Advances in Marine Ecosystem Dynamics from US GLOBEC: the Horizontal-Advection Bottom-Up Forcing Paradigm

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

Di Lorenzo, E., D. Mountain, H.P. Batchelder, N. Bond, and E.E. Hofmann. 2013. Advances in marine ecosystem dynamics from US GLOBEC: The horizontal-advection bottom-up forcing paradigm. Oceanography 26(4):22–33. doi:10.5670/oceanog.2013.73

10.5670/oceanog.2013.73 Oceanography Society

Version of Record http://hdl.handle.net/1957/47486

http://cdss.library.oregonstate.edu/sa-termsofuse SPECIAL ISSUE ON US GLOBEC: UNDERSTANDING CLIMATE IMPACTS ON MARINE ECOSYSTEMS Advances in Marine Ecosystem Dynamics from US GLOBEC The Horizontal-Advection Bottom-up Forcing Paradigm

BY EMANUELE DI LORENZO, DAVID MOUNTAIN, HAROLD P. BATCHELDER, NICHOLAS BOND, AND EILEEN E. HOFMANN

Southern Ocean GLOBEC cruise. Photo credit: Daniel Costa

22 Oceanography | Vol. 26, No. 4 ABSTRACT. A primary focus of the US Global Ocean Ecosystem Dynamics and atmosphere, the annular modes (GLOBEC) program was to identify the mechanisms of ecosystem response to large- are to first order uncoupled from ocean scale climate forcing under the assumption that bottom-up forcing controls a large variability and are associated with the fraction of marine ecosystem variability. At the beginning of GLOBEC, the prevailing strengthening and weakening of the bottom-up forcing hypothesis was that climate-induced changes in vertical transport atmospheric polar vortices. Together, modulated nutrient supply and surface primary productivity, which in turn affected these modes explain about one-third of the lower trophic levels (e.g., zooplankton) and higher trophic levels (e.g., fish) the global atmospheric variation that is through the trophic cascade. Although upwelling dynamics were confirmed to be responsible for the low-frequency vari- an important driver of ecosystem variability in GLOBEC studies, the use of eddy- ability in the ocean and ecosystem in resolving regional-scale ocean circulation models combined with field observations each of the GLOBEC regions. revealed that horizontal advection is an equally important driver of marine ecosystem Although the global nature of these variability. Through a synthesis of studies from the four US GLOBEC regions climate modes can allow for coher- (Gulf of Alaska, Northern California Current, Northwest Atlantic, and Southern ent climate signals across the different Ocean), a new horizontal-advection bottom-up forcing paradigm emerges in which GLOBEC study regions, in practice, large-scale climate forcing drives regional changes in alongshore and cross-shelf regional expressions of the modes lead ocean transport that directly impact ecosystem functions (e.g., productivity, species to distinct low-frequency signals in each composition, spatial connectivity). The horizontal advection bottom-up forcing region. For example, in the Northwest paradigm expands the mechanistic pathways through which climate variability and Atlantic, the regional sea level pressure climate change impact the marine ecosystem. In particular, these results highlight expression of the NAM, which is referred the need for future studies to resolve and understand the role of mesoscale and to as the North Atlantic Oscillation submesoscale transport processes and their relationship to climate. (NAO; Figure 1), dominates the forcing of changes in ocean conditions. In the LARGE-SCALE CLIMATE more regionally confined. The Aleutian Pacific sector, the surface expression of VARIABILITY IN Low and North Pacific High, and the the NAM is weaker, and regional sea level GLOBEC REGIONS Iceland Low and Azores High, are the pressure variability of the Aleutian Low Changes in marine ecosystems in large-scale atmospheric forcing for the (AL) is more strongly affected by remote regions studied by US GLOBEC have Northeast Pacific and Northwest Atlantic response (teleconnection) to ENSO. In been linked to changes in large-scale GLOBEC regions, respectively (Figure 1). the Southern Ocean, the lack of strong atmospheric circulation, which drive The circumpolar Southern Ocean allows continental boundaries makes the SAM important rearrangements in ocean development of meridional dipoles elon- the dominant forcing of ocean variability. circulation and in regional transport gated in the zonal direction, which give dynamics that impact ecosystem pro- rise to the strong westerlies that drive the OCEAN AND ECOSYSTEM cesses. Atmospheric variability is best Antarctic Circumpolar Current (ACC). RESPONSE TO understood in the context of sea level On seasonal, interannual, and decadal CLIMATE FORCING pressure deviations from the mean time scales, these atmospheric pres- The large-scale atmospheric modes oper- atmospheric circulation (Figure 1). In sure systems fluctuate substantially ate differently in each of the GLOBEC the extratropical latitudes over GLOBEC in association with three main global regions. However, they all modify the regions, atmospheric circulation is char- modes of atmospheric variability: the ocean circulation on multiyear periods, acterized by dipoles of high and low El Niño-Southern Oscillation (ENSO); with implications for the marine eco- pressure associated with the mean path the Southern Annular Mode (SAM); system. GLOBEC isolated a common of westerly winds (along the pressure and the Northern Annular Mode mechanism driving oceanic and eco- gradients). These strong highs and lows (NAM), also referred to as the Arctic system responses across the GLOBEC are particularly evident in the Northern Oscillation (AO). See Figure 1 and Box 1 regions, a mechanism we refer to as the Hemisphere where the presence of conti- for a more detailed definition of these “horizontal advection bottom-up forc- nental boundaries drives standing-wave climate modes. While ENSO involves ing” paradigm. It can be summarized patterns characterized by dipoles that are dynamical coupling between the ocean as follows: changes in atmospheric

Oceanography | December 2013 23 forcing changes in ocean currents changes in ocean transport driven by subpolar gyre, which is characterized by and horizontal transport changes in changes in the Aleutian Low winds. In coastal downwelling and upwelling mean marine population dynamics and distri- the Northwest Atlantic, changes in the conditions in the gyre. On interannual butions. From a dynamical point of view, AO, and in its regional expression, the and decadal time scales, changes in the each arrow indicates the direction of the NAO, forced similar changes in Arctic strength and position of the AL strongly forcing and is equivalent to an integra- Ocean circulation, with strong down- impact the physical and biological con- tion of the forcing function, so that the stream (Arctic outflow) impacts on the ditions of the shelf environment in the marine ecosystems have the potential to Northwest Atlantic ecosystems. In the CCS and the GOA. Changes in AL winds integrate several times the atmospheric Southern Ocean, changes in the SAM (Figure 2a) drive an oceanic response forcing, one by the ocean and a second produced changes of sea ice distribution that is captured in the Pacific Decadal one by the ecosystem (e.g., the double and ocean circulation that affect con- Oscillation (PDO) pattern (Chhak et al., integration hypothesis; Di Lorenzo and nectivity patterns and food web linkages. 2009). In the positive phase of the PDO Ohman, 2013). Statistically, each integra- Here, we report on selected findings in when the AL is stronger, the Northeast tion can act as a low-pass filter of the each region that show the importance Pacific is characterized by downwelling input function, which allows marine of the horizontal advection bottom-up wind anomalies, which drive positive ecosystems to amplify the low-frequency forcing paradigm. sea level height anomalies (SSHa) along fluctuations contained in the large-scale the entire eastern boundary (Figure 2b). atmospheric modes. THE NORTHEAST PACIFIC The adjustment to higher SSHa anoma- The horizontal advection bottom- In the Northeast Pacific, GLOBEC lies leads to poleward geostrophic flow up forcing paradigm was found to be focused on two oceanographically dis- anomalies in coastal regions. In the GOA, important in all GLOBEC regions. In the tinct regions, the California Current the higher freshwater runoff during Northeast Pacific, observed long-term System (CCS) eastern boundary upwell- stronger AL provides an additional con- changes in zooplankton were linked to ing system and the Gulf of Alaska (GOA) tribution 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.

The Mean Atmospheric Circulation Iceland Low Aleutian Low Index (AL) Aleutian Low Defined as the dominant North Atlantic mode of North Pacific SLPa, Oscillation Index (NAO) the AL is associated in its Northeastth st Defined as the dominant positive phase with a Pacific mode of North Atlantic SLPa, strengthening and Northeast the NAO is the surface expression North Pacific High southward shift of the Atlantic Azores High of the NAM. In its positive phase Aleutian Low pressure system. the NAO is associated with a strengthening of the gradient between the Iceland Low and Azores High. Southern Ocean

SLP mbar 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.

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)

24 Oceanography | Vol. 26, No. 4 by increasing the cross-shelf baroclinicity waters from the south (north) bring BOX 1: CLIMATE MODES (e.g., Weingartner et al., 2005). Stronger zooplankton species residing in those downwelling winds and coastal currents waters into the NCCS region, funda- AL: The Aleutian Low index, defined tend to be accompanied by enhanced mentally and profoundly altering the as the dominant mode of North mesoscale eddy activity in the GOA shelf ecosystem. Poleward-flowing sub- Pacific SLPa, is associated in its positive phase with a strengthening and (Combes and Di Lorenzo, 2007; Henson tropical waters (+PDO) typically have southward shift of the Aleutian Low and Thomas, 2008); episodes of relax- many species of small bodied, lipid-poor pressure system. ation in downwelling winds can result in copepods, each of relatively low abun- fluxes of nutrient-rich water being driven dance and biomass. Conversely, boreal ENSO: El Niño-Southern Oscillation onto the shelf by these eddies (Ladd waters advected from the north (–PDO) index is used to track changes in et al., 2007). The changes in circulation have fewer copepod species, but the tropical Pacific ocean/atmosphere associated with a stronger Aleutian Low, dominant species have relatively large climate. and resulting positive phase of the PDO, body size and are often lipid-rich (Hooff have been identified as an important and Peterson, 2006). The dependence of NAM or AO: Northern Annular bottom-up forcing of marine ecosystems zooplankton community composition on Mode, also known as the Arctic in the Northern California Current changes in transport has been success- Oscillation, tracks changes in the and Gulf of Alaska. fully tested both with eddy-resolving strength of the polar vortex. ocean model simulations (Keister et al., The Northern California 2011) and observational studies that use NAO: North Atlantic Oscillation is the dominant mode of North Current System satellite data to infer currents (Bi et al., Atlantic SLPa and is the surface In the Northern California Current 2011; Figure 3). The supply of boreal expression of the NAM. In its posi- System (NCCS), high productivity along neritic copepods to the Oregon shelf tive phase, the NAO is associated the coast is associated with seasonal serves as an energy-rich prey base for a with a strengthening of the gradient upwelling that supplies surface nutrients. food web that supports rapid growth and between the Iceland Low and the Although interannual changes in upwell- high survival of juvenile salmon during Azores High. ing and primary production have been –PDO periods (Bi et al., 2011; Figure 3c). linked to changes in the AL alongshore The PDO has been mechanistically NPGO: The North Pacific Gyre winds and the PDO, the mechanistic link linked to changes in the alongshore Oscillation is a climate mode of between the AL/PDO and higher trophic transport that control zooplankton variability in the North Pacific that levels is still being explored. Several species composition along the entire tracks changes in strength of the GLOBEC publications (Hooff and CCS (Bi et al., 2011; Keister et al., North Pacific Current. Peterson, 2006; Peterson, 2009) suggest 2011; Di Lorenzo and Ohman, 2013). that PDO-related changes in along-shelf However, vertical transport and upwell- PDO: The Pacific Decadal Oscillation is the dominant mode of SSTa vari- transport exert a more important control ing dynamics show different signals ability in the North Pacific. on higher trophic levels. For example, in the NCCS and Southern California during positive (negative) phases of Current System. While PDO forcing SAM: Southern Annular Mode, also the PDO, the anomalous transport of controls the low-frequency modulations known as the Antarctic Oscillation, SAM tracks changes in the strength Emanuele Di Lorenzo ([email protected]) is Professor of Ocean Sciences, School of Earth and of the low pressure polar vortex Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA. David Mountain over Antarctica. is an independent oceanographer in Tucson, AZ, USA. Harold P. Batchelder is Professor, College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA. Nicholas Bond is Research Scientist, Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, WA, USA. Eileen E. Hofmann is Professor, Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, Norfolk, VA, USA. Daniel Costa Daniel

Oceanography | December 2013 25 Oceanography | December 2013 25 of upwelling in the NCCS (north of identify the mechanisms of interaction 2009; Bi et al., 2011), and can have major 38°N), decadal changes in upwelling in among the PDO, NPGO, and ecosystem implications for the productivity of the the SCCS are more closely related to the responses, it is clear that regional cir- shelf ecosystem and for regional econo- North Pacific Gyre Oscillation (NPGO; culation changes associated with these mies dependent on that productivity. Figure 2d,e,f), a second mode of North climate modes control water mass Pacific climate variability (Di Lorenzo transports and upwelling dynamics that The Gulf of Alaska et al., 2008). The NPGO is strongly alter copepod community structure and The GOA subpolar gyre is a high- correlated with long-term changes in abundance, including mean body size, nutrient, low-chlorophyll region where surface salinity, nitrate, and chlorophyll biomass, and lipid (e.g., energy) content. iron is the limiting factor to primary in the CCS and in the Gulf of Alaska These processes in turn affect the trophic productivity (Martin et al., 1989). In (Di Lorenzo et al., 2009). transfer of energy to higher consumers, contrast, coastal GOA biological pro- Although more studies are needed to such as forage fish and salmon (Peterson, ductivity is high, despite the mostly downwelling-favorable winds there. NorNortheasttheast P Pacicacic High phytoplankton production on the shelf is attributed to cross-shelf mixing of nearshore waters rich in iron concentrations, with waters near the shelf break high in nitrate concentrations. The two water masses tend to contain different lower trophic level communities, with the coastal region dominated by large-celled diatoms and dinoflagellates and the oceanic waters by smaller-celled phytoflagellates and pennate diatoms (Strom et al., 2006). The shifting boundary between these waters, and the degree of cross-shelf exchange, is determined by mechanisms related to freshwater runoff, Ekman transports, and eddies and meanders (Stabeno et al., 2004). Strong cyclonic storm systems associated with variability in the AL are frequent during the cool season (e.g., Weingartner et al., 2005) and induce Ekman upwelling in the basin of the Gulf of Alaska, Figure 2. Atmospheric forcing and oceanic response of downwelling in the coastal zone due to the Northeast Pacific during positive phases of the Pacific AGAG = =Alask Alaskanan Gyr Gyree Decadal Oscillation (PDO) and North Pacific Gyre Oscillation CCCC = =Calif Californiaornia Cu Currrrenent t onshore-directed Ekman transports, and NPC = North Paci c Current (NPGO). The patterns are obtained by regressing the PDO NPC = North Paci c Current vigorous wind mixing in both regions. (left column) and NPGO (right column) indices with the AnomaliesAnomalies in in ocean ocean currents currents PDO The response of the GOA ecosystem (a, d) sea level pressure anomaly (SLPa) and wind stresses PDO NPGO (black vectors), the (b, e) Regional Ocean Modeling System NPGO to the AL is evident in the results from (ROMS) sea surface height anomaly (SSHa), and (c, f) sea a combined empirical orthogonal func- surface temperature anomaly (SSTa). In the SSHa panels (b, e), the black contours correspond to the long-term mean SSH (1950–2008), so that it is pos- tion (CEOF) analysis of satellite-based sible to visualize how the PDO and NPGO SSHa project on the mean circulation—the bold vectors SSHa and Chl-a distributions (see Brown in the SSHa panels indicate the direction of PDO (red vectors) and NPGO (blue vectors) contribu- and Fiechter, 2012, for a GOA applica- tions to the circulation. Figure reproduced from 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), tion and a description of methods). and mean SSH from Niiler et al. (2003) For example, the 1st CEOF shows that

26 Oceanography | Vol. 26, No. 4 during periods of stronger Aleutian that iron from freshwater runoff plays in trophic levels. Moreover, the responses Low, enhanced downwelling in the GOA offshore productivity (e.g., Fiechter et al., of higher-trophic level species to varia- (positive SSHa in Figure 4a) serves to 2009; Fiechter and Moore, 2012; Coyle tions in lower trophic level community inhibit productivity in the subpolar gyre et al., 2012, and references therein). composition require further investiga- interior (negative Chl-a in Figure 4b). Freshwater runoff in the coastal GOA tion. For example, juvenile pink salmon This mode of variability explains about is weakly correlated with the PDO (Oncohynchus gorbuscha) are generalist 50% of the variance in the GOA subpolar (< 25%) and more strongly controlled feeders, consuming the biomass domi- gyre (Figure 4b) and is characterized by by regional-scale variability associated nant plankton prey of appropriate size, strong decadal fluctuations that track with the Ketchikan-Seward atmospheric although prey composition seems to the PDO (Figure 4c, R = 0.66), which sea level pressure gradient (Weingartner impact juvenile survival (e.g., Armstrong in this region is the oceanic response to et al., 2005). Here, the freshwater run- et al., 2008). Other species may have AL forcing (Chhak et al., 2009). off not only controls iron supply but stricter prey preferences and/or habitat In addition to the immediate response also exerts an important control on requirements and be more sensitive to of the GOA to AL forcing, the adjust- mesoscale eddies in the coastal region variations in transport, especially at the ment to stronger downwelling in the by determining the baroclinicity of the larval stage (Doyle et al., 2009). Finally, coastal region energizes mesoscale Alaska Coastal Current. while cross-shelf exchanges of nutri- eddies along the coastal GOA (Combes The complex set of dynamical controls ents and organisms were the focus of and Di Lorenzo, 2007; Henson and on mesoscale eddy variability, freshwater GLOBEC, along-shore transport dynam- Thomas, 2008). These eddies enhance runoff, and iron supply makes it more ics and mesoscale eddy entrainment cross-shelf transport of shelf waters challenging to establish in the GOA a of larvae are likely to exert important (Okkonen et al., 2003; Stabeno et al., clear deterministic link between large- controls on GOA ecosystem function 2004; Combes et al., 2009; Janout et al., scale climate forcing and ecosystem (e.g., Stabeno et al., 2004; Bailey and 2009) and have been linked to cross-shelf responses in the lower and higher Picquelle, 2002; Atwood et al., 2010). The export of coastal iron and Chl-a into the gyre (Crawford et al., 2005; Ladd Northern California Current System

et al., 2007; Ueno et al., 2010; Fietcher ) –1 and Moore, 2012), which increases over- s a all open ocean Chl-a concentrations. Changes in This delayed response of eddy-induced Alongshore Transport cross-shelf transport to AL forcing is Ecosystem Response nd R=0.78 to PDO Changes evident in the 2 CEOF of SSHa/Chl-a alongshore transport (m Aleutian Low (AL) in Ocean Transport (Figure 4d), which shows clear signa- alongshore winds (m s–1) )

tures of the three large anticyclonic –3 b

m R=0.67 eddies in the Haida, Sitka, and Yakutat Figure 3. Observed ecosys- Observations in Lower Trophic tem response to changes regions (Okkonen et al., 2001; Ladd Levels the Oregon Shelf in Oregon Shelf trans- et al., 2007). This second mode explains Bi et al., 2011 port. (a) Correlation ansformed biomass

tr between alongshore wind ~ 15% of the Chl-a variance in the gyre data from the National (Figure 4e) and has a weaker time lag lipid rich copepods (mg alongshore transport (m s–1) Centers for Environmental (~ 6 months) correlation with the PDO Prediction and satellite AVISO alongshore transport c (Figure 4f, R = 0.47) compared to the data. (b) Correlation between te

st ra

1 CEOF. The weak correlation with the l Newport zooplankton biweekly va Higher Trophic AL/PDO reflects the chaotic nature of data and alongshore transport. Levels (c) Correlation between coho

eddy dynamics in the GOA, especially oho Salmon (%) C ocean survi R=0.76 salmon survival rate and Newport along the Alaskan Stream (Combes and zooplankton. Figure redrafted from Bi et al. (2011); see paper for details Di Lorenzo, 2007; Ueno et al., 2010), lipid rich copepods (mg m–3) cumulative biomass on data sources and it also reflects the important role

Oceanography | December 2013 27 role of mesoscale entrainment of larvae continental-scale coastal current sys- salinity anomalies around 1990. Time on higher trophic ecosystem dynamics tem that extends from the subarctic to series formed by combining data col- is only beginning to be explored and Cape Hatteras (Chapman and Beardsley, lected by GLOBEC with earlier obser- requires new, targeted field observations. 1989; Figure 5). An increase in NAM vations indicate that various biological A comparison of the CCS and the (see definition in Figure 1) in the late indices changed concurrently with the GOA with other Pacific Ocean boundary 1980s to mid-1990s led to increased changes in salinity between the 1980s current systems was conducted during export of low-salinity waters from the and the 1990s (Figure 5b–e). Specifically, the GLOBEC Pacific Ocean Boundary Arctic into the North Atlantic, which phytoplankton production in the Gulf Ecosystem and Climate Study (POBEX; enhanced the transport of the coastal of Maine increased, zooplankton com- http://www.pobex.org; Di Lorenzo current system at least as far south as munity structure in the Gulf of Maine et al., 2013, in this issue). POBEX found the GB/GoM (Greene and Pershing, and Georges Bank shifted to a domi- that changes in horizontal transport 2007). Direct current measurements by nance of smaller copepod species, and dynamics were important drivers of GLOBEC (Smith et al., 2001) indicate first-year survival rate for haddock low-frequency variability also in the that during the mid-1990s, the influx (Melanogrammus aeglefinus) larvae on Kuroshio-Oyashio Extension (Chiba to the GB/GoM from the shelf current Georges Bank increased threefold, while et al., 2013) and the Humboldt or Peru- system was twice that measured in the Atlantic cod (Gadus morhua) larval sur- Chile Current System. late 1970s to early 1980s. The measure- vival decreased threefold (not shown). ments also showed that, concurrently, These findings indicate that a major, THE NORTHWEST ATLANTIC the influx to the region of high-salinity decadal-scale shift in the regional eco- The Georges Bank/Gulf of Maine offshore oceanic waters was reduced by system fundamentally altered its physical region (GB/GoM) in the Northwest half. This change in the proportion of properties and transports; transformed Atlantic is a highly productive area the inflows resulted in reduced salinity the size structure, composition, and that supports many important com- throughout the GB/GoM compared to ecology of the lower trophic levels; and mercial fisheries. The local circulation the historical record (Figure 5a), with altered the survival of two commercially is part of a southwestward flowing, the transition to persistently negative important fish stocks.

Gulf of Alaska Ocean Transport (SSHa) Lower Trophic Level (CHL-a) o shore downwelling reduces reduced nutrients lead to lower nutrients in gyre productivity in the gyre immediate a b c downwelling range [-.8 .8] range [-.8 .8] R=0.66 response SSHa CHL-a Regression CorrCorrelatione SSHa/CHL-a MODE1 50% of CHL-a variance in gyre MODE1 SSHa/CHL-a (10%) Stronger PDO Index Aleutian Low def SSHa range [-.7 .7] CHL-a range [-.4 .4] R=0.47 Regression CorrelCorrelation SSHa/CHL-a MODE2 Alaskan Sitka & Haida 15% of CHL-a mesoscale MODE2 SSHa/CHL-a (13%) Stream Eddies Eddies variance in gyre PDO Index (lead 6months) delayed cross-shelf transport [cm] [R] Year response coastal downwelling energizes eddies export coastal iron and mesoscale eddies CHL-a to o shore

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.

28 Oceanography | Vol. 26, No. 4 Coupled models of circulation and the broad, bottom-up, ecosystem-level (Burns et al., 2004; Chapman et al., ecosystem dynamics (Ji et al., 2008) changes that GLOBEC documented and 2004; Širović et al., 2004; Thiele et al., showed that changes in stratification that are believed to be the local response 2004). The hotspot regions were in areas associated with the increased shelf water to atmospherically driven oceanic varia- influenced by Circumpolar Deep Water inflow and reduced surface layer salin- tions at high latitude that were advected (CDW), a relatively warm (1–1.5°C), ity resulted in changes in the seasonal to the GB/GoM region. salty (34.74) oceanic water mass that timing and magnitude of primary pro- moves onto the WAP shelf at depth. duction consistent with the observed THE WESTERN ANTARCTIC This water mass influences heat and salt changes. Bio-physical modeling of PENINSULA budgets (Dinniman and Klinck, 2004), large- and small-sized zooplankton The US Southern Ocean GLOBEC sea ice distribution (Dinniman et al., species (Ji et al., 2012) captured the (SO GLOBEC) program, focused on 2012), and food web structure (Prézelin spatial and seasonal patterns observed Marguerite Bay in the southern por- et al., 2004; Ballerini et al., in press). in those species and demonstrated that tion of the western Antarctic Peninsula For the Antarctic Peninsula region, the patterns were controlled largely (sWAP) continental shelf (Figure 6, a positive SAM index has been related through bottom-up trophic processes. inset). The program featured an end-to- to warming (Kwok and Comiso, 2002; Year-to-year changes in zooplankton end food web approach that included Thompson and Solomon, 2002; Van den abundance were difficult to replicate in predators and competitors of Antarctic Broeke and van Lipzig, 2004), strength- models, probably because overall abun- krill (Euphausia superba), as well as the ening of the westerlies (Marshall et al., dance is sensitive to top-down processes, influence of habitat (Hofmann et al., 2006), and decreases in sea ice extent particularly predation mortality, which 2004). Surveys identified biological and seasonal duration (Stammerjohn is difficult to measure accurately from hotspots, regions of sustained biological et al., 2008). Model investigations field observations. The model could not production in excess of average condi- showed that increased wind strength and replicate the shift in zooplankton com- tions (Costa et al., 2007), that had asso- ACC transport increased the volume munity structure toward smaller species ciated enhanced abundance of marine of CDW transported onto the western (Figure 5). The mechanisms responsible mammals and other top predators Antarctic Peninsula continental shelf, for the change in cod and haddock survival are not completely understood. Ecosystem Response to Subarctic Intrusion 1990s They may be related to prey preferences of the two species. Because larval and (a) Northwest Atlantic Changes in -0.5-0.5 juvenile haddock prefer smaller-sized SalinitySalinity Physical Circulation 0 AnomAnomalyaly Environment prey than do the early life stages of cod, 0.50.5 the increased abundance of smaller zoo- (b) 1 PhPhytoplanktonytoplankton plankton species such as occurred during InIndedexx 0 the later, lower-salinity period might (c) Copepod 1 have conferred a feeding advantage to AAbundancbundance Lower Trophic young haddock compared to young cod. 0 Levels -1-1 The changes in haddock and cod survival 1 ZoZoooplanktonplankton (d) CoCommunitmmunityy were reflected in the subsequent abun- 0 IndeIndexx Figure 5. Observed impacts of the sub -1-1 dance of the adult stocks, with haddock arctic intrusion in the Northwest Atlantic. (e) HaddockHaddock 1515 exhibiting strong stock recovery in the (a) Georges Bank annual salinity anomaly. SurSurvivalvival 1990s while the cod stock continued to (b) Gulf of Maine autumn phytoplankton 0 Higher Trophic index. (c) Gulf of Maine small copepod Levels decline. Strong management measures 19801980 19199090 2000 abundance anomaly. (d) Georges Bank Year limiting haddock catches probably zooplankton community structure index. also contributed to the increase of the (e) Georges Bank haddock survival (recruits per 104 hatched eggs). The horizontal dashed lines rep- resent the 1980s and 1990s decadal average for each parameter. Series a, d, and e are redrafted from haddock stock. However, the changes Mountain and Kane (2010), and series b and c from Greene and Pershing (2007). See paper for descrip- in the adult stocks likely were part of tion of data sources.

Oceanography | December 2013 29 and the increased heat input reduced SO GLOBEC study region, reproduc- CONCLUSION sea ice extent and duration, especially in tion and recruitment of Antarctic krill Marine ecosystem responses to climate winter (Dinniman et al., 2012). is more closely related to the spring variability in the four regions studied Simulated transport pathways phytoplankton bloom than to sea ice in the US GLOBEC program reflect obtained from Lagrangian particle track- (Daly, 2004; Marrari et al., 2008), so the the different physical characteristics of ing studies showed that the sWAP shelf biological hotspots may not be affected atmospheric and ocean forcings that receives inputs from upstream regions in the short term by climate-induced influence local biological processes, food and local areas (Figure 6), both of which changes in circulation. web structure, and overall productiv- are important in maintaining local shelf A mass balance food web model ity of the ecosystem. Although vertical Antarctic krill populations and the bio- developed for the SO GLOBEC region supply of nutrients remains an impor- logical hotspots (Piñones et al., 2011, (Ballerini et al., in press) showed that tant mechanism controlling primary 2013a). Lagrangian transport simula- Antarctic krill provide the majority of productivity and food web dynamics tions using circulation fields produced by energy needed to sustain seabird and through the trophic cascade, GLOBEC increased wind strength and ACC trans- marine mammal populations. Long-term identified a new set of bottom-up forcing port showed that the increased volume of environmental changes, particularly pathways that involve changes in along- CDW on the shelf contributed to the suc- decreasing sea ice, will possibly alter the shore and cross-shore ocean transport. cess of larval Antarctic krill, but only for distribution and availability of Antarctic Specifically, it was shown that changes limited areas of the sWAP shelf (Piñones krill. The additive effects of bottom-up in atmospheric forcing drive changes in et al., 2013b). The modified circulation resource control through changes in ocean currents and horizontal transport also increased advection of Antarctic krill phytoplankton assemblages (Antarctic that impact marine population dynamics larvae onto the shelf and enhanced reten- krill prey) and the top-down effects of and distributions. This new horizontal- tion, but the reduced winter sea ice and consumers of Antarctic krill may amplify advection bottom-up forcing paradigm shifts in ice cover distribution may com- the effects of climate change on the was isolated in each GLOBEC region by promise overwintering survival of larvae sWAP food web, thereby adding another combining long-term eddy-resolving and recruitment to adult stages (Piñones dimension to the conceptual model ocean models with field observations. et al., 2013b). On the other hand, in the developed for the US GLOBEC regions. This revelation was a direct result of the

Southern Ocean WAP Larvae Stronger Southern Annular Mode (SAM) increases transport of CDW onto shelf Juveniles Adults

) )

) la Shelf break Shelf break Shelf break C) su C insula PCC) n n (A t (AP Shelf 2 CDWC Shelf Pe Shelf Peni ic Peninsula mpolar Deep Water (CDW Region t Region ircumpolar Current (ACC) Region IntrusionsIntruI C arctic Circu Circumpolar Deep Water (CDW) Circumpolar Deep Water (CDW) nt arctic Circumpolar Current (ACC) Antarc Antarcstaltic Current A t a ntarctic An Coastal Current (APCC Antarctic Circumpolar Current (ACC Co A Coastal Curren Hotspots Hotspots line Hotspots Region Region Region 1 Krill Larvae Coastline Coast Coastline advected by ACC 3 Recirculation Trough Recirculation TroughT Recirculation TrT & Retention & Retention & Retention o ug 4 h Coastline Western Antarctica Coastline Western Antarctica Coastline Western Antarctica Peninsula (WAP) Peninsula (WAP) Peninsula (WAP) Krill Larvae advected by ACCIntrusion of CDW carries larvae Retention and transport of juveniles on the shelf provide source of krill on the shelf

Figure 6. 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 from Piñones et al., 2013b

30 Oceanography | Vol. 26, No. 4 three-dimensional regional modeling dynamics) and mesoscale transport processes resolved in the models. focus of the GLOBEC program. The processes (e.g. cross-shelf exchanges) Establishing the link among climate modeling studies showed that changes are also critical tools for investigating forcing, regional mesoscale, and sub in ocean transport impacted different the mechanistic links between changes mesoscale transport processes and aspects of ecosystem structure and func- in basin-scale circulation and coastal marine ecosystem dynamics raises tion (e.g., productivity, species composi- marine ecosystems. GLOBEC pioneered important scientific questions and poses tion, connectivity) in the four regions. the application of eddy-resolving models new technical challenges. For example, In the GOA, the cross-shelf advection of to understand the impact of regional it is well known that mesoscale and iron-rich coastal waters into the ocean’s transport dynamics on lower trophic submesoscale dynamics have an impor- interior was identified as a mechanism levels. It is now clear that this approach tant nondeterministic component. for enhancing productivity in the iron- must be extended in order to fully This implies that a fraction of ocean limited/nutrient-rich subpolar gyre explore the dynamics of higher trophic and ecosystem variability cannot be (Figure 7, impact on productivity). In levels. For example, ocean entrainment predicted or linked to climate forcing. the CCS, changes in the strength of the dynamics associated with mesoscale However, climate may have predictable alongshore southward currents were eddies are likely to have strong impacts impacts on the statistics of mesoscale found to increase the supply of boreal on fish life cycles via transport of larvae and submesoscale features. To this end, neritic copepod species, which provide and by altering connectivity patterns. applying ensemble approaches with energy-rich prey for the local food web Furthermore, submesoscale trans- eddy-resolving ocean models may pro- and enhance the survival of juvenile port processes that were not resolved vide a way to separate and quantify the salmon (Figure 7, impact on species in GLOBEC likely play an equally amount of intrinsic and deterministic composition). In the NWA, the south- important role in ecosystem dynam- variance in ocean and ecosystem pro- ward intrusion of Arctic water masses ics (e.g., hotspots) and will need to be cesses in relation to climate forcing. was linked to an increase in productivity. resolved in future studies. As we advance Finally, it is essential to continue inte- These effects propagated through the our modeling framework, it is also criti- grating state-of-the-art regional ocean entire food web to higher trophic levels cal to develop novel observational strat- models with Intergovernmental Panel (Figure 7, impact on species composition egies that allow constraining the new on Climate Change (IPCC) class models and fish). In the SO, regional transport dynamics on the shelf were shown to ATMOSPHERE play an important role in the retention 2 The “horizontal advection bottom of Antarctic krill larvae and the distribu- Climate up forcing” paradigm Forcing tion of juvenile krill (Figure 7, impact 1 The “bottom up forcing” on connectivity patterns). classical paradigm The GLOBEC recognition that upwelling and alongshore and cross- strati cation shelf circulation climate-induced changes in horizontal Vertical dynamics dynamics Horizontal OCEAN transport processes exert a dominant OCEAN Transport Transport Processes Processes 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 obser- Ecosystem primary productivity Processes primary productivity (GOA, NWA) vational strategies that account for the (GOA, CCS, NWA, SO) species composition (CCS, NWA) connectivity patterns and hotspots (SO) nonlocal nature of ecosystem variability MARINE ECOSYSTEM associated with basin-scale changes in Figure 7. Conceptual advances of GLOBEC. GLOBEC expanded the classical bottom-up forc- ocean circulation. Advanced modeling ing 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. frameworks that resolve regional-scale GLOBEC studies identified ocean horizontal transport processes as an equally important circulation (e.g., regional alongshore driver of marine ecosystem dynamics.

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