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

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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 re arrangements 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
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