11:15 AM - 11:30 AM
[AOS19-03] Winter large-scale atmospheric circulation during La Niña events: A comparison between the first year and second year of events
Keywords:multiyear La Niña, teleconnection, cold air mass
The sea surface temperature (SST) distribution associated with ENSO generates large-scale atmospheric teleconnection patterns, such as the PNA pattern characterized by a wave-train from the central North Pacific to North America, and results in affecting the global climate. Recent studies have reported that there is a temporal asymmetry between El Niño and La Niña, with La Niña tending to last for multiple years compared to El Niño. It is suggested that atmospheric response would differ between the first year and the following year of the La Niña. However, the understanding has not been obtained yet. In this study, we conducted statistical analysis using observational and reanalysis datasets since 1950 and numerical experiments using the linear baroclinic model (LBM) to investigate characteristics of winter large-scale atmospheric circulation associated with La Niña.
First, we conducted a composite analysis using observational and reanalysis datasets. As a result, in the first year of La Niña, we detected enhanced convective activity over the western equatorial Pacific and the South China Sea, and low-pressure anomalies were distributed over Japan. A wave-train in geopotential height fields was observed from Japan to North America. This atmospheric pattern was not consistent with existing teleconnection patterns. Surface air temperature showed significant cooling around Japan, as previously pointed out in past works. We investigated the cold air mass distribution and indicated that a large amount of cold air flowing into Japan from the continent was a cause of the cold winter in Japan. We conducted the same analysis for the second year of La Niña. Results represented that convection in the western equatorial Pacific was not as active as in the first year. The large-scale pressure pattern indicated the wave-train from the central North Pacific to North America, consistent with the negative PNA pattern. Negative surface air temperature anomalies around Japan were obtained, but its amplitude was smaller than that of the first year, and it was not significant.
This study revealed the atmospheric responses attributable to La Niña events differ between the first and second years. Also, the heating sensitivity experiments by the LBM showed almost identical atmospheric responses.
First, we conducted a composite analysis using observational and reanalysis datasets. As a result, in the first year of La Niña, we detected enhanced convective activity over the western equatorial Pacific and the South China Sea, and low-pressure anomalies were distributed over Japan. A wave-train in geopotential height fields was observed from Japan to North America. This atmospheric pattern was not consistent with existing teleconnection patterns. Surface air temperature showed significant cooling around Japan, as previously pointed out in past works. We investigated the cold air mass distribution and indicated that a large amount of cold air flowing into Japan from the continent was a cause of the cold winter in Japan. We conducted the same analysis for the second year of La Niña. Results represented that convection in the western equatorial Pacific was not as active as in the first year. The large-scale pressure pattern indicated the wave-train from the central North Pacific to North America, consistent with the negative PNA pattern. Negative surface air temperature anomalies around Japan were obtained, but its amplitude was smaller than that of the first year, and it was not significant.
This study revealed the atmospheric responses attributable to La Niña events differ between the first and second years. Also, the heating sensitivity experiments by the LBM showed almost identical atmospheric responses.