Holte James W.

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James W.

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Now showing 1 - 12 of 12
  • Article
    Structure and surface properties of eddies in the southeast Pacific Ocean
    (John Wiley & Sons, 2013-05-07) Holte, James W. ; Straneo, Fiamma ; Moffat, Carlos F. ; Weller, Robert A. ; Farrar, J. Thomas
    A number of studies have posited that coastally generated eddies could cool the southeast Pacific Ocean (SEP) by advecting cool, upwelled waters offshore. We examine this mechanism by characterizing the upper-ocean properties of mesoscale eddies in the SEP with a variety of observations and by estimating the surface-layer eddy heat flux divergence with satellite data. Cyclonic and anticyclonic eddies observed during two cruises featured deep positive salinity anomalies along the 26.5 kg m−3isopycnal, indicating that the eddies had likely trapped and transported coastal waters offshore. The cyclonic eddies observed during the cruises were characterized by shoaling isopycnals in the upper 200 m and cool near-surface temperature anomalies, whereas the upper-ocean structure of anticyclonic eddies was more variable. Using a variety of large-scale observations, including Argo float profiles, drifter records, and satellite sea surface temperature fields, we show that, relative to mean conditions, cyclonic eddies are associated with cooler surface temperatures and that anticyclonic eddies are associated with warmer surface temperatures. Within each data set, the mean eddy surface temperature anomalies are small and of approximately equal magnitude but opposite sign. Eddy statistics drawn from satellite altimetry data reveal that cyclonic and anticyclonic eddies occur with similar frequency and have similar average radii in the SEP. A satellite-based estimate of the surface-layer eddy heat flux divergence, while large in coastal regions, is small when averaged over the SEP, suggesting that eddies do not substantially contribute to cooling the surface layer of the SEP.
  • Article
    An Argo mixed layer climatology and database
    (John Wiley & Sons, 2017-06-12) Holte, James ; Talley, Lynne D. ; Gilson, John ; Roemmich, Dean
    A 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.
  • Article
    Subantarctic mode water in the southeast Pacific : effect of exchange across the Subantarctic Front
    (John Wiley & Sons, 2013-04-23) Holte, James W. ; Talley, Lynne D. ; Chereskin, Teresa K. ; Sloyan, Bernadette M.
    This study considered cross-frontal exchange as a possible mechanism for the observed along-front freshening and cooling between the 27.0 and 27.3 kg m − 3 isopycnals north of the Subantarctic Front (SAF) in the southeast Pacific Ocean. This isopycnal range, which includes the densest Subantarctic Mode Water (SAMW) formed in this region, is mostly below the mixed layer, and so experiences little direct air-sea forcing. Data from two cruises in the southeast Pacific were examined for evidence of cross-frontal exchange; numerous eddies and intrusions containing Polar Frontal Zone (PFZ) water were observed north of the SAF, as well as a fresh surface layer during the summer cruise that was likely due to Ekman transport. These features penetrated north of the SAF, even though the potential vorticity structure of the SAF should have acted as a barrier to exchange. An optimum multiparameter (OMP) analysis incorporating a range of observed properties was used to estimate the cumulative cross-frontal exchange. The OMP analysis revealed an along-front increase in PFZ water fractional content in the region north of the SAF between the 27.1 and 27.3 kg m − 3 isopycnals; the increase was approximately 0.13 for every 15° of longitude. Between the 27.0 and 27.1 kg m − 3 isopycnals, the increase was approximately 0.15 for every 15° of longitude. A simple bulk calculation revealed that this magnitude of cross-frontal exchange could have caused the downstream evolution of SAMW temperature and salinity properties observed by Argo profiling floats.
  • Article
    Arrival of new great salinity anomaly weakens convection in the Irminger Sea
    (American Geophysical Union, 2022-06-06) Biló, Tiago C. ; Straneo, Fiamma ; Holte, James W. ; Le Bras, Isabela A.
    The Subpolar North Atlantic is prone to recurrent extreme freshening events called Great Salinity Anomalies (GSAs). Here, we combine hydrographic ocean analyses and moored observations to document the arrival, spreading, and impacts of the most recent GSA in the Irminger Sea. This GSA is associated with a rapid freshening of the upper Irminger Sea between 2015 and 2020, culminating in annually averaged salinities as low as the freshest years of the 1990s and possibly since 1960. Upon the GSA propagation into the Irminger Sea over the Reykjanes Ridge, the boundary currents rapidly advected its signal around the basin within months while fresher waters slowly spread and accumulated into the interior. The anomalies in the interior freshened waters produced by deep convection during the 2017–2018 winter and actively contributed to the suppression of deep convection in the following two winters.
  • Article
    Mean conditions and seasonality of the West Greenland boundary current system near Cape Farewell
    (American Meteorological Society, 2020-09-18) Pacini, Astrid ; Pickart, Robert S. ; Bahr, Frank B. ; Torres, Daniel J. ; Ramsey, Andree L. ; Holte, James W. ; Karstensen, Johannes ; Oltmanns, Marilena ; Straneo, Fiamma ; Le Bras, Isabela Astiz ; Moore, G. W. K. ; de Jong, Marieke Femke
    The structure, transport, and seasonal variability of the West Greenland boundary current system near Cape Farewell are investigated using a high-resolution mooring array deployed from 2014 to 2018. The boundary current system is comprised of three components: the West Greenland Coastal Current, which advects cold and fresh Upper Polar Water (UPW); the West Greenland Current, which transports warm and salty Irminger Water (IW) along the upper slope and UPW at the surface; and the Deep Western Boundary Current, which advects dense overflow waters. Labrador Sea Water (LSW) is prevalent at the seaward side of the array within an offshore recirculation gyre and at the base of the West Greenland Current. The 4-yr mean transport of the full boundary current system is 31.1 ± 7.4 Sv (1 Sv ≡ 106 m3 s−1), with no clear seasonal signal. However, the individual water mass components exhibit seasonal cycles in hydrographic properties and transport. LSW penetrates the boundary current locally, through entrainment/mixing from the adjacent recirculation gyre, and also enters the current upstream in the Irminger Sea. IW is modified through air–sea interaction during winter along the length of its trajectory around the Irminger Sea, which converts some of the water to LSW. This, together with the seasonal increase in LSW entering the current, results in an anticorrelation in transport between these two water masses. The seasonality in UPW transport can be explained by remote wind forcing and subsequent adjustment via coastal trapped waves. Our results provide the first quantitatively robust observational description of the boundary current in the eastern Labrador Sea.
  • Article
    Slantwise convection in the Irminger Sea
    (American Geophysical Union, 2022-09-28) Le Bras, Isabela A.‐A. ; Callies, Jörn ; Straneo, Fiammetta ; Biló, Tiago C. ; Holte, James ; Johnson, Helen L.
    The subpolar North Atlantic is a site of significant carbon dioxide, oxygen, and heat exchange with the atmosphere. This exchange, which regulates transient climate change and prevents large‐scale hypoxia throughout the North Atlantic, is thought to be mediated by vertical mixing in the ocean's surface mixed layer. Here we present observational evidence that waters deeper than the conventionally defined mixed layer are affected directly by atmospheric forcing in this region. When northerly winds blow along the Irminger Sea's western boundary current, the Ekman response pushes denser water over lighter water, potentially triggering slantwise convection. We estimate that this down‐front wind forcing is four times stronger than air–sea heat flux buoyancy forcing and can mix waters to several times the conventionally defined mixed layer depth. Slantwise convection is not included in most large‐scale ocean models, which likely limits their ability to accurately represent subpolar water mass transformations and deep ocean ventilation.
  • Article
    The role of air-sea fluxes in Subantarctic Mode Water formation
    (American Geophysical Union, 2012-03-29) Holte, James W. ; Talley, Lynne D. ; Chereskin, Teresa K. ; Sloyan, Bernadette M.
    Two hydrographic surveys and a one-dimensional mixed layer model are used to assess the role of air-sea fluxes in forming deep Subantarctic Mode Water (SAMW) mixed layers in the southeast Pacific Ocean. Forty-two SAMW mixed layers deeper than 400 m were observed north of the Subantarctic Front during the 2005 winter cruise, with the deepest mixed layers reaching 550 m. The densest, coldest, and freshest mixed layers were found in the cruise's eastern sections near 77°W. The deep SAMW mixed layers were observed concurrently with surface ocean heat loss of approximately −200 W m−2. The heat, momentum, and precipitation flux fields of five flux products are used to force a one-dimensional KPP mixed layer model initialized with profiles from the 2006 summer cruise. The simulated winter mixed layers generated by all of the forcing products resemble Argo observations of SAMW; this agreement also validates the flux products. Mixing driven by buoyancy loss and wind forcing is strong enough to deepen the SAMW layers. Wind-driven mixing is central to SAMW formation, as model runs forced with buoyancy forcing alone produce shallow mixed layers. Air-sea fluxes indirectly influence winter SAMW properties by controlling how deeply the profiles mix. The stratification and heat content of the initial profiles determine the properties of the SAMW and the likelihood of deep mixing. Summer profiles from just upstream of Drake Passage have less heat stored between 100 and 600 m than upstream profiles, and so, with sufficiently strong winter forcing, form a cold, dense variety of SAMW.
  • Article
    Seasonal overturning of the Labrador Sea as observed by Argo floats
    (American Meteorological Society, 2017-10-17) Holte, James ; Straneo, Fiamma
    Argo floats are used to investigate Labrador Sea overturning and its variability on seasonal time scales. This is the first application of Argo floats to estimate overturning in a deep-water formation region in the North Atlantic. Unlike hydrographic measurements, which are typically confined to the summer season, floats offer the advantage of collecting data in all seasons. Seasonal composite potential density and absolute geostrophic velocity sections across the mouth of the Labrador Sea assembled from float profiles and trajectories at 1000 m are used to calculate the horizontal and overturning circulations. The overturning exhibits a pronounced seasonal cycle; in depth space the overturning doubles throughout the course of the year, and in density space it triples. The largest overturning [1.2 Sv (1 Sv ≡ 106 m3 s−1) in depth space and 3.9 Sv in density space] occurs in spring and corresponds to the outflow of recently formed Labrador Sea Water. The overturning decreases through summer and reaches a minimum in winter (0.6 Sv in depth space and 1.2 Sv in density space). The robustness of the Argo seasonal overturning is supported by a comparison to an overturning estimate based on hydrographic data from the AR7W line.
  • Article
    Heat and salinity budgets at the Stratus mooring in the southeast Pacific
    (John Wiley & Sons, 2014-11-28) Holte, James W. ; Straneo, Fiamma ; Farrar, J. Thomas ; Weller, Robert A.
    The surface layer of the southeast Pacific Ocean (SEP) requires an input of cold, fresh water to balance heat gain, and evaporation from air-sea fluxes. Models typically fail to reproduce the cool sea surface temperatures (SST) of the SEP, limiting our ability to understand the variability of this climatically important region. We estimate the annual heat budget of the SEP for the period 2004–2009, using data from the upper 250 m of the Stratus mooring, located at 85°W 20°S, and from Argo floats. The surface buoy measures meteorological conditions and air-sea fluxes; the mooring line is heavily instrumented, measuring temperature, salinity, and velocity at more than 15 depth levels. We use a new method for estimating the advective component of the heat budget that combines Argo profiles and mooring velocity data, allowing us to calculate monthly profiles of heat advection. Averaged over the 6 year study period, we estimate a cooling advective heat flux of −41 ± 29 W m−2, accomplished by a combination of the mean gyre circulation, Ekman transport, and eddies. This compensates for warming fluxes of 32 ± 4 W m−2 due to air-sea fluxes and 7 ± 9 W m−2 due to vertical mixing and Ekman pumping. A salinity budget exhibits a similar balance, with advection of freshwater (−60 psu m) replenishing the freshwater lost through evaporation (47 psu m) and Ekman pumping (14 psu m).
  • Article
    Subpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation
    (Nature Research, 2021-05-24) Li, Feili ; Lozier, M. Susan ; Bacon, Sheldon ; Bower, Amy S. ; Cunningham, Stuart A. ; de Jong, Marieke F. ; deYoung, Brad ; Fraser, Neil ; Fried, Nora ; Han, Guoqi ; Holliday, Naomi Penny ; Holte, James W. ; Houpert, Loïc ; Inall, Mark E. ; Johns, William E. ; Jones, Sam ; Johnson, Clare ; Karstensen, Johannes ; Le Bras, Isabela A. ; Lherminier, Pascale ; Lin, Xiaopei ; Mercier, Herlé ; Oltmanns, Marilena ; Pacini, Astrid ; Petit, Tillys ; Pickart, Robert S. ; Rayner, Darren ; Straneo, Fiamma ; Thierry, Virginie ; Visbeck, Martin ; Yashayaev, Igor ; Zhou, Chun
    Changes in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP—Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014–2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region.
  • Article
    Export of ice sheet meltwater from Upernavik Fjord, West Greenland
    (American Meteorological Society, 2022-03-01) Muilwijk, Morven ; Straneo, Fiamma ; Slater, Donald A. ; Smedsrud, Lars H. ; Holte, James W. ; Wood, Michael ; Andresen, Camilla S. ; Harden, Benjamin E.
    Meltwater from Greenland is an important freshwater source for the North Atlantic Ocean, released into the ocean at the head of fjords in the form of runoff, submarine melt, and icebergs. The meltwater release gives rise to complex in-fjord transformations that result in its dilution through mixing with other water masses. The transformed waters, which contain the meltwater, are exported from the fjords as a new water mass Glacially Modified Water (GMW). Here we use summer hydrographic data collected from 2013 to 2019 in Upernavik, a major glacial fjord in northwest Greenland, to describe the water masses that flow into the fjord from the shelf and the exported GMWs. Using an optimum multi-parameter technique across multiple years we then show that GMW is composed of 57.8% ± 8.1% Atlantic Water (AW), 41.0% ± 8.3% Polar Water (PW), 1.0% ± 0.1% subglacial discharge, and 0.2% ± 0.2% submarine meltwater. We show that the GMW fractional composition cannot be described by buoyant plume theory alone since it includes lateral mixing within the upper layers of the fjord not accounted for by buoyant plume dynamics. Consistent with its composition, we find that changes in GMW properties reflect changes in the AW and PW source waters. Using the obtained dilution ratios, this study suggests that the exchange across the fjord mouth during summer is on the order of 50 mSv (1 Sv ≡ 106 m3 s−1) (compared to a freshwater input of 0.5 mSv). This study provides a first-order parameterization for the exchange at the mouth of glacial fjords for large-scale ocean models.
  • Article
    Seasonality of the Meridional Overturning Circulation in the subpolar North Atlantic
    (Nature Research, 2023-05-25) Fu, Yao ; Lozier, M Susan ; Biló, Tiago Carrilho ; Bower, Amy S. ; Cunningham, Stuart A. ; Cyr, Frédéric ; de Jong, M. Femke ; deYoung, Brad ; Drysdale, Lewis ; Fraser, Neil ; Fried, Nora ; Furey, Heather H. ; Han, Guoqi ; Handmann, Patricia ; Holliday, N. Penny ; Holte, James ; Inall, Mark E. ; Johns, William E. ; Jones, Sam ; Karstensen, Johannes ; Li, Feili ; Pacini, Astrid ; Pickart, Robert S. ; Rayner, Darren ; Straneo, Fiammetta ; Yashayaev, Igor
    Understanding the variability of the Atlantic Meridional Overturning Circulation is essential for better predictions of our changing climate. Here we present an updated time series (August 2014 to June 2020) from the Overturning in the Subpolar North Atlantic Program. The 6-year time series allows us to observe the seasonality of the subpolar overturning and meridional heat and freshwater transports. The overturning peaks in late spring and reaches a minimum in early winter, with a peak-to-trough range of 9.0 Sv. The overturning seasonal timing can be explained by winter transformation and the export of dense water, modulated by a seasonally varying Ekman transport. Furthermore, over 55% of the total meridional freshwater transport variability can be explained by its seasonality, largely owing to overturning dynamics. Our results provide the first observational analysis of seasonality in the subpolar North Atlantic overturning and highlight its important contribution to the total overturning variability observed to date.