Highlight: The NEW Influence on Climate of the Warming Mid-latitude Oceans
Executive summary: Climate Impact Company now uses the 2013-2020 climatology, or aspects of that time period to combine with the ENSO regime to project and produce climate forecasts. The explanation is due to the emergence of “warm blobs” of ocean water across vast mid-latitude regions which influence climate and sometimes cause extremes. There are also cool water zones contributing to this new forecast methodology.
Discussion: Beginning mid-2013 a large region of surface and deep layer anomalous warmth emerged in the northeast Pacific Ocean. The intensity of this “warm blob” of ocean water peaked in August (Fig. 1). After weakening during northern hemisphere autumn, the “warm blob” returned late in 2013. In 2014, the “warm blob” expanded throughout the entire northeast Pacific basin and intensified reaching maximum strength in September (Fig. 2). Many “warm blobs” have followed since 2013 including the most-lengthy on record during the second half of 2020 ending in early 2021 (Fig. 3). However, the September 2014 episode was the strongest in the 2013-2021 “warm blob” climatology.
Fig. 1-3: Tracking the emergence of Northeast Pacific “warm blobs” initializing in August 2013 (upper left) and reaching peak strength in September 2014 (upper right) and slightly stronger than a “warm blob” regime last year (lower).
Climate scientists have thoroughly investigated the “warm blob” initially due to the “dramatic effect on natural resources including economically valuable fisheries” according to the California Current Integrated Ecosystem Assessment (CCIEA). However, the “warm blob” is linked to many high impact climate events including increased incidence of California drought (Fig. 4) and the “polar vortex” winter seasons of 2013-14 and 2015-16 (Fig. 5).
Fig. 4-5: The annual precipitation anomaly across the U.S. 2013-2020 identifies long-term dryness for California while the warming Western Atlantic has caused a wet bias in the East (left). The mid-winter 2014/2015 upper air pattern produced an upper ridge pattern near the warm SSTA in the Northeast Pacific compensated for downstream by a deep polar vortex over Ontario (right).
Given less attention (initially) was the emergence, at the same time, of the North Atlantic Warm Hole (NAWH). Since the mid-to-late 1990’s the North Atlantic basin has warmed as defined by the long-term pattern change of the Atlantic multi-decadal oscillation (AMO) to the positive phase (Fig. 6). The warmer North Atlantic basin has caused a sharp increase in the typical number of tropical storms and hurricanes during the past two decades (Fig. 7).
Fig. 6-7: The 1950-2021 monthly plot of the Atlantic multi-decadal oscillation and evolution of the warm phase since the late 1990’s (left). Increase in North Atlantic tropical cyclone activity climatology due to the warming North Atlantic (right).
However, beginning in 2013 a curious pool of cool water gathered south-southeast of Greenland believed to be the result of increasingly rapid runoff of ice melt in Greenland. The cool pool frequently has an ominous appearance (Fig. 8-9). The cool pool was unique to the otherwise warmer than normal North Atlantic basin therefore given the name “warm hole”.
Fig. 8-9: Vivid examples of the North Atlantic Warm Hole are evident in the annual 2014 and 2020 North Atlantic SSTA pattern.
The NAWH has caused a slowdown of the warm Gulf Stream paralleling the U.S. Atlantic Coast causing a persistent piling of anomalous warm water, a second “warm blob” typically most vivid off the Northeast U.S. Coast. Record warm SSTA was observed in February 2021 east of New England (Fig. 10). The warming of the western North Atlantic has also caused climate to change primarily increasing the tendency of high pressure ridging to produce a warmer climate but also increase low-level atmosphere moisture leading to wetter climate. Sometimes these high-pressure zones gain amplification and cause tropical cyclones to slow down and stall once inland the U.S. Coastline (Fig. 11) leading to historic flooding as observed with Harvey (2017) and Florence (2018).
Fig. 10-11: Record warm SSTA observed east of New England this past February (left). AUG/SEP 2017-18 upper air pattern revealed an upper ridge blocking the north turn of Harvey and Florence which lead to historical rainfall episodes.
The NAWH has been a leading contributor to Europe climate especially during the warm season as on average cool low-pressure aloft floats over the cool water zone and is compensated for by a hot high-pressure ridge over Europe or Western Russia (Fig. 12).
Fig. 12: Warm season 2013-2020 northern hemisphere upper air pattern identifying the upper ridge over the warm Northeast Pacific, the cool trough over the NAWH and compensating pressure systems downwind from each feature.
The northeast Pacific “warm blob” and NAWH are heavily considered generating climate forecasts due their effect on general circulation/jet stream patterns and tendency to produce climate extremes. However, the two mid-to-northern latitude sea surface temperature anomaly (SSTA) pattern(s) are not alone. Climate Impact Company has noticed an increasing incidence of “warm blobs” in both hemispheres in the 2013-2021 timeframe. A few more-dramatic examples include
One potential explanation is the slow down of ocean currents transporting warm water poleward blocked by cooler higher latitude waters as part of the (slowing) global thermohaline conveyor belt.
Across large areas of warm SSTA there is a tendency for a warmer atmosphere aloft leading to a warmer climate and increased low-atmospheric moisture associated with the warmer ocean temperatures which provides increased precipitation to storms whether entrained into a conventional mid-latitude storm or absorbed by a tropical cyclone. Precipitation extremes are increasing and the incidence of increased “warm blobs” of ocean water may be the explanation.
Summary: Climate Impact Company is now routinely considering “warm blobs” (or “warm holes”) when they are spread-out over large regions of ocean surface as to their influence on climate. This science is in its infancy and so-far has been considered for mainly North America and Europe. El Nino southern oscillation (ENSO) remains a leading influence on climate and climate forecasts. However, the mid-latitude warm and cool pools not only contribute to the forecast but also increase skill of predicting major climate events.