The Role of the Tropically Excited Arctic Warming Mechanism on the Warm Arctic Cold Continent Surface Air Temperature Trend Pattern

The December-February surface air temperature (SAT) trend is examined for all consecutive 20-year time periods between 1979 and 2017, from which a transition from a cold-Arctic-warm-continent toward a warm-Arctic-cold-continent trend pattern is evident. This transition is accompanied by a consistent transition in the sea level pressure trend pattern that supports warm air advection over the Arctic and cold air advection over the continents. The regression of the detrended December-February-average SAT onto a detrended index defined to quantify the east-west gradient of tropical Pacific sea surface temperature is characterized by a warm-Arctic-cold-continent pattern much like the SAT trend pattern observed in recent decades. A decadal timescale warming of the western tropical Pacific water has increased the east-west gradient of tropical Pacific sea surface temperature, thus contributing to the observed extratropical SAT trend transition. Plain Language Summary Over the last two decades, Arctic surface temperatures have been increasing rapidly-at a rate faster than anywhere else on the globe. Concurrent with this rapid increase of Arctic surface temperatures, surface temperatures over the North American and Eurasian continents have decreased. This simultaneous warming of the Arctic and cooling of the continents was preceded by a period of simultaneous Arctic cooling and continental warming. The transition from the period of Arctic cooling and continental warming toward the present period of Arctic warming and continental cooling was likely caused by southerly winds (flowing from south to north) over the Arctic and northerly winds (flowing from north to south) over the continents. These winds move warm air into the Arctic and cold air over the continents. We present evidence that these circulation changes, and the associated temperature changes, are driven in part by the warming of the western tropical Pacific water (relative to the eastern tropical Pacific), which started at about the same time that the above transition occurred. The chain of events that link warming of the western equatorial tropical Pacific Ocean to circulation changes over the Arctic and continents is discussed in previous studies.