Jeff Masters' Wunderblog: Record Atlantic SSTs continue in the hurricane Main Development Region

Record Atlantic SSTs continue in the hurricane Main Development Region   by Jeff Masters, Wunder Blog, May 15, 2010       Sea Surface Temperatures (SSTs) in the Atlantic's Main Development Region for hurricanes had their warmest April on record, according to an analysis of historical SST data from the UK Hadley Center. SST data goes back to 1850, though there is much missing data before 1910 and during WWI and WWII. The area between 10° N and 20° N, between the coast of Africa and Central America (20° W to 80° W), is called the Main Development Region (MDR) because virtually all African waves originate in this region. These African waves account for 85% of all Atlantic major hurricanes and 60% of all named storms.    When SSTs in the MDR are much above average during hurricane season, a very active season typically results (if there is no El Niño event present.) SSTs in the Main Development Region (10° N to 20° N and 20° W to 85° W) were an eye-opening 1.46 °C above average during April. This is the third straight record warm month, and the warmest anomaly measured for any month--by a remarkable 0.2 °C. The previous record warmest anomalies for the Atlantic MDR were set in June 2005 and March 2010, at 1.26 °C. Figure 1. The departure of sea surface temperature (SST) from average for May 13, 2010. Image credit: NOAA/NESDIS. What is responsible for the high SSTs? As I explained in detail in a post on record February SSTs in the Atlantic, the Arctic Oscillation (AO) and its close cousin, the North Atlantic Oscillation (NAO), are largely to blame for the record SSTs. The AO and NAO are climate patterns in the North Atlantic Ocean related to fluctuations in the difference of sea-level pressure between the Icelandic Low and the Azores-Bermuda High. If the difference in sea-level pressure between Iceland and the Azores is small (negative NAO), this creates a weak Azores-Bermuda High, which reduces the trade winds circulating around the High. During December-February, we had the most negative AO/NAO since records began in 1950, and this caused trade winds between Africa and the Lesser Antilles Islands in the hurricane Main Development Region to slow to 1-2 m/s (2.2-4.5 mph) below average. Slower trade winds mean less mixing of the surface waters with cooler waters down deep, plus less evaporational cooling of the surface water. As a result, the ocean heated up significantly, relative to normal, over the winter. Negative AO/NAO conditions have been dominant much of this spring as well, resulting in further anomalous heating of the MDR waters. This heating is superimposed on the very warm global SSTs we've been seeing over the past few decades due to global warming. Global and Northern Hemisphere SSTs were the 2nd warmest on record this past December, January, and February, the warmest on record in March, and will likely be classified as the warmest or second warmest on record for April, since NASA just classified April as the warmest April on record for the globe. We are also in the warm phase of a decades-long natural oscillation in Atlantic ocean temperatures called the Atlantic Multi-decadal Oscillation (AMO). This warm phase began in 1995 and has been partially responsible for the high levels of hurricane activity we've seen since 1995. What does this imply for the coming hurricane season? The high April SST anomaly does not bode well for the coming hurricane season. The three past seasons with record warm April SST anomalies all had abnormally high numbers of intense hurricanes. Past hurricane seasons that had high March SST anomalies include 1969 (0.90 °C anomaly), 2005 (1.19 °C anomaly), and 1958 (0.97 °C anomaly). These three years had 5, 7, and 5 intense hurricanes, respectively. Just two intense hurricanes occur in an average year. The total averaged activity for the three seasons was 15 named storms, 11 hurricanes, and 6 intense hurricanes (an average hurricane season has 10, 6, and 2). Both 1958 and 2005 saw neutral El Niño conditions, while 1969 had a weak El Niño. The SSTs are already as warm as we normally see in July between Africa and the Caribbean, and we have a very July-like tropical wave approaching the Lesser Antilles Islands this weekend. However, wind shear is still seasonably high, and the tropical waves coming off of Africa are still too far south to have much of a chance of developing. The GFS model is indicating that shear will start to drop over the Caribbean the last week of May, so we may have to be on the watch for tropical storms forming in the Caribbean then. For those of you interested in a more detailed look at the early season tropical weather outlook, consult the excellent wunderblogs of StormW and Weather456. I'll be back with a new post on Monday.   Link:  http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=1480

Casey Saenger et al., Nature Geosci., 2009, Surface-temperature trends and variability in the low-latitude North Atlantic since 1552

Nature Geoscience, published online 21 June 2009; doi:10.1038/ngeo552

Surface-temperature trends and variability in the low-latitude North Atlantic since 1552

Casey Saenger¹, Anne L. Cohen², Delia W. Oppo², Robert B. Halley³ and Jessica E. Carilli⁴

Abstract

Sea surface temperature variability in the North Atlantic Ocean recorded since about 1850 has been ascribed to a natural multidecadal oscillation superimposed on a background warming trend^{1, 2, 3, 4, 5, 6}. It has been suggested that the multidecadal variability may be a persistent feature^{6, 7, 8}, raising the possibility that the associated climate impacts may be predictable⁹. However, our understanding of the multidecadal ocean variability before the instrumental record is based on interpretations of high-latitude terrestrial proxy records^{7, 8}. Here we present an absolutely dated and annually resolved record of sea-surface temperature from the Bahamas, based on a 440-year time series of coral growth rates. The reconstruction indicates that temperatures were as warm as today from about 1552 to 1570, then cooled by about 1 ^°C from 1650 to 1730 before warming until the present. Our estimates of background variability suggest that much of the warming since 1900 was driven by anthropogenic forcing. Interdecadal variability with a period of 15–25 years is superimposed on most of the record, but multidecadal variability becomes significant only after 1730. We conclude that the multidecadal variability in sea-surface temperatures in the low-latitude western Atlantic Ocean may not be persistent, potentially making accurate decadal climate forecasts more difficult to achieve.

1. Massachusetts Institute of Technology and Woods Hole Oceanographic Institution Joint Program in Oceanography, Woods Hole, MA 02543, U.S.A.
2. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A.
3. US Geological Survey (retired) 13765 2600 Rd., Cedaredge, CO 81413, U.S.A.
4. University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA 92093, U.S.A.

Correspondence to: Casey Saenger¹ e-mail: csaenger@mit.edu

Link to abstract: http://www.nature.com/ngeo/journal/vaop/ncurrent/abs/ngeo552.html

Amato T. Evan et al., The role of aerosols in the evolution of tropical North Atlantic Ocean temperature anomalies

Published online March 26, 2009, Science DOI: 10.1126/science.1167404

Reports

(Submitted on October 20, 2008; Accepted on March 11, 2009.)

The role of aerosols in the evolution of tropical North Atlantic Ocean temperature anomalies

Amato T. Evan ^1*, Daniel J. Vimont ², Andrew K. Heidinger ³, James P. Kossin ⁴, Ralf Bennartz ²

¹ Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin, Madison, WI 53706, USA.; Department of Atmospheric and Oceanic Sciences, University of Wisconsin, Madison, WI 53706, U.S.A.
² Department of Atmospheric and Oceanic Sciences, University of Wisconsin, Madison, WI 53706, U.S.A.
³ National Oceanic and Atmospheric Administration (NOAA)/National Environmental Satellite, Data, and Information Service (NESDIS)/Center for Satellite Applications and Research (STAR), 1225 West Dayton Street, Madison, WI 53706, U.S.A.
⁴ NOAA/NESDIS/National Climatic Data Center, Madison, WI 53706, U.S.A.

Abstract

Observations and models demonstrate that northern tropical Atlantic surface temperatures are sensitive to regional changes in stratospheric volcanic and tropospheric mineral aerosols. However, it is unknown if the temporal variability of these aerosols is a key factor in the evolution of ocean temperature anomalies. Here, we elucidate this question by using 26 years of satellite data to drive a simple physical model for estimating the temperature response of the ocean mixed layer to changes in aerosol loadings. Our results suggest that 69% of the recent upward trend, and 67% of the detrended and 5-year low pass filtered variance, in northern tropical Atlantic Ocean temperatures is the mixed layer’s response to regional variability in aerosols.

^*To whom correspondence should be addressed: Amato T. Evan, e-mail: atevan@wisc.edu

Link to abstract: http://www.sciencemag.org/cgi/content/abstract/1167404v1

Amato Evan et al., Aerosol coverage causing more rapid temperature rises in the tropical North Atlantic Ocean

by Liz Kalaugher, editor of environmentalresearchweb, March 30, 2009

Aerosols cause a sea change

Temperatures in the tropical North Atlantic Ocean have been rising faster than in other tropical basins, despite projections that it should be warming more slowly than other areas. Now scientists in the US reckon the anomaly may be due to a decrease over recent years in the amount of atmospheric aerosols in the region from the Sahara desert and volcanoes, and a consequent fall in their cooling effect.

"To understand future climate change, or at least future changes in the temperature of the northern tropical Atlantic, we really need to be able to anticipate how dust storms will also be changing," Amato Evan of the University of Wisconsin Madison told environmentalresearchweb. "This is an effect that is not included in a dynamic way in, for example, the model runs of the IPCC report."

Some studies estimate that Atlantic dust cover could decrease by 40-60% if carbon dioxide levels were to double, which would lead to an additional 0.3-0.4 °C of warming in the northern tropical Atlantic.

Evan and colleagues from the University of Wisconsin Madison, NOAA, National Environmnetal Satellite, Data and Information Service (NESDIS), Center for Satellite Applications and Research, National Climatic Data Center, used 26 years' worth of satellite records, which included information about aerosol coverage, to drive a simple physical model of the temperature response of the ocean mixed layer to aerosol loading. The data ranged from 1982 to 2007; the maximum dust activity occurred in 1985 and the minimum in 2005.

"The findings suggest that on long time periods volcanoes and dust from Africa play a dominant role in shaping the variability of tropical Atlantic Ocean temperatures," said Evan. "For example, we find that 70% of the upward trend in temperatures over this period is due to month to month changes in volcanic and dust aerosols, with 25% of the trend attributed to dust and 45% attributed to the eruptions of El Chichon (Mexico 1982) and Mt Pinatubo (Philippines 1991)."

Temperatures in the tropical North Atlantic Ocean have been rising at nearly 0.25 °C per decade since 1980. Studies have proposed global warming, mean northern hemisphere temperature variations, and/or changes in the thermohaline circulation as the explanation for this trend. But Evan and colleagues' findings indicate that aerosol coverage that scatters sunlight and reduces the surface irradiation are the key factor.

The tropical North Atlantic is downwind of West Africa, the world’s largest source of dust, making it the only tropical ocean basin that often experiences heavy and extensive aerosol cover. Temperatures there are rising faster than in other tropical ocean basins despite the fact that projections indicate the tropical North Atlantic should be warming more slowly than the other tropical basins.

"We suggest this apparent disconnect between observations and models may be due to the influence of Atlantic dust cover," write the researchers in their paper in Sciencexpress.

According to Evan, upward trends in summertime northern tropical Atlantic Ocean temperature, in the region thought to be important to hurricane activity, appear to be totally forced by aerosol variability. "So, there may be some important ramifications for attributing causality to year-to-year changes in Atlantic hurricane activity," he said. Dusty years tend to have low hurricane activity, but the drivers for dust variability from year to year are not yet well understood.

Now, Evan says he is trying to improve his treatment of the satellite data. He would also like to combine the satellite observations with more sophisticated climate models to understand more completely how aerosols are shaping climate and climate change.

Link to article: http://environmentalresearchweb.org/cws/article/research/38427

T. Mochizuki, T. Awaji, N. Sugiura, GRL, 36, Possible oceanic feedback in the extratropics in relation to the North Atlantic SST tripole

Geophysical Research Letters, 36, L05710; doi:10.1029/2008GL036781

Possible oceanic feedback in the extratropics in relation to the North Atlantic SST tripole

Possible oceanic feedback in the extratropics in relation to the North Atlantic SST tripole

Takashi Mochizuki (Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan), Toshiyuki Awaji (Frontier Research Center for Global Change, JAMSTEC, Yokohama, and Department of Geophysics, Kyoto University, Kyoto, Japan), and Nozomi Sugiura (Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan)

Abstract

We analyze the results of 4-dimensional variational data assimilation experiments using a coupled general circulation model and identify signals from a possible extratropical oceanic feedback relating to the North Atlantic Sea Surface Temperature (SST) tripole. Examination of the optimized control variables (coupling parameters) and the resultant climate fields reveals that the model errors in the North Atlantic climate variations are very sensitive to the intensity of the extratropical air-sea thermal coupling. This results in the enhancement of the atmospheric responses to SST changes particularly around 40°N, 50°W, when the model errors are most effectively corrected. Since an adjoint approach enables us to detect the sensitivity to fluctuations in the model variables, our results suggest that this oceanic thermal feedback in the extratropics is a key physical process influencing the North Atlantic Oscillation and the associated North Atlantic SST tripole.

Received 24 November 2008, accepted 10 February 2009, published 14 March 2009.

Mochizuki, T., T. Awaji, & N. Sugiura (2009), Possible oceanic feedback in the extratropics in relation to the North Atlantic SST tripole, Geophys. Res. Lett., 36, L05710; doi:10.1029/2008GL036781.

Link to abstract: http://www.agu.org/pubs/crossref/2009/2008GL036781.shtml

Takashi Mochizuki et al., Understanding sea temperature-atmospheric pressure links in the North Atlantic (SST anomaly tripole)

Understanding sea temperature-atmospheric pressure links in the North Atlantic

ScienceDaily (Mar. 29, 2009) — Feedback effects between the ocean and atmosphere are important to understanding the mechanisms affecting climate variations.

Previous studies have found that atmospheric anomalies associated with a variation in atmospheric pressure above the North Atlantic Ocean called the North Atlantic Oscillation produce a three-part pattern (tripole) of sea surface temperature anomalies at midlatitudes. Scientists refer to such anomalies as the North Atlantic sea surface temperature tripole, and scientists have debated to what extent the atmosphere responds to these midlatitude sea surface temperature variations.

Reporting in the journal Geophysical Research Letters, Mochizuki et al. identify oceanic feedback signals poleward of the tropics, taking a new approach based on a model used in four-dimensional variational data assimilation to determine the sensitivity of the model to fluctuations in physical variables.

Their results reveal that oceanic thermal feedback beyond the tropics is an important process influencing the North Atlantic Oscillation, providing a better understanding of the factors affecting climate variations in the North Atlantic.

The authors include: Takashi Mochizuki, Toshiyuki Awaji, and Nozomi Sugiura: Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan; Awaji is also at Department of Geophysics, Kyoto University, Kyoto, Japan.

Mochizuki et al. Possible oceanic feedback in the extratropics in relation to the North Atlantic SST tripole. Geophysical Research Letters, 2009, 36 (5), L05710; DOI: 10.1029/2008GL036781

Link to article: http://www.sciencedaily.com/releases/2009/03/090325155634.htm