Persistent Atmospheric and Oceanic Anomalies in the North Atlantic Ocean from Summer 2009 to Summer 2010 Zeng-Zhen Hu and Arun Kumar Climate Prediction Center NCEP/NWS/NOAA 5200 Auth Road, Camp Springs, MD 20746 Email: Zeng-Zhen.Hu@noaa.gov Contributors: Bohua Huang, Wanqiu Wang, Yan Xue, Bhaskar Jha, …

Winter 2009-2010

E1: Record-breaking MDR SSTA since 1950 (MDR: Hurricane Main Development Region, 10N-20N, 20W-85W) Monthly SSTA 2010 JJA SSTA 1998 2005 1958

E2: Persistent negative NAO dominated since Apr 2008 with record-breaking negative NAOI in some months during 2009-2010 since 1950 Should be GRL 2010…to be submitted

E3: Strong El Nino event in 2009-2010

Questions: Are these anomalies (NAO, El Nino, and record-breaking MDR SSTA) physically connected? Q1: What physical processes cause the anomalies in the North Atlantic (for example the record-breaking MDR SSTA) in 2009-2010? Q2: What is the contribution of the NAO? Q3: What is the impact of ENSO on the North Atlantic anomalies? Q4: Is it a long-term trend or short-term fluctuation? Q5: Is there any application in prediction?

N. Atlantic Seasonal SSTA and Surface Wind Stress Anomalies: MAM2009-JJA2010 1. Persistent tripole/horseshoe pattern 2. Strongest SSTA and wind stress anomalies in DJF 2009/2010 3. Subtropical +SSTA developed and intensified from JJA 2009-MAM2010

NAO connected with SSTA tripole/horseshoe pattern Southward shift of both –SSTA , surface wind, & heat flux Connection of wind speed with NAO, SSTA tendency dSSTA is associated with HF Total HF is dominated by latent HF SH, SW (LW, not shown) intensify (damp) the SSTA

W E S Wind Speed SSTA Heat Flux Wind-evaporation-SST (WES) mechanism (Xie and Philander 1994, Chang et al. 1997): Wind speed increases (decreases) -> Evaporation increases (decreases) -> Latent HF enhanced (suppressed) -> Ocean cooling (warming) -> Feedback Heat Flux

Physical processes associated with the persistent negative NAO It seems that the atmospheric forcing dominated the SSTA by thermodynamical process (WES feedback). How about the feedback from SST on NAO? Actually, (1) the feedback enhances the persistency of the atmospheric and oceanic anomalies, however, (2) its contribution is relatively small.

Model (GFS) and simulations Atmospheric component of NCEP Global Forecast System (GFS) (Saha et al. 2006) 2: GFS forced by regional specified SSTA: Observed SSTA in the Atlantic Ocean from 30oS to the northern pole and climatology SST elsewhere January 2009-Auguest 2010 Nine ensemble number integrations * Anomalies are the departure from monthly mean climatology of 50 year integration forced by climatological SST SSTA during Jan-Dec 2009 SSTA during Jan-Aug 2010

Contribution of the local SST feedback (About 20-30% of observed amplitudes in the GFS simulations; R2: shading)

The phase of the NAO is well reproduced by the regional SSTA forced experiment, but its amplitude is largely underestimated

SST feedback on atmosphere through altering surface wind

What is the historical connection of the North Atlantic variability with ENSO and NAO? For the persistent oceanic anomalies in the North Atlantic during summer 2009-summer 2010, is it a long-term trend or short-term fluctuation?

Historical Connection of SST with Nino3.4 & NAO ENSO signals propagate into the tropical N. Atlantic in 3-8 months late, so the SSTA in the tropical N. Atlantic is a typical impact of ENSO in this period. ENSO affects the tread wind through atmosphere (PNA), then changes the SST through WES mechanism. NAO’s impact on SST is mainly with 0-3 month lag. NAO correlation is weaker than ENSO, It is a tripole or horseshoe pattern, consisting with the SSTA in 2009-2010. Preceding El Nino may slightly favor to the negative phase of NAO

Global Anomalies of SST and Wind Stress and their Linear Trends Mean SSTA in Jun2009-Aug2010: Warming in tropics and cooling in high latitudes Linear Trend of SSTA in Jan1979-Dec2009: Warming in North Atlantic, Western Pacific, Indian Ocean Cooling in eastern Pacific and high latitudes of SH Is this a global warming pattern? Central value of averaged SSTA is between -0.6 and -0.9 oC for the negative SSTA in the mid-latitudes, and 0.6 and 0.9 oC for the positive SSTA in the low latitudes. The corresponding values are between -0.9 and -1.2 oC, and between 0.3 and 0.6 oC after the linear trend removed. Thus, the long-term trend may account for about 1/3 of the warming in the tropical Atlantic and weakens the cooling in the middle latitudes of the North Atlantic by about 1/3.

Therefore, the combination of the El Nino, persistent negative phase of the NAO and the long-term trend result in the record-breaking MDR SSTA (MDR: Hurricane Main Development Region, 10N-20N, 20W-85W) Monthly SSTA 2010 JJA SSTA 1998 2005 1958

What is the application for prediction?

Using NIno3.4 (t-5) and NAO (t-1), the SSTA pattern can be well reconstructed The contribution of NAO (El Nino) is mainly in high (low) latitudes The amplitude of SSTA is largely underestimated

Historical Connection of SST with Nino3.4 & NAO ENSO signals propagate into the tropical N. Atlantic in 3-8 months late, so the SSTA in the tropical N. Atlantic is a typical impact of ENSO in this period. ENSO affects the tread wind through atmosphere (PNA), then changes the SST through WES mechanism. Historical Connection of SST with Nino3.4 & NAO NAO’s impact on SST is mainly with 0-3 month lag. NAO correlation is weaker than ENSO, It is a tripole or horseshoe pattern, consisting with the SSTA in 2009-2010. Preceding El Nino may slightly favor to the negative phase of NAO

Years with largest SSTA in MDR: 2010, 2005, 1998, 1958 (MDR: Hurricane Main Development Region, 10N-20N, 20W-85W) Monthly SSTA 2010 JJA SSTA 1998 2005 1958

Combination of El Nino and negative phase of NAO favors warming in MDR: Cases of 1958, 1998, 2005

Atlantic Hurricane Activity in 1958, 1998, 2005, 2010 The 4 years that had a similar juxtaposition of a warm ENSO and negative phase of the NAO, i.e., 1958, 1998, 2005, and 2010 had above normal Atlantic hurricane seasons, with 2005 having the most Atlantic hurricane activity in the recent record. The Atlantic Accumulated Cyclone Energy (ACE) index value in the North Atlantic (http://en.wikipedia.org/wiki/Accumulated_cyclone_energy) is 121 in 1958, 182 in 1998, 248 in 2005, and 160 in 2010. For 1950-2009, the mean of ACE is 101.2 and the median is 88.0. 1958 1998 2005 2010

Tripole/horseshoe SSTA pattern and NAO (1) Bulk of the NAO signal comes from the mid-latitudes. (2) A dipolar anomaly, with warm SST southeast of Newfoundland and cold SST to the northeast and southeast, precedes a positive phase of the NAO, and may provide a prediction of up to 15% of its monthly variance several months in advance. (3) There is a positive feedback between the SST tripole and the NAO. (4) The tropical forcing of the NAO is nearly uncorrelated with and weaker than that resulting from the mid-latitudes Fig. 2. Heterogeneous Z500 (thick contours, CI = 5 m with negative values dashed) and homogeneous SST (gray shading in K with white contours for positive values and black contours for negative values) covariance maps for the first MCA mode between Pan-Atlantic 500-mb height and SST anomalies in early winter (Z500 fixed in NDJ, SST lagged as indicated). The results are shown from lag −7 to +1. The correlation coefficient r between the SST and Z500 MCA time series, the SC fraction F, and the SC of the mode are given for each lag. The percentages in parentheses for r and SC give their estimated significance level. (From: Czaja, A., and C. Frankignoul (2002), Observed impact of Atlantic SST anomalies on the North Atlantic Oscillation, J. Clim., 15, 606– 623.)

Is NAO Predictable? Yes!? 15 % of monthly variance is predictable in a few months advance (Czaja et al. 2002) Czaja, A., and C. Frankignoul (2002), Observed impact of Atlantic SST anomalies on the North Atlantic Oscillation, J. Clim., 15, 606– 623. Most of them is NOT!? A few years are predictable may through reemergence or other mechanisms Hu, Z.-Z., and B. Huang (2006), On the significance of the relationship between the North Atlantic Oscillation in early winter and Atlantic sea surface temperature anomalies, J. Geophys. Res., 111, D12103, doi:10.1029/2005JD006339.

Summary I The SSTA in the tropical North Atlantic during summer 2009-summer 2010 represented a typical response to ENSO, preconditioned and amplified by the influence of a strong and persistent negative phase of the NAO. The long-term trends enhanced the warming in the high and low latitudes and weakened the cooling in the middle latitudes. The combination of these 3 factors resulted in the record-breaking positive SSTA in the MDR in the boreal spring and summer of 2010. The persistent negative phase of the NAO was associated with active thermodynamic air-sea interaction in the North Atlantic basin. Surface wind anomalies associated with the NAO altered the ocean surface heat flux and changed the SSTA, which was likely further enhanced by the positive wind speed-evaporation-SST feedback. The total heat flux was dominated by the latent and sensible heat fluxes, while the short-wave radiation contributed to the tropical SSTA to a lesser degree.

Summary II Sensitivity experiments with an AGCM forced by observed SST in the Atlantic Ocean alone suggested that the Atlantic SSTA, which was partly forced by the NAO, had some positive contribution to the persistent negative phase of the NAO. Therefore, the persistent NAO condition was partly an outcome of the global climate anomalies and the ocean-atmosphere feedback within the Atlantic basin. Based on the statistical relationship, the SSTA pattern in the North Atlantic can be reasonably well predicted by using preceding ENSO and NAO as predictors. Recent SSTA in North Atlantic was different from the observed trend signal. It was mainly a short-term fluctuation, but the long-term tend had significant contribution. Persistent NAO anomalies might be linked to the subsurface ocean temperature anomalies, and probably to the location and intensity of the Gulf Stream extension.

North Atlantic Climate Variability NAO, AMO, AMOC, TAV, Gulf Stream CITATION: Hu, Z.-Z., A. Kumar, B. Huang, Y. Xue, W. Wang, and B. Jha, 2010: Persistent atmospheric and oceanic anomalies in the North Atlantic from Summer 2009 to Summer 2010. J. Climate (submitted).