The future of the subsurface chlorophyll‐a maximum in the Canada Basin—A model intercomparison

Abstract

Six Earth system models and three ocean‐ice‐ecosystem models are analyzed to evaluate magnitude and depth of the subsurface Chl‐a maximum (SCM) in the Canada Basin and ratio of surface to subsurface Chl‐a in a future climate scenario. Differences in simulated Chl‐a are caused by large intermodel differences in available nitrate in the Arctic Ocean and to some extent by ecosystem complexity. Most models reproduce the observed SCM and nitracline deepening and indicate a continued deepening in the future until the models reach a new state with seasonal ice‐free waters. Models not representing a SCM show either too much nitrate and hence no surface limitation or too little nitrate with limited surface growth only. The models suggest that suppression of the nitracline and deepening of the SCM are caused by enhanced stratification, likely driven by enhanced Ekman convergence and freshwater contributions with primarily large‐scale atmospheric driving mechanisms. The simulated ratio of near‐surface Chl‐a to depth‐integrated Chl‐a is slightly decreasing in most areas of the Arctic Ocean due to enhanced contributions of subsurface Chl‐a. Exceptions are some shelf areas and regions where the continued ice thinning leaves winter ice too thin to provide a barrier to momentum fluxes, allowing winter mixing to break up the strong stratification. Results confirm that algorithms determining vertically integrated Chl‐a from surface Chl‐a need to be tuned to Arctic conditions, but likely require little or no adjustments in the future.

Do Strong Winds Impact Water Mass, Nutrient, and Phytoplankton Distributions in the Ice‐Free Canada Basin in the Fall?

“…In the Canada Basin, a subsurface chlorophyll a maximum generally develops at a depth of ~60 m, which corresponds to the deeper subsurface chlorophyll a maximum observed in the present study. This maximum has been well studied, including in terms of its distribution and seasonal dynamics (Brown et al, ; Carmack et al, ; Tremblay et al, ), biogeochemical significance (Martin et al, ), interannual changes in depth (McLaughlin & Carmack, ), and future projections (Steiner et al, ). However, the shallower subsurface chlorophyll a maximum at ~20 m has not been well studied to date.…”

“…This strong suppression of turbulent mixing occurred even in the unusually ice‐free and stormy summer of 2012 in the Canada Basin. In addition, based on numerical models, the nutricline and subsurface chlorophyll a maximum in the Canada Basin will continue to deepen in future climate scenarios (Steiner et al, ). Thus, the nutrient and phytoplankton distributions in the Canada Basin are likely to be unresponsive to strong wind events in the future, as in the present study.…”

Effects of Model Resolution and Ocean Mixing on Forced Ice‐Ocean Physical and Biogeochemical Simulations Using Global and Regional System Models

“…The initial conditions for other ecosystem components are from the recent version of CESM global 1° grid initial conditions (Moore et al, ). All model runs start from 1965 with no motion in the ocean and no sea ice to 1975, then restart with the sea‐ice field of 1975 but no motion in ocean and initial T and S. This approach was used in model simulations by Steiner et al () to reduce the effects of initial ice formation on the ocean T and S field. Only the model results from 1980 onward were used for model‐data comparison as observational data used in this study start from 1980, though most data were collected after 2000.…”

“…Subsurface chlorophyll‐a maxima (SCM) have been underestimated in satellite‐based estimations of Arctic Ocean PP (Arrigo & van Dijken, ). Failure to simulate SCM could be due to either too much nitrate and hence no surface limitation or too little nitrate with limited surface growth (Steiner et al, ). An extensive comparison of 21 regional and global ecosystem models against observations in the Arctic (1959–2011) by Lee et al () evaluated modeled net primary productivity (NPP) and environmental variables such as NO 3 , mixed layer depth (MLD), euphotic layer depth, and sea‐ice concentration.…”

What sea-ice biogeochemical modellers need from observers

“…More recently, however, several examples have generated constructive outcomes (e.g., projects known as BEST, SIMBA, INTERICE IV and Barrow 2009; Gibson and Spitz, 2011;Vancoppenolle et al, 2011;Moreau et al, 2014). An Arctic example of a project that has made progress towards coordinated collaboration between modellers and observers is the Forum for Arctic Modelling and Observational Synthesis (FAMOS, http://web.whoi.edu/famos/), which has evolved from the Arctic Ocean Model Intercomparison Project (AOMIP) and now includes marine biogeochemistry (Popova et al, 2012;Steiner et al, 2014Steiner et al, , 2015.…”