The Climate-Weather Interface and Ocean-Atmosphere Interaction

Both the atmosphere and ocean have weather — natural variability arising from the instabilities of the circulation. The interaction between atmospheric and oceanic weather and variability on seasonal and longer time scales is an important and challenging aspect of understanding and modeling the Earth system. Recent work in the Oceanography Section has focused not just on advancing our understanding of the processes involved in these scale interactions within each fluid, but how these scale interactions act across the ocean-atmosphere interface. Atmosphere weather systems (on scales of 100 to 1000 km) and fronts can have a strong impact on the upper ocean circulation and mixing and marine ecosystems. In turn, ocean weather (mesoscale eddies and submesoscale filaments, on scales less than about 100 km) can interact with the atmosphere, potentially affecting the atmosphere weather systems and storm tracks. Progress in this area is being facilitated by recently developed high-resolution versions of the Community Earth System Model (Small et al 2014,2019), along with emerging observational capabilities that allow sustained global simulation and monitoring of ocean-atmosphere interaction on these scales

Several decades of satellite observations of ocean color have revealed significant spatial structure in phytoplankton distributions on ocean weather scales (10-100 km). However, our view of the color of the oceans from space is biased to clear-sky conditions. More recently, bio-optical sensors from floats and elephant seal tags in the Southern Ocean have enabled us to grasp the pulse of phytoplankton blooms in the water column under all weather conditions and seasons. These new observations revealed significant vertical structure in phytoplankton abundance within the upper ocean mixed-layer that has been linked to synoptic-scale wind forcing (i.e., weather events). Atmospheric weather systems, storms and mobile anticyclones that come after storms, inject variability in the upper ocean on temporal scales of a few days, which resonate with phytoplankton growth timescales. While storms presumably enhance upper-ocean mixing, deepening the ocean mixed-layer and potentially entraining nutrients from subsurface waters into the mixed layer, periods of quiescence between storm events might be instrumental for density restratification and alleviation of light limitation that can enhance phytoplankton growth. ASP postdoc Magdalena Carranza and collaborators are using a CESM ocean-sea-ice model simulation with marine biogeochemistry to evaluate the processes through which weather systems impact upper-ocean physics and phytoplankton bloom development in the Southern Ocean. They are identifying and tracking atmospheric cyclones and anticyclones in the atmospheric forcing fields to construct storm centric composites of upper-ocean bio-physical properties. They find contrasting patterns between cyclones and anticyclones throughout the seasons (Fig 1) that are consistent with upper-ocean mixing impacting on phytoplankton growth rates through expected changes in nutrient supply and light availability. Thus, the alternation of periods of mixing and quiescence from atmospheric weather seem instrumental to create structure in phytoplankton distributions in the upper ocean through a balance of mixing and stratification where phytoplankton strive. Ongoing research is examining the role of ocean weather in modulating the biological response to atmosphere weather, as well as the implications for ocean heat and carbon uptake.

Atmosphere weather imprints on surface winds, ocean mixed-layer depth, and summer phytoplankton from CESMv1 daily output
Figure 1: Atmosphere weather imprints on surface winds (left panel), ocean mixed-layer depth (MLD, middle), and summer phytoplankton (right) from CESMv1 daily output. Atmosphere cyclones (i.e., storms), linked to high winds and cloudy conditions, are associated with deeper mixed layers, and enhance surface chlorophyll concentrations (Chl-a, a proxy for phytoplankton) in summer but reduce them in winter (not shown). On the other hand, atmosphere anticyclones linked to clear skies and calm wind conditions, exhibit shallower mixed layers that induce higher Chl-a in winter but lower in summer (not shown).

It has traditionally been thought that the mid-latitude the oceans respond in a passive manner to atmosphere weather forcing, integrating the high frequency forcing to a lower frequency response. New research is showing that the ocean weather (mesoscale eddies) can drive a large-scale response in the atmosphere. In regions of strong ocean weather activity such as the Gulf Stream and Kuroshio, the variability in the flux of heat to and from the atmosphere is strongly modulated by ocean eddies and current meanders. These findings have required the use of climate models with very fine grids that allow for representation of ocean eddies (10s of km), and have been verified from observations (Fig 2). In contrast, climate models with coarse grids are only able to correctly simulate the influence of the ocean on the atmosphere near the equator, and not in the middle latitudes.

A near-Global map of where the ocean transfers heat to or from the atmosphere on monthly timescales
Figure 2: A near-Global map of where the ocean transfers heat to or from the atmosphere on monthly timescales. a) Observational estimate, from J-OFURO3 group, b) a climate model with fine, eddy containing grid, c) a standard climate model with coarse grid. Areas shown in red are the key regions of heat transfer from ocean, and are far less common in the coarse grid model (bottom) panel than in the top two panels.

References

  • Small R.J. et al (2014) A new synoptic scale resolving global climate simulation using the Community Earth System Model. J. Adv. Mod. Earth Sys., 6, 1065-1094.
  • Small, R.J. et al (2019) Air-sea turbulent heat flues in climate models and observational analyses. What drives their variability? J. Climate. (in press).