Posing for a New York photographer in 1880, Winslow Homer looks the epitome of a dapper man about town. Sporting a trim boater, a well-cultivated moustache and an expensive three-piece suit, he seems ready to cut a dash in the most stylish Manhattan venues. But the 44-year-old painter shunned urban life in favour of wild, deserted locations. His growing professional success, along with membership of the Palette Club and other New York artists' groups, had not given him an appetite for big-city pleasures. On the contrary: Homer was a loner, a Boston boy who had spent several formative years covering the battlefields of the American civil war for Harper's Weekly.
By 1880 he had reached a turning point in his career. The documentary paintings of sharpshooters and prisoners had given way to scenes of rocky coasts and Adirondack lakes. Their outdoor freshness allied Homer with the French landscapes produced by the painters of the Barbizon school, whose work he admired during a trip to Paris in 1867. Besides this urge to celebrate nature, however, Homer was haunted by a growing melancholy. Soon after taking up watercolours in 1873, he produced a brooding panorama of a moonlit beach where two silhouetted figures gaze at the overwhelming immensity of the ocean beyond.
Displayed early on in the Dulwich Picture Gallery's exhibition, this troubled image is closer to Caspar David Fried- rich than to the French school. Homer was beginning to convey a mood of Germanic gloom. He never told anyone about the emotional pressures that may have caused this turbulence to emerge in his work. And although he is venerated in America as one of its greatest artists, he built a protective wall around himself that turned his private life into an enduring enigma. One San Francisco professor, writing in GLBTQ: an encyclopedia of gay, lesbian, bisexual, transgender and queer culture, has speculated that Homer was a repressed homosexual. The Dulwich catalogue prefers to suggest that he "sublimated all his (possibly low) sexual energies into his art".
The exhibition proves that Homer's sudden urge to visit England in 1881 was decisive for his art. Far from following Monet's example and basing himself in London, he settled for a year and a half in the small fishing village of Cullercoats, near Tynemouth and Newcastle. Homer's vision of the sea was transformed by the storm-racked north-eastern coast. Witnessing the shipwreck of a three-masted vessel called The Iron Crown drove him to produce an elaborate, doom-laden watercolour. The fishergirls who people his Cullercoats pictures seem oppressed by the relentless anger of the waves.
Fascinated by these robust and industrious young women, Homer claimed that "there are none like them in my country in dress, feature or form". Whether looking out over the beach at Tynemouth or staring at the breakers on a stark foreshore, they are burdened by the menace of the deep. Their anxiety becomes explicit in a dark etching called Perils of the Sea, where two fishergirls veiled in funereal clothes seem transfixed by the vastness of the white sea heaving behind them.
Although Homer returned to America in 1882, the remaining decades of his life were marked by the thunderous vision he had discovered in England. Moving to a bleak stretch of the Maine coast call- ed Prout's Neck, he lived in an exposed shingle-style home and studio, isolated on the promontory, until his death in 1910.
Homer was exhilarated by the extremity of life-or-death drama at sea. The most celebrated painting in the Dulwich show, The Life Line, is based on a visit to Atlantic City, where he watched an intrepid life-saving crew at work in a wild storm. Suspended by a rope between shipwreck and shore, a seemingly unconscious young woman is carried above the hungry waves. Her head falls back at a precipitous angle, and yet the man grasping her has bound her legs with thick straps. His brawny arm grips her urgently around the waist, locking their bodies together. A glimpse of white underwear beneath her wind-buffeted skirt increases the undertow of eroticism in an otherwise alarming scene.
But Homer obliterated the man's face with the woman's red shawl, reducing him to an anonymous rescuer. This billowing garment provides the privacy cherished by the artist himself, as he strove to define his maritime images in the loneliness of his Prout's Neck studio.
Even when he set out to make an optimistic painting such as The Lookout - "All's Well", the result is freighted with foreboding. Standing below the ship's mighty bell on a clear night, a bearded sailor shouts his reassuring message to the rest of the crew. The star-spattered sky seems to reinforce his optimism, and the waves beyond the bell speak of a placid sea. All the same, the sailor seems squeezed uneasily into a corner of the dark canvas, and the shadow cast over his eyes by a sou'wester gives him the air of an unwelcome messenger. Directly above, his sliced-off right hand hangs ominously in space like a warning. He may be yelling "All's well", but Homer's feeling for tragedy fills the image with a sense of impending catastrophe.
"Winslow Homer: poet of the sea" is at the Dulwich Picture Gallery, London SE21 (tel: 020 8693 5254) until 21 May