Abstract
The dominant patterns of the intraseasonal surface air temperature (SAT) over the Contiguous United States (CONUS) during wintertime (December–March) are investigated in the period of 1979–2014 in the present study. The first two leading modes revealed by the empirical orthogonal function (EOF) analysis sufficiently represent the patterns by explaining nearly 80% of the total variance. The EOF1 mode has a monopole pattern with large loadings in the central-north CONUS, while the EOF2 has a dipole pattern with loadings of opposing signs in the northwestern and southeastern CONUS. The monopole pattern of the EOF1 has a significant lead-lag correlation with the EOF2 dipole pattern. These dominant patterns are traced back to the convective anomalies of the Madden–Julian Oscillation (MJO) which has eight phases characterized by the revised real-time multivariate MJO index. The connections are established dynamically by Rossby wave trains propagating from the MJO convections to North America. Specifically, the monopole warming (EOF1) is well developed by lagging about two pentads from enhanced MJO convections during phase 4 over the Maritime Continent as the wave sources. The dipole pattern (EOF2) of “warm-West/cool-East” starts to develop during MJO phase 7 with enhanced convections over the Western Pacific and reaches a maximum about two pentads later. Such dynamic lead-lag relationships on intraseasonal time scales are revealed by both diagnostic analysis and numerical experiments using a linear baroclinic model, which are potentially useful for predicting intraseasonal variations in SAT over the CONUS.
Highlights
Subseasonal (~ 5–120 days) variations of surface air temperature (SAT) is closely related to warm and cold events over the Contiguous United States (CONUS) in wintertime (December–March; DFJM) to impact economy, water resources and society
Because the dominant patterns of the 5–20-day filtered SAT do not have a significant relationship with Madden–Julian Oscillation (MJO) convection in terms of Rossby wave fluxes, the present study focuses on the characteristics of the 20–100-day filtered SAT and their relationships with MJO activities
These modes are associated with mid-latitude atmospheric circulation anomalies in response to tropical MJO convections through Rossby wave flux transport
Summary
Subseasonal (~ 5–120 days) variations of surface air temperature (SAT) is closely related to warm and cold events over the Contiguous United States (CONUS) in wintertime (December–March; DFJM) to impact economy, water resources and society. Strong MJO convection generates and amplifies extratropical Rossby waves and modulates teleconnections such as the Pacific-North American (PNA) pattern (Mori and Watanabe 2008), which is known to influence the intraseasonal variability of winter-time SAT over the CONUS (Schreck III et al 2013). The SAT variability at the shorter end (5–20 days) of subseasonal variations may be controlled by mid-latitude atmospheric thermos-dynamics such as planetary waves and baroclinic instability more than the remote MJO convections. This less-important relationship with the MJO was disclosed before (e.g., Lin 2015) and can be elucidated by nearly a null correlation (not shown).
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