Increasing summertime penetrative shortwave radiation and its weakening effect on the seasonal cycle in mid-latitude oceans

doi:10.21203/rs.3.rs-5114025/v1

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Increasing summertime penetrative shortwave radiation and its weakening effect on the seasonal cycle in mid-latitude oceans

https://doi.org/10.21203/rs.3.rs-5114025/v1

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The seasonal cycle of sea surface temperature (SST) in the mid-latitude ocean is projected to be strengthened with greater warming in the summer hemisphere under global warming. Influenced by the phytoplankton pigment and mixed layer depth, a fraction of the shortwave radiation penetrates out of the bottom of the mixed layer (hereafter Q_pen), which significantly affects the seasonal cycle of the SST. However, how the Q_pen will change under a warmer climate and its effect on the seasonal cycle of SST remain unknown. Here, we show that the summertime Q_pen increases by 3.9 (3.7) ± 1.9 (1.6) W m^− 2 in the northern (southern) mid-latitude oceans during the second half of the 21st century by analyzing state-of-the-art Earth System Models simulations. This remarkable increase in Q_pen is mainly due to the shoaling of the mixed layer and partly to the reduction in chlorophyll concentration, which contributes significantly to the increase in Q_pen due to its greater sensitivity to shortwave penetration depth in mid-latitude oceans. The enhanced summertime Q_pen tends to efficiently weaken the amplitude of the summertime SST by ~ 0.3°C month^− 1, whereby it mitigates the increase in net surface heat flux associated with greenhouse gas warming. These significant changes in Q_pen highlight the significance of quantifying Q_pen effects and future ocean phytoplankton-induced heating feedbacks in mid-latitude seasonal cycles.

Earth and environmental sciences/Climate sciences/Ocean sciences/Physical oceanography

Earth and environmental sciences/Climate sciences/Ocean sciences/Marine biology

Since the Industrial Revolution, human-induced greenhouse gas emissions have significantly altered the physical and biogeochemical conditions of the global ocean. ^1–4. Climate model simulations suggest that in a warmer climate, low- to mid-latitude oceans will likely experience a more pronounced seasonal cycle in sea surface temperature (SST), indicating that the summer hemisphere will undergo greater warming, while the winter hemisphere will experience less significant warming ^5–9. Importantly, this human-induced impact has become distinguishable from natural variability ^10,11, exerting significant impacts on worldwide seasonal rainfall, occurrences of marine heatwaves, and phytoplankton growth ^12–17.

The enhancement of seasonal SST variability in the mid-latitude ocean can be attributed to a shallower mixed layer (ML) in association with enhanced density stratification and, to some extent, an increase in the net surface heat flux (hflx) due to the direct greenhouse gas effect ^7,18,19. The thinned ML plays an important role in amplifying the seasonal cycle by trapping hflx within a thin surface layer, leading to an increase in SST, particularly in summer when the mean ML is thinner ^18,19. However, on the opposite side of the coin, the thinned annual mean ML allows more penetrative shortwave radiation (Q_pen) to enter the subsurface layer and subsequently reduces the hflx trapped within the ML. Physically, the value of Q_pen is influenced by three factors according to the Lambert‒Beer law: net shortwave downwelling to the sea surface (SW), mixed layer depth (H_m), and penetration depth (H_p) ^20–22. The increase in density stratification due to global warming contributes to the shoaling of ML ^23,24; meanwhile, the abundance of Hp increases due to the reduction in chlorophyll pigments as the nutrient supply decreases ². Quantitatively, under a high emissions scenario, the global annual mean Q_pen can increase by 2.49 W m^− 2, which is comparable to the increase in hflx ²⁵. During summer, when the climatological ML is thin, the shoaled ML and increased H_p may significantly enhance the seasonal Q_pen; however, the seasonal changes of Q_pen under global warming still lack quantification

As for the impacts of Q_pen on the earth climate, Q_pen affects the subsurface thermal structure ²⁶, which can further modulate climate variability from the annual mean to interannual variability. This effect is well recognized from regional scales to global scales by various modeling studies in the current climate ^27–36. For example, on the seasonal timescale, indirect evidence reveals that the effects of colored dissolved organic matter on Q_pen can amplify seasonal SST extremes in the mid-latitude ocean ³⁷.

However, whether Q_pen can increase significantly during the summer is still uncertain, and more research is needed to identify its controlling factors and potential climatic effects, especially for the seasonal cycle of SST. We address these concerns by conducting a detailed investigation of how the summertime Q_pen will change in the mid-latitude ocean by the end of 2100 in future climate projections. Outputs from ten state-of-the-art Earth system models (ESMs) included in the sixth phase of the Coupled Model Intercomparison Project (CMIP6) under the SSP5-8.5 scenario and four emission scenarios from CESM2 are used to analyze the physical processes responsible for Q_pen changes and their impacts on the seasonal evolution of Q_pen.

Historical seasonal cycle of Q _pen and its impacts on SST

Historically, the seasonal cycle of the Q_pen has exhibited complex regional dependence, with considerable changes in the mid-latitude oceans, especially in the mode water formation region, with marked changes in the seasonal H_m (Fig. 1a). Interestingly, the minimum H_m corresponds well to the seasonal variation in the Q_pen (Fig. 1a, contour lines), suggesting that the summertime minimum H_m plays a dominant role in determining the seasonal cycle of the Q_pen. Furthermore, in the mid-latitude ocean, the spatial pattern of the seasonal variation in Q_pen resembles the structure of the subtropical gyre circulation (Fig. 1a), with a maximum value of 20 W m^− 2. In contrast, the seasonal variation in Q_pen is relatively small due to the weak seasonal change in H_m in tropical oceans and the extremely deep ML in high-latitude oceans (Figure S1). This case can be found in the Southern and North Atlantic Oceans, where the extremely deep annual mean ML prevents SW from any penetration out of the base of the ML. Biologically, the seasonal variation in H_p was more considerable in the oligotrophic region and in the North Atlantic Ocean (Figure S1), which can efficiently modulate the seasonal cycle of the Q_pen.

Due to the significant change in Q_pen in the mid-latitude ocean (Fig. 1a), we define two latitudinal bands (40°S-20°S and 20°N-45°N) in the northern (NH) and southern (SH) hemispheres as the regions of focus. The time series shows that the seasonal cycle of Q_pen peaks in January (July) in the SH (NH) at more than 40 W/m², which corresponds to approximately half of the hflx (Fig. 1b and 1c). The minimum value of Q_pen appears in July (January) in the SH (NH), which is accompanied by the deepest ML. Therefore, the effect of Q_pen is mainly in summer, i.e., during December-January-February (DJF) in the SH and June-July-August (JJA) in the NH. This significant change in the seasonal Q_pen can affect the seasonal SST evolution, which can be further quantified by a simplified mixed-layer heat budget. In particular, hflx-induced heating (Q_net) controls the mixed-layer temperature on a seasonal timescale, while ocean dynamical processes tend to partially offset hflx-induced heating (Figure S2). Furthermore, Q_pen reduces SST warming by directly decreasing the absorbed shortwave radiation in the mixed layer (Figure S2), and this cooling effect is comparable to hflx-induced warming during summer

Quantifying the factors contributing to Q _pen change

Overall, due to the combined influences of decreased H_m and increased H_p, the change in Q_pen under the SSP5-8.5 scenario exhibits a consistent seasonal increase, with the most pronounced change occurring in summer, reaching 4–6 W m^− 2 in the mid-latitude ocean (Fig. 1d and f). Quantitatively, the summertime Q_pen in the multimodel ensemble mean increased by 3.9 (3.7) ± 1.9 (1.6) W m^− 2 in the northern (southern) mid-latitude oceans during the twenty-first century. The increase in Q_pen is most pronounced in the subtropical Pacific and the North Atlantic Ocean, particularly in the northeastern subtropical Pacific, which differs from the region where climatological Q_pen is large in the present climate (Figs. 2a-b), reflecting the significant change in this region under a warmer climate. Moreover, in the mid-latitude SH oceans, the increase in Q_pen is mainly concentrated in the sub-Antarctic front, with the largest increase reaching 10 W m^− 2 (Fig. 2a).

Under the SSP5-8.5 scenario, the three factors that determine Q_pen exhibit great diversity in the mid-latitude ocean. For the SW, the mid-latitude oceans show negligible changes, with the most significant change occurring in the boreal spring, with an increase of 1.6%, corresponding to 4 W m^− 2 (Figure S3). Therefore, this contribution to the change in Q_pen can be ignored in the following analysis. H_m tends to decrease in the mid-latitude ocean, with the most pronounced decrease occurring in February in the NH and September in the SH, albeit with greater intermodel uncertainty. Note that the change in H_m during summer is relatively small and is accompanied by a shallow climatological mean ML. Therefore, the shallowing of the ML in summer can lead to a significant increase in Q_pen by permitting much shortwave radiation to enter the subsurface layer. H_p exhibits relatively uniform changes during all seasons, increasing by ~ 1 m. Nevertheless, the change in Q_pen was sensitive to the change in H_p due to the nonlinear relationship between Q_pen and H_p (Eq. 1 in the methods section), especially in summer.

The contribution to Q_pen can be further quantified in Fig. 2, with a focus on summer in the Northern Hemisphere (JJA) and Southern Hemisphere (DJF). In the mid-latitude ocean, the SW has a small contribution to the change in Q_pen (Figs. 2c and 2d). Most of the change in Q_pen is attributed to ML shoaling, accounting for approximately 75% of the variability, while H_p contributes 25% of the variability during the summer (Figs. 2e, 2f, and S4). In the mid-latitude SH oceans, the change in H_p also contributes less to Q_pen than to H_m by 5 W m^− 2 (Figs. 2g, 2h, and S4). Although the H_m and H_p exhibites considerable intermodel uncertainty (Figure S3), Q_pen exhibites a relatively small spread due to the mutual cancellation of the three factors.

By analyzing the contributions of H_m and H_p to Q_pen under the SSP5-8.5 scenario, we found that the change in H_p plays a significant role in influencing Q_pen in the mid-latitude ocean during summer, albeit with relatively small changes (Fig. 2g and 2h). Therefore, a sensitivity analysis is performed by a Taylor expansion for Eq. 1 according to ³⁸, which can be linearly decomposed into contributing terms as follows:

$$\:\varDelta\:{Q}_{pen}=\gamma\:{Q}_{sr}exp(-\frac{{H}_{m}}{{H}_{p}})\frac{1}{{H}_{p}}\left[\varDelta\:{H}_{m}-{H}_{m}\varDelta\:{H}_{p}\right]$$

;

where Q_sr, H_m, and H_p represent the historical climatological mean SW, H_m, and H_p fields, respectively. ΔQ_pen, ΔH_m, and ΔH_p denote their changes under the SSP5-8.5 scenario. This equation illustrates that the change in ΔQ_pen is determined by the relative magnitudes of ΔH_m and H_mΔH_p. Thus, the relatively small change in ΔH_p can cause a substantial change in Q_pen because its effect can be amplified by a multiple of H_m. Physically, the climatological H_m is extremely shallow in summer due to surface warming, which indirectly amplifies the effect of penetrative shortwave radiation induced by the increased H_p. The diagram of the effects of H_m-H_p on the Q_pen shown in Fig. 3 further confirms this theoretical analysis.

The H_m-H_p diagram plot shows that the change in Q_pen is sensitive to the changes in H_p and H_m, which is due to the climatological shoaling of the ML (Fig. 3). The CMIP6 ESMs can well capture the observed Q_pen during summertime and wintertime (Fig. 3a and 3b). More importantly, as shown in Eq. 1 derived from the Taylor expansion, the change in Q_pen is more sensitive to that in H_p under the SSP5-8.5 scenario than to that in H_m, which is well illustrated in Fig. 3c-d. In Fig. 3c, when there is a change in Q_pen, less change in H_p can achieve a change in Q_pen than in H_m during the austral summer, and similar conditions can be found in the NH during the boreal summer (Fig. 3d). In other words, under the global warming scenario, a subtle change in H_p can efficiently modulate the change in Q_pen during summer, contributing to the greater sensitivity of H_p to Q_pen under a relatively shallow ML. The high sensitivity of Q_pen to H_p indicates that a change in phytoplankton (e.g., a decrease in chlorophyll or a phytoplankton bloom) can significantly influence the Q_pen and the thermodynamics of the subsurface layer or even affect the evolution of SST by changing the timing of blooms¹⁴. In contrast, during winter, the deep ML can lead to a minimum value of Q_pen under the SSP5-8.5 scenario, indicating that the climate effect of Q_pen is negligible.

Impacts of increased Q _pen on the seasonal cycle of SST

The contributions of the increased Q_pen to the seasonal upper ocean temperature are quantified by a mixed-layer heat budget analysis. Due to the different vertical mixing processes among CMIP6 models and the lack of data in some selected CMIP6 simulations, oceanic processes (advection and diffusion) are considered residual terms in the Methods section. It should be acknowledged that the impact of increased Q_pen on specific ocean dynamic processes (e.g., vertical advection and circulation change) is relatively small compared to the changes in hflx and H_m at the ocean-basin scale. Therefore, we only emphasize the potential effect of Q_pen on the mixed layer temperature, and the contribution from an ocean dynamic process is considered to compensate for the heat flux change, as adopted in a previous study ^18,19.

The heat budget analysis reveals that the SST tends to increase under the SSP5-8.5 scenario, with an increase in SST emerging during the summertime, i.e., JJA in the NH and DJF in the SH (Fig. 4a and b). Moreover, relative cooling occurs during the winter in both the SH and NH. The mixed layer warming due to the increased hflx to the mixed layer (Q_net) plays a dominant role in determining the seasonal tendency of the SST. In contrast, ocean dynamic processes act to partially compensate for the changes induced by changes in hflx. The effect of Q_pen shows a relatively constant cooling effect due to its enhanced effect in a warmer climate, as shown in Figs. 3a-b. Similar to the marked change in Q_pen compared to Q_net in the annual mean ²⁵, the enhanced Q_pen causes a cooling effect that is comparable to the change in Q_net on the seasonal timescale, which is predominantly in summer and reaches 0.3°C month^− 1. The relative relationship between Q_net and Q_pen is shown in Figs. 4c and 4d, suggesting that the comparable contribution of Q_pen occurs mainly in the low- to mid-latitude ocean, especially in the northern Atlantic Ocean, the northeastern subtropical Pacific, and the entire mid-latitude Southern Ocean.

To highlight the sensitivity of Q_pen to radiation forcing, an analysis was performed using four emission scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5) from CESM2, which is considered the best model for reproducing the observed spatial pattern of Q_pen (Figures S5-S6). The results suggest that with increasing greenhouse gas emissions, Q_pen gradually increases in the Southern Hemisphere during the DJF period (Figure S7). Note that the magnitude of Q_pen even exceeds the change in hflx under SSP2-4.5, which is a relatively reasonable emissions pathway for the future. In contrast, the change in Q_pen during summer can reach half of the change in hflx in the NH during JJA. Nevertheless, the changes in Q_pen and hflx are not linear, with a decrease occurring in SSP3-7.0 due to the decrease from the SW in the northeastern Pacific Ocean.

Based on previous analysis, we identified three factors that affect Q_net under the SSP5-8.5 scenario in Eq. 3. These factors are hflx, H_m, and Q_pen. Each factor is separately analyzed to better understand its effects by using linear decomposition. The computational error due to linear decomposition is negligible (Figure S8). In the 30°N-60°N region during boreal summer (JJA), the change in H_m has the largest effect on surface warming, while hflx has a secondary effect (Fig. 4f). In the 0°-30°N region, the change in hflx dominates the SST warming, and the contribution of H_m is minimal (Fig. 4f), especially in the northeastern tropical Pacific (Figure S9). In contrast, an increase in Q_pen induces a cooling effect that gradually increases toward the north. In the SH, the contributions of these three factors exhibit a complex distribution (Fig. 4e). The change in SST due to hflx is relatively small north of 60°S. Moreover, the contributions of H_m and Q_pen tend to cancel each other out, especially in the subantarctic front (Figure S10), resulting in a slightly weak SST change in the SH during the austral summer. These results indicate that the comparable change in Q_pen with hflx can induce a remarkable decrease in the SST in the mid-latitude ocean during summer, albeit with the dominant role of H_m.

Tight relationship between Q _pen and SST changes in the mid-latitude ocean

Due to the remarkable cooling effect of Q_pen on the SST on the annual mean timescale under the global warming scenario ²⁵, the potential feedback between increased Q_pen and SST may also exist on a seasonal timescale. Figure 5 shows that the significant correlations are mainly located in the mid-latitude ocean, with R = 0.59 (P = 0.07) and R = 0.70 (P = 0.02) in the NH and SH summers (Figs. 5a-b), respectively. In contrast, the correlation between Q_pen and SST in the global and tropical oceans is insignificant in the annual mean field (Fig. 5c-d), where the seasonal cycle is relatively weak. As a significant positive correlation between SST and Q_pen changes in the mid-latitude ocean, an increase in SST corresponds to an increase in Q_pen. As discussed in the previous sections, increased Q_pen can result in surface cooling through more shortwave penetration into the subsurface layer (less shortwave absorption within the surface layer); the strong correlation between Q_pen and SST suggests that Q_pen may exert negative feedback on surface warming in mid-latitude oceans under a global warming scenario. Note that the observed correlation does not necessarily imply causation, which requires further numerical experiments to demonstrate it.

Our finding of an increased Q_pen under a warmer climate during summertime in the mid-latitude ocean is underpinned by the combined effect of shoaling ML and increased H_p associated with a reduction in surface chlorophyll. The high sensitivity of Q_pen to H_p leads to a comparable effect of H_p to shoaling ML, indicating that the potential feedback due to reduced chlorophyll on SST cannot be ignored. A further mixed layer heat budget demonstrates that the Q_pen plays a considerable role in affecting seasonal SST evolution and decreasing SST during summertime and subsequently weakens the seasonal cycle of SST in the mid-latitude ocean, which contrasts with the enhanced seasonal SST due to global warming. Therefore, future climate projections should consider the effect of the Q_pen on the seasonal cycle under global warming.

Some current shortcomings need to be emphasized. The climate model exhibits remarkable biases in the simulations of SW, H_p, and H_m (Figures S5-S6, Figure S11), which may further underestimate the projection of Q_pen over the mid-latitude ocean in climate models. In addition, the direct Q_pen effect cannot be directly quantified due to the z- or sigma-coordinate configuration of the current ocean general circulation model, which does not explicitly define H_m ³⁹. Therefore, the effects of increased Q_pen need to be investigated in climate models with sophisticated experiments to determine the direct effect of Q_pen.

a. Data

The global gridded net downward shortwave radiation dataset is obtained from ECMWF/ERA5, while the temperature and salinity dataset is obtained from EN4 ⁴⁰. Additionally, the surface chlorophyll dataset utilized was obtained from the GlobColor Project during 1998–2018 ⁴¹. This study utilized ten earth system models from CMIP6, focusing on the historical run (1850–2014) and the Shared Socioeconomic Pathways 8.5 (SSP5-8.5) run (Table S1). In addition, outputs of the other three scenarios (SSP1-2.6, SSP2-4.5, and SSP3-7.0) from CESM2 are also adopted to emphasize the sensitivity of the Q_pen change to the emission scenarios. Some variables are analyzed, including the monthly mean net surface downward shortwave radiation at the sea surface, surface chlorophyll concentration, potential temperature, salinity, and hflx, with the horizontal resolution being interpolated to 1 degree. These models can simulate the mean state and seasonal cycle well for physical and biogeochemical fields ²⁵. The differences in climatological fields between the twentieth (1951–2000) and twenty-first (2051–2100) centuries represent future changes.

b. Shortwave penetration calculation and attribution

The amount of solar radiation that penetrates through the bottom of the ML is determined by the net downward solar radiation (Q_sr), mixed layer depth (H_m), and penetration depth (H_p), with the Beer‒Lambert law stating that shortwave radiation decays exponentially with depth.

$\:{Q}_{pen}=Qsr[\gamma\:{exp}(-{H}_{m}/{H}_{p})]$ (1);

To determine the amount of solar radiation that penetrates through the bottom of the upper ocean layer, we consider factors such as net downward solar radiation, mixed layer depth, and penetration depth. The Beer–Lambert Law indicates that shortwave radiation decays exponentially with depth. In this context, we can use the variables SW and γ = 0.33, which denote the net downwelling solar radiation arriving at the sea surface and the fraction of solar radiation that penetrates the sea surface by several centimeters, respectively. Additionally, we define H_m as the upper-ocean layer depth at which the density changes by a threshold ∆ρ, which we calculated using ∆ρ = 0.03 kg m^− 3. We also define H_p as the inverse of the shortwave attenuation coefficient, which is calculated as follows: $\:{K}_{A}={K}_{w}+{K}_{C}Chl$, where K_A is the total attenuation coefficient for shortwave radiation, K_W=0.028 m⁻¹ is the seawater attenuation coefficient, and K_C=0.058 m^− 1 (mg m⁻³)⁻¹ is the attenuation coefficient for chlorophyll adopted from ⁴².

The individual contributions of the controlling factors to determining changes in the Q_pen (SW, H_m, and H_p) were further quantified using a linear decomposition method ^25,43; for example, the contribution of changes in H_m under the SSP5-8.5 scenario can be computed as follows:

$\:{\varDelta\:Q}_{pen}={Q}_{pen}\left(\stackrel{-}{SW},\stackrel{-}{{H}_{m}}+\varDelta\:{H}_{m},\stackrel{-}{{H}_{p}}\right)-{Q}_{pen}\left(\stackrel{-}{SW},\stackrel{-}{{H}_{m}},\stackrel{-}{{H}_{p}}\right)$ (2);

where “-” denotes the historical mean and “Δ” denotes the change under the SSP5-8.5 scenario.

c. Mixed layer heat budget

A simplified mixed layer heat budget analysis study ^25,44 is adopted to emphasize the role of Q_pen:

$\:\frac{\partial\:T}{\partial\:t}=\frac{hflx-{Q}_{pen}}{{\rho\:}_{0}{C}_{p}{H}_{m}}+Residual$ (3);

where T denotes the temperature in the mixed layer. $\:\frac{\partial\:T}{\partial\:t}$ denotes the temperature tendency term. The first term on the right-hand side denotes the net heat flux-induced heating within the mixed layer (Q_net); hflx denotes the net surface heat flux at the air-sea interface; Q_pen is the penetrative solar radiation computed in Eq. (1); H_m is the mixed layer depth; ρ₀ (= 1030 kg m^− 3) is the density of seawater; and c_p (= 4000 J kg^− 1 °C^− 1) is the heat capacity. The residual term denotes the contribution of ocean dynamical processes to the mixed layer temperature change. The individual effects from hflx, Q_pen, and H_m are considered separately by fixing each term as the historical climatological fields during the future scenario, similar to Eq. (2).

Competing interests

The authors declare no competing interests.

Author contributions

F. T. and R.-H. Z conceived the idea and proposed this study. F.T. conducted the data analyses. The manuscript was drafted by F.T. and R.-H. Z

Acknowledgments

Tian is supported by the National Natural Science Foundation of China (NFSC; Grant 42476010;42006001) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB42000000 (XDB42040100, XDB42040103)). Zhang is supported by NFSC (Grant No. 42030410), Laoshan Laboratory (No. LSKJ202202402) and the Startup Foundation for Introducing Talent of NUIST.

Data Availability Statement

We thank the National Oceanic and Atmospheric Administration (NOAA)/National Centers for Environmental Information (NCEI) (https://www.ncei.noaa.gov/products/world-ocean-atlas), the GlobColor Project (https://hermes.acri.fr/index.pH_p?class = archive), the Asia-Pacific Data-Research Center (APDRC) (http://apdrc.soest.hawaii.edu/data/data.pH_p), the European Centre for Medium-Range Weather Forecasts (ECMWF) (https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era-interim), and the Met Office Hadley Centre (http://hadobs.metoffice.com/en4/download.html) for their provision of the data. We also thank the climate modeling groups (listed in Table S1) for producing and making available their model outputs and the Earth System Grid Federation (ESGF) for archiving the CMIP6 data and providing access; CMIP6 outputs were downloaded from the ESGF-DOE/LLNL node (https://esgf-node.llnl.gov/search/cmip6/).

Code availability

The code used in this study is available from the corresponding author upon reasonable request.

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SeasonalQpenGWv4Supplementary0917.docx

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Increasing summertime penetrative shortwave radiation and its weakening effect on the seasonal cycle in mid-latitude oceans

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