A role for the equatorial undercurrent in the ocean dynamical thermostat

Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 31 (2018): 6245-6261, doi:10.1175/JCLI-D-17-0513.1.Reconstructions of sea surface temperature (SST) based on instrumental observations suggest that the equatorial Pacific zonal SST gradient has increased over the twentieth century. While this increase is suggestive of the ocean dynamical thermostat mechanism of Clement et al., observations of a concurrent weakening of the zonal atmospheric (Walker) circulation are not. Here we show, using heat and momentum budget calculations on an ocean reanalysis dataset, that a seasonal weakening of the zonal atmospheric circulation is in fact consistent with cooling in the eastern equatorial Pacific (EEP) and thus an increase in the zonal SST gradient. This cooling is driven by a strengthening Equatorial Undercurrent (EUC) in response to decreased upper-ocean westward momentum associated with weakening equatorial zonal wind stress. This process can help to reconcile the seemingly contradictory twentieth-century trends in the tropical Pacific atmosphere and ocean. Moreover, it is shown that coupled general circulation models (CGCMs) do not correctly simulate this process; we identify a systematic bias in the relationship between changes in equatorial surface zonal wind stress in the EEP and EUC strength that may help to explain why observations and CGCMs have opposing trends in the zonal SST gradient over the twentieth century.2019-01-1

Predicting Atlantic seasonal hurricane activity using outgoing longwave radiation over Africa

Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 43 (2016): 7152–7159, doi:10.1002/2016GL069792.Seasonal hurricane activity is a function of the amount of initial disturbances (e.g., easterly waves) and the background environment in which they develop into tropical storms (i.e., the main development region). Focusing on the former, a set of indices based solely upon the meridional structure of satellite-derived outgoing longwave radiation (OLR) over the African continent are shown to be capable of predicting Atlantic seasonal hurricane activity with very high rates of success. Predictions of named storms based on the July OLR field and trained only on the time period prior to the year being predicted yield a success rate of 87%, compared to the success rate of NOAA's August outlooks of 53% over the same period and with the same average uncertainty range (±2). The resulting OLR indices are statistically robust, highly detectable, physically linked to the predictand, and may account for longer-term observed trends.Alfred P. Sloan Foundation; Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution; Ocean and Climate Change Institute2017-01-0

The Equatorial Undercurrent and TAO sampling bias from a decade at SEA

Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Atmospheric and Oceanic Technology 21 (2014): 2015–2025, doi:10.1175/JTECH-D-13-00262.1.The NOAA Tropical Atmosphere Ocean (TAO) moored array has, for three decades, been a valuable resource for monitoring and forecasting El Niño–Southern Oscillation and understanding physical oceanographic as well as coupled processes in the tropical Pacific influencing global climate. Acoustic Doppler current profiler (ADCP) measurements by TAO moorings provide benchmarks for evaluating numerical simulations of subsurface circulation including the Equatorial Undercurrent (EUC). Meanwhile, the Sea Education Association (SEA) has been collecting data during repeat cruises to the central equatorial Pacific Ocean (160°–126°W) throughout the past decade that provide useful cross validation and quantitative insight into the potential for stationary observing platforms such as TAO to incur sampling biases related to the strength of the EUC. This paper describes some essential sampling characteristics of the SEA dataset, compares SEA and TAO velocity measurements in the vicinity of the EUC, shares new insight into EUC characteristics and behavior only observable in repeat cross-equatorial sections, and estimates the sampling bias incurred by equatorial TAO moorings in their estimates of the velocity and transport of the EUC. The SEA high-resolution ADCP dataset compares well with concurrent TAO measurements (RMSE = 0.05 m s−1; R2 = 0.98), suggests that the EUC core meanders sinusoidally about the equator between ±0.4° latitude, and reveals a mean sampling bias of equatorial measurements (e.g., TAO) of the EUC’s zonal velocity of −0.14 ± 0.03 m s−1 as well as a ~10% underestimation of EUC volume transport. A bias-corrected monthly record and climatology of EUC strength at 140°W for 1990–2010 is presented.The authors thank the NSF Physical Oceanography program (OCE-1233282) and the WHOI Academic Programs Office for funding.2015-03-0

Predicting ecosystem components in the Gulf of Mexico and their responses to climate variability with a dynamic Bayesian network model

Funding: This research was carried out in part under the auspices of the Cooperative Institute for Marine and Atmospheric Studies (CIMAS), a Cooperative Institute of the University of Miami and the National Oceanic and Atmospheric Administration, cooperative agreement #NA10OAR4320143. This paper is NOAA IEA Program contribution #2018_4. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Acknowledgments This research was carried out in part under the auspices of the Cooperative Institute for Marine and Atmospheric Studies (CIMAS), a Cooperative Institute of the University of Miami and the National Oceanic and Atmospheric Administration, cooperative agreement #NA10OAR4320143. This paper is a result of research, supported by the National Oceanic and Atmospheric Administration’s Integrated Ecosystem Assessment (NOAA IEA) Program. This paper is NOAA IEA Program contribution #2018_4.Peer reviewedPublisher PD

Oceanography : oxygen and climate dynamics

Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Climate Change 4 (2014): 862-863, doi:10.1038/nclimate2386.Low oxygen levels in tropical oceans shape marine ecosystems and biogeochemistry with climate change expected to expand these regions. Now, a study indicates that regional dynamics control tropical oxygen trends, bucking projected global reductions in ocean oxygen.2015-03-2

Observing the Galápagos–EUC interaction : insights and challenges

Author Posting. © American Meteorological Society, 2010. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 40 (2010): 2768–2777, doi:10.1175/2010JPO4461.1.Although sustained observations yield a description of the mean equatorial current system from the western Pacific to the eastern terminus of the Tropical Atmosphere Ocean (TAO) array, a comprehensive observational dataset suitable for describing the structure and pathways of the Equatorial Undercurrent (EUC) east of 95°W does not exist and therefore climate models are unconstrained in a region that plays a critical role in ocean–atmosphere coupling. Furthermore, ocean models suggest that the interaction between the EUC and the Galápagos Islands (92°W) has a striking effect on the basic state and coupled variability of the tropical Pacific. To this end, the authors interpret historical measurements beginning with those made in conjunction with the discovery of the Pacific EUC in the 1950s, analyze velocity measurements from an equatorial TAO mooring at 85°W, and analyze a new dataset from archived shipboard ADCP measurements. Together, the observations yield a possible composite description of the EUC structure and pathways in the eastern equatorial Pacific that may be useful for model validation and guiding future observation.Karnauskas acknowledges the WHOI Penzance Endowed Fund in Support of Assistant Scientists

An equatorial ocean bottleneck in global climate models

Author Posting. © American Meteorological Society, 2012. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 25 (2012): 343–349, doi:10.1175/JCLI-D-11-00059.1.The Equatorial Undercurrent (EUC) is a major component of the tropical Pacific Ocean circulation. EUC velocity in most global climate models is sluggish relative to observations. Insufficient ocean resolution slows the EUC in the eastern Pacific where nonlinear terms should dominate the zonal momentum balance. A slow EUC in the east creates a bottleneck for the EUC to the west. However, this bottleneck does not impair other major components of the tropical circulation, including upwelling and poleward transport. In most models, upwelling velocity and poleward transport divergence fall within directly estimated uncertainties. Both of these transports play a critical role in a theory for how the tropical Pacific may change under increased radiative forcing, that is, the ocean dynamical thermostat mechanism. These findings suggest that, in the mean, global climate models may not underrepresent the role of equatorial ocean circulation, nor perhaps bias the balance between competing mechanisms for how the tropical Pacific might change in the future. Implications for model improvement under higher resolution are also discussed.KBK gratefully acknowledges the J. Lamar Worzel Assistant Scientist Fund. GCJ is supported by NOAA’s Office of Oceanic and Atmospheric Research. RM gratefully acknowledges the generous support and hospitality of the Divecha Centre for Climate Change and CAOS at IISc, Bangalore, and partial support by NASA PO grants.2012-07-0

Interannual variability of sea surface temperature in the eastern tropical Pacific Ocean and Central American rainfall

Sea surface temperature (SST) in the east Pacific warm pool (EPWP) plays a potentially important role in Central American rainfall, tropical cyclogenesis, ocean biology, large-scale tropical heating, and the El Niño-Southern Oscillation (ENSO). The first part of this dissertation is aimed at understanding what processes govern the interannual variability of SST in the EPWP. Interannual wind stress, shortwave radiation, and precipitation were used as forcing to an ocean general circulation model. Shortwave heating was identified as the primary driver of the interannual SST tendency in the EPWP. The high correlation between the EPWP and the equatorial Pacific Ocean is explained by the fact that equatorial SST anomalies modify the distribution of atmospheric vertical motions and therefore cloud cover and shortwave heating. In a parallel set of experiments, the low-frequency variability of the Tehuantepec gap winds was also shown to have a considerable effect on that of SST in the EPWP. Motivated by the results of the first part of this dissertation, the second part offers significant improvements to the mean state of the equatorial Pacific Ocean in a climatology ocean model experiment by including the Galápagos Islands in the model topography. In this context, the equatorial cold bias is reduced. Furthermore, when the ocean model is coupled to the atmosphere through zonal wind stress, the problem of an excessively regular and biennial ENSO is also reduced. The change in ENSO timescale is a result of the same dynamics operating on a different mean state. The third part of this dissertation is aimed at understanding the role of the interannual variability of SST in the EPWP in that of Central American rainfall. An anomalously warm EPWP can trigger a rapid enhancement of the east Pacific intertropical convergence zone (ITCZ) in rainy seasons following peak ENSO events, which leads to a rainfall anomaly over Central America. Moreover, the timing and amplitude of the SST-enhanced ITCZ depends on the persistence of the ENSO event. The longer the equatorial SST anomaly persists, the longer the EPWP is subject to anomalous shortwave heating and thus the greater the subsequent SST enhancement of the ITCZ