long-term shifts in regional precipitation and ocean circulation. By examining the inter-decadal trend in teleconnections, this research showed that the inter-decadal trend in both tropical convection and stratospheric ozone drives this jet shift, with the former having about twice the impact of the latter. A similar investigation was performed to examine the mechanisms that drive the inter-decadal jet trend in the Northern Hemisphere. We found that tropical convection also drives a poleward jet shift, but that the loss in sea ice in the Arctic has the opposite influence (Fig. 2). The tropical convection effect dominated until the late 1990s, driving the jet poleward, but since then the sea ice effect has been stalling the poleward shift. Other findings in this research showed that (1) MJO tropical convection is associated with sudden stratospheric warming events (when the temperature about 40 km above the Earth's surface decreases by about 40C within several days), (2) ENSO influences the frequency of excitation of Southern Hemispheric teleconnections, (3) an inter-decadal trend in MJO convection is a likely contributor to a warming over Antarctica, (4) the linkage between North Atlantic teleconnections and tropical convection is often manifested through the excitation of a teleconnection pattern comprised of five high and lows across the Northern Hemisphere, (5) the impact of tropical convection on the North Pacific and North America is greatest when it coincides with a teleconnection pattern over eastern Asia, and (6) predictability in midlatitudes becomes greater when the MJO is active. Broader Impact of Research The broader impact of the research includes its application to medium-range (2 to 4 week) weather forecasting and to climate change research. For medium-range forecasting, this research has shown that taking into the MJO has the potential to substantially improve forecasts in the 2-4 week range, a time period which until the present has not been amenable to useful forecasts. For the study of climate change, an important implication of our research is the need for climate models to improve their representation of convection in the tropics. Our findings suggest that this particular improvement has the potential to lead to much better simulations of the climate change response to global warming. This research activity also contributed to educating a PhD student and two undergraduate students.
