Owen Doherty

An analysis of aeolian dust in climate models

Aeolian dust is a key aspect of the climate system. Dust can modify the Earths energy budget, provide long-range transport of nutrients, and influence land surface processes via erosion. Consequently, effective modeling of the climate system, particularly at regional scales, requires a reasonably accurate representation of dust emission, transport, and deposition. Here we evaluate African dust in 23 state-of-the-art global climate models used in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. We find that all models fail to reproduce basic aspects of dust emission and transport over the second half of the 20th century. The models systematically underestimate dust emission, transport and optical depth, and year-to-year changes in these properties bear little resemblance to observations. These findings cast doubt on the ability of these models to simulate the regional climate and the response of African dust to future climate change.

Ecosystem responses in the southern Caribbean Sea to global climate change

Over the last few decades, rising greenhouse gas emissions have promoted poleward expansion of the large-scale atmospheric Hadley circulation that dominates the Tropics, thereby affecting behavior of the Intertropical Convergence Zone (ITCZ) and North Atlantic Oscillation (NAO). Expression of these changes in tropical marine ecosystems is poorly understood because of sparse observational datasets. We link contemporary ecological changes in the southern Caribbean Sea to global climate change indices. Monthly observations from the CARIACO Ocean Time-Series between 1996 and 2010 document significant decadal scale trends, including a net sea surface temperature (SST) rise of ∼1.0 ± 0.14 °C (±SE), intensified stratification, reduced delivery of upwelled nutrients to surface waters, and diminished phytoplankton bloom intensities evident as overall declines in chlorophyll a concentrations (ΔChla = −2.8 ± 0.5%⋅y−1) and net primary production (ΔNPP = −1.5 ± 0.3%⋅y−1). Additionally, phytoplankton taxon dominance shifted from diatoms, dinoflagellates, and coccolithophorids to smaller taxa after 2004, whereas mesozooplankton biomass increased and commercial landings of planktivorous sardines collapsed. Collectively, our results reveal an ecological state change in this planktonic system. The weakening trend in Trade Winds (−1.9 ± 0.3%⋅y−1) and dependent local variables are largely explained by trends in two climatic indices, namely the northward migration of the Azores High pressure center (descending branch of Hadley cell) by 1.12 ± 0.42°N latitude and the northeasterly progression of the ITCZ Atlantic centroid (ascending branch of Hadley cell), the March position of which shifted by about 800 km between 1996 and 2009.

Saharan mineral dust transport into the Caribbean: Observed atmospheric controls and trends

[1]Each summer large amounts of mineral dust from the Sahara are transported across the Atlantic and arrive at the Caribbean with far-reaching implications for climate in this region. In this paper we analyze summer season interannual variability of North African mineral dust over the Caribbean using the Total Ozone Mapping Spectrometer (TOMS)/Nimbus 7 (1979‐1992) and TOMS/Earth Probe (1998‐2000) satellite aerosol data. We apply the ‘‘centers of action’’ approach to gain insight into the atmospheric controls on Saharan dust transport into the Caribbean and identify longitudinal displacement and pressure fluctuation of the Hawaiian High as well as longitudinal displacement of the Azores High as key players. In contrast, traditional indices such as the North Atlantic Oscillation and the Southern Oscillation are not correlated with the mineral dust variations over the Caribbean region. We utilize National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis to investigate the underlying physical mechanisms and to identify meteorological conditions that correspond to high and low dust loads. Our analysis shows that two different transport routes from distinct source regions are responsible for transporting mineral dust into the Caribbean: a northern mode in which dust mobilized from the Sahara travels westward controlled primarily by the Azores High and a southern mode in which intense dust clouds originating in the Sahel region travel over the Gulf of Guinea to reach the Caribbean. The latter is controlled primarily by teleconnections with the Hawaiian High.

Ocean warming since 1982 has expanded the niche of toxic algal blooms in the North Atlantic and North Pacific oceans.

Significance This study used high-resolution (daily, quarter-degree resolution) sea-surface temperature records to model trends in growth rates and bloom-season duration for two of the most toxic and widespread harmful algal bloom species indigenous to the North Atlantic and North Pacific oceans. Alexandrium fundyense synthesizes saxitoxin and Dinophysis acuminata synthesizes okadaic acid, which cause the human health syndromes paralytic and diarrhetic shellfish poisoning, respectively. The model provided hindcasts of harmful algal bloom (HAB) events that were consistent with in situ observations from long-term monitoring programs during the same time period. This study provides evidence that increasing ocean temperatures have already facilitated the intensification of these, and likely other, HABs and thus contribute to an expanding human health threat. Global ocean temperatures are rising, yet the impacts of such changes on harmful algal blooms (HABs) are not fully understood. Here we used high-resolution sea-surface temperature records (1982 to 2016) and temperature-dependent growth rates of two algae that produce potent biotoxins, Alexandrium fundyense and Dinophysis acuminata, to evaluate recent changes in these HABs. For both species, potential mean annual growth rates and duration of bloom seasons significantly increased within many coastal Atlantic regions between 40°N and 60°N, where incidents of these HABs have emerged and expanded in recent decades. Widespread trends were less evident across the North Pacific, although regions were identified across the Salish Sea and along the Alaskan coastline where blooms have recently emerged, and there have been significant increases in the potential growth rates and duration of these HAB events. We conclude that increasing ocean temperature is an important factor facilitating the intensification of these, and likely other, HABs and thus contributes to an expanding human health threat.

Control of Saharan mineral dust transport to Barbados in winter by the Intertropical Convergence Zone over West Africa

[1] The reasons for the inter-annual variability of dust transport from the Sahara across the Atlantic are not well-understood. Here we address this issue by defining three new climate indices that capture the position and intensity of the zone of near-surface convergence over West Africa, a part of the global Intertropical Convergence Zone (ITCZ). We then relate these indices to a 38-year record of mineral dust concentrations at Barbados focusing on the winter season. The results show that the latitudinal displacement of the ITCZ over West Africa and the dust load in Barbados are statistically significantly correlated with a correlation coefficient of r = � 0.69. A southward movement of the ITCZ corresponds to an increased dust load at Barbados. This correlation represents an improvement upon previous results, which focused on traditional teleconnection indices such as the North Atlantic Oscillation or the El-Nino-Southern Oscillation. From analyzing composites of wind and precipitation we conclude that for the winter season, the inter-annual variability of the Barbados dust load is related to changes in near-surface northeasterly winds in semi-arid regions in North Africa coincident with the movement of the ITCZ. Changes in precipitation appear to only play a minor role. Citation: Doherty, O. M., N. Riemer, and S. Hameed (2012), Control of Saharan mineral dust transport to Barbados in winter by the Intertropical Convergence Zone over West Africa, J. Geophys. Res., 117, D19117, doi:10.1029/2012JD017767.

Decadal Changes in the World's Coastal Latitudinal Temperature Gradients

Most of the worlds living marine resources inhabit coastal environments, where average thermal conditions change predictably with latitude. These coastal latitudinal temperature gradients (CLTG) coincide with important ecological clines,e.g., in marine species diversity or adaptive genetic variations, but how tightly thermal and ecological gradients are linked remains unclear. A first step is to consistently characterize the worlds CLTGs. We extracted coastal cells from a global 1°×1° dataset of weekly sea surface temperatures (SST, 1982–2012) to quantify spatial and temporal variability of the worlds 11 major CLTGs. Gradient strength, i.e., the slope of the linear mean-SST/latitude relationship, varied 3-fold between the steepest (North-American Atlantic and Asian Pacific gradients: −0.91°C and −0.68°C lat−1, respectively) and weakest CLTGs (African Indian Ocean and the South- and North-American Pacific gradients: −0.28, −0.29, −0.32°C lat−1, respectively). Analyzing CLTG strength by year revealed that seven gradients have weakened by 3–10% over the past three decades due to increased warming at high compared to low latitudes. Almost the entire South-American Pacific gradient (6–47°S), however, has considerably cooled over the study period (−0.3 to −1.7°C, 31 years), and the substantial weakening of the North-American Atlantic gradient (−10%) was due to warming at high latitudes (42–60°N, +0.8 to +1.6°C,31 years) and significant mid-latitude cooling (Florida to Cape Hatteras 26–35°N, −0.5 to −2.2°C, 31 years). Average SST trends rarely resulted from uniform shifts throughout the year; instead individual seasonal warming or cooling patterns elicited the observed changes in annual means. This is consistent with our finding of increased seasonality (i.e., summer-winter SST amplitude) in three quarters of all coastal cells (331 of 433). Our study highlights the regionally variable footprint of global climate change, while emphasizing ecological implications of changing CLTGs, which are likely driving observed spatial and temporal clines in coastal marine life.

Identification of a new dust‐stratocumulus indirect effect over the tropical North Atlantic

Over the tropical North Atlantic, during boreal summer, both stratocumulus clouds and mineral aerosols are ubiquitous. We find that low cloud fraction increases in response to high mineral aerosols loadings by 3% to 10% over much of the tropical North Atlantic, in International Satellite Cloud Climatology Project (ISSCP) and Pathfinder Atmosphere Extended (PATMOSx) observations. Using the single-column mode of the Community Earth System Model (CESM), we estimate that this indirect effect nets a surface cooling of approximately −3 Wm−2 to −12 Wm−2 per unit of dust optical depth (DAOD) increase in mineral aerosols, similar to observed radiative forcings of −4 Wm−2 to −6 Wm−2 per unit of DAOD in ISSCP and PATMOSx. Increases in stratocumulus clouds are linked to increases in atmospheric stability, reductions in boundary layer height, and moistening of the lower atmosphere in response to increased dust load. Mineral dust is shown to behave similarly to other absorbing aerosols in indirectly forcing a response in stratocumulus clouds.

Reply to Dees et al.: Ocean warming promotes species-specific increases in the cellular growth rates of harmful algal blooms

Recently, we (1) reported that, since 1982, several regions across the North Atlantic and North Pacific Oceans have experienced warming in specific seasons and locations that have significantly increased the potential cellular growth rates and bloom seasons of two harmful algae, Alexandrium fundyense and Dinophysis acuminata , and that new blooms caused by these species have emerged in these same regions. In their comment “Harmful algal blooms in the Eastern North Atlantic Ocean,” Dees et al. (2) examine the Continuous Plankton Recorder data from the North Eastern Atlantic and North Sea from 1982 to 2015 and find no relationship between Dinophysis spp. abundance and sea-surface temperature. This observation is consistent … [↵][1]1To whom correspondence should be addressed. Email: christopher.gobler{at}stonybrook.edu. [1]: #xref-corresp-1-1