Wind‐Driven Strain Extends Seasonal Stratification

Abstract

The onset and breakdown of stratification are key physical drivers of phytoplankton growth in shelf seas and the open ocean. We show how in the Celtic Sea, where seasonality in stratification is generally viewed as controlled by heat input, a cross‐shelf salinity gradient horizontally strained by the wind prolonged the stratified period by 5–6 days in autumn prior to full winter mixing, while in spring caused seasonal stratification to begin 7 days early. Salinity straining has important implications for setting light conditions during the start of the spring bloom and for the timing of bottom‐water ventilation in winter. Analysis of winds around the time of likely onset of spring stratification between 1979 and 2016 showed that in 60% of the years' wind conditions were favorable for salinity straining. Accurate knowledge of the horizontal salinity field and wind stress are required to correctly determine the onset and breakdown of stratification.

Sensitivity of Shelf Sea Marine Ecosystems to Temporal Resolution of Meteorological Forcing

“…The timing and magnitude of the autumn bloom, particularly across the outer shelf immediately before a period of net off-shelf transport during the winter (Ruiz-Castillo et al, 2019), could affect the amount of carbon annually exported off-shelf (Wihsgott et al, 2019). Although not captured by the 1-D model, wind stress plays an important role in seasonal shelf-scale circulation (Ruiz-Castillo et al, 2018) and can advance (delay) the onset (breakdown) of stratification by~1 week via a horizontal salinity straining mechanism, with corresponding adjustments to the spring and autumn bloom timings (Ruiz-Castillo et al, 2019). The changes in bloom timing reported here that result from differing temporal resolutions of the wind stress forcing are of the same magnitude.…”

“…Chiswell (2011) links the timing of the spring bloom to a reduction in wind-driven surface mixing with wind intensity estimated to explain up to 60% of the interannual variability in the timing of phytoplankton blooms along the Norwegian shelf (Vikebø et al, 2019). Changing wind conditions have also been shown to both advance and delay the onset of spring phytoplankton blooms (Follows & Dutkiewicz, 2002;Ruiz-Castillo et al, 2019;Sharples et al, 2006;Waniek, 2003). A decrease in wind stress is often correlated with an earlier phytoplankton bloom in open oceans such as in the Japan Sea (Kim et al, 2007;Yamada & Ishizaka, 2006), North Atlantic (González Taboada & Anadón, 2014;Henson et al, 2009;Ueyama & Monger, 2005), the open ocean off the South West Iberian peninsula (Krug et al, 2018), and shallower systems such as the North West European Shelf (González Taboada & Anadón, 2014) and Baltic Sea (Groetsch et al, 2016).…”

Control of a phytoplankton bloom by wind‐driven vertical mixing and light availability

“…From previous work, we know that positive heat input initiated seasonal stratification on 26 th March but, vertical differences in salinity (not temperature) controlled 50%–100% of water column stability between the 28th March and 7th April (Ruiz‐Castillo et al 2019 a ). Our gliders were deployed during this time (on 4th April) and we initially observe (and model) modest increases in phytoplankton biomass within the nutrient replete euphotic zone during the day, followed by re‐distribution of this biomass to the depth of the pycnocline by convective overturning at night (Fig.…”

“…The regime changed on the 9th April, coincident with temperature regaining full control of water column stability (Ruiz‐Castillo et al 2019 a ) and wind stress as opposed to convection becoming the dominant driver of the mixing length scale (Wihsgott et al 2019). The increased importance of wind‐driven mixing is consistent with the increase in correlation between z mix and wind stress that we find here, from 0.2 during the 5 d before the 9th April to 0.6 for the remainder of the deployment.…”

“…In this paper we present high-resolution observations from moorings and gliders collected in the central Celtic Sea, Northwest European Shelf, combined with analysis of a 1-D physics and phytoplankton growth model. Seasonal stratification at our study site started to build on the 26th March 2015 coincident with the beginning of sustained positive heat flux (Ruiz-Castillo et al 2019a). Surface chlorophyll a (Chl a) concentrations also started to increase above background winter levels on this day.…”