Add grenadine to shaker and muddle the plum pieces in it. Add tequila, lime juice, rose and ice and shake until cold. Strain either up into a chilled coupe or over crushed ice in a rocks glass. Double strain if you worry about plum bits in your glass.
An ounce of plum purée may be substituted for the half plum. If shaken with frozen plum purée rather than ice, the drink has a velvety mouthfeel almost like a frozen drink (and an absurdly rich red hue). If using purée, double-straining is a must.
Ross' El Niño calls for blanco tequila and strawberries. One day, I had neither and decided to double down on the drink's redness: red syrup and plum. It is the first tequila drink I actually enjoyed.
I assume Ross named the drink for the meteorological phenomenon. The phenomenon is named for the infant child of the Nativity—there's already a La Niña, so this is named in honor of the Blessed Mother, the Virgin of the Nativity.