It is no secret that post-rockers Sigur Rós make music that reflects their Icelandic roots: ethereally beautiful, pastoral, and momentous, yet glacial in its motions. The quartet distills the essence of their homeland into extended experimental records that occasionally pay lip service to classical and pop music, but nonetheless remain firmly rooted in their own world. The group never concede a shred of their perfectly-structured compositions for the sake of accessibility, and in fact, their lyrics are often sung in a gibberish language of the band’s invention.

Sigur Rós haven’t released an album since 2008’s “Með suð í eyrum við spilum endalaust,” and despite their indefinite hiatus, lead vocalist and guitarist Jón “Jónsi” Birgisson has stayed busy, releasing an album titled “Riceboy Sleeps” with his boyfriend, graphic artist Alex Somers, last year. Jónsi’s first solo effort, “Go,” features the instrumental arrangements of composer Nico Muhly, who has made a name for himself in popular music via contributions to Grizzly Bear’s “Veckatimest” and Antony and the Johnsons’ “Crying Light.” But despite Muhly’s classically-oriented input, “Go” is by far the poppiest, most accessible record Jónsi has ever released. It isn’t a bad change—“Go” retains most of the delicate beauty of Sigur Rós but drastically shifts the music’s emotional timbre. While Sigur Rós’ albums always seemed meant to be listened to on overcast winter days, “Go” is a record for springtime: a bright, hopeful statement intent on capturing some of the season’s natural vitality.

The first hint that this is a pop album at its heart comes at the very start: unlike nearly all of Sigur Rós’ more symphonic releases, the best song on “Go” is the first one. “Go Do” may back Jónsi’s familiar soaring falsetto with chiming percussion and orchestral flourishes, but it is nonetheless a consummate pop song—and a great one. From the opening, cheerfully syncopated vocal samples through the disconcertingly straightforward verse-chorus-verse structure, “Go Do” takes the listener on a compressed journey through the emotional high points of a seven- or eight-minute Sigur Rós track. It leaves out the gloom and heartache, however, instead crooning, “We should always know that we can do anything / Go do, you’ll know how to.”

The meat of the album occasionally gets caught up in the experimental flamboyance of Muhly’s compositions, but never strays far from the more immediate poppiness of “Go Do.” “Boy Lilikoi” features fluttering flutes and pounding snare drums which would feel perfectly at home in a symphony, and yet it feels far more down-to-earth than such instrumentals would suggest. The vocals and percussion, which are mixed unusually high throughout the record, dominate Muhly’s complex arrangement and contribute significantly to that unpretentious quality. “Go” is an album which is easy to appreciate on first listen, but its immediacy never detracts from the emotional rollercoaster Jónsi is so adept at creating.

Even at its most melancholic, “Go” is always more wistful than mournful. “Grow Till Tall,” the album’s longest track, is a meandering, spare number that constantly threatens to build to a crashing, desolate conclusion—in a manner reminiscent of many of Sigur Ros’ best songs—but never actually does so. Instead, it underpins Birgisson’s nostalgic wailing with a couple of whining violins and jittering electronics, ensuring lyrics like “You’ll really want to grow and grow till tall / They all, in the end, will fall” never sound fatalistic. Far from undermining the album’s generally buoyant mood, this emotional low point makes the more hopeful track that follows it, “Hengilás,” all the more cathartic.

It may seem limiting to constantly compare Jónsi to Sigur Rós, but Jónsi invites the association by making music that has so much in common with his band—his own heartrending falsetto, simple but perfectly arranged orchestral support, and an unabashedly emotional atmosphere—and then self-consciously moving away from much of what defines them. Where Sigur Rós were sprawling, Jónsi is concise. Where Sigur Rós were languorous, Jónsi is propulsive. Where Sigur Rós were grandiose, Jónsi is playful. “Go” still feels like an Icelandic record, but perhaps it reflects an Iceland feeling the effect of global warming. Jonsi has created an undoubtedly ambitious work that stretches the definition of “pop” to its very breaking point, but its unabashed optimism and the simple beauty of the music make it a thoroughly enthralling album.

—Staff writer Daniel K. Lakhdhir can be reached at daniel.lakhdhir@college.harvard.edu.