Advances in Marine Ecosystem Dynamics from US GLOBEC : The Horizontal-Advection Bottom-up Forcing Paradigm

This Article is brought to you for free and open access by the Center for Coastal Physical Oceanography at ODU Digital Commons. It has been accepted for inclusion in CCPO Publications by an authorized administrator of ODU Digital Commons. For more information, please contact digitalcommons@odu.edu. Repository Citation Di Lorenzo, Emanuele; Mountain, David; Batchelder, Harold; Bond, Nicholas; and Hofmann, Eileen E., "Advances in Marine Ecosystem Dynamics from US GLOBEC: The Horizontal-Advection Bottom-up Forcing Paradigm" (2013). CCPO Publications. 14. https://digitalcommons.odu.edu/ccpo_pubs/14

Atlantic, the regional sea level pressure expression of the NAM, which is referred to as the North Atlantic Oscillation (NAO; Figure 1), dominates the forcing of changes in ocean conditions.In the Pacific sector, the surface expression of the NAM is weaker, and regional sea level pressure variability of the Aleutian Low (AL) is more strongly affected by remote response (teleconnection) to ENSO.In the Southern Ocean, the lack of strong continental boundaries makes the SAM the dominant forcing of ocean variability.

OCEAN AND ECOSYSTEM RESPONSE TO CLIMATE FORCING
The large-scale atmospheric modes operate differently in each of the GLOBEC regions.However, they all modify the ocean circulation on multiyear periods, with implications for the marine ecosystem.GLOBEC isolated a common mechanism driving oceanic and ecosystem responses across the GLOBEC regions, a mechanism we refer to as the "horizontal advection bottom-up forcing" paradigm.It can be summarized as follows: changes in atmospheric

L ARGE-SCALE CLIMATE VARIABILIT Y IN GLOBEC REGIONS
Changes in marine ecosystems in regions studied by US GLOBEC have been linked to changes in large-scale atmospheric circulation, which drive important re arrangements in ocean circulation and in regional transport dynamics that impact ecosystem processes.Atmospheric variability is best understood in the context of sea level pressure deviations from the mean atmospheric circulation (Figure 1).In the extratropical latitudes over GLOBEC regions, atmospheric circulation is characterized by dipoles of high and low pressure associated with the mean path of westerly winds (along the pressure gradients).These strong highs and lows are particularly evident in the Northern Hemisphere where the presence of continental boundaries drives standing-wave patterns characterized by dipoles that are ABSTR ACT.A primary focus of the US Global Ocean Ecosystem Dynamics (GLOBEC) program was to identify the mechanisms of ecosystem response to largescale climate forcing under the assumption that bottom-up forcing controls a large fraction of marine ecosystem variability.At the beginning of GLOBEC, the prevailing bottom-up forcing hypothesis was that climate-induced changes in vertical transport modulated nutrient supply and surface primary productivity, which in turn affected the lower trophic levels (e.g., zooplankton) and higher trophic levels (e.g., fish) through the trophic cascade.Although upwelling dynamics were confirmed to be an important driver of ecosystem variability in GLOBEC studies, the use of eddyresolving regional-scale ocean circulation models combined with field observations revealed that horizontal advection is an equally important driver of marine ecosystem variability.Through a synthesis of studies from the four US GLOBEC regions (Gulf of Alaska, Northern California Current, Northwest Atlantic, and Southern Ocean), a new horizontal-advection bottom-up forcing paradigm emerges in which large-scale climate forcing drives regional changes in alongshore and cross-shelf ocean transport that directly impact ecosystem functions (e.g., productivity, species composition, spatial connectivity).The horizontal advection bottom-up forcing paradigm expands the mechanistic pathways through which climate variability and climate change impact the marine ecosystem.In particular, these results highlight the need for future studies to resolve and understand the role of mesoscale and submesoscale transport processes and their relationship to climate.Here, we report on selected findings in each region that show the importance of the horizontal advection bottom-up forcing paradigm.

THE NORTHEAST PACIFIC
In the Northeast Pacific, GLOBEC focused on two oceanographically distinct regions, the California Current System (CCS) eastern boundary upwelling system and the Gulf of Alaska (GOA) subpolar gyre, which is characterized by coastal downwelling and upwelling mean conditions in the gyre.On interannual and decadal time scales, changes in the strength and position of the AL strongly impact the physical and biological conditions of the shelf environment in the CCS and the GOA.Changes in AL winds (Figure 2a) drive an oceanic response that is captured in the Pacific Decadal Oscillation (PDO) pattern (Chhak et al., 2009).In the positive phase of the PDO when the AL is stronger, the Northeast Pacific is characterized by downwelling wind anomalies, which drive positive sea level height anomalies (SSHa) along the entire eastern boundary (Figure 2b).
The adjustment to higher SSHa anomalies leads to poleward geostrophic flow anomalies in coastal regions.In the GOA, the higher freshwater runoff during stronger AL provides an additional contribution to the stronger alongshore flow

Northern Annular Mode Index (NAM)
Also known as the Arctic Oscillation, the NAM tracks changes in the strength of the polar vortex.

North Atlantic Oscillation Index (NAO)
Defined as the dominant mode of North Atlantic SLPa, the NAO is the surface expression of the NAM.In its positive phase the NAO is associated with a strengthening of the gradient between the Iceland Low and Azores High.The Mean Atmospheric Circulation

Southern Annular Mode Index (SAM)
Also known as the Antarctic Oscillation, the SAM tracks changes in the strength of the low pressure polar vortex over Antarctica.

Aleutian Low Index (AL)
Defined as the dominant mode of North Pacific SLPa, the AL is associated in its positive phase with a strengthening and southward shift of the Aleutian Low pressure system.SLP mbar Figure 1.Climate modes in the atmosphere and their relationship to the mean circulation.The graphic shows the mean sea level atmospheric pressure for December-January-February (DJF).High-pressure regions (in both hemispheres) have anticyclonic flow, and low-pressure regions have cyclonic flow.SLPa = sea level pressure anomaly.SLP data are from the National Center for Environmental Prediction, Kistler et al. (2001) Oceanography | December 2013 25 by increasing the cross-shelf baroclinicity (e.g., Weingartner et al., 2005).Stronger downwelling winds and coastal currents tend to be accompanied by enhanced mesoscale eddy activity in the GOA (Combes and Di Lorenzo, 2007;Henson and Thomas, 2008); episodes of relaxation in downwelling winds can result in fluxes of nutrient-rich water being driven onto the shelf by these eddies (Ladd et al., 2007).The changes in circulation associated with a stronger Aleutian Low, and resulting positive phase of the PDO, have been identified as an important bottom-up forcing of marine ecosystems in the Northern California Current and Gulf of Alaska.

The Northern California Current System
In the Northern California Current System (NCCS), high productivity along the coast is associated with seasonal upwelling that supplies surface nutrients.
Although interannual changes in upwelling and primary production have been linked to changes in the AL alongshore winds and the PDO, the mechanistic link between the AL/PDO and higher trophic levels is still being explored.Several GLOBEC publications (Hooff and Peterson, 2006;Peterson, 2009) suggest that PDO-related changes in along-shelf transport exert a more important control on higher trophic levels.For example, during positive (negative) phases of the PDO, the anomalous transport of waters from the south (north) bring zooplankton species residing in those waters into the NCCS region, fundamentally and profoundly altering the shelf ecosystem.Poleward-flowing subtropical waters (+PDO) typically have many species of small bodied, lipid-poor copepods, each of relatively low abundance and biomass.Conversely, boreal waters advected from the north (-PDO) have fewer copepod species, but the dominant species have relatively large body size and are often lipid-rich (Hooff and Peterson, 2006).The dependence of zooplankton community composition on changes in transport has been successfully tested both with eddy-resolving ocean model simulations (Keister et al., 2011) and observational studies that use satellite data to infer currents (Bi et al., 2011; Figure 3).The supply of boreal neritic copepods to the Oregon shelf serves as an energy-rich prey base for a food web that supports rapid growth and high survival of juvenile salmon during -PDO periods (Bi et al., 2011; Figure 3c).
The PDO has been mechanistically linked to changes in the alongshore transport that control zooplankton species composition along the entire CCS (Bi et al., 2011;Keister et al., 2011;Di Lorenzo and Ohman, 2013).Although more studies are needed to identify the mechanisms of interaction among the PDO, NPGO, and ecosystem responses, it is clear that regional circulation changes associated with these climate modes control water mass transports and upwelling dynamics that alter copepod community structure and abundance, including mean body size, biomass, and lipid (e.g., energy) content.
These processes in turn affect the trophic transfer of energy to higher consumers, such as forage fish and salmon (Peterson, 2009;Bi et al., 2011), and can have major implications for the productivity of the shelf ecosystem and for regional economies dependent on that productivity.

The Gulf of Alaska
The GOA subpolar gyre is a highnutrient, low-chlorophyll region where iron is the limiting factor to primary productivity (Martin et al., 1989).In  King et al. (2011).SLP data from Kistler et al. (2001), SSTa from the reanalyses of Smith and Reynolds (2004), ROMS SSHa from Di Lorenzo et al. (2009), and mean SSH from Niiler et al. (2003) Northeast Low, enhanced downwelling in the GOA (positive SSHa in Figure 4a) serves to inhibit productivity in the subpolar gyre interior (negative Chl-a in Figure 4b).
This mode of variability explains about 50% of the variance in the GOA subpolar gyre (Figure 4b) and is characterized by strong decadal fluctuations that track the PDO (Figure 4c, R = 0.66), which in this region is the oceanic response to AL forcing (Chhak et al., 2009).
In addition to the immediate response of the GOA to AL forcing, the adjustment to stronger downwelling in the coastal region energizes mesoscale eddies along the coastal GOA (Combes and Di Lorenzo, 2007;Henson and Thomas, 2008).These eddies enhance cross-shelf transport of shelf waters (Okkonen et al., 2003;Stabeno et al., 2004;Combes et al., 2009;Janout et al., 2009)  and it also reflects the important role that iron from freshwater runoff plays in offshore productivity (e.g., Fiechter et al., 2009;Fiechter and Moore, 2012;Coyle et al., 2012, and references therein).
Freshwater runoff in the coastal GOA is weakly correlated with the PDO (< 25%) and more strongly controlled by regional-scale variability associated with the Ketchikan-Seward atmospheric sea level pressure gradient (Weingartner et al., 2005).Here, the freshwater run-

THE NORTHWEST ATL ANTIC
The Georges Bank/Gulf of Maine region (GB/GoM) in the Northwest Atlantic is a highly productive area that supports many important commercial fisheries.The local circulation is part of a southwestward flowing, continental-scale coastal current system that extends from the subarctic to Cape Hatteras (Chapman and Beardsley, 1989; Figure 5).An increase in NAM (see definition in Figure 1) in the late 1980s to mid-1990s led to increased export of low-salinity waters from the Arctic into the North Atlantic, which enhanced the transport of the coastal current system at least as far south as the GB/GoM (Greene and Pershing, 2007).Direct current measurements by GLOBEC (Smith et al., 2001) indicate that during the mid-1990s, the influx to the GB/GoM from the shelf current system was twice that measured in the late 1970s to early 1980s.The measurements also showed that, concurrently, the influx to the region of high-salinity   Mountain and Kane (2010), and series b and c from Greene and Pershing (2007).See paper for description of data sources.
Coupled models of circulation and ecosystem dynamics (Ji et al., 2008) showed that changes in stratification associated with the increased shelf water inflow and reduced surface layer salinity resulted in changes in the seasonal timing and magnitude of primary production consistent with the observed changes.Bio-physical modeling of large-and small-sized zooplankton species (Ji et al., 2012)

THE WESTERN ANTARCTIC PENINSUL A
The US Southern Ocean GLOBEC (SO GLOBEC) program, focused on Marguerite Bay in the southern portion of the western Antarctic Peninsula (sWAP) continental shelf (Figure 6, inset).The program featured an end-toend food web approach that included predators and competitors of Antarctic krill (Euphausia superba), as well as the influence of habitat (Hofmann et al., 2004).Surveys identified biological hotspots, regions of sustained biological production in excess of average conditions (Costa et al., 2007), that had associated enhanced abundance of marine mammals and other top predators (Burns et al., 2004;Chapman et al., 2004;Širović et al., 2004;Thiele et al., 2004).The hotspot regions were in areas influenced by Circumpolar Deep Water (CDW), a relatively warm (1-1.5°C),salty (34.74) oceanic water mass that moves onto the WAP shelf at depth.This water mass influences heat and salt budgets (Dinniman and Klinck, 2004), sea ice distribution (Dinniman et al., 2012), and food web structure (Prézelin et al., 2004;Ballerini et al., in press).
For the Antarctic Peninsula region, a positive SAM index has been related to warming (Kwok and Comiso, 2002;Thompson and Solomon, 2002;Van den Broeke and van Lipzig, 2004), strengthening of the westerlies (Marshall et al., 2006), and decreases in sea ice extent and seasonal duration (Stammerjohn et al., 2008).Model investigations showed that increased wind strength and ACC transport increased the volume of CDW transported onto the western Antarctic Peninsula continental shelf, and the increased heat input reduced sea ice extent and duration, especially in winter (Dinniman et al., 2012).
Simulated transport pathways obtained from Lagrangian particle tracking studies showed that the sWAP shelf receives inputs from upstream regions and local areas (Figure 6), both of which are important in maintaining local shelf Antarctic krill populations and the biological hotspots (Piñones et al., 2011(Piñones et al., , 2013a)).Lagrangian transport simulations using circulation fields produced by increased wind strength and ACC transport showed that the increased volume of CDW on the shelf contributed to the success of larval Antarctic krill, but only for limited areas of the sWAP shelf (Piñones et al., 2013b).The modified circulation also increased advection of Antarctic krill larvae onto the shelf and enhanced retention, but the reduced winter sea ice and shifts in ice cover distribution may compromise overwintering survival of larvae and recruitment to adult stages (Piñones et al., 2013b).On the other hand, in the SO GLOBEC study region, reproduction and recruitment of Antarctic krill is more closely related to the spring phytoplankton bloom than to sea ice (Daly, 2004;Marrari et al., 2008), so the biological hotspots may not be affected in the short term by climate-induced changes in circulation.
A mass balance food web model developed for the SO GLOBEC region (Ballerini et al., in press)  This revelation was a direct result of the three-dimensional regional modeling focus of the GLOBEC program.The modeling studies showed that changes in ocean transport impacted different aspects of ecosystem structure and function (e.g., productivity, species composition, connectivity) in the four regions.

Stronger Southern Annular Mode (SAM) increases transport of CDW onto shelf
In the GOA, the cross-shelf advection of iron-rich coastal waters into the ocean's interior was identified as a mechanism for enhancing productivity in the ironlimited/nutrient-rich subpolar gyre (Figure 7, impact on productivity).In the CCS, changes in the strength of the alongshore southward currents were found to increase the supply of boreal neritic copepod species, which provide energy-rich prey for the local food web and enhance the survival of juvenile salmon (Figure 7, impact on species composition).In the NWA, the southward intrusion of Arctic water masses was linked to an increase in productivity.
These effects propagated through the entire food web to higher trophic levels (Figure 7, impact on species composition and fish).In the SO, regional transport   2) is an equally important mechanism by which marine ecosystems respond to climate forcing.GLOBEC studies identified ocean horizontal transport processes as an equally important driver of marine ecosystem dynamics.The "bottom up forcing" classical paradigm primary productivity (GOA, CCS, NWA, SO) 1 (Curchitser et al., 2013, in this issue) to develop robust strategies for downscaling large-scale climate forcing to regional and coastal-scale ecosystem dynamics.This process also requires fostering and maintaining a robust dialogue among climate modelers, coastal physical oceanographers, marine ecosystem scientists, and social scientists.
This type of interdisciplinary exchange will improve the quality of ecosystemintegrated assessments, with benefits for policy decisions.
This is US GLOBEC contribution 739.

Oceanography|
December 2013 23 more regionally confined.The Aleutian Low and North Pacific High, and the Iceland Low and Azores High, are the large-scale atmospheric forcing for the Northeast Pacific and Northwest Atlantic GLOBEC regions, respectively (Figure 1).The circumpolar Southern Ocean allows development of meridional dipoles elongated in the zonal direction, which give rise to the strong westerlies that drive the Antarctic Circumpolar Current (ACC).On seasonal, interannual, and decadal time scales, these atmospheric pressure systems fluctuate substantially in association with three main global modes of atmospheric variability: the El Niño-Southern Oscillation (ENSO); the Southern Annular Mode (SAM); and the Northern Annular Mode (NAM), also referred to as the Arctic Oscillation (AO).See Figure 1 and Box 1 for a more detailed definition of these climate modes.While ENSO involves dynamical coupling between the ocean and atmosphere, the annular modes are to first order uncoupled from ocean variability and are associated with the strengthening and weakening of the atmospheric polar vortices.Together, these modes explain about one-third of the global atmospheric variation that is responsible for the low-frequency variability in the ocean and ecosystem in each of the GLOBEC regions.Although the global nature of these climate modes can allow for coherent climate signals across the different GLOBEC study regions, in practice, regional expressions of the modes lead to distinct low-frequency signals in each region.For example, in the Northwest forcing changes in ocean currents and horizontal transport changes in marine population dynamics and distributions.From a dynamical point of view, each arrow indicates the direction of the forcing and is equivalent to an integration of the forcing function, so that the marine ecosystems have the potential to integrate several times the atmospheric forcing, one by the ocean and a second one by the ecosystem (e.g., the double integration hypothesis; Di Lorenzo and Ohman, 2013).Statistically, each integration can act as a low-pass filter of the input function, which allows marine ecosystems to amplify the low-frequency fluctuations contained in the large-scale atmospheric modes.The horizontal advection bottomup forcing paradigm was found to be important in all GLOBEC regions.In the Northeast Pacific, observed long-term changes in zooplankton were linked to changes in ocean transport driven by changes in the Aleutian Low winds.In the Northwest Atlantic, changes in the AO, and in its regional expression, the NAO, forced similar changes in Arctic Ocean circulation, with strong downstream (Arctic outflow) impacts on the Northwest Atlantic ecosystems.In the Southern Ocean, changes in the SAM produced changes of sea ice distribution and ocean circulation that affect connectivity patterns and food web linkages.
Figure 2d,e,f), a second mode of North Pacific climate variability (Di Lorenzo et al., 2008).The NPGO is strongly correlated with long-term changes in surface salinity, nitrate, and chlorophyll in the CCS and in the Gulf of Alaska (Di Lorenzo et al., 2009).
Figure 2. Atmospheric forcing and oceanic response of the Northeast Pacific during positive phases of the Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO).The patterns are obtained by regressing the PDO (left column) and NPGO (right column) indices with the(a, d) sea level pressure anomaly (SLPa) and wind stresses (black vectors), the (b, e) Regional Ocean Modeling System (ROMS) sea surface height anomaly (SSHa), and (c, f) sea surface temperature anomaly (SSTa).In the SSHa panels (b, e), the black contours correspond to the long-term mean SSH, so that it is possible to visualize how the PDO and NPGO SSHa project on the mean circulation-the bold vectors in the SSHa panels indicate the direction of PDO (red vectors) and NPGO (blue vectors) contributions to the circulation.Figure reproduced fromKing et al. (2011).SLP data fromKistler et al. (2001), SSTa from the reanalyses ofSmith and Reynolds (2004), ROMS SSHa from DiLorenzo et al. (2009), and mean SSH fromNiiler et al. (2003) and have been linked to cross-shelf export of coastal iron and Chl-a into the gyre(Crawford et al., 2005;Ladd et al., 2007;Ueno et al., 2010; Fietcher   and Moore, 2012), which increases overall open ocean Chl-a concentrations.This delayed response of eddy-induced cross-shelf transport to AL forcing is evident in the 2 nd CEOF of SSHa/Chl-a (Figure 4d), which shows clear signatures of the three large anticyclonic eddies in the Haida, Sitka, and Yakutat regions (Okkonen et al., 2001; Ladd et al., 2007).This second mode explains ~ 15% of the Chl-a variance in the gyre (Figure 4e) and has a weaker time lag (~ 6 months) correlation with the PDO (Figure 4f, R = 0.47) compared to the 1 st CEOF.The weak correlation with the AL/PDO reflects the chaotic nature of eddy dynamics in the GOA, especially along the Alaskan Stream (Combes and Di Lorenzo, 2007; Ueno et al., 2010), Figure 3. Observed ecosystem response to changes in Oregon Shelf transport.(a) Correlation between alongshore wind data from the National Centers for Environmental Prediction and satellite AVISO alongshore transport data.(b) Correlation between Newport zooplankton biweekly data and alongshore transport.(c) Correlation between coho salmon survival rate and Newport zooplankton.Figure redrafted from Bi et al. (2011); see paper for details on data sources Figure 3. Observed ecosystem response to changes in Oregon Shelf transport.(a) Correlation between alongshore wind data from the National Centers for Environmental Prediction and satellite AVISO alongshore transport data.(b) Correlation between Newport zooplankton biweekly data and alongshore transport.(c) Correlation between coho salmon survival rate and Newport zooplankton.Figure redrafted from Bi et al. (2011); see paper for details on data sources offshore oceanic waters was reduced by half.This change in the proportion of the inflows resulted in reduced salinity throughout the GB/GoM compared to the historical record (Figure 5a), with the transition to persistently negative salinity anomalies around 1990.Time series formed by combining data collected by GLOBEC with earlier observations indicate that various biological indices changed concurrently with the changes in salinity between the 1980s and the 1990s (Figure 5b-e).Specifically, phytoplankton production in the Gulf of Maine increased, zooplankton community structure in the Gulf of Maine and Georges Bank shifted to a dominance of smaller copepod species, and first-year survival rate for haddock (Melanogrammus aeglefinus) larvae on Georges Bank increased threefold, while Atlantic cod (Gadus morhua) larval survival decreased threefold (not shown).These findings indicate that a major, decadal-scale shift in the regional ecosystem fundamentally altered its physical properties and transports; transformed the size structure, composition, and ecology of the lower trophic levels; and altered the survival of two commercially important fish stocks.

Figure 4 .Figure 5 .
Figure 4. Covariability between AVISO SSHa and SeaWiFS chlorophyll-a (Chl-a) data from a combined empirical orthogonal function (EOF) analysis from 1998 to 2012.(a,d) SSHa regression maps with time series of MODE1 and MODE2.(b,e) Chl-a correlation maps with time series of MODE1 and MODE2.(c,f) Comparison of PDO index with time series of MODE1 and MODE2.The PDO leads MODE2 by six months.The two combined EOF modes are not independent and explain similar amounts of variance (10% MODE1 and 13% MODE2).The order of the modes in the figures is based on the amount of Chl-a variance explained in the gyre interior (~ 50% MODE1 in panel b and ~ 15 MODE2 in panel e).The black arrows in (a,b,e) indicate the circulation of the mean Alaskan subarctic gyre.
captured the spatial and seasonal patterns observed in those species and demonstrated that the patterns were controlled largely through bottom-up trophic processes.Year-to-year changes in zooplankton abundance were difficult to replicate in models, probably because overall abundance is sensitive to top-down processes, particularly predation mortality, which is difficult to measure accurately from field observations.The model could not replicate the shift in zooplankton community structure toward smaller species (Figure 5).The mechanisms responsible for the change in cod and haddock survival are not completely understood.They may be related to prey preferences of the two species.Because larval and juvenile haddock prefer smaller-sized prey than do the early life stages of cod, the increased abundance of smaller zooplankton species such as occurred during the later, lower-salinity period might have conferred a feeding advantage to young haddock compared to young cod.The changes in haddock and cod survival were reflected in the subsequent abundance of the adult stocks, with haddock exhibiting strong stock recovery in the 1990s while the cod stock continued to decline.Strong management measures limiting haddock catches probably also contributed to the increase of the haddock stock.However, the changes in the adult stocks likely were part of the broad, bottom-up, ecosystem-level changes that GLOBEC documented and that are believed to be the local response to atmospherically driven oceanic variations at high latitude that were advected to the GB/GoM region.
showed that Antarctic krill provide the majority of energy needed to sustain seabird and marine mammal populations.Long-term environmental changes, particularly decreasing sea ice, will possibly alter the distribution and availability of Antarctic krill.The additive effects of bottom-up resource control through changes in phytoplankton assemblages (Antarctic krill prey) and the top-down effects of consumers of Antarctic krill may amplify the effects of climate change on the sWAP food web, thereby adding another dimension to the conceptual model developed for the US GLOBEC regions.CONCLUSION Marine ecosystem responses to climate variability in the four regions studied in the US GLOBEC program reflect the different physical characteristics of atmospheric and ocean forcings that influence local biological processes, food web structure, and overall productivity of the ecosystem.Although vertical supply of nutrients remains an important mechanism controlling primary productivity and food web dynamics through the trophic cascade, GLOBEC identified a new set of bottom-up forcing pathways that involve changes in alongshore and cross-shore ocean transport.Specifically, it was shown that changes in atmospheric forcing drive changes in ocean currents and horizontal transport that impact marine population dynamics and distributions.This new horizontaladvection bottom-up forcing paradigm was isolated in each GLOBEC region by combining long-term eddy-resolving ocean models with field observations.

Figure 6 .
Figure6.Impact of regional ocean transport on Antarctic krill populations along the western Antarctic Peninsula (WAP) continental shelf.The Antarctic Circumpolar Current (ACC), which flows along the shelf edge, transports Circumpolar Deep Water (CDW) and Antarctic krill larvae (1) that originated in upstream source regions.On-shelf intrusions of CDW (2) occur at specific sites, such as Marguerite Trough, and bring larvae onto the shelf, fueling localized shelf hotspots (3).Local retention regions created by shelf circulation retain larvae and juveniles that are produced on the WAP shelf (4).Transport of larvae to the mid and inner shelf provides a source for the juvenile krill populations in these areas.The Antarctic Peninsula Coastal Current (APCC) potentially provides exchanges along the inner shelf.Figure redrafted fromPiñones et al., 2013b dynamics on the shelf were shown to play an important role in the retention of Antarctic krill larvae and the distribution of juvenile krill (Figure 7, impact on connectivity patterns).The GLOBEC recognition that climate-induced changes in horizontal transport processes exert a dominant control on ecosystem function has important implications for understanding the impact of climate change on marine populations.Future studies will need to develop modeling and observational strategies that account for the nonlocal nature of ecosystem variability associated with basin-scale changes in ocean circulation.Advanced modelingframeworks that resolve regional-scale circulation (e.g., regional alongshore dynamics) and mesoscale transport processes (e.g.cross-shelf exchanges) are also critical tools for investigating the mechanistic links between changes in basin-scale circulation and coastal marine ecosystems.GLOBEC pioneered the application of eddy-resolving models to understand the impact of regional transport dynamics on lower trophic levels.It is now clear that this approach must be extended in order to fully explore the dynamics of higher trophic levels.For example, ocean entrainment dynamics associated with mesoscale eddies are likely to have strong impacts on fish life cycles via transport of larvae and by altering connectivity patterns.Furthermore, submesoscale transport processes that were not resolved in GLOBEC likely play an equally important role in ecosystem dynamics (e.g., hotspots) and will need to be resolved in future studies.As we advance our modeling framework, it is also critical to develop novel observational strategies that allow constraining the new processes resolved in the models.Establishing the link among climate forcing, regional mesoscale, and submesoscale transport processes and marine ecosystem dynamics raises important scientific questions and poses new technical challenges.For example, it is well known that mesoscale and submesoscale dynamics have an important nondeterministic component.This implies that a fraction of ocean and ecosystem variability cannot be predicted or linked to climate forcing.However, climate may have predictable impacts on the statistics of mesoscale and submesoscale features.To this end, applying ensemble approaches with eddy-resolving ocean models may provide a way to separate and quantify the amount of intrinsic and deterministic variance in ocean and ecosystem processes in relation to climate forcing.

Finally
Figure 7. Conceptual advances of GLOBEC.GLOBEC expanded the classical bottom-up forcing paradigm (1) by recognizing that horizontal advection bottom-up forcing paradigm (2) is an equally important mechanism by which marine ecosystems respond to climate forcing.GLOBEC studies identified ocean horizontal transport processes as an equally important driver of marine ecosystem dynamics.