For decades, scientists have observed the phenomena known as El Niño and La Niña. Both significantly impact the global climate and both pose a puzzle to scientists since they’re not completely understood. Now, a new study clarifies some of the obscurity surrounding El Niño and La Niña, which together are called the El Niño Southern Oscillation (ENSO). This new study examines ENSO frequency asymmetry during the phases of the Pacific Decadal Oscillation (PDO), a climate pattern in the North Pacific.