Deep Winter Mixed Layer Research Articles

Deep Winter Mixed Layer Anchored by the Meandering Antarctic Circumpolar Current: Cross-Basin Variations

Abstract A deep winter mixed layer forms north of the Antarctic Circumpolar Current (ACC) in the Indo-Pacific sectors, while the mixed layer depth (MLD) is shallow in the Atlantic. Using observations and a global atmospheric model, this study investigates the contribution of surface buoyancy flux and background stratification to interbasin MLD variations. The surface heat flux is decomposed into broad-scale and frontal-scale variations. At the broad scale, the meandering ACC path is accompanied by a zonal wavenumber-1 structure of sea surface temperature (SST) with a warmer Pacific than the Atlantic; under the prevailing westerly winds, this temperature contrast results in larger surface heat loss facilitating deeper MLD in the Indo-Pacific sectors than in the Atlantic. In the Indian sector, the intense ACC fronts strengthen surface heat loss compared to the Pacific. The surface freshwater flux pattern largely follows that of evaporation and reinforces the heat flux pattern, especially in the southeast Pacific. A diagnostic relationship is introduced to highlight the role of ACC’s sloping isopycnals in setting a weak submixed layer stratification north of ACC. This weak stratification varies in magnitude across basins. In the Atlantic and western Indian Oceans where the ACC is at a low latitude (∼45°S), solar heating, intrusions of subtropical gyres, and energetic mesoscale eddies together maintain relatively strong stratification. In the southeast Pacific, in comparison, the ACC reaches the southernmost latitude (56°S), far away from the subtropical front. This creates weaker stratification, allowing deep mixed layers to form, aided by surface buoyancy loss.

Primary Production and Respiration in the Norwegian Sea Estimated From Biogeochemical Argo Floats

AbstractBiogeochemical (BGC) Argo floats were used in this study to investigate phytoplankton blooms. We assessed the seasonal and annual rates of net primary and community production, along with respiration in the Norwegian Sea. The years 2020 and 2021 were contrasted to illuminate similarities and differences. In both years the onset of the bloom occurred at the beginning of February, coinciding with a deep winter mixed layer. However, during spring and summer the biological production appeared to develop differently. In 2020 the mixed layer depth shoaled quickly in April due to surface heating, triggering a strong spring bloom event. In contrast, a significant surface cooling in April 2021 triggered a substantial vertical mixing, that delayed the mixed layer's shoaling. This delay initiated cascading effects impacting nitrate consumption and bloom development that resulted in increased respiration in 2021 compared to 2020. In both years, vertical nitrate mixing from deeper layers to the surface emerged as a pivotal factor determining primary production. Using a mixed layer model in combination with Argo observations, we found that the upward nutrient fluxes due to mixing doubled net community production. These findings underscore the capability of BGC‐Argo floats, operating at a 5‐day resolution, to unveil the intricate interplay between hydrography, physical drivers, and biogeochemical processes in shaping phytoplankton dynamics and overall ecosystem productivity in the Norwegian Sea.

Intrathermocline Eddy with Lens-Shaped Low Potential Vorticity and Diabatic Forcing Mechanism in the South China Sea

Abstract Intrathermocline eddies (ITEs), characterized by subsurface lens-shaped low potential vorticity (PV), are pervasive in the ocean. However, the abundance and generation mechanisms of these low-PV lenses are poorly understood owing to their weak surface signals and awkward sizes, which present an observational barrier. Using in situ observations of the northern South China Sea (NSCS), a typical ITE with a lens-shaped low PV at a core depth of 30–150 m and a horizontal size of ∼150 km was captured in May 2021. Combined with PV budget analysis, we investigate the underlying generation mechanism of low PVs within these ITEs using high-resolution reanalysis products. The results suggest that wintertime surface buoyancy loss driven by atmospheric diabatic forcing rather than frictional forcing is a crucial favorable condition for the ITE formation. These enhanced surface buoyancy losses produce a net upward PV flux and decrease PV in the weakly stratified and deep winter mixed layer, which are preconditioned by anticyclonic eddies (AEs). While surface heating in the following spring tends to weaken the surface buoyancy loss and gradually causes a downward PV flux, the surface-injected high PV subsequently caps the low-PV water within the surface-intensified AEs and transforms them into ITEs. Approximately 22% of the 58 AEs detected by satellite altimetry in the NSCS are ITEs. More importantly, the lens-shaped low PVs within them are produced primarily by the enhanced surface buoyancy loss during wintertime. These findings provide a new dynamic explanation for the low-PV generation in ITEs, highlighting the crucial role of atmospheric diabatic forcing. Significance Statement Intrathermocline eddies (ITEs), characterized by a lens-like isopycnal structure that bounds low potential vorticity (PV), are active in the oceanic interior. Although a few previous studies revealed the existence of ITEs in the South China Sea, the source and dynamic generation mechanisms of the lens-shaped low PV still remain elusive. We find that the enhanced surface buoyancy loss due to atmospheric diabatic forcing drives an upward surface PV flux and is identified to produce the low PV. The preexisting anticyclonic eddy, combined with seasonal surface heating in spring, can be easily transformed into the ITE. This study provides a new dynamic understanding for the generation mechanism of ITEs’ low PVs and highlights the contribution of atmospheric diabatic forcing.

Sea Ice‐Driven Variability in the Pacific Subantarctic Mode Water Formation Regions

AbstractSubantarctic Mode Water (SAMW) forms north of the Subantarctic Front, in regions of deep winter mixed layers, and is important to the absorption and storage of anthropogenic CO2 and heat. Two SAMW pools exist in the Pacific, a lighter Central mode (CPSAMW), and a denser Southeast mode (SEPSAMW). Both have experienced significant interannual variability in thickness and properties in recent years. We compute mixed layer temperature and salinity budgets for the two SAMW formation regions, to determine the relative contribution of processes driving variability in the properties of mixed layers that subduct to form SAMW. The dominant drivers of temperature and salinity variability are shown to be surface fluxes, horizontal advection, and entrainment of deeper water. Salt advection into each SAMW formation region is found to be strongly correlated with changes in sea ice area in the northern Ross Sea, with lags of up to 2 years. Further correlation is found between meridional salt advection in the southeast Pacific formation regions, and sea ice area in the northern Amundsen/Bellingshausen seas, suggesting that freshwater derived from sea ice melt reaches the SEPSAMW formation region within 6 months. In 2016, strong advective freshening of the SEPSAMW formation region, linked to increased winter sea ice in the Amundsen/Bellingshausen seas, led to anomalously fresh mixed layers. However, a regime change in Antarctic sea ice in 2016 resulted in a subsequent lack of the usual advective freshening in the SEPSAMW formation region, driving increased salinity of the mixed layer the following year.

A Potential Energy Analysis of Ocean Surface Mixed Layers

AbstractTurbulent mixing in the ocean surface boundary layer leads to the presence of a surface mixed layer. This mixed layer is important for many phenomena including large‐scale ocean dynamics, ocean‐atmosphere coupling, and biological and biogeochemical processes. Analysis of the ocean mixed layer requires one to estimate its vertical extent, for which there are various definitions. Correspondingly, there are uncertainties on how to best identify an ocean surface mixed layer for a given application. We propose defining the mixed layer depth (MLD) from energetic principles through the potential energy (PE). The PE based MLD is based on the concept of PE anomaly, which measures the stratification of a layer of seawater by estimating its energetic distance from a well‐mixed state. We apply the PE anomaly to diagnose the MLD as the depth to which a given energy could homogenize a layer of seawater. We evaluate the MLD defined by common existing methods and demonstrate that they contain a wide range of PE anomalies for the same MLD, particularly evident for deep winter mixed layers. The MLD defined from the PE anomaly ensures a more consistent MLD identified for a large range of stratifications. Furthermore, the PE method relates to the turbulent kinetic energy budget of the ocean surface boundary layer, which is fundamental to upper ocean mixing processes and parameterizations. The resulting MLD is more representative of active boundary layer turbulence, and is more robust to small anomalies in seawater properties.

Controls on the Boundary Between Thermally and Non‐Thermally Driven pCO2 Regimes in the South Pacific

AbstractRegional and temporal patterns of air–sea carbon exchange are strongly linked to the surface ocean partial pressure of carbon dioxide (pCO2), which varies with sea surface temperature (SST), salinity, dissolved inorganic carbon (DIC), and alkalinity. It is well‐known that temperature controls the pCO2 seasonal cycle in the subtropics, whereas DIC dominates at high latitudes. The balance of mechanisms governing the boundary between these regimes, however, are not well characterized due to lack of year‐round pCO2 data. Here, we use autonomous biogeochemical float measurements from the South Pacific to investigate the processes that control meridional variations in pCO2 seasonality. We find that the transition between pCO2 regimes is linked to the poleward decrease in SST seasonal cycle amplitude, which is closely associated with the northern boundary of deep winter mixed layers. Processes that determine the annual SST range are, therefore, central to the response of oceanic carbon uptake to anthropogenic forcing.

Decreasing Wintertime Mixed‐Layer Depth in the Northwestern North Pacific Subtropical Gyre

AbstractThe deep winter mixed layer (ML), ∼240 m, in the northwestern North Pacific subtropical gyre exchanges heat and absorbs carbon dioxide from the atmosphere, and receives nutrients through the Kuroshio. The ML water spreads broadly in the subsurface ocean over the subtropical gyre. Compiling historical temperature measurements, we show that the winter ML depth (MLD) has decreased by ∼6% over the last six decades. Observations, atmospheric reanalyzes, simulation outputs from a one‐dimensional turbulent closure model, and climate projections reveal that the strengthening of the upper‐ocean stratification due to surface warming under global warming is responsible for the MLD decrease. Climate models project further decrease of winter MLD attributable to the marked strengthening of ocean stratification with climate warming, of up to 27%–40% by 2100 under the highest emission scenario. A monitoring system for winter ML should be established so that important changes for marine ecosystems can be foreseen.

Revisiting the Malvinas Current Upper Circulation and Water Masses Using a High‐Resolution Ocean Reanalysis

AbstractWe use 25 years of a high‐resolution ocean reanalysis (1/12°) to revisit the Malvinas Current (MC) from the South (Drake Passage) to the North (Brazil‐Malvinas Confluence) from the synoptic to interannual time scales. The Malvinas Plateau is home to active eddy mixing, eddy dissipation and deep winter mixed layers occasionally reaching 600 m depth. The MC is organized in two jets which merge around 44°S as the continental slope steepens. The upper 900 m transport mean decreases from 40 Sv at 51°S to 35 Sv at 41°S indicating offshore leakage along the MC path. The MC plays a minor role in the velocity variations observed at the Confluence at seasonal and interannual scales; those are driven by changes in the intensity of the Brazil Current over the slope (34–36°S). Computing MC transport time series at different latitudes requires care because the section eastern limits are embedded in an energetic region. Transport time series were produced at selected latitudes using different criteria and showed common features. They show little seasonality (relative seasonal standard deviation of 2%) and no significant trend. The MC is a steady current: the relative standard deviation is on the order of 10% increasing to 20% near and on the Malvinas Plateau and near the Confluence. In contrast, velocity trends are large in the Brazil Current with the overshoot migrating southward. The associated increase in mesoscale activity south of 44°S in the Argentine Basin might contribute to blocking events occasionally reducing the MC transport.

Submesoscale Fronts and Their Dynamical Processes Associated with Symmetric Instability in the Northwest Pacific Subtropical Ocean

AbstractSubmesoscale density fronts and the associated processes of frontogenesis and symmetric instability (SI) are investigated in the northwest Pacific subtropical countercurrent (STCC) system by a high-resolution simulation and diagnostic analysis. Both satellite observations and realistic simulation show active surface fronts with a horizontal scale of ~20 km in the STCC upper ocean. Frontogenesis-induced buoyancy advection is detected to rapidly sharpen these density fronts. The direct straining effect of larger-scale geostrophic flows is a primary influence on the buoyancy-gradient frontogenetic tendency and frontal baroclinic potential vorticity (PV) enhancement. The enhanced lateral buoyancy gradients in conjunction with atmospheric forced surface buoyancy loss can produce a negative Ertel PV and trigger frontal SI in the STCC region. Up to 30% of the mixed layer (ML) inside a typical eddy has negative PV in the high-resolution simulation. As a result, the cross-front ageostrophic secondary circulations tend to restratify the surface boundary layer and induce a large vertical velocity reaching ~100 m day−1, substantially facilitating the vertical communication of the STCC system. At the same time, the SI is identified to be responsible for a forward cascade of geostrophic kinetic energy in the STCC region, despite the coexistence of ML eddies and SI in the deep winter ML. Therefore, these active density fronts and their SI-associated submesoscale processes play important roles in the enhanced vertical exchanges (e.g., heat, nutrients, and carbon) and energy transfer to smaller scales in the eddy-active STCC upper ocean, as well as triggering phytoplankton blooms at the periphery of eddies.

Atlantic Water Modification North of Svalbard in the Mercator Physical System From 2007 to 2020

AbstractThe Atlantic Water (AW) inflow through Fram Strait, largest oceanic heat source to the Arctic Ocean, undergoes substantial modifications in the Western Nansen Basin (WNB). Evaluation of the Mercator system in the WNB, using 1,500 independent temperature‐salinity profiles and five years of mooring data, highlighted its performance in representing realistic AW inflow and hydrographic properties. In particular, favorable comparisons with mooring time‐series documenting deep winter mixed layers and changes in AW properties led us to examine winter conditions in the WNB over the 2007–2020 period. The model helped describe the interannual variations of winter mixed layers and documented several processes at stake in modifying AW beyond winter convection: trough outflows and lateral exchange through vigorous eddies. Recently modified AW, either via local convection or trough outflows, were identified as homogeneous layers of low buoyancy frequency. Over the 2007–2020 period, two winters stood out with extreme deep mixed layers in areas that used to be ice‐covered: 2017/18 over the northern Yermak Plateau‐Sofia Deep; 2012/13 on the continental slope northeast of Svalbard with the coldest and freshest modified AW of the 12‐year time series. The northern Yermak Plateau‐Sofia Deep and continental slope areas became “Marginal Convection Zones” in 2011 with, from then on, occasionally ice‐free conditions, 50‐m‐ocean temperatures always above 0 °C and highly variable mixed layer depths and ocean‐to‐atmosphere heat fluxes. In the WNB where observations require considerable efforts and resources, the Mercator system proved to be a good tool to assess Atlantic Water modifications in winter.

Formation of Central Mode Water based on two zonal hydrographic sections in spring 2013 and 2016

Two zonal high-density hydrographic sections along 41° N and 37.5° N east of Japan were occupied in April 2013 and June 2016 to examine the formation of Central Mode Water (CMW) and Transition Region Mode Water (TRMW) in relation to fronts and eddies. In the 41° N section traversing the meandering subarctic front, the denser variety of CMW (D-CMW) and TRMW was formed continuously on both sides of the front, except for the part of the section located south of the Kuroshio bifurcation front where the lighter variety of CMW (L-CMW) and D-CMW was formed instead. L-CMW and D-CMW were also formed in the eastern part of the 37.5° N section between the Kuroshio Extension front and the Kuroshio bifurcation front, but were hardly formed in the western part of the section west of the bifurcation point of the two fronts. D-CMW and TRMW pycnostads in the western part of the 41° N section observed in April 2013 tended to exhibit more than one core (vertical minimum of potential vorticity), which might be formed by destruction of deep winter mixed layers. Such multiple-core structure was also observed in L-CMW and D-CMW pycnostads in the eastern part of both the sections south of the Kuroshio bifurcation front in June 2016, being particularly abundant in three anticyclonic eddies. It was likely to be formed by the exchange of low-potential vorticity water among the eddies and the ambient region in association with eddy-to-eddy interaction, suggesting a new mechanism of mode water subduction.

Multi-Year Observations of Fluorescence and Backscatter at the Southern Ocean Time Series (SOTS) Shed Light on Two Distinct Seasonal Bio-Optical Regimes

This work presents insights from 6 years of chlorophyll-a fluorescence and backscatter (700 nm) data at the Southern Ocean Time Series (SOTS) moorings, located in the Subantarctic Zone southwest of Tasmania. Using local calibrations from available voyage data, the fluorescence and backscatter records were related to chlorophyll-a (Chl) and particulate organic carbon (POC), allowing us to estimate and interpret carbon:Chl ratios. Surprisingly, observed carbon:Chl ratios were higher in winter than in summer, indicating that photo-acclimation of phytoplankton to decreased light levels in the deep winter mixed layer is not the main signal. Instead, the data suggest a seasonal succession of two trophodynamic regimes at SOTS: a phytoplankton-dominated community in summer, while in winter the proportion of “non-phytoplankton” POC increases. The two regimes can also be differentiated in an optical index based on fluorescence and backscatter, indicating two distinct bio-optical populations. Seasonal iron limitation and deep winter mixing in the SAZ, reaching as deep as 600m, likely play key roles in setting the stage for the observed ecological succession of the two trophodynamic regimes.

Sea Surface Temperatures and Paleoenvironmental Variability in the Central Mediterranean During Historical Times Reconstructed Using Planktonic Foraminifera

AbstractThe ongoing anthropogenic‐induced warming assessment requires a robust background from regional sea surface temperature (SST) reconstructions. Planktonic foraminifera have yielded valuable insights into late Quaternary SST dynamics, but the techniques to estimate SST from fossil assemblages have only rarely been used in very recent sedimentary records (the last 2,000 years). Here we use two transfer function methods, modern analog technique and artificial neural networks, to reconstruct SST variability in two cores from the Central Mediterranean Sea that span the last five centuries. Both cores show similar and considerable changes in the planktonic foraminifera assemblages. However, the inferred mean annual SSTs only varied in a narrow range, in agreement with instrumental data that go back to 1850 CE. Our reconstructions extend this time frame and indicate that SST variability did not exceed 1.5 °C over the past three centuries. Rather than temperature, we suggest that the changes in the assemblages reflect switches between sea surface winter/spring productivity and a deep winter mixed layer, due to the atmosphere/ocean interplay that governs different productivity modes in neighboring mesoscale gyres.

Baroclinic Instability in the Presence of Convection

AbstractBaroclinic mixed-layer instabilities have recently been recognized as an important source of submesoscale energy in deep winter mixed layers. While the focus has so far been on the balanced dynamics of these instabilities, they occur in and depend on an environment shaped by atmospherically forced small-scale turbulence. In this study, idealized numerical simulations are presented that allow the development of both baroclinic instability and convective small-scale turbulence, with simple control over the relative strength. If the convection is only weakly forced, baroclinic instability restratifies the layer and shuts off convection, as expected. With increased forcing, however, it is found that baroclinic instabilities are remarkably resilient to the presence of convection. Even if the instability is too weak to restratify the layer and shut off convection, the instability still grows in the convecting environment and generates baroclinic eddies and fronts. This suggests that despite the vigorous atmospherically forced small-scale turbulence in winter mixed layers, baroclinic instabilities can persistently grow, generate balanced submesoscale turbulence, and modify the bulk properties of the upper ocean.

A Mechanism of Mixed Layer Formation in the Indo–Western Pacific Southern Ocean: Preconditioning by an Eddy-Driven Jet-Scale Overturning Circulation

AbstractThe formation of a narrow band of the deep winter mixed layer (hereinafter “mixed layer wedge”) in the Indo–western Pacific Southern Ocean is examined using an eddy-resolving Parallel Ocean Program (POP) model simulation. The mixed layer wedge starts to deepen in June, centered at 47.5°S, with a meridional scale of only ~2° latitude. Its center is located ~1° north of the model’s Subantarctic Front (SAF). The Argo-based observed mixed layer is similarly narrow and occurs adjacent to the observed SAF. In the small sector of 130°–142°E, where the SAF is persistent and the mixed layer is deepest, the formation of the narrow mixed layer wedge coincides with destratification underneath the mixed layer. This destratification can be attributed primarily to the downwelling branch of a jet-scale overturning circulation (JSOC). The JSOC, which was reported in an earlier study by the authors, is driven by eddy momentum flux convergence and is therefore thermally indirect: its descending branch occurs on the warmer equatorward flank of the SAF, promoting destratification during the warm season. The model-generated net air–sea heat flux reveals a similar wedge-like feature, indicating that the flux contributes to the mixed layer depth wedge, but again this feature is preconditioned by the JSOC. Ekman advection contributes to the formation of the mixed layer, but it occurs farther north of the region where the mixed layer initially deepens. These findings suggest that the eddy-driven JSOC associated with the SAF plays an important role in initiating the narrow, deep mixed layer wedge that forms north of the SAF.

Winter to summer oceanographic observations in the Arctic Ocean north of Svalbard

AbstractOceanographic observations from the Eurasian Basin north of Svalbard collected between January and June 2015 from the N‐ICE2015 drifting expedition are presented. The unique winter observations are a key contribution to existing climatologies of the Arctic Ocean, and show a ∼100 m deep winter mixed layer likely due to high sea ice growth rates in local leads. Current observations for the upper ∼200 m show mostly a barotropic flow, enhanced over the shallow Yermak Plateau. The two branches of inflowing Atlantic Water are partly captured, confirming that the outer Yermak Branch follows the perimeter of the plateau, and the inner Svalbard Branch the coast. Atlantic Water observed to be warmer and shallower than in the climatology, is found directly below the mixed layer down to 800 m depth, and is warmest along the slope, while its properties inside the basin are quite homogeneous. From late May onwards, the drift was continually close to the ice edge and a thinner surface mixed layer and shallower Atlantic Water coincided with significant sea ice melt being observed.

An Argo mixed layer climatology and database

AbstractA global climatology and database of mixed layer properties are computed from nearly 1,250,000 Argo profiles. The climatology is calculated with both a hybrid algorithm for detecting the mixed layer depth (MLD) and a standard threshold method. The climatology provides accurate information about the depth, properties, extent, and seasonal patterns of global mixed layers. The individual profile results in the database can be used to construct time series of mixed layer properties in specific regions of interest. The climatology and database are available online at http://mixedlayer.ucsd.edu. The MLDs calculated by the hybrid algorithm are shallower and generally more accurate than those of the threshold method, particularly in regions of deep winter mixed layers; the new climatology differs the most from existing mixed layer climatologies in these regions. Examples are presented from the Labrador and Irminger Seas, the Southern Ocean, and the North Atlantic Ocean near the Gulf Stream. In these regions the threshold method tends to overestimate winter MLDs by approximately 10% compared to the algorithm.

Submesoscale processes promote seasonal restratification in the Subantarctic Ocean

AbstractTraditionally, the mechanism driving the seasonal restratification of the Southern Ocean mixed layer (ML) is thought to be the onset of springtime warming. Recent developments in numerical modeling and North Atlantic observations have shown that submesoscale ML eddies (MLE) can drive a restratifying flux to shoal the deep winter ML prior to solar heating at high latitudes. The impact of submesoscale processes on the intraseasonal variability of the Subantarctic ML is still relatively unknown. We compare 5 months of glider data in the Subantarctic Zone to simulations of a 1‐D mixing model to show that the magnitude of restratification of the ML cannot be explained by heat, freshwater, and momentum fluxes alone. During early spring, we estimate that periodic increases in the vertical buoyancy flux by MLEs caused small increases in stratification, despite predominantly down‐front winds that promote the destruction of stratification. The timing of seasonal restratification was consistent between 1‐D model estimates and the observations. However, during up‐front winds, the strength of springtime stratification increased over twofold compared to the 1‐D model, with a rapid shoaling of the MLD from >200 m to <100 m within a few days. The ML stratification is further modified under a negative Ekman buoyancy flux during down‐front winds, resulting in the destruction of ML stratification and deepening of the MLD. These results propose the importance of submesoscale buoyancy fluxes enhancing seasonal restratification and mixing of the Subantarctic ML.

Discrepant estimates of primary and export production from satellite algorithms, a biogeochemical model, and geochemical tracer measurements in the North Pacific Ocean

AbstractEstimates of primary and export production (PP and EP) based on satellite remote sensing algorithms and global biogeochemical models are widely used to provide year‐round global coverage not available from direct observations. However, observational data to validate these approaches are limited. We find that no single satellite algorithm or model can reproduce seasonal and annual geochemically determined PP, export efficiency (EP/PP), and EP rates throughout the North Pacific basin, based on comparisons throughout the full annual cycle at time series stations in the subarctic and subtropical gyres and basin‐wide regions sampled by container ship transects. The high‐latitude regions show large PP discrepancies in winter and spring and strong effects of deep winter mixed layers on annual EP that cannot be accounted for in current satellite‐based approaches. These results underscore the need to evaluate satellite‐ and model‐based estimates using multiple productivity parameters measured over broad ocean regions throughout the annual cycle.

The annual cycle of gross primary production, net community production, and export efficiency across the North Pacific Ocean

AbstractWe measured triple oxygen isotopes and oxygen/argon dissolved gas ratios as nonincubation‐based geochemical tracers of gross oxygen production (GOP) and net community production (NCP) on 16 container ship transects across the North Pacific from 2008 to 2012. We estimate rates and efficiency of biological carbon export throughout the full annual cycle across the North Pacific basin (35°N–50°N, 142°E–125°W) by constructing mixed layer budgets that account for physical and biological influences on these tracers. During the productive season from spring to fall, GOP and NCP are highest in the Kuroshio region west of 170°E and decrease eastward across the basin. However, deep winter mixed layers (>200 m) west of 160°W ventilate ~40–90% of this seasonally exported carbon, while only ~10% of seasonally exported carbon east of 160°W is ventilated in winter where mixed layers are <120 m. As a result, despite higher annual GOP in the west than the east, the annual carbon export (sequestration) rate and efficiency decrease westward across the basin from export of 2.3 ± 0.3 mol C m−2 yr−1 east of 160°W to 0.5 ± 0.7 mol C m−2 yr−1 west of 170°E. Existing productivity rate estimates from time series stations are consistent with our regional productivity rate estimates in the eastern but not western North Pacific. These results highlight the need to estimate productivity rates over broad spatial areas and throughout the full annual cycle including during winter ventilation in order to accurately estimate the rate and efficiency of carbon sequestration via the ocean's biological pump.