This project will examine the physical processes associated with the evolution (formation, circulation, and destruction) of Eighteen Degree Water (EDW) within the subtropical gyre of the North Atlantic Ocean. EDW is the archetype for the anomalously thick and vertically homogenous mode waters that are typical of all subtropical western boundary current systems. EDW is associated with a shallow overturning circulation that carries heat northward and is an interannual reservoir of anomalous heat, nutrients and CO2. Understanding the annual cycle of EDW evolution, and in particular its associated circulation pathways and destruction mechanisms, is important because though EDW is isolated beneath the stratified upper-ocean at the end of each winter, it may reemerge in subsequent years to influence mixed layer properties and consequently air-sea interaction and primary productivity.
The pathways of EDW circulation, the processes of EDW destruction, and the role of EDW in modulating ocean-atmosphere heat exchange and nutrient supply to the euphotic zone will be investigated. The plan is to synthesize a broad spectrum of observations including a substantial in-situ dataset collected during the recently completed CLIVAR Mode Water Dynamics Experiment (CLIMODE). This coordinated field effort included a large array of profiling floats, a moored array, and several hydrographic surveys. To supplement this unique dataset the investigators will also examine contemporary and historical hydrography, Argo profiling floats, surface drifters, satellite altimetry and ocean color fields, and output from several eddy-resolving OGCM simulations.
Intellectual Merit: This project will test the long-hypothesized climatic importance of EDW as an element of the North Atlantic's shallow overturning circulation and as a short-term reservoir of heat, nutrients, and carbon. Significantly, this study will advance understanding of the various processes that destroy EDW. In addition, this study will, for the first time, compare and contrast the roles of the large-scale, low-frequency circulation and the mesoscale eddy field in EDW dispersal and destruction. Finally, the proposed work will provide a foundation for comparative studies since mode waters are found in every ocean basin.
Broader Impacts: Clarifying the role of EDW in the exchange of heat between the atmosphere and ocean will support ongoing community efforts to improve interannual-to-decadal predictability. For example, EDW and its associated air-sea heat flux may have a direct impact on the year-to-year variability of surface storm amplitude in the North Atlantic storm track near the Gulf Stream. In addition, understanding the role of EDW in the storage and exchange of nutrients will advance our understanding of interannual variability in primary productivity. Rigorous model-data comparisons using the unique CLIMODE dataset will also aid the broader community by providing an objective assessment of the fidelity of eddy-resolving numerical models. Finally, the project will support the training of two postdoctoral investigators and a graduate student.
This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) program.
The ocean and the atmosphere work in tandem to redistribute heat from one locale to another, which impacts regional and global climate. While both fluids advect, or carry, heat with their movement, only the ocean has the capacity to store heat for long periods because of its relatively high heat capacity. Our work studied whether the heat exchange between the atmosphere and the ocean during one winter impacts the heat exchange in subsequent winters. We found that there was a persistence of this thermal "memory" due to ocean storage, but that the memory was limited to one year. Our work will advance our understanding of how the ocean release of heat to the atmosphere in the vicinity of the Gulf Stream will impact regional climate. As part of this funded work, we also studied the pathways of the water mass that stores this heat from one winter to the next. We found that this water mass plays an important role in the ocean’s meridional overturning circulation, which is the large scale circulation of the Atlantic Ocean that moves heat from the tropical to the polar regions. For decades, oceanographers have understood the upper limb of the meridional overturning circulation to be comprised of surface waters, but our study revealed that the waters that are carried to the subpolar and polar regions of the North Atlantic actually come from ~200-300 meters at depth in the subtropical region. At this depth are the waters that were involved in the heat exchange with the atmosphere the previous winter. This work will impact our understanding of how changes in temperature at one location in the ocean impact changes in the ocean at another location. Since ocean temperature changes impact atmospheric temperatures and rainfall, our work will advance studies of regional climate change. Finally, as part of this work, we also studied how nutrients are supplied to the subtropical gyre. Nutrients are necessary for the production of phytoplankton in the marine environement and these phytoplankton serve as the foundation of the marine food web. The North Atlantic subtropical gyre is characterized by downwelling currents,i.e. currents that carry water from the surface to depth. This downwelling also removes nutrients from the surface waters where they are needed for phytoplankton growth. Our study revealed that the nutrients needed to explain observed features can be supplied from horizontal currents which transport nutrients from upwelling regions - where nutrients are brought to the sea surface from depth where they are plentiful. We found that the nutrient transport southward across the Gulf Stream can explain many of the observed features of the nutrient field
