Warm Air Over Eastern Shores Draw In the Cold

Atmospheric waves generated from ocean heat may provide a new explanation for why western cities are usually warmer than eastern cities despite similar latitudes.

Having traveled south of Europe as well as west, early settlers arrived in the New World assuming they were moving into a climate of warmer, more benign winters. This matter of simple geography was lethal miscalculation in the 17th Century, and researchers have been attracted to it ever since.

So why do London and Barcelona have warmer winters than more southerly New York?

Researchers since Benjamin Franklin first pointed to the Gulf Stream, the current of tropical warmth that flows from the Tropics along the eastern seaboard and then angles across the North Atlantic to bathe the shores of Europe.

Later investigations suggested it really had more to do with the flow of prevailing westerly winds carrying cold Canadian continental air over New York and the ocean's warmth over Europe.

Now, in the latest issue of the Journal Nature, comes a new idea based on large waves in the atmosphere of the planet. According to modeling work at the California Institute of Technology, the sharply contrasting winter climates are the result of waves of thermal energy rising from the warm Atlantic Ocean in a pattern that pulls colder air farther south over the eastern continental boundaries of North America. These semi-stationary waves in the atmosphere, rather than the ocean itself, shape the patterns of winter on both sides of the Atlantic.

The geophysicists, Yohai Kaspi and Tapio Schneider, who conducted the work also point to the same large atmospheric wave pattern occurring over the Pacific Ocean as responsible for cold winters along the eastern continental boundary of Asia. These atmospheric waves over both ocean basins also explain the warmer climates on the western boundaries of the continents, they add.

The Cal-Tech pair write that their modeling work provides "a plausible answer to the question of why the eastern continental boundaries of both Asia and North America are so cold, and why the extent of the cold regions on both continents is similar."

"Kaspi and Schneider's work provides fresh insight into processes that create a notable asymmetry in Earth's climate," wrote Yale University geophysicist William Boos in a separate evaluation of the study published in the same issue of Nature. It also raises "numerous issues" about how such waves affect climate patterns on longer time scales, he said.

IMAGE 1: This image of snow cover over North America and Europe was taken on 23 March 2003 by NASA's Terra satellite using the MODIS instrument. Approximate latitudes at 30° N, 45° N, 60° N and 75° N are marked. Note that northeastern North America is almost entirely snow covered between 45° N and 60° N, but that there is little snow at the same latitudes in western Europe. Also shown is sea-ice surface temperature (imaged by the MODIS instrument on NASA's Aqua satellite): pink, temperatures between 0 °C and −15 °C; purple, temperatures between −15 °C and −28 °C; white (in the deep Arctic), temperatures colder than −28 °C. Kaspi and Schneider invoke the influence of Rossby-wave plumes as a partial explanation of this transatlantic asymmetry. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio; George Riggs (NASA/SSAI), edited by Yohai Kaspi.

IMAGE 2: Sea surface temperatures off the eastern coast of North America as taken by the Advanced High Resolution Radiometer (AVHRR) on the NOAA polar-orbiting satellites. Credit: NOAA IMAGE 3: Sea surface temperatures off the eastern coast of North America as imaged by NASA's Terra satellite using the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument. The darkest reds are 32 degrees Celsius and the purple is -2 Celsius (the freezing temperature of ocean water). The main feature in this image is the gulf stream which flows northward along the eastern US from Florida up to North Carolina and then diverges from the coast into the Atlantic ocean. During winter these warm waters are in sharp contrast to the cold continent to the west. This specific image is a 8 day average from Sep 6th to 14th 2001. In the mechanism described in the paper these warm waters off the eastern continental boundaries drive atmospheric waves which lead to cooling of the continents to the west. Credit: Ronald Vogel, SAIC for NASA GSFC.