The Indian summer monsoon during peaks in the 11 year sunspot cycle

GEOPHYSICAL RESEARCH LETTERS, VOL. 39,, doi:10.1029/2012gl051977, 2012 The Indian summer monsoon during peaks in the 11 year sunspot cycle Harry van Loon 1,2 and Gerald A. Meehl 1 Received 9 April 2012; revised 1 June 2012; accepted 2 June 2012; published 3 July 2012. [1] An analysis of sea-level pressure anomalies at 14 sunspot peaks in the 11 year solar cycle in the Indian region in summer shows that the mean sea level pressure anomalies consist of relatively high pressure over land surrounded by low pressure anomalies over the sea. This signal is robust enough to appear when the data are divided into two segments. The accompanying mean rainfall anomalies, with anomalies as high as 20% above normal, have maxima on the coasts and over water and are an enhancement of the mean Indian monsoon rainfall. In the sunspot peaks the Findlater Jet appears to shift east and strengthen somewhat, consistent with the lower sea level pressure and stronger monsoon rainfall. Citation: van Loon, H., and G. A. Meehl (2012), The Indian summer monsoon during peaks in the 11 year sunspot cycle, Geophys. Res. Lett., 39,, doi:10.1029/2012gl051977. 1. Introduction [2] Three previous papers [van Loon et al., 2007; van Loon and Meehl, 2008, 2011] described the influence of the sunspot peaks on the atmospheric and ocean circulations in the Pacific Ocean. The results showed that for peaks in the 11 year sunspot cycle, the response of the Pacific climate system involved an enhancement of climatological mean conditions. That is, the SE-trades were stronger, the Hadley and Walker circulations strengthened and covered a wider domain, the stronger trade winds brought cooler water to the surface in the equatorial eastern Pacific with a wider dry zone, there was higher sea-level pressure (SLP) in the Gulf of Alaska, and there was a modulation of the QBO in the stratosphere. In the years outside the sunspot peaks (including the sunspot minima) there was no consistent signal, indicating any type of anomalies could happen. Therefore, a correlation analysis would be less useful to bring out the signal to the climate system response to peaks in the sunspot cycle. In the sunspot peaks the effects were physically coherent and statistically significant from the stratosphere to the ocean surface. [3] A consistent picture thus emerged regarding the influence of the sunspot peaks on the region that favored a strengthening of precipitation in the tropical convergence zones, especially in the Indonesian-North Australian summer monsoon and the Indian summer monsoon [van Loon, 2012]. 1 National Center for Atmospheric Research, Boulder, Colorado, USA. 2 Colorado Research Associates, Northwest Research Associates, Boulder, Colorado, USA. Corresponding author: G. A. Meehl, National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307, USA. (meehl@ucar.edu) 2012. American Geophysical Union. All Rights Reserved. 0094-8276/12/2012GL051977 Similar signals were seen in global coupled climate model simulations of the 20th century that included the 11 year cycle of total solar irradiance, and a mechanism was proposed to explain the response at the surface involving dynamically coupled air-sea interaction [Meehl et al., 2008]. That so-called bottom-up mechanism was shown to complement a top-down mechanism involving the ozone response in the stratosphere, such that both mechanisms could work together and reinforce each other to produce stronger convection in the climatological rainfall maxima in the tropical Pacific along with the other responses documented in the observations [Meehl et al., 2009]. [4] Those papers mainly addressed the climate system response in northern winter, but similar responses to peaks in the 11 year cycle should occur in northern summer. That is, it could be expected that the Indian monsoon climatological precipitation would be strengthened, and produce stronger low level monsoon winds over the Indian Ocean. Indeed, a previous paper [van Loon et al., 2004] indicated that the Indian monsoon could be strengthened during peaks in the 11 year solar cycle, and those results are not inconsistent with the ones shown here. However, in that paper only three peaks were available, and anomalies were shown as the difference between solar maxima and minima. It has subsequently been demonstrated that there is a larger and more consistent signal when peak solar years are composited and anomalies calculated as those composites minus the long term climatology [van Loon et al., 2007]. Since the effect of the sunspot peaks lasts, at least, from December January February of Year 1 till their peak year in December January February of Year 0, the years chosen here to study the Indian Monsoon are thus the intervening 14 July August average summer months from 1859, 1869, 1882, etc., till 1999. [5] The theme of Indian monsoon and the 11 year sunspot cycle has attracted many authors over the years from Lockyer and Gilbert Walker till the present, and it is impossible to list them all in a short note. A few of the recent papers are Reddy et al. [1989], Hiremath and Mandi [2004], and Kodera [2004], all of which contain relevant information, not inconsistent with our results. [6] The data used are from the valuable websites made available by the Physical Science Division, NOAA/ERL; they are maintained by Ms. Cathy Smith. The sunspot years are from the website: ftp://ftp.noaa.gov/stp/solar_ DATA/SUNSPOT_NUMBERS/INTERNATIONAL/maxmin/ new/max/min. [7] We have used the climate-mean 1981 2010 to compute the anomalies. This is the default mean in the NCEP/NCAR reanalyses [Kalnay et al., 1996] in which the last five solar peaks are available. The reanalyses are used to calculate precipitation and low level winds. The longer Hadley sea 1of5

Figure 1. (a) Sea-level pressure anomalies in five recent sunspot peaks, July August (hpa). (b) Sea-level pressure anomalies in nine early sunspot peaks, July August (hpa). Figure 2. (a) Precipitation rate anomalies, mm day 1,in five recent sunspot peaks. July August: (b) mean monsoon precipitation, 1948 2011, July August, mm day 1. 2of5

Figure 3. (a) Mean vector wind anomalies in five recent sunspot peaks, and magnitude of wind anomalies (color shading), m sec 1, July August; (b) same as Figure 3a except for long term mean climatology, 1948 2011. level pressure dataset allows 14 solar peaks to be included in the sea level pressure analysis. 2. Analyses of Sea-Level Pressure, Rainfall Rates, and Wind [8] For the Indian Sector (48 E to 100 E, the equator to 30 N), Figures 1a and 1b show, for nine and five early and recent sunspot peaks, respectively, weak negative anomalies on the west coast of India surrounded by stronger mean negative anomalies over the Arabian Sea and Bay of Bengal. These anomalies are about the magnitude of typical standard deviations for sea level pressure for the Asian-Australian monsoon region [e.g., Trenberth, 1984], with maximum values of about 0.8 hpa. Though not large, the anomalies are consistent, as shown by testing the robustness of the signal by dividing the Hadley2 sea level pressure data into two segments. The first is from the period before the NCEP/NCAR re-analysis, and the second from the period of the re-analysis (Figures 1a and 1b). The Hadley2 SLP differences are computed relative to the NOAA default climatological mean (1981 2010) for earlier and later segments. The pattern is similar in both periods: a relative SLP pressure ridge over the land surrounded by two troughs over the ocean. The pattern is therefore robust in the two periods and portrays the response of the Indian monsoon region to the 14 sunspot peaks. The negative anomalies over the Arabian Sea are larger than those of the Bay of Bengal, and there are also negative anomalies over northern India. It is therefore likely that these sea level pressure anomalies are associated with anomalies in Indian monsoon rainfall. The signal consists of a strengthening of the mean meridional pressure gradient that would also affect winds across the Indian Ocean. [9] To look into these other aspects of the monsoon response to the 11 year sunspot cycle, the precipitation anomalies in the five sunspot peaks during the NCEP period (Figure 2a) show that the mean anomalies are by and large an enhancement of the average monsoon precipitation (Meehl et al., 2012, and Figure 2b), with greatest positive anomalies over the Arabian Sea, Nepal, and Bay of Bengal. These rainfall anomalies are consistent with the negative sea level pressure anomalies in Figure 1. [10] Surface wind anomalies (Figures 3) are shown for the past five sunspot peaks. The Findlater Jet [Findlater, 1969] dominates the long-term mean northern summer season from the Somali coast into the Indian Ocean as a strong band of southerly winds that crosses the equator off the East African coast, and curves northeastward across the Arabian Sea to India [Pankajakshan et al., 2002] (and shown for the climatology in Figure 3b). In the five sunspot maxima since the one in 1956/1957 the wind anomalies show a strengthening and eastward shift of the Jet, with small anomalies near the Somali coast, and larger southerly and southwesterly anomalies over the Arabian Sea (Figure 3). These wind anomalies are consistent with the sea level pressure anomalies and precipitation anomalies in Figures 1 and 2. In general there is an anticyclonic, anomalous circulation over the Arabian Sea, Sri Lanka and southernmost India in agreement with the SLP anomalies in both the early and the later periods in Figure 1, as well as the rainfall anomalies in the latter period. All indicate a strengthening of the mean monsoon during peaks in the 11 year sunspot cycle. [11] Above normal rainfall in the Indian monsoon also is often found during Cold Events (also called La Niña events) of the Southern Oscillation [e.g., Meehl, 1987]. But the pressure anomalies in Cold Events are different from those in the solar peaks. The composite SLP anomalies in 11 Cold Events after the one in 1949 are shown in Figure 4. The pattern is similar in the two sets of anomalies, but in the Cold Events the amplitude of the positive SLP anomalies is larger over the Pacific Ocean. Though there are generally negative SLP anomalies over the Indian sector in both Cold Events and solar peaks, the latter have larger negative SLP anomalies over the Arabian Sea, northern India and the Bay of Bengal than the Cold Events that have larger amplitude negative SLP anomalies over the Indian Ocean. 3. Conclusion [12] There is greater rainfall over Arabian Sea, the west coast of India, Nepal, eastern India and the Bay of Bengal in 14 sunspot peaks. This pattern indicates an enhancement of mean monsoon precipitation. The average precipitation anomalies in the five recent sunspot peaks reach values as high as 20% above normal. This is also the case for the SLP 3of5

Figure 4. (a) Sea-level pressure anomalies in ten Cold Events (La Niña events) in the Southern Oscillation (hpa), July August. (b) Sea-level pressure anomalies in 14 sunspot peaks (hpa), July August. anomalies, with relatively larger negative values over the Arabian Sea and Bay of Bengal, and smaller negative anomalies over central India. Thus, the response to the 11 year cycle in sunspot maxima is an enhancement of the mean monsoon precipitation and sea level pressure patterns, with a strengthening and eastward shift of the Findlater Jet off the coast of Africa. [13] Acknowledgments. Portions of this study were supported by the Office of Science (BER), U.S. Department of Energy, Cooperative Agreement DE-FC02-97ER62402, and the National Science Foundation. The National Center for Atmospheric Research is sponsored by the National Science Foundation. HvL is grateful to Colorado Research Associates/ Northwest Research Associates for providing him with an office and access to a computer. [14] The Editor and the authors thank Lon Hood and one anonymous reviewer for constructive comments and assistance in the evaluation of this paper. References Findlater, I. (1969), A major low-level air current near the Indian Ocean during the northern summer, Q. J. R. Meteorol. Soc., 95, 362 380, doi:10.1002/qj.49709540409. Hiremath, K. M., and P. I. Mandi (2004), Influence of the solar activity on the Indian Monsoon, New Astron., 9, 651 662, doi:10.1016/j.newast. 2004.04.001. Kalnay, E., et al. (1996), The NCEP/NCAR 40-year reanalysis project, Bull. Am. Meteorol. Soc., 77, 437 471, doi:10.1175/1520-0477(1996) 077<0437:TNYRP>2.0.CO;2. Kodera, K. (2004), Solar influence on the Indian Ocean monsoon through dynamical processes, Geophys. Res. Lett., 31, L24209, doi:10.1029/ 2004GL020928. Meehl, G. A. (1987), The annual cycle and interannual variability in the tropical Pacific and Indian Ocean regions, Mon. Weather Rev., 115, 27 50, doi:10.1175/1520-0493(1987)115<0027:tacaiv>2.0.co;2. Meehl, G. A., J. M. Arblaster, G. Branstator, and H. van Loon (2008), A coupled air-sea response mechanism to solar forcing in the Pacific region, J. Clim., 21, 2883 2897, doi:10.1175/2007jcli1776.1. Meehl, G. A., J. M. Arblaster, K. Matthes, F. Sassi, and H. van Loon (2009), Amplifying the Pacific climate system response to a small 11-year solar cycle forcing, Science, 325, 1114 1118, doi:10.1126/ science.1172872. Meehl, G. A., J. M. Arblaster, J. Caron, H. Annamalai, M. Jochum, A. Chakraborty, and R. Murtugudde (2012), Monsoon regimes and processes in CCSM4. Part 1: The Asian-Australian monsoon, J. Clim., 25, 2583 2608, doi:10.1175/jcli-d-11-00184.1. Pankajakshan, T., C. Zhao, P. M. Muraleedharan, G. S. P. Rao, and Y. Sugimori (2002), Monsoon oscillations of the Findlater Jet and coastal winds of India, in Proceedings of the Sixth Pan Ocean Remote Sensing Conference (PORSEC): Remote Sensing and Ocean Science for Marine 4of5

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