My primary interests are in the ocean's role in climate, and in deducing the general circulation of the ocean and ocean/atmosphere/ice interactions through direct observations of the ocean circulation. I am also increasingly thinking about the interactions between physical aspects of the ocean circulation and biogeochemical properties of the ocean. I am very interested in the development of novel instrumental techniques that can be used to measure and understand the ocean circulation in new ways. I have several ongoing, funded research projects that are examining these issues:
- Argo. Examining large-scale air-sea interaction and the ocean's role in climate requires a global ocean observation system; the Argo profilng float array was created for this purpose. I am the PI of the UW float group, which is part of the US Argo Consortium, and I also sit on the International Argo Steering Team. As such, we are funded by NOAA to build and deploy approximately 120 Argo floats per year. All of our floats are Teledyne/Webb Apex floats. However, unlike most float groups in the world we purchase only the float components from Teledyne/Webb and carry out the construction, testing, and calibration of the floats in our lab at UW. In the past decade we have built and deployed over 1000 floats throughout the world ocean. Most of the UW floats deployed in the past decade have been deployed in the Pacific (mainly South Pacific) and Indian Oceans, largely because there were not many other Argo groups in the world deploying floats in these regions. Working in these remote regions has required new approaches to deploying floats: in the South Pacific, the UW group and the float group at Scripps Institution of Oceanography have worked together with our colleagues in New Zealand to charter the vessel Kaharoa, based at the NIWA Laboratory in Wellington, New Zealand. This workhorse vessel has deployed UW and Scripps floats throughout the South Pacific and the Indian Ocean.
- Ocean/atmosphere interaction in the near-surface layer of the ocean. All heat and freshwater exchanges between the ocean and the atmosphere occur in the upper few meters of the ocean. The upper 1 cm or so of the ocean can be sampled by satellites, but the next 5 meters below are difficult to study over extended times or on large spatial scales. Standard Argo floats do a good job of collecting data between depths of 2000 m and 5 m. Above 5 meters, the pump on the float CTD is shut off in order to keep surface contaminants out of the CTD cell; this precaution is necessary in order to insure that the CTD unit will continue to return high-quality data for times of 4-5 years. In the past several years we have deployed over 40 floats with an additional, auxillary CTD sensor that can measure temperature and salinity at high resolution in the upper 5 meters of the water column. This work, funded by NASA (I am a member of the NASA Ocean Salinity Science Team), allows the investigation of the near-surface layer of the ocean in great detail. We are using these observations to examine the exchange of heat and salt on both diurnal and seasonal time scales near the ocean surface. In 2011 the Aquarius satellite was launched, with the goal of mapping the surface salinity of the ocean from space, and the data collected from these floats and others helped to provide a good measure of the quality of the data from Aquarius before the satellite ceased operation in 2015.
- The wind-driven ocean circulation, and the cycle of freshwater in the ocean/atmosphere system. Wind and direct solar heating are the main driving mechanisms of the large-scale ocean circulation; with the resulting feedbacks from the ocean to the atmosphere, the ocean's contribution to climate is defined. In order to understand this contribution, the details of the ocean circulation must first be discerned. Since Argo floats measure temperature and salinity as functions of pressure as well as an absolute velocity at the float parking depth, Argo floats can be used to infer the absolute geostrophic circulation of the ocean above 2000 m, the deepest level that is sampled by the floats. Dr. Alison Gray (a former graduate student, now a postdoc at Princeton) has done this for the global ocean, and a complete estimate of the total geostrophic circulation above 2000 m now exists. This estimate can be used in many ways, including estimating heat and freshwater transports, examining meridional overturning, and testing the idea of Sverdrup balance. Since the salinity measurements from floats are generally of very high quality (errors no more than 0.01 PSU over several years), large-scale changes in the salinity over recent decades can be assessed using the data. This was done by Dr. Li Ren (PhD 2008) for the N. Pacific Ocean in her dissertation. As changes in salinity likely reflect changes in the freshwater cycle, we are examining the terms in the freshwater budget (i.e. evaporation, precipitation, Ekman pumping, geostrophic advection, and entrainment) in order to determine which factors have changed in recent decades and how these changes are connected to climate variability.
- Ocean circulation in the Antarctic seasonal ice zone. It is known that global climate signals are amplified at high latitudes, and the circulation in and around the seasonal ice zone in both hemispheres is crucial to understanding ocean-atmosphere interactions in these regions. Over 120 UW floats have been deployed in and around the seasonal ice zone in the Antarctic in the past several years. These instruments have been equipped with special software that allows the floats to profile under the ice in winter and store their data, then transmit all of the data in the austral spring when the ice disappears. These floats have performed very well, with losses not much different than for midlatitude floats; some floats have now survived through 5 winters. The data show a cold period under the ice, accompanied by a slow increase in salinity near the ocean-ice interface in the winter, followed by a rapid increase in temperature and decrease in salinity as soon as the ice melts in spring. Some floats that are equipped with dissolved oxgyen sensors show a very strong increase in dissolved oxygen near the surface as soon as the ice melts, perhaps indicative of a spring phytoplankton bloom. The analysis of these profiles is being carried out cooperatively with Dr. Annie Wong. The dataset is extensive, with observations around much of the Antarctic continent. We are continuing to deploy as many as 10 Argo floats in the Antarctic each year, as resources permit.
- Oxygen and nutrient cycling in the ocean. There is growing interest in examining the interactions of the physical properties of the ocean with biogeochemical aspects of the circulation. A number of investigators have begun adding biological and/or chemical sensors to floats and gliders. The UW float group has been very active in this area. We have been adding dissolved oxygen sensors to floats since 2003, and several publications have resulted from this work (Riser and Johnson, 2008; Martz et al., 2008). Additionally, in the past decade we have added dissolved nitrate sensors to many floats. This work has been funded by NSF and is being done cooperatively with Dr. Ken Johnson's group at MBARI in Monterey, California. These floats are essentially identical to Argo profiling floats (although with added sensors) but are not official Argo floats since they are funded outside the Argo progam (they are, however, classified as "Argo equivalents", and their data are available in real-time through the usual Argo channels). The first floats with dissolved oxygen allowed us to examine the seasonal variability of O2 in the upper ocean over a 3 year time period (near Hawaii and Tahiti) and to make an estimate of new biological production in the ocean over that time. The float data showed unmistakable evidence that the ocean is a net producer of dissolved oxygen, at least in the subtropics. At higher latitudes the problem is more difficult, since the air-sea interaction is stronger and the mixed layer is much deeper; nonetheless, the data proved to be very useful for estimating new production. The addition of nitrate has opened many new questions concerning the origin of nutrients used in new production (do they come from the same water where the O2 is formed, or somewhere else?), which are being addressed now (see the paper by Johnson, Riser, and Karl in Nature, 2010). This is an exciting area for research that is only beginning.
- Development of new sensors for profiling floats. One of the reasons that we fabricate our own floats from components supplied by Teledyne/Webb is so we can add new sensors and capabilities to the instruments. One of the most important of these additions has been the use of Iridium communcations; most Argo floats use the Service ARGOS system, which is reliable but slow. Iridium is much faster and allows much more data to be transferred (and much quicker) and allows a 2-way communication capability so that float missions can be changed even after deployment. The implementation of Iridium on UW floats has allowed the use of additional sensors. The use of nitrate (see above) requires Iridium, as do the high resolution measurements of temperature and salinity in the upper ocean (also see above). In the Antarctic, Iridium is essential for insuring that all of the stored profiles can be transmitted in a reasonable amount of time from under-ice floats. Additionally (in cooperation with Prof. Jeff Nystuen and Dr. Jie Yang at the UW Applied Physics Laboratory) we have used acoustic methods to estimate wind and rainfall from floats during their deep (1000 m) drift phase (this also requires Iridium). Through NSF, we have been funded cooperatively with Dr. Ken Johnson at MBARI to make measurements of pH from floats (in conjunction with O2 and nitrate measurements), an important step in beginning to assess ocean acidification. The combination of oxygen, nitrate, chlorophyll, and pH sensors on floats will likely be an important new tool in developing an understanding of carbon cycling. In this context, I am a co-PI on the NSF-funded Southern Ocean Carbon and Climate Observation and Modelling (SOCCOM) program, a 6-year program generously funded by the National Science Foundation. The goal of this program is to build and deploy 200 floats in the Antarctic and to use the data in conjunction with models to examine the ocean's uptake of carbon in this critical region. In general, we are always looking at new features and sensors that can be added to floats in order to further our overall understanding of the ocean.