The Photophysiological Response of Nitrogen-Limited Phytoplankton to Episodic Nitrogen Supply Associated With Tropical Instability Waves in the Equatorial Atlantic

In the Equatorial Atlantic nitrogen availability is assumed to control phytoplankton dynamics. However, in situ measurements of phytoplankton physiology and productivity are surprisingly sparse in comparison with the North Atlantic. In addition to the formation of the Equatorial cold tongue in the boreal summer, tropical instability waves (TIWs) and related short-term processes may locally cause episodic events of enhanced nutrient supply to the euphotic layer. Here, we assess changes in phytoplankton photophysiology in response to such episodic events as well as short-term nutrient addition experiments using a pair of custom-built fluorometers that measure chlorophyll a (Chl a) variable fluorescence and fluorescence lifetimes. The fluorometers were deployed during a transatlantic cruise along the Equator in the fall of 2019. We hypothesized that the Equatorial Atlantic is nitrogen-limited, with an increasing degree of limitation to the west where the cold tongue is not prominent, and that infrequent nitrate injection by TIW related processes are the primary source alleviating this limitation. We further hypothesized phytoplankton are well acclimated to the low levels of nitrogen, and once nitrogen is supplied, they can rapidly utilize it to stimulate growth and productivity. Across three TIW events encountered, we observed increased productivity and chlorophyll a concentration concurrent with a decreased photochemical conversion efficiency and overall photophysiological competency. Moreover, the observed decrease in photosynthetic turnover rates toward the western section suggested a 70% decrease in growth rates compared to their maximum values under nutrient-replete conditions. This decrease aligned with the increased growth rates observed following 24 h incubation with added nitrate in the western section. These results support our hypotheses that nitrogen is the limiting factor in the region and that phytoplankton are in a state of balanced growth, waiting to “body surf” waves of nutrients which fuel growth and productivity.

Wolkenverfolgung: Vom Experiment zur Realität

<p>Die Wolkenverfolgung ist die einzige Möglichkeit zur Beobachtung der zeitlichen Entwicklung von Wolken und zur Quantifizierung der Veränderung ihrer physikalischen Eigenschaften während ihrer Lebensdauer (Seelig et al., 2021). Der Schlüssel dazu sind zeitaufgelöste Messungen von Instrumenten an Bord geostationärer Satelliten. Experimente mit atmosphärenähnlicher Konfiguration treiben die Entwicklung von Messmethoden und Alghoritmen unter Laborbedingungen voran. Heutzutage ist es z.B. möglich zweidimensionale, zeitlich und räumlich hochaufgelöste Geschwindigkeitsfelder auf Basis der Verschiebung kleinster Partikel zu messen (Seelig and Harlander, 2015; Seelig et al., 2018). Die Methodik der Partikelgeschwindigkeitsmessung dient als Anfangsbedingung zum Verfolgen dieser Partikel und kann auf troposphärische Wolken angewendet werden. Diese Präsentation stellt die Analogie von Experiment zur Realität vor, beschreibt das Verfahren der Partikelgeschwindigkeitsmessung und die Anwendung auf Daten geostationärer Satelliten.</p> <p><strong>Literatur:</strong></p> <p>Seelig, T., Deneke, H., Quaas, J., and Tesche, M.: Life cycle of shallow marine cumulus clouds from geostationary satellite observations, J. Geophys. Res.: Atmos., 126(22), e2021JD035577, https://doi.org/10.1029/2021JD035577, 2021.</p> <p>Seelig, T., Harlander, U., and Gellert, M.: Experimental investigation of stratorotational instability using a thermally stratified system: instability, waves and associated momentum flux, Geophys. Astrophys. Fluid Dyn., 112, 239-264, https://doi.org/10.1080/03091929.2018.1488971, 2018.</p> <p>Seelig, T. and Harlander, U.: Can zonally symmetric inertial waves drive an oscillating zonal mean flow?, Geophys. Astrophys. Fluid Dyn., 109, 541-566, https://doi.org/10.1080/03091929.2015.1094064, 2015.</p>

A Case Study of Cloud-top Kelvin–Helmholtz Instability Waves near the Dendritic Growth Zone

Kelvin-Helmholtz instability (KH) waves have been broadly shown to affect the growth of hydrometeors within a region of falling precipitation, but formation and growth from KH waves at cloud top needs further attention. Here, we present detailed observations of cloud-top KH waves that produced a snow plume that extended to the surface. Airborne transects of cloud radar aligned with range height indicator scans from ground-based precipitation radar track the progression and intensity of the KH wave kinetics and precipitation. In-situ cloud probes and surface disdrometer measurements are used to quantify the impact of the snow plume on the composition of an underlying supercooled liquid water (SLW) cloud and the snowfall observed at the surface. KH wavelengths of 1.5 km consisted of ~750-m-wide up- and downdrafts. A distinct fluctus region appeared as a wave-breaking cloud top where the fastest updraft was observed to exceed 5 m s−1. Relatively weaker updrafts of 0.5-1.5 m s−1 beneath the fluctus and partially overlapping the dendritic growth zone were associated with steep gradients in reflectivity of −5 to 20 dBZe in as little as 500 m depths due to rapid growth of pristine planar ice crystals. The falling snow removed ~80% of the SLW content from the underlying cloud and led to a twofold increase in surface liquid equivalent snowfall rate from 0.6 to 1.3 mm hr−1. This paper presents the first known study of cloud-top KH waves producing snowfall with observations of increased snowfall rates at the surface.

Off-Equatorial Deep-Cycle Turbulence Forced by Tropical Instability Waves in the Equatorial Pacific

The equatorial Pacific cold tongue is a site of large heat absorption by the ocean. This heat uptake is enhanced by a daily cycle of shear turbulence beneath the mixed layer—“deep-cycle turbulence”—that removes heat from the sea surface and deposits it in the upper flank of the Equatorial Undercurrent. Deep-cycle turbulence results when turbulence is triggered daily in sheared and stratified flow that is marginally stable (gradient Richardson number Ri ≈ 0.25). Deep-cycle turbulence has been observed on numerous occasions in the cold tongue at 0°, 140°W, and may be modulated by tropical instability waves (TIWs). Here we use a primitive equation regional simulation of the cold tongue to show that deep-cycle turbulence may also occur off the equator within TIW cold cusps where the flow is marginally stable. In the cold cusp, preexisting equatorial zonal shear uz is enhanced by horizontal vortex stretching near the equator, and subsequently modified by horizontal vortex tilting terms to generate meridional shear υz off of the equator. Parameterized turbulence in the sheared flow of the cold cusp is triggered daily by the descent of the surface mixing layer associated with the weakening of the stabilizing surface buoyancy flux in the afternoon. Observational evidence for off-equatorial deep-cycle turbulence is restricted to a few CTD casts, which, when combined with shear from shipboard ADCP data, suggest the presence of marginally stable flow in TIW cold cusps. This study motivates further observational campaigns to characterize the modulation of deep-cycle turbulence by TIWs both on and off the equator.

Long-distance radiation of Rossby waves from the equatorial current system

Measurements from satellite altimetry are used to show that sea-surface height (SSH) variability throughout much of the North Pacific is coherent with the SSH signal of the tropical instability waves (TIWs) that result from instabilities of the equatorial currents. This variability has regular phase patterns consistent with freely propagating barotropic Rossby waves radiating energy away from the unstable equatorial currents, and the waves clearly propagate from the equatorial region to at least 30°N. The pattern of SSH variance at TIW frequencies exhibits remarkable patchiness on scales of hundreds of kilometers, which we interpret as being due to the combined effects of wave reflection, refraction, and interference. North of 40°N, more than 6000 km from the unstable equatorial currents, the SSH field remains coherent with the near-equatorial SSH variability, but it is not as clear whether the variability at the higher latitudes is a simple result of barotropic wave radiation from the tropical instability waves. Even more distant regions, as far north as the Aleutian Islands off of Alaska and the Kamchatka Peninsula of eastern Russia, have SSH variability that is significantly coherent with the near-equatorial instabilities. The variability is not well represented in the widely used gridded SSH data product commonly referred to as the AVISO or DUACS product, and this appears to be a result of spatial variations in the filtering properties of the objective mapping scheme.

Two-point radiation statistics from large-scale turbulent structures within supersonic jets

The noise from large-scale coherent turbulent structures within jets remains the dominant source. For the purpose of developing future control systems for the large-scale noise source, we investigate the statistics between upstream and downstream radiating waves. We investigate two off-design supersonic jet flows with instability theory and associated noise radiation, large-eddy simulation (LES), and experiments. We compare the auto-correlation, cross-correlation, coherence, and other statistics predicted by aeroacoustic instability theory. As instability waves are closely connected with the formation of large-scale turbulent structures, they yield insight into large-scale noise statistics. We investigate two nozzles at two supersonic off-design conditions. The first is a biconic nozzle operating at an unheated condition, and the second is a NASA nozzle operating at a heated condition. We find that for these jets, the noise from instability waves is coherent between 0.40 to 0.70 at large-scale radiation frequencies between the downstream and upstream radiation directions.

Effect of tropical instability waves on the eastern tropical Pacific basin: damping of TIWs in a high-resolution ocean circulation model

<p>Tropical instability waves (TIWs) are oceanic cusp-like features propagating westward along the northern front of the tropical pacific cold tongue. Observational and modelling studies suggest that TIWs may have a large impact on the eastern tropical Pacific background state from seasonal to interannual time-scales, through heat advection and mixing. However, observations are coarse or limited to surface data, and modelling studies are often based on the comparison of low- vs. high-resolution simulations. In this study, we perform a set of regional high-resolution ocean simulations (CROCO 1/12°) in which we strongly damp (NUDG-RUN) or not (CTRL-RUN) TIWs propagation, by nudging the mixed layer meridional current velocities in the TIWs active region toward their climatological values. We first show that this approach do not alter the model internal physics, in particular related to the equatorial wave dynamics. The impact of TIWs on the oceanic mean state (zonal current and heat budget) is then assessed by comparing CTRL-RUN to NUDG-RUN. This approach allows quantifying for the first time the rectified effect of TIWs without degrading the model horizontal resolution, and may lead to a better understanding of ENSO asymmetry and the development of accurate TIWs parameterizations in Earth system models.</p>

Subsurface Tropical Instability Waves in the Atlantic Ocean in Model and Observations

<div> <p><span>Tropical instability waves (TIWs) near the ocean surface are present in all tropical oceans and are known to be important for air-sea interactions and regional climate variability. Recent studies based on observations in the Pacific Ocean found that apart from TIWs at the surface, there also exist subsurface TIWs (subTIWs) which can alter vertical mixing. To date, most studies have focused on TIW related dynamics near the ocean surface. However, to properly assess vertical mixing in the upper ocean, improved understanding of the vertical structure of TIWs and the influence of subTIWs is needed. In this study, </span>we show subTIW<span> presence</span> in the Atlantic Ocean for the first time using mooring observations.<span>Further, we characterize subTIWs in the tropical Atlantic Ocean with a special focus on subTIW spatial and temporal variability and their effect on mixing. For this, data covering almost two decades are used that were generated from a comprehensive, global, high-resolution ocean model forced by the reanalysis ERA5. We find subTIWs between 40 m depth and the thermocline in both model and observations and unlike TIWs, subTIWs are frequently active both north and south of the Equator. The results of our study suggest that subTIWs induce a multi-layer shear structure which has the potential to destabilize the mean flow and thereby cause mixing. These effects are strongest north of the Equator where TIWs and subTIWs act simultaneously, implying possible TIW/subTIW interactions. </span>We conclude that subTIWs are a feature of the tropical Atlantic Ocean with regionally varying implications for vertical mixing and heat fluxes. <span>In addition, subTIWs differ from TIWs in their temporal and regional occurrences Therefore, subTIWs should be considered in </span>f<span>u</span>ture assessments of upper ocean dynamics, particularly in subTIW dominated regions.</p> </div>