Publications

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2018
Amaya, DJ, Siler N, Xie SP, Miller AJ.  2018.  The interplay of internal and forced modes of Hadley Cell expansion: lessons from the global warming hiatus. Climate Dynamics. 51:305-319.   10.1007/s00382-017-3921-5   AbstractWebsite

The poleward branches of the Hadley Cells and the edge of the tropics show a robust poleward shift during the satellite era, leading to concerns over the possible encroachment of the globe's subtropical dry zones into currently temperate climates. The extent to which this trend is caused by anthropogenic forcing versus internal variability remains the subject of considerable debate. In this study, we use a Joint EOF method to identify two distinct modes of tropical width variability: (1) an anthropogenically-forced mode, which we identify using a 20-member simulation of the historical climate, and (2) an internal mode, which we identify using a 1000-year pre-industrial control simulation. The forced mode is found to be closely related to the top of the atmosphere radiative imbalance and exhibits a long-term trend since 1860, while the internal mode is essentially indistinguishable from the El Nio Southern Oscillation. Together these two modes explain an average of 70% of the interannual variability seen in model "edge indices" over the historical period. Since 1980, the superposition of forced and internal modes has resulted in a period of accelerated Hadley Cell expansion and decelerated global warming (i.e., the "hiatus"). A comparison of the change in these modes since 1980 indicates that by 2013 the signal has emerged above the noise of internal variability in the Southern Hemisphere, but not in the Northern Hemisphere, with the latter also exhibiting strong zonal asymmetry, particularly in the North Atlantic. Our results highlight the important interplay of internal and forced modes of tropical width change and improve our understanding of the interannual variability and long-term trend seen in observations.

2015
Rasmussen, L, Bromirski PD, Miller AJ, Arcas D, Flick RE, Hendershott MC.  2015.  Source location impact on relative tsunami strength along the US West Coast. Journal of Geophysical Research-Oceans. 120:4945-4961.   10.1002/2015jc010718   AbstractWebsite

Tsunami propagation simulations are used to identify which tsunami source locations would produce the highest amplitude waves on approach to key population centers along the U.S. West Coast. The reasons for preferential influence of certain remote excitation sites are explored by examining model time sequences of tsunami wave patterns emanating from the source. Distant bathymetric features in the West and Central Pacific can redirect tsunami energy into narrow paths with anomalously large wave height that have disproportionate impact on small areas of coastline. The source region generating the waves can be as little as 100 km along a subduction zone, resulting in distinct source-target pairs with sharply amplified wave energy at the target. Tsunami spectral ratios examined for transects near the source, after crossing the West Pacific, and on approach to the coast illustrate how prominent bathymetric features alter wave spectral distributions, and relate to both the timing and magnitude of waves approaching shore. To contextualize the potential impact of tsunamis from high-amplitude source-target pairs, the source characteristics of major historical earthquakes and tsunamis in 1960, 1964, and 2011 are used to generate comparable events originating at the highest-amplitude source locations for each coastal target. This creates a type of ``worst-case scenario,'' a replicate of each region's historically largest earthquake positioned at the fault segment that would produce the most incoming tsunami energy at each target port. An amplification factor provides a measure of how the incoming wave height from the worst-case source compares to the historical event.

Miller, AJ, Song H, Subramanian AC.  2015.  The physical oceanographic environment during the CCE-LTER Years: Changes in climate and concepts. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 112:6-17.   10.1016/j.dsr2.2014.01.003   AbstractWebsite

The California Current System (CCS) has been studied by the California Cooperative Oceanic Fisheries Investigations program for many decades. Since 2004, the Southern California Bight (SCB) and the oceanic region offshore has also been the site for the California Current Ecosystem (CCE) Long-Term Ecological Research (LTER) program, which has established long-term observational time series and executed several Process Cruises to better understand physical biological variations, fluxes and interactions. Since the inception of the CCE-LTER, many new ideas have emerged about what physical processes are the key controls on CCS dynamics. These new perspectives include obtaining a better understanding of what climate patterns exert influences on CCS physical variations and what physical controls are most important in driving CCE ecological changes. Physical oceanographic and climatological conditions in the CCS varied widely since the inception of the CCE-LTER observational time series, including unusual climate events and persistently anomalous states. Although the CCE-LTER project commenced in 2004 in the midst of normal ocean conditions near the climatological means, over the following decade, El Nino/Southern Oscillation conditions flickered weakly from warm to cold, with the Pacific Decadal Oscillation (PDO) generally tracking that behavior, while the North Pacific Gyre Oscillation (NPGO) evolved to persistent and strong positive conditions after 2007, indicative of enhanced upwelling from 2007 to 2012. Together the combined impact of the negative PDO state (La Nina conditions) and positive NPGO state (increased upwelling conditions) yielded remarkably persistent cool conditions in the CCS from late 2007 to early 2009 and from mid-2010 through 2012. The broad-scale climate variations that occurred over the North Pacific and CCS during this time period are discussed here to provide physical context for the CCE-LTER time series observations and the CCE-LTER Process Cruises. Data assimilation fits, using the Regional Ocean Modeling System four-dimensional data assimilation framework, were successfully executed for the 1-month time period surrounding each of the Process Cruises. The fits provide additional information about how the physical flows evolve during the course of the multi-week Process Cruises. Relating these physical states to the numerous biological measurements gathered by the CCE-LTER time series observations and during the Process Cruises will yield vital long-term perspective of how changing climate conditions control the ocean ecosystem in this region and information on how this important ecosystem can be expected to evolve over the coming decades. (C) 2014 Elsevier Ltd. All rights reserved.

2013
DeFlorio, MJ, Pierce DW, Cayan DR, Miller AJ.  2013.  Western US extreme precipitation events and their relation to ENSO and PDO in CCSM4. Journal of Climate. 26:4231-4243.   10.1175/jcli-d-12-00257.1   AbstractWebsite

Water resources and management over the western United States are heavily impacted by both local climate variability and the teleconnected responses of precipitation to the El Nino-Southern Oscillation (ENSO) and Pacific decadal oscillation (PDO). In this work, regional precipitation patterns over the western United States and linkages to ENSO and the PDO are analyzed using output from a Community Climate System Model version 4 (CCSM4) preindustrial control run and observations, with emphasis on extreme precipitation events. CCSM4 produces realistic zonal gradients in precipitation intensity and duration over the western United States, with higher values on the windward side of the Cascade Mountains and Sierra Nevada and lower values on the leeward. Compared to its predecessor CCSM3, CCSM4 shows an improved teleconnected signal of both ENSO and the PDO to large-scale circulation patterns over the Pacific-North America region and also to the spatial pattern and other aspects of western U.S. precipitation. The so-called drizzle problem persists in CCSM4 but is significantly improved compared to CCSM3. In particular, it is found that CCSM4 has substantially less precipitation duration bias than is present in CCSM3. Both the overall and extreme intensity of wintertime precipitation over the western United States show statistically significant linkages with ENSO and PDO in CCSM4. This analysis provides a basis for future studies using greenhouse gas (GHG)-forced CCSM4 runs.

2009
Kim, HJ, Miller AJ, McGowan J, Carter ML.  2009.  Coastal phytoplankton blooms in the Southern California Bight. Progress in Oceanography. 82:137-147.   10.1016/j.pocean.2009.05.002   AbstractWebsite

Surface chlorophyll (CHL) measured at the Scripps Pier in the Southern California Bight (SCB) for 18 years (1983-2000) reveals that the spring bloom occurs with irregular timing and intensity each year, unlike sea-surface temperature (SST), which is dominated by a regular seasonal cycle. In the 1990s, the spring bloom occurred earlier in the year and with larger amplitudes compared to those of the 1980s. Seasonal anomalies of the Pier CHL have no significant correlation with local winds, local SST, or upwelling index, which implies that classical coastal upwelling is not directly responsible for driving chlorophyll variations in nearshore SCB. The annual mean Pier CHL exhibits an increasing trend, whereas the Pier SST has no evident concomitant trend during the CHL observation period. The interannual variation of the Pier CHL is not correlated with tropical El Nino or La Nina conditions over the entire observing period. However, the Pier CHL was significantly influenced by El Nino/Southern Oscillation during the 1997/1998 El Nino and 1998/1999 La Nina transition period. The Pier CHL is highly coherent at long periods (3-7 years) with nearby offshore in situ surface CHL at the CalCOFI (California Cooperative Fisheries Investigations) station 93.27. (C) 2009 Elsevier Ltd. All rights reserved.

2006
Auad, G, Miller A, Di Lorenzo E.  2006.  Long-term forecast of oceanic conditions off California and their biological implications. Journal of Geophysical Research-Oceans. 111   10.1029/2005jc003219   AbstractWebsite

[1] The impact of global warming due to an increased content of atmospheric CO(2) is studied by forcing a numerical eddy-resolving ocean model with wind stresses, heat fluxes, and open boundary conditions obtained from a state-of-the-art coupled model. Specifically, we have compared the 1986 - 1996 and 2040 - 2050 decades to describe and analyze the changes attained by several oceanographic variables in the California Current System. A richer atmosphere in CO(2) leads to increased sea surface and near-surface temperatures in the model domain and to an increased stratification along the coast that, however, is not strong enough to overcome the effect of increased upwelling favorable winds. A mild oceanic cooling is forecast below the 70-m depth, in agreement with recent studies of global warming trends. Near-surface vertical velocities increase by about 30% in April, but our simulations also forecast anomalous offshore transports in most of the coastal areas. The eddy kinetic energy decreases, on an annual mean, along the California Current main path with maximum negative anomalies in winter. The upward displacement of the 26.5 isopycnal surface, especially in the northern half of our study area, leads to an increase in the concentration of nutrients in the subsurface. The agreement between some results of this forecasting study and recent published findings would suggest that the current global warming trend would persist in the study area with similar changes to those observed over the last half century.

Seo, H, Jochum M, Murtugudde R, Miller AJ.  2006.  Effect of ocean mesoscale variability on the mean state of tropical Atlantic climate. Geophysical Research Letters. 33   10.1029/2005gl025651   AbstractWebsite

A regional coupled ocean-atmospheric model is used to investigate the effect of oceanic mesoscale features on the mean climate of the tropical Atlantic. It is shown that, compared to a non-eddy resolving ocean model, resolving oceanic mesoscale variability leads to a cooler mean equatorial cold tongue and a cooler coastal upwelling zone. This changes the meridional SST gradient, and the resulting weaker low-level convergence reduces the mean of rainfall in the marine Inter-Tropical Convergence Zone (ITCZ). The reduced rainfall and the cooler coastal upwelling regions represent a clear improvement of the model solution.

1997
Miller, AJ, White WB, Cayan DR.  1997.  North Pacific thermocline variations on ENSO timescales. Journal of Physical Oceanography. 27:2023-2039.   10.1175/1520-0485(1997)027<2023:nptvoe>2.0.co;2   AbstractWebsite

The North Pacific thermocline (250 to 400 m) is studied using XBT observations acquired during the 1970s and 1980s. Interannual variations (3-5 yr timescales) in thermocline temperature, with O(0.1 degrees C) amplitude at 400 m, are found to exhibit westward propagation throughout the extratropical North Pacific up to 45 degrees N. Southward of 30 degrees N, the features propagate intact across the basin from the eastern boundary to the western boundary. Northward of 30 degrees N, the features can be observed to propagate only as far as the date line. The observed midlatitude thermocline anomalies are often related to tropical ENSO events in that they occur most strongly after the development of tropical El Nino or La Nina conditions and propagate westward from near the eastern boundary in the midlatitudes. But it is found that the observed midlatitude thermocline anomalies have larger phase speeds than theoretically predicted free baroclinic Rossby waves. Also, the observed anomalies have larger wavelength and faster propagation speeds than baroclinic Rossby waves that radiate from coastal Kelvin-like waves near the eastern boundary in well-known high-resolution models. Large-scale thermocline fluctuations that have spatial scale and phase speeds similar to the observations are also found in a coarse-resolution model of the Pacific Ocean forced by observed wind and heat Aux anomalies over the 1970-88 period. In the midlatitudes, north of 30 degrees N, large-scale Ekman pumping by interannual wind stress curl variations provides a significant driving mechanism for the modeled large-scale thermocline anomalies. The modeled ocean response is a combination of the static thermocline response to large-scale Ekman pumping plus a train of westward traveling Rossby waves, which accounts for part of the propagating temperature fluctuations. A tropical, remotely forced component is prominant near the eastern boundary, but this only contributes weakly in the model open ocean.

1996
Miller, AJ, Lermusiaux PFJ, Poulain PM.  1996.  A topographic-Rossby mode resonance over the Iceland-Faeroe Ridge. Journal of Physical Oceanography. 26:2735-2747.   10.1175/1520-0485(1996)026<2735:atmrot>2.0.co;2   AbstractWebsite

An array of current meter moorings along 12 degrees W on the southern side of the Iceland-Faeroe Ridge reveals a narrowband barotropic oscillation with period 1.8 days in spectra of velocity. The signal is coherent over at least 55-km scales and propagates phase with shallow water on the right (toward the northwest). Velocity ellipses tend to be elongated (crossing contours of f/H) and rotate anticyclonically. Solutions of the rigid-lid barotropic shallow-water equations predict the occurrence of a topographic-Rossby normal mode on the south side of the ridge with spatial Scales exceeding 250 km and with intrinsic period near 1.84 days. This fundamental mode of the south side of the ridge has predicted spatial structure, phase propagation, and velocity ellipses consistent with the observed oscillation. The frictional amplitude e-folding decay time for this normal mode is estimated from the observations to be 13 days. The observed ocean currents are significantly coherent with zonal wind stress fluctuations (but not with wind stress curl) in the relevant period band, which indicates the oscillation is wind forced. This appears to be the first clear evidence of a stochastically forced resonant barotropic topographic-Rossby normal mode in the ocean.