Hawai'i Ocean Time-Series (HOT) and
Woods Hole HOT (WHOTS) Research
HOT is a nearly 30-year long research program that collects baseline vitals of a deep ocean site, named Station ALOHA, 100 km north of O'ahu. I worked for this lab for two summers, deploying instruments at sea, analyzing data in the lab, and writing websites and blogs for outreach material.
When:
Summers of 2011 and 2012
Where:
Honolulu, Hawai'i
For:
Dr. Lukas, University of Hawai'i, Department of Physical Oceanography
Process
At Sea
The HOT Program conducts monthly cruises to Station ALOHA (which stands for A Long-term Oligotrophic Habitat Assessment) and deploys an annual mooring, called the WHOTS mooring, in order to gather consistent and constant data on basic ocean parameters such as salinity, temperature, acidity, and current speed and direction. I went to sea several time on HOT and WHOTS research cruises over the course of those two summers, during which I deployed instruments and collected samples.
In July 2011, I was lucky enough to score a bunk on the research cruise to install the Aloha Cabled Observatory, the deepest observatory of its kind which connects oceanographic instruments to the end of a retired telecommunications cable at Station ALOHA. This cable brings data back from the sea floor in real time - the first of its kind to be installed in the deep ocean. The installation cruise was three weeks long and a series of instrument malfunctions interrupted the installation process causing the final installation of the observatory to be completed only hours before the vessel had to be back in port. It was an exciting trip in which I had the opportunity to help troubleshoot with engineers from my lab and from the Woods Hole Oceanographic Institute Remotely Operated Vehicle (ROV) Jason Team. While on that cruise, I blogged almost daily to keep the followers on land abreast of the deployment of instruments. You can read my blog about the deployment of the Aloha Cabled Observatory online.
On Land
When I returned to the lab in 2012, Dr. Lukas had 7 years of unanalyzed data, collected from the WHOTS mooring, that he asked me to dig into. In particular, he directed me to use Empirical Orthogonal Functions (EOFs) to determine the prevailing temporal and spatial (depth) patterns in the data gathered from instruments suspended beneath the buoy and to determine the prevailing trends over the nearly decade of collection.
Fig. 1. Low-frequency temperature variability of the upper 55 m of the water column. A low-pass filter was run on 7 years of temperature data collected from from six instruments attached to the mooring at depths between 1 and 55 m. The oscillations' peaks and troughs coincided with El Nino and La Nina events in the Pacific. Notice the deepest downturn occurs during 2008 and 2009 - a year of significant upheaval in multiple ocean variables in the Pacific.
EOFs take a 2-D matrix of data, such as that of a single variable down the depth profile of the mooring and across 7 years of sampling, and compute eigenvectors and corresponding amplitude vectors that explain variance - they show the data changes over time and space. The eigenvectors show you depth profiles characteristic of the data set and the amplitude vector show you how dominant, or how much variance is explained by this eigenvector over time. The first eigenvector corresponds to the primary pattern in the data. In the case of temperature, for example, the first eigenvector shows the mixed layer - upper 50 m of the ocean which is fairly homogenous and sensitive to surface heat flux - and a steep gradient below. The corresponding eigenvalue shows the seasonal variation of that thermocline accounting for warmer summers and colder winters. See Figs. 2 and 3 below.
Fig. 2. Eigenvector of first mode of EOF run on the total temperature time-series from all deployments. This eigenvector showed the mixed layer above 50 m with a steep gradient below.
Fig. 3. Amplitude of first mode of the EOF of cumulative temperature dataset exhibit the seasonal variation of the mixed layer
This preliminary research resulted in findings that elucidated the functions and changes of natural variability in the Pacific, including the El Nino Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) - about which surprisingly little is known. These results will be incorporated into future publications from the HOT lab.
Why is this important?
For me:
I gained advanced technical expertise in analyzing and interpreting oceanographic data. At sea, I was also exposed to engineers who taught me how they developed instrument casings and deployment systems. I loved the hands-on aspect of that work and the process of troubleshooting when things went wrong.
For others:
The HOT and WHOTS Research has an important role to play in providing baseline, observational data in the North Pacific. It has implications for climate change research in that it provides long-term trends for ocean parameters such as temperature and pH.