Surface flux variability over the North Pacific and Atlantic Oceans
The re-emergence of SST anomalies in the North Pacific Ocean
Over the past few years I have participated in studies that have focused on midlatitude atmosphere-ocean interaction and interactions between the surface and deeper layers in the ocean. As the ocean evolves much more slowly than the atmosphere, a better understanding of these interactions may lead to improved seasonal and longer forecasts of the climate system. In addition, it is important to understand processes that influence the physical state of the upper ocean, which has a strong affect on marine organisms from phytoplankton to salmon. I've found that in studying midlatitude air-sea interaction, I've embarked on a career that often involves looking for small signals among a lot of noise.
When I first joined the Climate Diagnostic Center at CIRES about five years ago I was finishing a series of studies that examined the development of sea surface temperature (SST) anomalies in the North Pacific Ocean during El Niño events in the tropical Pacific. Previous observational studies indicated that the central North Pacific tended to be colder and the eastern Pacific warmer than normal during El Niño events. How did these anomalies form? The hypothesis I tested was that the atmosphere would act as a bridge between the tropical and midlatitude parts of the ocean: warm SSTs in the tropical Pacific associated with El Niño could alter the atmospheric circulation in midlatitudes, which in turn, could force the cold central/warm east SST anomaly pattern to form in the North Pacific. The experiment design consisted of an atmospheric general circulation model (GCM) that was attached to a model of the North Pacific Ocean but was forced by prescribed warm SSTs representative of El Niño conditions in the tropical Pacific. These model experiments appeared to confirm this hypothesis: SST anomalies developed in the North Pacific Ocean model which resembled those in the real world during El Niño events.
In the second part of the experiment, SST anomalies that developed in the North Pacific were allowed to feed back upon the overlying atmosphere. The results suggested that the North Pacific SST anomalies tended to damp the response to the tropical SSTs. However, the true impact of the North Pacific was unclear due to the high degree of variability among five similar model experiments.
During my first year at CIRES I became intrigued by a pair of articles from the early 1970's by Jerome Namias and Robert Born who noted a tendency for midlatitude SST anomalies to recur from one winter to the next without persisting through the intervening summer. They speculated that temperature anomalies that form at the surface and extend over the relatively deep ocean mixed layer in winter, could remain intact in the summer seasonal thermocline, a strongly stratified and thus stable layer. The anomalies could then re-emerge at the surface when the mixed layer again deepened by entraining water from below during the following fall and winter. In complimentary studies Clara Deser, Cecile Penland and I examined this Re-emergence mechanism using both subsurface temperature data and mixed layer model simulations at Ocean Weather Stations in the North Atlantic and North Pacific. Ocean modeling experiments indicated that entrainment plays an important role in the ocean heat budget by regulating the mixed layer depth in addition to controlling the heat flux through the base of the mixed layer. Lead-lag correlations showed that temperature anomalies beneath the mixed layer in summer were associated with the temperature anomalies in the mixed layer in the previous winter/spring and following fall/winter but are unrelated or weakly opposed to the temperature anomalies in the mixed layer in summer, These results supported the re-emergence mechanism and opened the possibility that thermal anomalies stored within the upper ocean may return to the surface and influence the atmospheric circulation in subsequent seasons.
In a recent examination of upper ocean observations Clara, Mike Timlin and I found that abnormally cold water first created at the surface in the central North Pacific during the mid 1970's, propagated down into the ocean, reaching depths of 400 m about 2 years later. Over a period of 15 years this cold water moved slowly down and southward with time following the general circulation of the North Pacific ocean. This work was very exciting as it is one of the first observational studies to show the evolution of subsurface ocean temperature anomalies and because it supports recent theories of how water enters and flows within the permanent thermocline.
For the past ~2 years I have participated in a cooperative research project between several Universities and NOAA's Geophysical Fluid Dynamics Lab (GFDL). As part of that project, James Scott and I have developed an atlas of atmospheric fields using a recent GFDL GCM simulation and along with Julia Collins have made the atlas available on the World Wide Web. The atlas includes standard fields such as the mean and standard deviation of precipitation and more unique analyses such as the model's representation of the 30-60 day wave in the tropics and the mid-winter minimum in storm activity over the North Pacific Ocean. Please contact me (maa@cdc.noaa.gov) if you are interested in the print atlas; the Web version is located at "http://www.esrl.noaa.gov/psd/gfdl/".
In future work, I plan to use observations and models to study air-sea interactions on different time scales and processes which cause ocean temperature anomalies to develop and evolve. I also plan to work closely with Joshua, my two-year-old son, who is a big signal among a lot of noise.