The Madden-Julian oscillation (MJO) has long been identified as an important source of midlatitude weather predictability on subseasonal-to-seasonal (S2S) timescales (∼2 weeks to 3 months) via its teleconnections to higher latitudes. Tropical convection associated with the MJO slowly propagates eastward in a quasiperiodic manner, taking ∼20-90 days to circumnavigate the globe and complete a cycle (Madden & Julian, 1971, 1972; Zhang, 2013). MJO activity can excite Rossby waves which propagate out of the tropics and into the midlatitudes, modifying the large-scale circulation and weather patterns. Because it can take 10-15 days for the teleconnection to reach the midlatitudes, knowing the state of the MJO today can provide information about the evolution of the midlatitude flow in the coming weeks. In fact, multiple studies have demonstrated that the MJO can be used to make skillful forecasts of weather up to 5 weeks in advance across the

The Madden-Julian oscillation (MJO) has long been identified as an important source of midlatitude weather predictability on subseasonal-to-seasonal (S2S) timescales (∼2 weeks to 3 months) via its teleconnections to higher latitudes. Tropical convection associated with the MJO slowly propagates eastward in a quasiperiodic manner, taking ∼20-90 days to circumnavigate the globe and complete a cycle (Madden & Julian, 1971, 1972; Zhang, 2013). MJO activity can excite Rossby waves which propagate out of the tropics and into the midlatitudes, modifying the large-scale circulation and weather patterns. Because it can take 10-15 days for the teleconnection to reach the midlatitudes, knowing the state of the MJO today can provide information about the evolution of the midlatitude flow in the coming weeks. In fact, multiple studies have demonstrated that the MJO can be used to make skillful forecasts of weather up to 5 weeks in advance across the

Because continuous meteorological observations across Antarctica did not start until the middle of the twentieth century, little is known about the full spatial pattern of pressure variability across the extratropical Southern Hemisphere (SH) in the early twentieth century, defined here as the period from 1905 to 1956. To fill this gap, this study analyzes pressure observations across the SH in conjunction with seasonal pressure reconstructions across Antarctica, which are based on observed station-to-station statistical relationships between pressure over Antarctica and the southern midlatitudes. Using this newly generated dataset, it is found that the early twentieth century is characterized by synchronous but opposite-signed pressure relationships between Antarctica and the SH midlatitudes, especially in austral summer and autumn. The synchronous pressure relationships are consistent with the southern annular mode, extending its well-known influence on SH extratropical pressure since 1957 into the early twentieth century. Apart from connections with the southern annular mode, regional and shorter-duration pressure trends are found to be associated with influences from tropical variability and potentially the zonal wavenumber 3 pattern. Although the reduced network of SH observations and Antarctic reconstruction captures the southern annular mode in the early twentieth century, reanalysis products show varying skill in reproducing trends and variability, especially over the oceans and high southern latitudes prior to 1957, which stresses the importance of continual efforts of historical data rescue in data-sparse regions to improve their quality.

Two distinct features of anthropogenic climate change, warming in the tropical upper troposphere and warming at the Arctic surface, have competing effects on the mid-latitude jet stream’s latitudinal position, often referred to as a “tug-of-war”. Studies that investigate the jet’s response to these thermal forcings show that it is sensitive to model type, season, initial atmospheric conditions, and the shape and magnitude of the forcing. Much of this past work focuses on studying a simulation’s response to external manipulation. In contrast, we explore the potential to train a convolutional neural network (CNN) on internal variability alone and then use it to examine possible nonlinear responses of the jet to tropospheric thermal forcing that more closely resemble anthropogenic climate change. Our approach leverages the idea behind the fluctuationdissipation theorem, which relates the internal variability of a system to its forced response but so far has been only used to quantify linear responses. We train a CNN on data from a long control run of the CESM dry dynamical core and show that it is able to skillfully predict the nonlinear response of the jet to sustained external forcing. The trained CNN provides a quick method for exploring the jet stream sensitivity to a wide range of tropospheric temperature tendencies and, considering that this method can likely be applied to any model with a long control run, could lend itself useful for early stage experiment design.

Because continuous meteorological observations across Antarctica did not start until the middle of the 20th century, little is known about the full spatial pattern of pressure variability across the extratropical Southern Hemisphere (SH) in the early 20th century, defined here as the period from 1905-1956. To fill this gap, this study analyzes pressure observations across the SH in conjunction with seasonal pressure reconstructions across Antarctica, which are based on observed station-to-station statistical relationships between pressure over Antarctica and the southern midlatitudes. Using this newly generated dataset, it is found that the early 20th century is characterized by synchronous, but opposite signed pressure relationships between Antarctica and the SH midlatitudes, especially in austral summer and autumn. The synchronous pressure relationships are consistent with the Southern Annular Mode, extending its well-known influence on SH extratropical pressure since 1957 into the early 20th century. Apart from connections with the Southern Annular Mode, regional and shorter-duration pressure trends are found to be associated with influences from tropical variability and potentially the zonal wavenumber three pattern. Although the reduced network of SH observations and Antarctic reconstruction capture the Southern Annular Mode in the early 20th century, reanalyses products show varying skill in reproducing trends and variability, especially over the oceans and high southern latitudes prior to 1957, which stresses the importance of continual efforts of historical data rescue in data sparse regions to improve their quality.