Observing intermittent biological productivity and vertical carbon transports during the spring transition with BGC Argo floats in the western North Pacific

To investigate changes in ocean structure during the spring transition and responses of biological activity, two BGC-Argo floats equipped with oxygen, fluorescence (to estimate chlorophyll a concentration – Chl a), backscatter (to estimate particulate organic carbon concentration – [POC]), and nitrate sensors conducted daily vertical profiles of the water column from a depth of 2000 m to the sea surface in the western North Pacific from January to April of 2018. Data for calibrating each sensor were obtained via shipboard sampling that occurred when the floats were deployed and recovered. During the float-deployment periods, repeated meteorological disturbances passed over the study area and caused the mixed layer to deepen. After deep-mixing events, the upper layer restratified and nitrate concentrations decreased while Chl a and POC concentrations increased, suggesting that spring mixing events promote primary productivity through the temporary alleviation of nutrient and light limitation. At the end of March, POC accumulation rates and nitrate decrease rates within the euphotic zone (0–70 m) were the largest of the four events observed, ranging from +84 to +210 mmol C m−2 d−1 and –28 to –49 mmol N m−2 d−1, respectively. The subsurface consumption rate of oxygen (i.e., the degradation rate of organic matter) after the fourth event (the end of March) was estimated to be –0.62 micromol O2 kg−1 d−1. At depths of 300–400 m (below the mixed layer), the POC concentrations increased slightly throughout the observation period. The POC flux at a depth of 300 m was estimated to be 1.1 mmol C m−2 d−1. Our float observation has made it possible to observed biogeochemical parameters, which previously could only be estimated by shipboard observation and experiments, in the field and in real time.

Oscillation, synchrony, and multi-factor patterns between cereal aphids and parasitoid populations in southern Brazil

In different parts of the world, aphid populations and their natural enemies are influenced by landscapes and climate. In the Neotropical region, few long-term studies have been conducted, maintaining a gap for comprehension of the effect of meteorological variables on aphid population patterns and their parasitoids in field conditions. This study describes the general patterns of oscillation in cereal winged aphids and their parasitoids, selecting meteorological variables and evaluating their effects on these insects. Aphids exhibit two annual peaks, one in summer–fall transition and the other in winter-spring transition. For parasitoids, the highest annual peak takes place during winter and a second peak occurs in winter–spring transition. Temperature was the principal meteorological regulator of population fluctuation in winged aphids and parasitoids during the year. The favorable temperature range is not the same for aphids and parasitoids. For aphids, temperature increase resulted in population growth, with maximum positive effect at 25°C. Temperature also positively influenced parasitoid populations, but the growth was asymptotic around 20°C. Although rainfall showed no regulatory function on aphid seasonality, it influenced the final number of insects over the year. The response of aphids and parasitoids to temperature has implications for trophic compatibility and regulation of their populations. Such functions should be taken into account in predictive models.

Bowen ratio and daily temperature range thresholds: Are they signals of transient seasons?

<p>The winter into spring and the summer into autumn transition seasons can last several weeks. Leaf emergence in midlatitude climates decreases the ratio of sensible (<em>H</em>) and latent heat (<em>LE</em>) fluxes - the Bowen ratio (<em>B</em>).  Because there are many more surface climate stations than flux-measuring sites, researchers seek to link the state variables at standard climate station heights to the leaf development.  Schwartz (1996) found out that, during the midlatitude onset of spring, the DTR trend rapidly increases for several weeks and then levels off.  Adopting an alternate approach, Fitzjarrald et al. (2001) linked changes in <em>H</em> and <em>LE</em> to <em>B </em>to the state variable daily change tendencies. This approach is based on assuming that the surface climate alters as a small fraction of the surface fluxes converge on average into the lower atmosphere.</p><p>Schwartz’ approach has the advantage of not requiring information from directly-measured fluxes, but station’s representativeness during the daytime (<em>T<sub>max</sub></em>) greatly exceeds the area that the <em>T<sub>min</sub></em> would describe. What’s more, daytime cloudiness depreses <em>T<sub>max</sub></em> but nocturnal cloudiness enhances <em>T<sub>min</sub></em>. The Fitzjarrald et al. approach requires long-term day-to-day averages to determine the times of the year when the surface state variables identify the consequences of leaf emergence.</p><p>            Here we seek to refine methods to relate plant characteristics to surface climate state, with emphasis on the spring transition at Harvard Forest (HF, MA, USA). At HF, J. O’Keefe kept a careful log of significant phenological events (Klosterman et al., 2018). The transition to the ‘growing season’ begins with bud break (mid-April), ending with nearly fully leafed crowns ("95%") in most species by mid-May.</p><p>We revisited the HF data and found that DTR, from the start of spring transition until the end of autumn, changes along with daily sensible heat flux changes, particularly during the period from sunrise until the daily maximum air temperature occurs. Since the seasonal course of daily temperature <em>T<sub>d </sub> </em>(°C) follows the latent heat flux trend, we <em>normalized</em> the DTR (DTR/<em>T<sub>d</sub></em>) and found that DTR/<em>T<sub>d</sub></em> ≈ 1 approximately at budbreak and again at "95%”. When the DTR next approaches <em>T<sub>d</sub></em><sub>,</sub> the autumn transition is beginning. We use the METAR data to identify cloudy/clear periods and assess the sensitivity of DTR to this effect.</p><p>We examined the utility of using DTR/<em>T<sub>d</sub></em> ratio as an indicator of spring and autumn transition, exploring  temperature measurements and phenological observations from the HF and PIS network (Lalic et al., 2020). Preliminary results indicate that this approach can identify significant effects of leaf state on local surface climate without the need for averaging over a decade or longer.</p><p>Fitzjarrald et al., 2001, 1175/1520-0442(2001)014<0598:CCOLPI>2.0.CO;2.</p><p>Klosterman et al., 2018, 1007/s00484-018-1564-9</p><p>Lalic et al., 2020, 1007/978-3-030-37421-1.</p><p>Schwartz, 1996, 1175/1520-0442(1996)009<0803:ETSDID>2.0.CO;2</p>

Timing of the VLF October effect in relation to mesospheric wind dynamics

<p>The seasonal variation of the daytime lower ionosphere, monitored using the propagation of Very Low Frequency (VLF) radio waves, shows an asymmetry when comparing the spring and autumn transitions. Considering the solar zenith angle variation, it can explain the spring transition but not the autumn one. The climatological variation exposes that the maximum of the VLF deviation is around the beginning of October. Thus, the deviation is called “the October effect”. This study aims to understand the possible atmospheric phenomena behind this effect. We use VLF signals transmitted from USA (NAA, f = 24 kHz), UK (GQD, f=19.6 kHz) and Iceland (NRK, f = 37.5 kHz) recorded in Northern Finland from 2011 to 2019. We compare our results with the Whole Atmosphere Community Climate Model with the thermosphere-ionosphere eXtension (WACCM-X) data. The October effect is separated into climatological earliest and latest effect according to WACCM-X climatological earliest and latest transitions from eastward to westward mean zonal winds</p>

A new classification of the Arctic spring transition in the middle atmosphere

<p>In the middle atmosphere, spanning the stratosphere and mesosphere, spring transition is the time period where the zonal circulation reverses from winter westerly to summer easterly which has a strong impact on the vertical wave propagation influencing the tropospheric and ionospheric variability. The spring transition can be rapid in form of a final sudden stratospheric warming (SSW, mainly dynamically driven) or slow (mainly radiatively driven) but also intermediate stages can occur. In most studies spring transitions are classified either by their timing of occurrence or by their vertical structure. However, all these studies focus exclusively on the stratosphere and can give only tendencies under which pre-winter conditions an early or late spring transition takes place and how it takes place. Here we classify the spring transitions regarding their vertical-temporal development beginning in January and spanning the whole middle atmosphere in the core region of the polar vortex. This leads to five classes where the timing of the SSW in the preceding winter and a downward propagating Northern Annular Mode (NAM) plays a crucial role. The results show distinctive differences between the five classes in the months before the spring transition especially in the mesosphere allowing a certain prediction for some of the five spring transition classes which would not be possible considering the stratosphere only.</p>

PSVIII-26 Extending the grazing season for grass-fed beef production into the spring transition period

In the Gulf Coast region, the spring transition period is a 45–60 d period between late April and mid-June. Red and white clovers’ growth pattern is delayed compared to winter grasses making them suitable for this transition period; however, an appropriate rest period allowing stockpiling is needed. Three treatments were evaluated on pastures planted in September of three consecutive years: 1) grazed until mid-February (MF); 2) grazed until first week of March (EM); and 3) grazed until last week of March (LM). Grazing re-started on May 1. Pasture was a mixed of annual ryegrass, red, white and berseem clover. Each year, 24 crossbred steers (330 ± 11 kg) were blocked by BW, allotted to 1 of 6 groups (2 replicates/treatment), and continuously stocked at 995 kg BW/ha. Forage mass at the beginning of the grazing period was greater (P < 0.05) in MF, followed by EM and LM. This represented a forage allowance of 2.0, 1.6, and 1.1 kg DM/kg BW. On d0, the proportion of annual ryegrass was greater (P < 0.05) in MF than in EM and the smallest in LM. Proportion of clovers was greater (P = 0.04) in EM in Year 2 while MF and LM were similar but greater for MF in Year 1. Berseem clover represented 59% of the clover biomass in MF while red clover was 72% of the clover biomass in LM. Proportion of clovers decreased with time while annual ryegrass became mature affecting its palatability. Steers that grazed on MF and EM had greater ADG (1.83 and 1.71 kg) than those on LM (1.41 kg). Grazing season was longer (P = 0.03) for MF (66 days) than for LM (39 days) while EM was intermediate (50 days). A rest period from early March to late April would allow grazing of high-quality pastures during the spring transition period.