Try this — 
load('sample.mat')
whos('-file','sample.mat')
  Name        Size            Bytes  Class        Attributes
  sample       -              25859  timetable              
sample
sample = 50x32 timetable
          time          Var1    Var2    Var3    Var4     Var5    Var6    Var7    Var8     Var9    Var10    Var11    Var12    Var13     Var14      Var15    Var16    Var17    Var18    Var19    Var20    Var21    Var22    Var23    Var24    Var25    Var26    Var27    Var28    Var29    Var30    Var31    Var32
    ________________    ____    ____    ____    _____    ____    ____    ____    _____    ____    _____    _____    _____    _____    ________    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____
    1990-01-01 00:00     1       1       0      -11.5    71.5    92.1    16.5    104.9     6.7      4        3        0        1      1.99e+09    {'T'}      1        0        0        0        0        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-01 12:00     2       1       0      -11.8    71.8    88.8    16.1    125.1     8.4      6        3        0        4      1.99e+09    {'T'}      1        1        1        0        0        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-02 00:00     3       1       0      -12.5      72    95.4      17     82.8     4.7      2        0        0        7      1.99e+09    {'F'}      1        1        0        0        0        0        1        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-01 00:00     1       2       0        -11      87    87.2    15.3     89.4     5.3      1        0        0        3      1.99e+09    {'F'}      0        1        0        0        0        0        1        0        1        0        0       60       50       85       70      12.5      14  
    1990-01-01 12:00     2       2       0        -12      86      90    15.4     52.3     8.5      1        0        0        5      1.99e+09    {'F'}      1        0        0        0        0        0        1        1        0        0        0       60       50       85       70      12.5      14  
    1990-01-02 00:00     1       3       0        -18     170    90.1    16.6     70.9     3.5      1        0        0        8      1.99e+09    {'F'}      0        0        0        0        1        0        1        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-03 00:00     2       3       0        -17     177    89.1    15.1     64.7     1.6      1        0        0       17      1.99e+09    {'F'}      0        1        0        0        1        0        1        0        1        0        0       60       50       85       70      12.5      14  
    1990-01-02 12:00     1       4       0      -11.5      86    95.2    16.4    103.8     9.8      2        2        0        9      1.99e+09    {'T'}      0        0        0        0        0        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-03 00:00     2       4       0        -11      86    93.4    16.5    154.1       6      4        3        0       12      1.99e+09    {'T'}      0        1        0        0        1        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-03 12:00     3       4       0      -13.9    86.1    91.3    16.3    182.4    11.3     10        9        0       20      1.99e+09    {'T'}      0        0        1        0        0        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-04 00:00     4       4       0      -16.1    85.2    92.7    16.6    200.6      15     13       12        0       22      1.99e+09    {'T'}      0        0        0        0        1        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-04 12:00     5       4       1      -17.8      85    94.9      17    225.2    16.1     10       10        0       25      1.99e+09    {'T'}      0        0        0        0        0        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-05 00:00     6       4       1      -20.1    84.2    97.8    16.4    244.4    16.9     10        5        0       28      1.99e+09    {'T'}      0        1        0        0        5        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-05 12:00     7       4       1      -21.7    84.3    99.5    16.3    301.9    17.7      6        2        0       33      1.99e+09    {'T'}      0        1        0        0        4        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-06 00:00     8       4       1        -22    82.5    97.8    16.6    235.7    10.3      4        0        0       38      1.99e+09    {'F'}      0        1        0        1        4        0        1        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-06 12:00     9       4       1      -21.7    85.7    97.5    16.5    161.6    15.6      3        1        0       42      1.99e+09    {'F'}      1        0        0        2        0        0        1        0        0        0        0       60       50       85       70      12.5      14  
count = cumsum([1; diff(sample.Var1) < 0]);
ID = year(sample.time) + "S" + count;
sample = addvars(sample, ID, 'Before',1)
sample = 50x33 timetable
          time             ID       Var1    Var2    Var3    Var4     Var5    Var6    Var7    Var8     Var9    Var10    Var11    Var12    Var13     Var14      Var15    Var16    Var17    Var18    Var19    Var20    Var21    Var22    Var23    Var24    Var25    Var26    Var27    Var28    Var29    Var30    Var31    Var32
    ________________    ________    ____    ____    ____    _____    ____    ____    ____    _____    ____    _____    _____    _____    _____    ________    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____    _____
    1990-01-01 00:00    "1990S1"     1       1       0      -11.5    71.5    92.1    16.5    104.9     6.7      4        3        0        1      1.99e+09    {'T'}      1        0        0        0        0        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-01 12:00    "1990S1"     2       1       0      -11.8    71.8    88.8    16.1    125.1     8.4      6        3        0        4      1.99e+09    {'T'}      1        1        1        0        0        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-02 00:00    "1990S1"     3       1       0      -12.5      72    95.4      17     82.8     4.7      2        0        0        7      1.99e+09    {'F'}      1        1        0        0        0        0        1        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-01 00:00    "1990S2"     1       2       0        -11      87    87.2    15.3     89.4     5.3      1        0        0        3      1.99e+09    {'F'}      0        1        0        0        0        0        1        0        1        0        0       60       50       85       70      12.5      14  
    1990-01-01 12:00    "1990S2"     2       2       0        -12      86      90    15.4     52.3     8.5      1        0        0        5      1.99e+09    {'F'}      1        0        0        0        0        0        1        1        0        0        0       60       50       85       70      12.5      14  
    1990-01-02 00:00    "1990S3"     1       3       0        -18     170    90.1    16.6     70.9     3.5      1        0        0        8      1.99e+09    {'F'}      0        0        0        0        1        0        1        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-03 00:00    "1990S3"     2       3       0        -17     177    89.1    15.1     64.7     1.6      1        0        0       17      1.99e+09    {'F'}      0        1        0        0        1        0        1        0        1        0        0       60       50       85       70      12.5      14  
    1990-01-02 12:00    "1990S4"     1       4       0      -11.5      86    95.2    16.4    103.8     9.8      2        2        0        9      1.99e+09    {'T'}      0        0        0        0        0        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-03 00:00    "1990S4"     2       4       0        -11      86    93.4    16.5    154.1       6      4        3        0       12      1.99e+09    {'T'}      0        1        0        0        1        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-03 12:00    "1990S4"     3       4       0      -13.9    86.1    91.3    16.3    182.4    11.3     10        9        0       20      1.99e+09    {'T'}      0        0        1        0        0        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-04 00:00    "1990S4"     4       4       0      -16.1    85.2    92.7    16.6    200.6      15     13       12        0       22      1.99e+09    {'T'}      0        0        0        0        1        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-04 12:00    "1990S4"     5       4       1      -17.8      85    94.9      17    225.2    16.1     10       10        0       25      1.99e+09    {'T'}      0        0        0        0        0        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-05 00:00    "1990S4"     6       4       1      -20.1    84.2    97.8    16.4    244.4    16.9     10        5        0       28      1.99e+09    {'T'}      0        1        0        0        5        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-05 12:00    "1990S4"     7       4       1      -21.7    84.3    99.5    16.3    301.9    17.7      6        2        0       33      1.99e+09    {'T'}      0        1        0        0        4        0        0        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-06 00:00    "1990S4"     8       4       1        -22    82.5    97.8    16.6    235.7    10.3      4        0        0       38      1.99e+09    {'F'}      0        1        0        1        4        0        1        0        0        0        0       60       50       85       70      12.5      14  
    1990-01-06 12:00    "1990S4"     9       4       1      -21.7    85.7    97.5    16.5    161.6    15.6      3        1        0       42      1.99e+09    {'F'}      1        0        0        2        0        0        1        0        0        0        0       60       50       85       70      12.5      14  
The logic is straightforward.  The ‘count’ assignment cumulatively adds every instance where the difference in ‘Var1’ decrements, so the result is true or 1.  Logic values become numeric when used in calculations, so ‘count’ increments accordingly.  The ‘ID’ variable is easy to create using the year and the value for ‘count’ and string variable syntax.  (If you do not have string variables, use sprintf to create  a similar array.)  
I kept all the other variables here.  Use removevars to eliminate ‘Var1’ if you no longer want it in your timetablle.  
.


