National Geographic : 2016 Jan
Into Thin Ice 111 GREENLAN D WARMAIRFROMSOUTHPACIFIC Sea-surface temperature anomalies Warmer Cooler JETSTREAM JET STREA M Temperature 90̊ Pole 30̊ Midlatitudes 90̊ Pole 30̊ Midlatitudes LATITUDE LATITUDE STRONGWEAK ATLANTIC OCEAN PACIFIC OCEAN LOW PRESSURE LOW PRESSURE HIGH PRESSURE HIGH PRESSURE ASIANORTHAMERICAHOTANDDRYCOLDANDWETCOLDA ND WETWavierjetstreamStraighterjetstream The Arctic The Arctic is warming faster than the rest of the planet, in part because of the feedback caused by the loss of sea ice. According to a controversial theory, Arctic warming is causing the jet to slow down and meander more. The result: unseasonable weather that sits in one place for a long time. STRAIGHTER JET STREAM The jet stream gets most of its energy from the temperature contrast between the air masses it separates. A strong jet is a straighter jet that keeps cold air bottled up in the Arctic. WAVIER JET STREAM As the Arctic warms faster than the midlatitudes, the tempera- ture contrast decreases. That weakens the jet, letting Arctic air flow south—over eastern North America, for instance. Extreme Persistence Whatever their cause, big jet stream meanders move slowly around the planet. That means the weather they carry stays in one place for a long time. Think of California parched below a persistent high-pressure ridge or New England buried under polar vortex snows in early 2015. LAUREN E. JAMES, JASON TREAT, AND RYAN WILLIAMS, NGM STAFF ART: NICK KALOTERAKIS SOURCES: JENNIFER FRANCIS, RUTGERS UNIVERSITY; DENNIS HARTMANN, UNIVERSITY OF WASHINGTON Early winter 2012 ice extent shown GREENLAN D WARMAIRFROMSOUTHPACIFIC Sea-surface temperature anomalies Warmer Cooler JETSTREAM JET STREA M Temperature 90̊ Pole 30̊ Midlatitudes 90̊ Pole 30̊ Midlatitudes LATITUDE LATITUDE STRONGWEAK ATLANTIC OCEAN PACIFIC OCEAN LOW PRESSURE LOW PRESSURE HIGH PRESSURE HIGH PRESSURE ASIANORTHAMERICAHOTANDDRYCOLDANDWETCOLDA ND WETWavierjetstreamStraighterjetstream The polar jet stream is a high-altitude air current that separates low-pressure Arctic air—aka the polar vortex—from warmer, high-pressure air to the south. When the jet dips far south, it can deliver blasts of cold and snow to temperate latitudes; a north-jutting ridge promotes heat and drought. Such extreme weather has been happening a lot lately. Scien- tists are debating whether that’s primarily due to the shifting cycles of the Pacific—or whether the melting Arctic plays a key role. The Pacific It’s considered the main influence on the polar jet stream’s path—and on weather patterns globally—because of the huge amount of solar heat its tropical regions absorb and release. If the jet is indeed getting wavier, some scientists argue, the Pacific is overwhelmingly to blame. NORTH PACIFIC MODE Every decade or so, warm air rises from a horseshoe pattern across the Pacific and pushes the jet stream far north into Alaska. EL NIÑO When warm water sloshes into the eastern Pacific during an El Niño, the heat it releases can draw the jet stream south, carrying rain to California. Arctic Heat Pump Ice-free water absorbs more solar heat in summer, then releases it in winter, as the water refreezes. Heat and water vapor rising from the ocean raise air pressure and moisture and may affect the jet. Extreme Weather: An Arctic Connection?