Confirmation for and Predictability of Distinct U.S. Impacts of El Niño Flavors

Whether distinct wintertime U.S. climate conditions exist for central-Pacific (CP) versus eastern-Pacific (EP) El Niño events is explored using atmospheric and coupled ocean–atmospheric models. Results using the former agree with most prior studies indicating different U.S. temperature and precipitation patterns associated with El Niño flavors. Causes are traced to equatorial rainfall sensitivity to both magnitudes and spatial patterns of sea surface temperatures (SSTs) distinguishing CP and EP cases. Warmer east equatorial Pacific Ocean SSTs during EP than CP events, specifically for strong EP cases, are responsible for greater east equatorial Pacific rainfall, which displaces tropospheric circulation anomalies eastward over the Pacific–North American region. Weak-amplitude EP cases and all CP events since 1980 fail to excite east equatorial Pacific rainfall, thus not initiating the dynamical chain of effects characterizing strong EP cases. Over the contiguous United States, the difference in tropospheric circulations between strong EP and CP events describes a cyclonic pattern that renders the former colder and wetter. Regional signals include notably colder western and warmer eastern U.S. surface temperatures during EP versus CP events, and higher southwestern and southeastern U.S. precipitation during EP events. We demonstrate the important result—new to studies of observed El Niño flavor impacts—that coupled models largely reproduce the sensitivities of atmospheric models. Confirmed hereby is the realism of prior estimates of El Niño flavor impacts that relied on atmospheric models alone. We further examine predictability of El Niño flavors using coupled forecasts, demonstrating that SST distinctions between CP and EP events and their diverse U.S. wintertime impacts are predictable at least a season in advance.

Simulating the Mutual Forcing of Anomalous High Southern Latitude Atmospheric Circulation by El Niño Flavors and the Southern Annular Mode*

Numerical simulations using the National Center for Atmospheric Research Community Atmosphere Model (CAM) are conducted based on tropical forcing of El Niño flavors. Though these events occur on a continuum, two general types are simulated based on sea surface temperature anomalies located in the central (CP) or eastern (EP) tropical Pacific. The goal is to assess whether CAM adequately represents the transient eddy dynamics associated with each of these El Niño flavors under different southern annular mode (SAM) regimes. CAM captures well the wide spatial and temporal variability associated with the SAM but only accurately simulates the impacts on atmospheric circulation in the high southern latitudes when the observed SAM phase is matched by the model. Composites of in-phase (El Niño–SAM−) and out-of-phase (El Niño–SAM+) events confirm a seasonal preference for in-phase (out of phase) events during December–February (DJF) [June–August (JJA)]. Modeled in-phase events for both EP (during DJF) and CP (during JJA) conditions support observations of anomalous equatorward momentum flux on the equatorward side of the eddy-driven jet, shifting this jet equatorward and consistent with the low phase of the SAM. Out-of-phase composites show that the El Niño–associated teleconnection to the high southern latitudes is strongly modulated by the SAM, as a strong eddy-driven jet is well maintained by high-latitude transient eddy convergence despite the tropical forcing. A regional perspective confirms that this interaction takes place primarily over the Pacific Ocean, with high-latitude circulation variability being a product of both tropical and high-latitude forcing.

El Niño Flavors and Their Simulated Impacts on Atmospheric Circulation in the High Southern Latitudes*

Two El Niño flavors have been defined based on whether warm sea surface temperature (SST) anomalies are located in the central or eastern tropical Pacific (CP or EP). This study further characterizes the impacts on atmospheric circulation in the high latitudes of the Southern Hemisphere associated with these types of El Niño events though a series of numerical simulations using the National Center for Atmospheric Research Community Atmosphere Model (CAM). Comparing results with the Interim ECMWF Re-Analysis (ERA-Interim), CAM simulates well the known changes to blocking over Australia and a southward shift in the subtropical jet stream across the eastern Pacific basin during CP events. More importantly for the high southern latitudes, CAM simulates a westward shift in upper-level divergence in the tropical Pacific, which causes the Pacific–South American stationary wave pattern to shift toward the west across the entire South Pacific. These changes to the Rossby wave source region impact the South Pacific convergence zone and jet streams and weaken the high-latitude blocking that is typically present in the Amundsen-Bellingshausen Seas during EP events. Anticyclonic flow becomes established farther west in the south central Pacific, modifying high-latitude heat and momentum fluxes across the South Pacific and South Atlantic associated with the ENSO–Antarctic dipole.